Category: GE

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.

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 USB port of any personal

computer.

A summary of the available functions and a single-line diagram of protection and control

features is shown below. For a complete understanding of each feature operation, refer to

the About Setpoints chapter, and to the detailed feature descriptions in the chapters that

follow. The logic diagrams include a reference to every setpoint related to a feature and

show all logic signals passed between individual features. Information related to the

selection of settings for each setpoint is also provided.

Overview

The relay features generator unbalance, generator differential, over excitation, loss of

excitation, 3rd harmonic neutral undervoltage, over and under frequency, synchrocheck

and other essential functions with a basic order option. Additionally available with an

advanced order option are overall differential (to protect the transformer-generator

combined), directional overcurrent elements, restricted ground fault, 100% stator ground,

out-of-step protection, rate of change of frequency, power factor, harmonic detection,

frequency out-of-band accumulation and others. An optional RTD module allows for

thermal protection and monitoring. An optional analog inputs/outputs module allows for

monitoring of generator excitation current, vibration and other parameters.

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.

Operation

To put the 515 Blocking & Test Module into operation, additional parts have been provided:

• 1 package containing at least 28 terminal nuts

• 14 white tags for identification of each of the switches

The 515 provides a means of trip blocking, relay isolation, and testing of GE Multilin relays.

The 515 accomplishes this with a total of 14 switches. There are 6 single pole throw

switches for use with the output relays, and 4 groups of 2 switches each for use with the

current transformers as illustrated in Figure 1: Typical Wiring.

Isolation or opening of the relay’s output circuits is accomplished with the six switches

at terminals 1 through 6 and 15 through 20. These switches can be used to simply open

the protection relay’s output contacts and thus provide a means of blocking trips.

The 4 remaining groups of 2 switches at terminals 7 through 14 and 21 through 28 are

used for shorting of the CT inputs, injection of test current, and measuring of CT current.

The four groups correspond to Phase 1 Current, Phase 2 Current, Phase 3 Current, and Ground Current.

Each group is made up of two switches. The first switch for the Phase 1 group is at

terminal 7 and 21 and is configured as shown on the right:

Note that terminals 7 and 21 are shorted together regardless of whether the switch is open or closed.

The second switch for the Phase 1 group is at terminals 8 and 22. It is configured as shown on the right:

Note that this switch operates as make before break. When this switch is closed terminals 8 and 22 are

shorted together.

abs will be sufficient

to hold the 515 module securely in place. If additional fastening is desired bend out the clamping

screw tabs at both ends of the chassis at right angles.

Insert the #8 screws provided in the accessory pouch into the tapped holes with the vibration

proof nut between the tab and the panel. Tighten each screw until the end of the screw butts firmly against the front panel.

Ensure the nut is installed tightly against the bent tab. Nylon inserts in these nuts prevent them from vibrating loose.

The 515 test switch module should now be securely mounted to the panel with no movement ready for rear terminal wiring.

When completed, place the front cover over the mounted 515 test module and turn

the fasteners at both ends ¼ turn to lock it in place, as shown in Figure 3.

As a safety precaution, a ground screw located on the bottom-right of the

rear side of the module is available to be connected to panel chassis ground.

This in turn shorts the Phase 1 CT. It is essential that the CT is connected to the shorted side 

of the switch as shown on the right. Otherwise, dangerously high voltages would be present

at the open circuited current transformer. Also note that currents can be

injected into the protection relay from a secondary injection test set.

With the switch between terminals 7 and 21 open and the switch between terminals 8 and 22 closed as shown below,

a 515 test plug can be inserted between terminals 7 and 21 to monitor the CT current.

See the diagrams below for details. Note that the 515 test plug is made up of two conductors separated by an insulator.

Installation

Shorting switches are provided for connection of 3 phase CTs (current transformers) and

a separate core balance ground fault CT or 3 phase CTs connected for residual ground fault sensing.

When each CT switch is opened, the CT is shorted. It is essential that the CT is connected to the

shorted side of the switch as shown in the following figure,

otherwise dangerously high voltages would be present from the open circuited CTs.

When the switches are open, test plugs can be inserted to either inject signals into the

relay wired to the switches or monitor signals such as CT current from the switchgear.

The 515 Blocking and Test Module consists of a metal chassis attached to the 515 test switches that slides into the panel.

A single cutout in the panel, as per the dimensions shown in Figure 3, is required to mount the 515 test switches.

Slide the metal chassis attached to the 515 test switches into the cutout from the front of the panel.

While firmly applying pressure from the front of the 515 module to ensure the chassis fits snugly,

bend out the retaining tabs as shown in Figure 3. Usually the retaining tabs will be sufficient

to hold the 515 module securely in place. If additional fastening is desired bend out the clamping

screw tabs at both ends of the chassis at right angles.

Introduction

The GE Multilin 515 Blocking and Test Module has the following features:

• 14 Pole switchbank

• CT inputs short when current switches are opened

• Current injection for each phase

• Ground terminal

• Ability to visually isolate (open) trip relay output circuits

• Cover provided

• Suitable for utility and industrial use

• 515 test plugs available

Description

The 515 Blocking and Test Module provides an effective means of trip blocking,

relay isolation and testing of GE Multilin relays. By opening the switches and inserting test plugs,

phase and residual currents from the primary CTs can be monitored.

Currents can be injected into the relay from a secondary injection test set during commissioning.

Prior to testing, the trip and auxiliary circuits must first be opened to prevent nuisance tripping;

CTs can then be shorted. Conversely, when the test is complete and the relay put back into operation,

the CT switches should be closed first to ensure normal operation of the relay, prior to closing the trip and auxiliary circuits.

Closing time delay

The minimum closing time, set at 100 ms, is actually 160 ms, as it is necessary to add the time required by

the measurement units and the operation of the output relays to the set 100 ms. It should be kept in

mind that during the first cycles, from the time of a closure, the voltage is stabilizing both in the line as

well as in the buses and it is no desirable to allow a closure under these conditions. What is more, it is

necessary to take into consideration the response of both the high voltage transformers and the internal

transformers of the relay.

There is also a time for sustaining the permission signal, which always has a fixed value of some 130 ms.,

being the result of a prefixed delay of 100 ms. added to the dropout time of the measurement units (one

cycle = 20 ms. at 50 Hz.) and the deactivation of the output relay (10 ms.).

Minimum settings

Barring any special requirements, it is not recommended to use settings that near the minimum limits for

the relays, (2 V for the voltage difference, 2º for the angle difference), so as to avoid being too restrictive

in close enabling, given the real characteristics of accuracy in the installations, measurement transformers,

etc.

Influence of harmonics

The pillar of the MLJ measurement calculation is the discrete Fourier transform, which is in essence a

harmonics filter. For this reason the voltage and line measurements are not affected by frequencies other

than the fundamental.

The rejection of harmonics is added to the independence of measurements, both magnitude and phase,

relative to frequency signal variations, which is very important in a synchronism checking relay which, by its

own nature, works in variable frequencies.

Given that in power systems, synchronization or synchronism checking is carried out in a steady state, that

is with voltage magnitudes near or equal to the rated value, close enable is not emitted for very low voltages.

Therefore, for voltage of less than 9 volts, the relay stops measuring phase and frequency, not giving

permission to close under such conditions.

The MLJ also offers additional insensitivity to frequency measurement concerning harmonics, since this is

done via a hardware circuit, a zero-cross detector, with an intrinsic harmonics filter. Furthermore, it has a

software filter which operates by double-period measurement, both between the rising and falling edges,

averaging them out and allowing for better performance of algorithm frequency (improving security and

response).

DESIGN CHARACTERISTICS

Measurement accuracy

The differential angle measurement of the MLJ is high precision and is limited solely by errors in available

voltage transformers.

The measurement of the angle is practically independent of the voltage.

In the MLJ the measurement is obtained via a numerical calculation done on digital voltage samples, thus

achieving high precision. This allows for a rating of 2º, which is clearly better than the possible rating using

other technologies.

Influence of harmonics

The pillar of the MLJ measurement calculation is the discrete Fourier transform, which is in essence a

harmonics filter. For this reason the voltage and line measurements are not affected by frequencies other

than the fundamental.

The rejection of harmonics is added to the independence of measurements, both magnitude and phase,

relative to frequency signal variations, which is very important in a synchronism checking relay which, by

its own nature, works in variable frequencies.

Given that in power systems, synchronization or synchronism checking is carried out in a steady state, that

is with voltage magnitudes near or equal to the rated value, close enable is not emitted for very low voltages.

Therefore, for voltage of less than 9 volts, the relay stops measuring phase and frequency, not giving

permission to close under such conditions.