Product Description; Introduction; General Overview - GE Multilin HID Instruction Manual

PRODUCT DESCRIPTION

2. PRODUCT DESCRIPTION
2.1

INTRODUCTION

2.1.1.

GENERAL OVERVIEW

HID modules provide resistors together with voltage limiters (MOV) to be used with high-speed overcurrent relay in
order to get a high impedance differential protection scheme.
The overcurrent relay unit connected in series with the resistors provides high-speed operation for busbar faults
involving high-magnitude currents. A voltage-limiting element is connected in parallel to avoid excessively high CT
secondary voltages, which can damage current inputs when bus faults occurred. Since the overcurrent unit is relied
on only for high magnitude currents, its pickup can easily be set high enough to avoid operation for external faults.
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 CT's should have the same rating, and should be a type, like a bushing CT with a
distributed secondary winding, that has little or no secondary leakage reactance.
A voltage-limiting element is connected in parallel, in order to prevent excessively high CT secondary voltages when
bus faults occurred.
The HID includes voltage-limiting resistors. This feature makes the HID better suited to those applications where high
internal fault currents can be encountered.
Typical applications include high impedance differential protection for busbars and electrical machines, such as
transformers, generators or motors, as well as restricted earth fault protection. HID modules are available in single-
phase models for applications such as restricted earth fault protection in a transformer winding, models with two
resistors for applications in transformers with two grounded windings, and three-phase models for busbar high
impedance differential protection.
Each HID module incorporates 2000-Ohm resistors that provide the associated high impedance relay with stability
against external faults, and varistors (MOV – Metal Oxide Varistors) in order to limit the voltage peak in the secondary
under 2 kV during fault conditions. 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.
The HID front plate incorporates a Reset pushbutton to reset the latching relay once the fault has been cleared, in
order to eliminate the resistors short circuit.
The external overcurrent unit may also be used to supplement with the implementation of breaker failure protection,
since it may include timers, auxiliary contacts, digital inputs and programmable logic.
The combination of HID module with a high-speed overcurrent relay can be applied for bus protection in most cases
where CTs having negligible leakage reactance are used. This generally includes any kind of current transformers
with a toroidal core if the windings (on the tap used) are completely distributed about the core.
Additional functionality provided by the additional overcurrent device includes: time stamped sequence of event
recorder, waveform capture, data logger, and communication to remote control centers.6
GEK-113064 HID High Impedance Differential Module 2-1