Power quality Problems and mitigation techniques (EX-802 (A)) - Unit 1 Short Notes

UNIT 1: POWER QUALITY PROBLEMS AND MITIGATION TECHNIQUES

1.0 INTRODUCTION TO POWER QUALITY

1.1 Definition and Concept of Power Quality (PQ)

• Power Quality refers to the concept of ensuring that the voltage, current, and frequency of the electrical power supply remain within specified limits, allowing electrical equipment to operate as intended without performance degradation or damage.

• It encompasses all issues related to voltage, current, and frequency deviations that can cause malfunction, failure, or premature aging of end-use equipment.

1.2 Importance and Need for Power Quality

• Increased Sensitivity: Modern equipment (computers, PLCs, drives) is more sensitive to PQ disturbances.

• Economic Impact: PQ problems cause production losses, equipment damage, and increased maintenance costs.

• Reliability: Critical facilities (hospitals, data centers) require high-quality, uninterrupted power.

1.3 Major Power Quality Issues (Overview)

• Voltage Disturbances: Sags, Swells, Interruptions, Fluctuations, Transients.

• Waveform Distortions: Harmonics, Interharmonics.

• Frequency Variations: Deviations from nominal frequency (50/60 Hz).

• Voltage Imbalance: Unequal magnitude or phase shift in three-phase systems.

1.4 Reasons for Increased Concern

• Proliferation of non-linear loads (rectifiers, UPS, drives).

• Use of power electronics in control and conditioning.

• Economic penalties for poor power factor or harmonic distortion (utilities).

• Automation in industries making processes intolerant to disturbances.

1.5 General Causes of Power Quality Disturbances

• Natural: Lightning strikes.

• System Faults: Short circuits, line-to-ground faults.

• Load Switching: Inrush currents from motor starting, capacitor switching.

• Non-linear Loads: Draw non-sinusoidal currents, creating harmonics.

• Poor Coordination: Of protection devices and capacitor banks.

2.0 VOLTAGE DISTURBANCES

2.1 Voltage Sag (Dip)

• Definition: A decrease in RMS voltage to between 0.1 and 0.9 pu for a duration of 0.5 cycles to 1 minute.

• Common Causes:

  • Faults in the power system (most common).

  • Motor starting (large inrush current).

  • Load switching (especially inductive loads).

• Mitigation Techniques:

  • Ride-through for sensitive equipment (UPS, DVR).

  • Dynamic Voltage Restorer (DVR).

  • Improved system design (reducing impedance, adding parallel paths).

  • Soft starters for motors.

• Voltage Sag Performance Estimation:

  • Sag Magnitude: $$\displaystyle V_{sag} = \frac{V_{during}}{V_{prefault}} \times 100\% $$

  • Sag Duration: Time voltage remains below threshold.

  • Sag Frequency/Probability: Estimated via site-specific monitoring or statistical methods (e.g., fault-tree analysis).

  • Indices: Sag Frequency Index (SFI), Sag Severity Index (SSI).

[!TIP] Exam Focus: Sag vs. Swell, Mitigation methods (DVR, UPS ride-through), and estimation indices are frequently asked.

2.2 Voltage Swell

• Definition: An increase in RMS voltage to between 1.1 and 1.8 pu for a duration of 0.5 cycles to 1 minute.

• Common Causes:

  • Fault clearing (especially single-line-to-ground faults).

  • Load rejection (sudden disconnection of a large load).

  • Capacitor bank switching (improper timing).

• Difference between Voltage Surge and Swell:

  | Feature | Voltage Swell | Voltage Surge | |-------------------|--------------------------------------------|--------------------------------------------| | Nature | RMS increase (sustained, 0.5 cyc - 1 min) | Transient impulse (microseconds) | | Duration | Longer (cycles to minutes) | Very short (µs to ms) | | Waveform | Sustained sinusoidal increase | High-frequency oscillation/impulse | | Typical Cause | Fault clearing, load rejection | Lightning, capacitor switching, load dump |

2.3 Voltage Interruption (Outage)

• Definition: Voltage drops to less than 0.1 pu.

• Classification:

  • Momentary: < 1 sec (often cleared by reclosing).

  • Temporary: 1 sec to 1 min (requires manual reset).

  • Sustained: > 1 min (until repair).

2.4 Voltage Fluctuation

• Definition: Oscillations in voltage magnitude, typically below 25 Hz.

• Common Causes:

  • Varying loads: Arc furnaces, rolling mills, large motor starting/stopping.

  • Intermittent welding operations.

• Flicker:

  • Definition: The perception of light fluctuation by the human eye/brain due to voltage fluctuations.

  • Measurement: IEC Flicker Meter (Pst - short-term severity, Plt - long-term severity).

  • Perception Threshold: ~0.5% voltage fluctuation at 8.8 Hz.

2.5 Transient Overvoltages

• Definition: High-frequency, short-duration (µs to ms) overvoltages superimposed on the normal waveform.

• Types:

  • Impulsive Transients: Unidirectional (e.g., lightning).

  • Oscillatory Transients: Bidirectional ringing (e.g., capacitor switching, fault clearing).

• Common Causes:

  • Lightning (direct or indirect).

  • Switching operations (capacitor banks, reactors, fault clearing).

  • Ferroresonance.

• Factors Affecting Transient Recovery Voltage (TRV):

  • System voltage level.

  • Fault location and type.

  • Source impedance (inductance and capacitance).

  • Breaker characteristics (opening speed).

• Mitigation - Zero-Voltage Crossing Switching for Capacitor Banks:

  • Switching at voltage zero minimizes the instantaneous voltage difference across the capacitor, drastically reducing the transient magnitude and frequency. This is the optimal switching instant to prevent harmful inrush currents and overvoltages.

2.6 Voltage Spike / Impulse Voltages

• Definition: See Transient Overvoltages (often used interchangeably).

• Protection Methods for High Voltage Equipment:

  • Lightning Arresters / Surge Arresters.

  • Shielded Cables and proper grounding.

  • Isolation Transformers with electrostatic shields.

  • RC Snubbers across switching devices.

• Surge Protection Devices (SPDs):

  • Operation Principle: Clamp the voltage to a safe level by presenting a low impedance path to ground for transient energy once the voltage exceeds a clamping voltage.

  • Types:

    1. Gas Tube Arresters: High discharge capacity, slow response.

    2. Metal Oxide Varistors (MOVs): Fast response, energy absorption, degrades with surges.

    3. Semiconductor (TVS Diodes): Fastest response, low energy capacity, precise clamping.

  • Coordination: Type 1 (direct lightning, service entrance), Type 2 (main distribution), Type 3 (equipment protection). Must be coordinated in a cascaded manner.

3.0 HARMONONIC DISTORTIONS

3.1 Fundamentals of Waveform Distortion

• A non-sinusoidal periodic waveform can be decomposed into a fundamental component (at system frequency, e.g., 50 Hz) and a series of harmonic components (integer multiples of the fundamental frequency).

• Fourier Series Representation: $$\displaystyle v(t) = V_1 \sin(\omega t + \phi_1) + \sum_{h=2}^{\infty} V_h \sin(h\omega t + \phi_h) $$

3.2 Harmonic Sources

• Industrial Loads:

  • Arc Furnaces (major source, highly non-linear).

  • Rectifiers (AC/DC conversion for drives, electrolysis).

  • Adjustable Speed Drives (ASDs) - PWM inverters.

  • Saturation in transformers (magnetizing current).

• Commercial Loads:

  • Computers, UPS systems, Switched-mode power supplies (SMPS).

  • Fluorescent lighting with electronic ballasts.

  • Elevators, HVAC controls.

• Primary Cause: Non-linear loads that draw current in non-sinusoidal pulses, distorting the current waveform, which then propagates as voltage distortion through system impedance.

3.3 Harmonic Indices and Definitions

• Individual Harmonic Distortion (IHD): $$\displaystyle IHD_h = \frac{I_h}{I_1} \times 100\% $$ (for current) or $$\displaystyle \frac{V_h}{V_1} \times 100\% $$ (for voltage).

• Total Harmonic Distortion (THD): Ratio of RMS value of all harmonics to RMS value of fundamental.

  • Current THD: $$\displaystyle THD_I = \frac{\sqrt{\sum_{h=2}^{\infty} I_h^2}}{I_1} \times 100\% $$

  • Voltage THD: $$\displaystyle THD_V = \frac{\sqrt{\sum_{h=2}^{\infty} V_h^2}}{V_1} \times 100\% $$

  • \boxed{THD = \sqrt{\sum_{h=2}^{\infty} (IHD_h)^2}}

• Total Demand Distortion (TDD): THD of current relative to the maximum demand current ($$\displaystyle I_{L} $$) at the fundamental frequency. Used for setting IEEE 519 limits.

  • $$\displaystyle TDD = \frac{\sqrt{\sum_{h=2}^{\infty} I_h^2}}{I_L} \times 100\% $$

• Crest Factor (CF): Ratio of peak value to RMS value of a waveform. $$\displaystyle CF = \frac{I_{peak}}{I_{rms}} $$. High CF (>1.5) indicates significant harmonic content.

• Interharmonics: Frequency components that are not integer multiples of the fundamental frequency. Caused by static frequency converters, cycloconverters, and arcing loads. Can cause flicker and torsional oscillations.

3.4 Effects of Harmonics

• Heating in Rotating Machines & Transformers: Core losses (hysteresis & eddy current ∝ $$\displaystyle f^2 $$) and additional copper losses ($$\displaystyle I_h^2R $$). Causes overheating and reduced efficiency.

• Capacitor Bank Failures: Resonance between system inductance and capacitor bank capacitance can amplify harmonic currents, leading to overloading, overheating, and failure.

• Misoperation of Protective Relays & Meters: Harmonics cause incorrect sensing of RMS values, zero-crossing, and phase angles, leading to nuisance tripping or failure to trip.

• Communication Interference: Harmonic currents in power lines induce noise in nearby telephone and signal lines (electromagnetic interference - EMI).

3.5 Harmonic Standards and Limits

• IEEE 519-2014: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.

  • Sets limits on THD and individual harmonics for both voltage (at PCC) and current (based on TDD and $$\displaystyle I_L/I_{sc} $$ ratio).

  • Key Principle: Limits are stricter for smaller customers (lower $$\displaystyle I_L/I_{sc} $$ ratio).

• IEC 61000-4-30: Defines methods for power quality measurement.

• IEC 61000-3-2/4: Sets emission limits for harmonic currents for equipment.

4.0 MITIGATION TECHNIQUES: PASSIVE METHODS

4.1 Passive Filters

• Shunt Passive Filters:

  • Principle: Low-impedance path for selected harmonic frequencies, shunting them away from the load/system.

  • Types:

    • Tuned Filters (Single/Double-Tuned): Series LC circuit tuned to a specific harmonic (e.g., 5th, 7th). Disadvantage: Risk of detuning and resonance with system.

    • High-Pass Filters (C-Type, etc.): Provide a low-impedance path for a wide band of high-frequency harmonics. C-Type filter reduces losses at fundamental frequency.

  • Advantages: Simple, reliable, cost-effective for single or few dominant harmonics.

  • Disadvantages: Fixed compensation, can cause resonance, performance degrades with system impedance changes.

• Series Passive Filters:

  • Principle: High impedance at harmonic frequencies, blocking harmonic currents from entering the system or load.

  • Application: Often used with tuned shunt filters to prevent resonance.

  • Disadvantages: High fundamental voltage drop, large size, rarely used alone.

4.2 Passive Series Compensation

• Principle: Inserting a series capacitor to compensate for line inductance, improving voltage profile and stability.

• Working: The series capacitor cancels a portion of the line's inductive reactance ($$\displaystyle X_L $$), reducing the total transfer reactance ($$\displaystyle X_{total} = X_L - X_C $$). This increases power transfer capability and improves voltage regulation.

• Types: Fixed series capacitor, Thyristor-Controlled Series Capacitor (TCSC - part of FACTS).

4.3 Capacitor Banks for Power Factor Correction

• Purpose: Compensate for lagging reactive power (Q) drawn by inductive loads, improving power factor ($\cos\phi$).

• Advantages of P.F. Correction:

  • Reduced system losses ($$\displaystyle I^2R $$).

  • Better utilization of transformers, cables, generators (reduced KVA loading).

  • Improved voltage regulation.

  • Avoidance of utility penalties for low PF.

• Installation Locations:

  • Individual Load Correction: Capacitor at each large inductive load (motor).

  • Group/Feeder Correction: Capacitor bank on a distribution feeder.

  • Central/System Correction: Large capacitor bank at the main substation or load center.

• Reactive Power Compensation Penalty: Utilities charge penalties for excessive reactive power demand because it increases current flow for the same real power, causing higher losses and reduced capacity in the utility's system.

4.4 Reactors

• Line Reactors: Series inductors to limit fault currents, inrush currents, and harmonic currents.

• Bus Reactors: Connected between buses to limit voltage transients during switching and isolate harmonic frequencies.

• Tuning Reactors: Used in detuned filter banks to shift the resonance frequency away from dominant harmonics (e.g., 4.7th order detuning for 5th harmonic).

5.0 MITIGATION TECHNIQUES: ACTIVE METHODS

5.1 Active Power Filters (APF)

• Basic Principle: Uses power electronics (VSC) to inject a compensating current (or voltage) that is equal and opposite to the harmonic current (or voltage distortion) drawn by the load. Dynamic and selective compensation.

• Shunt Active Filter:

  • Operation: Connected in parallel with the load. Senses load current, calculates the reference compensating current (harmonic + reactive), and injects it via a current-controlled VSC.

  • Reference Current Calculation: Methods include Instantaneous Reactive Power Theory (p-q theory), Synchronous Reference Frame (d-q) theory, or Fourier analysis.

• Series Active Filter:

  • Connected in series with the load. Injects a compensating voltage to cancel voltage harmonics/sags/swells. Acts as a voltage source.

• Hybrid Active Filters: Combination of active filter + passive filter (e.g., shunt APF with tuned shunt passive filter). APF improves the passive filter's performance by detuning it and compensating for its limitations.

• Advantages of Active Harmonic Filters:

  • Dynamic compensation for multiple harmonics and reactive power.

  • No resonance risk with system.

  • Can compensate for flicker and unbalance.

• Disadvantages:

  • High cost and complexity.

  • Power losses in the converter.

  • Limited capacity (typically up to a few MVA).

5.2 Flexible AC Transmission Systems (FACTS) Controllers

• Concept of "Flexible": Refers to the ability to rapidly and continuously control AC transmission system parameters (impedance, voltage, phase angle) using power electronics to increase power transfer capability, enhance stability, and improve reliability.

• Why Capacitance is Taken as Shunt in Transmission Lines? Shunt capacitors provide local reactive power (VAR) support, which boosts voltage magnitude at the point of connection and reduces current flow for the same real power, thereby increasing transfer capability and improving stability.

• Static VAR Compensator (SVC):

  • Principle: A thyristor-based device that dynamically controls the equivalent shunt reactance.

  • Main Components:

    • Thyristor-Controlled Reactor (TCR): Inductor with thyristor valve controlling conduction angle → continuously variable inductive VAR.

    • Thyristor-Switched Capacitor (TSC): Capacitor banks switched in/out by thyristors → stepwise capacitive VAR.

    • TCR-TSC Combination: Provides continuous, bidirectional VAR control over a wide range.

  • V-I Characteristic: Slope (equivalent reactance) is positive (inductive). Dead band prevents continuous switching.

• STATCOM (Static Synchronous Compensator):

  • Principle: A Voltage Source Converter (VSC) based device, connected in shunt, that acts as a controllable voltage source behind a small interfacing reactor.

  • Operation: By controlling the magnitude and phase of its output voltage ($$\displaystyle V_{STATCOM} $$), it can generate or absorb reactive power.

    • $$\displaystyle V_{STATCOM} > V_{bus} $$ → Capacitive (injects Q).

    • $$\displaystyle V_{STATCOM} < V_{bus} $$ → Inductive (absorbs Q).

  • Comparison: SVC vs. STATCOM:

    | Feature | SVC | STATCOM | |----------------------|--------------------------------------------|-------------------------------------------| | Technology | Thyristor-switched/reactor (L-C) | VSC (GTO/IGBT) + DC capacitor | | Reactive Output | Decreases with decreasing voltage (∝ V²) | Constant over a wide voltage range | | Response Speed | Fast (1-2 cycles) | Very Fast (<1 cycle) | | Harmonic Generation | Generates more harmonics (needs filters) | Minimal harmonics (PWM switching) | | Size/Cost | Larger (large inductors/capacitors) | More compact for same rating | | Low Voltage Performance | Poor (VAR output ∝ V²) | Superior (constant current capability) |

  • Difference between Active Power Filter and STATCOM:

    • APF: Primarily designed for harmonic current compensation (and sometimes fundamental reactive power). Reference is harmonic current.

    • STATCOM: Primarily designed for dynamic reactive power/voltage support at fundamental frequency. Its main output is fundamental reactive current. It can be augmented with harmonic control to function as an APF, but that's a secondary function.

5.3 Unified Power Quality Conditioner (UPQC)

• Principle: Integration of a Series Active Power Filter (DVR) and a Shunt Active Power Filter (APF) connected back-to-back via a common DC bus capacitor.

  • Shunt APF: Compensates for load current harmonics, unbalance, and reactive power.

  • Series APF (DVR): Compensates for supply voltage sags, swells, harmonics, and unbalance.

• Working and Operation:

  • Power Flow: The shunt APF supplies the load's fundamental reactive power and harmonic currents, and also supplies the real power consumed by the series APF (which injects voltage). The DC bus capacitor provides the energy storage for real power exchange between the two converters.

  • Control: Two independent controllers. Shunt controller regulates DC bus voltage and generates compensating current. Series controller regulates the load-side voltage to be sinusoidal and balanced.

• Classification of UPQC (Topologies):

  • Back-to-Back: Standard configuration described above.

  • Interline UPQC (I-UPQC): Two series APFs and one shunt APF to compensate for two different feeders.

  • Multilevel UPQC: Uses multilevel converters for higher voltage applications.

• Advantages of UPQC:

  • Comprehensive solution for both voltage and current-related PQ problems.

  • Simultaneous mitigation of sags, swells, harmonics, flicker, and unbalance.

  • Improved power factor and load balancing.

• Power Quality Problems Mitigated by UPQC:

  • Voltage Sags & Swells.

  • Voltage Harmonics & Unbalance.

  • Current Harmonics & Reactive Power.

  • Voltage Flicker.

  • Supply interruptions (with energy storage).

• Differences between UPQC and DSTATCOM:

  • UPQC: Has both series and shunt converters. Mitigates voltage and current disturbances.

  • DSTATCOM: Is essentially a shunt-only STATCOM. Primarily provides dynamic reactive power/voltage support and current harmonic compensation. Cannot directly compensate for voltage sags/swells on the supply side (that's DVR's job).

6.0 POWER FACTOR CORRECTION

6.1 Power Factor: Definition, Importance

• Definition: Power Factor (PF) = $$\displaystyle \frac{Real Power (P)}{Apparent Power (S)} = \cos\phi $$ (for sinusoidal, balanced systems). It measures the effectiveness of power utilization.

• Importance:

  • Reduced I²R losses in conductors and transformers.

  • Better utilization of system capacity (KVA rating).

  • Improved voltage regulation.

  • Avoidance of utility penalties.

6.2 Methods of Power Factor Improvement

• Static Capacitors (Shunt/Series):

  • Shunt: Most common. Connected across load/feeder to supply leading VAR.

  • Series: Used for voltage regulation in long distribution lines or for specific loads.

• Synchronous Condensers: Over-excited synchronous motor running without mechanical load. Provides smooth, continuous, and adjustable VAR. Can also aid in system stability.

• Phase Advancers: AC exciter mounted on the same shaft as an induction motor to provide leading excitation to the motor's rotor, improving its PF.

• Active PFC Circuits: Used in switch-mode power supplies (SMPS). Boost converters with active control to draw sinusoidal current in phase with voltage (PF > 0.99). Common in computers, chargers.

6.3 Economic Considerations and Penalties

• Cost-Benefit Analysis: Savings from reduced losses and deferred capacity upgrades vs. cost of correction equipment.

• Penalties: Utilities impose kVA demand charges or PF penalty multipliers on bills if PF falls below a specified limit (e.g., 0.9 or 0.95 lagging). Penalties are based on reactive energy (kVARh) consumed.

7.0 PROTECTION SYSTEMS FOR POWER QUALITY

7.1 Voltage Sag Protectors / Ride-Through Schemes

• Need: Sensitive electronic equipment (computers, PLCs) can malfunction or shut down during sags even if magnitude is above their nominal rating.

• Types:

  • Electronic: UPS (Uninterruptible Power Supply) with battery backup. DVR (Dynamic Voltage Restorer) injects boosting voltage.

  • Magnetic: Ferroresonant transformers (constant voltage transformers) use core saturation to maintain secondary voltage during sags.

  • Hybrid: Combination of technologies.

7.2 Surge Protective Devices (SPDs)

• Operation Principle: Clamp the voltage to a safe level by switching to a low-impedance state when voltage exceeds a threshold (clamping voltage). Divert transient energy to ground.

• Types & Coordination: (See 2.6)

  • Type 1 (Direct): Lightning arresters, service entrance.

  • Type 2 (Main): Distribution boards, SVC/STATCOM DC bus.

  • Type 3 (Equipment): Point-of-use, receptacle-mounted.

• Coordination: Cascaded installation (Type 1 → Type 2 → Type 3) to share energy and achieve lower overall clamping voltage.

8.0 APPLICATIONS AND CASE STUDIES

8.1 Power Quality in Electric Vehicles (EVs)

• PQ Challenges in EV Charging Stations:

  • Large, non-linear loads (AC/DC converters) draw harmonic currents.

  • Fluctuating and unbalanced loads (multiple chargers).

  • Voltage sags due to sudden high inrush during charging start.

  • Impact on distribution grid (voltage drop, transformer loading).

• Mitigation Requirements:

  • Active Front-End (AFE) converters with PWM to minimize harmonics.

  • On-site reactive power compensation (STATCOM, capacitor banks).

  • Energy storage systems to buffer grid during sags and manage peak demand.

  • Careful planning of charging station location and capacity.

8.2 Power Quality in Photovoltaic (PV) Water Pumping Systems

• PQ Issues due to Inverters:

  • Harmonic injection from inverter switching (especially if not using sinusoidal PWM).

  • DC injection into AC grid if inverter control is poor.

  • Voltage fluctuations due to variable solar irradiance.

  • Low power factor if inverter operates at unity PF only.

• Mitigation Strategies:

  • Use high-frequency, high-switching-frequency inverters with filter inductors.

  • Implement active harmonic filtering or multi-pulse inverter topologies.

  • Inverter control to provide reactive power support (grid-supportive mode).

  • Proper grounding and shielding to reduce EMI.

8.3 Industrial Applications (Sensitive Manufacturing, Data Centers)

• Challenges: Process control failures, scrap production, downtime.

• Solutions: Dedicated UPS/DVR systems, on-site generation with power electronics, comprehensive harmonic filtering, isolated grounding systems.

8.4 Commercial Applications (Hospitals, Office Buildings)

• Challenges: Life-support equipment failure, data loss, lighting flicker.

• Solutions: Essential power systems (generators, UPS), centralized harmonic filters, proper wiring practices, SPD installation.

9.0 MEASUREMENT, MONITORING, AND STANDARDS

9.1 Power Quality Analyzers / Monitors

• Operation: Digitize voltage and current waveforms at high sampling rates (e.g., > 10 kHz). Use DSP/FPGA to compute parameters in real-time.

• Parameters Measured:

  • RMS voltage/current, frequency.

  • Sags, Swells, Interruptions (magnitude, duration, phase).

  • Harmonics (individual, THD, TDD up to 50th/100th).

  • Flicker (Pst, Plt).

  • Unbalance (voltage/current).

  • Transients (capture waveform).

  • Energy (kWh, kVARh).

9.2 Key Standards

• IEEE 519-2014: Harmonic control limits (voltage & current).

• IEC 61000-4-30: Testing and measurement techniques for PQ parameters (defines measurement methods, accuracy, reporting).

• IEC 61000-4-15: Flicker measurement.

• IEC 61000-4-7/11: Harmonic and interharmonic measurements.

9.3 PQ Monitoring Practices and Locations

• Point of Common Coupling (PCC): Mandatory for utility-customer interface to assess compliance with standards (IEEE 519).

• Critical Load Buses: Within a facility to identify sensitive equipment issues.

• Before/After Mitigation: To verify effectiveness of installed solutions.

• Continuous vs. Periodic Monitoring: Continuous for critical facilities; periodic for surveys.

[!TIP] Exam Focus: Be prepared to define all disturbance types (sag, swell, interruption, fluctuation, transient) with their IEEE/IEC standard duration/magnitude ranges. Know the key formulas for THD, TDD, Crest Factor. Differentiate clearly between SVC and STATCOM, UPQC and DSTATCOM, surge vs swell. Understand the principle of zero-voltage crossing switching and SPD coordination.