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example 7/1.04mm (6mm2) for the bus wire ring and the
CT connections to it. The cable from the ring to the relay
need not be of the larger section.
When the reserve bar is split by bus section isolators and
the two portions are protected as separate zones, it is
necessary to common the bus wires by means of auxiliary
contacts, thereby making these two zones into one when
the section isolators are closed. 15.8.6 Summary of Practical Details
This section provides a summary of practical
considerations when implementing a highimpedance
busbar protection scheme.
15.8.6.1 Designed stability level
For normal circumstances, the stability level should be
designed to correspond to the switchgear rating; even if
the available shortcircuit power in the system is much
less than this figure, it can be expected that the system
will be developed up to the limit of rating. Busbar P rotection needed for the check zone in buscoupler and bussection breakers. 15.8.6.2 Current transformers
Current transformers must have identical turns ratios,
but a turns error of one in 400 is recognised as a
reasonable manufacturing tolerance. Also, they should
preferably be of similar design; where this is not possible
the magnetising characteristics should be reasonably
matched.
Current transformers for use with high impedance
protection schemes should meet the requirements of
Class PX of IEC 600441.
15.8.6.3 Setting voltage
The setting voltage is given by the equation
Vs > If (RL + RCT)
where: • 245 • Vs = relay circuit voltage setting
If = steadystate through fault current RL = CT lead loop resistence
RCT = CT secondary winding resistance • 15 • 15.8.6.4 Kneepoint voltage of current transformers secondary condition is: This is given by the formula
VP = VK ≥ 2Vs
15.8.6.5 Effective setting (secondary) ...Equation 15.8 VK = knee  point voltage IS = relay circuit current setting Any burden connected across the secondary will reduce
the voltage, but the value cannot be deduced from a
simple combination of burden and exciting impedances. IeS = CT excitation current at voltage setting
n = number of CT’s in parallel
For the primary fault setting multiply IR by the CT turns
ratio.
15.8.6.6 Current transformer secondary rating
It is clear from Equations 15.4 and 15.6 that it is
advantageous to keep the secondary fault current low;
this is done by making the CT turns ratio high. It is
common practice to use current transformers with a
secondary rating of 1A. Busbar P rotection VK Iek = exciting current at knee  point voltage where: It can be shown that there is an optimum turns ratio for
the current transformers; this value depends on all the
application parameters but is generally about 2000/1.
Although a lower ratio, for instance 400/1, is often
employed, the use of the optimum ratio can result in a
considerable reduction in the physical size of the current
transformers.
15.8.6.7 Peak voltage developed by current transformers
Under inzone fault conditions, a high impedance relay
constitutes an excessive burden to the current
transformers, leading to the development of a high
voltage; the voltage waveform will be highly distorted
but the peak value may be many times the nominal
saturation voltage.
When the burden resistance is finite although high, an
approximate formula for the peak voltage is:
V P = 2 2 V K (V F − V K I ek If = fault current IR = IS + nIeSIR 15 • If where: The effective setting of the relay is given by • 2 ) ...Equation 15.7 where:
VP = peak voltage developed
VK = kneepoint voltage
VF = prospective voltage in absence of saturation
This formula does not hold for the open circuit condition
and is inaccurate for very high burden resistances that
approximate to an open circuit, because simplifying
assumptions used in the derivation of the formula are
not valid for the extreme condition.
Another approach applicable to the open circuit These formulae are therefore to be regarded only as a
guide to the possible peak voltage. With large current
transformers, particularly those with a low secondary
current rating, the voltage may be very high, above a
suitable insulation voltage. The voltage can be limited
without detriment to the scheme by connecting a
ceramic nonlinear resistor in parallel with the relay
having a characteristic given by:
V = CIβ
where C is a constant depending on dimensions and β is
a constant in the range 0.20.25.
The current passed by the nonlinear resistor at the relay
voltage setting depends on the value of C; in order to
keep the shunting effect to a minimum it is
recommended to use a nonlinear resistor with a value of
C of 450 for relay voltages up to 175V and one with a
value of C of 900 for setting voltages up to 325V.
15.8.6.8 High impedance relay
Instantaneous attracted armature relays are used. Simple
fastoperating relays would have a low safety factor
constant in the stability equation, Equation 15.5, as
discussed in Section 15.8.1. The performance is improved
by seriestuning the relay coil, thereby making...
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This note was uploaded on 02/18/2013 for the course EE 45 taught by Professor Kjald during the Spring '13 term at Aachen University of Applied Sciences.
 Spring '13
 kjald
 Impedance

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