ABB REL 356 Current Differential Protection
4-1
Measuring Elements and Operational Logic
Measuring Elements and Operational Logic
Composite Sequence Filter
The REL 356 combines the phase currents (IA, IB and IC) measured at the protective relaying terminal into a single
quantity. This quantity, IT, is an output of a symmetrical component filter that is proportional to the weighted sum of
the sequence components. The quantity IT is defined as:
0
0
2
2
1
1
I
C
I
C
I
C
IT
+
+
−
=
The quantity IT is a sine wave that is proportional to the fault current. The C1 (positive sequence coefficient), C2
(negative sequence coefficient) and C0 (zero sequence coefficient) are system settings that control the sensitivity of the
relaying system.
An example of a single phase to ground, AG, fault with the corresponding IT quantity is shown in Figure 4-1. The first
graph shows the three phase currents and the second the IT filter output for settings C1 = 0.1, C2 = 0.7 and C0 = 0.0.
Figure 4-1. IT Filter Output for an A-G Fault
Current Differential Algorithm
For the comparison process, two quantities are generated from the local and remote IT waveforms. The operate quantity
is derived by vector addition of the local and remote quantities. The restraint (bias) quantity is obtained by adding the
local and remote quantities on a magnitude basis. The resultant output is opposite in polarity to the operate quantity. The
operate and restraint quantities are combined and the result fed to a level detector which produces a trip signal.
The quantity IT is converted into the phasor quantity ITL using discrete Fourier transform (DFT) cosine filter (see Figure
4-2). The quantity ITL is applied to
·
Communication transmitter and sent to the remote receiver where it is received as ITR. Note that
due to channel propagation delay, ITR is delayed with respect to ITL.
·
Current differential protection algorithm after compensation for the channel propagation delay, LDT.
Summary of Contents for REL 356
Page 23: ...ABB REL 356 Current Differential Protection 1 10 Product Overview and Specifications ...
Page 83: ...ABB REL 356 Current Differential Protection 3 36 Settings and Application ...
Page 127: ...ABB REL 356 Current Differential Protection 5 28 Testing ...
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