Fixed series compensation
Fixed Series Compensation (FSC) is a proven transmission solution that increases the power transfer capability and overall efficiency of high-voltage networks.
By optimizing how transmission lines behave electrically, FSC enables utilities to deliver more power using existing infrastructure—supporting lower system costs, higher operational flexibility and smoother integration of renewables.
A key element of the FSC solution is the fast protective device CapThor™, featuring a hermetically sealed design. It overcomes the limitations of conventional spark gaps and delivers higher operational reliability.
Hitachi Energy is a pioneer in series compensation technology. The first FSC was delivered in 1948. Today, we have more than 340 FSCs installed worldwide, representing over 75 years of experience with this technology. These installations operate reliably across a wide range of demanding environments, including:
Ambient temperatures from –50°C to +50°C
Areas with high pollution levels
Regions with stringent seismic requirements
How it works
FSC works by managing the impedance of a transmission line. By reducing the line’s series reactance, the solution enhances the flow of active power. This makes FSC an important tool for strengthening existing corridors and improving the performance of long-distance transmission systems. Enhanced voltage and angular stability ensures that generators and other grid components remain synchronized, minimizing the risk of outages
Design overview
All main components of the FSC, including the capacitor bank, Current Limiting Damping Equipment (CLDE), measuring and overvoltage protection devices, are installed on a fully insulated steel platform at system voltage. The CapThor™ Fast Protective Device (FPD) has a hermetically sealed design which addresses all the drawbacks of traditional spark gaps. The control and protection systems are located at ground level and connects via fiber optic links to the platform.
Key Advantages
Affordability
Lower upfront and ongoing costs of delivering power compared to building new transmission infrastructure
Timeliness
Accelerates project timelines with faster deployment and minimal construction delays
Sustainability
Reduces environmental impact by optimizing existing resources instead of expanding footprint
Fixed Series Compensation is the most efficient and sustainable way of boosting the transmission capacity of a transmission line. It increases active power flow, improves grid stability that ensures reliable and quality power supply to the consumers.
Frequently Asked Questions
The degree of series compensation (k = XC / XL) is typically between 30% and 60%, depending on line characteristics and stability margins.
Higher compensation increases power transfer but can also introduce sub synchronous resonance (SSR) risks if not carefully engineered.
Under certain network conditions, FSC can introduce Transient Recovery Voltage (TRV) challenges for line circuit breakers. Hitachi Energy's patented CapThor™ technology is designed to address and mitigate this issue effectively.
Gapped FSC provides overvoltage protection by using a fast protective device (FPD) in parallel with metal oxide varistors (MOVs). In contrast, Gapless FSC does not include a fast protective device (FPD); instead, it relies solely on MOV-based protection complemented by a fast bypass switch.
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