Alstom SHPM 101, Service Manual

Page: 20 / 336

SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 4 of 27
is ganged to the Zone 3 setting. This situation is only likely to arise on long lines
where the type A (mho) relay will be in use. Full advantage will need to be taken
of the Zone 3 and power swing blocking lenticular facility in order to avoid load
encroachment problems.
Section 8. POWER SWING BLOCKING FEATURE (PSB)
The power swing blocking feature can be enabled or disabled by an option switch
and the individual distance zones to be blocked are also selectable via switches.
The inner power swing blocking impedance characteristic is formed by the A – B
Zone 3 phase fault characteristic. The additional PSB starter characteristic has its
settings automatically ganged to the relay Zone 3 settings, so that the PSB
characteristic is concentric with the Zone 3 characteristic, but its dimensions are
such that its forward reach is 30% greater than the Zone 3 forward reach.
If the power system A – B phase impedance locus enters the operating area of the
starter characteristic, but takes longer than 50ms to pass through into the Zone 3
operating area, then the PSB unit will block the selected zones if the A – B phase
impedance does eventually pass into the Zone 3 operating area.
If the PSB feature is to be used, then when setting the Zone 3 impedance
characteristic, it must be ensured that the resulting PSB starter characteristic will not
be encroached upon by the minimum phase to phase load impedance (a 10%
impedance safety margin should be observed).
Section 9. CHOICE OF RESISTIVE REACH OF QUADRILATERAL CHARACTERISTIC
The resistive reach should be set to cover the desired level of ground fault
resistance, which would comprise arc resistance and tower footing resistance.
A 10% impedance margin should be observed between the resistive reach and the
minimum load impedance.
In addition to ensure Zone 1 reach accuracy, the resistive reach should not be set
greater than 15 times the Zone 1 ground loop reach.
Section 10. AUTOMATIC COMPENSATION OF QUADRILATERAL REACH
LINE ANGLE
For the quadrilateral ground fault characteristics, the phase current is in phase with
the resistive axis of the R/X diagram. The reach line for the Zone 1 characteristic is
in phase with the residual or neutral current measured (with a –3
°
droop). Thus if
there is a difference in angle between the measured phase current and neutral
current, then the Zone 1 reach line will be at a corresponding angle to the resistive
axis. This feature ensures that resistive ground faults on a double end fed loaded
system will not result in underreach or overreach of the relay.
The Zone 2 and Zone 3 reach lines and the Zone 3 offset lines are in phase with
the resultant angle of the measured neutral current, plus the relevant phase current.
A qualitative analysis of the purpose of reach line compensation for Zone 1 is
given in Figure 2.