GE Description of the 889 Generator Protection System

Description of the 889 Generator 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 digital 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 offnominal

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 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 through 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.

Processing of AC Current Inputs

The DC Decaying Removal Filter is a short window digital filter, which removes the DC

decaying 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 use the

same DC Decaying Removal Filter. This filter ensures no overreach of the overcurrent

protection.

The Discrete Fourier Transform (DFT) uses exactly one cycle of samples to calculate a

phasor quantity which represents the signal at the fundamental frequency; all harmonic

components are removed. All subsequent calculations (e.g. power, etc.) are based upon the

current and voltage phasors, such that the resulting values have no harmonic

components. RMS (root mean square) values are calculated from one cycle of samples

prior to filtering.

Protection Elements

All voltage, current and frequency protection elements are processed eight times every

cycle to determine if a pickup has occurred or a timer has expired. The voltage and current

protection elements use RMS current/voltage, or the magnitude of the phasor.