CONVERTER FAULTS AND PROTECTION 5.1 INTRODUCTION ‘As in AC systems, the faults in a DC system are caused by (i) the malfunctioning of the equipment and controllers and (ii) the failure of insulation caused by external sources such as lightning, pollution, ete. The faults have to be detected and the system has to be protected by switching ‘and control action such that the disruption in the power transmission is minimized. Apart from disrupting the normal operation, the various faults that can occur also cause the stressing of the equipment due to overcurrents and overvoltages. In a converter station, the valves are the most eritical equipment which need to be protected against damage caused by the rise in the junction temperature of thyristors, which is caused by excessive losses in the device and sensitivity to overvoltages. In this chapter, the faults occurring in a converter station, their causes and effects are described. The protection against overcurrents and overvoltages is also discussed. It is seen that the converter control plays a major role in protecting the equipment. _/ 52 QONVERTER FAULTS” Sener: ‘There are three basic types of faults that can occur in converters as given below: 1. Faults due to malfunctions of valves and controllers (i) Are backs (or backfire) in mercury are valves Gi) Are through (fire through) tii) Misfire (iv) Quenching or current extinction 2. Commutation failure in inverters 8, Short circuits in a converter station. “The are back is the failure of the valve to block in the reverse direction and results in the temporary destruction of the rectifying property of the valve due to conduction in the reverse direction. This is a major fault in mereury are valves and is of random nature. This is a nonself clearing fault and results in severe stresses on transformer windings as the incidence of are backs is common. Fortuns are valves f discussion. Some o clearing if cause a maj Because of value of the ‘The ov rreduetion in angle and 1 incoming v carrying th The fi bridge as b 4 is succes: 6 is fired. reverse bia conducting by 60°, for 5 will ben Ifthe normal ex ‘The volta voltage ac (ey ~e) 4 is turnay instance | commutat voltages a ‘Thef failure’ If in the con earlier). § instead o 6 is rever in Fig. 5. single cotSs AND of the equipment uch as lightning, fed by switching mized occur also cause ter station, the mage caused by ve losses in the and effects are d. It is seen that dresults in the Hin tho reverse his is a nonself Bcidence of arc Converter Faults and Protection Fortunately, thyristors don’t suffer from are backs which has led to the exclusion of mercury are valves from modern converter stations. Hence this fault will not be considered in further discussion, ‘Some of the converter faults such as commutation failure, are through and misfire are self clearing if the causes that led to these faults are of transient nature. However, they can still cause a major disturbance unless the system including the controllers is properly designed. ¢ Because of the turn-off time requirements of thyristors, there is need to maintain a minimum value of the extinction angle defined by y=180-a-u - (u) is a function of the commutation voltage and the DC turrent. The reduction in the voltage or increase in the current or both can result in an inerease in the overlap angle and reduction of below yin. This gives rise to commutation failure. The current in the incoming valve (say valve 8) will diminish to zero and the outgoing valve (valve 1) will be left ‘The overlap angl carrying the full link current, The firing of the next valve in sequence (valve 4) will result in the short circuit of the bridge as both valves in the same arm will conduct, If the commutation from valve 2 to valve 4s sucessful, only valves 1 and 4 are left conducting and this state continues until the valve 6 is fired. The firing of valve 5 prior to the firing of valve 6 is unsuccessful as the valve 5 is, reverse biased at the time of firing. This happens as the voltage across valve 5 (when valve 1 is, conducting) is (e, ~¢, ) instead of the nofmal commutation voltage (¢, ~¢, ). Since é, leads &4, by 60°, for normal values of the firing angles for the inverter ((}< 60° ), the voltage across valve 5 will be negative and hence reverse biased. If the commutation from valve 4 to valve 6 is successful, the conduction pattern returns to normal except that the bridge voltage is negative at the instant where valve 4 ceases conduction. ‘The voltage across valve 6, just before it starts conducting is (e, ~e, =~e,, |. However, the voltage across the bridge after valve 4 is turned off (and when valves 1 and 6 are conducting) (ee If ~ ey, >0 for valve 6 to successfully turn-on, the bridge voltage when valve 4 ig turned off is negative. If the causes which led to commutation failure in valve 1 in the first instance have disappeared, the bridge operation returns to normal state. Thus, a single commutation failure is said to be self clearing. The waveforms of the bridge voltage and valve voltages are shown in Fig. 6.1 The failure of two successive commutation in the same eycle, is called ‘double commutation failure’. If the commutation failure occurs when valve 4 is fired also, the valves 1 and 2 are left in the conducting state until the instant in the next eycle when valve 3 will be fired. The valve 5 is reverse biased at the instant when firing pulse is sont as ¢,, leads ey by 60° (as mentiot .arlier). Similarly, the voltage across valve 6 when firing pulse is applied to valve 6 is ~(e, stead of the normal commutation voltage ~ (e, ~¢, ). Since — e,, leads ~ ¢,, by 60°, the valve 6 is reverse biased at the instant of firing. The bridge voltage waveform for this case is shown in Fig. 6.2 and it can be seen that the double commutation failure is more severe than the single commutation failure. TERTime ‘a er Time Time , (4 gs | g : f Time Time a Fig, 6.1: Voltage waveforms for a single commutation falurensmilssion Systems Converter Faults and Protection ‘The following are the effeets of a single commutation failure: : 1. The bridge voltage remains zero for a period exceeding 1/8 of a cycle, during which the DC current tends to increase 2. There is no AC current for the period in which the two valves in an arm are left conducting. ‘The recovery from a commutation failure depends on the following factors: 1, “he response of the gamma controller at the inverter 2. ‘The current control in the link and 8, The magnitude of the AC voltage. If, on detection of a commutation failure, the angle of advance i is increased, there is a ‘good chanee that subsequent commutation failures may be averted. However, this also depends ‘upon the control of DC current and the magnitude of AC voltage. The initial rate of rise of current, in the inverter is limited by the smoothing reactor and the current controller at the rectifier helps to limit the current in the case of persistent commutation failure. It may be even necessary to reduce the current reference to limit the overlap angle in the case of low voltages caused by faults in the AC system, While in most cases, commutation failures are self clearing, in the case of persistent ‘commutation failures, the converter differential protection helps to take the converter out of service. This protection is based on the comparison of DC and the valve side AC currents. During commutation failures when the two valves in an arm of a bridge are left conducting, the AC current goes to zero while the DC current continues to flow. ‘The commutation failure in a bridge can lead to consequential commutation failures in the series connected bridges unless the rate of rise of current is sufficiently limited by the series connected smoothing reactors. {[Compute the maximum voltage dip at the converter bus that will not result in a commutation failure, [Assume that the voltage dip occurs at the instant immediately after firing the incoming valve. Consider (a) symmetrical three phase voltage dip and (b) single phase yoltage dip. CHAPTER 5HVDC Power Transmission Systems It is assumed that the converter is initially operating in steady state with I, = Ij.y = Yo and E = Ey (where F is the line to line AC voltage at the eonverter bus. As shown in chapter 3 (see example 1), the expression for the DC current can be expressed as Ty+ly BY Wag Banke e087, ~c088] Where, Buy is the rated (line to line) voltage of the converter transformer at the secondary (valve) side, ys th rated DC current, X, isthe p.u leakage reactance of the transformer (on its own base) ‘and 1! is the DC current at the end of commutation ‘Expressing Ty =Tag + Mla, B B,-A8, We can write the current equation as Note that Yq is the minimum extinction angle (typically) that ensures successful commutation, ‘The current equation during steady state is given by oak Tay’ By = c0s'yy cos : aE ‘Subtracting the above equation from the previous equatfon and simplifying, we ean solve for ‘g, as 1 aly a7 (cost -on)+(} (605% ~€08Y0)~ Ey Tet us denote e, and ¢, as B ite 8, [Bese 60r0 «2 ‘Thon the voltage across valve 1 just after it has turned off is given by cos(cot 60") Es 2K sinot a ‘The phasor diagram of voltages E, , B, and Hy, in steady state is shown in Fig. 5.3.EES ee ’=(B3+ AV} -BE)AV,)* ‘There is also a phase shift (advance inthis case) of @in Fy defined by ion. | ay, sing=sins0r- “Ye p= sin30°- | ‘The effect of the phase shift, inthe valve voltage is equivalent to increasing the minimum extinction an from 10949 U8 | Ts Gea ear ces eres Ym = 8° = 15°, Aly = 0, = 1.0 and X, = 0.2 Megs AE = 008 frase fal If y=" thn = 0184, Thin how tha operat eee eee es ccna ame ee | eee eee tees ee cg ie Oh icles este ies xia ater ae arate celeste ipetoe ators na, tassel anteater limit on X, is determined by the surge current rating of the valves. | It is to be noted that if there is a single phase fault resulting in the voltage dip in phase b (instead Beidaresyetreremer ge oe ane ie ea | rar pose ais Bikes eg ea 9 toed cyt PCO | tavanced to prevent commutation uur, In general, it ean be suid that ifthe voltage dip occurs in cos RRS aca epg wr an converses MD es nett eri Wee cance ce tray capa a aun oes pt ae ra oak oc pacha vata Gate ag | reactor value is less for back to back HVDC links. 2) | CHAPTER 5120 HVDC Power Transmission Systems ‘An inverter experiences an (aymmetrieal, three phase) AG voltage dip when operating at the rated current, What is the converter voltage (in pu) below which persistent commutation failures occur? Data: Gin =90°, Enen(= margin angle)= 8,%. =0.2 It ean be shown (see chapter 8) that I, = jee Fler ~ cos] while operating at mode 1 (overlap angle below 60°), For f)=90° we have +30 1nd ence Ynig ~30% Emin = 38° ‘Thus E, is given by we get B=0.254pu ‘Note that the converter control had advanced the firing angle to Ga... However, the bus voltage is +0 small to prevent persistent commutation failures. The VDCOL can help by reducing the operating DC current. It-should be noted that reducing «increases the reactive power requirements of the converter and | can depress the AC voltage further (depending on the short circuit ratio), While the reduction of the DC current can overcome this problem, the reduction in the power flow may not be acceptable (even. under transient conditions).In these situations an optimal control strategy that can reduce the incidence of commutation failures during dynamic conditions (after about one eyele following the fault initiation, is desirable (15,16). In reference [15] the authors used predictive extinetion angle control | based on the detection of the d-q components of the network voltage using PLL. The required firing | angle to maintain the desired commutation margin is calculated based on the prediction of the | magnitude and phase angle of the commutation voltages. A similar approach is used in [16) when single phase faults are considered and control cireuits based on Digital Signal Processor (DSP) are used, | __ It isto be noted that faults and commutation failures can introduce harmonic components in the network voltages whieh ean complicate the analysis. A third harmonic voltage component with adverse phase in relation to the fundamental component ean increase the incidence of the commutation failures. This is a fault likely to occur mainly at the inverter station, where the valve voltages are positive ‘most of the time (when they are not conducting). A malfunction in the gate pulse generator or the arrival of a spurious pulse can fire a valve which is not supposed to conduct, but is forward biased. For example, in a bridge, when valve 1 has successfully commutated its current to valve 8, the initial voltage across it is negative (for the duration of the margin angle) and then becomes positive. If valve 1 is fired at this time, the current will transfer back to valve 1 from valve 3. Converter Faults and P ‘The effects of an are th the bridge falls as valve when valve 2 current ga to normal operation aft sequence. Thus a single ‘The are throughs i the probability of which is also through the conv While an are through is the required gate pulse the occurrence of misfi converter controls, mon ainsmission Systems Haling at the rated] fon failures occur? St mode 1 (overlap ithe bus voltage is icing the operating Fthe eonverter and he reduction of the acceptable (even at can reduce the fllowing the fault ion angle contro! he required firing (Prediction of the ised in [16] when feessor (DSP) are mponents in the ment with adverse the commutation lages are positive ulse generator or at, but is forward Seurrent to valve ind then becomes @1 from valve 3, Converter Faults and Protection 121 The effects of an are through are similar to that of a commutation failure—the voltage across the bridge falls as valve 4 is fired (with valve 1 conducting) and the AC current goes to zero when valve 2 current goes to zero, The firing of valve 5 is unsuccessful and the bridge recovers ‘to normal operation after valve 6 is fired and the subsequent firings are according to the normal sequence. Thus a single are through is also self -clearing if the causes that led ta it are removed. The are throughs in thyristor valves can occur due to malfunction in the control system, the probability of which is very small. Anyway, the protection against persistent arc throughs is also through the converter differential protection scheme, ‘While an are through is caused by the presence of an unwanted gate pulse, misfire oceurs when the required gate pulse is missing and the incoming valve is unable to fire, The probability of the occurrence of misfire is very small in modern converter stations because of duplicated converter controls, monitoring and protective firing of valves, o 2 4 6 8 Ww 2 4 16 18 Time ~ Sec. '27,776-09) o 2 4 6 8 10 2 1 16 18 Time-sec ('27.776-08) CHAPTER 5122 HVDC Power Transmission Systems While misfire can occur in rectifier or inverter stations, the effects are more severe in the latter case. This is due to the fact that in inverters, persistent misfire leads to the average bridge voltage going to zero, while an AC voltage is injected into the link. This results in large current and voltage oscillations in the DC link as it presents a lightly damped oscillatory circuit, viewed from the converter, The DC current may even extinguish and result in large overvoltages, ‘across the valves. The waveforms of the DC voltage and current in the link for persistent misfire in the inverter are shown in Fig. 5.4. The overvoltages can be controlled by controller ‘modifications, ‘The effects ofa single misfire are similar to those of commutation failure and are through. When valve 3 in a bridge misfires, the valves 1 and 2 are left conducting until valve 4 is fired. However, at the end of the cycle, the normal sequence of valve firings is restored. Thus a single misfire is also self-clearing. 5.2.5 Current Extinction ‘The extinction of current can oecur in a valve ifthe current through it falls below the holding current. This can arise at low values of the bridge currents when any transient ean lead to current extinetion. The current extinction can result in overvoltages across the valve due to current chopping in an oscillatory circuit formed by the smoothing reactor and the DC line capacitanee. ‘The problem of current extinction is more severe in the case of short pulse firing method discussed in the last chapter. However, in modern converter stations, the return pulses coming from thyristor levels to the valve group control, indicate the build up of voltage across the thyristors and initiate fresh firing pulses when the valve ig supposed to be conducting. It may happen that a number of firing pulses may be generated during a cycle when the link current is low. 5.2.6 Short Circuit in a Bridge ‘This fault also has very low probability as the valves are kept in a valve hall with air conditioning. However, bushing flashover can lead to a short eircuit across the bridge and produce lange current peaks in the valves that are conducting. ‘The short circuit currents are significant only in rectifier bridges, ‘The worst case is when the short circuit oceurs at the instant of firing a valve at a= 0. Assuming that there is no inductance in series with the bridge, the peak short cireuit current. Cigsax) is given by 62) where ig, = the de current at the instant of firing the valve and T, = V2E, /2X, ‘The bridge current i, waveform is shown in Fig. 5.6. firing of valve 8), the valve currents q } ‘The expressions for |and are through. fil valve 4 is fired. d, Thus a single below the holding asient can lead to the valve due to and the DC line firing method n pulses coming gltage across the ducting. It may prt circuit current Converter Faults and Protection Proof: , ‘Assume that the valves 1 and 2.are conducting and valve 8 is fired at ¢ = 0, corresponding to a =0. The supply voltages ¢,,¢, and ¢, in the circuit shown in Fig. 5.5 are given by ; ; ase ast 5 (ssa ‘These expressions result in ¢,, = V2E,,, sineat . When a bridge fault is applied at ¢ = 0 (after firing of valve 3), the voltages at the terminals p and n become zero (due to symmetry). Thus the valve currents i, and i, are obtained by solving the following equations ‘The expressions for i, and i are obtained on i, = 7esin(oe +60°)-sin60") +i, 2 21, ; ig = 22 Isin(or ~60°)+ sin60"] ae eee! CHAPTER 5124 HVDC Power Transmission Systems ‘The bridge current, i, is given by 2 igh t= Fl sinat tig, ‘The current in valve 1 becomes zero at wt=u defined by 2 0= = L,lsin(u + 60°) sin 60" pil , 1+iuy _ 63) Since i, >0, it can be observed that u > 60°, For wt =u, we have the following equation as only valves 2 and 8 are conducting, VBEj, sin(wt + 60°) ‘The solution of this equation gives a) Where i, =iy(ot =u) is given by sin(u B07) +iay fay = Fela sinu tig ‘The bridge current is at its peak when of =120°and ij i given by fpeae =I, Sint -30°)+ 27, sina + pee =e Ty lesinu the 64) It ean be shown that A 1 1,sinu—1, sina 30°) = 44 situ +60°) anne Tasial 8 2 From Eq. (6.3), we ean derive Fgsintu + 60%) = 4 ingo° ide 2 tun, 22 From the above, we can derive the final expression for jaa, given in Bq, (5.2). In Eq. (5.2), the effect of network impedance in limiting the current is neglected. The maximum peak current in a valve results when it is conducting into a valve fault. For example, the maximum current in valve 3, when it starts conducting with short eircuit across valve 1, is given by BK = (1+ cosa) + (lg/21,) 65) ‘The peak currents are of the order of 10 to 12 times the rated current and the thyristor valves must have surge current ratings above this value. The fault clearing is performed by Haieeretenona? blocking the pulses and provided the voltage ac: additional loops of overc ‘The detection of br DC currents. In this ca 8:3 PROTECTION, ‘The overcurrent protec systems. The factors tha i) sonsitivity (i) relia ‘The main feature o action (in less than 20 selectivity is also enha transformer. Further, t protection system must Consider a convert ‘The protection system u DC line faults, undervo ‘The basic protectio by valve group differen converter transformer ‘The differential prote overcurrent protection be set higher than that outside the station (theion Systems eted. The example, svalve 1, thyristor ormed by Converter Faults and Protection 125 blocking the pulses and when the fault current goes to zero, the valve assumes blocking state provided the voltage across it is not high. If the valve is unable to block the forward voltage, additional loops of overeurrents result and this can be avoided only by tripping the AC breaker. ‘The detection of bridge or valve short circuit is also performed by comparing the AC and DC currents. In this case, the DC current goes to zero while AC current tends to increase. 46.3 PROTECTION AGAINST OVERCURRENTS The overcurrent protection in converters is based on principles similar to those used in AC systems. The factors that must be considered in designing a protection system are (i) selectivity Gi) sensitivity (i) reliability and (iv) backup : ‘The main feature of converter protection is that itis possible to clear faults by fast controller action (in less than 20 msec) by blocking gate pulses or current regulation and control. The selectivity is also enhanced by high impedances of the smoothing reactor and the converter transformer. Further, the converters are divided into independent valve groups such that the | protection system must be able to switch off only the affected valve group (or bridge). Consider a converter station with a 12 pulse converter per pole (2 valve groups per pole) ‘The protection system used for a pole is shown in Fig. 5.6. This does not show protection against DC line faults, undervoltage or transformer protection (oce] [ver ica} poP| (ocr | [var OCP Over Current Protection \VGP Valve Group Protection s POP Pole Dieronta Protection 6B Circuit Breaker ‘The basic protection against converter faults (considered in the previous section) is provided by valve group differential protection, which compares the rectified current on the valve side of converter transformer to the DC current measured on the line side of the smoothing reactor. ‘The differential protection is employed because of its selectivity and fast detection. The overcurrent protection circuit is used as back-up. The level of overcurrent required to trip must be set higher than that of the valve group differential protection to avoid tripping with faults outside the station (that can be cleared by the control action). CHAPTER 5126 HVDC Power Transmission Systems ‘The pole differential protection is used to detect ground faults which may not be otherwise detected, such as faults at the neutral bus. ‘The fault clearing action of these protection circuits is to block the valves and at the same time trip the AC breaker of the affected group or pole. The fast tripping sequence is used for internal faults where there is a danger of valve damage. This involves forced retard (increasing the delay angle of the rectifier to about. 160°) combined with the signal to trip the AC breaker. ‘The pulses are blocked after 20 msec. This allows the inverter action (by forced retard) at the rectifier station to try to reduce the current before the converter is blocked, ‘The faults producing overcurrent are classified into 3 categories: 1. Internal faults which cause high overcurrents but are very infrequent. The thyristor surge current ratings must be chosen to withstand these overcurrents, 2. Line faults which cause overeurrents in the range of 2 to 3 p.u. These are limited by current control. The protection against line faults which are frequent will be discussed in the next chapter. 3. Commutation failures at inverters may be quite frequent. Howev , the overeurrents are small and limited by current control, However, because of continuous eonduetion during commutation failures, the current reference has to be reduced In case 1, normal operation may commence only after the in of the valves to check for damages. In cases 2 and 8, normal operation can commence soon after fault clearing. p 5.4.1 General The overvoltages in a converter station are caused by (i) disturbances originating on the AC side (ii) disturbanees originating on the DC side (iii) internal faults in the converter. ‘The type of overvoltages, as in a AC system, can be classified into three categories: 1. The switching overvoltages (with wavefront times of more than 100 msec.) 2. Temporary overvoltages (lasting few seconds) 3. Stgep front overvoltages (with wavefront times in the range of 0.3 to 8 msec) 5.4.2 Disturbances on the AC Side The lightning strokes in the AC network cause steep-fronted high overvoltages which are, however, reduced in magnitude and steepness by AC filters. After they pass through the converter transformer, they appear only as highly damped switching surges across the converter ‘The initiation and clearing (by switching action) of the faults in the AC system result in switching surges and temporary overvoltages. The energization of a converter transformer ean cause high (up to 1.6 p.u) overvoltage due to the inrush magnetizing currents and last up to 100 cycles. The voltage is also distorted due to even harmonics (typically 4th harmonic). This type of temporary overvoltage ean cause severe stresses on the metal oxide surge arrestors due to excessive energy dissipation. Pre-insertion resistors in circuit breakers energizing the converter transformer, are very effective in controlling these overvoltages, ‘The temporary overvoltages due to load rejection can be quite serious for converter stations connected to weak AC systems. The load rejection may be caused by blocking of the converters in response to the action of the protection system. During the load rejection, there is a possibility from the AC filters excitation of synch regulator) action. 5.4.3 The steep wavefront * when they reach the ‘The switching su DC link. Owing to¢# on the healthy pole. of voltage at the setting of the ‘The overvolt by sudden jump in th or injection of AC Wo ‘The switching and overvoltages 5.4.4 Over The series connet result in overval to be taken into ac parameters. The result in repetiti ‘Transient o ‘as a ground fault switching surge’ transformer. by deblocking the in the initial ll the valves were of active sparky protective levels eee With the d gap has disappe were first appli number of othe: Ri sien not be otherwise and at the same quence is used for retard (increasing ip the AC breaker. peed retard) at the it. The thyristor ts. are limited by will be discussed p the overcurrents uous conduction ye valves to check ult clearing, categories: B msec) hich are, however, the converter harmonic). This fee arrestors due energizing the verter stations jofthe converters is a possibility Converter Faults and Protection 127 of self excitation in case of the isolated generating plant supplying the converters. This arises from the AC filters which appear as capacitive at fundamental frequency and result in self excitation of synchronous generators which cannot be controlled by AVR (automatic voltage regulator) action. The only solution is to switch off capacitor and filter banks, ‘The steep wavefront surges in DC overhead lines are produced by lightning strokes. However, when they reach the converter through the smoothing reactor they appear as switching surges, ‘The switching surges at the converter are also caused by ground faults on a pole of bipolar DC link. Owing to capacitive and inductive coupling between conductors, the surges also oceur on the healthy pole. The magnitude and the wave shape of these surges arriving at the terminal fare dependent on the type of termination inductive, capacitive or resistive. The rate of decrease of voltage at the terminal is a variable that is usually utilized in line fault detegtion and the setting of the threshold value is based on the knowledge of the voltage waveforms. ‘The overvoltages can also arise from the oscillations, of current and voltage in the line caused by sudden jump in the converter voltage (due to commutation failure and other converter faults) or injection of AC voltages of fundamental frequency and second harmonic. ‘The switching of DC filter branches, parallel connection of poles can cause transient currents and overvoltages which will mainly stress the neutral bus and filter reactors. ‘The series connection of thyristors and the spread in the delay times of the thyristor turn-on result in overvoltages across the device during turn-on. However, these are repetitive and have to be taken into account in the valve design and the choice of grading circuit (snubber circuit) parameters. The spread in the reverse recovery charges and the commutation overshoot also result in repetitive overvoltages. ‘Transient overvoltages of very steep front may result from internal converter faults, such as a ground fault at the valve side of the smoothing reactor. The ground faults can also produce switching surge type overvoltages, for example, a firult between the valve bridge and the converter transformer. The firing of bypass pairs or closing of the bypass switch across one converter generates overvoltages across the remaining converters, The energization of the DC line from the rectifier sid¢ with the remote terminal blocked can cause high overvoltages at the inverter which is open ended. Such events must be avoided by deblocking the inverter first and limiting the rate of decrease of the delay angle. 5.5 SURGE ARRESTERS In the initial stages of application of DC technology, DC surge arresters were not available and the valves were protected by the spark gaps connected across them. Later, with the development of active spark gaps, it was possible to extinguish the arrester current without exceeding the protective level and DC arresters were made of nonlinear resistors in series with the active spark gaps. With the development of metal oxide resistors with high nonlinearity, the need for a series ‘gap has disappeared and the present DC arresters are gapless arresters. The metal oxide elements ‘were first applied in AC arzesters in 1976. Comprising primarily of zine oxide, but containing number of other metal oxides (such as Biz0s, Sb,05, MnO,, Cr,09), a8 additives, the material CHAPTER 5extremely nonlinear voltage-current characteristics. A typical disc that conducts less than lliampere of current at normal operating voltage can carry currents of thousands of amperes vice the normal voltage. This property makes it possible to eliminate series connected spark s and reduce the voltage margins due to the constancy of protective levels. The voltampere for a typical zine-oxide dise is shown in Fig. 5.7 for different levels of operating temperature. It is seen that the temperature coefficient of the material is slightly negative at low rents, but becomes positive at currents above a.few amperes. This makes it possible to ie the zine oxide elements in parallel to discharge high energy surges. The long term lity of the material is satisfactory although it is influenced considerably by disc composition processing, Vottage (kv) —> N 38 We gsc eerie igs en ioe Curent (Amps) Fig. 6.7: Volt ampere curve for a typical zinc oxide disc (Source: Reference 13) ‘The properties of the material are such that it is possible to design arresters to control amie overvoltages in addition to switching surges. This results in economic insulation dination. However, proper design of the arrester based on the evaluation of the energy losses ssential, The ultimate limit on the energy dissipation capability of a disc is imposed by the king of the dise under thermal shock. A single column arrester is capable of absorbing around J per kV of the maximum continuous operating voltage (MCOV). In many DC applications, the energy capability of a single column of dises is inadequate | multiple columns are used. A parallel column arrester is made up by selecting discs such t the voltage for each column is the same at a predetermined current. The maximum erence in the currents of parallel columns can be made less than 0.5%. General basic principles of overvoltage protection is the same in DC systems as in AC systems. ce aro given below: 1. The overvoltage stresses in ¢ at all times by providing su lower than the breakdown 2. Self restoring insulation st danger to the safety of the) ‘The operation of surge art Frequent discharges of art level of arrestors must be! 4, There must be proper coord parts of the system, taking of overvoltages, ete, ‘The overvoltages generated or ‘on the AC side. The overvoltages and neutral bus arresters. The ¢ arresters connected close to the & §.6.2 Overvoltage Protection 7he typical arrangement of surg 5.8, For a system with two 12-pu AC bus arrester Fig. 8.8: Typical arrangenTransmission Systems that conducts less than Hhousands of amperes Series connected spark levels. The voltampere foperating temperature ghtly negative at low jp makes it possible to Surges. The long term by dise composition arresters to control feconomic insulation jf the energy losses is imposed by the jofabsorbing around Of diss is inadequate selecting dises such nt. The maximum as in AC systems. 129 (Converter Faults and Protection 1. The overvoltage stresses in equipment with non self-restoring insulation must be limited at all times by providing surge arresters. The protection level of the arresters must be lower than the breakdown voltage of the insulation. 2. Self restoring insulation such as air may be allowed to breakdown where there is no danger to the safety of the personnel. 3. The operation of surge arresters or flashover of air insulation must not be frequent. Frequent discharges of arresters may damage them. This implies that the protective level of arresters must be higher than the maximum operating voltage in the system. 4, "There miist be proper coordination of the insulation and overvoltage protection in different parts of the system, taking into account the characteristics of the insulation, the nature of overvoltages, ete, ‘The overvoltages generated on the AC side should, as far as possible, be limited by arresters on the AC side. The overvoltages generated on the DC side must be limited by DC line, DC bus ‘and neutral bus arresters. The critical components such as valves are directly protected by arresters connected close to the components. Abe typical arrangement of surge arresters in a converter station (for a pole) is shown in Fig. 5.8. Fora system with two 12-pulse converters per pole, there are about 40 arresters per pole vaNe oc Teartor ie ta De tine none pci larester be titer xf arrester Mid point DC bus re Neural AO reactor arester CHAPTER 5HVDC Power Transmission Systems Converter Fail ‘The arresters are selected with adequate energy dissipation capabilities which vary with the AC volta location of the arresters. For example, the valve arrester protecting the commutation group at ica ee the highest potential can be subjected to higher energies than other arresters when a ground Erphess ikl fault occurs between the valve and the converter transformer in the upper bridge. This is due care Se ae a ee | ‘The closing of a bypass switch across a converter results in inereasing the DC voltage across rood a the remaining converter. The converter unit arrester is stressed in such cases. Be Atternateiy el ‘The protective firing of a valve is the backup protection that is available for overvoltages: back to back Df in the forward direction. ‘The pool / 571 PROTECTION AGAINST FAULTS IN A VOLTAGE SOURCE CONVERTER pact ‘The protection requirements are similar to those of an LCC based HVDC link. The objectives of farm | the protection scheme are: 1. Protect against damage by an external fault or overvoltage 2 | 2, Limit the damage against internal faults and — | 3, Feed the fault to permit the operation of fault detection relays in an isolated system. 1. Kauferleds ‘The last requirement is unique to a VSC based HVDC station. Since VSC based links can CIGRE Pa supply passive loads and the current is normally held close to the nominal current even at low 2. Kautldlf network voltages, it may be required to feed AC faults to allow the operation of overcurrent relays. Conf, Pall A VSC is equipped with free-wheeling diodes which prevent overvoltages in the reverse mission’ direction impacting the IGBT valves. However, it may be necessary to provide overcurrent 3. Bowles JB protection (against inrush current when the DC voltage is reduced). Also, DC voltage-clamping Inrwsh Cap devices are provided to protect against overvoltages in the forward direction. 4. Gieaner For temporary external faults, the converter may be blocked temporarily to protect IGBT Proc. EB) and free-wheeling diodes from overcurrents, After a few milliseconds, the valves are deblocked 5. Kimbarkll and an attempt is made to restart. If automatic deblocking fails one or more times, permanent Theory til blocking is also carried out, Permanent blocking is also carried out for internal faults along 6. Hingor with tripping of AC breakers. After permanent blockings, deblocking is done manually. A fault race ‘across the converter requires tripping of the AC cireuit breaker to prevent overcurrents and ae isolate the converter against voltage stresses. tee a : ‘A specific requirement in a VSC station isthe protection against progressive failure of VSC oe DC capacitor elements, which results in the reduction of the capacitance value and the ue asymmetric distribution of the DC voltage. Monitoring the voltages across each capacitor helps ah in detecting the failures as the voltage ripple increases as the capacitance value decreases. » ee However, change in the capacitance value is not so critical compared to that in a harmonic of HVDC) filter. Ifthe reduction in the DC eapacitance is significant, the AC breaker is tripped. 10. Peterson Unlike in the ease of a CSC based HVDC link, moderate changes in the AC bus voltages current can be handled by the control action of the converter (in regulating the injected current). If 1. ee ‘ there is a failure in the control system, there can be overcurrents due to dips in the AC voltage. ‘The overvoltages due to lightning and switching are handled by surge arresters. 12. Thio CV. ‘The sudden shifting in the phase of the AC bus voltage results on a sudden power change and Field on a VSC that requires a fast acting PLL to counter the phase shift. Otherwise, the overcurrent Winniper detection will result on VSC blocking and tripping.n Systems y with the n group at a ground his is due ERTER jectives of i system, {links can ven at low ent relays. he reverse vereurrent clamping tect IGBT deblocked permanent lts along lly. A fault rrents and ure of VSC we and the itor helps decreases. | harmonic ed. ns voltages current). If AC voltage. wer change wereurrent Converter Faults and Protection 131 AC voltage unbalance can lead to large ripples in the DC capacitor voltage. The control system needs to be designed to inject negative sequence voltage temporarily to reduce the impact of phase imbalance. In the worst case, the converter has to be tripped. Interestingly, the dip in the AC voltage at the inverter can cause overvoltage across the ‘DC capacitor due to energy imbalance (the power output of the inverter falling below the input received from the reetifier). Voltage clamping circuits can be used to regulate the DC voltage. Alternately, control action ean be used to reduce the power level at rectifier (particularly in back to back DC links). ‘The post-fault recovery of VSC based HVDC links is very crucial when they are used to evacuate power from wind forms. If the VSC based HVDC link is stopped for too long,the wind generator will be tripped by its over-speed protection resulting in the tripping of the e farm, wind REFERENCES AND BIBLIOGRAPHY Kauferle J. and Povh D., ‘Concepts of Overvoltage and Overcurrent Protection of HVDC Converters’, CIGRE Paper 14-08,1978. Kauthold W. and Povh D. ‘Recovery of HVDC Transmission after Faults in the AC System’EE Inrush Currents’, IEEE Trans., Vol. PAS-98, 1974, pp. 487-498. Giesner D.B. and Arzillaga J. Behaviour of HVDC Links under Unbalanced AC Fault Conditions’ Proc. IEE, Vol. 119, No. 2, 1972, pp. 209-215, Kimbark EW, ‘Transient Overvoltages Caused by Monopolar Ground Faults on Bipolar DC Lines— Theory and Simulation’, IEEE Trans., Vol. PAS-89, 1970, pp. 584-592. Hingorani N.G., Transient Overvoltages on Bipolar HVDC Overhead Lines Caused by DC Line Faults’. ibid, 1970, pp. 597-610. Clerici A and Taschini A. Transient Overvoltages Caused by Earth Fault on Bipolar DC Lines’, IEE Conf. Publication No. 107, on HVDC/AC Power Transmission, Nov. 1973, pp. 196-200, Melvold DL, Odam P.C. and Vithayathil,J.J., ‘Transient Overvoltages on HVDC Bipolar Line uring Monopolar Line Faults’, 1EEE Trans., Vol. PAS-96, No. 2, March/April, 1977, pp. 591-601. CIGRE Study Committee No. 38, ‘Application Guide for Insulation Coordination and Protection of HVDC Converter Stations’, 33.83 (SC 03-21, WD). Peterson H.A., Phadke A.G. and Reitan D.K, ‘Transients in EHV DC Power Systems, Part [Rectifier ‘currents’, IREE Trans., Vol. PAS-88, No. 7, July 1969, pp. 981-989, Reove J. and Kapoor S., ‘Analysis of Transient Short Circuit Currents in HVDC Power Systems’, IBEE Trans., Vol. PAS-90, No. 3, 1971, pp. 1174-1182, Thio CLV. et al., ‘Switching Overvoltage on the Nelson River HVDC System-Studies, Experienee ‘and Field Tests’, IEEE. Conf. on ‘Overvoltages and Compensation on Integrated AC- DC Systems’, Winnipeg, 1980, pp. 15-26 8, Bowles J-P., Overvoltages in HVDC Transmission Systems Caused by Transformer Magnetizing 10. nL. 12, CHAPTER 5 Esc