ABB RELION REX640, Техническое руководство, Страница: 1854 / 1960

10.4.6 Application
Voltage unbalance is one of the basic power quality parameters.
Ideally, in a three-phase or multiphase power system, the frequency and voltage
magnitude of all the phases are equal and the phase displacement between any two
consecutive phases is also equal. This is called a balanced source. Apart from the
balanced source, usually the power system network and loads are also balanced,
implying that network impedance and load impedance in each phase are equal. In
some cases, the condition of a balance network and load is not met completely,
which leads to a current and voltage unbalance in the system. Providing unbalanced
supply voltage has a detrimental effect on load operation. For example, a small
magnitude of a negative-sequence voltage applied to an induction motor results in
a significant heating of the motor.
A balanced supply, balanced network and balanced load lead to a better
power quality. When one of these conditions is disturbed, the power quality
is deteriorated. VSQVUB monitors voltage unbalance conditions in power
transmission and distribution networks. VSQVUB calculates two sets of measured
values, a three-second and a ten-minute non-sliding average value. The three-second
average value is used for continuous monitoring while the ten-minute average value
is used for percentile calculation for a longer period of time. It can be applied
to identify the network and load unbalance that may cause sustained voltage
unbalance. A single-phase or phase-to-phase fault in the network or load side can
create voltage unbalance but, as faults are usually isolated in a short period of time,
the voltage unbalance is not a sustained one. Therefore, the voltage unbalance may
not be covered by VSQVUB.
Another major application is the long-term power quality monitoring. This can
be used to confirm a compliance to the standard power supply quality norms.
The function provides a voltage unbalance level which corresponds to the 95
th
percentile of the ten minutes' average values of voltage unbalance recorded over
a period of up to one week. It means that for 95 percent of time during the
observation period the voltage unbalance was less than or equal to the calculated
percentile. An alarm can be obtained if this value exceeds the value that can be set.
The function uses five different methods for calculating voltage unbalance.
• Negative-sequence voltage magnitude
• Zero-sequence voltage magnitude
• Ratio of negative-sequence to positive-sequence voltage magnitude
• Ratio of zero-sequence to positive-sequence voltage magnitude
• Ratio of maximum phase voltage magnitude deviation from the mean voltage
magnitude to the mean of phase voltage magnitude.
Usually, the ratio of the negative-sequence voltage magnitude to the positive-
sequence voltage magnitude is selected for monitoring the voltage unbalance.
However, other methods may also be used if required.
10.4.7 Signals
Power quality measurement functions 1MRS759142 F
1854 REX640
Technical Manual