Protection riddle No.59 - Low and High impedance differential protection
What is difference of low and high impedance relay and their advantage in practical cases?

#1
Wed, December 2nd, 2009 - 18:45
Low-impedance protection is provided with new numerical or microprocessor-based protection relays. Generally, relay manufacturers employ different methods to provide protection. In most cases, operation of the low-impedance protection is based on the fundamental current, after filtering removes all harmonic currents.
The most important difference between classical high impedance protection and new low-impedance protection is the input impedance. As with all numerical relays, the input impedance of the low-impedance  is very low compared to high-impedance relays. For example, a low impedance relay typically has an input impedance of 0.1 VA. At 1 A nominal rating, this computes to 0.1 W. On the other hand, for a high-impedance relay with a voltage setting of 100 V and a 20 mA operating current, the input impedance is 5 kW. This is a significant difference. Low-impedance protection does not have the same inherent stability against CT saturation for external faults as does high-impedance protection.
A second significant difference is that the operating current of the low-impedance protection is not realized by CT connection. With low-impedance, the relay measures all CTs necessary to realize the element.
A very important advantage of low-impedance protection is the fact that the CT ratios for the phase CTs which covered protection zone do not have to be the same.
Most low-impedance relays use an operating and a restraint current. The difference between different relays from different manufacturers lies in the way these relays determine the restraint quantities and in the CT saturation detection algorithm of each relay. Note that, in the case of low-impedance protection, there is no inherent immunity to CT saturation, as is the case with high-impedance protection.
In restricted earth fault protection, because of the nature of the protection, classical low impedance REF protection cannot be used as a balanced earth fault protection on an unearthed transformer or on a transformer with only three phase CTs and no neutral CT. In short, it is because the low-impedance REF protection requires a restraint and an operating current, at least one of which is also derived from the neutral CT. When a neutral CT is not provided, a low-impedance REF protection cannot be used to protect an unearthed transformer. However, most numerical relays provide a number of protection elements for each winding.
To realize balanced earth-fault protection on an unearthed star-connected transformer or a delta-connected transformer, Because of the inherently unstable nature of the low impedance element, it may misoperate during external faults, especially in the case of faults not involving earth as phase-to-phase and three-phase faults, when one of the phase CTs saturates. Various manufacturers of protection relays have each developed additional supervision elements to improve security during external faults while improving sensitivity during in-zone faults.

Some advantage and disadvantage of two various systems are described as follow:

High-impedance protection

.Relays relatively cheap – offset by expensive CTs
.Simple and well proven
.Fast ,15–45 ms
. Stability and sensitivity calculations – easy providing data is available
.Stability can be guaranteed

Very dependent on CT performance
CT saturation could give false tripping on through faults
Sensitivity must be decreased
DC offset of CTs unequal – use filters
Expensive class X CTs – same ratio
Vknp = 2 times relay setting
Primary effective setting (30–50%)
Limited by number of circuits
Z-earthed system difficult for earth fault
Duplicate systems – decreased reliability
Require exact CT dataV knp, R sec, I mag, V setting
High voltages in CT circuits (+-2.8 kV) limited by volt-dependent resistors

Retrofitting:

Additional CTs six per circuit
Space problems on metal clad switchgear
Long shutdowns
CT performance important
Class X
Vknp = 2 times setting
Rsec must be low
Limit on number of circuits
CT polarity checks required
Primary injection tests required
Compete switchboard
Separate relay cubicle
Differential relays
Auxiliary relays
CT cabling
Busbar tripping cabling

Biased low and medium-impedance protection

High speed 8–13 ms
Fault sensitivity +-20%
Excellent stability for external faults
Normal CTs can be used with minimal requirements
Other protection can be connected to same CTs
No limit to number of circuits
Secondary voltages low (medium impedance)
Well proven 10 000 systems worldwide
Any primary system configuration
No need for duplicate systems
Retrofitting easy
No work on primary CTs
Biasing may prevent possibility of achieving a sensitive enough earth fault setting of Z-earthed systems