Transformer Selection and Sizing in Substation Design

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In electrical power system, transformer is one of the most important components, as it helps grid operators to increase or decrease the voltage levels safely and efficiently to provide reliable power supply to consumers.

Read this chapter to gain a good understanding of the following fundamental concepts of Substation Design −

• An overview of transformer definition and working principle
• Needs of transformer in power system
• Different types of transformers used in Indian power system
• Selection of a right transformer for specific application
• Transformer size calculation for a given load

Apart from these key topics, we will also learn about common transformer sizes used in India and some Indian standards related to transformers.

What is a Transformer?

An electrical transformer, simply called a transformer, is an electromagnetic machine that transfers electrical energy from one circuit to another through the magnetic medium at a different voltage level. Transformers work on the principle of Mutual Inductance and Electromagnetic Induction.

A transformer is a static machine having no moving components, instead it has three key components namely,

• Primary winding
• Secondary winding
• Magnetic core

Note − A transformer does not generate any power; it only transfers the power from one circuit to another with adjusted voltage level.

The above-mentioned three components of the transformer performs these functions −

• Primary winding receives the input voltage form one circuit at a certain voltage level.
• Secondary winding delivers the output voltage to another circuit at a different voltage.
• Magnetic core provides a low reluctance path to magnetic flux.

The voltage transformation equation of a transformer is given by,

$$\mathrm{\frac{V_{2}}{V_{1}} \:=\: \frac{N_{2}}{N_{1}}}$$

Here, V₁ and V₂ are primary and secondary side voltages and N₁ and N₂ are primary and secondary winding turns.

From this equation, it is clear that,

• If the secondary winding has more turns than the primary winding, then it increases the voltage and it is called step-up transformer.
• If the secondary winding has less number of turns than the primary winding, then it decreases the voltage and it is called step-down transformer.

Why Do We Need Transformers in Power Systems?

Most electrical power systems are segmented in three different parts namely, Generation, Transmission, and Distribution. All these segments operate at different voltage ratings. For example, in India,

• Generation of electricity takes place at 11 kV – 15 kV.
• Transmission takes place at 66 kV to 765 kV AC.
• Distribution takes place at 230 V to 33 kV.

Therefore, transformers are required for the following key purposes −

• Voltage step-up or step-down
• Electrically isolate one circuit from another.
• Fulfil the voltage level requirement of different sections of the power system.

Types of Transformers Used in Power Systems

There are several different types of transformers used in a typical electrical power system to meet the system demand. But the following two are widely found everywhere −

• Distribution Transformer
• Power Transformer

Distribution Transformer

These are generally step-down transformers and are mainly used in locations near to the load. In Indian power system, the common rating of distribution transformer lies between 25 kVA and 2500 kVA. The distribution transformers can be easily seen near to our homes, offices, or farms.

Distribution transformers commonly step-down 11 kV or 33 kV voltage to 440 V (3-phase) and 230 V (1-phase).

Power Transformer

A power transformer can be either a step-up or step-down depending on the network requirement. These transformers are generally found in generation substations, grid substations, transmission substations, etc. Their ratings lie between 5 MVA and 500 MVA or more.

Power transformers are generally used to transform voltages like 66 kV to 132 kV, 220 kV to 33 kV, 765 kV to 132 kV, etc.

Selection of Transformer

In electrical power system, transformer selection is a process of choosing the most appropriate type of transformer for a given electrical load. The select of transformer significantly affects the efficiency, maintenance requirement, reliability, and cost of the power system.

Given below are some of the major factors that play a vital role in selection of transformer for specific applications −

• Application − First of all, we need to understand the application type, whether we need to increase the voltage or decrease the voltage, accordingly we will select either a step-up or step-down transformer.
• Voltage Class − After that we will consider the voltage class, i.e. the transformer voltage ratings must match the grid voltages.
• VA Rating − It is another critical factor that must be considered. We have to calculate the VA rating, generally in kVA and MVA, depending on our load requirement and power factor.
• Cooling System Type − We consider a transformer with proper cooling system depending on its installation location, safety, and size.
• Winding Material − If higher efficiency and lower losses are our preference, then copper winding should be used. On the other side, if we can go with lower cost, higher losses, and larger physical size, then we can use aluminum winding.
• Environmental Conditions − If transformer is being used in high ambient temperature, dusty, or humid areas, then it must meet the proper insulation class. For example, for high-temperature areas, the transformer must meet class H insulation.
• Tap Changer − Depending on the requirement of voltage regulation, we select either on-load tap changer for real-time voltage regulation or off-load tap changer for occasional voltage regulation.
• Standards and Certifications − Transformer must comply with some standards like IS 2026 for power and distribution transformers, IS1180(Part 1): 2014 for energy efficient distribution transformers, CEA regulations, BEE star labeling for efficiency, etc.

Sizing of Transformer

Transformer sizing can be defined as a process of finding or calculating the correct power rating in kVA or MVA of a transformer so that it can be used to supply a given load safely and efficiently.

For a given load, selecting a transformer of correct size is important because of the following reasons −

If we select an undersized transformer, then it can cause −

• Transformer overheating, fire, or explosion
• Reduces the life span of the transformer
• Risk of frequent failures

If we select an oversized transformer, then it is also problematic because −

• It causes higher no-load losses and energy wastage
• Unnecessary expense due to higher transformer’s initial cost

After getting a basic overview, let us now see the process of calculating transformer size for a given load.

Transformer Size Calculation

First of all, it must be noted that transformer’s rating or size is specified in Volt Ampere (VA), like kVA or MVA rather than kW (kilo watt). It is because transformer acts like a source device when used in an electrical system and it does not mean to be a load.

Another reason is that it handles both active power and reactive power which are combined known as apparent power and it is given by,

$$\mathrm{kVA \:=\: \sqrt{kW \:+\: kVAr}}$$

Let us now see how we can calculate correct size of a transformer for a given load.

Step 1 − Total Connected Load

Firstly, calculate the total load that has to be connected to the transformer. It is basically sum of power rating of devices that will receive power from the transformer.

For example,

Lighting load = 50 kW

Motor load = 70 kW

Total Load = 50 + 70 = 120 kW

Step 2 − Accounting Load Power Factor

All load devices have a power factor and in India it varies between 0.85 and 0.95 for most devices. Considering power factor, we can write,

$$\mathrm{kVA \:=\: \frac{kW}{Power\: Factor}}$$

For example, let us consider a power factor of 0.90, then

$$\mathrm{kVA\:Rating \:=\: \frac{120}{0.90} \:=\: 133.33\:kVA}$$

Step 3 − Safety Margine Consideration:

There is always a scope of load increase in future and also inductive loads like motors take a very high inrush current while starting. To handle all these situations, it is recommended to add a safety margin of about 25%, so the transformer can be protected from overloading.

Hence the actual size of the transformer will be,

$$\mathrm{Transformer\: Size\: (kVA) \:=\: \frac{kW}{PF} \:+\: \left(\frac{kW}{PF} \:\times\: 25\% \right)}$$

For example,

Transformer size for the given 120 kW load will be,

kVA Rating of Transformer = 133.33 + (133.33 × 25%)

kVA Rating of Transformer = 133.33 + 33.33 = 166.66 kVA

Hence, the transformer must be of a kVA rating of greater than or equal to 166.66 kVA. Generally, we found 200 kVA transformer in market that can be used to power this load.

Numerical Example − A town has to be supplied by a power transformer. The load of the town is around 20 MW and the average power factor is 0.85. If we consider a safety margin of 25% for future expansion, then calculate the size of the transformer required.

Solution − Given data,

Load=15 MW

PF=0.85

Safety Margin = 25%

Then,

$$\mathrm{Transformer\: Size \:=\: \frac{15}{0.85} \:+\: \left(\frac{15}{0.85} \:\times\: 15\% \right)}$$

$$\mathrm{Transformer\: Size\:=\:17.65 \:+\: 4.41 \:=\: 22.06 \:MVA}$$

Hence, this load can be supplied by using a power transformer of 22 MVA.

Standard Sizes of Transformers in India

The following table gives commonly available standard sizes of distribution and power transformers along with their applications −

Conclusion

In this chapter, we covered the basic concepts of transformer selection and sizing. We provided a step-by-step guide to calculate the correct size of a transformer matching the load requirements. The most important point that the readers should always remember is that the transformer size is calculated in kVA or MVA and not in kW.