Consideration has to be given to the PES supply voltage to the consumer as it will affect the design of the maximum motor sizes and the starting methods employed.
The supply from PESs in England will normally be at 400/230V (nominal voltage) where the consumer’s site load does not exceed 500kVA, above 500kVA and below 12MVA the supply will be at MV which will normally be 11kV although some PESs offer 33kV, above 12MVA the supply will generally be at 33kV, and above 40MVA at 132kV.
PES LV distribution systems will normally be TN-S or TN-C. A TN-C supply will be converted at the consumers's terminals to TN-S. In a TN-S system the neutral and protective conductor are separate.
Since the PES LV supply is 415/240V ±6% (the old statutory limits in England) that equates to a range of 390/225V to 440/254V which is, no surprises, within the range of 360/207V to 440/254V as laid down by the statute that reflects EU regulations, a number of companies have designed devices that keep the voltage at the nominal 400/230V.
These devices act as automatic tap changing auto transformers, the manufacturers claim energy saving when they are included in the consumers network as the voltage is kept at the nominal level or even lower than the nominal level; great care must be taken with volt drops where this is the case; this is true for resistive loads but is not true for motors where lowering the motor terminal voltage increases the motor current therefore the power output and input remains constant.
Care must also be taken that when these units are incorporated into a system, the fault levels on the load side must be sufficient to operate the protective devices of the downstream equipment within the times laid down by the IET Wiring Regulations.
It must also be noted that these devices are not 100% efficient as they will have impedance, which means that some of the claimed gains will be offset by the losses due to that impedance, that energy loss is via heat gain into the room in which the device is situated.
Where energy is used for cooling and ventilating these units, that energy must also be taken into account when assessing their overall efficiency.
Where consideration is given to including these devices their impedance must be taken into account when determining the maximum motor sizes to be connected as these devices cannot operate instantaneously.
Thus inclusion of these devices will reduce the maximum size of motor that may be connected to a consumers’ network whilst still retaining the volt drop limits laid down.
For MV and HV the LV system will be created by the consumer, it is a legal requirement to use a TN-S system. For full details of distribution systems see the primary distribution article on this website.
Where the PES supply is at MV the consumer will at some point in their network transform the incoming voltage to the nominal LV voltage (400/230V). If the supply is at 11kV the transformers will normally have a number of fixed taps usually 0% ±2½% and ±5% of the nominal voltage.
The output voltage from the transformer will depend on the load, it is normal to set the taps to give a no load voltage of 417V which gives the transformer’s full load voltage of 400V, this is due to the transformer’s impedance the majority of which is the winding resistance. Since transformers are invariably oversized it is advisable to carry out a second adjustment of the taps so that the transformer’s output voltage is 400V between phases measured at its associated LV switchboard busbars when the load is at its actual not theoretical maximum.
If the supply is at 33kV or higher then transformers equipped with automatic tap changers should be installed. These transformers should have taps set at steps of 1% with the automatic tap changers set for an output voltage of 400V this will keep the LV voltage within the range of 396/228V to 404/232V. The voltage reference point for an automatic tap changer should be at the incoming circuit breaker of the switchboard fed by the transformer.
Where ever transformers are used the transformer impedance will affect the consumer’s network short circuit level.
It should be noted that transformer impedance is given in a table of BS EN 60076 however that is only a recommendation and transformer manufacturers can vary that impedance to some degree to suit the requirements of the network design. So if the network is designed for 80kA on the LV network and the PES supply impedance is known it is possible to achieve the design objective by requiring transformers of a given impedance to be supplied. However before committing to a large variation to the British Standard impedance figures the prospective transformer manufacturers should be consulted.
For motors with outputs of 120kW or greater, consideration should be given to using a MV or HV motor. Consideration should also be given to using a separate transformer with an output of 3.3kV or 11kV for a MV MCC dedicated to motors with outputs of 120kW or greater as this will allow the 3% volt drop to be used at the MCC although the volt drop at the point of connection to the supply switchboard must be limited to 1%.