What are Numerical Protection Relays?

Numerical protection relays protect power transformers and distribution systems from various types of faults. For power transformers, these faults include protection from distance, line differential, pilot wire, low-impedance busbar, high-impedance differential, frequency, voltage, failure of circuit breaker, auto reclosing, and synchronism faults. For power distribution systems, these faults include protection from overcurrent, under or overvoltage, directional overcurrent’s, and feeder manager relay faults.

Numerical protection relays are digital systems in constant communication with substation automation systems through menu-driven interfaces. They have configurable binary inputs, outputs, and programmable logic. They monitor, measure, and record electrical values, fault and disturbances, and events. Numerical protection relays feature high-speed operation and multi-functionality, offering improved selectivity and stability. As they detect faults with automatic supervision, they bring high reliability to power systems, while at the same time being compact in size and consuming very low power.

Numerical protection relays have a multiple microprocessor design. Each microprocessor within the relay performs software functions such as executing protection algorithms and scheme logic, processing signals from sensors, controlling output relays, and handling the human interface.

The relay handles several analog inputs such as phase control inputs, phase voltage inputs, and residual current inputs. Depending on the type of relay, the number of analog inputs may vary.

Internal auxiliary transformers isolate the electronics from the high voltage on the system—isolating, generating step down voltages, and conditioning the inputs from the voltage and current transformers. Analog to digital converters transform these analog signals into digital data, which the microprocessors can process further.

The front panel of a numerical protection relay is a liquid crystal display (LCD) along with pushbutton keys providing local access to the relay menu. Light emitting diodes (LEDs) on the panel visually indicate the present status of the relay.

Three types of communication ports are available on a typical numerical protection relay-an RS232C port for locally connecting to a PC, an RS485 port for connecting to a remote PC, and an IRIC-B port for connecting an external clock.

The LCD exhibits information the relay is measuring continuously and simultaneously displays the same on the local PC, and the remote PC when connected. For instance, this information shows several voltages and currents such as phase, phase-to-phase, their symmetrical components, frequency, and active and reactive power. The type of relay defines the parameters it will measure and display. Users can monitor locally as well as remotely the element output of the relay and input/out binary values.

Within the relay, the software program records several events such as tripping operations, alarms, change of relay settings, change of state of each binary input/output, and failure detected by automatic supervision. Typically, the relay stores each time-tagged event with a 1 ms resolution. The user may define additional events for the system to record.

Apart from providing a date and time for tagging of records, a numerical protection relay records faults initiated by a relay trip and logs data such as date and time of the trip operation, operating phase, protection scheme that triggered the trip, and measured current data. The relay stores the eight most recent faults, time-tagged to 1 ms resolution.