To determine the volt drops on an electrical system it is necessary to obtain the supply impedance. In England the Public Electricity Suppliers (PES), also known as the District Network Operators (DNOs), normally quote the fault level of their supply point equipment not the actual fault level. This is satisfactory for determining the fault levels of the downstream electrical equipment however it is useless for determining the volt drops on the downstream electrical network. If the PES refuse to give accurate impedance figures or the actual fault level and the X/R ratio then a fault level of 50% of the declared value should be assumed for use in the volt drop calculations with the R+jX values for the volt drop calculations being determined by using a power factor of 0.15.
Although this is not entirely satisfactory, when consideration is given to normal PES operational switching and power station alternator availability the supply impedance is not going to be a constant therefore the use of the 50% figure and power factor of 0.15 can be justified. The experience gained when reviewing completed projects against the design criteria used may require those figures of 50% and 0.15 to be adjusted.
The supply network downstream of the PES supply point can be more accurately assessed as each component part’s R+jX values are known quantities. As installation of the consumer’s network proceeds the theoretical R+jX values, taken from manufacturer’s data, used in the design should be replaced by the actual values determined by measurement or test.
The pdf version of this article contains the calculation for the determination of the maximum load that may be connected to an electrical distribution system whilst limiting the volt drop to a set percentage at the point of common coupling to the system.
Motors connected to generators require special attention as to impose a load on a generator will cause it to slow down and its output voltage to lower. Different generators have different mechanical and electrical characteristics that affect their recovery abilities when loads are imposed or removed. Therefore each generator or generating system must be treated individually; where the generator or generating system is to be supplied as part of the installation it will be necessary to incorporate into the specifications for the generator or generating system notification of the largest loads that will be imposed or removed in one step, and of the maximum acceptable voltage and frequency deviation during that step load change and the maximum recovery time to steady state conditions. Where there is an existing generator or generating system it will be necessary to request of the manufacturer the maximum load that may be applied or removed for a stated maximum voltage and frequency deviation.
For outline design purposes the maximum size of a motor for connection to a generating system should be no more than 25% of the normal running capacity of the generating system.
Mention must be made of lifts which use regenerative breaking, or for that matter any other motor where regenerative breaking, is used as part of its’ control system. Regenerative breaking can cause stability problems with a generator or generating system and it must be ascertained from the generator manufacturer the maximum regenerative load that may be applied if there are no other loads on the generator or generating system.
Mention must also be made of soft starters these use either waveform modification or pulse techniques to vary the starting current, both of these techniques have effects on generators. Generators with electronic speed governors can be affected by these electronic controls and the harmonics generated by them, as can alternator electronic voltage regulators. Alternators are not immune from the harmonics generated by electronic equipment such as soft start starters as these cause increased heating of the alternator. It is therefore important that where a generator or generating system is to be supplied as part of a project the specifications for the generator(s) include information regarding any electronics loads to which they will be subjected. Where there is an existing generator or generating system it will be necessary to request advice from the manufacturer as to the suitability of the generator or generating system for the proposed electronic loads.