3.5 Unbalanced current
The generator can be in a situation where it is not delivering its rated load, but the current is very high in one of the phases. This can
be caused by an unbalanced load. When a generator load is unbalanced, the stress on the generator will be higher than normal.
The heat in one of the windings can also be very high. Unbalanced load can also develop if a cable has been damaged or dropped
off, or if a fuse to a single phase has blown. To protect the generator from unnecessary stress, the protection against unbalanced
load can be used. It is located in parameters 1501 to 1506. Parameter 1203 is also related to these parameters. Parameter 1203
defines how the calculations should be done, and it can be set to nominal or average.
If parameter 1203 is set to nominal, the AGC uses the maximum and the minimum current and subtracts the values. Then it will
compare this to the nominal current typed in parameter 6003, 6013, 6023 or 6033, depending on which of the nominal settings is
activated. The comparison to the nominal current will give a percentage that is related to parameter 1501.
Example: A genset is rated at 400 A and is supplying a load. The currents of the three phases are: 115 A, 110 A and 100 A. The
AGC will use the maximum and the minimum current, in this case 115 A and 100 A. The calculation will now be: ((115 -
100)*100)/400 = 3.75 %. If parameter 1501 is set to 4 %, the genset will keep running. If parameter 1501 is set to 4 %, and the
genset's rated current is 400 A, it can be calculated how unbalanced the genset is allowed to be: (4*400)/100 = 16 A. When the
phases are loaded more than 16 A, the generator breaker will be tripped. This is independent of the size of the load.
Parameter 1203 can also be set to average. The AGC will then calculate an average of the phases and compare how unbalanced
the load is between them.
Example: An genset is rated at 400 A and is supplying a load. The currents of the three phases are: 115 A, 110 A and 100 A. The
AGC will now calculate an average of these currents, take the one that differs most from the average and calculate a percentage of
deviation: (115 + 110 + 100)/3 = 108.3 A. Then the AGC will analyse which of the currents that differs most. In this example, it will be
the 100 A. The maximum difference will be compared to the average current: ((108.3 - 100)*100)/108.3 = 7.7 %. If the load had been
bigger, this calculated percentage would have been smaller. If the phase currents were 315 A, 310 A and 300 A, the average would
be: (315 + 310 + 300)/3 = 308.3 A. This would give a deviation of:
((308.3 - 300)*100)/308.3 = 2.7 %.
3.6 Unbalanced voltage
As well as having an unbalanced current protection, the AGC also has an unbalanced voltage protection. The AGC will measure on
each of the phase voltages and compare them to each other. If the genset is mounted in an application with capacitors to
compensate and a failure occurs in one of the capacitors, a difference in voltage may appear. The windings for this phase will be
overheated and thus exposed to heavy stress. To prevent this, the unbalanced voltage protection can be set.
The percentage set in parameter 1511 is a percentage of deviation compared to the average voltage in the three phases. The
average comparison is described with an example below.
Example: Phase L1 to L2 is 431 V, phase L2 to L3 is 400 V and phase L3 to L1 is 410 V. The three voltages must be added up to
find an average voltage: (431 + 400 + 410)/3 = 414 V. Now the voltage with the biggest voltage difference must be subtracted, in this
case L1 to L2: 431 - 414 = 17 V. Now the biggest voltage deviation in percent can be calculated: (17/414)*100 = 4.1 %.
This means that if parameter 1511 is set to 4.1 %, it is allowed to have a voltage difference of 31 V in this application, before the
unbalanced voltage protection can be activated.
In the example, phase-phase measurements have been used. Phase-phase is selected as default, but it can also be phase-neutral
measurements, and this can be changed in parameter 1201. (Parameter 1201 will be described later).
INFO
Be aware that when parameter 1201 is changed, it will influence other protections.
DESIGNER'S HANDBOOK 4189341275A EN Page 71 of 196
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