Alstom MiCOM P54 Series, Техническое руководство, Страница: 229 / 1024

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-55
OP
The directional criteria with negative sequence polarization are given below:
Directional forward
-90
o
< (angle(I2) - angle(V2+180
o
) - RCA) < 90
o
Directional reverse
-90
o
> (angle(I2) - angle(V2+180
o
) - RCA) > 90
o
1.22.3 Delta directional comparison principle and setup (Distance option only)
Note:
The characteristic angle set in this section is also used by the
DISTANCE PROTECTION. This is because distance zones are
directionalized by the delta decision.
Delta directional comparison looks at the relative phase angle of the superimposed current

I compared to the superimposed voltage

V, at the instant of fault inception. The delta is
only present when a fault occurs and a step change from the prefault steady-state load is
generated by the fault itself. The element will issue a forward or reverse decision, which can
be used to input into an aided channel unit protection scheme.
Under healthy network conditions, the system voltage will be close to Vn nominal, and load
current will be flowing. Under such steady-state conditions, if the voltage measured on each
phase now is compared with a stored memory from exactly two power system cycles
previously (equal to 96 samples), the difference between them will be zero. Zero change
equals zero “delta” (

V = 0). The same will be generally true for the current (

I = 0), except
when there are changes in load current etc.
When a fault occurs on the system, the delta changes measured will be:

V = fault voltage (time “t”) - prefault healthy voltage (t-96 samples)

I = fault current (time “t”) - prefault load current (t-96 samples)
The delta measurements are a vector difference, resulting in a delta magnitude and angle.
Under healthy system conditions, the prefault values will be those measured 2 cycles earlier,
but when a fault is detected, the prefault values will be retained for the duration of the fault.
The changes in magnitude are used to detect the presence of the fault, and the angles are
used to determine whether the fault is in the Forward or Reverse direction.
Consider a single phase to ground fault as shown in Figure 31 below.