66/89 Device handbook A
PLUS
with / without LED display
,
157
679-19, 04/2016
A3 System imbalance
Measured quantity
p
res
en
t
m
ax
m
in
1L
2L
3L
b
3L
u
3
L
u
.A
4L
b
4
L
u
.O
4L
u
UR1: Positive sequence [V]
●
√ √ √
√
UR2: Negative sequence [V]
●
√ √ √
√
U0: Zero sequence [V]
●
√
U: Imbalance UR2/UR1
● ●
√ √ √
√
U: Imbalance U0/UR1
● ●
√
IR1: Positive sequence [A]
●
√
√ √
IR2: Negative sequence [A]
●
√
√ √
I0: Zero sequence [A]
●
√
√ √
I: Imbalance IR2/IR1
● ●
√
√ √
I: Imbalance I0/IR1
● ●
√
√ √
Available via interface only
Imbalance in three-phase systems may occur due to single-phase loads, but also due to failures, such as
e.g. the blowing of a fuse, an earth fault, a phase failure or an isolation defect. Also harmonics of the 3rd,
9th, 15th, 21st etc. order, which add in the neutral wire, may lead to imbalance. Operating resources
dimensioned to rated values, such as three-phase generators, transformers or motors on load side, may
be excessively stressed by imbalance. So a shorter life cycle, a damage or failure due to thermical stress
can result. Therefore monitoring imbalance helps to reduce the costs for maintenance and extends the
undisturbed operating time of the used resources.
Imbalance or unbalanced load relays use different measurement principles. One of them is the approach
of the symmetrical components, the other one calculates the maximum deviation from the mean-value of
the three phase values. The results of these methods are not equal and don't have the same intention.
Both of these principles are implemented in the A
PLUS
.
Symmetrical components (acc. Fortescue)
The imbalance calculation method by means of the symmetrical components is ambitious and intensive to
calculate. The results may be used for disturbance analysis and for protection purposes in three-phase
systems. The real existing system is divided in symmetrical system parts: A positive sequence, a negative
sequence and (for systems with neutral conductor) a zero sequence system. The approach is easiest to
understand for rotating machines. The positive sequence represents a positive rotating field, the negative
sequence a negative (braking) rotating field with opposite sense of direction. Therefore the negative
sequence prevents that the machine can generate the full turning moment. For e.g. generators the
maximum permissible current imbalance is typically limited to a value of 8...12%.
Maximum deviation from the mean value
The calculation of the maximum deviation from the mean value of the phase currents resp. phase
voltages gives the information if a grid or substation is imbalanced loaded. The results are independent of
rated values and the present load situation. So a more symmetrical system can be aspired, e.g. by
changing loads from one phase to another.
Also failure detection is possible. The capacitors used in compensation systems are wear parts, which fail
quite often and then have to be replaced. When using three phase power capacitors all phases will be
compensated equally which leads to almost identical currents flowing through the capacitors, if the system
load is comparable. By monitoring the current imbalance it's then possible to estimate if a capacitor failure
is present.
The maximum deviations are calculated in the same steps as the instantaneous values and therefore are
arranged there (
see A1
).