Q1 Explain transmission line interconnection. Also explain factors limiting transmission line loading capacity?

Transmission Line Interconnection

Transmission line interconnection refers to the electrical linkage between two or more power systems or grids through transmission
lines. These interconnections allow for:
1.​ Power Exchange – Transfer of surplus electricity from one region to another.
2.​ Reliability – Backup during outages or maintenance.
3.​ Economic Operation – Use of cheaper power generation resources from interconnected regions.
4.​ Load Sharing – Balancing power demand and supply across regions.
Types of Interconnection:
1.​ Radial Interconnection – One-way power flow, used in small or isolated systems.
2.​ Loop or Ring Interconnection – Power can flow in multiple paths, offering redundancy.
3.​ Mesh Interconnection – Complex network allowing multiple pathways for power, common in large grids.
4.​ Inter-regional or Inter-national Interconnections – Connect grids across regions or countries.
Factors Limiting Transmission Line Loading Capacity
The loading capacity of a transmission line—how much power it can carry—is limited by several key factors:
1.​ Thermal Limits
a.​ Conductors heat up with current flow. Excessive current causes temperature rise, leading to conductor sag,
weakening of materials, and risk of line damage.
b.​ Determined by ambient temperature, wind speed, and conductor material.
2.​ Voltage Drop / Voltage Stability
a.​ Higher loading leads to greater voltage drops across the line.
b.​ Can result in voltage instability or collapse, especially in long-distance lines.
3.​ Angle Stability (Power Angle Limit)
a.​ Power flow depends on the sine of the angle difference between sending and receiving ends.
b.​ Beyond a certain point, the system becomes unstable, leading to loss of synchronism.
4.​ Insulation and Clearance
a.​ Overloading may lead to overvoltage conditions.
b.​ Adequate clearance and insulation are necessary to avoid flashovers or arc faults.
5.​ Corona Loss and Radio Interference
a.​ At higher voltages and loading, corona discharge can increase, causing power loss and interference.
6.​ System Protection Limits
a.​ Circuit breakers, relays, and other protection systems are rated for specific load capacities.
b.​ Exceeding limits can cause tripping or equipment damage.
7.​ Environmental Constraints
a.​ Weather (ice, wind), terrain, and surrounding infrastructure may limit conductor size or tower design, thus limiting
capacity.

Q2 Explain switching converter type VAR generator. Explain its basic operating principle?

1. Static VAR Compensator (SVC) 2. STATCOM

A switching converter type VAR generator, also known as a Static VAR Generator (SVG) or Static Synchronous Compensator
(STATCOM), is a type of power electronic device used to provide reactive power compensation in electrical power systems. It
improves power factor, voltage regulation, and power quality.
What is VAR?
1.​ VAR (Volt-Ampere Reactive) is a unit of reactive power.
2.​ Reactive power is necessary for the operation of inductive loads like motors and transformers but doesn't do actual work.
3.​ A VAR generator supplies or absorbs reactive power to keep the voltage stable.
What is a Switching Converter Type VAR Generator?
It is a power electronic-based device that uses high-speed switching converters (like IGBTs or MOSFETs) to dynamically inject or
absorb reactive power (VARs) into the power system. It's an active compensator, unlike passive capacitor or reactor banks.
Basic Operating Principle:
1.​ Voltage Source Converter (VSC):
a.​ At the heart of the system is a Voltage Source Converter, which creates a controllable AC voltage from a DC
source.
b.​ It uses high-frequency switching devices (like IGBTs).
2.​ Reactive Power Control:
a.​ By adjusting the magnitude and phase of the converter output voltage relative to the grid voltage:
i.​ If converter voltage > grid voltage → Injects reactive power (capacitive)
ii.​ If converter voltage < grid voltage → Absorbs reactive power (inductive)
3.​ DC Link Capacitor:
a.​ Maintains a stable DC voltage for the converter.
b.​ Stores and supplies energy for conversion.
4.​ Control System:
a.​ Fast digital control (DSP or microcontroller-based) determines how much reactive power is needed and adjusts
switching accordingly.
b.​ Responds within milliseconds to load changes.
Key Features:
1.​ Fast response time (much faster than capacitor banks).
2.​ Bidirectional VAR control (can supply and absorb reactive power).
3.​ Improves power factor, reduces harmonics, and stabilizes voltage.
4.​ Useful in renewable energy systems, industrial loads, and distribution networks.
Simplified Diagram (Description):
Grid ↔ Line Reactor ↔ VSC (Switching Converter) ↔ DC Link Capacitor
↑
Control System

Q3 Explain switching converter type series' compensator (SSSC).

The Switching Converter Type Series Compensator (SSSC) is a type of FACTS (Flexible AC Transmission System) device used in
power systems to improve power flow control, voltage regulation, and system stability.
What is the SSSC?
The Static Synchronous Series Compensator (SSSC) is a power electronics-based device that is connected in series with a
transmission line. It is based on Voltage Source Converter (VSC) technology and acts as a controllable voltage source.
How It Works:
1.​ The SSSC injects a synchronously generated AC voltage (in series) with the transmission line.
2.​ This injected voltage is in quadrature (90° phase shift) with the line current.
3.​ It can be capacitive or inductive, depending on whether the power needs to be boosted or limited.
Internal Structure:
1.​ Voltage Source Converter (VSC): Converts DC to controlled AC voltage.
2.​ DC Energy Storage (Capacitor or Battery): Maintains a stable DC link voltage.
3.​ Coupling Transformer: Injects the output voltage of the VSC into the transmission line.
Purpose and Benefits:
1.​ Controls Power Flow: Changes the impedance of the transmission line dynamically.
2.​ Improves Transient Stability: Helps during sudden faults or load changes.
3.​ Mitigates Sub-synchronous Resonance (SSR).
4.​ Reduces Line Losses and increases transmission capacity.
Oh, it's not my bad, it's just my hidden talents that make me a critical thinker, said no one ever.

Comparison to Other FACTS Devices:

Device Type Connection Control

SVC (Static VAR Shunt Parallel Voltage control

Compensator)

TCSC (Thyristor-Controlled Series Series Impedance control

Series Capacitor)

1
 SSSC Series Series Voltage injection (more
flexible and faster response)

STATCOM Shunt Parallel Reactive power compensation

Summary: The Static Synchronous Series Compensator (SSSC) is a modern FACTS device that injects a variable voltage in series
with a transmission line to control power flow, enhance system stability, and improve overall grid performance using VSC
technology.

Q4 Explain thyristor controlled phase angle regulator (TCPAR)

FACTS devices: (a) SVC. (b) TCVR. (c) TCSC. (d) TCPST. (e) UPFC.
A Thyristor Controlled Phase Angle Regulator (TCPAR) is a type of power electronic device used for controlling the output voltage
and power flow in AC systems by adjusting the firing angle of thyristors. It is commonly used in industrial motor drives, heating
control, and AC voltage regulation.
Working Principle:
TCPAR works by delaying the firing angle (α) of a thyristor in each AC cycle, which controls the portion of the waveform that is
applied to the load.
1.​ In an AC waveform, voltage naturally varies sinusoidally.
2.​ The thyristor (SCR) remains off until a gate pulse is applied.
3.​ By changing the point (angle α) at which the gate pulse is triggered, you control how much of the AC waveform is allowed
to pass to the load.
4.​ The larger the delay (firing angle), the less power is delivered to the load.
Components:
1.​ Thyristors (SCRs): Semiconductor switches that conduct only when triggered.
2.​ Control Circuit: Generates gate pulses to control the firing angle.
3.​ Load: Typically a resistive or inductive load (motor, heater, etc.).
Waveform Behavior:
1.​ For 0° firing angle: Full voltage is delivered (like a regular AC supply).
2.​ For 90° firing angle: Only the second half of each half-cycle reaches the load.
3.​ For greater than 90°: Even less power is transferred.
This phase angle control effectively regulates RMS voltage and hence the power.
Applications:
1.​ Speed control of AC motors
2.​ Light dimming
3.​ Industrial heaters
4.​ Soft starting of motors
5. Voltage regulation in power systems

Q5 Explain the operating principle of the thyristor controlled reactor (TCR) in detail?

Operating Principle of Thyristor Controlled Reactor (TCR) – Detailed Explanation

A Thyristor Controlled Reactor (TCR) is a key component of Flexible AC Transmission Systems (FACTS) used in power systems
to regulate reactive power and improve voltage stability. It consists of a reactor (inductor) connected in series with a bidirectional
thyristor valve (two thyristors in anti-parallel), which is then connected to the power system.
1. Basic Components

2
 1.​ Inductor (L): Limits current flow and stores reactive power (VAR).
2.​ Thyristor Valve: Two thyristors connected in anti-parallel to allow bidirectional current control.
3.​ Control Circuit: Provides gate signals to control firing angle of thyristors.
2. Working Principle
The TCR operates by controlling the conduction angle of the thyristors, which in turn controls the amount of current flowing
through the reactor and hence the reactive power absorbed.
Firing Angle Control:
1.​ The firing angle (α) is the angle (in electrical degrees) from the zero-crossing of the AC voltage waveform at which the
thyristors are triggered.
2.​ Range: 90° ≤ α ≤ 180°
a.​ At α = 90° → Full conduction (maximum current through reactor, max inductive VAR absorption).
b.​ At α = 180° → No conduction (thyristors never fire, zero current through reactor).
Conduction Mechanism:
1.​ When the gate pulses are applied at the desired firing angle (α), the thyristors conduct for the remainder of the half-cycle.
2.​ Since the reactor limits the current, the waveform of the current is not purely sinusoidal but lagging.
3.​ By varying α from 90° to 180°, the RMS current through the inductor changes, hence controlling the reactive power
absorbed.
3. Reactive Power Control
The reactive power absorbed by the TCR is:
QTCR=V2/ωL⋅f(α)
Where:
1.​ V = RMS supply voltage
2.​ L = Inductance of the reactor
3.​ ω=2πf Angular frequency
4.​ f(α) = Firing angle-dependent function (non-linear)
So by adjusting α, the system operator can finely regulate the amount of inductive VARs absorbed from the system, which helps:
1.​ Maintain voltage stability
2.​ Balance reactive power
3.​ Improve power factor
4. Key Advantages
1.​ Smooth and fast control of reactive power.
2.​ No mechanical switching, hence higher reliability and speed.
3.​ Used in Static VAR Compensators (SVCs) in conjunction with capacitors (TSCs).
5. Applications
1.​ Voltage control in transmission systems
2.​ Compensation of reactive power in industrial loads
3.​ Stabilization of weak or long transmission lines
​

Q6 Explain various basic FACTS controllers on the basis of their connection with needful diagram. Give one example in
each category.

3
FACTS (Flexible AC Transmission Systems) controllers are devices used to enhance the power flow in transmission systems,
providing better control over the electrical grid. They are mainly categorized into series controllers, shunt controllers, combined
series-shunt controllers, and voltage-source converters. Let me explain each category along with an example and provide diagrams.
1. Series FACTS Controllers
These controllers are connected in series with the transmission line and help to improve the voltage profile, reduce transmission line
losses, and control power flow.
Example:
●​ Thyristor Controlled Series Capacitor (TCSC): A TCSC is a variable series capacitor used to control power flow and
enhance voltage stability. It adjusts the reactance of the line to optimize power flow.
Diagram:
[Generator] ---|---[TCSC]---|---[Load]
2. Shunt FACTS Controllers
Shunt controllers are connected in parallel (shunt) with the transmission line. They are used to improve voltage stability, reactive
power compensation, and maintain the required voltage levels in the system.
Example:
●​ Static Var Compensator (SVC): SVC is used to absorb or supply reactive power to maintain a stable voltage level in the
system. It uses thyristor-controlled reactors (TCR) or thyristor-switched capacitors (TSC).
Diagram:
[Generator] ----[SVC]-----[Load]
3. Combined Series-Shunt Controllers
These controllers combine the functions of both series and shunt compensation. They are used for power flow control and voltage
regulation simultaneously.
Example:
●​ Unified Power Flow Controller (UPFC): UPFC is a combined controller that can control both the active and reactive power
flow in the system. It consists of two voltage-source converters, one connected in series and the other in parallel.
Diagram:
[Generator] --|--[UPFC]--|--[Load]
| Series | Shunt
4. Voltage Source Converter (VSC) Controllers
VSC controllers use voltage-source converters to control both active and reactive power. These controllers can work for both
transmission and distribution systems.
Example:
●​ Flexible AC Transmission System (FACTS) Controller using VSC: The VSC is used for both active and reactive power
compensation in a transmission system, and can operate in both grid-connected and isolated modes.
Diagram:
[Generator] --[VSC]--[Load]
Summary of Key FACTS Controllers:
Type Example Function

Series Controllers TCSC (Thyristor Controlled Series Controls power flow, reduces losses
Capacitor)

Shunt Controllers SVC (Static Var Compensator) Voltage regulation, reactive power
compensation

4
 Combined Series-Shunt UPFC (Unified Power Flow Controller) Active and reactive power control

Voltage Source Converters FACTS using VSC Both active and reactive power
compensation
These controllers are essential for maintaining the stability and efficiency of electrical transmission systems.

Q7 Explain power factor correction in a single phase system.

Power factor correction in a single-phase system is the process of improving the power factor of the system to make it closer to
unity (1).
What is Power Factor?
Power factor (PF) = Real Power (kW) / Apparent Power (kVA)
1.​ Real Power (kW): Actual power used to do useful work.
2.​ Reactive Power (kVAR): Power stored and released by inductive or capacitive elements.
3.​ Apparent Power (kVA): Combination of real and reactive power.
In a single-phase system, a low power factor (e.g., 0.6–0.8) often occurs due to inductive loads like motors, fans, or transformers,
which draw more reactive power.

Why Correct the Power Factor?

A low power factor means:
1.​ More current is needed for the same real power.
2.​ Increased losses in the system (more heat).
3.​ Heavier burden on generators, transformers, and cables.
4.​ Possible penalty from utility companies.
How is Power Factor Corrected?
To improve (raise) the power factor, capacitors are added to the system.
1.​ Inductive loads create lagging power factor (current lags voltage).
2.​ Capacitors provide leading reactive power (current leads voltage).
3.​ When added in parallel, capacitors cancel out some of the reactive power drawn by inductive loads.
Practical Example:
Let’s say your single-phase load has a power factor of 0.7 lagging.
1.​ You add a capacitor bank in parallel.
2.​ The reactive power supplied by the capacitor reduces the total reactive demand from the supply.
3.​ The new power factor rises closer to 1.
Summary:
Power factor correction in a single-phase system is typically done by adding shunt capacitors to counteract the lagging reactive
power from inductive loads, thus improving efficiency, reducing losses, and optimizing the system.

Q8 What is instantaneous PQ theory?

Instantaneous PQ Theory (also called the instantaneous power theory or p-q theory) is a mathematical method used primarily for
real-time power analysis and control in three-phase power systems, especially in active power filters and reactive power
compensation systems.
It was introduced by Akagi, Kanazawa and Nabae in 1983 to describe and control power in nonlinear and unbalanced electrical
systems.
Purpose of PQ Theory
To decompose total instantaneous power into:
1.​ Active power (p): the real power used for doing work.
2.​ Reactive power (q): associated with energy exchange between source and load.

5
Unlike traditional average power theory, PQ theory works in the time domain and captures instantaneous behaviors—making it
suitable for fast-switching power electronics.
How It Works (Basic Overview)
1.Three-phase voltages and currents (a, b, c) are converted into α-β components using the Clarke Transformation:

2.Then, instantaneous power components are calculated:

1.​ Instantaneous active power: p(t)=VαIα+VβIβ
2.​ Instantaneous reactive power: q(t)=VαIβ−VβIα
3.These quantities are used to design control strategies in active filters, compensators, and power management systems.

Applications
1.​ Active power filters: For harmonic and reactive power compensation.
2.​ Custom power devices: Such as STATCOM, DVR, UPQC.
3.​ Real-time power quality monitoring.
4.​ Power factor correction in complex, dynamic loads.
Advantages
1.​ Works with unbalanced and non-sinusoidal systems.
2.​ Suitable for dynamic compensation.
3.​ Real-time application possible.

Q9 Explain the power flow in the Mesh system.

Power Flow in a Mesh System : In electrical power systems, a mesh system (also known as a meshed network) is a type of
interconnected grid where multiple paths exist between power sources (generators) and loads (consumers). This configuration
is common in transmission and distribution networks for improved reliability and load sharing.
Characteristics of a Mesh System:
1.​ Multiple paths for power to flow.
2.​ High reliability-if one line fails, power can reroute through others.
3.​ Better voltage regulation and lower losses than radial systems.
4.​ More complex analysis and control.
Power Flow in a Mesh System: Step-by-Step
1.​ Generation:
a.​ Power is generated at different buses (nodes) in the network.
2.​ Load Demand:
a.​ Loads are connected at various buses across the system.
b.​ Each bus has specific real power (P) and reactive power (Q) demands.
3.​ Power Distribution via Multiple Paths:
a.​ Power flows through multiple interconnected transmission lines.
b.​ The direction and amount of power flow depend on:
i.​ Line impedances (resistance and reactance).
ii.​ Bus voltages and phase angles.
iii.​ System topology.
4.​ Loop Currents / Mesh Currents:

6
 a.​ Currents circulate through various loops (meshes) formed by the network.
b.​ Kirchhoff’s Voltage Law (KVL) is applied around each mesh to calculate current and power flow.
5.​ Power Flow Analysis:
a.​ Solving power flow involves calculating:
i.​ Voltage magnitude and angle at each bus.
ii.​ Power flowing through each line.
b.​ Techniques used:
i.​ Newton-Raphson method (most common).
ii.​ Gauss-Seidel method.
iii.​ Fast Decoupled Load Flow (FDLF).
6.​ Reactive Power and Voltage Support:
a.​ Reactive power flows are carefully controlled to maintain voltage stability.
b.​ Capacitors, reactors, and FACTS devices are often used.

Power Flow Equations (Simplified)

For a line between bus i and j:

Where:
❖​ Pij,Qij : Active and reactive power from bus i to j.
❖​ Vi,Vj : Voltages at buses i and j.
❖​ Xij : Reactance of the line.
❖​ δi,δj : Voltage phase angles.
Advantages of Mesh System
1.​ High reliability and redundancy.
2.​ Load sharing among generators.
3.​ Power rerouting capability in case of faults.
Disadvantages
1.​ Complex to analyze and protect.
2.​ Higher initial cost (more conductors, switches).

Q10 Explain various parameters which limit the loading capabilities of the transmission line.
The loading capability of a transmission line is influenced by several parameters that ensure safe, efficient, and stable power
delivery.
1. Thermal Limit : Each conductor has a maximum current-carrying capacity determined by its thermal limit. Exceeding this limit
can cause excessive heating, leading to conductor sagging, damage to insulation, or even failure. This is often the primary constraint
for short transmission lines.

7
2. Voltage Drop Limit : As power flows through a transmission line, voltage drops occur due to the line's impedance. Excessive
voltage drops can result in poor voltage regulation at the receiving end, affecting the performance of connected equipment. Utilities
typically aim to keep voltage drops within 5–10% of the nominal voltage.
3. Stability Limit : For long transmission lines, system stability becomes a critical factor. The power transfer capability is limited
by the ability to maintain synchronism between generators and loads. Exceeding the stability limit can lead to system oscillations or
blackouts.
4. Surge Impedance Loading (SIL) : SIL represents the natural loading level of a transmission line where the reactive power
generated by the line's capacitance equals the reactive power consumed by its inductance. Operating significantly above or below
SIL can lead to voltage stability issues.
5. Line Length : Longer transmission lines have higher reactance and capacitance, which can exacerbate voltage drops and stability
issues. For instance, lines longer than 320 km may face steady-state stability limitations, restricting their loading capacity.
6. Conductor Sag and Clearance : As conductors heat up under increased loading, they expand and sag. Excessive sag can reduce
ground clearance, posing safety hazards and potentially violating regulatory standards. This physical limitation restricts the
maximum current the line can carry.
7. Environmental Conditions : Ambient temperature, wind speed, and solar radiation affect the cooling of conductors. Higher
ambient temperatures and low wind conditions reduce the conductor's ability to dissipate heat, thereby lowering its thermal limit.
8. Insulation and Dielectric Strength : The insulation of transmission lines must withstand operational and transient overvoltages.
Overloading can increase the risk of dielectric breakdown, especially during switching operations or lightning strikes.
9. Reactive Power Compensation : Inadequate reactive power support can lead to voltage instability and limit power transfer
capability. Devices like shunt capacitors, reactors, and FACTS (Flexible AC Transmission Systems) are employed to manage
reactive power and enhance loadability.
10. System Protection Constraints : Protection systems are designed to operate within specific current and voltage thresholds.
Overloading a line can trigger protective relays, leading to line tripping and potential system outages. Therefore, protection settings
inherently limit the maximum permissible loading.
Understanding these parameters is essential for the design, operation, and upgrading of transmission systems to ensure reliability
and efficiency.

Q11 Explain the capacitor-based circuit for power factor correction.

❖​ A capacitor-based power factor correction (PFC) circuit enhances the efficiency of AC electrical systems by reducing the
phase difference between voltage and current. This is particularly important in systems with inductive loads, such as motors
and transformers, which cause the current to lag behind the voltage, resulting in a lagging power factor.
❖​ The power factor (PF) is the ratio of real power (kW) to apparent power (kVA). A lagging power factor indicates
inefficiency, as more current is drawn than necessary. Capacitor-based PFC circuits address this by introducing capacitors
that supply leading reactive power, counteracting the lagging reactive power from inductive loads.
❖​ In a single-phase system, a capacitor is connected in parallel with the inductive load. This configuration allows the capacitor
to supply leading reactive power, thereby improving the overall power factor.
To determine the appropriate capacitance for power factor correction:​
1.​ Calculate the reactive power (Q) needed to correct the power factor:
○​ Q=P×(tan⁡ϕ1−tan⁡ϕ2)
Where:
P = Real power (kW)
ϕ1 = Angle corresponding to initial power factor
ϕ2 = Angle corresponding to desired power factor
Determine the required capacitance (C):
C=Q/2πfV2
Where:

8
f = Supply frequency (Hz)
V = Supply voltage (V)
This calculation ensures that the capacitor provides the necessary reactive power to achieve the desired power factor.

Advantages of Capacitor-Based PFC

1.​ Improved Efficiency: Reduces the current drawn from the source, minimizing losses.
2.​ Cost Savings: Lower energy bills due to reduced demand charges.
3.​ Enhanced Capacity: Frees up capacity in the electrical system for additional loads.
4.​ Voltage Stabilization: Helps maintain voltage levels within desired ranges.​
Implementation Considerations
1.​ Capacitor Sizing: Accurate calculation is crucial to avoid over or under-compensation.
2.​ Harmonics: Capacitors can resonate with system inductance, potentially amplifying harmonics.
3.​ Switching: In systems with varying loads, automatic capacitor banks with switching mechanisms may be employed.​

Q12 Classify in detail the power filter used for harmonic elimination and explain any one with advantages and
disadvantages.
Power Filters for Harmonic Elimination
Power filters are used to reduce or eliminate harmonics in power systems. Harmonics are unwanted frequencies that distort the
sinusoidal waveform of the voltage or current. They are generated by non-linear loads such as rectifiers, variable speed drives, and
power supplies. Power filters are classified into various types based on their configuration and functionality:
1. Passive Filters : These filters are made from passive components like inductors (L), capacitors (C), and resistors (R). They are
designed to eliminate specific harmonic frequencies.
Types of Passive Filters:
1.​ Single-Tuned Filters: Designed to target a single harmonic frequency.
2.​ Band-Pass Filters: Can target a range of harmonic frequencies.
3.​ High-Pass and Low-Pass Filters: Used to either block lower or higher frequency harmonics.
4.​ Multi-Tuned Filters: Used for systems with multiple harmonic frequencies.
2. Active Filters : Active filters use power electronics (like transistors or thyristors) and are more flexible than passive filters. They
dynamically adjust their impedance to cancel out harmonic currents.
Types of Active Filters:
1.​ Series Active Filters: Connected in series with the load and designed to block specific harmonic frequencies.
2.​ Shunt Active Filters: Connected in parallel to the load, injecting compensating currents to cancel out harmonics in the
system.
3.​ Hybrid Filters: A combination of both passive and active filters. The passive component reduces the low-frequency
harmonics, and the active component deals with higher-order harmonics.

9
3. Hybrid Filters : Hybrid filters combine the advantages of both active and passive filters to achieve efficient harmonic mitigation.
These filters are especially useful when high efficiency, cost-effectiveness, and broad harmonic range elimination are needed.
4. Resonant Filters : These filters are specifically tuned to resonate at a particular harmonic frequency. They use the principle of
resonance, where the impedance of the circuit becomes very high at the resonant frequency, thereby trapping and dissipating
harmonic energy.
5. Notch Filters : A notch filter is designed to block a narrow band of frequencies, particularly one or two harmonic orders, by
reducing the power at those frequencies without affecting others. These are generally more specific in their application.
Explanation of Shunt Active Filter : A Shunt Active Filter is a device designed to mitigate harmonics by injecting compensating
currents into the power system. It is placed in parallel with the load and actively detects and compensates for harmonic currents.
Working Principle:
1.​ The filter continuously monitors the total harmonic distortion (THD) in the system.
2.​ It uses a controller (usually based on Fourier analysis) to calculate the harmonic content.
3.​ Based on this information, the filter injects counteracting currents into the system to cancel out the harmonic currents.
4.​ The filter can eliminate both current harmonics (distorted load current) and voltage harmonics (distorted supply voltage).
Components of a Shunt Active Filter:
1.​ Power Electronic Switching Device (e.g., IGBT, MOSFET): Allows fast switching of the compensating current.
2.​ Controller: An algorithm that calculates the harmonic currents to inject and adjusts the switching device accordingly.
3.​ DC Link: A DC voltage source (typically a capacitor) that powers the switching device.
Advantages of Shunt Active Filters:
1.​ Dynamic Compensation: Can respond in real-time to changing loads and harmonic currents.
2.​ Broad Harmonic Coverage: Capable of compensating for a wide range of harmonic orders (e.g., 3rd, 5th, 7th).
3.​ Improved Power Quality: Reduces both current and voltage harmonics, improving the overall power quality.
4.​ Compact and Flexible: Unlike passive filters, they can be easily integrated into existing systems and adjusted for varying
loads.
5.​ No Need for Tuning: Active filters are adaptable and don’t require recalibration for different harmonic frequencies.
Disadvantages of Shunt Active Filters:
1.​ High Cost: Shunt active filters are generally more expensive than passive filters, both in terms of initial investment and
maintenance.
2.​ Complex Control Systems: Requires sophisticated controllers and algorithms, which can add complexity to the system.
3.​ Power Losses: The power electronics in the filter can introduce losses, reducing efficiency.
4.​ Size and Weight: Although compact, the active components and cooling systems can be relatively bulky for large-scale
applications.
5.​ Response Time: Although fast, the filter may have a slight delay in response to rapidly changing load conditions.

10
Conclusion : Shunt Active Filters are highly effective for harmonic elimination in modern electrical systems, especially when
dealing with varying loads and multiple harmonic frequencies. While they offer high performance and flexibility, the costs and
complexity involved make them more suited for larger systems or applications where precision and real-time adjustment are crucial.

Q13 What is the flicker in power quality?

Flicker in power quality refers to the perceptible fluctuations in light intensity caused by rapid and repetitive variations in the
supply voltage. These voltage fluctuations can lead to noticeable dimming or brightening of lighting systems, particularly
incandescent and fluorescent lamps, which are sensitive to such changes.
Understanding Flicker : Flicker is primarily caused by rapid voltage changes, often resulting from:
1.​ Large motor starts or stops
2.​ Arc furnaces
3.​ Welding machines
4.​ Large air conditioning units
These events introduce sudden and significant changes in the load, leading to voltage dips or swells that propagate through the
electrical distribution system. The human eye detects these variations as flickering lights, especially when the frequency of voltage
changes falls within a certain range.
Measuring Flicker : Flicker is quantified using specific indices:
1.​ Pst (Short-Term Flicker Severity): Assesses the severity of flicker over a short period (typically 10 minutes).
2.​ Plt (Long-Term Flicker Severity): Evaluates the flicker severity over a longer period (usually 2 hours).
These indices are standardized by organizations like the IEC (International Electrotechnical Commission) and IEEE to ensure
consistent measurement and assessment of flicker in power systems.
Impacts of Flicker
1.​ Human Perception: Continuous exposure to flicker can cause discomfort, eye strain, and headaches.
2.​ Equipment Performance: Sensitive electronic equipment may experience malfunctions or reduced lifespan due to constant
voltage variations.
3.​ Regulatory Compliance: Utilities are often required to maintain flicker levels within specified limits to meet power quality
standards.
Mitigation Strategies : To reduce or eliminate flicker:
1.​ Install Power Conditioning Equipment: Devices like voltage stabilizers or dynamic voltage restorers can smooth out voltage
fluctuations.
2.​ Implement Active Filters: Active power filters can dynamically compensate for rapid voltage changes.
3.​ Enhance System Design: Proper sizing and placement of transformers, capacitors, and other equipment can help manage
load variations more effectively.
4.​ Regular Maintenance: Ensuring that all equipment is in good working condition can prevent issues that lead to flicker.

11
Q14 Explain Ideal Compensator and its conditions
An Ideal Compensator in power systems is a theoretical device designed to improve system performance by dynamically injecting
or absorbing reactive power. It is often modeled as an ideal current source that supplies or absorbs only reactive power, with no real
power exchange. This idealization helps in understanding the potential benefits of compensation without the complexities of
real-world limitations.
Characteristics of an Ideal Compensator
1.​ Reactive Power Only: An ideal compensator injects or absorbs only reactive power, with no real power exchange.
2.​ Instantaneous Response: It can adjust its reactive power output instantaneously to match system requirements.
3.​ No Losses: Assumed to operate without any losses, making it 100% efficient in its reactive power compensation role.
4.​ No Voltage Drop: It does not cause any voltage drop or phase shift in the system, maintaining the voltage profile.
5.​ Ideal Current Source: In models, it is represented as an ideal current source that can supply or absorb reactive power
without limitations.
Conditions for an Ideal Compensator : For an ideal compensator to function as intended, certain conditions must be met:
1.​ No Real Power Exchange: It must inject or absorb only reactive power, with no real power exchange.
2.​ Infinite Bandwidth: It should be capable of responding to changes in reactive power demand without any delay.
3.​ No Harmonic Generation: It must not introduce harmonics into the system, ensuring pure sinusoidal waveforms.
4.​ Perfect Control: It should have perfect control over its reactive power output, with no overshoot or oscillations.
5.​ No Physical Limitations: It operates without any physical constraints, such as size, cost, or thermal limits.
Applications in Power Systems : Ideal compensators are used in theoretical analyses to:
1.​ Model System Behavior: Understand how compensation affects system stability and performance.
2.​ Design Compensation Strategies: Develop strategies for reactive power compensation in real systems.
3.​ Benchmark Performance: Set performance benchmarks for real compensating devices.
Limitations : While ideal compensators are useful for theoretical studies, they have limitations in practical applications:
1.​ Unrealistic Assumptions: Assume no real power exchange and perfect control, which is not achievable in real systems.
2.​ No Physical Existence: They do not correspond to any physical device currently available.
3.​ Simplified Models: Their simplicity may overlook complex dynamics present in actual power systems.
Practical Counterparts : In real-world applications, devices that approximate the behavior of ideal compensators include:
1.​ Static VAR Compensators (SVC): Thyristor-controlled devices that provide fast-acting reactive power compensation.
2.​ Static Synchronous Compensators (STATCOM): Voltage-source converter-based devices offering dynamic reactive power
support.
3.​ Synchronous Condensers: Synchronous motors running without mechanical load, providing reactive power support.
These devices, while not ideal, offer practical solutions for reactive power compensation in modern power systems.

12
Q15 Explain the merits and demerits of transmission interconnections
Transmission interconnections in power systems refer to the linking of separate electrical grids or regions through high-voltage
transmission lines, enabling the transfer of electricity between them. These interconnections offer several advantages and
disadvantages, which are crucial for understanding their impact on power system operations.
Merits of Transmission Interconnections
1.​ Enhanced Reliability and Security : By connecting multiple power systems, interconnections provide backup options during
outages, improving the overall reliability and security of the electricity supply.
2.​ Economic Efficiency : Interconnections allow for the sharing of electricity between regions, enabling the use of lower-cost
generation sources and reducing the need for expensive peaking power plants.
3.​ Better Load Management : They facilitate the balancing of loads across different areas, helping to manage peak demands
more effectively and reducing the risk of overloading local systems.
4.​ Integration of Renewable Energy : Interconnections enable the integration of renewable energy sources by allowing excess
power generated during peak production times to be transmitted to areas with higher demand.
5.​ Reduced Reserve Capacity Requirements : With interconnected systems, the need for reserve capacity is reduced, as power
can be imported from neighboring regions during shortages, leading to cost savings.
6.​ Improved Voltage Stability : The sharing of reactive power through interconnections can enhance voltage stability across
the network.
7.​ Optimized Generation Dispatch : Operators can dispatch generation more efficiently by utilizing the most cost-effective
sources available across interconnected regions.
8.​ Facilitation of Market Competition : Interconnections promote competition among electricity suppliers, potentially leading
to lower prices and improved service quality for consumers.
9.​ Support for Emergency Assistance : In case of local generation failures or natural disasters, interconnected systems can
provide emergency power support to affected areas.
10.​ Enhanced System Flexibility : Interconnections offer greater flexibility in system operations, allowing for more dynamic
responses to changing demand and generation conditions.
Demerits of Transmission Interconnections
1.​ Risk of Cascading Failures : A fault in one part of the interconnected system can propagate, leading to widespread outages
across multiple regions.
2.​ High Infrastructure Costs : Building and maintaining interconnection infrastructure, such as transmission lines and
substations, can be expensive.
3.​ Complex Coordination Requirements : Effective operation of interconnected systems requires sophisticated coordination
among multiple operators, which can be challenging.
4.​ Transmission Congestion : High demand on interconnection lines can lead to congestion, limiting the ability to transfer
power efficiently and potentially causing bottlenecks.
5.​ Regulatory and Policy Challenges : Different regions may have varying regulations and policies, complicating the
management and operation of interconnected systems.​

6.​ Environmental and Land Use Concerns : The construction of new transmission lines for interconnections can raise
environmental and land use issues, including habitat disruption and public opposition.
7.​ Security Vulnerabilities : Interconnected systems can be more susceptible to cyberattacks and other security threats, as a
breach in one area can affect the entire network.
8.​ Operational Complexity : Managing the flow of electricity across interconnected systems adds complexity to grid
operations, requiring advanced monitoring and control systems.
9.​ Potential for Unequal Benefits : Some regions may benefit more from interconnections than others, leading to disparities in
service quality and costs.
10.​ Dependency on Neighboring Systems : Over-reliance on neighboring systems for power supply can be risky if those
systems experience issues or outages.
Transmission interconnections play a vital role in modern power systems by enhancing reliability, economic efficiency, and the
integration of renewable energy sources. However, they also introduce challenges such as increased risk of cascading failures, high
infrastructure costs, and complex coordination requirements. Balancing these advantages and disadvantages is crucial for the
effective design and operation of interconnected power systems.

13
Q16 State objectives of series compensator.
A series compensator is a device used in power systems to improve the transmission efficiency and stability of electrical power
flow. It typically involves inserting a capacitor in series with the transmission line to reduce the overall reactance, thereby enhancing
the system's performance.
Objectives of Series Compensation
1.​ Increase Power Transfer Capability : By reducing the total reactance of the transmission line, series compensation allows
for higher power transfer for a given voltage and current, effectively increasing the line's capacity.
2.​ Improve Voltage Stability : The insertion of capacitive reactance helps maintain voltage levels along the transmission line,
especially under heavy load conditions, thereby improving voltage stability.
3.​ Enhance System Stability : Series compensation contributes to the overall stability of the power system by increasing the
synchronizing and damping torques, which helps in maintaining system stability during disturbances.
4.​ Reduce Transmission Losses : By optimizing the impedance of the transmission line, series compensation minimizes power
losses during transmission, leading to more efficient power delivery.
5.​ Damp Power Oscillations : Series compensators can help in damping power oscillations that may occur due to disturbances,
improving the dynamic response of the system.
6.​ Mitigate Subsynchronous Resonance (SSR) : Properly designed series compensation can help in mitigating SSR, a
phenomenon where interactions between series capacitors and turbine-generators can lead to system instability.
7.​ Facilitate Parallel Line Loading : In systems with multiple parallel transmission lines, series compensation can help in
balancing the load distribution, preventing overloading of individual lines.
8.​ Enhance Frequency Response : Series compensation can improve the frequency response of the power system, aiding in
maintaining frequency stability during load variations.
In summary, series compensation plays a crucial role in modern power systems by enhancing power transfer capabilities, improving
voltage and system stability, reducing losses, and mitigating potential oscillatory phenomena. Its implementation, however, requires
careful design to avoid issues such as subsynchronous resonance.

14
Q17 With the neat phasor diagram explains power factor compensation using capacitor.

Concept: In AC systems, inductive loads (like motors and transformers) draw lagging reactive power (VARs), which lowers the
power factor. Capacitors supply leading reactive power, which can offset the lagging VARs and thus improve the power factor
toward unity.
Phasor Diagram Explanation:
In the phasor diagram:
1.​ Active Power (P) – Lies along the horizontal axis (real power).
2.​ Reactive Power (Q) – Lies along the vertical axis.
a.​ Lagging (Inductive Load): Points upward.
b.​ Leading (Capacitor): Points downward.
3.​ Apparent Power (S) – The vector sum of P and Q; it's the hypotenuse of the triangle.
4.​ After Compensation:
a.​ Capacitor provides leading Q, reducing the net Q.
b.​ This brings the apparent power vector closer to the real axis, increasing power factor (cosθ).
Effect of Compensation:
1.​ Reduces the angle θ between voltage and current.
2.​ Brings current and voltage more in phase.
3.​ Decreases apparent power S.
4.​ Improves system efficiency and reduces losses.

Q18 Enlist common power quality issues. Explain any five of them.
Common Power Quality Issues:
1.​ Voltage sag (dip)
2.​ Voltage swell
3.​ Transients (impulses/spikes)
4.​ Harmonics
5.​ Flicker
6.​ Interruptions (short and long)
7.​ Frequency variations
8.​ Voltage imbalance
9.​ Noise
Explanation of Five Power Quality Issues:
1.​ Voltage Sag (Dip): A short-duration reduction in RMS voltage, typically lasting from a few milliseconds to a few seconds.
It is often caused by the starting of large motors or faults in the power system. Sensitive equipment may malfunction or shut
down during sags.
2.​ Voltage Swell: A temporary increase in RMS voltage, usually caused by a sudden reduction in load or switching off a large
load. Swells can damage electronic components and reduce the lifespan of electrical equipment.
15
 3.​ Transients (Spikes/Impulses): Sudden, high-frequency disturbances often caused by lightning strikes, switching
operations, or faults. They can lead to insulation breakdown and damage to sensitive electronic equipment.
4.​ Harmonics: These are voltage or current waveforms that are multiples of the fundamental frequency (50 or 60 Hz).
Harmonics are usually caused by nonlinear loads like variable frequency drives or rectifiers. They can lead to overheating,
equipment malfunction, and increased losses.
5.​ Interruptions: Interruptions refer to complete loss of voltage or current. They can be short-term (lasting up to 1 minute) or
long-term (lasting more than 1 minute). They may result from equipment failure, faults, or tripping of protection devices,
and can lead to downtime and loss of productivity.

Q19 Comment how harmonics affect the rotating machines, transformer & cable.
1.​ Rotating Machines (Motors and Generators):
a.​ Increased Losses: Harmonics cause additional core (iron) and copper losses, especially due to eddy currents and
skin effect.
b.​ Overheating: The extra losses lead to excessive heating, which can shorten insulation life and reduce the motor's
lifespan.
c.​ Torque Pulsations: Harmonic currents can cause fluctuations in torque, resulting in mechanical vibrations and noise.
d.​ Reduced Efficiency and Performance: Motors operate less efficiently under harmonic distortion and may
experience derating.
2.​ Transformers:
a.​ Increased Eddy Current Losses: Harmonics, especially of higher orders, induce eddy currents in the core and
windings, increasing losses.
b.​ Overheating: Due to increased core and copper losses, transformers may overheat, even under rated load conditions.
c.​ Reduced Life Expectancy: Continuous overheating accelerates insulation aging, reducing transformer life.
d.​ De-rating: Transformers may need to be derated in harmonic-rich environments to prevent overloading.
3.​ Cables:
a.​ Increased I²R Losses: Harmonics raise the RMS value of the current, increasing resistive heating.
b.​ Skin and Proximity Effects: Higher frequency harmonic currents tend to flow on the surface of the conductor (skin
effect), effectively increasing the resistance and resulting in more heating.
c.​ Overheating and Insulation Damage: Sustained exposure to harmonic currents can cause insulation degradation and
premature cable failure.

Q20 Explain different causes of voltage and current harmonics

Harmonics in electrical systems are primarily caused by non-linear loads—devices whose current consumption is not proportional
to the applied voltage. Here’s a breakdown of their causes:
1. Power Electronic Devices:
Examples: Variable Frequency Drives (VFDs), inverters, rectifiers, UPS systems.
Cause: These devices switch power rapidly (typically using semiconductors like IGBTs or thyristors), generating non-sinusoidal
current waveforms that contain harmonics.
2. Non-Linear Loads:
Examples: Computers, printers, LED lighting, battery chargers, microwave ovens.
Cause: These devices draw current in pulses rather than smooth sine waves, distorting the overall current waveform.
3. Saturated Magnetic Devices:
Examples: Transformers, reactors, inductors operating near saturation.
Cause: When magnetic cores saturate, their inductance becomes non-linear, distorting the current waveform.
4. Arc Furnaces and Welding Equipment:
Cause: The arcing process is inherently non-linear and erratic, generating large harmonic currents and voltage disturbances.
5. Fluorescent Lamps with Electronic Ballasts:
Cause: Ballasts chop the current at high frequencies to regulate power, introducing high-frequency harmonic components into the
current.
6. Switching Power Supplies:
Examples: Found in most modern electronics.
Cause: These draw current in sharp spikes, rich in harmonics, especially the 3rd, 5th, and 7th.
7. Unbalanced Loads in Three-Phase Systems:
Cause: Load imbalance causes zero-sequence harmonic currents (like the 3rd harmonic) to circulate in the neutral conductor,
distorting the voltage waveform.
8. Industrial Drives and Servo Motors:
Cause: Their control electronics use switching to vary speed or torque, injecting harmonics back into the supply.

Q21 Distinguish between power outages and interruptions.

16
Distinction Between Power Outages and Interruptions:
Aspect Power Outage Power Interruption

Definition Complete loss of power supply to a Temporary loss or reduction of power

system or area. (partial or complete).

Duration Usually longer, from minutes to hours or Typically shorter, ranging from
even days. milliseconds to a few minutes.

Cause Major faults, natural disasters, equipment Switching operations, faults, or

failure. protective device actions.

Scope Often affects a larger area or grid May affect specific circuits or loads
segment. locally.

Impact Can halt operations completely and for May cause minor disruptions, equipment
longer. resets, or flickers.

Types No formal classification. Classified as momentary, temporary, or

sustained interruptions.
A power outage is a prolonged and often unexpected loss of power, whereas a power interruption is typically a short, often planned
or transient event in the power system.

17