The aim of protection coordination is that, during a fault, only the protective device closest to the fault operates while the rest of the network stays energised. Poor coordination either causes unnecessary widespread outages or clears the fault too slowly and damages equipment.
The four attributes of a protection system
- Selectivity: disconnect the minimum faulty section.
- Speed: reduce fault energy and thermal and dynamic stress.
- Sensitivity: detect the lowest expected fault current, such as a resistive fault.
- Reliability: operate when needed (dependability) and not operate unnecessarily (security).
Study inputs
- Short-circuit study: maximum and minimum fault current at each bus (three-phase, phase-to-phase and single-phase)
- Transformer data (rating, impedance, vector group, inrush) and motor data (starting current and time)
- CT ratio and accuracy class, cable data (thermal withstand curve), existing fuses and relays
- Relay curve characteristics and operator constraints
Choosing pickup
- Phase overcurrent (50/51): pickup is typically 1.1 to 1.5 times the maximum load (or rated) current, above load and self-starting current and below the minimum fault current at the end of the protected zone. A ratio Ifault,min / Ipickup of at least about 1.5 is recommended.
- Instantaneous (50): usually set above the maximum fault current at the next (downstream) bus (about 1.2 to 1.3 times) to remain selective.
- Earth fault (50N/51N): set according to neutral earthing (solid, resistance, reactance or isolated) and can be far more sensitive than phase overcurrent.
- Sensitive (SEF) and directional (67N) earth fault: for resistance-earthed networks and parallel feeders.
Coordination time interval (CTI)
An adequate interval is needed between upstream and downstream relays to cover breaker time, relay overshoot and curve error.
| Relay type | Typical CTI |
|---|---|
| Digital relay with modern breaker | 0.2 to 0.3 s |
| Electromechanical relay | 0.35 to 0.5 s |
| Upstream relay with downstream fuse | Check against the fuse curve (about 0.2 s) |
IEC 60255-151 inverse-time curves
Operating time is calculated by t = TMS × k / (M^α − 1). SI, VI, EI and LTI curves are suited to general networks, networks with a large difference between load and fault current, coordination with fuses, and networks with long motor starting times respectively. In industrial networks with large motors, the curve must not cross the motor starting current and its duration.
Coordination steps
- Draw the single-line diagram and identify protection points from load to source.
- Calculate fault and load currents at each point.
- Set the relay at the end of the network (lowest level) with the fastest possible time.
- Set upstream relays respecting the CTI to the downstream device.
- Check for clashes with transformer inrush, motor starting and cable and transformer withstand curves.
- Plot the time–current (TCC) graph and perform the final review.
- Issue the settings table and agree it with the operator.
Software such as ETAP, DIgSILENT PowerFactory and SKM is used to calculate and plot curves.
Proof in practice
A correct coordination on paper is no guarantee until the settings are actually loaded into the relays and tested by secondary injection. After network changes (load growth, added generation, a different short-circuit level), the study must be repeated.
Common mistakes
- Setting only for maximum fault and forgetting minimum fault (no sensitivity)
- Ignoring the effect of reduced source strength on operating time
- Using factory-default settings without a study
- No coordination between a relay and a fuse, or between relays of different manufacturers
- CT saturation at high fault currents
ENP provides short-circuit and coordination studies, settings tables, and testing and application of settings in the relays.