In a widely circulated letter, the management of the Nigerian Electricity Management Services Agency (NEMSA) has given a notice of enforcement of strict compliance with the extant regulations regarding the subject matter in the Nigerian Electricity Supply Industry (NESI) commencing from 1st July, 2021.
For many years now, the supply voltage for single phase AC systems in the Nigerian Electricity Supply Industry (NESI) has been 240 +/-6%, giving a possible spread of voltage from 226V to 254V. For three-phase supplies, the voltage was 415V +/-6%, the spread being from 390V to 440V. This is largely because British Engineers built the Nigerian Power System after their own and so it was logical that we adopted their principles and practises.
However, as Britain became part of the European Union in the 1970s, it became necessary to harmonize the supply voltage systems in the whole of Europe to facilitate trade in their common market as most of continental Europe had been on 220/380V. In 1988, an agreement was reached to harmonize supply system voltages in the whole of Europe. They settled for a unified voltage supply system of 230V at the supply terminal of single phase systems and 400V for 3-phase systems. It took 7 years for the harmonization to hold. In both cases, the tolerance levels had to be changed to +10/-6% to accommodate the voltage supply regulations in the British power system. Thus the voltage range became 216V – 253V for single phase supplies and 376V – 440V for 3-phase supplies. 8 years later, precisely on 1st of January, 2003, the tolerance levels were changed to +/-10% to cover the ranges 207V – 253V for single phase AC systems and 360V – 440V for three phase systems. This was principally due to the insistence of British power engineers who opined that any harmonization must protect British consumers and suppliers. In the end, the tolerance levels accommodated all concerned parties.
Standard Types of Supply in the NESI
The regulation that governs the supply and installation standards in the NESI is known as the NESIS REGULATIONS 2015. It tends to conform with the requirements of international standards bodies such as the international electro- technical commission (IEC). There is nothing bad in this except that, Africa should endeavour to set up it’s own regional standard body to be equivalent to IEC and other regional standard bodies or organizations to factor in some of our unique peculiarities. Saying that, the IEC allows individual nations to elect to stick to standards based on their unique circumstances.
Section 5.1.6 of the NESIS REGULATIONS 2015 deals with Systems of Supply. Standard A.C voltages shall be 230V +/-6% between phase and neutral conductor and 400V +/-6% between phase conductors. Primary distribution high voltage shall be 33,000V +/-6% and secondary distribution high voltage shall be 11,000+/-6% it states. This however conflicts with table 7.2.2.1 of the same document regarding the permissible voltage range at 11,000V.
The regulation further provides that “the supply voltage for industrial purposes may be given at higher voltages either for transformation or for direct supply to motors or any other agreed voltage between the distribution licensee and the user”.
Section 5.1.6, sub-section ii stipulates the voltage at the supply terminal (at the user’s main switchboard) to be:
- 230V for single phase AC systems
- 400V between phases for 3-phase four-wire AC systems.
The voltage shall be maintained within +/-6% of the nominal voltage.
This gives a possible spread of 216.2V to 243.8V for single phase AC systems and 376V to 424V for 3-phase supplies.
IMPLICATION OF A 230V/400V SUPPLY SYSTEM CHANGE IN NESIS REGULATIONS 2015
Since the supply voltages in the NESI before this regulations lie within the acceptable limits, the distribution companies (DisCos) are not likely to reduce their voltages in the near future. This is because such an action would immediately reduce the energy used by consumers and therefore lower the income from consumers with prepaid meters by more than 8%. Hence, it can be safely assumed that supply voltage will remain at 240/415V.
However, manufacturers may supply appliances rated at 230V for use in Nigeria. When they do so, there will be problems.
A 230V linear appliance used on a 240V supply will take 4.3% more current and will consume almost 9% more energy. For example, a 230V rated 3kW immersion heater, will actually provide almost 3.27kW when fed at 240V. This means that the water will heat a little more quickly. Even at that, there is unlikely to be a serious problem other than that the life of the heater may be reduced, the level of reduction being difficult to quantify.
Life reduction is easier to specify in the case of filament lamps. A 230V rated lamp used at 240V will achieve only 55% of its rated life (it will fail after about 550 hours instead of the average of 1,000 hours) but will be brighter and will run much hotter, possibly leading to overheating problems in some luminaires. The starting current for large concentrations of discharge lamps will increase dramatically, especially when they are very cold. High pressure sodium and metal halide lamps will show a significant change in colour output when run at higher voltage than their rating, and rechargeable batteries in 230V rated emergency lighting luminaires will overheat and suffer drastic life reductions when fed at 240V
There could be electrical installation problems here in the future!
THE IMPACT ON INDUSTRIAL MOTORS
What about when 400V motors are run on 415V systems? The question is whether it is acceptable for 400V motors to run on a 415V system.
Basically, if you run a motor above or below its nominal voltage, it can overheat and be less efficient. However, this depends on various factors including but not limited to its loading, enclosure or environmental conditions, over-temperature controls, and protection to mention a few.
In practice, what to determine is whether the voltage will be out of specification in terms of the voltage limits earlier discussed.
If 400V is the nominal voltage of the motors, and the voltage at the supply terminal to the motor varies in the range 376V – 424V, manufacturers generally take this into account by designing for what we call the Utilization Voltage Range. As a rule of thumb, manufacturers should at least design for +10 % / – 15 % of nominal voltage (e.g. 340 V to 440 V). But what manufacturers do vary.
Similarly, the electrical supply will have a Supply Variation, and the electrical installation will also have a Voltage Drop. e.g. in the UK, a 400 V supply in accordance with the relevant legislation is +10 % / – 6 % of nominal, and the voltage drop in the installation can cause a further 5 % drop in voltage in the motor circuits. This means that the installation delivers a Design Utilization Voltage variation of + 10 % / – 11 % (356 V to 440 V). The previous UK requirements before harmonization were 415 V +/- 10 % (373.5 V to 456.5 V – and there seem to be few worries about the additional 16.5 V when replacing motors with the new 400 V ones, as this is often taken up in the volt-drop in the installation).
The ideal thing is to determine the Utilization Voltage Range the motor has been designed for, and for a particular installation, the design Utilization Voltage Variation at the point of connection of the motors, and then correlate the two. The main issue is more on starting of motors due to volt drop in the installation, but matching equipment voltage to supply voltage variations (accounting for volt drop) is quite important.
Even using a motor at up to 10% change in supply voltage will not result in any great deal of difference except if the supply voltage were fixed.
Operationally, heating comes from two sources and one of them is fairly small. When designing an induction motor the primary controlling factor is not the supply but the necessary power output (mechanical). On no-load the motor will be perfectly happy to turn and thus cool on a wide range of supply voltage, and the current will be mainly controlled by the winding inductance and iron loss from the basic magnetising current. As mechanical load is applied, the current will rise and the loss increase mainly as resistive loss in the windings which generate heat. The supply voltage is not the controlling factor, except that it must be sufficient to sustain the losses and provide enough mechanical output. A 10% change in supply voltage will not result in any great design difference, but will enable the motor to provide more power under overload conditions which may well cause overheating. As long as the overload trip is set correctly, there is nothing much to worry about. Most induction motors have a nominal operating voltage specified, and this is really only a guide.
To be sure, if you connect a 400V motor to a 600V supply it will not fail instantly, and will probably be quite satisfactory until such time as it gets overloaded, then it will get far too hot! The magnetising current will however be higher than expected (the no-load current), but not extreme.
Nowadays, the application of variable speed drives (VSDs) gives better control of the load and power factor. This means you can optimise performance, and at the same time save on electricity consumption. The return on investment in VSDs is quickly realised by savings and operational flexibility.
It is important to request for the motor type test report to confirm the impact when you connect a 400V motor to a 415V supply.
Parameters to watch for include the efficiency, power factor, electrical input power to motors, reactive power consumption, and possible investment in capacitor banks for power factor correction.
Manufacturers produce motors for 400V, 415V and 440V (the three common voltages in the range) and typically have a single production line for all three, in which case, the 400V motor will be exactly the same as a 415V motor. On the other hand, if a manufacturer only produces for 400V systems, it is possible that the motor insulation will be slightly overstressed by the increase in voltage and its life may be reduced. As the increase is less than 4%, the risk is very small.
Connecting the 400V motor to a 415V system will slightly change the motor characteristics, lead to brisker start up, probably higher starting current/faster acceleration, less prone to stall, but the overall impact is likely to be an improvement in operation.
Another consideration should be the mode of starting of the industrial motors. Direct on line starters are now commonly replaced by variable frequency drives (VFDs) which have become cheap enough to really take off as the way to run sites with large number of high current motors most economically. Also, with VFDs, the supply voltage is even less important, as the electronics sort things out.
Low Voltage three phase supplies in the UK are still in reality about 415 Volts most of the time in most places, despite harmonization at 400 Volts. The same motors are used in Continental Europe on supplies that tend to be 380 volts actual and in the UK on 415 volts actual, despite both being called 400 volts.
It is important to check what the actual supply voltage is at your point of common coupling with the low voltage supplies popular in our distribution system in Nigeria. If it is within the new range, then it is fine.
Generally, three phase induction motors should tolerate a supply from 90% of nominal up to 110% of nominal voltage, or from 360 volts up to 440 volts for nominal 400 volt units. Long term reliability may decline a bit at the extremes of the range if used on a 415V system.
Higher than nominal voltage will increase the magnetising current and result in extra heat. 5% over voltage should be fine, and 10% should be acceptable under favourable conditions, but high temperatures, poor ventilation, or other site conditions may reduce reliability.
Summarily, consideration should be given to a revision of the tolerance levels in the NESIS Regulation to +/-10% at 230/400V as there will be no material need for unexpected capital expenditure (CAPEX) on the part of consumers. Also, a stakeholder engagement with inputs from seasoned power engineers should have come before the notice for compliance perhaps there may be a review of the NESIS Regulations 2015.
This process of taking expert review on the technical aspects of power systems should be the norm and not the exception in a power system like ours. This will avoid implementation problems that occur when we place the cart before the horse.
Engineer Idowu Oyebanjo is a UK chartered power systems engineer
