Power quality Problems and mitigation techniques (EX-802 (A)) - Unit 5 Short Notes

UNIT 5: Power Quality Problems and Mitigation Techniques

1. Fundamentals and Classification of Power Quality

Power Quality (PQ) is the concept of maintaining the voltage, current, and frequency of the electrical supply within specified limits to ensure that customer equipment operates as intended without degradation or malfunction.

Reasons for Increased Concern in Power Quality (May 2023):

• Proliferation of sensitive electronic loads (computers, PLCs, medical equipment).

• Increased use of non-linear loads (power electronics, variable frequency drives).

• Economic impact of downtime, equipment damage, and production losses.

• Stricter standards (IEEE 519, IEC 61000) and utility penalties for poor PQ.

• Competitive electricity markets requiring higher reliability.

Major Power Quality Issues (May 2023):

1. Voltage Disturbances: Sags, Swells, Interruptions, Fluctuations.

2. Waveform Distortions: Harmonics, Interharmonics, Notching.

3. Transients/Overvoltages: Impulses, Spikes, Switching transients.

4. Frequency Variations.

5. Voltage Imbalance.

Causes of Power Quality Disturbances (May 2023):

• Natural: Lightning, weather.

• System Operations: Capacitor switching, fault clearing, load switching.

• Load Characteristics: Non-linear loads (rectifiers, arc furnaces), rapidly varying loads (welding, motors starting).

• External: Electromagnetic interference (EMI).

2. Voltage Disturbances: Sags, Swells, Fluctuations, and Interruptions

Voltage Sag: A decrease in RMS voltage to between 0.1 and 0.9 pu for a duration of 0.5 cycles to 1 minute. Voltage Swell: An increase in RMS voltage to between 1.1 and 1.8 pu for a duration of 0.5 cycles to 1 minute.

Key Difference (Jun 2025, May 2023):

• Sag: Magnitude ↓, Duration short.

• Swell: Magnitude ↑, Duration short.

Voltage Fluctuation (Jun 2025):

• Cause: Rapid, repetitive changes in load current (e.g., arc furnaces, motor cycling, welding).

• Effect: Light dimming ("flicker") in incandescent lamps. Quantified by P_st (short-term flicker) and P_lt (long-term flicker).

Voltage Interruption: RMS voltage drops to < 0.1 pu for a period up to 1 minute. Momentary Outage is a short interruption (< few seconds).

Sources of Sags and Interruptions (May 2023):

• Remote faults on the transmission or distribution system (most common).

• Local faults (equipment failure, insulation breakdown).

• Large motor starting (causes voltage drop, not typically a sag).

• Lightning strikes.

Mitigation of Voltage Sag (May 2023):

• Dynamic Voltage Restorer (DVR).

• Static Synchronous Compensator (STATCOM).

• Uninterruptible Power Supply (UPS) for critical loads.

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

Estimation of Voltage Sag Performance (May 2023):

• Voltage Sag Calculator based on fault positions and system impedance.

• Statistical methods: Using sag density and sag magnitude distribution.

• Monte Carlo simulation of faults and network configurations.

Voltage Sag Protectors & Protection Schemes (Jun 2025):

• Sag protector / Mitigator: Devices like DVR that inject voltage to compensate for the sag.

• Protection Scheme: Under-voltage relays to disconnect vulnerable loads during deep/long sags. Coordination with ride-through capabilities of modern equipment is essential.

3. Harmonic Distortions

Sources of Harmonics (May 2023):

• Industrial Loads: Arc furnaces, welders, rolling mills, induction furnaces, rectifiers in HVDC, electrolysis.

• Commercial Loads: UPS systems, computers, FAX/printers, fluorescent lighting (magnetic ballasts), AC/DC drives in HVAC.

• Non-linear Loads Causing PQ Problems (May 2023): Any load where current is not proportional to voltage (I ≠ kV). Examples: power electronic converters (diodes, thyristors, IGBTs), saturated transformers, ferromagnetic cores.

Harmonic Indices and Definitions (May 2023):

• Harmonic: Frequency component integer multiple of fundamental (50/60 Hz). Order h.

• Individual Harmonic Distortion (IHD): $$\displaystyle \text{IHD}_h = \frac{V_h \text{ or } I_h}{V_1 \text{ or } I_1} \times 100\% $$

• Total Harmonic Distortion (THD):

$$ \text{THD} = \frac{\sqrt{\sum_{h=2}^{\infty} (X_h)^2}}{X_1} \times 100\% $$

where X is V or I.

• Total Demand Distortion (TDD):

$$ \text{TDD} = \frac{\sqrt{\sum_{h=2}^{\infty} (I_h)^2}}{I_{L}} \times 100\% $$

where $$\displaystyle I_L $$ is demand current (maximum average over a billing period). TDD is used for current distortion limits (IEEE 519).

• Crest Factor (CF):

$$ \text{CF} = \frac{\text{Peak Value}}{\text{RMS Value}} $$

High CF (>1.5) indicates significant harmonic content.

• Interharmonics: Frequency components not integer multiples of fundamental. Caused by static frequency converters, cycloconverters, arc furnaces, PWM inverters with non-integer carrier ratios.

Fundamentals of Waveform Distortion (May 2023):

• Periodic non-sinusoidal waveforms can be decomposed into DC component, fundamental, and harmonic components via Fourier Series.

• Distortion causes additional losses (in conductors, transformers, motors), resonances, misoperation of protective relays and meters, capacitor failure, and EMI.

Harmonic Mitigation Methods:

• Passive Filters (May 2023): Tuned LC circuits (shunt, series, or C-type) to absorb specific harmonic frequencies. Advantages: Simple, low cost. Disadvantages: Fixed tuning, can cause resonance with system impedance, performance degrades with system changes.

• Active Harmonic Filters (Jun 2025, May 2023): Power electronic converters (VSCs) that inject equal and opposite harmonic currents to cancel distortion. Shunt-active filter is most common.

Shunt-Active Filter: Principle, Operation, Advantages, Disadvantages (Jun 2025):

• Principle: Detect load harmonic currents ($$\displaystyle I_{Lh} $$) using harmonic extraction (e.g., instantaneous p-q theory, FFT). Control a VSC to inject $$\displaystyle I_{Fh} = -I_{Lh} $$ into the system, making source current sinusoidal.

• Operation: Connected in parallel with non-linear load. Uses current control loop to generate reference compensating current.

• Advantages: Dynamic compensation, tunable for multiple harmonics, no resonance risk, can also compensate for reactive power and unbalance.

• Disadvantages: Higher cost, complex control, power losses in converter, limited to low-medium power applications (though scaling is possible).

4. Transients and Overvoltages

Causes of Transient Voltages (May 2023):

• External: Lightning (direct strike or induced).

• Internal:

  • Capacitor switching (most common in distribution).

  • Inductive load switching (motors, transformers).

  • Faults and fault clearing (TRV - Transient Recovery Voltage).

  • Load rejection (e.g., loss of large load).

  • Back-fed voltages from motors/inductors.

Factors Affecting Transient Recovery Voltage (TRV) (Jun 2025):

TRV is the voltage across a circuit breaker immediately after current interruption.

• System voltage level and grounding.

• Fault type (3-phase, line-to-ground).

• Network configuration (radial, meshed).

• Presence of capacitors or reactors near the fault.

• Breaker location relative to the fault.

Power Spikes and Surge Protection (Jun 2025):

• Power Spike/Surge: A sub-cycle, high-magnitude transient (µs to ms) caused by lightning or major switching.

• Surge Protection: Surge Protective Devices (SPDs) installed at service entrance, sub-panels, or equipment level.

  • Type 1: For direct lightning (installed at service entrance).

  • Type 2: For indirect surges (main distribution).

  • Type 3: For equipment protection (close to load).

  • Mechanism: Metal Oxide Varistors (MOVs) clamp voltage by becoming conductive at a specified clamping voltage.

Mitigation of Harmful Transients (Jun 2025):

• For capacitor switching: Close capacitor breakers at or near voltage zero-crossing to minimize inrush current and transient overvoltage. Use pre-insertion inductors or controlled switching.

• General: SPDs, RC snubbers across switch contacts, surge capacitors, proper grounding.

Methods to Protect/Withstand Impulse Voltages for HV Equipment (Jun 2025):

• Insulation Coordination: Design equipment insulation with Basic Impulse Level (BIL) higher than expected impulse voltages.

• Lightning Arresters / Surge Arresters: Installed on all incoming lines and critical equipment. Metal-Oxide Arresters (MOA) are standard.

• Shielding: Faraday cages for sensitive equipment, shielded cables.

• Proper Grounding: Low-impedance ground grid to divert surge currents safely.

5. Reactive Power and Power Factor Correction

Power Quality Problems Due to Reactive Power Component (May 2023):

• Increased current magnitude for same real power → higher I²R losses, larger conductor size.

• Voltage drop along lines → poor voltage regulation.

• Reduced system capacity (transformers, cables carry reactive current).

• Poor power factor leads to utility penalties.

• Voltage instability in severe cases.

Benefits of Power Factor Correction Services (Jun 2025):

• Reduced system losses → energy savings.

• Improved voltage profile → better regulation.

• Increased capacity of existing infrastructure (transformers, cables).

• Avoidance of utility PF penalties.

• Reduced demand charges (if based on kVA).

Installation Locations for Power Factor Correction Capacitor Banks (Jun 2025):

1. Individual Load Compensation: Capacitors directly at load terminals (e.g., motor terminal). Best for large, constant loads.

2. Group Compensation: Capacitors for a group of similar loads (e.g., in a panel).

3. Central/System Compensation: At main distribution bus or substation. Easiest to control but doesn't reduce distribution losses as much.

4. Automatic Switching: Capacitor banks with contactors/thyristors switched by PF relay to follow load variation.

Reactive Power Compensation Penalty (Jun 2025):

• Utilities often charge a penalty if PF < 0.9 (lagging) or sometimes if PF > 0.95 (leading).

• Penalty is based on kVA demand instead of kW demand, increasing the bill.

• Purpose: To discourage customers from drawing excessive reactive power, which burdens the utility's system.

6. Flexible AC Transmission Systems (FACTS) for Power Quality

Meaning of "Flexible" in FACTS (Jun 2025):

• Refers to the ability to rapidly and continuously control AC transmission system parameters (voltage, impedance, phase angle) using power electronics.

• Enables dynamic response (milliseconds) to system disturbances, improving stability, capacity, and power flow control.

Role of Shunt Capacitance in Transmission Lines (Jun 2025):

• Primary role: Provide leading VARs to support voltage along the line, especially under light load.

• Compensates for the inductive reactance of the line.

• Improves voltage profile and increases power transfer capability by reducing net series reactance.

• In FACTS, shunt capacitors are part of devices like SVC (TCR+TSC) and STATCOM for dynamic VAR support.

Comparison: SVC vs STATCOM (Jun 2025):

Difference Between Active Power Filter and STATCOM (Jun 2025):

• Active Power Filter (APF): Primary goal is to eliminate harmonics and/or compensate unbalance/reactive power from specific non-linear loads. Designed for current injection.

• STATCOM: Primary goal is dynamic voltage support and reactive power compensation at a bus (like a synchronous condenser). It stabilizes voltage. While a STATCOM can filter harmonics if controlled to do so, its main control objective is voltage regulation, not harmonic elimination. An APF's main control is harmonic current cancellation.

7. Custom Power Devices: DSTATCOM and UPQC

DSTATCOM (Distribution STATCOM):

• Operation & Working Principle: A VSC-based shunt device connected in parallel with the distribution system. It injects/absorbs reactive current to regulate the voltage magnitude at its connection point. By controlling the magnitude and phase of its output voltage ($$\displaystyle V_{DSTATCOM} $$) relative to the system voltage ($$\displaystyle V_{sys} $$), it can generate or absorb both active and reactive power from the DC bus.

• Sketch Description:

  DiagramA single-line diagram showing a DSTATCOM (box with VSC and DC capacitor) connected to a PCC (Point of Common Coupling) via a coupling transformer. It shows the DSTATCOM injecting current $$\displaystyle I_{inj} $$ to compensate for load current $$\displaystyle I_L $$, making source current $$\displaystyle I_s $$ sinusoidal and in-phase with voltage.

• Advantages: Fast response, dynamic voltage support, can also mitigate flicker and unbalance, smaller footprint than capacitor banks.

• Disadvantages: Higher cost, requires DC source (capacitor), limited real power capacity (unless paired with energy storage).

Unified Power Quality Conditioner (UPQC) (Jun 2025, May 2023):

• Principle, Working, and Operation: A series-shunt combination of two VSCs (or one split DC bus) that simultaneously mitigates voltage and current-related PQ problems.

  1. Series Part (DVR-like): Injects a voltage in series to compensate for voltage sags, swells, unbalance, harmonics, and flicker.

  2. Shunt Part (DSTATCOM-like): Injects a current in parallel to compensate for load current harmonics, unbalance, and reactive power.

  • Common DC Bus: Both converters share a DC capacitor. Real power exchange between series and shunt parts balances the DC bus.

• Classification of UPQC (May 2023):

  • UPQC-Q (Shunt-Shunt): Both parts inject current (less common).

  • UPQC-P (Shunt-Series): Shunt injects current, series injects voltage (most common).

  • UPQC-S (Series-Shunt): Series injects voltage, shunt injects current (back-up power flow).

  • UPQC-Q (Series-Series): Both inject voltage (for dual series compensation).

• Advantages of UPQC (May 2023):

  • Comprehensive solution: Mitigates both voltage and current disturbances.

  • Improves voltage profile and source current quality simultaneously.

  • Can provide load voltage regulation and power factor correction.

  • Protects sensitive loads from upstream disturbances.

• Comparison: UPQC vs DSTATCOM (May 2023):

  | Feature | DSTATCOM | UPQC | | :--- | :--- | :--- | | Configuration | Shunt-only | Series + Shunt | | Primary Function | Voltage support (reactive power) at PCC | Both voltage support and current harmonic mitigation | | Compensates | Voltage sags (partially), flicker, unbalance | Voltage sags/swells, harmonics, unbalance, flicker (voltage & current) | | Cost/Complexity | Lower | Higher | | DC Power | Needs external source or limited | Self-sustaining via power exchange between converters |

• Power Quality Problems Mitigated by UPQC (May 2023):

  • Voltage Sags & Swells

  • Voltage Harmonics & Unbalance

  • Current Harmonics & Reactive Power

  • Voltage Flicker

  • Voltage Interruptions (with backup energy storage)

8. Passive Compensation Techniques

Classification and Working of Passive Series Compensation (May 2023):

• Series Capacitor Compensation: A capacitor bank connected in series with a transmission line.

  • Working: The capacitive reactance ($$\displaystyle X_C $$) cancels part of the line inductive reactance ($$\displaystyle X_L $$), reducing net series reactance.

  • Effect: Increases power transfer capability ($$\displaystyle P \propto \frac{V_s V_r}{X} \sin \delta $$), improves stability (by reducing power angle $\delta$ for same P), and controls load flow.

  • Risk: Can cause sub-synchronous resonance (SSR) with turbine-generator shaft. Requires protection (series breaker, damping filter).

• SSSC (Static Synchronous Series Compensator): A VSC-based series device (FACTS) that injects a controlled voltage in series. More flexible than fixed capacitor, can also damp oscillations.

Passive Shunt Compensation (Capacitor Banks):

• Working: Capacitor banks connected in parallel (shunt) to supply leading reactive power (VARs) locally.

• Purpose: Correct power factor, support voltage, reduce line losses.

• Types: Fixed or switched ( mechanically or thyristor-switched).

• Limitation: Can cause resonance with system inductance at harmonic frequencies. Requires detuning (adding reactors) or harmonic filtering.

9. System Analysis and Monitoring

Operation of Harmonic Analyzer (May 2023):

• A digital instrument or software-based tool that measures and quantifies harmonic distortion in voltage and current waveforms.

• Basic Operation:

  1. Signal Conditioning: Voltage/current transducers (CTs, PTs) scale signals.

  2. A/D Conversion: Samples waveform at high rate (e.g., >10 samples/cycle).

  3. Digital Signal Processing (DSP): Applies Fast Fourier Transform (FFT) to decompose waveform into harmonic frequency components (fundamental, 2nd, 3rd, ...).

  4. Calculation: Computes THD, TDD, individual harmonic magnitudes/phases, crest factor, flicker indices.

  5. Display/Recording: Shows numerical values, bar charts of harmonic spectrum, and trends. Often compliant with IEC 61000-4-7 (measurement standards).

Measurement and Quantification of Power Quality Indices:

• Voltage Sags/Swells: Measure RMS voltage over a sliding window (e.g., 1/2 cycle). Record magnitude, duration, phase-angle jump.

• Interruptions: Detect when RMS voltage < 0.1 pu.

• Harmonics: Use FFT-based analyzers to compute THDv, THDi, TDD as per IEEE 519/IEC 61000-4-7.

• Flicker: Measure P_st (short-term) and P_lt (long-term) using standardized weighting filters (per IEC 61000-4-15).

• Unbalance: Calculate Voltage Unbalance Factor (VUF) or Negative Sequence Voltage.

• Transients: High-speed digitizers (MHz sampling) capture fast impulses; measure peak amplitude, rise time, duration.

10. Integrated Solutions and System-Level Considerations

Coordination of Multiple Mitigation Devices:

• Hierarchical Approach: Install local devices (APFs, DVRs) for critical loads, bus-level devices (STATCOM, UPQC) for area correction, and system-level (FACTS) for bulk power flow.

• Avoid Conflict: Ensure devices do not counteract each other (e.g., shunt capacitor switching conflicting with DSTATCOM VAR output).

• Control Coordination: May require master controller to set references for multiple devices based on system conditions.

• Location Optimization: Place devices where they have maximum impact (e.g., UPQC at PCC of sensitive plant, STATCOM at weak bus).

Economic Aspects and Penalty Avoidance:

• Cost-Benefit Analysis: Compare investment & O&M cost of mitigation device vs. avoided losses and penalties.

• Penalty Avoidance: Correcting PF (avoid kVA demand penalty), reducing harmonics (avoid IEEE 519 violation fines), improving voltage sag performance (avoid process downtime costs).

• Increased Revenue: Ability to serve more load with existing infrastructure (deferred upgrades).

Application-Specific Solutions:

• Sensitive Industrial Processes (semiconductor, pharma): UPQC or DVR + APF for sag and harmonic protection.

• Arc Furnaces: Active filters for flicker/harmonics, series reactors for current limitation.

• Data Centers: Multiple levels of UPS with harmonic mitigation, generator sets for backup.

• Renewable Integration (PV/Wind): STATCOM/DSTATCOM for voltage support and low-voltage ride-through (LVRT).