Power quality Problems and mitigation techniques (EX-802 (A)) - Unit 3 Short Notes

UNIT 3: POWER QUALITY PROBLEMS AND MITIGATION TECHNIQUES

I. INTRODUCTION TO POWER QUALITY

• Definition: Power Quality (PQ) refers to the characteristics of electricity at a given point on an electrical system, evaluated against a set of reference parameters such as voltage magnitude, frequency, and waveform purity.

• Reasons for Increased Concern:

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

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

  • Automation and process continuity requirements.

  • Economic losses due to downtime and equipment damage.

• Major PQ Issues: Voltage disturbances (sags, swells, interruptions, fluctuations, transients), harmonic distortion, reactive power issues, frequency variations, voltage unbalance.

• Primary Causes: Lightning, capacitor switching, fault clearing, motor starting, arc furnaces, rectifiers/inverters, large load switching.

• Key Standards: IEEE 519 (Harmonic Control), IEC 61000 series (Electromagnetic Compatibility).

II. VOLTAGE DISTURBANCES

Voltage Fluctuations and Flicker

• Definition: Repeated or random variations in voltage magnitude. Flicker is the perceptible visual effect of these fluctuations on lighting (dimming).

• Causes: Cyclic loads with rapidly varying current demand (e.g., arc furnaces, large HVAC compressors, sawmills).

• Mitigation: Static Var Compensators (SVCs) for fast reactive power support; dedicated supply lines; reducing source impedance.

Voltage Sags and Swells

• Definitions:

  • Sag: A short-duration reduction in RMS voltage to between 0.1 and 0.9 p.u. (duration 0.5 cycles to 1 min).

  • Swell: A short-duration increase in RMS voltage to between 1.1 and 1.8 p.u. (duration 0.5 cycles to 1 min).

• Causes:

  • Sags: System faults (remote or local), motor starting, inrush currents.

  • Swells: Fault clearing, sudden load rejection, single-phase faults in 3-phase systems.

• Key Difference: Sag = voltage drop; Swell = voltage rise.

• Performance Estimation: Site Index (probability of sag at a location), Equipment Tolerance (CBEMA/ITIC curve defines withstand capability).

• Mitigation:

  • Dynamic Voltage Restorer (DVR): Injects voltage in series to compensate.

  • Uninterruptible Power Supply (UPS): Provides backup during interruptions/sags.

  • Switching capacitor banks at voltage zero-crossing to avoid transients that can cause sags/swells.

Voltage Surges and Spikes

• Definitions:

  • Surge: A transient wave of voltage/current on a power line, typically > 1.2/50 μs duration.

  • Spike: A very fast, high-amplitude transient (nanoseconds).

• Causes: Lightning strikes, capacitor bank switching, load rejection, fault clearing.

• Key Difference: Surge is a broader transient category; spike is a specific, fast, high-frequency component.

• Protection: Surge Protective Devices (SPDs) (e.g., Metal Oxide Varistors - MOVs) shunt excess energy to ground.

Transient Overvoltages

• Sources: Switching transients (capacitor/inductor), fault clearing (TRV - Transient Recovery Voltage), lightning.

• Factors Affecting TRV: System grounding, fault location, source impedance, breaker characteristics.

• Mitigation: RC Snubbers across contacts, surge arresters, zero-voltage crossing switching for capacitors.

Voltage Interruptions

• Causes: Faults, equipment failure, human error, planned maintenance.

• Classification:

  • Momentary: < 5 sec (often cleared by reclosing).

  • Temporary: 5 sec to 1 min (manual restoration).

  • Sustained: > 1 min.

• Mitigation: Backup generators, UPS systems, microgrids with islanding capability.

III. HARMONIC DISTORTION

Fundamentals of Waveform Distortion

• Harmonics: Sinusoidal components with frequencies that are integer multiples ($$\displaystyle h \times f_1 $$, $$\displaystyle h=2,3,4... $$) of the fundamental frequency $$\displaystyle f_1 $$.

• Interharmonics: Components with frequencies that are not integer multiples of the fundamental.

• Subharmonics: Components with frequencies below the fundamental.

• Primary Source: Non-linear loads where current is not proportional to voltage (diodes, thyristors, saturated magnetics).

  • Industrial: Rectifiers (AC/DC drives), arc furnaces, cycloconverters.

  • Commercial: Computers, LED drivers, UPS, electronic ballasts.

Harmonic Indices and Standards

• Total Harmonic Distortion (THD):

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

where $X$ is voltage or current. \boxed{THD}

• Total Demand Distortion (TDD): THD referred to the rated fundamental current of the device, not the instantaneous fundamental. Used in IEEE 519.

• Crest Factor (CF): Ratio of peak value to RMS value of a waveform. High CF indicates significant high-frequency content.

• Individual Harmonic Distortion (IHD):

$$IHD_h = \frac{X_h}{X_1} \times 100\%$$

• Standards: IEEE 519-2014 sets limits on harmonic voltage distortion (bus) and current distortion (consumer). IEC 61000-4-7 for measurement.

Effects of Harmonics

• Equipment Heating: Additional losses in transformers ($$\displaystyle I^2R $$), motors (stator/rotor core losses), and conductors.

• Resonance & Capacitor Failure: Harmonic frequencies can excite system/capacitor bank resonance, causing overcurrent and failure.

• Misoperation: Protective relays (over/under-current), energy meters, and control systems can malfunction.

• Communication Interference: Induced voltages in telephone/communication lines.

Harmonic Mitigation Methods

• Passive Filters: Tuned LC circuits to shunt specific harmonic frequencies.

  • Tuned Filters: Single-tuned for dominant harmonic (e.g., 5th, 7th).

  • High-pass Filters: Broadband, damped, for high-order harmonics.

  • C-type Filters: For 2nd harmonic, with low losses at fundamental.

  • Limitation: Fixed tuning, can resonate with system impedance changes.

• Active Power Filters (APF): Inject equal-but-opposite harmonic currents. Shunt APF is most common.

• Phase Shifting Transformers: Use delta-wye or zigzag connections to cancel triplen (3rd, 9th...) harmonics.

• Detuning/De-rating Capacitors: Use series reactors to detune capacitor banks from harmonic frequencies.

• Best Method: Hybrid approach (e.g., passive filter for dominant harmonics + active filter for dynamic/variable loads). No single "best" for all; depends on load profile, system impedance, and cost.

IV. REACTIVE POWER AND POWER FACTOR

• Reactive Power Issues: Causes increased current flow, higher system losses ($$\displaystyle I^2R $$), voltage drops, and reduced capacity.

• Power Factor Correction (PFC) Benefits:

  1. Reduced losses in transformers/lines.

  2. Improved voltage profile.

  3. Increased system capacity (release kVA).

  4. Avoidance of utility reactive power compensation penalties (tariffs charge for low PF, often below 0.9 lag).

• Capacitor Bank Installation Locations:

  • Individual Load: At motor terminals (most effective, reduces current in entire upstream circuit).

  • Load Bus/Feeder: For groups of similar loads.

  • Substation/Main Bus: For system-wide correction, but less effective for local loss reduction.

• Shunt vs. Series Compensation: Shunt capacitors are used for voltage control/PFC because they inject leading current locally, directly countering lagging reactive current. Series capacitors are used for power transfer capability/stability.

• Switching Transient Mitigation: Zero-voltage crossing switching is optimal to prevent inrush current and transient overvoltages when energizing capacitor banks.

• PFC Techniques: Fixed capacitors, switched capacitor banks (contactors/thyristors), synchronous condensers.

V. POWER QUALITY MITIGATION DEVICES

Static Var Compensator (SVC)

• Principle: Fast, thyristor-based reactive power compensation. Combines Thyristor-Controlled Reactor (TCR) and Thyristor-Switched Capacitor (TSC) banks.

• Operation: TCR provides continuously variable inductive VARs; TSC provides stepwise capacitive VARs. Net output is continuously controllable.

• Advantages: Fast response (~ms), continuous control, good for flicker mitigation.

• Limitations: Generates harmonics (especially TCR), limited reactive power range at low voltages, requires large reactors/capacitors.

STATCOM (Static Synchronous Compensator)

• Principle: Uses a Voltage Source Converter (VSC) with a DC capacitor to generate a controllable AC voltage. Acts as a synchronous condenser but solid-state.

• Operation: By controlling the magnitude/phase of the output voltage ($$\displaystyle V_{STATCOM} $$) relative to system voltage ($$\displaystyle V_{sys} $$), it can generate or absorb reactive power ($$\displaystyle Q \propto (V_{STATCOM} - V_{sys}) $$).

• Comparison: SVC vs STATCOM

  | Feature | SVC | STATCOM | | :--- | :--- | :--- | | Response | Fast (ms) | Very Fast (sub-ms) | | Reactive Range | Limited at low V | Excellent at low V (linear to V²) | | Harmonics | Generates (needs filters) | Minimal (PWM switching) | | Footprint/Cost | Larger, cheaper | Smaller, more expensive | | Best For | Flicker, large industrial | Weak grids, dynamic support |

Active Power Filter (APF)

• Shunt Active Filter Principle: Acts as a current source. Detects load harmonic currents (using e.g., Instantaneous Power Theory $p-q$ or $d-q$), then injects equal-and-opposite harmonic currents from a VSC, so only fundamental current is drawn from the grid.

• Advantages: Excellent dynamic performance, filters multiple harmonics, can compensate for unbalance and PF.

• Disadvantages: High cost, complex control, limited power rating compared to passive filters.

• Comparison: APF vs STATCOM

  • STATCOM: Primary function is reactive power/voltage support. Can provide some harmonic mitigation if control is designed for it, but not its primary purpose.

  • APF: Primary function is harmonic/current distortion cancellation. Can provide some reactive support, but typically smaller rating for that purpose.

  • Key Difference: STATCOM controls voltage; APF controls current.

Unified Power Quality Conditioner (UPQC)

• Principle: Integration of a series active filter (SAF) and a shunt active filter (SAF) connected back-to-back via a common DC capacitor.

  • Shunt Part: Compensates load harmonics, reactive power, and unbalance (like an APF).

  • Series Part: Injects voltage to correct supply sags/swells, harmonics, and unbalance (like a DVR).

• Working: Shunt APF ensures load current is sinusoidal; Series APF ensures load voltage is sinusoidal and regulated.

• Classification:

  • By Converter Topology: Voltage Source (VSC) or Current Source (CSC) based.

  • By Control Strategy: Unified controller or separate controllers.

• Advantages: Comprehensive mitigation (sags, swells, harmonics, flicker, unbalance) in a single device; high flexibility.

• PQ Problems Mitigated: Voltage sags/swells, harmonics, voltage unbalance, reactive power, flicker.

• Comparison: UPQC vs DSTATCOM

  • DSTATCOM: Shunt-only VSC device. Provides reactive power support and some harmonic mitigation (like a STATCOM with harmonic control).

  • UPQC: Series + Shunt. Can both inject voltage (for sags/swells) and inject current (for harmonics). UPQC has series element; DSTATCOM does not.

DSTATCOM (Distribution STATCOM)

• Operation: VSC-based shunt compensator for distribution systems. Controls reactive power flow to regulate voltage at the point of common coupling (PCC).

• Applications: Voltage support in weak distribution feeders, industrial parks, renewable integration (PV/wind farms).

VI. PROTECTION AND TRANSIENT MITIGATION

• Surge Protection (SPDs): Connected line-to-ground (Class I/II) or line-to-line (Class III). MOVs are common; they clamp voltage by becoming conductive at a specified "clamping voltage."

• Voltage Sag Protectors: Use ride-through schemes for critical equipment. May use stored energy (capacitors) or switch to alternate source (static transfer switch).

• High Voltage Equipment Protection:

  • Insulation Coordination: Selecting BIL (Basic Impulse Level) based on expected overvoltages and safety margins.

  • SI (Switching Impulse) & LI (Lightning Impulse) withstand levels.

  • Safety: De-energize, ground, use appropriate PPE and procedures.

• Transient Mitigation Techniques:

  • Zero-Voltage Crossing Switching: For capacitor banks, ensures switching occurs when voltage is near zero, minimizing inrush current and transient overvoltage.

  • RC Snubbers: Resistor-Capacitor networks across switching contacts to damp transients.

  • MOVs: Clamp overvoltages to safe levels.

VII. POWER QUALITY MEASUREMENT AND ANALYSIS

• Harmonic Analyzers: Use FFT (Fast Fourier Transform) on sampled waveforms to decompose into harmonic spectrum. Must meet IEC 61000-4-7 class accuracy (Class 1 for precision, Class 2 for general).

• Power Quality Monitors: Continuously record parameters: RMS voltage/current, harmonics (up to 50th/100th), sags/swells (magnitude/duration), frequency, unbalance, flicker (Pst/Plt).

• Sag/Swell Estimation: Statistical methods (probability density functions), Monte Carlo simulation of faults, site-specific measurements.

• Audit Strategy: Identify sensitive loads, measure at PCC and critical points, log disturbances, correlate with system events.

VIII. APPLICATIONS AND CASE STUDIES

• Industrial PFC: Large induction motors (individual capacitor banks), rectifier loads (AC/DC drives - use 12-pulse converters + filters).

• Renewable Energy: PV inverters and wind turbine converters are non-linear sources; require grid-tie harmonic filters and sometimes STATCOM/UPQC for grid support.

• Electric Vehicle (EV) Charging: Significant harmonic current draw (especially 3rd, 5th, 7th) and potential for voltage drop in local distribution. Mitigation: On-board PFC, dedicated supply, active filters at charging station.

• PV Water Pumping: Inverter-based pump system. PQ issues: inverter harmonics, voltage fluctuations due to solar irradiance changes. Mitigation: Filtered inverter, proper cable sizing, sometimes small capacitor bank.

• Capacitor Bank Switching Transients: Caused by random switching. Mitigation: Zero-voltage crossing switching, pre-insertion resistors, controlled switching devices (TSC).

• Critical Facilities (Data Centers, Hospitals): Require UPQC or combination of DVR + APF to protect against sags (ITIC curve) and harmonics from large UPS/IT loads.

IX. COMPARATIVE ANALYSIS AND SELECTION CRITERIA

Selection Flow:

1. Identify dominant PQ problem (Harmonics? Sags? Low PF?).

2. Characterize load (fixed/variable, size, harmonic spectrum).

3. Assess system strength (short circuit ratio).

4. For Harmonics: Passive filter if dominant harmonic is fixed & large; APF/Hybrid if dynamic/variable.

5. For Sags/Swells: DVR or UPQC.

6. For Reactive Power/Flicker: SVC or STATCOM (STATCOM preferred for low voltage support).

7. For Comprehensive Solution (Critical Load): UPQC.

8. For Simple PF Correction: Switched capacitor banks (with zero-crossing switching).

Exam Tip: Questions often ask for "best method" or "difference between". Structure answers with a clear comparison table or bullet points. For mitigation, always link cause → effect → solution. Remember: UPQC = DVR + APF; STATCOM ≠ APF (voltage vs current control). Zero-voltage crossing is the gold standard for capacitor switching to avoid transients.