Capacitor Bank Integration with Switchgear Systems

Point-on-Wave Switching vs Legacy Systems Before advanced point-on-wave switching (PoW) became widely adopted, capacitor bank energisation was typically managed using zero-voltage crossing switching. While effective in certain cases, these systems only operate correctly when capacitor banks are fully discharged. This creates a key limitation. Banks must be left to discharge before they can be safely re-energised, restricting their use in dynamic applications. Point-on-wave switching removes this constraint. By precisely controlling the switching instant relative to both the voltage waveform and the capacitor bank’s trapped charge, PoW significantly reduces inrush currents and enables rapid, controlled re-energisation without waiting for discharge. This unlocks a new capability: Using capacitor banks in dynamic reactive power control, a role traditionally reserved for STATCOM systems. When combined with modern turbine control systems, PoW-switched capacitor banks can deliver comparable dynamic performance, with lower upfront cost and minimal ongoing maintenance. We applied this approach on the Fallago Rig project, replacing an ageing STATCOM with a PoW-based solution. If you are interested in how this was implemented in practice, the case study is linked in the first comment.

#LT Distribution system This is a single-line diagram of an LT (Low Tension) main distribution panel that receives power from two transformers and a DG set, then distributes it to sub‑LT panels and power factor correction (capacitor bank / APFC) panels. #Main incomers and bus bars - On the left and right top sides are incomers from Transformer‑1 and Transformer‑2, each coming through an ACB (Air Circuit Breaker) and meter into the main LT Panel‑1 bus bars (three colored lines for three phases plus neutral). - These incomers feed a common bus system; the horizontal colored lines across the diagram represent the main LT bus bars that carry power to all outgoing feeders and capacitor banks. #Bus coupler and redundancy - In the middle is a bus coupler ACB labelled “Bus Coupler Arrangement,” which can connect or isolate the left and right sections of the LT bus. - With the bus coupler open, each transformer side can operate independently; with it closed, both transformer incomers can share the total load, improving reliability and flexibility during maintenance or heavy demand. #Outgoing feeders to sub‑LT panels - Several ACBs on each side of the bus feed “Sub‑LT Panel‑1 Switchgear” and “Sub‑LT Panel‑2 Switchgear,” which in turn supply Sub‑LT Panel‑1 and Panel‑2 via outgoing cables. - These sub‑LT panels further distribute power to various loads and distribution boards (machinery, lighting, HVAC, etc.), acting as secondary distribution centers in the plant or facility. #APFC panels - Below each main bus section are “Capacitor Bank‑1” and “Capacitor Bank‑2” connected to the LT bus via ACBs and linked to APFC (Automatic Power Factor Control) panels. - The APFC panel measures power factor via CTs/PTs and automatically switches these capacitor banks in or out to supply reactive power, keeping the overall power factor near unity and reducing kVA demand and penalties. #DG set incomer and operation - At the bottom, a DG set incomer ACB connects the diesel generator output to the same LT bus bars, providing backup or standby supply when transformer (utility) power is unavailable or insufficient. - During mains failure, the DG ACB closes and, depending on the scheme, one or both transformer incomers open, allowing the DG to feed selected sub‑LT panels while maintaining proper protection and power factor correction via the same bus and capacitor arrangement. ~Thank you Mayuk Shome(DEE)

●●● Medium Voltage (MV) capacitor bank protection involves a comprehensive scheme using several methods and dedicated relays to detect both external faults and internal component failures. ● Key Protection Methods: The primary methods for protecting MV capacitor banks are: • Individual Capacitor Fusing: Each capacitor unit or element is typically protected by its own internal or external current-limiting fuse. The fuses are primarily selected for short-circuit protection and must withstand normal inrush currents when the bank is energized. When an element fails, its fuse blows, isolating it from the rest of the bank to prevent case rupture and a cascading failure. • Unbalance Protection 46 : When a fuse blows, it changes the capacitance of a phase, causing an imbalance in the system. Relays (e.g., using overcurrent relay function 46C or a neutral current unbalance function 50UB) detect this asymmetry by measuring current in the neutral connection of a double-star configuration or across a center-tapped reactor. The unbalance relay is set to: - Initiate an alarm when a minimum number of elements fail, prompting maintenance. - Initiate a trip command to isolate the entire bank if the imbalance exceeds a maximum safe limit to prevent overvoltage across healthy units. • Overcurrent and Short Circuit Protection 50/51: Standard two- or three-phase overcurrent relays and earth fault relays are applied on the line side of the bank to protect against external phase-to-phase or phase-to-ground faults. • Overload Protection 49 : While modern capacitors have low losses, overload can occur due to excessive total peak voltage caused by fundamental frequency voltage and harmonic currents. Relays designed for this purpose (e.g., function 49OL) measure the current and transform it into an equivalent voltage across the elements to ensure the 110% continuous overvoltage rating is not exceeded. • Overvoltage Protection 59 : Relays may monitor busbar or capacitor voltage to protect against sustained overvoltage conditions. Circuit Breaker: A dedicated circuit breaker is used to switch the bank on and off and to disconnect it during a fault condition. The circuit breaker must be rated to handle the potentially high inrush currents and prevent restrikes. • Current-Limiting Reactors: Inrush currents can be very high (up to 25 times the rated current) but typically last for a very short duration (< 1/4 cycle). Current-limiting reactors may be installed in series with the capacitor bank to limit these surges and other excessive currents. • Lock out relay 86 • earthing fault relay 64