Quality of supply voltage

An adequate level of voltage at the consumers supply-service terminals is essential for satisfactory operation of equipment and appliances. Practical values of current, and resulting voltage drops in a typical LV system, show the importance of maintaining a high Power Factor as a means of reducing voltage drop.

The quality of the LV network supply voltage in its widest sense implies:

• Compliance with statutory limits of magnitude and frequency
• Freedom from continual fluctuation within those limits
• Uninterrupted power supply, except for scheduled maintenance shutdowns, or as a result of system faults or other emergencies
• Preservation of a near-sinusoidal wave form

In this Sub-clause the maintenance of voltage magnitude only will be discussed.

In most countries, power-supply authorities have a statutory obligation to maintain the level of voltage at the service position of consumers within the limits of ± 5% (or in some cases ± 6% or more-see table C1) of the declared nominal value.

Again, IEC and most national standards recommend that LV appliances be designed and tested to perform satisfactorily within the limits of ± 10% of nominal voltage. This leaves a margin, under the worst conditions (of minus 5% at the service position, for example) of 5% allowable voltage drop in the installation wiring.

The voltage drops in a typical distribution system occur as follows: the voltage at the MV terminals of a MV/LV transformer is normally maintained within a ± 2% band by the action of automatic onload tapchangers of the transformers at bulk-supply substations, which feed the MV network from a higher-voltage subtransmission system.

If the MV/LV transformer is in a location close to a bulk-supply substation, the ± 2% voltage band may be centered on a voltage level which is higher than the nominal MV value. For example, the voltage could be 20.5 kV ± 2% on a 20 kV system. In this case, the MV/LV distribution transformer should have its MV off-circuit tapping switch selected to the + 2.5% tap position.

Conversely, at locations remote from bulk supply substations a value of 19.5 kV ± 2% is possible, in which case the off-circuit tapping switch should be selected to the - 2.5% position.

The different levels of voltage in a system are normal, and depend on the system powerflow pattern. Moreover, these voltage differences are the reason for the term “nominal” when referring to the system voltage.

Practical application

With the MV/LV transformer correctly selected at its off-circuit tapping switch, an unloaded transformer output voltage will be held within a band of ± 2% of its no-load voltage output.

To ensure that the transformer can maintain the necessary voltage level when fully loaded, the output voltage at no-load must be as high as possible without exceeding the upper + 5% limit (adopted for this example). In present-day practice, the winding ratios generally give an output voltage of about 104% at no-load, when nominal voltage is applied at MV, or is corrected by the tapping switch, as described above. This would result in a voltage band of 102% to 106% in the present case.

A typical LV distribution transformer has a short-circuit reactance voltage of 5%. If it is assumed that its resistance voltage is one tenth of this value, then the voltage drop within the transformer when supplying full load at 0.8 power factor lagging, will be:

V% drop = R% cos φ + X% sin φ

= 0.5 x 0.8 + 5 x 0.6

= 0.4 + 3 = 3.4%

The voltage band at the output terminals of the fully-loaded transformer will therefore be (102 - 3.4) = 98.6% to (106 - 3.4) = 102.6%.

The maximum allowable voltage drop along a distributor is therefore 98.6 - 95 = 3.6%.

This means, in practical terms, that a medium-sized 230/400 V 3-phase 4-wire distribution cable of 240 mm² copper conductors would be able to supply a total load of 292 kVA at 0.8 PF lagging, distributed evenly over 306 metres of the distributor. Alternatively, the same load at the premises of a single consumer could be supplied at a distance of 153 metres from the transformer, for the same volt-drop, and so on...

As a matter of interest, the maximum rating of the cable, based on calculations derived from IEC 60287 (1982) is 290 kVA, and so the 3.6% voltage margin is not unduly restrictive, i.e. the cable can be fully loaded for distances normally required in LV distribution systems.

Furthermore, 0.8 PF lagging is appropriate to industrial loads. In mixed semi-industrial areas 0.85 is a more common value, while 0.9 is generally used for calculations concerning residential areas, so that the volt-drop noted above may be considered as a “worst case” example.