Converter controlled series capacitors (CCSC PLUS)

Converter controlled series capacitors (CCSC PLUS) provide dynamic series compensation and advanced power flow control for high-voltage AC transmission grids. CCSC PLUS is part of Siemens Energy's Flexible AC Transmission Systems (FACTS) portfolio. It combines proven series capacitor technology with a converter branch – drawn from Siemens Energy's SVC PLUS® and HVDC expertise – to deliver dynamic power flow control directly in series with the transmission line, with no high-voltage transformer required. The platform is designed for transmission corridors operating at up to 400 kV and line current ratings up to 4,000 A.

Three forces are converging on transmission networks at the same time: demand is rising faster than new lines can be built, generation is moving further from load, and power flows are becoming harder to predict. The result is congestion on some circuits while parallel paths run below their thermal limit.

Growing demand is straining existing infrastructure. Transmission system operators (TSOs) are facing a sharp increase in electricity demand, driven by additional loads from data centers, the electrification of industrial processes, and EV charging. Much of this demand lands on infrastructure that was planned for a different load profile.

The construction of new power lines, or the refurbishment of existing ones, is very costly and time-consuming. Therefore, improving the utilization of existing infrastructure can help mitigate investment costs. Permitting, land rights and construction span years rather than months, so TSOs increasingly need to release capacity from the corridors they already own before committing to new build.

Congestion is no longer an occasional peak-hour event – it is a structural condition of networks carrying large volumes of variable generation:

• As more decentralized and renewable generation enters the grid, power flows become less predictable and harder to manage
• Thermal overloads on individual lines coexist with underutilized parallel paths – a structural inefficiency of conventional devices cannot be solved.
• Redispatch interventions are costly and becoming more frequent; TSOs need tools that manage congestion automatically and in real time. 

Fixed Series Capacitors (FSC) and phase-shifting transformers (PST) remain key assets in modern grid infrastructure, but neither delivers fast dynamic series compensation: FSC impedance is set at commissioning and cannot respond to changing conditions, and a PST responds in seconds to minutes. 

Thyristor controlled series compensation (TCSC) adds controllability, but injects harmonic content that requires filtering and offers only a limited inductive range.

A new class of grid enhancing technology is needed: one that combines the reliability of series capacitors with fast, converter-based control, and that stays cost-effective at the line ratings where modular series compensators become impractical.

Because no high-voltage transformer is required, CCSC PLUS offers a shorter lead time, a competitive installed cost and a smaller substation footprint than transformer-coupled alternatives – while providing full dynamic control capability.

Key positioning points:

• Part of Siemens Energy's FACTS portfolio, alongside SVC PLUS® (STATCOM), SVC PLUS® FS (E-STATCOM), FSC, and Synchronous Condensers.
• Platform-based: benefits from containerized SVC PLUS® (Prefab) converter technology.
• Designed for line voltages up to 400 kV and line current ratings up to 4000 A (higher voltage available on request).

CCSC PLUS combines two well-established Siemens Energy building blocks on a single line-voltage platform: a series capacitor bank and a containerized voltage source converter. The sections below cover the system architecture, the two control modes, and the additional grid-support functions the converter branch enables.

CCSC PLUS has two main elements installed on a single platform:

• Fixed capacitor branch: installed directly in series with the transmission line. Provides capacitive compensation via fixed impedance, increasing the power flow capability of the line.
• Converter branch (VSC): installed in parallel with the capacitor bank on the same platform. A containerized Voltage Source Converter (VSC) based on Modular Multilevel Converter (MMC) technology controls the voltage across the capacitor, dynamically adjusting the effective impedance seen by the line.

No high-voltage transformer is required. The platform design draws directly on Siemens Energy’s FSC and containerized SVC PLUS® experience, both of which are in service in transmission networks worldwide.

Additional grid-support functions

Beyond steady-state power flow control, CCSC PLUS provides:

• Power Oscillation Damping (POD): an additional control loop acts against inter-area power oscillations, improving dynamic stability across the transmission corridor.
• Sub-synchronous resonance (SSR) mitigation: dynamic impedance control counteracts SSR and SSTI (sub-synchronous torsional interaction), which can damage generator shafts, as well as SSCI (sub-synchronous control interaction) between series compensation and inverter-based resources such as wind and solar plants.
• Emergency Dispatch: in N-1 fault scenarios (loss of a parallel line), the system can be commanded to maximize power flow on the remaining line, supporting grid resilience and N-1 security requirements.
• Line overload management: impedance adjustment redistributes flow away from thermally constrained circuits in real time, without operator intervention.

CCSC PLUS is applied wherever a transmission corridor is limited by how power distributes itself rather than by the physical rating of the conductors. The four scenarios below are the most common starting points for a system study.

Where two or more lines run in parallel, power flow naturally concentrates on the lower-impedance path – leaving the higher-impedance path underutilized. CCSC PLUS redistributes flow dynamically between parallel circuits, eliminating overloads without new infrastructure. This directly reduces redispatch frequency and cost.

Large offshore wind farms and solar hubs often connect into grid corridors with limited dynamic control capability. CCSC PLUS enables higher renewable penetration by redirecting power flows away from bottlenecks in real time, improving hosting capacity without line upgrades.

• POD (Power oscillation damping): CCSC PLUS damps inter-area oscillations that can destabilize interconnected grids – a function critical as synchronous generation declines and grid inertia falls.
• SSR/SSTI (Sub-Synchronous Resonance / Sub-Synchronous Torsional Interaction) mitigation: Particularly relevant for grids with long radial lines and thermal generation, and for corridors with high shares of inverter-based generation, where sub-synchronous interactions pose a risk to generator shafts and to converter controls.
• Emergency dispatch after N-1 faults: Following the loss of a parallel line, CCSC PLUS can immediately maximize power transfer on the remaining circuit, maintaining supply security.

By extracting more capacity from existing lines, CCSC PLUS enables TSOs to defer or avoid capital-intensive grid reinforcement programs – delivering the economic case for grid enhancing technology investment at a fraction of the cost of new construction.

The current CCSC PLUS platform covers the majority of high-voltage transmission applications. Applications above these ratings can be discussed with the Siemens Energy engineering team.