Intro - GE Multilin MIB Instruction Manual

2.2 INTRO

2.2INTRO
MIBis a microprocessor-based relay designed for the following applications:
• High-Impedance differential protection for Bus bars
• Restricted Ground Fault (RGF) protection on electrical machines with grounded-wye windings
• Differential protection for generators
• Differential protection for reactances
2
In applications where all the CTs have the same ratio.
The protection combines a well-proven principle, as the high-impedance one, with the advantages of microprocessor
technology. The criteria used in high impedance differential protection are very simple. The high impedance path using a
stabilizing resistor R (2000 Ohm resistors), produces a voltage and current during an internal fault. Measuring the value,
the unit can determine if there is an internal or external fault. If the voltage or current value is above the set pickup value,
the fault is internal. If the value is under the pickup value, then the fault is external. The pickup value becomes the critical
point. This setting must contemplate the possibility of a complete CT saturation. Selecting an adequate pickup value, the
stability of the scheme is ensured including operation under extreme conditions.
To cover the mentioned application, MIB is comprised of the following modules:
• High Impedance Module: Stabilising Resistors and voltage limiters. The High-Speed over current module
connected in series with the stabilizing resistors provide high speed operation for bus faults involving high-magnitude
currents. A voltage limiting element (MOV) is connected in parallel to avoid excessively high CT secondary voltages
that can damage the current inputs when the relay fault occurred.
• High-Speed overcurrent module: Differential protection for up to 3 differential currents.
• Lockout Relay & Push Button: Mechanical lockout after relay operation and Reset Button.
The procedure for determining the necessary settings and the resulting sensitivity to low-current bus faults is very simple
and straightforward, requiring only knowledge of the CT secondary excitation characteristics and their secondary
impedance.
For the best possible results, all CTs should have the same rating, and should have a characteristic similar to a bushing CT
with a distributed secondary winding, that has little or no secondary leakage reactance.
The overcurrent-relay unit connected in series with the stabilizing resistor voltage limiter provides high-speed operation for
bus faults involving high-magnitude currents. Since the overcurrent module unit is relied on only for high magnitude
currents, its pickup can easily be made high enough to avoid operation for external faults.
Additionally, a latching relay is incorporated, whose contacts are aimed to short circuit the resistors once the associated
relay has tripped. This way, the fault current is prevented from circulating through the resistors after the relay has tripped.
The MIB relay provides two configurable digital inputs and three configurable contact outputs. Additionally, it provides two
fixed outputs, one associated to the TRIP command and the other one to the SYSTEM ALARM.
Also it provides communications with a computer by means of two communication ports, one located in the
front (RS232 type) and one located in the rear (RS485 type). Both faceplate RS232 port and rear RS485 port
may be used to connect a PC for programming settings, monitoring actual values and for retrieving stored
information (list of events, oscillography, etc.). All serial ports use the Modbus® RTU protocol and may be
connected to system computers with baud rates from 300, 600, 1200, 4800, 9600 and 19200 bps. The rear
RS485 port can be converted into an RS232 port or into a fibre optic (plastic or glass) serial port by using the
GE DAC300 module. The EnerVista MII Setup communication software is the Windows® based program used
to communicate with the relay.
MIB The following one line (single line) diagram (Figure 2–1:) illustrates the relay functionality using ANSI (American
National Standards Institute) device numbers.
2-2 MIB High Impedance Bus Differntial Relay
2 PRODUCT DESCRIPTION
GEK-106426B