Application - ABB RELION REX640 Technical Manual

Section 10
Power quality measurement functions
10.4.6
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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
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.
1MRS759142 C
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percentile
REX640
Technical Manual