HVDC & FACTS (EX-702 (B)) - Unit 5 Short Notes

UNIT 5: HVDC & FACTS

I. Fundamentals of High Voltage Technology

High Voltage (HV) technology refers to the generation, transmission, distribution, and utilization of electrical energy at voltages significantly higher than those used in common domestic and industrial applications.

Significance in Modern Power Systems:

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

• Facilitates bulk power transfer over large geographical areas, enabling grid interconnection.

• Essential for EHV/UHV networks to handle increasing power demands efficiently.

Voltage Classification (as per Indian standards/commonly used):

Advantages of High Voltage Transmission:

• Reduced current → Lower line losses ($$\displaystyle P_{loss} = I^2R $$).

• Reduced conductor cross-section → Economical transmission.

• Improved voltage regulation and stability over long distances.

Limitations:

• Increased insulation costs (clearances, creepage distances).

• Higher corona losses and radio interference.

• Complex and expensive substation equipment (circuit breakers, transformers).

• Greater safety challenges and required clearances.

Applications:

• Power Systems: Transmission (HV/EHV/UHV lines), Distribution (HV distribution), HVDC converter stations.

• Non-Power Applications:

  • Industry: X-ray machines, electron beam welding, electrostatic painting, ozone generation.

  • Medical: CT scanners, radiation therapy (linear accelerators).

  • Research: Particle accelerators, plasma physics, high-voltage insulation testing labs.

Need for HV Generation in Laboratories:

• To simulate service conditions and test the dielectric strength of equipment (insulators, cables, transformers, circuit breakers) under controlled overvoltages (AC, DC, Impulse).

• To calibrate high-voltage measuring instruments (dividers, sphere gaps).

• To conduct research on breakdown phenomena in gases, liquids, and solids.

II. Breakdown Mechanisms in Dielectrics

A. Gaseous Dielectrics

1. Primary & Secondary Ionization:

• Primary Ionization: Free electron (from cosmic rays/background radiation) gains energy from electric field, collides with neutral gas molecule → ionization (electron + positive ion). The electron is accelerated again.

• Secondary Ionization: Positive ions drift to cathode, strike it with high energy → secondary electron emission (from cathode surface). These electrons initiate new avalanche. Crucial for self-sustaining discharge.

2. Townsend Discharge Theory:

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

• Second Ionization Coefficient (γ): Number of secondary electrons emitted from cathode per incident positive ion.

• Townsend Breakdown Condition: Discharge becomes self-sustaining when the total number of electrons multiplies sufficiently.

$$1 = γ (e^{αd} - 1)$$

where $d$ = gap distance. For large $αd$, breakdown occurs when:

$$\boxed{γ e^{αd} = 1}$$

3. Streamer Mechanism (for non-uniform fields):

• Improvement over Townsend: Explains rapid breakdown in non-uniform fields (e.g., rod-plane gap).

• Mechanism: Avalanche head develops a space charge that distorts the local electric field, enhancing it at the avalanche tip. This creates a highly conductive ionized channel (streamer) that propagates rapidly to the cathode, causing breakdown.

• Key: Space charge field $$\displaystyle E_{sc} $$ aids the applied field $$\displaystyle E_a $$ at the streamer tip: $$\displaystyle E_{total} = E_a + E_{sc} $$.

4. Statistical & Formative Time Lags:

• Statistical Time Lag ($$\displaystyle t_s $$): Time from voltage application to first free electron appearance (random process, depends on background radiation).

• Formative Time Lag ($$\displaystyle t_f $$): Time from first electron to complete breakdown (deterministic, depends on $E/p$, $d$).

• Total Breakdown Time Lag: $$\displaystyle t = t_s + t_f $$.

• > [!TIP] Exam Focus: Distinguish between the two lags. $$\displaystyle t_s $$ is probabilistic; $$\displaystyle t_f $$ is the physical growth time of the discharge.

5. Paschen's Law:

• Statement: Breakdown voltage ($$\displaystyle V_b $$) in a uniform field gas gap is a unique function of the product $pd$ (pressure × gap distance).

$$V_b = f(pd)$$

• Derivation (Simplified): Using Townsend's condition ($$\displaystyle γ e^{αd} = 1 $$) and $$\displaystyle α = A p e^{-Bp/E} $$, with $$\displaystyle E = V_b/d $$, solving leads to:

$$V_b = \frac{B p d}{\ln(A p d) - \ln[\ln(1 + \frac{1}{γ})]}$$

• Paschen Minimum: $$\displaystyle V_b $$ vs. $pd$ curve has a minimum. At this point, $pd$ is such that $$\displaystyle t_f $$ is minimum, and $$\displaystyle t_s $$ dominates.

  • For air: $$\displaystyle V_{b(min)} ≈ 327 V $$ at $pd ≈ 0.567$ Torr·cm.

• Practical Implications:

  • Insulation Design: For a given $$\displaystyle V_{max} $$, minimum clearance $d$ is determined at operating pressure (often 1 atm). $d$ must be > $$\displaystyle d_{min} $$ at Paschen minimum.

  • Vacuum Systems: Avoid operating in the Paschen minimum region for gas-filled equipment.

  • High-Altitude Equipment: Lower air pressure shifts Paschen curve → requires larger clearances for same $$\displaystyle V_b $$.

6. Breakdown in Vacuum & Cavity Breakdown:

• Vacuum Breakdown: Not due to gas ionization. Mechanisms include:

  • Field Emission: Electrons emitted from micro-protrusions on electrodes.

  • Micro-discharges: From adsorbed gases or particulates.

  • Closure of Vacuum Gap: By molten metal bridges from cathode spots.

• Cavity/Body Breakdown: In solid dielectrics with internal voids/cavities. Gas in cavity breaks down at lower $$\displaystyle V_b $$ than solid, leading to partial discharge (PD) and eventual insulation failure.

B. Solid and Liquid Dielectrics

1. Intrinsic Strength of Solid Dielectrics:

• Definition: The maximum electric field a perfect, defect-free dielectric material can withstand before breakdown. Typically very high ($$\displaystyle 10^6 $$ - $$\displaystyle 10^7 $$ V/cm).

• Determined by: Electronic polarization limits and bond strength.

• Practical Note: Real solids have impurities, voids, defects → operational strength is far lower than intrinsic.

2. Electron Avalanche & Breakdown in Solids:

• Mechanism: High field accelerates free electrons (from injection or ionization) to energies > band gap → impact ionization creates electron-hole pairs → avalanche.

• Thermal Runaway: Avalanche generates heat → more charge carriers → further heating → thermal breakdown.

• Electromechanical Breakdown: Electrostatic pressure compresses dielectric → if stress > mechanical strength → collapse.

• Treeing: Partial discharge in voids creates carbonized, branching paths ("trees") leading to final breakdown.

3. Partial Discharge (PD):

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

• Significance: PD is a symptom and cause of insulation deterioration. It erodes insulation over time, leading to failure.

• Measurement: Used as a diagnostic tool for insulation health (e.g., in transformers, cables, motors).

III. High Voltage Generation

1. Series Resonant Circuit:

• Principle: At series resonance ($$\displaystyle X_L = X_C $$), circuit impedance is minimum ($$\displaystyle Z = R $$). For a given input voltage $$\displaystyle V_s $$, current $$\displaystyle I = V_s/R $$ is maximum. This high current flows through the test object (capacitive load $$\displaystyle C_t $$) and the HV capacitor ($C$).

• Voltage Magnification: $$\displaystyle V_{HV} = I \cdot X_C = I \cdot \frac{1}{ωC} $$. Since $I$ is large at resonance, $$\displaystyle V_{HV} >> V_s $$.

• Resonance Condition Derivation:

  Impedance $$\displaystyle Z = R + j(ωL - \frac{1}{ωC}) $$. At resonance, $$\displaystyle Im(Z) = 0 $$:

$$ωL = \frac{1}{ωC} \quad \Rightarrow \quad ω_0 = \frac{1}{\sqrt{LC}} \quad \Rightarrow \quad f_0 = \frac{1}{2π\sqrt{LC}}$$

• > [!TIP] Exam Tip: Always state that the test object capacitance $$\displaystyle C_t $$ is in parallel with $C$, so effective capacitance $$\displaystyle C_{eff} = C + C_t $$. Resonance condition becomes $$\displaystyle ω_0 = 1/\sqrt{L C_{eff}} $$.

2. Cockcroft-Walton (C-W) Voltage Multiplier:

• Construction: Cascaded stages of diode-capacitor networks. Each stage multiplies peak AC voltage.

• Working: During positive half-cycle, capacitors $$\displaystyle C_1, C_3,... $$ charge through diodes $$\displaystyle D_1, D_3,... $$. During negative half-cycle, charged capacitors add in series to charge $$\displaystyle C_2, C_4,... $$ via diodes $$\displaystyle D_2, D_4,... $$. Output $$\displaystyle V_o ≈ 2n V_{peak} $$ (n = stages), minus ripple.

• Advantages: Simple, no transformer needed for high voltage, good for DC HV generation.

• Limitations: High output impedance, poor regulation under load, high ripple voltage, limited current.

3. Tesla Coil (Resonant Transformer):

• Construction: Two LC circuits: primary (low inductance $$\displaystyle L_1 $$, low capacitance $$\displaystyle C_1 $$, few turns) and secondary (high inductance $$\displaystyle L_2 $$, high capacitance $$\displaystyle C_2 $$, many turns). Both tuned to same resonant frequency.

• High-Frequency Operation: Spark gap in primary acts as a switch, exciting secondary at its resonant frequency → voltage magnification.

• Current Flow Near Coil: Due to resonant oscillation and electromagnetic coupling, high voltage appears across secondary terminals. "Skin effect" at high frequency causes current to flow on the outer surface of the coil conductor.

4. Impulse Generator (Marx Circuit):

• Construction: $n$ stages, each with a capacitor $C$ charged in parallel to $V$ via charging resistors $$\displaystyle R_c $$. All stages discharged in series through triggering gaps and a main gap to the load.

• Triggering Methods (Three-Electrode Gap):

  • Gap: Three electrodes: Trigger electrode (T), Main electrode (M1), Triggered electrode (M2). A high-voltage pulse applied to T initiates breakdown between T and M2, which then triggers the main gap M1-M2.

  • Why Preferred? Provides precise, simultaneous triggering of all stages → clean, reproducible impulse waveform. Essential for standardized testing (1.2/50 μs standard impulse).

• Control Tripping: The triggering pulse (from a pulse generator) controls the exact moment of discharge, allowing synchronization with oscilloscopes.

IV. High Voltage Measurement

A. Potential Dividers

Conditions for Impulse Voltage Measurement:

1. Frequency Response: Divider must have flat response up to frequencies in impulse waveform (MHz range).

2. Low Capacitance: To avoid loading the circuit and distorting the waveform.

3. Low Inductance: To prevent ringing and overshoot.

4. Matching: Input impedance of divider >> impedance of HV circuit.

> [!TIP] Exam Focus: For impulse measurement, RC or C dividers are used. Pure R dividers are unsuitable due to capacitive loading.

B. Sphere Gap

• Principle: Breakdown voltage between two spheres of diameter $D$ and gap $S$ is a function of $D$ and $S$, relatively independent of wave shape (for uniform field). Measures peak value of AC, DC, or impulse voltage.

• Construction: Two polished brass spheres of specified diameter (common: 12.5 cm, 25 cm, 50 cm, 100 cm). Mounted on insulating supports. Gap $S$ adjustable.

• Operation: Voltage increased until sparkover. Mean of several readings taken. Look up breakdown voltage from standard tables (IEC 60052) based on $D$ and $S$.

• Factors Influencing Accuracy:

  • Humidity: Significant effect, correction factors applied.

  • Sphere Surface Condition: Must be clean, smooth.

  • Gap Setting Accuracy: Precise measurement of $S$.

  • Voltage Waveform: For non-standard impulses, correction may be needed.

  • Surrounding Objects: Must be at least $2D$ away from spheres.

C. Generating Voltmeter

• Principle (for High DC Voltage): A rotating vane or disk driven by a small motor. The electrostatic force between fixed and rotating electrodes causes a deflecting torque proportional to $$\displaystyle V^2 $$. This torque is balanced by a spring or gravitational torque. The rotation speed (or pointer deflection) is read and calibrated to give $$\displaystyle V_{rms} $$ or $$\displaystyle V_{dc} $$.

• Advantage: No direct connection to HV circuit → high insulation, no loading.

D. Electrostatic Voltmeter

• Construction: Fixed electrode (connected to HV) and a lightweight moving vane (attached to pointer/spring) inside a grounded case.

• Working: Electrostatic attraction between fixed and moving electrodes causes deflection. Deflection ∝ $$\displaystyle V^2 $$. Used for AC and DC. Very high input impedance, negligible loading.

E. Surge Current Measurement

• Methods/Instruments:

  • Rogowski Coil: Air-cored toroidal coil around conductor. Output voltage $$\displaystyle v_o = M \frac{di}{dt} $$, where $M$ = mutual inductance. Integrator needed to get $i(t)$.

  • Shunt Resistor (Low-Value): Direct measurement of $$\displaystyle V_{shunt} = i \cdot R_{shunt} $$. Must have very low inductance.

  • Magnetic Field (Hall Effect) Sensors.

• Challenges:

  • Very High di/dt: Requires extremely fast response (ns-μs).

  • Bandwidth: Instrument must have sufficient frequency response.

  • Isolation: Sensor must be insulated from high-voltage circuit.

  • Insertion Impedance: Sensor must not disturb the surge current path.

V. Testing of High Voltage Equipment

General Principle: Tests simulate overvoltages (lightning, switching) and sustained overvoltages to verify insulation strength.

A. Circuit Breakers

• Short-Circuit Test (Making & Breaking):

  • Objective: Verify ability to make and break rated short-circuit currents without excessive arcing, contact welding, or failure.

  • Procedure: Breaker connected to a synthetic test circuit (source + current injection) or a direct test station. Current is passed through breaker, and arc extinction is observed.

• Dielectric Test (Withstand Test):

  • Objective: Verify insulation strength between open contacts and to ground.

  • Procedure: Apply power-frequency AC voltage (or DC) for 1 minute (or specified time) at a value > rated voltage. No breakdown should occur. Often includes impulse voltage test (1.2/50 μs).

B. Isolators (Disconnect Switches)

• Electrical Tests:

  • Contact Resistance Test: Measure resistance of main circuit using micro-ohmmeter (DC current). Should be very low (< 100 μΩ).

  • Dielectric Strength Test: Power-frequency withstand voltage applied between contacts (open) and to ground.

  • Mechanical Operation Test: Check opening/closing time, speed, synchronism.

C. Insulators

• Mechanical Strength Test: Apply tensile, compressive, bending, or torsional load as per standard until failure. Determines ultimate strength.

• Puncture Voltage Test: Voltage applied across the insulator body (between pin and cap, with shell shorted). Tests internal dielectric strength. Should be > flashover voltage.

• Flash-Over Voltage Test: Voltage applied along the insulator surface (between line and ground terminals). Tests surface insulation. Measured as 50% flashover voltage (statistical).

D. Power Transformers

• High Voltage Test (Induced Overvoltage Test / AC Withstand):

  • Purpose: Verify main insulation (winding-to-winding, winding-to-ground) strength against power-frequency overvoltages (e.g., from switching).

  • Procedure: Apply AC voltage at twice rated voltage (or as per standard) for 60 seconds (or specified) to the winding while other windings are grounded. No breakdown.

E. High Voltage Cables

• Partial Discharge (PD) Tests:

  • Principle: Apply AC voltage (typically 1.5-2 times rated) and measure PD magnitude (pC) and discharge inception/extinction voltages. PD activity indicates insulation defects.

  • Fault Location Techniques:

    1. Time Domain Reflectometry (TDR): Send fast voltage pulse, reflections from impedance discontinuities (faults) indicate location.

    2. Bridge Methods (Murray Loop, Varley Loop): For low-resistance faults.

    3. High-Voltage Radar (HV-TDR): For high-resistance or open-circuit faults.

    4. Partial Discharge Location: Using multiple sensors and time-difference-of-arrival (TDOA).

F. Test Comparisons

G. Insulation Techniques

• Pouring Methods for Motor Coil Insulating Paint (Varnish):

  1. Dip & Bake: Coil assembly dipped into varnish tank, then baked in oven. Simple, good penetration.

  2. Vacuum-Pressure Impregnation (VPI): Coil placed in pressure vessel, evacuated, varnish injected under pressure. Excellent void elimination, high reliability.

  3. Resin-Rich (Pre-Impregnated): Pre-impregnated glass tape/sleeve wrapped on coil, then cured (heat/pressure). Used for high-voltage machines.

VI. HVDC Transmission Systems

A. Introduction

• Merits:

  • Long Distance/Undersea Cables: Lower cost & losses for >~600 km (OHL) or >~50 km (cable).

  • Asynchronous Interconnection: Connect grids of different frequencies/stabilities.

  • Controllable Power Flow: Fast, independent control of active power.

  • No Skin Effect: Current uniformly distributed in conductors.

  • Lower Right-of-Way: Fewer conductors for same power.

• Demerits:

  • High Converter Station Cost: Expensive AC/DC converters (thyristor/IGBT valves).

  • Reactive Power Consumption: Converters absorb large reactive power (~40-60% of active power).

  • Harmonics Generation: 12-pulse, 24-pulse converters produce characteristic harmonics (12th, 24th...).

  • Complex Control & Protection.

  • Lack of Overload Capability: Unlike transformers, limited overload.

B. Converter Station Layout & Major Equipment

• Layout: AC switchyard → AC Filter bus → Converter Transformer (often with tertiary winding for filters) → Valve Hall (containing thyristor valves in series/parallel) → DC Switchyard → DC line/cable.

• Major Equipment:

  • Converter Transformers: Special design for harmonic currents, often with two secondary windings (12-pulse bridge).

  • Thyristor Valves: Stacked in series/parallel to form converter bridge (6-pulse/12-pulse). Housed in valve hall with cooling, insulation.

  • AC & DC Filters: Tuned LC circuits to absorb harmonics.

  • DC Smoothing Reactor: Reduces ripple in DC current.

  • Surge Arresters: Protect against overvoltages.

  • Control & Protection Systems.

C. DC Links

D. System Control

• Principle: Control firing angle (α) of thyristors to control DC voltage $$\displaystyle V_d = V_{do} \cos α - I_d R_{line} $$.

  • α < 90°: Power flow from AC to DC (rectifier).

  • α > 90°: Power flow from DC to AC (inverter).

• Converter Control Characteristics:

  • Constant Current (CC) Control: Maintains $$\displaystyle I_d $$ constant (primary control).

  • Constant Extinction Angle (CEA) Control: Maintains inverter extinction angle γ > limit (for stability). γ = π - α - μ (μ = overlap angle).

  • Current Margin: Rectifier in CC mode, inverter in CEA mode with a current margin (e.g., 10%). Prevents mode jumping.

  • DiagramCANVAS: Sketch a graph with DC voltage (V_d) on Y-axis and DC current (I_d) on X-axis. Show two lines: Rectifier CC characteristic (downward slope from left) and Inverter CEA characteristic (downward slope from right). Intersection is operating point. Indicate current margin between them.

E. Multiterminal DC Systems (MTDC)

• Series Configuration: Terminals connected in series on DC side. Same current flows through all. Power control by voltage control at one terminal (master) and current control at others. Complex control, one terminal failure disrupts all.

• Parallel Configuration: Terminals connected in parallel on DC side. Same voltage. Power control by current control at each terminal. Simpler control, one terminal failure does not affect others directly.

• Applications: Tapping power from main HVDC link to intermediate loads, connecting offshore wind farms, multi-infeed HVDC grids.

F. Power Reversal

• Concept: Reversing direction of power flow without changing physical polarity of DC line.

• Implementation: Change firing angle control:

  • Rectifier Mode: α < 90° (e.g., 15°).

  • Inverter Mode: α > 90° (e.g., 160°).

  • Process: One end (usually the one with stronger AC system) switches to inverter mode first, then the other switches to rectifier mode. DC voltage polarity remains same, but current reverses.

G. Harmonics and Filters

• Types of Harmonics in HVDC:

  • AC Side: 12-pulse → $12k±1$ (11th, 13th, 23rd, 25th...). 24-pulse → $24k±1$.

  • DC Side: Ripple frequency = $$\displaystyle 6f_{ac} $$ for 12-pulse → 12th harmonic on DC side.

• AC Filters: Tuned LC branches (single-tuned, high-pass, C-type) connected between AC bus and ground. Absorb specific harmonic frequencies (e.g., 11th, 13th) and provide reactive power.

• DC Filters: Connected in series with DC line or between DC line and ground. Tuned to major DC side harmonics (e.g., 12th, 24th). Often part of DC smoothing reactor circuit.

VII. FACTS Controllers

A. Introduction to FACTS

• Definition: Flexible AC Transmission Systems. Power electronics-based controllers that enhance controllability and increase power transfer capability of AC transmission systems.

• Role: Improve stability, reduce losses, alleviate congestion, enable better utilization of existing infrastructure.

B. Conventional Reactive Power Compensators

• Principle: Generate/absorb reactive power (Q) to control voltage at a point.

  • Shunt Capacitors: Generate Q → raise voltage.

  • Shunt Reactors: Absorb Q → lower voltage.

  • Series Capacitors: Compensate line inductance → increase power transfer ($$\displaystyle P ∝ \frac{V_1 V_2}{X} \sinδ $$).

  • Limitation: Stepped control (on/off), slow (mechanical switches).

C. Static Var Compensator (SVC)

• Definition: A shunt-connected FACTS controller that provides continuous, dynamic reactive power control.

• Types & Operation:

  1. Thyristor-Controlled Reactor (TCR): Reactor in series with bidirectional thyristor valve (anti-parallel). Controls fundamental current by phase angle control (firing angle α). Absorbs variable Q.

  2. Thyristor-Switched Capacitor (TSC): Capacitor bank switched on/off by thyristor valve at current zero (to avoid transients). Generates fixed Q steps.

  3. Combined TCR + TSC: Most common. TCR provides continuous control within the range defined by TSC steps. Smooth VAr characteristic.

• Operation: SVC controls bus voltage by injecting/absorbing Q to maintain $$\displaystyle V = \text{constant} $$ (droop characteristic).

D. Thyristor-Controlled Series Capacitor (TCSC)

• Detailed Operation: Series capacitor ($C$) shunted by a TCR (reactor $L$ + thyristor valve). The TCR is phase-angle controlled.

• Effective Impedance: The TCSC presents a variable, controllable inductive impedance in series with the fixed capacitive reactance.

  • Bypass Mode (α ≈ 0°): TCR fully on → $L$ and $C$ in parallel → net impedance is inductive (low).

  • Blocking Mode (α ≈ 180°): TCR off → net impedance = capacitive reactance $$\displaystyle -X_C $$.

  • Control Range: Between capacitive and inductive modes. Virtually increases line reactance ($$\displaystyle X_{line} + X_{TCSC} $$) to control power flow.

• Advantages:

  • Damp power oscillations (PSS role).

  • Increase steady-state power transfer.

  • Improve transient stability.

  • Disadvantages: Complex control, harmonic generation (from TCR), resonance risks (with system reactance).

E. Static Synchronous Series Compensator (SSSC)

• Construction: Voltage Source Converter (VSC) (IGBT-based) connected in series with transmission line via a coupling transformer.

• Operation: VSC generates a synchronous voltage $$\displaystyle V_{inj} $$ that is in quadrature with line current $I$. Injects this voltage in series.

  • $$\displaystyle V_{inj} ∝ I $$ → acts as a variable inductive reactance ($$\displaystyle X_{SSSC} = V_{inj}/I $$).

  • Can also inject voltage with different phase → can also control active power flow directly.

• Use: Series compensation (like TCSC but faster), power oscillation damping, loop flow control. No susceptibility to subsynchronous resonance (SSR) like TCSC.

F. Static Synchronous Compensator (STATCOM)

• Construction: VSC (IGBT) connected in shunt to the bus via a coupling transformer.

• Operation in Reactive Power Compensation:

  • VSC generates a synchronous voltage $$\displaystyle V_{conv} $$ at its terminals.

  • By controlling magnitude of $$\displaystyle V_{conv} $$ relative to bus voltage $$\displaystyle V_{bus} $$:

    • $$\displaystyle |V_{conv}| > |V_{bus}| $$ → capacitive (injects Q).

    • $$\displaystyle |V_{conv}| < |V_{bus}| $$ → inductive (absorbs Q).

  • Advantages over SVC:

    • Faster response (μs vs ms).

    • Better performance at low voltages (can provide more Q at low $$\displaystyle V_{bus} $$).

    • Smaller footprint (no large capacitor banks/reactors).

    • No resonance with system (VSC based).

G. Unified Power Flow Controller (UPFC)

• Schematic Diagram:

  DiagramSEARCH: UPFC schematic diagram
  (Shows: two VSCs (Converter 1 & 2) connected back-to-back via a DC capacitor. Converter 1 is series connected via transformer. Converter 2 is shunt connected. Both share common DC link.)

• Principle of Operation:

  • Shunt Converter (STATCOM): Provides reactive power support to the bus, controls bus voltage $$\displaystyle V_{bus} $$, and supplies active power to the DC capacitor.

  • Series Converter (SSSC): Injects a controllable voltage $$\displaystyle V_{inj} $$ in series with the line. By controlling magnitude and phase of $$\displaystyle V_{inj} $$, it can:

    1. Control line active power flow ($$\displaystyle P ∝ V_1 V_{inj} \sinδ / X $$).

    2. Control reactive power at both terminals.

    3. Damp oscillations.

  • Key: The two converters are decoupled via DC capacitor. Shunt converter provides/absorbs the active power needed by the series converter (plus losses). Enables independent control of voltage, impedance, and phase angle → comprehensive power flow control.