UNIT 4: Power Quality Problems and Mitigation Techniques

I. Introduction to Power Quality

Definition and Concept

Power Quality (PQ) is the concept of ensuring that the voltage, current, and frequency of an electrical supply system remain within specified limits, allowing connected equipment to operate as intended without degradation or malfunction.

[!TIP] Exam Focus: Definitions are often the first part of long answers. Always state the IEEE/IEC definition clearly.

Reasons for Increased Concern

• Sensitive Electronic Loads: Microprocessors, PLCs, medical equipment.

• Digital Equipment: Computers, communication systems require clean sine waves.

• Industrial Processes: Automation, precision manufacturing are sensitive to voltage variations.

• Economic Impact: Downtime, equipment damage, production losses.

Classification of Power Quality Disturbances

Causes and Sources

• Nonlinear Loads: Rectifiers (AC/DC drives), arc furnaces, switch-mode power supplies (SMPS), LED drivers.

• Switching Events: Capacitor bank switching, motor starting/stopping, load rejection, fault clearing.

• System Faults & Lightning: Short circuits, insulation breakdown, direct/indirect strikes.

• Renewable Energy Integration: Inverter-based interfaces (PV, wind) introducing harmonics and voltage fluctuations.

II. Voltage Disturbances

Voltage Fluctuations (Flicker)

• Definition: Systematic variations of voltage envelope or magnitude.

• Causes: Varying loads (arc furnaces, welders, large motor starting/stopping).

• Measurement:

  • Pst (Short-term Flicker Severity): Assessed over 10 minutes.

  • Plt (Long-term Flicker Severity): Assessed over 2 hours (cumulative of Pst).

• Impact: Visible flickering of incandescent lamps, nuisance to humans, possible malfunction of sensitive equipment.

Voltage Sags, Swells, and Interruptions

[!TIP] Key Difference: Voltage Surge is a transient overvoltage (microseconds to milliseconds), while Swell is a rms increase lasting longer (0.5 cycles to 1 min).

Mitigation of Voltage Sags

• Need: Sensitive loads (computers, ASICs, process controllers) can malfunction or trip on sags.

• Voltage Sag Protectors: Devices that detect sag and quickly connect a backup energy source (like a capacitor bank) to bridge the dip.

• Estimation of Sag Performance:

  • Site Indices: Frequency of sags at a specific location (Pssag).

  • System Indices: Expected sag magnitude/duration at a bus due to faults elsewhere (using fault-position method, voltage drop calculations).

• Mitigation Devices:

  • UPS (Uninterruptible Power Supply): Provides complete isolation; battery-backed.

  • DVR (Dynamic Voltage Restorer): Voltage-source converter based; injects voltage to correct sag/swell.

  • Voltage Regulators: Tap-changing transformers (slow), static regulators (faster).

III. Transients and Overvoltages

Causes of Transient Overvoltages

• Capacitor Switching: Inrush currents cause oscillatory transients.

• Lightning: Direct strike or induced surges on lines.

• Fault Clearing: Interruption of fault current causes TRV.

• Load Rejection: Sudden loss of large load (e.g., generator tripping).

Transient Recovery Voltage (TRV)

• Definition: Voltage that appears across circuit breaker contacts immediately after current interruption.

• Factors Affecting TRV:

  • System voltage level and grounding.

  • Source and load impedances.

  • Nature of fault (single-phase, three-phase).

  • Cable vs. overhead line characteristics (cable → higher TRV rates).

Protection Against Transients

• Surge Protectors:

  • Gas Discharge Tubes (GDT): Spark over at a defined voltage; high surge capacity.

  • Metal Oxide Varistors (MOV): Voltage-dependent resistor; clamps overvoltage by increasing conductance.

• Impulse Voltage Withstand (BIL - Basic Impulse Level): Maximum voltage equipment can withstand without breakdown. Tested with standardized impulse waves (1.2/50 µs).

• Zero-Voltage Crossing Switching: Optimal for capacitor banks. Switching at voltage zero minimizes inrush current and transient overvoltage because the capacitor voltage is initially zero, eliminating the sudden voltage step.

[!TIP] Why Zero-Crossing? At voltage zero, the capacitor is uncharged. Closing the switch connects it to a source with near-zero voltage difference → minimal transient.

IV. Harmonics and Waveform Distortion

Fundamentals of Waveform Distortion

• Harmonics: Sinusoidal components with frequencies that are integer multiples of the fundamental frequency ($$\displaystyle f_1 $$). Order $$\displaystyle h = n \cdot f_1 $$.

• Interharmonics: Components with frequencies not integer multiples of fundamental (e.g., 150 Hz in a 50 Hz system).

• Crest Factor (CF): Ratio of peak value to RMS value of a waveform.

$$CF = \frac{|V_{peak}|}{V_{rms}}$$

• Harmonic Indices:

  • THD (Total Harmonic Distortion): For voltage or current.

$$THD_V = \frac{\sqrt{\sum_{h=2}^{\infty} V_h^2}}{V_1} \times 100\%$$

$$THD_I = \frac{\sqrt{\sum_{h=2}^{\infty} I_h^2}}{I_1} \times 100\%$$

• TDD (Total Demand Distortion): Similar to THD but denominator is rated fundamental current ($$\displaystyle I_{1,rated} $$), used for current distortion limits in standards (IEEE 519).

$$TDD = \frac{\sqrt{\sum_{h=2}^{\infty} I_h^2}}{I_{1,rated}} \times 100\%$$

Sources of Harmonics

Effects of Harmonics

• Increased Losses: $$\displaystyle I^2R $$ losses in conductors, transformers, motors (due to RMS current increase).

• Heating in Machines: Core losses in transformers/motors increase with frequency.

• Resonance: Parallel/series resonance between system inductance and capacitor banks → amplification of specific harmonics.

• Misoperation: Protective relays (overcurrent, distance), meters, communication interference.

• Nuisance Tripping: Circuit breakers and fuses may trip due to harmonic-induced overcurrents.

Mitigation of Harmonics

1. Passive Filters

• Shunt Passive Filters: Tuned LC circuits (series resonant) connected in parallel to provide low impedance path for specific harmonic frequencies (e.g., 5th, 7th).

  • Tuned Filters: Single or multiple tuned.

  • C-Type Filters: Modified high-pass filter with better damping, lower losses at fundamental.

• Passive Series Compensation: Series-connected LC filters (trap filters) to block harmonic currents from entering sensitive loads or the source.

2. Active Power Filters (APF)

• Shunt Active Filter (SAF) Basic Principle:

  • Detects load current harmonics using current transformers and signal processing.

  • A Voltage Source Converter (VSC) with DC capacitor injects a compensating current ($$\displaystyle i_c $$) that is equal and opposite to the harmonic current drawn by the load.

  • Result: Source current ($$\displaystyle i_s $$) becomes sinusoidal ($$\displaystyle i_s = i_{load} + i_c $$).

• Advantages of Active Harmonic Filters:

  • Dynamic, fast response (<1 cycle).

  • Can compensate for multiple harmonics simultaneously.

  • Can also compensate for reactive power and unbalance.

  • No resonance risk with system.

• Disadvantages:

  • High initial cost.

  • Complex control circuitry.

  • Limited power rating compared to passive filters.

  • Requires maintenance (power electronics).

3. Hybrid Filters

• Combination: Passive filter (handles major harmonics, provides fundamental reactive power) + Active filter (improves filtering performance, dampens resonance, handles dynamic loads).

• Types: Shunt passive + shunt active; series active + shunt passive.

4. Best Methods to Eliminate Harmonics

• Source Control: Use 12-pulse or higher-pulse rectifiers, active front ends.

• Passive Filtering: For large, fixed industrial loads (cost-effective for specific harmonics).

• Active Filtering: For dynamic loads, critical facilities, where resonance is a concern.

• Hybrid Approach: Most effective for complex systems (e.g., steel plants, large commercial buildings).

[!TIP] Exam Comparison: Be ready to contrast Passive vs. Active filters in a table format.

V. Reactive Power and Power Factor

Problems Due to Reactive Power

• Increased Current: For same real power, low PF → higher RMS current.

• Increased System Losses: $$\displaystyle I^2R $$ losses in lines, transformers increase.

• Voltage Drop: Reactive current causes additional voltage drop ($I \cdot X$).

• Reduced Capacity: Transformers and cables rated for current; higher reactive current reduces capacity for real power.

• Penalty: Utilities charge for low PF (usually below 0.9 lagging).

Power Factor Correction

Benefits

• Loss reduction in distribution system.

• Release of capacity in transformers/cables.

• Improved voltage regulation.

• Avoidance of utility penalties.

• Reduced demand charges (if based on kVA).

Installation of Capacitor Banks

• Individual vs. Bank: Individual → precise correction; Bank → economical for multiple small loads.

• Fixed vs. Switched: Fixed for constant loads; Switched (contactors) for varying loads to avoid overvoltage at light load.

Reactive Power Compensation Penalty

• Reason: Utilities must supply both real and reactive power. Low PF from customers increases system current, requiring larger infrastructure (transformers, lines) and higher losses. Penalty incentivizes customers to correct PF locally.

Techniques

• Capacitors: Most common (shunt).

• Synchronous Condensers: Rotating machines (synchronous motors) operated at leading PF to generate reactive power. Provide inertia, but have losses and maintenance.

• STATCOM: Power electronic-based (VSC); faster, better dynamic response, continuous control, no harmonic generation (if properly designed).

VI. Custom Power Devices and FACTS

Flexible AC Transmission Systems (FACTS)

• What is "Flexible"? Refers to controllability and fast response (milliseconds) of power electronic devices to control AC transmission system parameters (voltage, impedance, phase angle) for increased power transfer, stability, and reliability.

• Why Shunt Capacitance in Transmission Lines? Shunt capacitors (or reactors) provide voltage support and reactive power compensation. They control voltage magnitude along the line, reduce losses, and increase power transfer capability by managing the reactive power balance.

Comparison of FACTS Devices

SVC vs. STATCOM

Active Power Filter vs. STATCOM

DSTATCOM (Distribution Static Compensator)

• Operation & Working Principle:

  • A VSC-based device connected in shunt at distribution level.

  • DC capacitor provides DC link voltage.

  • VSC generates a AC output voltage ($$\displaystyle V_{DSC} $$) slightly different from the PCC voltage ($$\displaystyle V_{PCC} $$).

  • The difference drives a current ($$\displaystyle I_{DSC} $$) that can be capacitive (leading) or inductive (lagging) to control voltage at PCC.

  • Reactive Power: $$\displaystyle Q_{DSC} \approx \frac{V_{PCC} \cdot (V_{DSC} - V_{PCC})}{X} $$ (for small angle difference). Controls $$\displaystyle V_{DSC} $$ to regulate $Q$.

• Advantages over SVC:

  • Faster response (<1 cycle).

  • Better performance at low voltages (can inject rated current even at 0.2 p.u. voltage).

  • Smaller footprint (no large reactors/capacitors).

  • No resonance with system (if properly controlled).

  • Can also provide small active power support (if DC source present).

Unified Power Quality Conditioner (UPQC)

• Principle of UPQC: A series-shunt combination of two VSCs sharing a common DC capacitor. Series VSC acts as a voltage source to inject voltage, Shunt VSC acts as a current source to inject current.

• Working and Operation:

  • Series Part: Injects a voltage ($$\displaystyle V_{inj} $$) to compensate for voltage sags, swells, unbalance, and harmonics on the supply side. Ensures load voltage ($$\displaystyle V_L $$) is sinusoidal and at desired magnitude.

  • Shunt Part: Compensates for load current harmonics, reactive power, and unbalance. Ensures source current ($$\displaystyle I_S $$) is sinusoidal and in phase with supply voltage.

  • Power Flow: Real power exchange between series and shunt converters via DC link. Small external source may supply DC losses.

• Classification of UPQC:

  • UPQC-Q (UPQC-Q): Series VSC controls voltage; shunt VSC controls reactive power.

  • UPQC-S (UPQC-S): Both VSCs share reactive power compensation.

  • UPQC-P (UPQC-P): Series VSC provides voltage regulation; shunt VSC provides active power filtering.

  • UPQC-SP, UPQC-QP: Combinations based on control strategies.

• Advantages of UPQC:

  • Comprehensive mitigation: sags, swells, harmonics, flicker, unbalance, reactive power.

  • Improves both voltage and current quality simultaneously.

  • Can also provide active power transfer during interruptions (with backup).

• Differences Between UPQC and DSTATCOM:

  | Aspect | UPQC | DSTATCOM | |---------------------|-------------------------------------------|---------------------------------------| | Configuration | Series VSC + Shunt VSC (dual converters) | Single Shunt VSC | | Primary Role | Both voltage & current compensation | Primarily voltage support (reactive) | | Compensates | Sags, Swells, Harmonics, Unbalance | Voltage fluctuations, flicker | | DC Link | Shared between both converters | Single VSC's DC capacitor |

• Power Quality Problems Mitigated by UPQC:

  1. Voltage Sags & Swells (series injection).

  2. Voltage Harmonics & Unbalance (series).

  3. Current Harmonics (shunt injection).

  4. Reactive Power & Current Unbalance (shunt).

  5. Voltage Flicker (dynamic response).

  6. Provides some isolation from supply disturbances.

VII. Measurement, Monitoring, and Standards

Power Quality Monitoring Tools

• Power Quality Analyzers / Recorders: Measure voltage/current waveforms, calculate indices (THD, TDD, flicker), detect transients, and log events.

• Harmonic Analyzers: Specifically measure harmonic spectrum (up to 50th/100th order) and indices (THD, TDD). Use FFT algorithms on sampled waveforms.

• Flicker Meters: Measure Pst and Plt according to IEC 61000-4-15.

• Transient Recorders: High-speed sampling (MHz) to capture impulsive transients.

Estimation of Voltage Sag Performance

• Site Indices: Measured at a specific bus. Count of sags below a given magnitude threshold (e.g., $$\displaystyle P_{ssag}(V_{min}) $$).

• System Indices: Predicted performance for a new customer location. Uses:

  • Fault-Position Method: Assume faults at all critical points on the system, calculate resulting sag magnitude at the point of interest using voltage divider rule.

  • Statistical Methods: Combine fault rates with system impedance model.

Relevant Standards

• IEEE 519-2014: IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. Sets limits for harmonic voltage and current distortion at PCC.

• IEC 61000-4 Series: Electromagnetic Compatibility (EMC) - Testing and measurement techniques.

  • IEC 61000-4-4: Electrical fast transient/burst immunity test.

  • IEC 61000-4-5: Surge immunity test.

  • IEC 61000-4-11: Voltage dips, short interruptions, and voltage variations immunity tests.

  • IEC 61000-4-15: Flicker meter - Functional and design specifications.

• IEC 61000-2-2: Compatibility levels for low-frequency conducted disturbances in public power supply systems.