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IEC 60358-2: Coupling Capacitors and Capacitor Dividers — From PLC to Electronic Instrument Transformers
Coupling Capacitors on HV Lines — Carrying Carrier Signals While Serving as Voltage Transformers
IEC 60358-2:2013 specifies coupling capacitors and capacitor dividers. HV transmission line coupling capacitors (CC) serve dual roles: (1) providing high-frequency signal injection for Power Line Carrier (PLC, 30–500 kHz) communication; (2) combining with an electromagnetic unit to form a Capacitor Voltage Transformer (CVT) for protection and metering voltage signals.
| Component | Function | Key Parameter |
|---|---|---|
| C1 (HV capacitor) | Withstands system voltage | Typically 5,000–20,000 pF |
| C2 (LV capacitor) | Provides division ratio | C1/C2 determines ratio |
| Electromagnetic Unit | Steps intermediate voltage down to standard secondary | 100/√3 or 110/√3 V |
The CVT transient response problem: During a primary voltage collapse (e.g., short-circuit), energy stored in the capacitor divider must discharge through the intermediate transformer — producing a decaying oscillation lasting several cycles. This is critical for distance protection — if the CVT transient response is too slow, the distance relay may overreach or underreach due to distorted voltage signals.
TNLab — The CVT is the most overlooked error source in protection and metering systems. Its transient response directly impacts distance protection reliability.
