What is Static VAR Compensator (SVC)? Circuit & Operation in PF Correction

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What is a Static VAR Compensator (SVC)?

A Static VAR Compensator (SVC), also termed a Static Reactive Compensator, is a critical device for enhancing the power factor in electrical power systems. As a type of static reactive power compensation equipment, it injects or absorbs reactive power to maintain optimal voltage levels, ensuring stable grid operation.

An integral part of the Flexible AC Transmission System (FACTS), an SVC comprises a bank of capacitors and reactors controlled by power electronics like thyristors or Insulated Gate Bipolar Transistors (IGBTs). These electronics enable rapid switching of capacitors and reactors to inject or absorb reactive power as required. The SVC's control system continuously monitors system voltage and current, adjusting the device's reactive power output in real time to counteract fluctuations.

SVCs primarily address reactive power variations caused by fluctuating load demands or intermittent generation (e.g., wind or solar power). By dynamically injecting or absorbing reactive power, they stabilize voltage and power factor at the point of connection, ensuring reliable power delivery and mitigating issues like voltage sags or swells.

Construction of SVC

A Static VAR Compensator (SVC) typically comprises key components including a Thyristor-Controlled Reactor (TCR), Thyristor-Switched Capacitor (TSC), filters, a control system, and auxiliary devices, as detailed below:

Thyristor-Controlled Reactor (TCR)

The TCR is an inductor connected in parallel with the power transmission line, regulated by thyristor devices to control inductive reactive power. It enables continuous adjustment of reactive power absorption by varying the thyristor firing angle.

Thyristor-Switched Capacitor (TSC)

The TSC is a capacitor bank also connected in parallel with the grid, controlled by thyristors to regulate capacitive reactive power. It provides discrete reactive power injection in steps, ideal for compensating steady-state load demands.

Filters and Reactors

These components mitigate harmonics generated by the SVC's power electronics, ensuring compliance with power quality standards. Harmonic filters typically target dominant frequency components (e.g., 5th, 7th harmonics) to prevent grid contamination.

Control System

The SVC's control system monitors grid voltage and current in real time, adjusting TCR and TSC operations to maintain target voltage and power factor. It features a microprocessor-based controller that processes sensor data and sends firing signals to thyristors, enabling millisecond-level reactive power compensation.

Auxiliary Components

Includes transformers for voltage matching, protective relays for fault isolation, cooling systems for power electronics, and monitoring instruments to ensure reliable operation.

Working Principle of Static VAR Compensator

An SVC regulates voltage and reactive power in power systems using power electronics, operating as a dynamic reactive power source. Here’s how it functions:

Reactive Power Management
The SVC combines a TCR (inductive) and TSC (capacitive) in parallel with the grid. The TCR can absorb reactive power by adjusting thyristor firing angles, while the TSC injects reactive power in discrete steps. This combination allows bidirectional reactive power control:
- Voltage Sag: When grid voltage drops, the SVC injects capacitive reactive power via TSC to raise voltage.
- Voltage Surge: When voltage exceeds the setpoint, the SVC absorbs reactive power via TCR to lower voltage.
Continuous Monitoring & Adjustment
Sensors measure real-time voltage and current, feeding data to the control system. The controller calculates the required reactive power and adjusts thyristor firing angles to maintain voltage stability within ±2% of the nominal value.
Harmonic Mitigation
The TCR’s switching action generates harmonics, which are filtered by passive LC filters (e.g., 5th, 7th harmonic filters) to ensure grid compliance.

Advantages of SVC

Enhanced Power Transmission: Increases line capacity by up to 30% through reactive power compensation.
Transient Stability: Damps voltage fluctuations during faults or load changes, improving system resilience.
Voltage Control: Manages steady-state and temporary overvoltages, ideal for renewable energy integration.
Reduced Losses: Improves power factor (typically to >0.95), cutting resistive losses by 10–15%.
Low Maintenance: Solid-state design with no moving parts, reducing operational costs.
Power Quality Improvement: Mitigates voltage sags/swells and harmonic distortion.

Applications of SVC

High-Voltage Transmission Grids: Stabilizes voltage in EHV/UHV lines (380 kV–1,000 kV) and compensates for long-line capacitive charging.
Industrial Plants: Corrects power factor in heavy inductive loads (e.g., steel mills, mining equipment) to reduce utility costs.
Renewable Energy Integration: Mitigates voltage fluctuations from wind farms or solar parks.
Urban Distribution Networks: Improves voltage stability in densely populated areas with fluctuating loads.
Railway Systems: Compensates for reactive power variations in electrified rail networks.

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