User manual

5 Power quality - a guide
85
Fig. 37. Example of lightning surge.
Surges caused by atmospheric discharges may have destructive effects due to the very high
energy triggered during the discharge. Most of surges of this type observed in networks, result from
voltage induced by close but not direct lightning stroke. In the area of lightning stroke, a very strong
electromagnetic field is generated and long overhead/underground lines induce high voltage that
penetrate into the distribution network. These surges have pulse nature with rise time on the order
of microseconds. An example of the lightning impulse recorded by PQM-703 analyzer, with ampli-
tude of approx. 6.6 kV is shown in Fig. 37.
Tests of ICT devices carried out before introducing them into markets, include immunity tests
for simulated lightning surges. AC power connections are tested with ±2 kV pulses applied between
power lines and grounding lines, and ±1 kV pulses applied directly between power supply lines.
Standardized pulse has voltage rise time of 1.2 µs and voltage fall time of 50 µs. For the measuring
devices that may be connected directly to distribution networks at the distribution boards or at LV
transformers, authorities defined a measurement category (overvoltage category), which informs
about the device protection level against surges. For example, to be included into measurement
category IV 600 V (the category of PQM-702 and PQM-703 analyzers), the devices must be immune
to impulses of 1.2 μs/50 μs with 8 kV amplitude, applied directly between test terminals at source
impedance of 2 Ω. Peak current during surge may be therefore equal to 4 kA.
The main protection measures against such surges include the circuits limiting the maximum
voltage such as gas discharge tubes (GDTs) and varistors. Their construction must ensure receiving
impact energy and limiting voltage penetrating the device circuits to a safe level.
Transients caused by switching compensation capacitance, as opposed to lightning strokes,
have their source within the distribution network. The compensation is used to improve the power
factor and efficiency of energy transfer to the load. At the moment of switching on, a capacitor is a
short circuit for the network, thus initially there is a sudden voltage drop almost to zero, followed by
fast recovery and an overshoot when voltage much higher than the nominal is reached (usually it