G2010 A 10.4ST ABB Operation panel display screen

Description

G2010 A 10.4ST An internal ground fault on an impedance grounded wye winding (see second figure

above) produces a fault current (I F) dependent on the value of the ground impedance and

the position of the fault on the winding with respect to the neutral point. The resultant

primary current (I P) will be negligible for faults G2010 A 10.4ST on the lower 30% of the winding since the

fault voltage will not be the system voltage but the result of the transformation ratio

between the primary windings and the percentage of shorted turns on the secondary.

Therefore, the resultant differential currents could be below the slope threshold of the

percent differential element and thus the fault could go undetected. The graph below

shows the relationship between the primary (I P) and fault (I F) currents as a function of the

distance of the fault point from the neutral and FIGURE 5–25: RGF and percent differential

zones of protection outlines the zones of effective protection along G2010 A 10.4ST the winding for an

impedance grounded wye.

The 745 implementation of restricted ground fault (shown below) is a low impedance

current differential scheme where “spill” current due to CT tolerances is handled via load

bias similar to the percent differential. The 745 calculates the vectorial difference of the

residual and ground currents (i.e. 3I 0 – I g) and divides this by the maximum line current

(I max) to produce a percent slope value. The slope setting G2010 A 10.4ST allows the user to determine the

sensitivity of the element based on the class and quality of the CTs used. Typically no more

than 4% overall error due to CT “spill” is assumed for protection class CTs at nominal load.

Note

The restricted ground fault protection is also available for delta windings with ground

inputs as shown in table 3.2.

The issue of maloperation due to heavy external G2010 A 10.4ST faults resulting in CT saturation is handled

by a programmable timer. The timer provides the necessary delay for the external fault to

be cleared by the appropriate external protection with the added benefit that if the RGF

element remains picked up after the timer expires, the 745 operates and clears the fault.

This approach provides backup protection. Since the restricted ground fault element is

targeted at detecting low magnitude internal winding fault currents, the time delay for

internal faults is of little consequence, since sensitivity and security are the critical

parameters.

For example, consider a transformer with the following specifications:

10 MVA, 33 kV to 11 kV, 10% impedance, delta/wye 30,

Rg = 6.3 ohms,

Phase CT ratio = 600 / 1 A,

Rated load current I rated = 10 MVA / (

× 11 kV) = 525 A,

Maximum phase-to-ground fault current I gf(max) = 11 kV / (

× 6.3) = 1000 A.

For a winding fault point at 5% distance from the neutral:

(EQ 5.10)

From FIGURE 5–24: Fault currents vs. points from neutral on page 5–73, we see that the I p

increase due to the fault is negligible and therefore 3I o = 0 (approximately). Therefore, the

maximum phase current = I max = I rated = 525 A (approximately), and

(EQ 5.11)

The winding 1 restricted ground fault setpoints are described below:

W1(3) RESTD GND FAULT PICKUP: Enter the minimum level of ground differential

current (in units of phase CT primary associated G2010 A 10.4ST with the winding, where the restricted

ground fault is set) for the winding 1(3) restricted ground fault element.

W1(3) RESTD GND FAULT SLOPE: Enter a slope percentage (of ground differential

current to maximum line current).

W1(3) RESTD GND FAULT DELAY: Enter the time that the winding 1(3) restricted

ground fault element must remain picked up before the element operates.

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