more than half of the nominal frequency period and less than one minute (European
Standard EN 50160 and IEEE Std 1159-1995).
These short-duration voltage variations are almost always caused by a fault
condition. Depending on where the fault is located, it can cause either a temporary
voltage rise (swell) or voltage drop (dip). A special case of voltage drop is the
complete loss of voltage (interruption).
PHQVVR is used for measuring short-duration voltage variations in distribution
networks. The power quality is evaluated in the voltage waveform by measuring the
voltage swells, dips and interruptions.
U_B
interruption
dip
swell
duration
max duration
magnitude
Voltage Int set
Voltage dip set
Voltage swell set
V Var Dur point 1
V Var Dur point 2
min duration
Figure 584: Duration and voltage magnitude limits for swell, dip and interruption
measurement
Voltage dips disturb the sensitive equipment such as computers connected to
the power system and may result in the failure of the equipment. Voltage dips
are typically caused by faults occurring in the power distribution system. Typical
reasons for the faults are lightning strikes and tree contacts. In addition to fault
situations, the switching of heavy loads and starting of large motors also cause
dips.
Voltage swells cause extra stress for the network components and the devices
connected to the power system. Voltage swells are typically caused by the earth
faults that occur in the power distribution system.
Voltage interruptions are typically associated with the switchgear operation related
to the occurrence and termination of short circuits. The operation of a circuit
breaker disconnects a part of the system from the source of energy. In the case of
overhead networks, automatic reclosing sequences are often applied to the circuit
breakers that interrupt fault currents. All these actions result in a sudden reduction
of voltages on all voltage phases.
Due to the nature of voltage variations, the power quality standards do not specify
any acceptance limits. There are only indicative values for, for example, voltage dips
1MRS757644 H
Power quality measurement functions
620 series
Technical Manual
1101
