UNIT 3: HVDC & FACTS – Comprehensive Study Notes

I. HIGH VOLTAGE ENGINEERING FUNDAMENTALS

A. Introduction to High Voltage Technology

• Significance: Enables efficient bulk power transfer over long distances, reduces current (I = P/V), minimizing I²R losses and allowing thinner conductors.

• Voltage Classifications:

  | Abbreviation | Range (AC) | Typical Use | |--------------|------------------|--------------------------------------| | HV | 33 kV – 230 kV | Regional transmission | | EHV | 345 kV – 765 kV | Long-distance, bulk power transfer | | UHV | > 800 kV (AC) | Very long distances, large capacity|

• Applications:

  • Power: Transmission lines, substations, HVDC converter stations.

  • Non-Power: X-ray generators, particle accelerators, ozone generation, electrostatic precipitators.

[!TIP] Exam Focus: Be ready to cite specific voltage levels for HV/EHV/UHV as per Indian/International standards.

B. Need for High Voltage Generation in Laboratories

• Purposes:

  1. Insulation Testing: Dielectric withstand, impulse voltage tests.

  2. Simulation: Replicating lightning, switching overvoltages.

  3. R&D: Characterizing new insulating materials, insulation coordination studies.

• Testing Scenarios: High potential (hipot) tests, partial discharge measurements, aging studies.

C. Breakdown in Dielectrics

1. Gaseous Dielectrics

• Townsend's First Ionization Coefficient (α): Number of ionizing collisions per electron per unit length. Depends on E/p (field strength/pressure).

• Townsend's Second Ionization Coefficient (γ): Number of secondary electrons emitted from cathode per ion impact.

• Condition for Townsend Breakdown: When electron multiplication reaches a critical value, leading to self-sustaining discharge.

$$ \boxed{e^{(\alpha d)} = 1 + \frac{1}{\gamma}} $$

where d = gap distance.

• Streamer Mechanism: Explains breakdown in non-uniform fields. Avalanche develops a space charge that distorts the electric field, creating a conductive ionized channel (streamer) that bridges the gap. More relevant for practical gaps than Townsend theory.

2. Vacuum Breakdown

• Mechanism: In ultra-high vacuum, breakdown is not due to gas ionization but field emission from microprotrusions on electrode surfaces.

• Causes of Arcing:

  • Outgassing: Adsorbed gases desorb under high field.

  • Microprotrusions: Enhance local electric field (field enhancement factor β).

  • Particle Ejection: Microscopic particles vaporize/ionize, initiating arc.

3. Solid Dielectrics

• Intrinsic Strength: Maximum electric field a perfect, defect-free dielectric can withstand before electronic breakdown (~10⁸ V/m for many polymers).

• Electronic Breakdown: High field accelerates electrons to energies sufficient to ionize lattice atoms via collision, causing avalanche.

• Cavity Breakdown (Voids): Air-filled voids within insulation have lower breakdown strength. Partial discharges (PD) occur in voids, progressively damaging insulation.

4. Time Lags in Breakdown

• Total Breakdown Time = Statistical Time Lag + Formative Time Lag.

D. Paschen's Law

• Statement: Breakdown voltage in a uniform field gas gap is a unique function of the product of gas pressure (p) and gap distance (d).

$$ \boxed{V_b = f(pd)} $$

• Derivation Sketch: From Townsend breakdown condition, with α = A p exp(-B p / E), solving yields V_b as function of pd.

• Paschen Minimum: The curve V_b vs. pd has a minimum. For air, minimum ~327 V at pd ≈ 0.567 Torr·cm.

  Significance: Indicates easiest breakdown condition. For a given voltage, there is a minimum safe clearance (pd product must be above the minimum point for safety).

• Practical Implications:

  • Design of insulation clearances in gas-insulated switchgear (GIS).

  • Choice of gas (SF₆ has higher dielectric strength, shifts minimum to higher pd).

  • High-altitude equipment (lower pressure → need larger clearances).

E. High Voltage Generation

1. Series Resonant Circuit

• Principle: At series resonance (ωL = 1/ωC), impedance is minimum (purely resistive), current is maximum. Voltage across capacitor (V_C) = Q × V_input, where Q = quality factor.

• Resonance Condition:

$$ \boxed{\omega_0 = \frac{1}{\sqrt{LC}}} $$

• Use: High voltage testing of cables, transformers where high current is needed.

2. Cockcroft-Walton Generator

• Construction: Cascaded voltage doubler stages using diodes and capacitors.

• Working: Each stage doubles peak AC voltage. Output V_out ≈ 2nV_p (n = stages, V_p = peak transformer secondary voltage), minus ripple and regulation losses.

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

• Limitations: High ripple, poor regulation under load, limited current.

3. Tesla Coil

• Construction: Primary LC circuit (low voltage, high current), secondary LC circuit (high turns ratio, high voltage). Coupled via mutual inductance.

• Working: Primary oscillates at resonant frequency, transfers energy to secondary via resonant coupling. Secondary voltage stepped up to very high values.

• Why HF Allows Current Flow:

  • Capacitive Coupling: High frequency electric field can induce currents in nearby objects without direct contact.

  • Skin Effect: HF current flows on surface, but for nearby objects, displacement currents dominate.

F. High Voltage Measurement

1. Sphere Gaps

• Principle: Breakdown voltage across a sphere gap is a well-defined function of gap spacing and sphere diameter for AC, DC, and standard impulse waves.

• Construction: Two identical metal spheres. Gap spacing adjustable.

• Operation: Voltage increased until breakdown occurs. Read breakdown voltage from calibrated curves.

• Factors Influencing Accuracy:

  • Sphere diameter (standard: 6.25 cm, 12.5 cm, 25 cm).

  • Gap spacing (must be within specified range for given sphere size).

  • Humidity (affects air density).

  • Polarity (for DC and impulse, positive vs negative breakdown differs).

2. Potential Dividers

• Conditions for Impulse Work:

  • Frequency response must cover impulse spectrum (up to ~100 kHz).

  • Adequate damping to prevent oscillations.

  • Low inductance construction.

3. Electrostatic Voltmeters

• Principle: Attraction force between fixed and moving plates proportional to V². Direct reading for DC and RMS for AC (if calibrated).

• Use: Medium to high DC voltages (kV range).

4. Generating Voltmeters

• Principle: Rotating vane or motor-driven electrode system generates current proportional to voltage. Current measured by sensitive galvanometer.

• Use: Very high DC voltages (MV range), where direct connection is impractical.

G. High Voltage Testing

1. Impulse Testing

• Impulse Generator Triggering:

  • Three-electrode gap: Main gap between trigger electrode and one terminal. Trigger pulse applied to trigger electrode creates spark, ionizing main gap.

  • Why Preferred: Provides precise, synchronized triggering, essential for wave shape control and reproducibility.

• Surge Current Measurement:

  • Methods: Rogowski coil (integrates di/dt to get current), shunt resistors (low inductance).

  • Instruments: High-bandwidth oscilloscopes, impulse recorders.

  • Challenges: Very high di/dt, amplitude (kA to MA), bandwidth requirements (>10 MHz), electromagnetic interference (EMI).

2. Tests on Power Equipment

• HV Test vs Insulation Resistance Test:

  | Feature | High Voltage Test | Insulation Resistance Test (Megger) | |------------------|---------------------------------------|--------------------------------------| | Voltage | High (kV to MV) | Low (500 V to 5 kV) | | Purpose | Test dielectric strength under overvoltage stress. | Measure bulk insulation resistance, detect moisture/contamination. | | Nature | Destructive if failed. | Non-destructive. |

3. Cable Testing

• Partial Discharge (PD) Tests:

  • Procedure: Apply AC or impulse voltage. Detect electrical pulses from PD in voids/defects using couplers (capacitive, inductive).

  • Fault Location: Time-domain reflectometry (TDR) or PD pulse arrival time difference from both ends locates defect.

4. Insulation Techniques

• Pouring Methods for Motor Coil Insulating Paint:

  1. Dip & Bake: Coil dipped in varnish, then baked.

  2. Vacuum-Pressure Impregnation (VPI): Coil placed in vacuum, varnish introduced under pressure → eliminates voids completely.

  3. Powder Coating: Electrostatic application of powdered resin, then cured.

II. HVDC TRANSMISSION SYSTEMS

A. Overview and Comparison with AC Transmission

• Merits of HVDC:

  • No skin effect → better conductor utilization.

  • Asynchronous interconnection possible.

  • Controllable power flow (fast, independent of phase angle).

  • Lower losses for very long distances/submarine cables (no charging current).

  • Can connect grids with different frequencies.

• Demerits of HVDC:

  • High converter station cost (thyristor/IGBT valves).

  • Requires reactive power compensation at converter stations.

  • Generates harmonics (needs filters).

  • Lack of overloading capability (thermal limits).

• Typical Applications: Long-distance (>600 km overhead, >50 km submarine), interconnections (e.g., India-Sri Lanka, USA-Canada), offshore wind integration.

• Types of DC Links:

  | Type | Configuration | Use Case | |------------|----------------------------------------|-----------------------------------| | Monopolar | Single conductor + ground/sea return. | Economical for long distances. | | Bipolar | Two conductors (± polarity). | Common; allows metallic return, redundancy. | | Homopolar | Multiple conductors at same polarity. | Rare; for specific applications. |

B. HVDC Converter Station

• Layout: AC switchyard → Converter transformer → Valve hall (12-pulse bridge) → DC switchyard → DC line/ground electrode.

• Key Equipment:

  • Converters: Thyristor/IGBT valves in bridge configuration (usually 12-pulse).

  • Converter Transformers: Provide required voltage, isolate AC/DC, have high short-circuit impedance.

  • AC Filters: Tuned to 5th, 7th, 11th, 13th harmonics; provide capacitive reactive power.

  • DC Filters: Smooth DC ripple, suppress high-frequency noise.

  • Arresters: Protect against overvoltages (lightning, switching).

  • DC Switchgear: For line maintenance, pole isolation.

• Filters Purpose:

  • Harmonic Suppression: Prevent AC system distortion, avoid resonance.

  • Reactive Power Compensation: AC filters supply capacitive vars to offset converter's reactive power consumption (~40-60% of active power).

C. HVDC System Control

• Principles:

  • Current Control: Primary control; maintains constant current (I_order) to prevent commutation failure.

  • Extinction Angle Control (γ-control): Secondary; ensures minimum γ (typically 15°-20°) for reliable commutation.

  • Voltage Control: Tertiary; adjusts DC voltage via tap changers or reactive power.

• Converter Control Characteristics:

  • Rectifier: Constant current (CC) at low voltage, constant extinction angle (CEA) at high voltage.

  • Inverter: Constant current (CC) at low voltage, constant voltage (CV) at high voltage (to maintain stable DC voltage).

  • Power Reversal: Achieved by changing firing angle α > 90° (inverter mode) at one end while other remains rectifier. Power flow reverses without reversing DC voltage polarity.

    Significance: Enables bidirectional power flow control, essential for multi-terminal and grid support.

D. Multi-terminal HVDC (MTDC)

• Applications: Offshore wind farms (multiple turbines to single onshore station), multi-infeed HVDC in large grids (e.g., China, Europe).

E. Harmonics in HVDC Systems

• Characteristic Harmonics: Determined by pulse number (p): h = kp ± 1 (k=1,2,...). For 12-pulse: 11th, 13th, 23rd, 25th.

• Non-characteristic Harmonics: Due to unbalanced conditions, firing angle variations, transformer saturation.

• Role of Filters:

  • AC Filters: Tuned to characteristic harmonics, provide capacitive vars.

  • DC Filters: At converter station, suppress high-frequency harmonics on DC side (from valve switching).

III. FLEXIBLE AC TRANSMISSION SYSTEMS (FACTS)

A. Introduction to FACTS Controllers

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

• Need: To optimize existing infrastructure, defer upgrades, manage congestion, integrate renewables.

• Classification:

  • Shunt: SVC, STATCOM (inject/absorb reactive power).

  • Series: TCSC, SSSC (modify line impedance).

  • Combined: UPFC (simultaneous shunt & series control).

B. Conventional Reactive Power Compensators

• Principle: Mechanically switched capacitors/inductors (e.g., capacitor banks, reactors). Slow (seconds), stepwise control, cause switching transients.

C. Static Var Compensator (SVC)

• Basic Concept: Thyristor-based switchable reactive power source.

• Types:

  • TCR (Thyristor Controlled Reactor): Continuous reactive power absorption via phase-angle control.

  • TSC (Thyristor Switched Capacitor): Stepwise capacitive reactive power injection.

  • TCR-TSC Combination: Provides continuous control range.

D. Thyristor Controlled Series Capacitor (TCSC)

• Construction: Capacitor bank in parallel with a thyristor-controlled reactor (TCR).

• Operation:

  • Thyristors control reactor current, effectively changing net series reactance.

  • Can vary from capacitive (cap. dominates) to inductive (reactor dominates), even bypassing capacitor during faults.

• Advantages:

  • Modulates line impedance → controls power flow, damps oscillations.

  • Modular, relatively low cost.

• Disadvantages:

  • Resonance risk with system (sub-synchronous resonance, SSR).

  • Control complexity, harmonic generation.

E. Static Synchronous Series Compensator (SSSC)

• Operation: Voltage Source Converter (VSC) in series with line via transformer. Injects a controllable AC voltage in series with line.

• Use:

  • Independent control of magnitude and angle of injected voltage.

  • Can act as variable series impedance (like TCSC) or even negative impedance.

  • Faster response than TCSC (no LC resonance).

F. Static Synchronous Compensator (STATCOM)

• Operation: VSC-based shunt device. Maintains constant bus voltage by generating/absorbing reactive power.

  • Reactive Power Generation: VSC output voltage > bus voltage → capacitive.

  • Reactive Power Absorption: VSC output voltage < bus voltage → inductive.

• Comparison with SVC:

  | Feature | SVC | STATCOM | |------------------|----------------------------------|----------------------------------| | Response | Slower (ms) | Very fast (μs) | | Low Voltage | Reactive power drops with V². | Constant current region → better support at low V. | | Size | Large (capacitors/inductors). | Compact (VSC, smaller footprint).| | Harmonics | More (TCR switching). | Less (PWM switching). |

G. Unified Power Flow Controller (UPFC)

• Schematic Diagram:

  
  AC System ──[Shunt VSC]───(DC Link)───[Series VSC]─── Load
  
                │                             │
  
                └───────────[Transformer]─────┘
  
  

• Basic Operation:

  • Shunt VSC: Provides DC link voltage, exchanges active/reactive power with AC bus.

  • Series VSC: Injects voltage with controllable magnitude and phase in series with line.

• Capabilities:

  • Simultaneous control of bus voltage (shunt), line impedance (series), and power flow (phase angle via series voltage).

  • Can also regulate active power flow independently.

END OF UNIT 3 NOTES
Focus on derivations (Paschen), diagrams (UPFC, converter control), and comparisons (SVC vs STATCOM, MTDC types) as per past papers.