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Pickup Thresholds
Application
Examples
Delays
7UM62 Manual
C53000-G1176-C149-3
The DC voltage/DC current protection can be set to operate for overvoltage or
undervoltage (address 7203 DC >/<).
Depending on the whether current or voltage input has been set at address 0295
TRANSDUCER 1, one of the following parameters is available, whereas the other is
masked out:
− Voltage measurement threshold: 7204 U DC ><
− Current measurement threshold: 7205 I DC ><
When setting the pickup values (address 7204), the ratio of a voltage divider – if fitted
– has to be considered.
When used for excitation voltage monitoring, the DC current protection is configured
to operate for undervoltage; the pickup threshold is set to approx. 60 % to 70 % of the
no-load excitation voltage. Users should be aware that normally a voltage divider is
connected between the protection and the excitation voltage (see Figure 2-113).
Another typical application is the earth fault protection for the startup converter of a
gas turbine set. In the case of an earth fault in the DC circuit, half of the DC voltage is
present between the transformer starpoint and the earth if the transformer starpoint is
not earthed. This voltage can be considered as the voltage feeding the earth current.
As the transformer starpoints are earthed, the current flowing is determined by the
feeding voltage and the ohmic resistance of all transformers that are galvanically
connected to the converter set and earthed. This DC current is normally between
about 3 and 4 A.
For a startup converter having a startup transformer with U
bridge circuit, there will be a DC voltage of U
an earth fault in the intermediate circuit, the „displacement voltage"will be half of the
= 0.5 ⋅ U
DC voltage (U
DC, fault
If we assume that the earthing transformer has an ohmic winding resistance of R ≈
150 Ω, a DC current of I
Note: The ohmic winding resistances of earthing and neutral transformers can differ
widely depending on the type. For a concrete application, they should be obtained
from the manufacturer, or determined by measurements.
If not tripped, the earth fault current would cause a temperature overload that would
destroy the wye-connected voltage transformers and the earthing transformer. To
ensure that the protection will pick up reliably, it is set to a value of less than half the
fault current, in this example to 2 A. With the shunt and shunt converter used in the
example, this current causes a secondary current of 4 mA (see Figure 2-114) (fault
current ≈ 6 A, selected pickup value = 2 A, setting value = 4 mA).
The tripping delay can be set at address 7206 T DC. The set time is an additional
time delay not including the operating time of the protective function.
For the startup earth fault current protection, T DC is determined by the permissible
temperature load of the earthing and/or neutral transformer. A value of 2 s or less is
quite common.
Note: It should be noted that in operating condition 0, the operating times for pickup
and dropout are 4 times longer due to the more complex filter procedure needed to
eliminate disturbances.
DC Voltage/DC Current Protection (ANSI 59NDC/51NDC)
DC
= 945 V).
DC
= 945 V/150 Ω = 6,3 A will flow through its starpoint.
0
≈ 1.4 kV and a 6-pulse
N, AT
≈ 1.35 ⋅ U
= 1.89 kV. In case of
N, AT
237
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