Power quality Problems and mitigation techniques (EX-802 (A)) - Unit 2 Short Notes

UNIT 2: POWER QUALITY PROBLEMS AND MITIGATION TECHNIQUES

A. FUNDAMENTALS OF POWER QUALITY

• Definition & Concept: Power Quality (PQ) refers to the characteristics of voltage and current at a given point on a power system, measured against established standards. It encompasses any deviation from nominal voltage (magnitude, frequency, waveform) that may cause malfunction or failure in customer equipment.

• Reasons for Increased Concern:

  • Economic Impact: Downtime in industries, data corruption, reduced equipment lifespan.

  • Sensitive Loads: Proliferation of electronics (computers, PLCs, medical equipment) with low tolerance to disturbances.

• Major PQ Issues (Classification Overview):

  • Voltage Disturbances: Sags, swells, interruptions, fluctuations, transients.

  • Waveform Distortion: Harmonics, interharmonics, notching.

  • Voltage Unbalance: Phase magnitude/angle deviation.

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

• Causes of PQ Disturbances:

  • Utilities: Faults, switching operations, capacitor bank switching, load dispatch errors.

  • Customers: Large motor starting, non-linear loads (rectifiers, arc furnaces), faulty equipment.

  • Environment: Lightning strikes, weather-related faults, animal contact.

[!TIP] Exam Focus: Be prepared to list and briefly explain each major PQ issue category and attribute a typical cause to each.

B. VOLTAGE DISTURBANCES

Voltage Fluctuations & Flicker

• Voltage Fluctuation: A series of voltage changes or a cyclic variation of the voltage envelope.

• Light Flicker Mechanism: Rapid, repetitive voltage fluctuations cause the light output of incandescent lamps to vary perceptibly (dimming). The human eye integrates these variations, and if the frequency and magnitude are within certain ranges, the flicker becomes annoying.

• Causes: Cyclic loads with significant power demand variations (e.g., arc furnaces, large refrigeration compressors, elevators, welding machines).

Voltage Sags, Swells, and Interruptions

• Definitions:

  • Voltage Sag: A short-duration reduction in RMS voltage to between 0.1 and 0.9 pu, lasting from 0.5 cycles to 1 minute.

  • Voltage Swell: A short-duration increase in RMS voltage to above 1.1 pu, lasting from 0.5 cycles to 1 minute.

  • Interruption: A condition where voltage drops to less than 0.1 pu for a period not exceeding 1 minute.

• Causes:

  • Sags & Interruptions: Remote faults (most common), nearby faults, motor starting (inrush current causes voltage drop), load switching.

  • Swells: Load rejection (sudden disconnection of a large load), capacitor bank switching (energization), single-phase faults in a three-phase system.

• Sources of Sags & Interruptions: Primarily faults on the transmission or distribution system. The severity at a facility depends on fault location, system impedance, and transformer connections.

• Mitigation Techniques for Voltage Sags:

  • Capacitor Injection: Series capacitors can boost voltage during a sag.

  • Voltage Regulators: Tap-changing transformers (OLTC) provide slow correction.

  • Dynamic Voltage Restorer (DVR): A power electronics-based device that injects a compensating voltage in series with the supply to maintain load voltage.

• Estimation of Voltage Sag Performance:

  • Site Survey: Monitoring voltage at the point of common coupling (PCC) over time.

  • Indices: Sag Frequency (number of sags below a threshold per year), Sag Magnitude-Duration plots (e.g., CBEMA curve), SARFI (System Average RMS Variation Frequency Index).

Transient Overvoltages

• Definition: A high-frequency, short-duration (microseconds to milliseconds) voltage spike or impulse superimposed on the normal sinusoidal waveform. Also called a power spike or transient.

• Common Causes:

  • Lightning: Direct strike or induced surges on lines.

  • Capacitor Switching: Inrush current and recovery overvoltage.

  • Fault Clearing & Load Rejection: Interruption of fault current or sudden load drop.

• Factors Affecting Transient Recovery Voltage (TRV): System grounding, fault location, breaker characteristics, cable vs. overhead line parameters.

• Mitigation/Protection Methods:

  • Surge Protective Devices (SPDs) / TVSS: Clamp transient voltage to a safe level by diverting energy to ground. Types: Type 1 (lightning, service entrance), Type 2 (main distribution), Type 3 (point-of-use).

  • Zero-Voltage Crossing Switching: Switching capacitive loads (e.g., capacitor banks) at or near the zero-crossing of the voltage waveform minimizes the magnitude of the transient overvoltage. This is optimal for harmful transient mitigation.

  • Impulse Withstand: Proper insulation coordination, use of arresters, and equipment rated for expected BIL (Basic Impulse Level).

Voltage Surge vs. Swell: Clear Differentiation

C. HARMONICS AND WAVEFORM DISTORTION

Fundamentals of Waveform Distortion

• Harmonics: Sinusoidal components with frequencies that are integer multiples of the fundamental frequency (50/60 Hz). e.g., 5th harmonic = 250/300 Hz.

• Interharmonics: Components with frequencies that are not integer multiples of the fundamental. Can cause flicker and motor vibration.

• Harmonic Sources:

  • Industrial Loads: Rectifiers (AC/DC drives), arc furnaces, welding equipment, saturated transformers, cycloconverters.

  • Commercial Loads: Uninterruptible Power Supplies (UPS), personal computers, fluorescent lighting with electronic ballasts, AC/DC drives in HVAC.

• Non-linear Loads are the primary cause—they draw current only during part of the voltage cycle, distorting the current waveform.

Harmonic Indices and Quantification

• Individual Harmonic Distortion (IHD): Ratio of RMS value of a specific harmonic to the RMS value of the fundamental.

$$IHD_h = \frac{V_h}{V_1} \times 100\% \quad \text{(Voltage)} \quad IHD_h = \frac{I_h}{I_1} \times 100\% \quad \text{(Current)}$$

• Total Harmonic Distortion (THD): Measure of overall harmonic content.

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

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

\boxed{THD = \sqrt{\sum_{h=2}^{\infty} (IHD_h)^2} \times 100\%}

• Total Demand Distortion (TDD): THD of current relative to the maximum demand current (I_d), not the fundamental. Used for billing and system planning.

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

• Crest Factor (CF): Ratio of peak value to RMS value of a waveform. High CF (>2.5) indicates significant harmonic content and can stress equipment.

Effects of Harmonics

• On Equipment:

  • Overheating: Increased RMS current in transformers, motors, and capacitors (due to harmonic currents).

  • Nuisance Tripping: False operation of thermal relays and circuit breakers.

  • Bearing Currents & Fluting: In motors, due to capacitive coupling of high-frequency currents.

  • Metering Errors: Electromechanical and some electronic meters read incorrectly.

• On System:

  • Resonance: Harmonic frequencies can excite system capacitance/inductance, causing severe voltage amplification.

  • Neutral Overloading: In 3-phase 4-wire systems, triplen harmonics (3rd, 9th, 15th) add in the neutral conductor.

  • Protection Misoperation: Harmonics can cause relay miscoordination.

Harmonic Mitigation Techniques

• Passive Filters:

  • Shunt Tuned Filters: LC circuits tuned to a specific harmonic frequency (e.g., 5th, 7th). Provide low impedance path for targeted harmonic currents to ground.

  • Series Filters: Block harmonic currents from entering a sensitive load.

  • High-Pass Filters: Broadband mitigation for high-order harmonics.

  • Advantages: Simple, reliable, cost-effective for large, fixed harmonic sources.

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

• Active Harmonic Filters (Shunt Active Filter - SAF):

  • Basic Principle: Uses power electronics (VSC) to inject a current waveform that is equal and opposite to the harmonic current drawn by the load, resulting in a sinusoidal source current.

  • Advantages: Dynamic, adaptive to changing loads, can mitigate multiple harmonics simultaneously, no resonance risk.

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

• Comparison: Passive vs. Active Filters

  | Feature | Passive Filters | Active Filters | | :--- | :--- | :--- | | Compensation | Fixed, for specific harmonics | Dynamic, broad spectrum | | Resonance Risk | Yes | No | | Cost | Low (for high power) | High | | Size/Weight | Large (magnetics) | Compact | | Maintenance | Low | Moderate |

• Operation of a Harmonic Analyzer: A digital device that samples voltage/current waveforms, performs Fast Fourier Transform (FFT) to decompose the signal into harmonic components, and displays magnitude and phase angle for each harmonic order, along with THD and TDD values.

[!TIP] Exam Focus: "Best method to eliminate harmonics?" – Answer depends on context. For a large, fixed industrial load (e.g., 6-pulse rectifier), tuned passive filters are cost-effective. For dynamic, varying loads or multiple harmonic sources, an active filter is superior. Often a hybrid system (passive + active) is used.

D. REACTIVE POWER & POWER FACTOR

• Power Quality Problems due to Reactive Power:

  • Increased system current for same real power → higher I²R losses.

  • Voltage drop along lines, leading to poor voltage regulation.

  • Reduced capacity of generators, transformers, and cables.

  • Potential for voltage instability under heavy reactive power demand.

• Power Factor Correction (PFC) Benefits:

  • Reduced system losses and improved efficiency.

  • Improved voltage profile (less drop).

  • Release of system capacity (same KVA rating delivers more kW).

  • Avoidance of utility reactive power penalty charges.

• Installation Locations for Capacitor Banks:

  • Transmission Level: For system voltage support and stability (often via SVC/STATCOM).

  • Distribution Level: At substations or on feeders to correct overall PF.

  • Load-Side (Most Common): Directly at the load terminals (e.g., motor starter) for individual correction or in banks for a group of loads.

• Methods of Reactive Power Compensation:

  • Shunt Capacitors: Most common, simple, and economical.

  • Synchronous Condensers: Rotating machines (synchronous motors) operated at leading PF to generate reactive power. Provide inertia and can absorb/produce vars.

  • Static VAR Compensators (SVC): Power electronics-based, fast-acting (see Section E).

• Reason for Reactive Power Compensation Penalty: Utilities must supply both real (kW) and reactive (kVAR) power. Low PF means they must generate and transmit more current for the same real power delivery, increasing their losses and requiring larger infrastructure. The penalty incentivizes customers to correct their PF.

E. FLEXIBLE AC TRANSMISSION SYSTEMS (FACTS)

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

• Role of Shunt Capacitance in Transmission Lines: Shunt capacitors (or SVC/STATCOM) provide leading vars to support voltage, especially under light load conditions where line capacitance generates excess vars (Ferranti effect). They counteract the lagging vars consumed by inductive loads and lines.

• Key FACTS Devices for Voltage & Reactive Power Control:

  • Static VAR Compensator (SVC):

    • Principle: Combines a Thyristor-Controlled Reactor (TCR) and a Thyristor-Switched Capacitor (TSC). The TCR provides continuously variable inductive vars; the TSC provides stepped capacitive vars. Net reactive power output is the difference.

    • Characteristics: Fast response (cycles), but its reactive power output decreases with decreasing system voltage (∝ V²).

  • STATCOM (Static Synchronous Compensator):

    • Principle: Based on a Voltage Source Converter (VSC) connected in parallel (shunt) with the system. It acts as a controlled voltage source behind a small reactor. By adjusting the magnitude and phase of its output voltage, it can generate or absorb reactive power independent of system voltage.

    • Advantages over SVC:

      1. Faster response (sub-cycle).

      2. Better performance at low voltage: Can maintain full capacitive output even when voltage is low (unlike SVC's V² dependence).

      3. Smaller footprint (no large harmonic filters usually needed).

      4. Better harmonic performance.

• SVC vs. STATCOM Comparison:

  | Feature | SVC | STATCOM | | :--- | :--- | :--- | | Core Technology | TCR + TSC (Thyristors) | VSC (IGBTs/GTOs) | | Reactive Power vs. Voltage | Decreases with V² | Nearly constant down to low V | | Response Speed | Fast (cycles) | Very Fast (sub-cycle) | | Harmonics | Generates harmonics, needs filters | Minimal inherent harmonics | | Cost/Complexity | Lower for high power | Higher, but decreasing | | Best Application | Bulk power transmission, large vars | Dynamic support, weak grids, low voltage ride-through |

• Difference between Active Power Filter and STATCOM:

  • Primary Function: An Active Power Filter (APF) is designed primarily for harmonic mitigation (injecting harmonic currents). A STATCOM is designed for dynamic reactive power/voltage support (injecting fundamental frequency vars).

  • Control Objective: APF control targets specific harmonic currents. STATCOM control targets system voltage magnitude or power factor at fundamental frequency.

  • Power Rating: STATCOMs are typically rated in MVAr for system support. APFs can be smaller (kVAr) for individual load correction.

[!TIP] Exam Focus: "What is best, SVC or STATCOM?" – STATCOM is generally superior in performance (speed, low-voltage support). However, SVC can be more economical for very high power ratings (>200 MVAr) and is a mature technology. The "best" depends on application, required response, and budget.

F. CUSTOM POWER DEVICES & INTEGRATED SOLUTIONS

Unified Power Quality Conditioner (UPQC)

• Basic Principle: An integration of a Series Compensator (DVR) and a Shunt Compensator (APF or STATCOM) connected back-to-back via a common DC bus capacitor. It provides comprehensive mitigation for both voltage- and current-related PQ problems.

• Working and Operation:

  • Shunt Part (APF/STATCOM): Compensates for load current harmonics, unbalance, and reactive power. It ensures the current drawn from the source is sinusoidal and in phase with voltage.

  • Series Part (DVR): Injects a voltage in series with the supply to compensate for voltage sags, swells, unbalance, and harmonics at the load terminals.

  • Common DC Bus: Provides power exchange between the two parts during disturbances (e.g., DVR uses energy from DC bus to inject voltage during a sag).

• Classification of UPQC:

  • Based on Power Rating: Low-power (tens of kVA), Medium-power (hundreds of kVA), High-power (MVA).

  • Based on Compensation Strategy: UPQC-Q (prioritizes reactive power compensation), UPQC-P (prioritizes active power/voltage sag support).

• Advantages of UPQC:

  • Single device solves multiple PQ problems (sags, swells, harmonics, unbalance, flicker).

  • Improves voltage regulation and source power factor.

  • Protects sensitive loads from upstream disturbances.

• Power Quality Problems Mitigated by UPQC: Voltage sags/swells/interruptions, harmonics, voltage unbalance, flicker, voltage distortion.

• Differences between UPQC and DSTATCOM:

  • UPQC has both series and shunt compensation capability.

  • DSTATCOM is shunt-only (like a STATCOM). It can only inject current to correct load-side current harmonics and provide voltage support at its PCC, but cannot directly inject voltage in series to mitigate sags for a specific downstream load.

Voltage Sag Protectors / Ride-Through Schemes

• Need for Protection Schemes: Voltage sags are the most frequent and costly PQ disturbance for industries (causing process trips, data loss). Protection schemes allow equipment to ride through short sags without disconnection.

• Operation of Voltage Sag Protectors:

  • Solid-State Transfer Switch (SSTS): Uses power electronics (like a back-to-back thyristor bridge) to quickly (<4 ms) transfer the load from a sagging source to a standby source (e.g., UPS, generator).

  • DVR-Based Protectors: A small DVR dedicated to a critical load or group of loads. It detects a sag and injects the required voltage to maintain the load voltage within tolerance.

  • Ride-Through Schemes: Adjusting control parameters of drives and processes to tolerate deeper/longer sags (e.g., using DC bus capacitors in VSDs).

G. COMPARATIVE ANALYSIS & SYSTEM-LEVEL PERSPECTIVE

Comparative Evaluation of Mitigation Technologies

Selection Criteria for Mitigation Devices

1. Type of PQ Problem: Harmonic distortion? → Filter (Passive/Active). Voltage sag? → DVR/UPQC/SSTS. Reactive power? → Capacitors/SVC/STATCOM.

2. Location in System: Transmission (FACTS), Distribution (SVC/STATCOM), Load-side (APF, capacitors, UPQC).

3. Dynamics Required: Static (capacitors) vs. Dynamic (SVC/STATCOM/APF) vs. Ultra-fast (UPQC/DVR).

4. Cost-Benefit Analysis: Capital cost vs. cost of downtime/equipment damage.

5. System Constraints: Space, existing infrastructure, coordination with protection.

[!TIP] Exam Winning Strategy: For "difference between" or "best method" questions, always structure your answer:

1. Define both terms/devices.

2. List 3-4 key differentiating points in a table or bulleted list.

3. Conclude with the context-dependent best choice (e.g., "STATCOM is best for dynamic low-voltage support, but SVC may be economical for high-power bulk correction").