Figure 90: High Impedance Ref Ct Requirement - GE P642 Technical Manual

Chapter 8 - Restricted Earth Fault Protection
The approach with older electromechanical high impedance relays was to use an universally safe K factor of 1.0,
but the older relays operated quickly with a lower V
optimum K factor for stability, so that the required V
kneepoint voltage requirements worse than traditional requirements.
The high impedance REF CT requirements are shown in the following figure. They are valid for:
5 £ X/R £ 120 and 0.5In £ 40In
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
0
V00752

Figure 90: High Impedance REF CT requirement

Using the above graphical tool:
1. Locate the required average operation time on the right-hand side of the vertical axis.
2. Draw a horizontal line across until it intersects the red curve
3. Draw a vertical line from the above intersection point
4. Read off the the Vk/Vs value from the horizontal axis
5. Where this vertical line intersects the blue curve, read off the K value on the left hand side of the vertical
axis
If we take the example where the average operating time is 38 ms, the above graphical operations reveal that
Vk/Vs = 4 and K = 0.95.
For best accuracy we recommend class X or class 5P current transformers (CTs). The CT requirements for high
impedance REF protection are generally lower than those for differential protection. If the line CTs for high
impedance REF protection are the same as those used for differential protection, the differential CT requirements
cover both differential and high impedance REF applications. However if the line CTs for high impedance REF are
not the same as those used for differential protection, the high impedance REF CT requirements can be obtained
by using the graph shown above.
190
Unstable
Stable
1 2 3 4 5 6
/V ratio. With more modern IEDs, it is desirable to identify the
K s
/V ratio for stability and operating speed will not make CT
K s
Average operating time (red curve)
K (blue curve)
7 8 9 10 11 12
V /V
K S
0.06
0.05
0.04
0.03
0.02
0.01
0
13 14 15 16
P64x
P64x-TM-EN-1.3