GE 850 Feeder Protection System

GE 850 Feeder Protection System

Overview

Each relay provides protection, control, and monitoring functions with both local and  remote human interfaces. They also display the present trip/alarm conditions, and most of  the more than 35 measured system parameters. Recording of past trip, alarm or control  events, maximum demand levels, and energy consumption is also performed.

These relays contain many innovative features. To meet diverse utility standards and  industry requirements, these features have the flexibility to be programmed to meet  specific user needs. This flexibility will naturally make a piece of equipment difficult to  learn. To aid new users in getting basic protection operating quickly, setpoints are set to  typical default values and advanced features are disabled. These settings can be  reprogrammed at any time.

Programming can be accomplished with the front panel keys and display. Due to the  numerous settings, this manual method can be somewhat laborious. To simplify  programming and provide a more intuitive interface, setpoints can be entered with a PC  running the EnerVista 8 Setup software provided with the relay. Even with minimal  computer knowledge, this menu-driven software provides easy access to all front panel  functions. Actual values and setpoints can be displayed, altered, stored, and printed. If  settings are stored in a setpoint file, they can be downloaded at any time to the front panel  program port of the relay via a computer cable connected to the serial port of any  personal computer.

Description of the 850 Feeder Protection System

CPU

Relay functions are controlled by two processors: a Freescale MPC5125 32-bit  microprocessor that measures all analog signals and digital inputs and controls all output  relays, and a Freescale MPC8358 32-bit microprocessor that controls all the advanced  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 11.5 kHz  low pass analog 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 16-bit A/D converter before finally being passed on to the CPU for analysis.

The '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 8 Series Setup software for display and diagnostics.

Frequency

Frequency measurement is accomplished by measuring the time between zero crossings  of the composite signal of three-phase bus voltages, line voltage or three-phase currents.  The signals are passed through a low pass filter to prevent false zero crossings. Frequency  tracking utilizes the measured frequency to set the sampling rate for current and voltage  which results in better accuracy for the Discrete Fourier Transform (DFT) algorithm for off nominal frequencies.

The main frequency tracking source uses three-phase bus voltages. The frequency  tracking is switched automatically by an algorithm to the alternative reference source, i.e.,  three-phase currents signal or line voltage for the configuration of tie-breaker, if the  frequency detected from the three-phase voltage inputs is declared invalid. The switching  will not be performed if the frequency from the alternative reference signal is detected  invalid. Upon detecting valid frequency on the main source, the tracking will be switched  back to the main source. If a stable frequency signal is not available from all sources, then  the tracking frequency defaults to the nominal system frequency.

Phasors, Transients, and Harmonics

All waveforms are processed eight times every cycle with a DC decaying removal 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 and one that will not overreach.