Application and Setting Notes
If reverse power occurs in a power plant, the turbo-generator set must be disconnected from the electrical
power system. Operating the turbine without the minimum steam flow (cooling effect) is dangerous. For a
gas turbine cogeneration unit, the motor load can also be too great for the electrical power system.
Parameter: Threshold
•
Recommended setting value (_:991:3) Threshold = -1.00 %
The friction losses that have to be overcome mainly determine the active power consumed by a turbo-gener-
ator set. Depending on the plant, the consumed active power is of the following ranges:
Steam turbines P
reverse
/S
rated
= 1.00 % to 3.00 %
Diesel drives P
reverse
/S
rated
> 5.00 %
Gas turbines P
reverse
/S
rated
= up to 30.00 %
You can measure the reverse power of the turbo-generator set yourself in a primary test by using the protec-
tion function. Set the setting value P
reverse
, for example, to 0.5 times the value of the measured reverse
power. This power is shown additionally in the functional measured values in the Reverse-power protection
function (P1avg). For large machines with very small reverse power, you must use the angle-error correction
option of the current and voltage transformers.
NOTE
In the current-input selection, you can select protection current transformers or instrument transformers
on the device side. Since the transformers are calibrated at the factory, there are hardly any differences
between the transformers for the Reverse-power protection function. You can thus always select device
inputs with a protection current transformer.
In contrast, a primary current transformer significantly influences the measuring accuracy via its angle
error. The angle error of a class 0.2 instrument transformer is significantly smaller than the angle error of a
class 5P protection current transformer. A separate function group is necessary for the connection to an
instrument transformer. Use a Voltage/current 3-phase function group and load the Reverse-power
protection function from the library.
Do not connect the Reverse-power protection function to a linear core-type transformer (for example, a
TPZ type), as these transformers have a large angle error (for example, approx. 180 min).
Settings
Addr.
Parameter C Setting Options Default Setting
Stage 1
_:991:1 Stage 1:Mode
•
off
•
on
•
test
off
_:991:2 Stage 1:Operate &
flt.rec. blocked
•
no
•
yes
no
_:991:3 Stage 1:Threshold -30.00 % to -0.30 % -1.00 %
_:991:4 Stage 1:Dropout ratio 0.40 to 0.99 0.60
_:991:102 Stage 1:Dropout delay 0.00 s to 60.00 s 0.00 s
_:991:6 Stage 1:Operate delay 0.00 s to 60.00 s 10.00 s
_:991:103 Stage 1:Operate delay
stop valve
0.00 s to 60.00 s 1.00 s
6.38.4.2
6.38.4.3
Protection and Automation Functions
6.38 Reverse-Power Protection
SIPROTEC 5, Overcurrent Protection, Manual 815
C53000-G5040-C017-8, Edition 07.2017
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