Toshiba GRZ100 B Series, Instruction Manual, Page: 110 / 447

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2.5.2 Phase Selection Element UVC
The phase selection element has the undervoltage characteristic shown in Figure 2.5.2.1 and is
used to select a faulty phase in case of a single-phase-to-earth fault.
I
V
Vs
IZs
θ
Figure 2.5.2.1 Phase Selection Element
The characteristic is obtained by a combination of the equations below. If equation (1) or equation
(2), or both equations (3) and (4) are established, the UVC operates.
|V|
≤
Vs (1)
|V
−
IZs|
≤
Vs (2)
−
Vs
≤
V sin
θ ≤
Vs (3)
0
≤
V cos
θ ≤
|IZs| (4)
where,
V = fault voltage
I = fault current
θ
= angle difference between V and IZs
Zs = impedance setting
Vs = undervoltage setting
When the value and angle of Zs are set to those similar to the impedance of the protected line, the
phase selection element will detect all single-phase earth faults that have occurred on the protected
line even with a strong source and the voltage drop is small.
As a result of current compensation, the operating zone expands only in the direction leading the
current by the line impedance angle. Therefore, the effect of current compensation is very small
under load conditions where the current and voltage have almost the same phase angle.
2.5.3 Directional Earth Fault Elements DEFF and DEFR
There are two types of directional earth fault element, the forward looking element (DEFF) and
reverse looking element (DEFR). Their characteristics are shown in Figure 2.5.3.1.
Both the DEFF and DEFR use a residual voltage as their polarizing voltage and determine the fault
direction based on the phase relationship between the residual current and polarizing voltage.