Micom P721, Техническое руководство

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P72x/EN AP/C21 Application Guide
Page 16/52 MiCOM P721/P723
FIGURE 11: EXTERNAL FAULT – SINGLE BUSBAR
Consider the current distributions during an internal fault in the single busbar diagram shown
in FIGURE 12. During an internal fault, the high impedance path (stabilizing re relay
burden) forces most of the secondary current through the CT magnetizing branch. The
voltage developed across the magnetizing branch is very high and it is highly distorted. The
peak voltage may be many times the nominal saturation voltage. This voltage is across the
high impedance path; therefore enough current flows through the relay to make it trip. If the
Measurement Filter is set to Fourier, the P72x would be tuned at the system frequency.
Therefore, dc components and harmonics are filtered.
The resistance, R, is much smaller than the stabilizing resistor, R
ST
; therefore, it can be
neglected. Since the P72x burden is also smaller than R
ST
, it can be neglected.
The high impedance differential scheme requires that R
CT
+ R
L
is kept low. To keep R
L
as
low as possible, it is recommended that the junction point (physical connection of the current
transformers secondaries) is as close as possible to the locations of the CTs. The current
transformers secondaries should also be equidistant. The CTs should have the same ratio
and should operate on the full winding.
FIGURE 12: INTERNAL FAULT – SINGLE BUSBAR
The connected CTs should be grounded at only one point as shown in FIGURE 11 and
FIGURE 12 to avoid improper relay operation and damage to the CT interconnections. If
grounds are made at two or more locations, currents may flow in the differential circuit
because of differences of potential between the grounding points, due to the flow of large
ground currents during ground faults.
The correct practice is to interconnect the neutrals of all differentially connected CTs.
The neutral interconnection is grounded at one point only. The grounding is for the
protection of personnel, therefore the best place to make the ground is at the
differential relay panel.