Design overview and analysis of 11/0.415 kV, 500 kVA power substation in Nigeria

Layout and arrangement drawing

The practical design of factors such as the location of protection, distribution distance to source on the low-voltage side, and other general factors will need to be considered. First, put together a good design consistent with good construction practices, including the ground grid and equipment foundations. It is difficult to replace a failed or weak foundation after the equipment has been installed.

Verify the credentials of the people doing the work and their safety records and review references. It is good to have a licensed engineer perform the engineering work and be available during construction.

It’s essential to take into consideration a single line diagram and type of protections against phase to phase and phase to earth faults, main characteristics of electrical equipment, solution for the compensation of the reactive energy, and earthing system.

Information and Requirements to Be Provided

These include the following:

• Level of voltage supply by overhead line,
• Supply by underground cables,
• Single-line service,
• Ring type supply,
• Parallel feeders, etc.
• Rated values concerning the voltage,
• Rated value concerning the current,
• Details concerning the applicable tariff and the billing of the electrical energy,
• Comments on drawings and information provided by the consumer, and
• Specific requirements applicable to the substation.

The detailed studies of the substation must take all these parameters into account and other requirements.

Commissioning, Testing and Energizing

When required by the local authority, commissioning tests and checking must be successfully completed to get the authorization to energize a new installation. The following tests and checking are generally mandatory and applicable to the whole installation:

Electrical Installation Works:

These include Insulation Resistance test, Verification of polarity, Continuity test, Ring Circuit Test, Effectiveness of earthing test.

Electricity Projects:

• Insulation Resistance Test performed by Megger tester
• Transformation Ration Test
• Short Circuit test
• Excitation Test
• Transformation Oil Dielectric strength Test
• Earth Resistance Test

As soon as the conformity official document is issued, the utility proceeds with the energizing of the installation.

Transformer Tests

On completion of the design of 500 kVA 11/0.415 kV/kV some forms of inspections and testing with suitable instruments by qualified person (installation inspector or engineer or technician) are desirable and necessary. These are to ensure suitability, fitness and the reliability of materials and equipment used for the completed electrical installation and electricity projects before they are commissioned for use.

They are also to ensure the safety of operation of the installation or network.

However, the inspection and testing that are carried out on a completed installation works are not a complete and all time guarantee of the safety and quality of the electrical installation works after the initial testing and commissioning is necessary to maintain the installation in proper and safe times.

More importantly, such regular inspection and testing help to ensure as far as practicable that the safety requirements as specified in the regulations have been complied with.

Things to note before carrying out any test

1. Ensure the reliability of the measuring instrument.
2. Ensure that the three J&P fuses (Drop-out fuses) are isolated from the high tension.
3. Ensure that the fuses in the feeder pillar are all deloaded from the circuit unit.
4. Avo-meter must be used to check if there is any unidentified source, if there is any returning current.
5. Test must not be carried out when it is raining and also when there is lightning, because when transformer is wet it gives wrong reading and for lightning for safe working condition.
6. The neutral cable must be loose from the secondary side of the transformer before testing.

Open and short circuit testing

Open and short circuit tests are performed on a transformer to determine:

1. Equivalent circuit of transformer
2. Voltage regulation of transformer
3. Efficiency of transformer

The power required for these Open Circuit test and Short Circuit test on transformer is equal to the power loss occurring in the transformer.

Open Circuit Test on Transformer

A voltmeter, wattmeter, and an ammeter are connected in LV side of the transformer. The voltage at rated frequency is applied to that LV side with the help of a variac of variable ratio auto transformer. The HV side of the transformer is kept open.

Now with help of variac applied voltage is slowly increase until the voltmeter gives reading equal to the rated voltage of the LV side. After reaching at rated LV side voltage, all three instruments reading (Voltmeter, Ammeter and Wattmeter readings) are recorded.

As the transformer is open circuited, there is no output hence the input power here consists of core losses in transformer and copper loss in transformer during no load condition. But as said earlier, the no load current in the transformer is quite small compared to full load current so copper loss due to the small no load current can be neglected.

Hence the wattmeter reading can be taken as equal to core losses in transformer.