Functions
2.40 DC Voltage/Current Protection (ANSI 59NDC/51NDC)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
273
2.40.2 Setting Notes
General
The DC voltage protection is only effective and available if set to Enabled at address 172 DC PROTECTION.
If the function is not required, Disabled is set. For the associated measuring transducer 1, address 295
TRANSDUCER 1 was set to one of the alternatives 10 V, 4-20 mA or 20 mA (see section 2.5).
Jumpers X94, X95 and X67 on the C-I/O-6 module are used to set in the hardware whether the measuring
transducer input will be a voltage or a current input (see section 3.1.2 in the chapter „Installation and Commis-
sioning“). Their setting must correspond to the setting at address 295. If it does not, the device is blocked and
issues an annunciation to that effect. When the relay is delivered from the factory, the jumpers and configuration
parameters are set to voltage measurement.
Address 7201 DC PROTECTION serves to switch the function ON or OFF or to block only the trip command
(Block relay).
Measurement Method
Normally, the arithmetic mean of a DC voltage is detected. A high ripple content or non-periodic peaks in the
measurement voltage are averaged in this manner. The polarity of the measured voltages is of no concern
since the absolute value is taken.
Alternatively, the RMS of a sinusoidal AC voltage can be measured (address 7202 MEAS.METHOD = RMS). The
protection then multiplies the rectified average value with 1.11. The frequency of the AC voltage must match
the frequency of other input quantities, because the latter determine the sampling rate. The maximum AC am-
plitude must not exceed 10 V, so that for r.m.s. value measurement a maximum setting of 7.0 V
rms
is reason-
able. The resulting higher secondary voltage can be reduced by means of a voltage divider.
The DC voltage/DC current protection can be set to operate for overvoltage protection at address 7203 DC >/<
= DC > or undervoltage protection = DC <.
Pickup Thresholds
Depending on whether current or voltage input has been set at address 295 TRANSDUCER 1, one of the fol-
lowing parameters is available, whereas the other is hidden:
• 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.
Application Examples
When used for excitation voltage monitoring, the DC current protection is configured to operate for undervolt-
age; 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
above).
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 with a startup transformer of U
N, ST
≈ 1.4 kV and a 6-pulse bridge circuit, there will be a
DC voltage of U
DC
≈ 1.35 · U
N, ST
= 1.89 kV. In case of an earth fault in the intermediate circuit, the „displacement
voltage“ will be half of the DC voltage (U
DC, fault
= 0.5 U
DC
= 945 V).
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