Power Electronics Application to Power System (EX-702 (A)) - Unit 5 Short Notes

UNIT 5: POWER ELECTRONICS APPLICATION TO POWER SYSTEM

I. HIGH VOLTAGE ENGINEERING

A. Introduction to High Voltage Technology

High Voltage (HV) technology deals with generation, measurement, and application of voltages significantly above normal operating levels (typically > 1000 V AC or 1200 V DC).

Significance in Modern Power Systems:

• Enables long-distance power transmission with reduced $$\displaystyle I^2R $$ losses.

• Essential for insulation coordination and equipment design.

• Facilitates integration of large, remote generation (e.g., hydro, wind farms).

Voltage Level Classification:

Applications:

• Power Applications: Transmission lines, substations, switchgear, transformers, insulation testing.

• Non-Power Applications: X-ray tubes, particle accelerators, electrostatic precipitators, ozone generation, food sterilization.

Need for Laboratory HV Generation:

• Insulation Testing: Withstand voltage, impulse voltage testing of equipment.

• Research & Development: Studying breakdown mechanisms, new insulating materials.

• Calibration: Standardizing HV measuring instruments.

• Simulation: Recreating system overvoltages (switching, lightning) in controlled conditions.

B. Breakdown in Dielectrics

1. Gaseous Dielectrics

Primary Ionization: Free electrons gain energy from an electric field, collide with neutral gas molecules, and ionize them, creating more electron-ion pairs.

• Townsend's First Ionization Coefficient (α): Number of ionizing collisions produced by one electron per unit path length. $$\displaystyle \alpha = A p e^{-Bp/E} $$, where $p$ = pressure, $E$ = field.

Secondary Ionization: Processes that generate new electrons at the cathode, sustaining the discharge.

• Townsend's Second Ionization Coefficient (γ): Average number of secondary electrons produced per incident positive ion, photon, or metastable atom.

• Secondary Effects: Positive ion bombardment, photon interaction (photoelectric effect), metastable atom de-excitation.

Townsend Discharge Theory:

The current growth in a gap is: $$\displaystyle i = i_0 e^{\alpha d} / \left(1 - \gamma (e^{\alpha d} - 1)\right) $$ Breakdown Condition: When denominator → 0, i.e., $$\displaystyle \gamma (e^{\alpha d} - 1) = 1 $$. For $$\displaystyle \alpha d >> 1 $$, the Townsend breakdown criterion is:

$$ \alpha d = \ln\left(\frac{1}{\gamma}\right) + \ln\left(\frac{1}{1+1/\gamma}\right) \approx \ln\left(\frac{1}{\gamma}\right) $$

[!TIP] Limitation: Fails for large gaps & non-uniform fields where space charge distorts the field.

Streamer Mechanism (Non-Uniform Fields):

1. Avalanche Growth: Electron avalanche develops near the high-field electrode.

2. Space Charge Formation: The avalanche head has a net positive space charge, the tail a net negative charge.

3. Field Distortion: This space charge locally enhances the electric field ahead of the avalanche tip.

4. Streamer Formation: The enhanced field causes new independent avalanches, forming a conductive plasma channel (streamer) that bridges the gap rapidly.

Improvement over Townsend: Explains fast breakdown in large gaps by accounting for space charge field enhancement.

Statistical & Formative Time Lags:

• Statistical Time Lag ($$\displaystyle t_s $$): Time for an initial electron to appear (from natural background radiation). Random.

• Formative Time Lag ($$\displaystyle t_f $$): Time for the electron avalanche to develop into a self-sustaining streamer. Deterministic, depends on $E/p$ and gap geometry.

• Total Breakdown Time ($$\displaystyle t_b $$): $$\displaystyle t_b = t_s + t_f $$.

Paschen's Law:

Breakdown voltage $$\displaystyle V_b $$ in a uniform field gap is a function of the product of gas pressure ($p$) and gap distance ($d$).

$$ V_b = f(pd) $$

Derivation (Simplified): From Townsend criterion, assuming $$\displaystyle \alpha/p = A e^{-Bp/E} $$ and $$\displaystyle E = V_b/d $$, solving leads to $$\displaystyle V_b \propto Bpd / \ln(Apd) - \ln[\ln(1+1/\gamma)] $$. Paschen Minimum: The curve $$\displaystyle V_b $$ vs. $pd$ has a minimum. At this minimum, $pd$ is smallest and $$\displaystyle V_b $$ is lowest.

Practical Implication: For a given $$\displaystyle V_b $$, there is an optimum $pd$ (pressure × distance). This guides insulation design: for a fixed pressure, there is a critical gap distance below which breakdown voltage drops. Equipment must be spaced wider than this critical distance.

Breakdown in Deep Vacuum:

• Mechanism: Not gas ionization. Caused by field emission of electrons from microprotrusions on electrode surfaces.

• Process: High field at protrusion tip → electron emission → micro-arc → localized heating → vaporization of electrode material → vacuum arc.

• Cause of Arcing: Surface imperfections (roughness, contaminants) create field enhancement sites.

2. Solid and Liquid Dielectrics

Intrinsic Breakdown Strength: The maximum electric field a perfect, defect-free dielectric can withstand before electronic breakdown. Determined by electronic (band theory) or ionic (lattice vibration) processes. Typically very high ($\sim$ MV/cm).

Breakdown Mechanisms in Solids:

• Electronic (Intrinsic): Direct band-to-band transition at very high fields.

• Thermal Breakdown: Localized heating from dielectric losses ($$\displaystyle I^2R $$) leads to runaway thermal instability.

• Electromechanical Breakdown: Electrostatic pressure compresses the material, reducing thickness and increasing field.

• Discharge (Avalanche) Breakdown: In porous/impure solids, gas-filled voids break down first, eroding the solid.

Partial Discharge (PD):

• Definition: Localized electrical discharge that only partially bridges the insulation between conductors. Occurs in voids, cracks, or at interfaces.

• Causes: High local field in a cavity due to lower permittivity of gas vs. solid.

• Effects: Progressive erosion and degradation of insulation, leading to eventual failure. Generates electrical noise, chemical byproducts (O₃, NOₓ), and heat.

Partial Discharge Tests on Cables:

1. Procedure: Apply AC voltage above inception level. Use a coupling capacitor to isolate PD pulses from the high-voltage source.

2. Detection: Measure PD pulses (magnitude, repetition rate) using a PD detector/meter (oscilloscope, audio/radio frequency monitors).

3. Interpretation: PD magnitude (pC) indicates severity. Repetition rate relates to void size/condition.

4. Fault Location (Time Domain Reflectometry - TDR): Inject a fast-rising voltage step. Measure reflected pulse from impedance discontinuity (fault/void). Time delay $\Delta t$ gives distance: $$\displaystyle x = (v \cdot \Delta t)/2 $$, where $v$ = wave propagation velocity in cable.

3. Vacuum Breakdown

(Covered in B.1.6 above: Field emission from microprotrusions leading to vacuum arc).

C. High Voltage Generation

Series Resonant Circuit for HV Testing:

• Principle: At series resonance ($$\displaystyle \omega L = 1/\omega C $$), the circuit impedance is purely resistive and minimal. The same current flows through L and C, but the voltage across C ($$\displaystyle V_C = I \cdot X_C $$) can be much larger than the input voltage due to the high reactance $$\displaystyle X_C $$.

• Resonance Condition: $$\displaystyle \omega_0 = 1/\sqrt{LC} $$.

• Advantages: Generates high AC voltage with a low-power, small-sized input source (tuning transformer). Ideal for capacitive loads (cables, capacitors).

• Limitation: Only works at/near resonant frequency. Load must be capacitive.

Cockcroft-Walton (CW) Voltage Multiplier:

• Construction: Cascaded stages of diodes and capacitors. Each stage: capacitor charged to peak input $$\displaystyle V_{max} $$ during alternate half-cycles.

• Working: During positive half-cycle, C1 charges to $$\displaystyle V_{max} $$ via D1. During negative half-cycle, C2 charges to $$\displaystyle 2V_{max} $$ (C1 + input). Subsequent stages add $$\displaystyle V_{max} $$ each.

• Output Voltage (No Load): $$\displaystyle V_{out} \approx 2n V_{max} $$, where $n$ = number of stages.

• Advantages: No high-voltage transformer needed. Compact, lightweight, produces high DC voltage from low-voltage AC.

• Limitations: Significant ripple and poor voltage regulation under load (voltage drops with load current). Capacitor size/voltage rating increases with stages.

Tesla Coil:

• Construction: Two resonant LC circuits (primary & secondary) loosely coupled via a spark gap. Primary: capacitor + few-turn coil. Secondary: many-turn coil + toroidal terminal.

• Working: Capacitor in primary charges, sparks across gap, oscillating current in primary inductively couples energy to secondary, which resonates at much higher frequency (MHz range).

• Why Current Flows in Nearby Objects: The high-frequency, high-voltage secondary terminal creates a strong, oscillating electric field. Nearby objects (human hand, light bulb) experience:

  1. Displacement Currents: AC field induces currents in non-conductors.

  2. Skin Effect: HF current flows on conductor surfaces.

  3. Corona/Streamer: Field ionizes air, creating a conductive path.

  Result: A visible, often audible, corona discharge or spark occurs to the object.

Impulse Generator Triggering:

• Method: Three-electrode gap arrangement.

  • Electrodes: Charging electrode (H.V.), triggering electrode (T), grounded electrode (G).

  • Operation: Main gap (H.V.-G) is over-voltaged. A separate, low-energy trigger pulse (from a small capacitor) is applied to T, creating a localized spark that rapidly ionizes the main gap, causing synchronous breakdown.

• Why Preferred: Provides precise, reproducible timing of the impulse wavefront. Independent of statistical time lag of the main gap. Essential for consistent testing and synchronization with measurement equipment.

D. High Voltage Measurement

Potential Dividers:

Used to scale down high voltage to a measurable low voltage. Must have known, frequency-independent ratio.

Conditions for Impulse Dividers:

1. Damping Ratio (ζ): Should be critically damped (ζ ≈ 0.7) to avoid overshoot/oscillation.

2. Rise Time Response: Step response must have rise time < 1/3 of impulse wavefront time (e.g., for 1.2/50 μs wave, $$\displaystyle t_r < 0.4 \mu s $$).

3. Impedance Matching: Input impedance must be much higher than the source impedance (HV circuit) to avoid loading.

Sphere Gap:

• Construction: Two precisely machined, identical metallic spheres with adjustable gap.

• Working Principle: Breakdown voltage across the gap is a well-established, reproducible function of gap distance, sphere diameter, and waveform (AC, DC, impulse). Standardized by IEC/ISO.

• Measurement: Measures peak value of voltage. For AC, it's the peak of the sine wave. For impulse, it's the amplitude.

• Factors Influencing Measurement:

  • Sphere diameter (larger = higher voltage range, less surface irregularity effect).

  • Gap distance (use standard tables/curves).

  • Humidity: Increases breakdown voltage (water vapor electronegative).

  • Polarity: Positive polarity gives lower breakdown voltage for same gap (in air).

  • Waveform: Different tables for DC, AC, Switching Impulse (250/2500 μs), Lightning Impulse (1.2/50 μs).

  • Surrounding Objects: Must be at least 2.5× sphere diameter away to avoid field distortion.

Generating Voltmeter (for High DC):

• Principle: Rotating electrode (vanes) in a high-voltage field. The electrostatic force causes a torque proportional to $$\displaystyle V^2 $$. The rotation is opposed by a spring. A counter (or integrator) measures the total revolutions over time, which is proportional to the average current and hence to $$\displaystyle V^2 $$. $V \propto \sqrt{\text{revolutions per unit time}}$.

• Construction: High-voltage electrode, rotating vane assembly, spring, gear train, revolution counter.

• Operation: No direct electrical contact with HV. Measures average value, not instantaneous.

Electrostatic Voltmeter:

• Principle: Attraction force between fixed and moving electrodes in a high-voltage field. Force $$\displaystyle F \propto V^2 $$. Movable pointer deflects against spring tension.

• Construction: Fixed stator, movable vane/plate, pivot, spring, scale.

• Use: For AC/DC HV measurement. Very high input impedance (pF level), draws negligible current.

• Advantages: True RMS for AC, no loading, robust.

Surge Current Measurement:

• Methods:

  1. Rogowski Coil: Air-cored toroidal coil around conductor. Output $$\displaystyle v_o \propto di/dt $$. Requires integration to get $i(t)$. Advantages: No magnetic core saturation, wide bandwidth, isolated.

  2. Shunt Resistor: Low-value, non-inductive resistor. $$\displaystyle V_{shunt} = i(t) \cdot R_{shunt} $$. Challenges: Must handle high $di/dt$, low inductance, high power dissipation, isolation.

  3. Magnetic Field Sensors (Hall Effect, B-dot probes): Measure magnetic field around conductor, proportional to current.

• Challenges:

  • High $di/dt$: Requires very high bandwidth (>10 MHz) measurement system.

  • Electromagnetic Interference (EMI): Surge generates strong EMI, needs shielding and careful grounding.

  • Safety & Isolation: Measurement circuit must be fully isolated from HV circuit.

  • Bandwidth vs. Sensitivity: Trade-off in sensor design.

E. High Voltage Testing

High Voltage Test vs. Insulation Resistance Test:

Tests on Power System Equipment:

• Circuit Breaker:

  • Short-Circuit Test (Making/Breaking Capacity): Tests ability to close on and interrupt fault currents. Measures arcing time, re-ignition, current chopping, dielectric recovery.

  • Dielectric Test: Insulation withstand test (power frequency, impulse) to verify clearance and insulation integrity.

  • Objective: Ensure reliable operation under worst-case fault conditions.

• Power Transformer:

  • Purpose of HV Test: Verify main and longitudinal insulation strength against overvoltages.

  • Tests Include:

    • Applied Voltage Test: Tests insulation between windings and ground.

    • Induced Voltage Test: Tests inter-turn insulation and insulation between windings.

    • Impulse Test: Simulates lightning surge, tests overall insulation coordination.

    • Corona & Partial Discharge Tests: Detect local defects in windings and bushings.

• Insulators:

  • Mechanical Strength Test: Apply ultimate tensile/compressive/bending load to verify mechanical integrity (for suspension, strain, pin insulators).

  • Puncture Voltage Test: Apply voltage across the insulator body (electrodes on metal fittings) to test bulk dielectric strength.

  • Flashover Voltage Test: Apply voltage across surface (clean/wet) to test surface insulation and arcing distance.

  • Necessity: Ensure reliability against mechanical failure (wind, weight) and electrical failure (flashover, puncture) in service.

• Isolator (Disconnect Switch):

  • Electrical Tests: Insulation resistance, contact resistance (micro-ohmmeter), dielectric withstand test (power frequency).

  • Mechanical Tests: Operation cycles, torque, interlocks.

  • Note: Isolators are not designed to break load current, only to provide visible isolation. Testing focuses on insulation and mechanical operation.

Partial Discharge Testing (Covered in B.2.3).

Motor Coil Insulating Paint Pouring Methods:

1. Dip Painting: Manual dipping.

2. Brush Application: Manual painting.

3. Spray Application: Automated or manual spraying.

4. Vacuum Pressure Impregnation (VPI): Most effective. Coil placed in vacuum to remove air/moisture, then immersed in resin under pressure. Ensures complete penetration, eliminates voids, excellent mechanical strength.

F. High Voltage Power Equipment Overview

II. POWER ELECTRONICS APPLICATIONS IN POWER SYSTEMS

A. High Voltage DC (HVDC) Transmission Systems

Merits of HVDC:

• Lower Losses for Long Distance: No skin effect, no reactive power flow, lower line charging current.

• Asynchronous Interconnection: Can connect AC systems of different frequencies or non-synchronized grids.

• Controllable Power Flow: Power can be rapidly and precisely controlled (within ms).

• Stability Improvement: Does not contribute to short-circuit ratio (SCR) issues. Can provide dynamic support (e.g., power modulation to damp oscillations).

• No Charging Current: For cables, allows longer subsea/underground links.

Demerits of HVDC:

• High Converter Station Cost: Expensive converters (valve hall), converter transformers, AC/DC filters.

• Harmonic Generation: Converters generate characteristic (12-pulse: 12k±1) and non-characteristic harmonics on AC and DC sides.

• Reactive Power Consumption: Converters consume large amounts of reactive power (≈ 40-60% of rated power). Requires AC filters (which also provide harmonic mitigation) and sometimes SVC/STATCOM.

• Limited Overload Capability: Converters have thermal limits (typically 10-20% overload for short time).

• Complex Control & Protection: Requires sophisticated control systems and DC circuit breakers (still developing for multiterminal).

HVDC Converter Station Layout & Major Equipment:

[AC Bus] --> [AC Filter] --> [Converter Transformer] --> [Valve Hall (Thyristor Valves)] --> [DC Smoothing Reactor] --> [DC Line/Electrode]

                                     ^

                              [Control & Protection System]

• Converters: Typically 12-pulse (two 6-pulse bridges with 30° phase shift via transformer) to reduce dominant 5th & 7th harmonics.

• Converter Transformers: Special design for high harmonic currents, often with two secondary windings for 12-pulse configuration.

• AC/DC Filters: Tuned LC filters (single/double-tuned, high-pass) to meet harmonic standards (IEEE 519, IEC 61800).

• DC Switchgear: Isolators and fast mechanical switches. HVDC circuit breakers are complex (use forced current zero via capacitor discharge or resonant circuits).

• Surge Arresters: Protect against DC switching overvoltages and lightning.

• Control System: Master control, pole control, valve firing control.

Types of DC Links:

• Monopolar: One conductor (+ or -) + earth/sea return. Simple, cheaper for long distances.

• Bipolar: Two conductors (+ and -). Allows unbalanced operation (one pole grounded, other floating). More flexible, no earth current issues.

• Homopolar: Both poles on same conductor, return via earth. Rarely used.

Multiterminal DC Systems (MTDC):

• Series MTDC: Terminals connected in series on same DC line. Current same everywhere, voltage adds. Advantages: Simple control, inherent current limiting. Disadvantages: One terminal fault affects all, difficult to add terminals.

• Parallel MTDC: Terminals connected in parallel to same DC bus. Voltage same, currents add. Advantages: High reliability (one terminal fault doesn't collapse voltage), easy to add terminals. Disadvantages: Complex voltage control coordination, requires DC circuit breakers.

• Applications: Multi-infeed HVDC (e.g., large cities, industrial hubs), grid integration of remote renewables (offshore wind, desert solar), black-start capability for AC grids.

HVDC Control:

• Principle: Control rectifier to maintain constant current (to avoid commutation failure). Control inverter to maintain constant extinction angle (γ) or constant DC voltage.

• Converter Control Characteristics:

  • Rectifier: Constant Current (CC) control is primary. Characteristic: $$\displaystyle I_d = I_{d0} $$ (constant), $$\displaystyle V_d = V_{d0} - I_d X_c $$ (sloping line).

  • Inverter: Constant Extinction Angle (CEA) control is primary. Characteristic: $$\displaystyle \gamma = \gamma_0 $$ (constant), $$\displaystyle V_d = V_{d0} + I_d X_c \cos \gamma_0 $$ (sloping line).

  • Power Flow Direction: Reversing voltage polarity (bipolar) or current direction (monopolar with earth return swap).

Power Reversal in HVDC:

• Method: For bipolar links, simply reverse the polarity of both poles (swap + and -). For monopolar, swap the polarity of the single conductor and reverse earth return current.

• Application: Bidirectional power flow. Enables regenerative braking (e.g., from a remote wind farm during high wind, or from a connected metro system).

Harmonics in HVDC Systems:

• Characteristic Harmonics: Determined by pulse number $p$. For 12-pulse: $$\displaystyle h = 12k \pm 1 $$ (11th, 13th, 23rd, 25th...). Always present.

• Non-Characteristic Harmonics: Caused by unbalances (AC system impedance, transformer saturation, firing angle errors). Include even harmonics (2nd, 4th) and other odd harmonics (3rd, 5th, 7th).

• Effects: Transformer overheating, capacitor overloading/failure, communication interference, misoperation of protective relays, torque pulsations in motors.

• Standards: IEEE 519 (harmonic control), IEC 61800 (adjustable speed drives, relevant for converter harmonics).

AC and DC Filters:

• Purpose:

  1. Harmonic Mitigation: Reduce harmonic currents injected into AC system and DC line.

  2. Reactive Power Compensation: Provide capacitive vars to offset converter consumption.

• Design:

  • Tuned Filters: Single/double-tuned LC filters for specific dominant harmonics (e.g., 11th/13th).

  • High-Pass Filters (C-type, damped): Attenuate higher-order harmonics (23rd+).

• Placement: Connected between AC bus and converter transformer (AC side) and sometimes on DC line (DC side filter for high-frequency harmonics).

B. Flexible AC Transmission Systems (FACTS)

FACTS Controllers:

• Definition: Power electronics-based systems that enhance controllability of AC power systems and increase power transfer capability.

• Classification:

  • Shunt Controllers: Connected in parallel with line (e.g., SVC, STATCOM). Control voltage and reactive power.

  • Series Controllers: Connected in series with line (e.g., TCSC, SSSC). Control line impedance and power flow.

  • Combined Controllers: Both shunt and series (e.g., UPFC). Control voltage, impedance, and angle simultaneously.

• Purpose: Increase transfer capability, improve stability (voltage, transient, small-signal), damp oscillations, reduce losses, optimize power flow.

Principle of Conventional Reactive Power Compensators:

• Static VAR Compensator (SVC): Uses thyristors to switch/control reactors (TCR) and capacitors (TSC).

  • TCR (Thyristor Controlled Reactor): Inductor with anti-parallel thyristors. By phase-angle control, varies effective reactance, continuously absorbing vars.

  • TSC (Thyristor Switched Capacitor): Capacitor bank switched at current zero by thyristors. Provides step-wise capacitive vars.

  • SVC Operation: Combines TCR (continuous inductive) and TSC (stepped capacitive) to provide continuous, fast reactive power/voltage control over a range (lagging to leading).

Thyristor Controlled Series Capacitor (TCSC):

• Construction: A series capacitor bank ($C$) shunted by a thyristor-controlled reactor ($L$).

• Operation: By controlling thyristor firing angle ($\alpha$) in the reactor branch, the net impedance $$\displaystyle Z_{net} = -j/(\omega C) + j\omega L_{eff}(\alpha) $$ is varied.

  • Blocked Mode ($$\displaystyle \alpha = 90° $$): Thyristors off. Net capacitance = $C$.

  • Capacitive Mode ($$\displaystyle \alpha < 90° $$): Partial conduction. Net capacitance increases ($$\displaystyle L_{eff} $$ capacitive).

  • Inductive Mode ($$\displaystyle \alpha > 90° $$): Net impedance becomes inductive (used for SSR mitigation).

• Advantages:

  • Increases power transfer capability by dynamically compensating line reactance.

  • Damps power oscillations (PSS function).

  • Enhances transient stability.

  • Can mitigate Sub-Synchronous Resonance (SSR) in inductive mode.

• Disadvantages:

  • Complex control, harmonic generation (needs filters).

  • SSR risk in capacitive mode if not properly controlled.

  • Limited dynamic range compared to VSC-based devices.

Static Synchronous Series Compensator (SSSC):

• Construction: A Voltage Source Converter (VSC) (usually IGBT-based) with a DC capacitor, connected in series with the transmission line via a coupling transformer.

• Operation: The VSC injects a controlled AC voltage $$\displaystyle V_{inj} $$ in series with the line. This voltage is orthogonal to the line current $I$.

  • Injected Voltage: $$\displaystyle V_{inj} = jX_{SSSC} I $$, where $$\displaystyle X_{SSSC} $$ is the equivalent series reactance injected.

  • Effect: The line's net impedance becomes $$\displaystyle X_{line} + X_{SSSC} $$. By varying $$\displaystyle X_{SSSC} $$ (positive or negative), power flow is controlled.

• Use:

  • Power Flow Control: Increase or decrease active power flow.

  • Stability Improvement: Damp oscillations, improve transient stability.

  • Voltage Regulation: Can also provide some voltage support.

Static Synchronous Compensator (STATCOM):

• Construction: A VSC (IGBT) with a DC capacitor, connected in shunt to the AC bus via a coupling transformer.

• Operation: Acts as a controllable voltage source behind a small impedance (transformer leakage).

  • Reactive Power: If STATCOM voltage $$\displaystyle |V_{STAT}| > |V_{AC}| $$, it injects capacitive vars (leading current). If $$\displaystyle |V_{STAT}| < |V_{AC}| $$, it absorbs inductive vars (lagging current).

  • Dynamic Response: Much faster than SVC (milliseconds vs. cycles). Provides symmetric reactive power capability over a wide range.

• Compensation Principle:

  • Voltage Regulation: Maintains bus voltage by injecting/absorbing vars.

  • Dynamic Reactive Power Support: During faults or load changes, provides immediate var support.

  • Power Factor Correction: Improves PF at the point of connection.

  • Stability Enhancement: Increases short-circuit ratio (SCR), improves voltage stability margin.

Unified Power Flow Controller (UPFC):

• Schematic: Combines a STATCOM (shunt VSC) and an SSSC (series VSC) sharing a common DC capacitor.

  • Shunt VSC: Provides/absorbs vars at the bus, controls bus voltage.

  • Series VSC: Injects voltage in series, controls line impedance/angle.

  • Common DC Link: Allows power exchange between shunt and series converters.

• Functions (Simultaneous & Independent Control):

  1. Control Bus Voltage (via shunt converter).

  2. Control Line Active Power Flow (via series voltage magnitude/angle).

  3. Control Line Reactive Power Flow (via series voltage quadrature component).

  4. Damp Power Oscillations.

• Capability: The most powerful FACTS controller, capable of independent control of all basic power flow parameters (V, θ, Z).