Bus Capacitor Bank
How is a bus capacitor bank differentiated from feeder or load capacitors in design and protection?
A bus capacitor bank is connected at the bus level (e.g., main switchgear bus), seeing aggregate load behavior and exposed to system-wide transients and harmonics. It requires coordinated assemblies with integrated switching, overcurrent protection, and unbalance detection (e.g., ABB REV615 relay). Feeder or motor capacitors have simpler protection and lower switching duty.
What are the key design parameters for a bus capacitor bank in accordance with CSA/IEC standards?
Key parameters include rated voltage (matching system voltage, e.g., ≤46 kV per CSA C22.2 No. 190), frequency (15–60 Hz for power factor, up to 50 kHz for induction heating), reactive power rating (kVAR based on desired power factor), insulation/BIL levels, connection type (e.g., wye, double-Y, H-bridge), discharge devices, and enclosure type (open-rack, enclosed-terminal, or metal-enclosed).
What transient and harmonic phenomena are specific to bus capacitor banks, and how are they mitigated?
Frequent switching causes high inrush currents and overvoltages, while harmonic resonance on buses with non-linear loads amplifies harmonics. Mitigation includes using detuned banks (capacitor + reactor) to shift resonance, sequencing switching with pre-insertion resistors, employing protection relays for unbalance/overcurrent, and performing a harmonic impact assessment before installation.
A bus capacitor bank is an assembly of one or more power-factor-correction or filter capacitors, complete with bus conductors, interconnections, fusing/switching devices, discharge means, and supporting structure, connected directly to an electrical bus (e.g., switchgear, MCC, panelboard, or substation bus). It provides reactive power, voltage support, or harmonic filtering for all loads served by that bus, typically in shunt connection.
On a shop floor, bus capacitor banks are installed on main distribution or MCC buses to improve power factor and reduce line current for inductive loads like motors and welders, avoiding utility penalties. They may be fixed or automatically switched based on bus power factor. For plants with non-linear loads (e.g., VFDs), detuned or tuned filter banks (capacitors with reactors) are used to prevent harmonic resonance. In HV substations, they support voltage and VARs, often with protection relays. Safety requires isolation, discharge using sticks, and maintaining approach distances to prevent shock or arc-flash incidents.
Inadequate discharge and verification before work: assuming built-in bleeder resistors are sufficient without time verification or using discharge sticks on each HV bushing, leading to shock or arc-flash risks.
Installing banks on non-linear load buses without harmonic assessment: standard shunt banks on VFD or rectifier buses can cause resonance, overcurrent, and equipment damage, violating due diligence obligations.
Access, guarding, and approach-distance violations around HV bus banks: bypassed interlocks, missing barriers, or workers entering prohibited boundaries without authorization or PPE, contrary to electrical safety rules.