HVDC & FACTS (EX-702 (B)) - Unit 4 Short Notes

UNIT 4: HVDC & FACTS SYSTEMS

I. HVDC Transmission Systems

Introduction to HVDC Transmission

• Definition: High Voltage Direct Current (HVDC) transmission uses DC for bulk power transfer over long distances or for interconnecting asynchronous AC systems.

• Comparison with AC:

  • HVDC: Lower line losses (no skin effect, reactive power), no stability limits, controllable power flow, suitable for long distances/submarine cables.

  • AC: Higher losses over long distances, stability issues, simpler terminal equipment.

• Historical Development: First commercial HVDC (1954, Sweden–Gotland). Modern systems use Line-Commutated Converters (LCC) and Voltage Source Converters (VSC).

• Modern Applications:

  • Long-distance power transmission (> 600 km overhead, > 50 km submarine).

  • Interconnection of asynchronous grids (e.g., US–Canada, Europe).

  • Power supply to islands/isolated areas.

  • Integration of remote renewable generation (offshore wind).

[!TIP] Exam Focus: HVDC is economical for long distances despite high converter station cost due to lower line cost and losses. Always mention asynchronous interconnection as a key application.

Merits and Demerits of HVDC

[!TIP] Common Pitfall: HVDC does not eliminate the need for reactive power; converter stations consume reactive power (LCC) or can generate/absorb (VSC).

Applications of HVDC Systems

1. Long-distance power transmission: E.g., China’s Changji-Guquan (±1100 kV, 3300 km).

2. Interconnection of asynchronous grids: E.g., UK–France (IFA), US–Canada.

3. Submarine cable links: E.g., NorNed (Norway–Netherlands, 580 km).

4. Power supply to isolated areas: E.g., Tasmania–Australia (Basslink).

5. Grid reinforcement and congestion relief: Urban areas where AC lines are saturated.

6. Renewable integration: Offshore wind farms (VSC-HVDC).

HVDC Converter Station

Layout & Major Equipment:

DiagramSEARCH: HVDC converter station layout single line diagram

1. Converters:

   • LCC (Line-Commutated): Thyristor valves, require AC voltage for commutation, generate harmonics.

   • VSC (Voltage Source Converter): IGBTs, self-commutating, independent control, lower harmonics.

2. Converter Transformers: Special design (high impedance, harmonic filtering), multiple windings for 12-pulse configuration.

3. AC Switchgear: Circuit breakers, disconnectors for AC side isolation.

4. DC Switchgear: DC disconnectors, arresters (lightning protection).

5. Filters:

   • AC Filters: Tuned to 12-pulse harmonics (12, 24, 36...), provide reactive power.

   • DC Filters: Smooth DC ripple (50/100 Hz, 150/250 Hz).

6. Reactive Power Compensation: Shunt capacitors (LCC), or inherent in VSC.

7. Control and Protection: Hierarchical control (system, station, valve level); protection against overcurrent, overvoltage, commutation failure.

[!TIP] Exam Question: "Draw layout and discuss equipment" → Label all 7 components above. Emphasize 12-pulse converter (two 6-pulse bridges with phase-shifting transformer) to reduce harmonics.

Types of HVDC Links

[!TIP] Bipolar is standard for overhead lines; monopolar for submarine cables (no corrosion issues with ground return).

Multiterminal HVDC (MTDC) Systems

• Series MTDC:

  • Converters connected in series on DC side.

  • Current constant throughout, voltage varies per terminal.

  • Complex control, lower losses.

• Parallel MTDC:

  • Converters connected in parallel on DC side.

  • Voltage constant, current varies.

  • Easier control, higher losses.

• Comparison:

  | Aspect | Series MTDC | Parallel MTDC | |------------------|----------------------------------|----------------------------------| | Control | Complex (voltage sharing) | Simpler (current sharing) | | Losses | Lower | Higher | | Fault Impact | Affects all terminals | Localized | | Common Use | Rare | More common (e.g., China) |

• Applications:

  • Multi-infeed HVDC (e.g., Shanghai grid).

  • Offshore wind farm collection.

  • Interconnecting multiple AC grids.

[!TIP] Past Paper: "Compare series and parallel MTDC" → Use table above. Parallel is more common due to easier control.

Power Reversal in HVDC Systems

• Concept: Changing direction of power flow without physical reconnection.

• Necessity: Bidirectional power exchange (e.g., import/export), emergency control.

• Methods:

  1. Polarity Reversal (Monopolar): Reverse DC voltage polarity → power reverses.

  2. Current Reversal (Bipolar): Change firing angles to reverse current while keeping voltage polarity fixed (preferred for bipolar).

• Control Implications:

  • Requires coordinated control at both ends.

  • Protection settings must accommodate reversal (e.g., DC fault detection).

  • Reversal time: seconds to minutes (LCC slower, VSC faster).

[!TIP] Power reversal in bipolar HVDC is done by current reversal to avoid grounding issues. VSC-HVDC can reverse power in < 100 ms.

II. HVDC System Control and Harmonics

Principle of HVDC System Control

• Basic Control Objectives:

  1. Power Control: Set desired active power transfer.

  2. Current Control: Limit current to protect valves.

  3. Voltage Control: Maintain DC voltage stability.

• Control Hierarchy:

  • System Level: Power dispatch between stations.

  • Station Level: Current/voltage regulation (CC, CEA, CV).

  • Valve Level: Firing pulse generation (phase-locked oscillator).

• Control Modes:

  • Constant Current (CC): Primary control; limits current.

  • Constant Extinction Angle (CEA): For inverters; ensures reliable commutation.

  • Constant Voltage (CV): Maintains DC voltage (often at one terminal).

  • Constant Power (CP): Derived from CC and CV.

[!TIP] Rectifier typically runs in CC, inverter in CEA or CC with lower current limit to prevent commutation failure.

Converter Control Characteristics

• LCC Control: Firing angle α (0°–180°). Power $$\displaystyle P = V_d I_d $$, where $$\displaystyle V_d = V_{d0} \cos \alpha - \frac{3}{\pi} X_c I_d $$.

• Characteristics:

  • CC Characteristic: Horizontal line (current constant).

  • CEA Characteristic: Sloping line (α increases as I_d decreases).

  • CV Characteristic: Vertical line (voltage constant).

• Multi-Control Operation: Overlapping regions; priority settings (e.g., CC takes precedence over CEA).

  
  DiagramSEARCH: HVDC converter control characteristics rectifier inverter
  
  

  Rectifier: CC region dominant; Inverter: CEA region dominant; overlap for power reversal.

[!TIP] Past Paper: "Draw converter control characteristics" → Sketch α vs I_d for rectifier and inverter, show CC, CEA, CV limits, and overlap region.

Harmonics in HVDC Systems

• Sources: Non-linear converter switching (thyristor turn-on/off).

• Types:

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

  • Non-Characteristic: Due to unbalance, misfiring, control asymmetry (e.g., 5th, 7th, even harmonics).

• Effects:

  • Heating in transformers, capacitors.

  • Interference with communication lines.

  • Capacitor overloading, resonance.

  • Misoperation of protective relays.

[!TIP] 12-pulse converters eliminate 5th/7th harmonics but produce 11th/13th. Filters are mandatory for compliance with IEEE 519.

AC and DC Filters in HVDC

• Purpose:

  • AC Filters: Tune out AC side harmonics, provide reactive power (LCC).

  • DC Filters: Reduce DC ripple (ripple voltage on DC line).

• Types:

  • Tuned Filters: Single-tuned (e.g., 11th, 13th), damped tuned.

  • High-Pass Filters: C-type, damped high-pass (for high-order harmonics).

• Design Considerations:

  • Harmonic spectrum from converter.

  • Reactive power compensation requirement.

  • Avoid resonance with system impedance.

• Role: Harmonic mitigation (to meet standards), reactive power support (AC filters), improve power quality.

[!TIP] AC filters for LCC are dual-purpose: harmonic filtering + reactive power supply. VSC-HVDC needs smaller filters due to PWM switching at higher frequencies.

III. FACTS Controllers

Introduction to FACTS (Flexible AC Transmission Systems)

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

• Need: To relieve congestion, improve stability, damp oscillations, optimize power flow without new lines.

• Classification:

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

  • Series: TCSC, SSSC (inject series voltage to control line impedance).

  • Combined: UPFC (shunt + series).

• Role: Increase thermal limit, reduce loop flows, improve voltage profile, enhance transient/dynamic stability.

[!TIP] FACTS devices do not generate active power; they control power flow by modulating line impedance or voltage.

Conventional Reactive Power Compensators

• Principle: Fixed or mechanically switched capacitors/reactors.

• Devices:

  • Shunt Capacitors: Generate reactive power (Q ∝ V²), stepped switching.

  • Shunt Reactors: Absorb reactive power (for light load).

  • Series Capacitors: Compensate line inductance, increase transfer capability.

• Limitations:

  • Slow response (mechanical switches).

  • No continuous control.

  • Can cause subsynchronous resonance (series capacitors).

  • Overvoltage issues on light load (capacitors).

Static Var Compensator (SVC)

• Types:

  • TCR (Thyristor Controlled Reactor): Continuous reactive power absorption by controlling firing angle (α).

  • TSC (Thyristor Switched Capacitor): Stepwise reactive power generation by full/zero conduction.

• Operation: Combines TCR and TSC for continuous V-I characteristic.

• V-I Characteristic:

  
  DiagramSEARCH: SVC V-I characteristic
  
  

  • Sloping line due to slope control (droop).

• Advantages: Fast response (ms), continuous control, improves voltage stability.

• Limitations:

  • Generates harmonics (TCR → 3rd, 5th, 7th).

  • Requires filters.

  • Limited operating range (especially at low voltage).

  • Large footprint (magnetic components).

[!TIP] SVC is current-source device; its reactive power output ∝ system voltage (decreases at low voltage).

Thyristor Controlled Series Capacitor (TCSC)

• Construction: Capacitor bank in series with line, bypassed by thyristor-controlled reactor (TCR).

• Working Principle: Vary net series reactance $$\displaystyle X_{net} = X_C - X_L(\alpha) $$.

• Operation Modes:

  1. Bypass Mode: TCR fully on → low impedance (fault protection).

  2. Blocking Mode: TCR off → full capacitive reactance.

  3. Controlling Mode: Partial thyristor conduction → variable capacitive reactance.

• Advantages:

  • Increase power transfer capability.

  • Damp power oscillations (PSS effect).

  • Reduce short-circuit current.

• Disadvantages:

  • Harmonic generation (TCR).

  • Risk of subsynchronous resonance (SSR) with turbine shafts.

  • Complex control.

• Applications: Power flow control, stability enhancement, SSR mitigation.

[!TIP] TCSC modulates line impedance → changes power flow $$\displaystyle P \propto \frac{V_1 V_2}{X} \sin \delta $$. Used for damping oscillations by fast impedance modulation.

Static Synchronous Compensator (STATCOM)

• Principle: VSC-based (IGBTs) shunt device, behaves as a controllable voltage source.

• Operation:

  • DC capacitor → VSC → AC voltage injection.

  • Reactive power: $$\displaystyle Q = \frac{V_{stat}^2 - V_{sys}^2}{X} $$ (where $$\displaystyle V_{stat} $$ is STATCOM voltage magnitude).

  • Controls voltage by adjusting $$\displaystyle V_{stat} $$ magnitude and phase.

• Voltage Regulation: Can generate/absorb reactive power over wider range than SVC.

• Advantages over SVC:

  1. Faster response (< 1 ms vs 10–20 ms).

  2. Wider operating range (works even at low voltage).

  3. Smaller footprint (no large reactors/capacitors).

  4. Lower harmonics (PWM switching at high frequency).

  5. Can provide active power if DC source added (e.g., battery).

• Limitations: Higher cost, losses in VSC.

[!TIP] STATCOM is voltage-source; reactive power output independent of system voltage (unlike SVC). Ideal for weak grids.

Static Synchronous Series Compensator (SSSC)

• Principle: VSC in series with line, injects controlled AC voltage in quadrature with line current.

• Operation:

  • Injects voltage $$\displaystyle V_{inj} \perp I_{line} $$ → changes effective line impedance $$\displaystyle X_{eff} = X_{line} \pm \Delta X $$.

  • Power flow: $$\displaystyle P = \frac{V_1 V_2}{X_{eff}} \sin \delta $$.

• Use:

  • Power flow control (like TCSC but faster).

  • Damping of power oscillations (modulate $\Delta X$).

  • Loop flow control.

• Advantages over TCSC: Faster response, no SSR risk, independent of line current magnitude.

• Limitations: Requires DC source (capacitor), higher cost.

[!TIP] SSSC can reverse power flow by injecting negative reactance ($-\Delta X$), unlike TCSC which only reduces net reactance.

Unified Power Flow Controller (UPFC)

• Schematic Diagram:

  
  DiagramSEARCH: UPFC schematic diagram
  
  

  • Main Components:

    1. Shunt Converter (STATCOM): Connected to bus, provides voltage support.

    2. Series Converter (SSSC): Connected in series with line.

    3. Common DC Link: Shared capacitor.

• Operation: Simultaneous control of:

  • Voltage (via shunt converter).

  • Line Impedance (via series converter voltage injection).

  • Phase Angle (via series voltage phase shift).

• Applications:

  • Flexible power flow management.

  • Voltage stability improvement.

  • Damping of inter-area oscillations.

  • Load sharing in parallel lines.

• Advantages: Most versatile FACTS device; decouples control of P, Q, V.

[!TIP] UPFC combines STATCOM + SSSC → can control all three power flow parameters (V, X, δ). Most expensive but most flexible.

Comparison of FACTS Devices

Selection Criteria:

• Need shunt vs series vs combined.

• Required response speed.

• Budget constraints.

• Grid strength (weak grid → STATCOM/UPFC).

• Specific issue (voltage instability → shunt; power flow → series; both → UPFC).

[!TIP] For voltage support in weak grids, choose STATCOM over SVC. For simultaneous control of P and Q, choose UPFC. TCSC is obsolete in new designs due to SSR risk; prefer SSSC.

END OF UNIT 4 NOTES
Focus on diagrams from past papers: Converter station layout, control characteristics, UPFC schematic, MTDC configurations.