5 DELTA DIRECTIONAL ELEMENT
Where Distance protection is being applied, a ‘Delta’ algorithm is provided to directionalize the distance elements.
If used in conjunction with aided schemes, this Delta algorithm can also provide additional protection in the form
of directional comparison protection.
5.1 DELTA DIRECTIONAL PRINCIPLE AND SETUP
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 DI (delta I) compared to
the superimposed voltage DV (delta V), at the instant of fault inception. The delta is only present when a fault
occurs and a step change from the pre-fault steady-state load is generated by the fault. The element issues a
forward or reverse decision which can be input into an aided channel unit protection scheme.
Under healthy network conditions the system voltage is close to Vn nominal and load current flows. 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, the difference between them is zero. Zero voltage change (DV = 0) and
zero current change (DI = 0), except when there are changes in load current.
When a fault occurs on the system, the delta changes measured are:
D
V = fault voltage (time “t”) - pre-fault healthy voltage (t-2 cycles)
D
I = fault current (time “t”) - pre-fault load current (t-2 cycles)
The delta measurements are a vector difference, resulting in a delta magnitude and angle. Under healthy system
conditions the pre-fault values are those measured 2 cycles earlier. When a fault is detected the pre-fault values
are 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.
The following figure shows a single phase to earth fault.
Chapter 7 - Distance Protection P543i/P545i
176 P54x1i-TM-EN-1
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