Low Impedance Networks

They have lower loss and therefore lower heat output. They have better voltage regulation with varying loads. They offer better noise reduction and higher immunity to load injected transients. Etc., etc.

These positive aspects of low impedance networks are undisputed and this must not be seen as an argument against them. However, when tracing power quality problems, especially equipment damage, one must be aware of the pitfalls (Eh!) of low impedance networks.

The first (and probably the major) problem is fault currents. Accidents happen, and these include unwanted short circuits. During the short, especially if this happens some way up the cycle, the fault current will equal the voltage / impedance, and on a low impedance network the result is a high value. And then the breakers let go! This is when the breaker takes a serious beating even though it is only doing its job.

The short is removed and the breaker returned to normal...... but some time down the road further 'funnies' start occurring. An expert is called in who simply discovers that the breaker contacts have, at some time, been badly burned and their resistance is high (which defeats the object of a low impedance network!).

Further problems can exist in the form of electro-mechanical shock as a result of incoming voltage changes. Any motorised machinery where smooth operation is paramount will be put under severe strain should the voltage suddenly jump from one cycle to the next during e.g. a tap change on the main network. The low impedance supply does not have any 'give' allowing the motors to slowly build up to the new speeds and phase relationship of the higher voltage.

Electronic equipment is also prone to damage as, again, incoming voltage rises are suddenly 'shoved' into the equipment meaning the equipment must adjust, which could mean exceptionally high currents for a cycle peak or two (this was covered at length in "input diode damage").

As said, this is not an argument against low impedance networks - far from it. It was merely intended as a guide to areas where such a network could have influence in creating problems down the road, and what should be catered for at design level.

High Network Impedances  >>

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© 01.05.03