Reactive Power Control Lama “The necessity of reactive power contol, production and absorption of reactive powor, reactive powor requ power taco contl and wotage regulation andthe loading capably cure of synctrencus danas, peso yen ‘controller, ‘Series compensation: reactor and capacitor, TCSC, SSSC. ‘Shunt compensation: reactor and capacitor, STATCOM, FC-TCR. ‘Series and shunt compensation: UPFC. (FACTS devices: working principle, circuit diagram, VI charactoristics, applications) 2.1 Terminologies and Definitions ‘The terms used in reactive power control are defined by the IEEE standard dictionary of Electrical ang Electronics Terms (IEEE - Std. 100-88) ‘+ Apparent power is the product of rms voltage and rms current and fs denoted by, VA = E+ Reactive power is an amplitude of power oscillation with no net transfer of energy and is caused by energy ‘storage components, such as a capacitor and an inductor. Active power is the power delivered as an output in the form of electric power, mechanical power, thermal power, ete. In normal balanced sinusofdal ac. circuit, VA = Active Power + Reactive Power ‘+ The right hand side is the vector addition. {In non-sinusoidal circuits there is an additional power, termed as distortion power. In this case, then VA = Active Power + Reactive Power + Distortion Power * Scalar power again applies to non-sinusoidal circuits and is the vector sum of active and reactive power butis equal to apparent power. — Active Power _ ‘ower Factor = “Reactive Power = °S® ‘Where $ is the phase angle between the starts of current and voltage signal for sinusoidal circuits. For non-sinusoidal circuits, the following additional terms are used, Displacement Factor = Fundamental frequency power factor Pe Distortion factor = < dll Scanned with CamScannerpeer System Opener co ) Reactive Power Control Power System Operation and Control (SPPU} Active power Where, Q = Reactive power SS = Apparent power Inthis case the power factor Is given by Displacement factors x Distortion factor. 21.1 Significance of Definitions Active Power (VA) This refers to the maximum value of current and voltage, which the equipment can handle, + Inpractice the capital cost ofthe equipment is directly related to ts VA rating. + TheVArrating being very small it can be express in kVA (VAX 103) or MVA (VA x 10), + Forbilling purpose, one of the component is kVA demand, Le, Maximum Demand of the consumer. + Mathematically maximum demand exists without.a direction (positive or negative). Active Power + This power is unidirectional and exists physically. It flows from the source Is the equipment connected in a closed circuit. Reactive Power + The energy storage component is responsible for Reactive Power. It does not Participate in the energy transfer Process, but itIoads the equipment. Power Factor * The power factor is an indicator of power utilization, + Ithasa great recognition in the power system. + Theutlty imposes the penalty tothe consumer, ift goes below the prescribed limit set up by the utility. 21.2 Impedance and Power Triangle * The present day power system is of complex nature, * The utility, as a rule, has to supply uninterrupted, rated voltage and rated frequency electricity within the tolerance limits, * With the advancement in industrial growth and use of electronics devices, the load is more non linear, * _Thenon linearity in the power system network generates harmonics. * Its therefore, necessary to take the review ofthe inductive network to simplify the analysis. * Fie 211 shows a simple ac. series circuits with parameters R (Resistance) and L (inductance) in series, ‘Supplied from rated voltage, frequency source. Vel, i Vole MeV Vem 1 ° 4‘ 'n=Va Fig. 2.1.1: R-L series circuit Fig. 2.1.2 : Phasor Diagram Impedance Triangle Fig-2.1.2s the phasor diagram of the circult shown in Fig 2.4.1. Scanned with CamScannerW_ Power System Operation and Control (SPPU)__2-3 Reactive a IA OAB is considered, they represent the voltage across: parameters. V-type =i +, In another words, OArepresents R AB represents Xy And OA represents 2. IR ‘Angle ¢ is the phase angle between V and I. Therefore the power factor cos = 77 =7. ‘OAB, hence, is known as Impedance Triangle. Power Triangle Fig. 2.1.3 shows a power triangle and is obtained by multiplying each current phasor by V, _ TruePower _Vicos¢ ‘Thus, cos = pparent Power = VI Pp S_ = Apparent Power = VI P cos lo Q = Reactive power = Vising Fig, 2:13 leads to show that St = Pag Fig. 2.1.3 : Power Triangle Ex.2.1.1: An inductive circuit consists of a resistance of 8 ohms and an inductive reactance of 6 ohms in sea supple trom a 200 V, 50 Hz source. Find (8) Curent lin the circuit (0) Phase angle between V and (©) Power factor ‘True Powe Soin, : cose = 7 Power consumed = Vi cos @=200x 200.8 =3200 watts 2.13 Reactive Power Control Q. Enlist the reasons for reactive power control. EEE Q. Wy is it necessary o control reactive power? What aro the ways of controling reactive power? ss 'Q. What is the necessity of reactive power control? Discuss the various sources of reactive power. SESE Scanned with CamScannerPower Al Jamia Mohammediyah reoce ns Reactive Power Control ‘em Operation and Control (SPPU) For maximum utilization of power peneration, It is necessary to have power factor unit somof the However the generation, transmission and distribution systems need reactive power for the operation o equipments connected in the network. ‘The Indian Electricity Act prescribes the limits on voltage regulation and are different in case of law, extra high tension lines. ‘There is also a limit on frequency variation of the supply. ‘Thus, there are many factors associated with the reactive power control Maximum Demand and Power factor being the major components of the power tariffs. ‘There is one more aspect of electric supply and it relates to power quality. Reactive power control, therefore, plays a vital role in the power systems network. The static capacitors are used for controlling the power factor. Every kVA added by the capacitor bank has to be included in the costing of reactive power. Power factor, ifleads is ofnouse since it unnecessarily Increases the cost of reactive power. ‘The reactive power control, in general, should lead to the following objectives. 1. _ Revenue requirement of all the sectors, Le. generation, transmission and distribution. 2. Economy in transmission because of optimal reactive power flow. 3. Operational flexibility, 4. Cost effective solution high and 2.13(A) Reactive Power Management Reactive power is the combination of forward moving real power combined with imaginary flow. Reactive ower is required to maintain the voltage to deliver active power. Reactive power deficiency causes the voltage to drop down, Reactive power refers to the circulating power in the grid which is not consumed and has strong effect on system voltages. Excess or deficit reactive power can cause voltage stability issues, heating oscillations ultimately causing the system to become unstable. Hence it is important to control and manage the reactive effectively to increase system stability and efficiency. ‘Various ways of reactive power control are through : (Synchronous condensers (6) Capacitors /Capacitor Banks (Gil) Shunt Reactors (i) Series compensators () sTATCOM (vi) Various types of Static VAR compensators like TCR, TSC, UPFC ete. ‘The effect of variation in excitation of synchronous machine on reactive power management. Synchronous condensers have the capability of supplying/absorbing VAR's based on its operating conditions and thereby achieving reactive power management or Improving power factor. Such operation is performiéd by varying the field/excitation voltage using a voltage regulator. ‘V-curves’ of the synchronous machine helps to understand the concept better. ‘An over-excited synchronous motor/condenser has a leading power factor which is useful in power factor correction. Scanned with CamScannerReactive Power Contry fon and Control Power System Oj Excitation Fig. 2.1.4 power factor correction as well as about 150% VAR’ joy synchronous condensers provides step-less compensation. By controling the excitation of the machine the amount of VAR's to be absorbed/provided can be controlleg lenthout any switching transient and harmonic issues. Thus ft helps in Improvement of power factor, VAR compensation thereby increasing system stability, regulation and control. 2.1.4 Power Quality work associated with its complex nature poses many problems in the operation ang ‘© Large power system nel control of the system. «The consumers operate with different types of load and the abnormalities in the system depend upon the sizeof the load, ie. heavy industry, street lighting, domestic etc. dynamic or transient and hence the factors © The loading may be classified into categories like steady state, responsible for the disturbances in the system are discussed in brief. L Impulses «These are usually because of the lightning, The parameters of the power system control use peak voltages and the duration. ‘* Thevoltage wave dies down exponentially. ‘The high voltage of the lighting stroke may cause the dielectric breakdown of the equipments connected to the affected network. 2. Transients All inductive and capacitive devices in the power system network, when switched on and off, the transient currents flow, which have an appreciably high valve. Though the possibility of the dielectric breakdown is less, it does stress the system and hence the adequate ‘measures are required to be taken to protect the system. The remedial measures include the insertion of resistance or inductor in series in order to reduce the current peak. 3. Sags and Swells Whenever there is a short circuit across the line, the voltage drops down suddenly. It is necessary to operate the protective devices in order to isolate the faulty section and restore the system to normal condition. This voltage change is called as sag which exists in the system for a short duration. Scanned with CamScanneri ‘Power System Operation and Control (SPP 2 Reactive Power Control + Such voltage sags make the electronic equipments mal-operate and hence the provision of Uninterrupted Power Supply (UPS) be made, + Sudden unloading of the system responds in reverse way, Le. it causes the voltage to increase suddenly called swells. + The effect persists only for a very short time a swells is more then there may be damage to th 4, Under and Over Voltages ind may not cause major damage. If, however, the frequency of he connected appliances. Under woltages inthe system under steady state condition may affect the equipments in adverse ‘way. ‘They may cause overloading of the equipments and result in poor power factor. + On the other hand, the over voltages in the system may not only raise the temperature of the equipments but also make the equipment damage, 215 Power Factor Correction 2.1.5(A)_ Shunt Capacitors + Power triangle shown in Fig. 2.1.3 shows that the apparent power * power P and the reactive power Q. having two components, namely the true ‘The true power has to be maximum in order to utilize the power optimally. ‘The necessity of the reactive power to operate the equipments is undebatable, ‘The limiting maximum value of cos 4 = 1 means there is no reactive power, * Similarly when cos @ = 0, the active power is zero, The power factor of the eircult is the ability of utilizing the apparent power. ‘The true power P is given by, P = Vicos¢ ‘The equipments to be connected in the system are desi igned for rated voltage and frequency and have fixed value of power P. It is obvious from the equation that Teo cos Low power factor means the circuit draws more current demanding high current ating means large kVA rating, For higher current to flow in the circuit, the conductor size is more. The copper loss is proportional to I? and hence copper loss is also more. The voltage regulation is also poor. In power system network the industrial and other lagging. ‘The utility prescribes limiting value to the power factor. loads are mostly inductive and hence the power factor is power factor is below the set value the consumer has to pay the penalty. Similarly ifthe power factor Is greater that the set value, the consumer gets the concession in billing. The consumer has to make the necessary provision for improving the power factor, Wines all Scanned with CamScannerJ Reactive Power Contry | tion and Control (SPPU) Power System Oj The better way to improve the power factor Is to use the capacitor across the crcult.The capactor draws ye Jeading current and hence reduces the lagging reactive power. + By suitably selecting the value of capacttor for certain load, the lagging reactive power can completely 5, nllified. © Theprocedure for selecting the capacitor value is explained, Let the circuit be as shown in Fig. 2.1.5. R XM 1 | +24 1 Fig. 2.1.5: 1-9 A.C. series circuit + Rand are the resistance and inductive reactance connected In series. When connected to ac, Source of voltage ‘Vand frequency f, it draws a current I; with a lagging power factor cos $4. ‘+ Itis desired to improve the power factor to cos $2 by connecting a capacitor ‘C’ across the series combination sp that current I remains same and an additional leading current Ic flows through the capacitor branch. Fig. 216 shows the phasor diagram wherein the active power remains unchanged. c fe Q v SN, & | 7 D DN Fig. 2.1.6 : Phasor Diagram for circuit in Fig. 2.1.5 “OA’ the active power P, is given by, Pe [cos 61 lz cos $2 ‘0B is the original reactive power Sj and Is given by Sy Ising The effect of connecting the capacitor is shown by current Ic leading V by 90°. This component of current, being ‘opposite of lagging component, reduces the current to lp at same power but different power factor cos ¢2. sInsingg = Iysingy=Ie Alternatively, ele = Insingy~Ipsin 6 Also, Xe eb Ie “ov and is the value of capacitance required to Improve the power factor ofthe circult from cos 4 to cost ¢z- The theoretical derivation given above can well be explained through the example, wise Scanned with CamScanner an neepower System Operation and: Reactive Power Control 1.2.12! single phaso induction motor connected to 200 V, 50 Hz a.c. supply draws a current of SA at 0.8 power factor lagging. It the power factor is to be improved to unity, what value of capacitor be connected in parallel with the motor? AB cos Q = 2005 0.6 = 600 VAR (lagging) 0. * 8 > la ‘« Fig. P. 2.1.2: Power Triangle for example ‘When improving the power factor from 0.8 to 1.0, the whole of the reactive power has to be nullified by the capacitor. = GOOVAR (leading) I¢= current through capacitor 600 50 234 viv Bat ke = yeep sankey 2k 2axS0x200 = 472 HF Ex213: AS-~, 10 KW induction motor runs at 0.8 power factor lagging. It is desired to improve the power factor to (0.9 lagging. It bank of capacitors is used in delta acress the source, what will be the RVAR capacity of capacitors per phase 7 o o2 cost08=3687° tang = 0.75 cost 0.9 =25.84° tan 2 = 0.4842 EVAR supplied by the capacitor (leading) = P (tandy—tan gz) = 10(0.75 - 0.4842) = 2658 = KVAR perphase = “°° = 0.886 kVAR ‘The example solved above shows that cos! xy = power factor existing cos"! xp = power factor improved KVAR (tangy ~ tan gz) = “ey ‘These expressions can help in preparing the ready-reckoner as shown in Table P.2.1.1, Scanned with CamScannerjon and Control (SPPU Reactive Power Contra, Power System Oj le P.2.2.1 : Ready-Reckoner to find out KVAR/KW (typl New Power factor Existing power | 09 | 091 | 092 | 0.93 | 0.94 | 0.95 | 0.96 | 0.97 | 0.98 | 099 | 199 factor 6251 | 0.657 | 0.691 | 0.729 | 0.769 | 0811 | 0978 | 192 0.70 | 0536 | 0.5647 05345 | 05641 [05949 | 0627 | 0.663 | 0.701 | 0741 | 0.703 | 0.850 | oo92 071 | 0508 0.72 [0479 | 0s0v2 | 05370 | 05606 | 0.601 | 0.634 | 0.672 | 0.712 | 0.754 | 0821 | 0963 0.73 [0.452 | 0.4806 0.74 | 0.425 | 0.4533 075 | 0.398 | 0.4263 0.76 | 0.371 | 03996 | 0.4292 077 | 0345 | 0.373 | 0.4026 0.78 | 0319 | 03467 | 0.3763 | 04071 0.79 _| 0.292 | 0.3205 | 0.3501 | 0.3809 0.80 | 0266 | 0.2944 | 0.324 | 0.3548 | 0.387 0.81 | 0.240 | 0.2684 | 0.298 | 0.3288 | 0.361 os2 | 0.214 | 0.2474 | 0.272 | 0.3028 | 0.335 | 0.369 | 0.407 023 | 0118 | 0.2164 |0.226 | 0.2768 | 0.309 | 0.343 | 0.381 | 0.421 o¢ | 0162 | 0.1903 | 0.2199 | 0.2507 | 0.283 | 0.317 | 0.355 | 0.395 | 0.437 | 0.478 | 0.645 osi02z | 05410 | 0573 | 0507 | 0.645 | 0.605 | 0.727 | 0.794 | 0936 04929 | 05137 | 0546 | 0.580 | 0.618 | 0.658 | 0.700 | 0.740 | 0909 04559 | 04867 |os19 | 0553 | 0591 | 0.631 | 0673 | 0713 | ona 0.4600 | 0.492 | 0526 | 0.564 | 0.604 | 0.652 | 0.687 | oass 0.4334 | 0.466 | 0500 | 0.538 | 0.578 | 0.620 | 0.661 | 0829 0439 | 0.474 | 0.512 | 0.552 | 0594 | 0.633 | ofg 0413 | 0.447 | 0495 | 0525 | 0567 | 0.608 | 0.76 0.421 | 0.459 | 0.499 | 0541 | 0582 | 0750 0.395 | 0.433 | 0473 | 051s | 0556 | 0.724 0.447 | 0.489 | 0.530 | 0.698 0.463 | 0.504 | 0.672 The above example can be solved by using the ready-reckoner. Corresponding to existing 0.8 and| improved 0.9 the (KVAR/kW) = 0.266 KVAR supplied by capacitor = kW x 0.266 = 10 x 0.266 = 2.66kW 2.1.5(B) Series Capacitors Q. Explain the problems associated with the series compensation. SUES * Capacitor is connected in series with the supply. Load being inductive in nature can be expressed as Z = R+j% ‘The whole circuit is supplied from a source of voltage V and frequency Fin Hz. % = aE co RAK, {toad} Vs Fig. 2.1.7 : Capacitor connected in series a Scanned with CamScannerpower System Operation and Control (SPP 210 Reactive Power Control L ‘re total impedance of the circutt is the r= RIK =Xe) Fig 2.1.7 shows the clrcult. Let 1 be the current drawn from the source. The phasor dlagram for the circults Is shown in Fig. 2.1.8, Fig. 2.1.8 : Phasor Diagram «grand ¢z re the power factor angles of the circult before and after the power factor Improvement. + Itcan be noted from the diagram that the capacitance carries the same current as the load. With the change in load the current also changes and hence the kVAR supplied by the capacitor. The advantage with serfes capacitors are: Good voltage regulation ‘The stability of the system is better L 2. Thevariation in current with load reduces the transients 3. 4. Controls the power flow ‘The series capacitor connected in the circuit carries short circuit current, which is for more than the rated current. Designing for a series capacitor for short circuit current is highly uneconomical, Itis always better to supply the reactive power at the load point and hence shunt compensation is more suitable. Depending upon the situation the design engineer has to decide the option of series or shunt compensation. 215(C) Capacitor Banks ‘The standard sizes of the capacitors are manufactured by the manufacturers. ‘The size of the capacitor is decided from the load and the power factor of the consumer. Since the load varies during 24 hours of the day and the seasons of the year. In order to provide the flexibility in operation the capacitor value needed for the consumer is assembled in the form of the capacitor bank consisting of several capacitors in series or parallel. Depending upon the load current the sensing relays are provided to sense the current automatically so select the total KVAR that the preset power factor is maintained. ‘The equipment is known as Automatic Power Factor Controller (APFC). The simplicity in connection and operation of APFC {s versatile and can be used right from residential complexes tothe industries, The advantages of using APFC provide better operation of the system at reasonable cost and with economy. Fig. 2.1.9 shows a schematic representation of a Power Factor Correction (PFC) system with filter circuit Teactors for harmonic reduction. Wrttmietgs Scanned with CamScanner‘ower System Operation and Control se pt —— Fig. 2.1.9: PFC Controller Schematic Diagram 2.1.5(D) Ratings of Bank Capacitor banks are rated at 1. Reactance in ohms/phase 2. Percentage compensation 3 MVAR 4. Overload currents with duration ‘The parameters mentioned above are inter connected and can be drawn from the load flow studies. Of all the electrical equipments the capacitors have the highest dielectric stress. A normal high tension shunt capacitor has voltage stress of 60 V/micron. ‘The overload capacity of series capacitors may go as high as 2.5. 2..5(E) Compensated Transmission Lines ‘The compensation of transmission lines is the process of increasing the power transmission capability bY ‘changing its electrical characteristics. While compensating the transmission lines it does not merely satisfies al the constraints of electricity act and performs the following functions : (9 Provides at voltage profile (il) Improves stability (ill) Meets reactive power requirements ‘The flat voltage profile ts achieved by modifying the effective surge impedance to a virtual value. The Surs® impedance of an uncompensated line is, % = ft c ¥. Scanned with CamScannerReactive Power Control ‘stem Operation and Control (SPPU} fy appropriately modifying the Inductance and capacltance of serles or shunt capacitance, % = VX FP’ and 2) are the virtual power and surge impedance, then 2 = % visintiw % This method fs known as ‘Virtual Surge Impedance Compensation’, The stability can be Improved by effectively reducing the virtual value of 0 (transmission angle) by using serles or shunt compensation. This method is known as ‘0 compensation method’, Another way of compensating transmission line Is to sectionalize the line into number of short-sections, 5(F) Concept of Sub Synchronous Resonance (SSR) De (©. Explain Explain the concept of sub - synchronous resonance in detail SEDI EAET Let the line be compensated by series capacitor, The generator, transformer and the line have Ly Lz and Ly as their inductance respectively. Let C be the capacitance connected In series, Interms of reactances they can be written as Xq = X, + X; + X and Xsc = series capacitor reactance. ‘The natural frequency of oscillation will be given by, 1 fo 2nafUcsc naflcse = 4 =f [Xe Pa \ se 2R\ | and Bt Nec Xi ‘The term xccrepresents the degree of series compensation. Since fy ‘< f there is an occurrence of sub- synchronous resonance. Because of large disturbances oscillations take place in the system and even ifthey are damped down the system 's subjected to sub-synchronous resonance ultimately resulting in ‘Low cycle fatigue’. 2.15(G) Compensators Cs - What are different types of compensations used in power system? = ETE SS FTIETN ES Q. _ Discuss various ways of providing shunt compensation, 2. Shunt Compensation * This consists of a static VAR compensator comy iposed of thyristor-switched capacitors and thyristor controlled reactors. With proper coordination of the capacitor switching and reactor control, controlled between the capacitive and inductive ratin, voltage to a desired value. the VA output can be continuously ig of the equipment. It regulates the transmission line Scanned with CamScannerBE _Power System Operation and Control (SPPU) 2-13 pcr AY POET Coy ru 2. Thyristor Controited Reactor (TCR) © The shunt connected thyristor controlled Inductor can be used for continu « Tthetps in reducing the response ime, The current inthe circu Is essentially reactive lagging the yop, ' 90°. * This type of reactor is useful in continuous cont rol with no transients, uous varlation of the reactan Ke, however, Itgenerates harmonic, ‘Thyristor Switched Capacitors (TSC) steps Depending upon the VAR requirem, ent circult or can be excluded. ea + The capacitor connected Is used to vary the capacitance In number of capacitors are used which can be included into th + Thesensing device, to know the, VAR, Is used. The major characteristic f SC are (a) Stepped contro! (b)Notranstents (©) Noharmonics (Reduced losses (©) Flexibility in operation 2.1.5(H) Series Compensator University Questions @. Explain the advantages of series compensation. Also stato the ca tion of capacitor used in the series compensation, EAE SPPU- May 19.5 SEL 1r bank is controlled by a thyristor srated at voltage and current zero crossing. ed by the current in the thyristor controlled switched off. ©. What are different types of compensations used in powor system? Q, Explain the problems associated with series compensation. © In this scheme the capacitor banks are connected in series. Each capacito bypass switch. The thyristor switch is ope! «The degree of series compensation can be increased or decreast reactor. The compensation is minimum when the TCR is EsVe ‘The power transfer, P= sind Where, V, = sending end voltage V, = receiving end voltage 5 = angle between V, and V; K = MX Series compensation Is useful for optimizing power flow between two areas having different loadings. This scheme has following characteristics (1) Minimum system losses (i) Over toad elimination (iil) Optimum sharing between parallel paths (iv) Power flows directed along requisite path 2.1.5(1) Comparison of Compensating Schemes OT Q. What are the diferent types of compensations used in powor systom? ‘The following Table gives the comparison : Scanned with CamScannerower system operation and Contra (SPPU Reactive Power Control compensating Equipment Advantages Disadvantages scR ‘Simple construction Value fixed ssc Simple construction Fixed capacitor switching transients sub harmonics Series Capacitor Simple, Sensitive, Good -over load | Slow response high maintenance capacity synchronous condenser | Fully controllable, Low harmonics Strong foundation necessary sensitivity changes with Jocatton. saturated Reactor Rugged construction, Large overload | Value fixed, sensitive to location capacity, Low harmonics TCR Fast response fully controllable Harmonics, sensitive to location repairable TSC Repairable, No harmonics No over voltage limit controls, low frequency sensitive to location 2.2 Production and Absorption of Reactive Power Q. What is synchronous condenser ? State its advantages and disadvantages and compare it with equivalent capacitor ‘and reactor shunt compensation. FURIE EAE ERENT Importance of Reactive Power + Inc. power network there are many equipments connected. For transformation of energy from one form to another there is a need to establish the magnetic field. All through the magnetic field is required to operate these equipments. The setting up of variable flux in the ‘power system is because of the reactive power and hence reactive power is essential. ‘The utility provides electricity to the consumer in the form of active and reactive power. The flow of active Power is from utility to consumer, however, the utility as well as the consumer generate reactive power and hence the reactive power flow is bi-directional. + reactive power i excess, itIeads to poor power factor, high current and hence the connected equipments in the power system network will have larger volume, more losses and high temperature rise. This needs to control the reactive power so that maximum utilization of the energy will be possible, the current will be reduced, losses will be reduced and the system will be more efficient. ‘The synchronous machine has a unique feature of running at lagging as well as leading power factor. All the enerating stations use synchronous generator for generating electricity. ‘These generators (alternators) are generating capacity utpo 1000 MVA. The importance of the synchronous machine can be underlined through its modelling, equivalent circuit, phasor diagrams and characteristics. Scanned with CamScannerReactive Power, Control (SPPU)__2- Power System Operation. 2.2.1 Modelling of Synchronous Machine Cylindrical Rotor Machine (Generator) ‘© Fig.2.2:1 shows the pictorial representation of synchronous machine, he rotor Is uniform, surface such that the airgap between the stator and t © Therotor covers the whole: the axis of which is at 90° to the coil. ‘The R phase, for simplicity is shown by coll R-R', 4if synchronous machine Rgaxis of field mi > axis of phase R Fig, 2.2.1: Synchronous Machine «Theaxis of the field winding, similarly, is also shown. The flux ¢ set up by the excitation winding and its MMF y, are shown in Fig. 2.2.1. + Theemssetupin the coil of phase Ris eyand is given by er Where, N= number of turns in the coil = drcosast = synchronous speed in radians second t = time in seconds ser = -NA Greasy = No, orsin at ©. Interms of rms. value substitute, sin wt = 1 Ey = V2ntNey @, = 2nf oy = flux/pole ‘The stator winding is distributed and hence winding factor can be added in the expression : Ey = V2mkof Np Or Where, ky = winding factor Non = N=No.ofturns/Phase The stator winding, when acting as a generator, supplies a balanced 3-phase load. The armature current sets"? flux and its magnetomotive force (mmf) which is sinusoidally distributed in space and rotates at synchron™ | speed. The mmf so produced is called armature reaction, | wise Scanned with CamScannerpower System Operation and Control (SPPU) Reactive Power Cor ‘Axis of fold Fig. 2.2.2: MMF diagram Ia and Ey are in phase ‘The mmf diagram is shown in Fig. 2.2.2. Field mmf Myinduces em. Ey- Fy induces emf Ey Fig. 2.2. fs self explanatory and the mmf diagram for any position on load (lag) can be drawn similarly. Fig. 223 shows the phasor diagram depicting the flux and voltage positions for a lagging current l, Ea = eter Fig. 2.2.3 : Phasor diagram ‘The equivalent circuit for the phasor diagram shown in Fig. 2.2.3 yields to Fig. 2.2.4. Xr, 5 5 Fig. 2.2.4 : Equivalent circuit without armature parameter ‘The equivalent circuit does not account for the armature resistance ra and its leakage reactance x, which are present in the realistic synchronous machine. Er is the induced emf because of the air gap flux 6, and is called air gapems. When the armature parameters are added in equivalent circuit then the terminal voltage is given by Ve = Ep-lata~ Slax & Mg Fig. 2.2.5: Equivalent circult with armature parameters Scanned with CamScanner7 Pe Cy ances ave limped topsther ee Reactive f the two ‘©The equivalent circult then is presented tn Fit reactance, is called the ‘synchronous Reactance’ | xy = art XD «Further more the Synchronous Impedance’ (Zs) isdefined #5 Te = tat IX «The final equivalent circuit fs shown in Fig, 22.6. " 5 M Fig. 2.2.6: Equivalent circutt in terms of synchronous impedances «In practice, while analysing the parameters of synchronous machine, the armature resistance 1s very small ang sma be neglected, The saturation effect fs also neglected so that the parameters are linear. 2.2.2 Synchronous Reactance and Short Circuit Ratio = While deciding the value of the synchronous reactance, the open circuit test and the short circuit test are carried out on synchronous generator and the characteristic are shown In Fig. 2.2.7- «From the characteristics shown, the value of synchronous reactance is given By, __ Open circuit voltage (Voc) Xs = Short circuit current (Isc) Both Voc and Isc correspond to the same excitation current. Open circuit voltage Voc Veatea ° 5 ra 1, = Field current Fig. 22.7 : Open circult (o.c.c) and short cireuit (s.c.c) characteristics of synchronous generator The Short Circuit Ratio (SCR) Is defined as the ratio of fi jold current required to generate rated voltage on OCC field current required to obtain rated 1, under short circuit condition on SCC. 5 me Hence ser = of” + Ifthe calculations are made on p.u. basis, then 1 SCR = Xs (p.u.) ‘The SCR is the representative of the synchronous reactance hence decides the stability limits. weet Scanned with CamScannergoer System Operation and Control (SPPU) 2-10 Reactive Power Control 323. Loading Capability Curve of Synchronous Generator sketch and explain the loading capability of synchronous generator. Also state the constraints for generation of ° lagging and leading MVAR. SPPU May 12, Dec.12, May 13, May 14, Dec. 14, Dec. 15, 10 Marks. Describe the loading capability curve of generator. PIT ‘Sketch and explain the loading capabilty curve of synchronous generator, ECR Capability curve indicates the optimum operating conditions of the generator for its safe operation. It is derived from the phasor diagram of the machine. The limiting values are plotted on the following performance indices. (1) Optimum values of MVA and MW loadings (2) Steady state stabitity limit (3) Optimum value of excitation current + The optimum value of the MVA loading is decided by the generator rating, If the MVA rating exceeds then the current and hence the losses will increase resulting the temperature rise beyond limit. This causes the insulation to get damaged. + The MW output has consideration of MVA and power factor. The generator output is controlled by the prime mover and hence due care is to be taken that the prime mover rating should not be exceeded. + The field current flowing in rotor circuit causes the heating of rotor or fleld winding. If excessive current flows the rotor copper losses will increase and there is excessive heating of the rotor eircult, Ultimately it will damage the insulation provided. ‘+ lfthese limiting conditions are not followed strictly, the generation will be stopped and {t will be required to be isolated. This situation will lead to heavy revenue loss. * Neglecting armature resistance, he following phasor diagram shown in Fig. 2.2.8 is considered to derive the capability curve. Fig. 2.2.8 : Phasor diagram of cylindrical synchronous generator neglecting ry * Fit-228 shown above is to be converted into power triangle and hence all the sides of the phasor diagram are 3, multiplied by &) to represent the power triangle in 3-phase values. Fig, 2.2.9 depicts the change. VX Ia Vile ~ h Scanned with CamScanner2:19 n and Control (SPPU) Reactive owe Conny My P =H oxy ly cos=3Vela cos? as a ry fy sind=3 V4 ysind axis Fig. 2.2.9 : Power Triangle ince indices with proper scale. «The capability curve i drawn with limiting values of the perform Pha (Paral 19 Onyn) ft” Soe Qaxis Fig. 2.2.10 : Capability curve for synchronous generator by drawing the Bpay Pav Snax and Ey (max) line as stated in the diagram the capability curve can be drawn The inside hatched portion is the safer limit of operating the synchronous generator. Requirement of Reactive Power for Power Factor Control and Voltage Regulation 2.3 2.3.1 Sources and Sinks of Reactive Power Reactive power Is generated or absorbed by number of devices or equipments connected to the power syste network. The flow of reactive power through the network is therefore controlled by these devices. Let us discs about these reactive power sources. Generators ‘The Synchronous machines are able to produce or absorb reactive power depending on the DC excitation giv to its field winding, The machine generates the reactive power when over-excited and absorbs reactive power whe under-excited. It is most commonly used source of reactive power for voltage control. Capacitors and Reactors To control the reactive power, the capacitive and Inductive devices are used in series and shunt compensatst techniques in order to regulate system voltage and stability. Scanned with CamScanneree og poner System Operation and Control SPPU) 2.20 Reactive Power Control ‘A capacitive compensator produces reactive power, and an inductive compensator absorbs reactive pows Series capacitor compensation fs normally applied for transmission lines to produce reactive power when It is required while shunt capacitors are installed at substations In load areas to produce reactive power to maintain voltage within limits. ansmission Lines and Underground Cables The transmission lines and also the cables absorb and generate reactive power. If the transmission lines are tenily loaded then it consumes reactive power, which decreases the voltage of the line. If transmission lines are righty loaded ‘then it will generates reactive power, which increases the voltage of the line. sold State Converters ‘There are different types of solid state converters in-use in power system operation, like HVDC converters. ‘These types of converters always consume reactive power when they are in operation, Due to this reason, most of the converters use reactive compensation devices to limit reactive power requirement of the converters. ‘Transformers For the production of magnetic field in a transformer a reactive power is required, therefore transformer absorbs the reactive power. The reactive power consumption of a transformer depends on transformer rating and current loading. 23.2 Importance of Reactive Power ‘+ _ Reactive power plays an important role in the electrical power system for various functions such as it improves the voltage profiles, it decreases the losses in a network, it provides sufficient reserve to ensure system security in emergencies, and other different functions, + Letus discuss some of the reasons in brief which will makes reactive power so important. Voltage Control Normally all the electrical equipment's are so designed to operate satisfactorily within specified limits of rated ‘voltage. The changes in voltage are mainly caused due to change in load on power system source. * Ifthe load on energy source increases, the voltage drop in power system elements increases hence the voltage at the consumer terminal decreases, and vice-versa. These types of voltage changes on the supply system is undesirable as it deviates the actual working of the devices at the consumer end such as lamps, motors and other electrical equipment’s which are sensitive to voltage variations. Hence the power system is therefore, must be designed so as to maintain those voltage variations by providing voltage control devices at suitable places. The most common method of maintaining voltage profile is the injection and absorption of reactive power. In general, increasing reactive power causes the system voltage to | rise while decreasing reactive power causes voltage to fall. ‘The voltage control devices are placed at two or more than two places in the power system network because ‘there may be different voltage drops in different sections of transmission and distribution systems and the load characteristics may also be different in various circuits of the power system, Most commonly this type equipment’s are placed at generating stations, transmission substations and feeders. ‘The different methods are used to control voltage in the transmission line such as excitation control, use of tap changing transformers, use of shunt capacitors, series capacitors, synchronous condensers, and boosters. Each technique has its own advantages and disadvantages. Dependls on the suitability, ‘methods are employed for controlling the receivers end voltage level. In case of high loading condition more Current is drawn from the supply that results in to sudden fll In receivers end voltage. If there exists large voltage drop, it causes to trip the generating units, fllure in equipment and overheating of motors. availability and cost, these Wrens Scanned with CamScannerReactive P tive Power ¢ ontro rol (SPU) ‘rating mechanism or rela bring the voltage back t Power System Operation and Conti fe the reactive power equipment, This will also be achieved yj th During this condition, automatic ope ys will activat that reactive power will increase to fo Its rated value. series reactors and series capacitors. toaded condition recelvers end volta ation, lower power factor and autor: ines is compensated by automat en control by alternator, shunt capactors go ncreases toa greater val. This wil aug Be eipping of equipment’. Under this cond if tye power compensation devices sy ind reactors. ss) ‘© During the case of light damage in machines insu! additional reactive power in the I as synchronous condensers, excitat Satisfy Reactive Power Demand tion of some loads or devices such a5 transformers and HVDC converters, requires reacyg power demand, ,e drop will take place. ‘maintain the power, due to Which lings ‘This will result in to voltage collapse y ator, the voltag core current from the supply t© 1es will reduces further. © For the proper opera power. When the load has large reactive «Due to voltage drop, system will draw m consume more reactive power and hence voltae t voltage drops very low. This collapse in voltage will cause to tripping of generators sane es the system ang 4 , jue to the reason th ot + oo vpnected to the power system. This voIaE® C& lapse is due acl png of on ee load which is not being fulfilled due to shortage of power system is unable to supply reactive Power demand of I reactive power generation and transmission- «inorder to resolve this issue, reactive power § unctive power is required by the loads. Howe¥ehs reactive power demand ifthe Toads draw excessive rea is connected to the load locally where iil charge consumers as a penalty for .e allowable reactive power demand cource like series capacitor utility companies wi .ctive power above thi limit. Reduce Electrical Blackouts Inadequate reactive power ‘As discussed, insufficient amount of reactive of generating stations and various equipment Sitondon in Aug 28, 2003; at Sweden and Denmark in Sep 23, Produce Magnetic Flux Most inductive loads e.g. motors, order to produce a magnetic field. In every el is consumed for production and maintenance of order to obtain the high value of power factor, capacitors a devices to supply the reactive power. 2.4 Types of FACTS Controller Ee Explain the importance of FACTS controllers in power system.. (CTS controllers following two different methods are considered. The first method ches as controlled elements, the sources. Different types of fac as been a major reason in power outages in worldwide {t ultimately results in to the shutdown 1 in power system network hi tage collapse that ts include, at Tokyo in July 23, 1987; power causes vol 1's, Some examples of such blackot 2003 ete. transformers, ballasts and induction heating devices require reactive powers fectrical machine, some part of input electric energy, i. reactive power ff magnetic flux to do so. However, it leads to lower the power factors re used which are generally connected across these Aa «For the development of FA ‘employs reactive impedances or a tap changing transformer with thyristor swit second method employs sel-commutated static converters as controlled voltage controllers in power system are as 1. Series controllers 2. Shunt controllers 3. Combined series series controllers 4. Combined series shunt controllers. Scanned with CamScannerpower System Operation and Control (SPU) Reactive Power Control The general symbol for the different types of which shows a thyristor ‘dance, such as capacitor, facts Controllers are shown in Fig. 2.4.1. arrow inside a box. The series controller of Fig. 2.4.1 15.0b could be a vatlable Impe M |r (1) General symbol for FACTS controller (FC) (2)Series controter 6 Une sor (9) Shunt conrater “aes conver v (8) Coordinated Series and shunt controller 24.1 Different FACTs Controllers (6) Unified Series and shunt controler ‘The shunt controller shown in Fig. 2.4.1(3) may be of variable impedance, variable source or a combination of both, All shunt controllers are used to inject current into the system atthe point of connection, Combined series- Series controllers of Fig. 24.1(4) could be a combination of separate series controllers which may be controlled ina coordinated manner to each other or it could be a unified controller. ‘The combined series-shunt controllers are either controlled in a coordinated manner shown in fg 2.41(5) or a unified Power Flow Controller with series and shunt elements as in Fig2.4.1(6). In case of unified controller, there can be an exchange of real power between the series and shunt controllers via the de powerlink. Energy storage devices such as a capacitor, battery, superconducting magnet, or any other source of energy can be connected in parallel through an electronic interface to fill up the converter’ de storage as shown with dotted lines in Fig.24.1(2), A controller with storage device Is much more effective for controlling the system dynamics ‘than the corresponding controller without storage. ‘The group of FACTS controllers which uses switching converter based synchronous voltage sources Includes the STATIC synchronous compensator (STATCOM), the static synchronous series compensator (SSCC), the unified Power flow controller (UPFC) and the latest, the Interline Power Flow Controller (IPEC)- Scanned with CamScanner10s and disadvantages. nat is evios compensation? Stats tAVATIS 7 voltage by connecting a capacitor In sey rh uit The method to Improve the system Is Injected In series with the transmisso ransfer capability of the ine, te ‘series compensation Is t transmission line. ries compensation rei sm, Whiel high voltage tine. active power Jn will improves the power t © Inother words, in se! {improving the impedance ofthe syste This is mostly used in extra and ultra 2.5.1 Reactor and Capacitor ©The power transfer through the transmission line isgiven by V3 x Ve P= Ty Sand ny Power transferred per phase ‘Sending-end phase voltage Receiving-end phase voltage Series reactance of the line Phase angle between Vs and Vr Where, Pr Vs - Ve - % - ae Ifa capacitor of capacitance reactance Xe is connected in series with the line, then the reactance of the ie i transfer for the circuit is given by, reduced from X, to ( X,~ Xc). Hence the equation of power Xs Xu Xe Xe Xa P Vs Va Fig. 2.5.1: Transmission line system with capacitor Vs VR Pp = i 2 = Ox 0d (253) Divide equation (2.5.1) by (2.5.2) esd 054 Where + The factor kis called asa d vs legree of compensati es ation or compensation factor. Thus, per unit compensation s 8% x -% xpPu i | Scanned with CamScannerwy powersystem Operation and Control (SPrU) _ 2.24 Reactive Power Control ‘compensation equation ts X K = SEpux ro0% where, Xs the total series inductive reactance ofthe line per phase Xcis the eapacitive reactance of the capacitor bank per phase Ingeneral, the value ofk lies between 0.4 and 0.7, For k= 05, Pe wh od, 1" T-K"T-o5*2 Hence the power transfer is doubled by 50 % compensation. 252 Thyristor Controlled Series Compensation (TCSC) Ion Explain operating principle and working of TCSC with the help of ercuit diagram and characerai, 1. Wit diagram, explain working of Thytistor Controlled Sates Capactor[TCSC], 2. Explain the working of TCSC with its characteristics, ‘+ Thyristor Controlled Series Compensation (TCSC) Is used in reactance of the lines which provide sufficient load compens: control the amount of compensation of a transmission line, distinguishing qualities are very desirable since loads are co + TCSC operates in the same way as like Fixed Series Compensation, reactance absorbed by the capacitor device. The basic structure of a TCS td cB Power systems transmission line to control the ation. The benefits of TCSC are like, its ability to and its ability to operate in different modes. These mstantly changing and cannot always be predicted. but it provide variable control of the iC can be seen below: Fig. 2.5.2: TCSC compensation Scanned with CamScannerB Reactive Power, 2.25 W Power System Operation and Control (SPPU compensator (TESe) «made up ofa series capactance which has papa “TesC will operate in different modes depending on when thy aeaaerae gered forthe inductive branch. The different modes of operation of TCSC areas, (0) Blocking Mode in this mode the thyristors are not triggered thus opening the Inductive bran mode the scheme actually operates as fixed sertes capacitor (FSC). {Bypass Mode: tn this mode the thyristors are always turned ON. In sucl a mode the capactor any arein parallel helping in reducing the current through —- wet — ay je he thyristors are triggered slightly before capacitor voltage cros, : (9 ot an me he a it ‘ce of TCSC without the need of larger capacitor in TCSC. nected to a constant voltage source but in TCSC the TCR ts connected across capacitor jp ince. The best part of TCSC is that the effective capacitive Impedance can be raised, eye wfadditional voltage of TCR appears across capaciygy ft + Athyristor controlled serte Including a thyristor controlled reactor. capacitan © ATCRalone ts con of fixed voltage sout actual capacitor by controlling the firing angle making ‘TCSC carvies all the benefits of FSC but in addition also carries the increased compensation. TCSC also, alin ability to limit line current which is typically important In event of a fault. TCSC also carries an additionay of the ability of damping of sub-synchronous resonance due to the parallel LC circuit combination, Ths raat ability to transfer more power and multi area connections. The TCSC will allow different operating mast depending on system requirement; TCSC is convenient for several reasons. mada + Inaddition to number of benefits of FSC, TCSC allows for increased compensation simply by working in di rode of operation, as well as controls the line current in the event ofa fault. Also TCSC Is used for danpie sub synchronous resonance caused due to tensional oscillations and inter area vibrations. ie + Theabilty to suppress this typeof oscillation fs due to the control mechanism controlling the compensator. Ty, will ultimately results in to transfer more power, and the possibility of connecting the power systems ¢ different areas over long distances. 25.2(A) TCSC V-I Characteristics The V-I characteristics of a TCSC is shown in Fig. 2.5.3. This characteristic is illustrated for continuous tay applications, short-duration implementations (30 min), and 1-10 s. In both capacitive and inductive zones ty ‘operation is generally constrained between the minimum and maximum reactance limits. MOV Protective Level (Capactive) oR996 (09 Xe han) F i Carent (pv om has) Fig. 25.3: The V-1 characteristics of TCSC Scanned with CamScannerPower System Operation and Control Limitations of TCSC Reactive Power Control The design of TCSC Is based on the application requirements then also, the some limits are determined by the characteristics of different TCSC components. Some of the Important limits are : 4, Voltage limits : Voltage limits Is the maximum amount of voltage across any operating equipment 1s determined by the equipment’s insulation level. The constraint on voltage may vary with the duration of application of voltage. For short durations the overvoltage limit is more critical than that of the eapacltor. 2. Current Himits: tis required to control the currents in the thyristor valve, FC, and surge inductor to prevent overheating, Harmonics also cause heating and therefore have some effect on the operation of TSC. 3. Firing-angle Timits: It is required for the thyristors, which must be carefully handled so that the TCSC does not go Into the resonant region, 25.2(C) Applications of TCSC Ina power system, the device TCSC has huge potential n applications because TCSC can improve power system performance including the control of power flow, electricity transfer capability improvement of the transmission system, improvement in transient stability and also mitigation in sub-synchronous resonance (SSR) etc. 25.3 Static Synchronous Series Compensator (SSSC) + The static synchronous series compensator (SSC) Is a modern FACTS device that consists of voltage source converter which is connected in series to a transmission line through a transformer. + The working of SSSC is like a controllable series capacitor and series inductor. The primary difference Is that the voltage injected by SSSC is not related to the line intensity and can be managed independently. + Due to this property the SSSC will work satisfactorily with high loads as well as with lower loads. + The Static Synchronous Series Compensator (SSSC) has three basic components, 1. Voltage Source Converter (VSC) - this is the main component SSSC 2 Transformer - transformer is used to couples the SSSC to the transmission line 3. Energy Source ~ the energy source provides voltage across the DC capacitor and compensate for device losses 2.5.3(A) Working of SSSC ‘+ Static synchronous series compensator works like a STATCOM, except that SSSC Is connected in series instead of shunt. ‘+ SSC is able to transfer both active as well as reactive power to the system which will allow it to compensate for the resistive and reactive voltage drops maintaining high effective X/R which will not depends on the degree of series compensation. However, this is costly as It requires relatively large energy source. * On the other hand if control is only for reactive compensation then a small supply is also enough. In such case only the voltage is controllable because the voltage vector forms 90° with the line Intensity. Subsequently the series injected voltage will leads or lags the line current that means the SSSC can be uniformly conéfolled in any value. Scanned with CamScannerPower System Operation and Control (SPPU) © The SSSC when works with the proper magnitude but opposite in phase any the effect of the voltage drop on © Asa result of this wides fast control and synchronous series compensator pro sssc Static Synch Fig. 2.5.4 2.5.3(8) VI characteristics of SSSC ine is constant but its net the inductive reactance the capacitive reactance + Generally the reactance value of li ‘The line current will decreases as the line current will increases with ‘energy supply can Ini le with the voltage developed Reactive Power ject a voltage COMPONEN, Which 4 ge cross the line. power transmission ts offset. In addition, yy Js essentially neutral to sub-synchronous Fes ronous Series Compensator (SSSC) effect can be controlled by means of voltage i compensation level increases from 0% to 100% gy, compensation level from 0% to 33%. 7 Vv, v wu We = Veer i Vs Va Vs, Ya 6 3 ‘Normal mode of operation Inductive mode of operation ‘Capacitive mode of operation Fig. 2.5.5 : SSS Mode of Operation From the above Fig. 2.5.5 it is observed that the static synchronous series compensator does not only increst the transferable power but it can also decrease it, by means of simply interchanging the polarity of the inject voltage. This reversed polarity voltage is directly fed in to the line voltage drop as if the Impedance of the Bt was increased, 1n short, the effects of reactance compensation on normal power flow in the transmission line are as, © When the reactance is capacitive the active and reactive power flow will increases and the effet reactance will decreases as the reactance compensation increases in the positive direction. © When the reactance Is inductive the active and reactive power flow will reduces and the effective reaco™™ {increases as the reactance compensation increases in the negative direction 2.5.3(C) Applications and Advantages The SSSC Is generally used to correct the voltage d juriny 4 umber of ad¥antages during normal operating conditions a Rtn he por satan ewe SS ‘SSC is used for power factor correction through continuous l a properly structured controller, infection of voltage and is used in combination ™ Scanned with CamScanner wt hf ; 3 =ag rower System Operation and Control (SPP) 28, irisused in load balancing in interconnected distribution system. ‘sssC can also be used to cover the capacitive and reactive power demand. it{salso used In power flow control, $55C is used to reduce harmonte distortion by active fltering. 2.6 _ Shunt Compensation + Theshunt compensation is applied to the line by using shunt capacitors and shunt reactors that are permanently connected to the network or it can be switched on and off according to operating conditions. ‘+ Shunt capacitors helps to increase the system load ability and reduces the voltage drop In the line by improving the power factor. Shunt reactors are used to limit the Increase in voltage under both open line and light load conditions. 261 Reactor and Capacitor Incase of shunt compensation FACTS devices are connected in parallel with the power system transmission line. Ik operates like a controllable current source. A reactive current Is fed into the line to maintain constant voltage magnitude by changing shunt impedance, Hence the transmitted active power Is raised but at the cost of increasing the reactive power demand, There are two methods of shunt compensations as, 1, Shunt capacitive compensation : The Shunt capacitive compensation method is used for power factor correction. When there is an inductive load connected to the transmission line the power factor will lags because of lagging load current. To compensate this shunt capacitor is connected which can draw current which is leading to the source voltage. The net results will helps in improvement of power factor. Shunt inductive compensation : The shunt inductive compensation method is used either when charging the transmission line or when there is very less load at the receiving end, Due to very less load or no load, a small current flows through the transmission line. The shunt capacitance in the transmission line causes amplification of voltage. The receiving end voltages (VR) can double the sending end voltage (Vs). To compensate it, shunt inductors are connected across the transmission line. 2.6.2 Static Shunt Compensator : STATCOM @. Explain working of STATCOM with diagram, There are many devices under the FACTS family the STATCOM is one of them. The STATCOM is a regulating device which can be used to control the flow of reactive power in the system independent of other system Parameters. STATCOM has no long term energy support on the de side and it cannot uses for exchange of active power with the ac system. In the transmission systems, STATCOM mainly handles only basic reactive power exchange and gives voltage support to buses by modulating bus voltages during dynamic disturbances in order to provide better transient characteristics an improves the transient stability margins and to damp out the system oscillations due to these disturbances. The STATCOM consists of a three phase inverter normally a PWM inverter using SCR's, MOSFET's or IGBT’s, a D.C capacitor which is used to provide the D.C voltage for the Inverter, a reactor is used to link the inverter ‘output to the ac supply side, filter components are used to filter out the high frequency components due to the PWM inverter. From the dc. side capacitor, a three phase voltage Is produced by the inverter. Which is synchronized with the ac supply. The inductor Is used to link thls voltage to the a.c supply side. This isthe basic principle of operation of STATCOM. "Scanned with CamScannerPower System Operation and Control (SPPU) 29 Reactive ‘System bus Moc Coupling 1C-AC Switching Converter Vide Fig. 2.6.1: STATCOM V-I characteristics of statcom Transient Rating (t< 1 3) % " Transient i rating | r 10 Heat {_—__—+—, om | f+ oso | | joa; Ct Tomax Capacitive 2 0 Ina a Inductive Fig. 2.6.2 : V-I characteristics of statcom Scanned with CamScannerranslent rating in both the capacitive- and the ble transtent overcurrent in the capacitive region Is ty of the converter switches, inabove 262 It Is observed that tn the inductive re the transient current rati achteval determined by the maximum current tur off capa '#lon the converter switches are naturally commutated, 8 COM Is limited b le junction swit by the maximum allowable Junctio temperature of i converter switches. In Normal practice the Semiconductor switches of the converter are not lossless nes the enerey stored in the de capacitor fs eventually sal we et the Internal losses of the converterand the de capacitor voltage decreases, therefore Ink of the STAT 263 Fixed Capacitor, Thyristor-Controlled Reactor (FC-TCR) A basic reactive power generator arrangement using a fixed capacitor with a thyristor controlled reactor (Fc TCR) is shown functionally in Figure 2.6.2(a). The current in the reactor is changes due to the method of firing Gelay angle control. Generally the fixed capacitor in practice is substituted fully or partially, by a filter network that has the Decessary capacitive reactance at the fundamental frequency to generate the reactive power required, but it Provides 2 low reactance at selected frequencies to shunt the dominant harmonics produced by the TCR. xe _K) {—— ne iw) x pH SECON i) Kt Fig. 2.6.3 : Basic FC-TCR type static var generator The FC-TCR type var generator may be essentially considered to consist of a variable reactor (controlled by delay angle a) and a fixed capacitor, with an overall reactive power demand versus reactive power output characteristic as shown In Fig, 2.6.4. i Scanned with CamScannerReactive Power Coy ¥ Power System Operation and Control (SPPU ————————— oy itput characteristic the fixed capacitor Is opposed by the vay to give the total reactive power output reactor is off (a= 90"), demand versus var ou eration (Qc) of reactoF, the thyristor-controlled creased by reducing delay angle (a). At2r, recome equal and hence the capacitive xy tion of angle (a), the inductive current will ince Fig. 26.4: var’ ‘As seen the constant capacitive reactive POM gen reactive power absorption (QL) of the thyristor-controlled required, At the maximum capacitive reactive power output, «To decrease the capacitive output the current In the reactor is in reactive power output, the capacitive and inductive currents b Inductive reactive powers cancel out. With a further reduc than the capacitive current, resulting na net inductive reactive power output. thyristor-controlied reactor conducts current over the full 180° degree inter tive power output that is equal to the difference between «done which absorbed by the fully conducting reactor. The court © When delay angle zero, th which results in to maximum inductive reat reactive powers generated by the capacitor an fhe thyristor-controlled reactor in the FC-TCR type reactive power generator needs to provide four bat functions. ing reactive powers through thyrisz “The FC-TCR system provides continuously controllable lagging to lead control of reactor current. Leading reactive powers are supplied by two or more fixed capacitor banks. Ts rating of TCR is generally larger than the total of fixed capacitance so that net lagging reactive power ‘can alsol supplied. The change of current through the reactor is obtained by phase angle control of back to back part thyristors connected in series with the reactor. 2.6.3(A) VI Characteristics of FC-TCR © Fig 265sh 1 i Fe? 265 shows a elcharaerits for re the operating area of FC-TCR is define as the maximum a ance and by the voltage and ct tenet urrent rating of major ent TF S se Pesan Is as ‘mportant element for maintaining the stability of the , wih eet ins, but this wall be reflected in additional costs. Wh: eae trying to optimize the regulation means, Win detest gt ten ad v. Scanned with CamScannertal sg_foer System Operation and Control (SPPU Reactive Power Control Isve(pu) Fig. 2.6.5 : VI characteristics of FC-TCR 264 Applications of SVC «Below mentioned are some important uses/applications of SVC: 1. Used for rapid voltage control and dynamic over voltages due to faults, loss of load or disturbances. 2. Increased power transfer capability of the line/system through VAR control and voltage regulation. 3. _ Improved system stability for small and transient disturbances by damping power oscillations. 4. Improved compensation and stability with location on the line. 5. Lowering the short circuit current levels thereby not increasing the circuit breakers capacity. ‘+ The drawback of SVC that is worth mentioning here is the production of harmonics and to minimize the injected harmonic currents tuned filter circuits are needed which increase the cost, size and can also affect the time response of SVC. 27__ Series and Shunt Compensation of Transmission Line Series and shunt compensation of transmission lines results in to improvement in the system stability and Voltage Control Method, also it resulting in increasing the efficiency of power transmission, and it also facilitates line energization and reducing temporary and transient over voltages. 27.1 Unified Power Flow Controller (UPFC) 2 Explain untied power flow controller [UPFC] along with phasor diagram. UPFC isthe combination of static synchronous compensator (STATCOM) and a static series compensator (SSC), the STATCOM and SSSC are coupled with each other via a common dc link, to allow bidirectional flow of or reactive Power between the series output terminals of the static series compensator (SSSC) and the shunt output terminals of the static synchronous compensator (STATCOM). Principle of Operation ‘The UPFC is the most flexible FACTS controller amongst other devices, this device will have capabilities of voltage regulation, series compensation, and phase shifting. Scanned with CamScannerPower System Operation and Control (SPPU) _2-33 Reactive Power Cony y The UPFC can control individually and very rapidly the both real and reactive power flows in a transmi, lines. The configuration of UPFC is shown in Fig.2.7.1 and consists of two voltage source converters pe through a common de terminal. 7 = TL Soran Tanemisson Une Sates ty supply g transtonmer L Converter? ¥ convertor rai roller (UPFC) Fig. 2.7.1 : Unified Power Flow Cont rected in shunt with the power line through a coup, © One voltage source converter (converter) is connt converter2) Is connected In series withthe transmis transformer; and the other voltage source converter line through an interface transformer. © The required de voltage supply for both conver the converters. The series converter Is respon: which can be varied from 0 to Vg max. Furthermore, the phase angle of Vpq ges both active and reactive power with the transmission Ing ‘bsorbed by the series converter, and the active poy capacitor. a common capacitor bank connected ‘a voltage phasor, Vpq in series with the ng can be varied from 0° to 3609, ters is provided by sible to injection of During this process, the series converter exchan} Here the reactive power is internally generated or al generation or absorption is made feasible by the de energy storage devi cer 1) is used mainly to supply the active power demand of conven, Jon line itself. The shunt converter is used to maintain constay ‘drawn from the ac system is equal to the losses in the ty, ‘* The converter connected in shunt (convert 2, and this power is taken from the transmiss voltage of the de bus. Thus the resultant active power converters and their coupling transformers. Power Function of UPFC The Various Power Function of UPFC are as follows 1. Voltage regulation 2. _ Series Compensation 3. Phase Shifting 1. Phasor Diagram for series voltage injection Fig. 2.7.2 Vorav 20 Vo-avV 20 2. Phasor Diagram for Series Compensation + Here, Vpq is the summation of a voltage regulating component AVo and series compensation provi voltage component of V_ which lags behind the line current by 90°. [re vo Fig. 2.7.3 : Phasor Diagram for Serles Compensation net — i Scanned with CamScannerstem Operation and Control (SPPU) 4 ay power SIE Reactive Power Control wert AVo.~ Voltage regulating components Vc + Series compensation providing voltage + Thecomponent Ve will lag behind the line current by 90° phasor Diagram for Phase shifting + Inthe process of phase shifting, the UPEC wilt regulating component AVp and phase shifting volt V0 Va Benerate voltage Veq which Is the combination of voltage tage component Var Vo" vot Wa Fig. 2.7.4: Phasor Diagram for Phase shifting Fig. 2.7.5: Phasor representation of UPFC UPFC uses a shunt connected and a series transformer, both interconnected toa DC eapacttor link via voltage source converters (WSC's)1 and 2, UPFC Is by far the most versatile member of the FACTS family. It can be termed as combination of STATCOM (Shunt controller) and SSSC (Series Controller) Interconnected though a common DC bus/link. + Such a series-shunt controller topology provides much greater flexibility for control of real, reactive and Gareping of the system. UPFC acts asa shunt compensating and phase shifting device simultaneously. The DC Cirevit allows exchange/transfer of active power between shunt and series transformers and controls the phase shif ofthe series voltage. Referring to the phasor diagram in 2.755, if the magnitude of injected voltage (Vs) is kept constant and if its Phase angle ¢ with respect to V1 is varies from 0° to 360®, the locus formed at the end of vector (V2 = V1 » Vs) is # circle on the phasor diagram. As ¢ is varying the phase shift 5 is also varying between the transmission line voltages. Hence real as well as reactive power can be controlled. 's addition to control of real and reactive power, UPFC will also contrl voltage V1 by absorbing or generating Feactive power. The shunt converter controls the AC voltage at terminal and the voltage of DC bus, The series ‘converter is used for control of reactive power and control of DC bus voltage. This combination also improves the dynamic and transient stability of the system while also effectively damping oscillations at fundamental frequency, Thus speaking about the capabilities of the UPFC, it can independently/simultaneously control real power, Feactive power and voltage. Quick response is another featured characteristics of UPFC due to which it finds lot f applications and installations in substations where one FACTS device can help in controlling multiple Parameters at the same time and enhance reliability of the system as a whole. The disadvantage with UPFC is that in case of no supply /fault at input side, UPFC will not function, Secondly Its cost will be higher. ‘As Compared to SVC, STATCOM or other FACTS devices, UPFC Is the most versatile in performance and is gaining Wide acceptance. Applications of UPFC * The capacity power transmission is determined by the transient stability considerations of the 345 kV line, ‘The UPFC Is connected in the 138 kV network. A 3-e phase to ground fault is applied on the 345 KV line for four les, and the lines disconnected after the fault. Ws Scanned with CamScanner3 35 Reactive Power Cony. x * However, the power transfer can be improved with the UPFC from 181 MW (10396) to 357 MW. Although power can be increased further by improving the UPFC rating, the increase in power Is relatively lover than) increase in the UPFC rating, thereby showing that the practical limit on the UPFC size has been attained, *) ¥F_Power System Operation and Control (SPPU) 2 2.8 _ Synchronous Motor | @. Explain synchronous motor as a source of reactive power withthe help capacitive compensation. EEL IDEE | @. Explain the synchronous motor as a reactive power compensator. EECEEDE ight process can be developed and Is shows x) ‘of phasor diagram and compare i with suc * The synchronous motor equivalent circuit, on similar thou Fig. 2.8.1. 4 's x oc excitation Field & “ Synchronous Motor Equivalent Circuit inding from the dc. source. The flux produced by te Fig. 2 «The motor is supplied from voltage source and the field wii he field winding interact with each other and rotor tries to move. Jther run as an induction motor using damper winding and the: ‘ed. Sometimes the provision of a pony motor is made to rot armature winding and t The motor is not a self starting and hence it ise! pulled into step and rotates at synchronous spe the rotor and then pulled into step as before. 28.1 Features “The constructional features of the motor and generator are same. Synchronous motor has certain characterstss and are laid down below: (2) eis notseifstarting Its usually started as induction motor using damper winding. Damper winding consis the cage bars shorted by end rings and are placed on rotor poles. The provision of damper winding makes when the stator is supplied from the rated voltage 22! synchronous machine to run as induction motor, ym de frequency source, at a speed less than synchronous speed. When the dc. excitation winding is excited fro ‘source, the rotor is pulled into step to get locked with the poles of rotating magnetic field. Under this condition works only on synchronous speed and the damper bars then have no role to play during normal operstst conditions. | (2) When working as synchronous motor, it runs only at synchronous speed governed by the frequency and Be | number of poles ie. | thee, Ny = Sylronme sped pm { = Frequency of the source in Hz | P= Novolpoleson stator | In never runs at sub or super synchronous speed. (3) The synchronous motor enjoys a special characteristic of variable power factor ranging from lagging to leading when the excitation is varied. This characteristic of motor makes it to operate as synchronous condenser. wim 4 Scanned with CamScannerReactive Power Contral 282, Working When the stator windin, 8 Is supplied from speed and back em. sls developed i cot Voltage (V) and frequency source (f) it runs at synchronous ve “conductors. The armature current Isthem, Woe Thisback ems. n case of synchron * fac, motors. The net votta Be res) cemfEy,and the impedance, }Ous mr Danaher Is because of the rotor excitation and not on speed as in the case OF armature current is the vector difference of voltage V and back + Onno-load, V and Ey, are in phase °} the net voltage is zero and no armature current wilt back ems. Ey falls back by certain I the losses are neglected. This means, shown in Fig.2.8.2 under no load co: © are losses taking place at no load, the 3. Is then not zero, The phasor diagram Is dition, 6: Snes byWheh Els bade Vand istoad ange ~ Power factor angle 0 - ; Xs internal angle is given by tan-1 Ry When R, is negli igi Armature resistance / phase Synchronous reactance / phase Rated voltage / phase Back ems. / phase V-E, Ra+ jks P, Motor: input per phase = V I, cos Pay mechanical power developed in the rotor = Ej I, cos (8 - ¢) 6-9) is the angle between I, and Ey reversed. Friction, winday ddition 1,2R, '8¢ and iron losses, when accounted for, the total power available “lanatory to j at the shaft will be less, In S the armature copper loss in the motor. The power flow diagram showa in Fig. 283 is sett indicate the power loss and power availability. Weg Scanned with CamScannertron, friction. Stator copperloss _windago and 1A, excitation loss ram in synchronous motor Fig, 2.8.3 : Power flow diagt the phasor diagram takes the form shown in ip 254 c In practice, when Rj is negligible as compared to Xs, Fig. 2.8.4: Phasor diagram of cylindrical motor synchronous AB = CD=Eysin5=I,Xs cos 2. Vi, Xqcoso = VEp sind ve Vigeos9 = —y~sind When stator copper loss is neglected P_ = Pm=Gross mechanical power <. The torque developed by the motor in Nm 55Pmy TH, Nm ‘A cylindrical rotor synchronous motor has folowing specifications. Ex. 281: = 100, V = 440 (Line), f= 50 Hz, Phases = 3, Connection star Cos 6 = 0.8 (lead) Efficiency of motor = 95% X-=5 ohms No. of poles = 4 ‘The motor runs at rated operating conditions. Calculate : (@) Mechanical power developed, Pry (c) Back EMF, E (e) Maximum Torque, Tm (b) Armature current, la (d) Power angle, Soin. : (2) Mechanical Power Developed, Pm = 105.263 kW Scanned with CamScannerSS rove L ‘armature CUrTENt, Ty e Reactive Power Control = 105263 V3 x 40009 *1904 fg) BAeKEME, Ey The magnitude and the phase of the back emf can be derived from the OB = OAcosg=Veos 4 ABYAC=Vsing +1, x, OC = Es 254y Oc? = OB? +Bcz SB = (? costo)? + (V sing + ta x2 ~(cos $= 0.8) (sing = 0.6) = (Weostg)24 (sing)? +(x 242 Via Xe sing Substituting the values By? = (203.23)2+ (252.4) + (950) +2254 190x506 41209 + 23226 + 902500 + 209560 = 1256405 1121 volts / 4 BC _Vsing+IaX, 15244950 G+0) = 7955" 9=cos-108=36.87 #8 = 7955-3687 =42.69° Pm = a sind and is maximum when 3 = 90°, . Eyv “Pam = EY For 3 motor itis three times Pmmax = tH21X254x3 _saeiay 5 9.55 Pin max Nt 1206 9:55%171513 | an 20 = 1500 rpm = SSX 71513 « 1092 Nam, Scanned with CamScannerHF _vower System operation and Control (SPPU)_2.39 React Power 2, Synchronous Motor Operation y The excitation to the synchronous motor plays a vital role In controlling the operating characteris, | motor. The excitation can be classified into three categories as follows + m 1. Under Excitation Ey . | First two operating conditions refer to the armature current of lagging nature, whereas In the thir e, power factor of the armature current is leading. These three cases are discussed in detall with the help gp tha diagrams. Casel: Under Excitation (Ey V) Fig.2.9.2 shows the phasor diagram. Fig. 2.9.2 : Over Excited Synchronous Motor. From the phasor diagram, OA = ¥, AB = Ey, OB = E, = [ly AB = AC+{BC 2 By = V+ Ecos (180 - 0-9) + jE, sin (180 - 0-9) and 8 = tan-t BS aC Scanned with CamScannercer system Operation and Control (SPPU) 2.40 Reactive Power Control ae ma Unity Power Factor Fig. 2.9.3 : Synchronous Motor cos 6 = 1 0c = LR, BCHIX, E, = AC+jBC=(V-R,I,) +) 1X, 5 case 5: Variable Excitation (Constant Load) characteristics ‘The earlier article shows the operations of the synchronous motor under different power factor conditions. If a raph between and I, is drawn, it will be as shown in Fig. 2.9.4. Over Under "Normal excitation excitation excitation Fig. 2.9.4: V~ Curves Three curves; for no load, half load and full load are drawn and they shape like letter ‘V' and hence are called as ‘V curves. If the graph of ly versus power factor is drawn then the curve obtained is inverted V (\) and hence are called as inverted V curves. 2.10 Synchronous Condenser or Inductor Sam O._Epizin synchronous condenser in context with reactive power management. SITET Eee The popularity of synchronous motor is because of its special characteristics of operation under variable “ciation ranging from lagging to leading power factor. Over-excited synchronous motor runs at leading power ‘ctor. Synchronous motor, therefore, {s used for improving the power factor of the power system network in industries, ‘Atnoload with losses neglected. * B=0 This means that the supply voltage and the back e.m.or excitation voltage are in phase. AE rateiett Scanned with CamScannerPower System Operation ani oaetive Payer Cony, W aah May 2019 powar ayatenn? (Beettone 9.1.04), BALAN) are BBC) (0 Margy, What aro difforont typen of compennations used It a.10 2.14 Why tt noconeary to contol reactive pow? iia ave the ways ‘ot oontreig ranative power? (Section 2.1.3) (6 Marty) 0.12. Explain operating principe al workig of TCC wit tho We? orotcutt agra are chanaoterito, (Section 2.6.1) (8Marty) 0.13. Explain tho Importance of FACTS controltors tn powor ayator, (Botton aay (6 Marky) @.14 Explain eynchronous motor ae a foureo of feacve power aha tno pan aga art corpare wth ceapactive componeation, (Gectton 2.0) (5 Marky) Oct. 2019 0.15 Explain ho synchronous motor as a raetve power compensa: (Goatton 2.0) 0.16 Whats series componsation? Stato Is advantagn avd londvantagos, (Becton 2.) (6 Marta) 0.17 Whats the necossty of rnctivo power contol? Discuss IN” ‘various aoureas of raactiva powers (Section 2.1.3) (8.Ma 0.18 Skotch and explain tho loading capably cuve of oynelvoneu® gonorator, (Becton 2.2.3) (8 Marks) 10.19 Wit diagram, explain tho working of SVC. (Betton 2.6.) (6 Marka) £0.20 With diagram, explain working of Thyltor Conrllod Gaon Copacttor (TOSC}. (Bectton 2.6.1) (6 Marka) (0.21 Explain working of STATCOM with diagram. (Sectlon 2.6.1) (5 Marka) 0.22 Explain unfiod power flow controler {UPEC] along wih nhavor diagram, (Soctton 2.7.1) (6 Marks) Dec. 2019 0.22 Explain the working of TOSC wi ts charactoriaten. (Section 2.8.1) (6 Marks) 0.24 Explain he problems associated wih serie componcalion. (Section 2.1,6(H)) (6 Marks) 2.25 Discuss various ways of providing shunt compensation (Section 2.1,6(6)) (Marks) gaa Scanned with CamScanner