GE 350 Feeder Protection System

350 Feeder Protection System

The 350 is a microprocessor-based relay for primary and backup over-current protection of medium and low voltage distribution feeders. The relay is also suitable for providing over-current and backup protection for small and medium size motors, transformers, generators, and distribution bus-bars. The small footprint and the withdrawable option make the 350 relay ideal for panel mounting on either new or retrofit installations. The combination of proven hardware, a variety of protection and control features, and communications, makes the relay ideal for total feeder protection and control. Equipped with serial (RS485), USB, and Ethernet ports with the possibility of adding redundancy (IEC62439, PRP and HSR), and a wide selection of protocols such as Modbus, DNP3.0, IEC 60870-5-103, 60870-5-104, IEC61850 GOOSE, OPC-UA, the 350 relay is the best-in-class for MCCs and PCCs, SCADA and inter-relay communications. The 350 relay provides excellent transparency with respect to power system conditions and events, through its four-line 20-character display, as well as the EnerVista SR3 Setup program. Conveniently located LEDs provide indication of relay operation, alarm, and pickup, as well as breaker, and relay status.

The 350 relay provides the following key benefits:

•Withdrawable small footprint – saves on rewiring and space. (non-draw out version is also available)

•Multiple protection groups with the added flexibility of switching through a wide selection of overcurrent protection and control features.

•Fast setup (Quick Setup) menu for power-system setup and a simple overcurrent protection configuration.

•Large four-line LCD display, LEDs, and an easy-to-navigate keypad.

•Multiple communication protocols for simultaneous access when integrated into monitoring and control systems.

CPU

Relay functions are controlled by two processors: a Freescale MPC5554 32-bit microprocessor measures all analog signals and digital inputs and controls all output relays; a Freescale MPC520B 32-bit microprocessor controls all the Ethernet communication protocols.

Analog Input and Waveform Capture

Magnetic transformers are used to scale-down the incoming analog signals from the source instrument transformers. The analog signals are then passed through a 960 Hz low pass anti-aliasing filter. All signals are then simultaneously captured by sample and hold buffers to ensure there are no phase shifts. The signals are converted to digital values by a 12-bit A/D converter before finally being passed on to the CPU for analysis.

Both current and voltage are sampled thirty-two times per power frequency cycle. These ‘raw’ samples are scaled in software, then placed into the waveform capture buffer, thus emulating a fault recorder. The waveforms can be retrieved from the relay via the EnerVista SR3 Setup software for display and diagnostics.

Frequency

Frequency measurement is accomplished by measuring the time between zero crossings of the Bus VT phase A voltage . The signals are passed through a low pass filter to prevent false zero crossings. Sampling is synchronized to the Va-x voltage zero crossing which results in better co-ordination for multiple 350 relays on the same bus.

Phasors, Transients, and Harmonics

Current waveforms are processed four times every cycle with a DC Offset Filter and a Discrete Fourier Transform (DFT). The resulting phasors have fault current transients and all harmonics removed. This results in an overcurrent relay that is extremely secure and reliable; one that will not overreach.

Processing of AC Current Inputs

The DC Offset Filter is an infinite impulse response (IIR) digital filter, which removes the DC component from the asymmetrical current present at the moment a fault occurs. This is done for all current signals used for overcurrent protection; voltage signals bypass the DC Offset Filter. This filter ensures no overreach of the overcurrent protection.

The Discrete Fourier Transform (DFT) uses exactly one sample cycle to calculate a phasor quantity which represents the signal at the fundamental frequency; all harmonic components are removed. All subsequent calculations (e.g. RMS, power, etc.) are based upon the current and voltage phasors, such that the resulting values have no harmonic components.