AC capacitor bank CCSC PLUS

high-currenthigh-voltagefor three-phase

Overview
Converter Controlled Series Capacitors (CCSC PLUS) provide dynamic series compensation and advanced power flow control for high-voltage AC transmission grids. CCSC PLUS combines proven series capacitor technology with a converter branch to deliver dynamic power flow control directly in series with the transmission line, without a high-voltage transformer. The platform is designed for transmission corridors operating at up to 400 kV and line current ratings up to 4,000 A.

Why transmission grids need dynamic power flow control
Three converging forces increase the need for dynamic power flow control: rising demand (faster than new lines can be built), generation moving further from load, and less predictable power flows caused by increasing variable generation. The result is structural congestion on some circuits while parallel paths remain underutilized. Improving utilization of existing infrastructure mitigates investment costs and avoids lengthy new-build permitting and construction.

Limitations of conventional series compensation

• Fixed Series Capacitors (FSC): fixed impedance set at commissioning and cannot respond to changing conditions.
• Phase Shifting Transformers (PST): respond in seconds to minutes, not fast enough for dynamic control.
• Thyristor Controlled Series Compensation (TCSC): adds controllability but injects harmonic content and offers a limited inductive range.

What is CCSC PLUS?
CCSC PLUS is a platform-based solution for dynamic series compensation in AC transmission grids. It pairs a fixed series capacitor bank with a containerized voltage source converter branch based on modular multilevel converter (MMC) technology to enable real-time, bidirectional impedance control (capacitive and inductive) and advanced grid-support functions. By eliminating the need for a high-voltage transformer, CCSC PLUS reduces footprint, installed cost and lead time compared with transformer-coupled alternatives.

System architecture
CCSC PLUS combines two main elements on a single platform:

• Fixed capacitor branch: installed directly in series with the transmission line to provide capacitive compensation via fixed impedance.
• Converter branch (VSC): containerized Voltage Source Converter (VSC) based on Modular Multilevel Converter (MMC) technology, installed in parallel with the capacitor bank on the same platform to control the voltage across the capacitor and dynamically adjust effective impedance seen by the line.

Control modes
The converter branch enables two primary control modes:

• Impedance control mode: maintains a constant impedance setpoint regardless of changes in line current to redistribute power flow between parallel transmission paths.
• Line current control mode: adjusts impedance compensation continuously to maintain a target line current setpoint for precise power flow management.

Additional grid-support functions
Beyond steady-state power flow control, CCSC PLUS provides:

• Power Oscillation Damping (POD) to act against inter-area oscillations and improve dynamic stability.
• Sub-synchronous resonance (SSR) and sub-synchronous torsional interaction (SSTI) mitigation to protect generator shafts and converter-interfaced plants.
• Emergency Dispatch: maximize power flow on the remaining circuit during N-1 contingencies to support supply security.
• Line overload management: redistribute flow away from thermally constrained circuits in real time without operator intervention.

Use cases
Common applications where CCSC PLUS is applied:

• Congestion management on parallel transmission corridors by dynamically redistributing flows.
• Integrating large renewable hubs (offshore wind, solar) into constrained grids, increasing hosting capacity without line upgrades.
• Grid stabilization and contingency response (POD, SSR mitigation, emergency dispatch after faults).
• Deferring costly grid reinforcement by extracting more capacity from existing lines.

Benefits for grid operators and engineers

• Maximize existing grid capacity without new line construction; reduce redispatch costs via automated, millisecond-speed flow redistribution.
• Smaller substation footprint and simpler civil works due to no high-voltage transformer requirement.
• Platform-based design (containerized converter technology) for factory testing, reduced commissioning time and predictable EPC scope.
• Freely programmable control enabling updates to control modes and setpoints as grid conditions and codes evolve.

Engineering, project and lifecycle services
CCSC PLUS is supplied and supported across the full asset lifecycle: system studies and grid integration (load flow, stability, SSR/SSTI analysis, validated models), full EPC scope including factory acceptance testing (FAT) and site commissioning, integration with SCADA/EMS systems, and lifecycle support (remote monitoring, diagnostics, software/control updates, spare parts and preventive maintenance).

Caractéristiques / spécifications techniques

• System voltage: Up to 400 kV (higher voltages on request)
• Line current rating: Up to 4,000 A
• Impedance control: Capacitive and inductive (bidirectional)
• Additional functions: Power Oscillation Damping (POD), SSR / SSTI mitigation, Emergency dispatch, Line overload management
• Converter technology: Voltage Source Converter (VSC) / Modular Multilevel Converter (MMC)
• HV transformer: Not required
• Platform design: Containerized / site-assembled
• Grid code compliance: N-1 security, power oscillation damping requirements
• Control system: Based on proven SVC PLUS® (STATCOM) control platform
• Typical installation: Series with transmission line; converter branch (VSC) in parallel with capacitor bank on same platform