Protection and control
Protection relays
Numerical relays that measure currents and voltages from instrument transformers and trip the breaker when the scheme's conditions are met.
- Standard
- IEC 60255 seriesmeasuring relays and protection equipment
- Inputs
- current and voltage transformersratios and classes set by the scheme
- Functions
- overcurrent, earth fault, differential and moreset by the protected equipment
- Communication
- protocol set by the substation system
- Settings
- applied at commissioningagreed with the network operator where required
A protection relay is specified by the equipment it protects and the scheme it sits in. A feeder relay commonly provides overcurrent and earth fault protection. A transformer relay adds differential protection, and a motor relay adds thermal and starting supervision.
Specifications often name the functions by their ANSI device numbers, for example 50 and 51 for overcurrent and 87 for differential protection.
The relay depends on what feeds it. The current and voltage transformer ratios and classes have to match, and so do the auxiliary supply voltage and the communication protocol of the substation system.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Protected equipment | Feeder, transformer, motor, generator, busbar | The single line diagram |
| Protection functions | Overcurrent, earth fault, differential, voltage, frequency | The protection scheme |
| Current and voltage inputs | 1 A or 5 A secondary, voltage inputs | The instrument transformers |
| Auxiliary supply | DC or AC voltage | The station battery or auxiliary system |
| Communication | Substation protocol and ports | The control system |
| Mounting | Flush panel mounting, cut out size | The panel, especially for replacements |
| Inputs and outputs | Number of binary inputs and output contacts | Interlocking and alarm needs |
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 60255-1 | The common requirements for the relay as a product |
| IEC 60255-151 | Functional requirements for over and under current protection, the functions most feeder relays carry |
| IEC 61869-2 | The current transformers that feed the relay |
| IEC 61850 | Communication networks and systems for power utility automation, where the substation system uses it |
What makes a relay usable on the day
| Item | Why it matters |
|---|---|
| Firmware version and settings file format | Your configuration tool has to read and write the relay |
| Communication configuration | The protocol and data model your station system expects |
| Cut out and terminal drawing | A replacement has to fit the existing panel door and wiring |
What a protection relay request needs
- The protected equipment, and the functions by name or ANSI number
- Current transformer secondary, 1 A or 5 A, and the voltage inputs
- The auxiliary supply voltage
- The station's communication protocol
- For a replacement: the old relay's model, firmware and cut out size
Questions buyers ask
Who calculates the protection settings?
Your protection engineer calculates them from the network study, and the party that commissions the scheme applies them. The relays we source carry the functions and interfaces that scheme needs.