Electric Transformer Rating (Explained for Beginners)

Electric transformer rating makes or breaks your electrical project. This is because under rating the transformer can cause it to fail, breakdown or completely damaged. It happened in my work, one of the most critical transformers had a serious damage, and we had to replace it ASAP to reconnect loads.

In this article I will discuss, how to rate a transformer, with examples for beginners. Let’s dive into details.

Standard KVA Rating of Transformers

We use KVA for transformer rating, (KVA means Kilo volt Ampere).  To determine the size of the transformer, it is important to determine the load by KVA.

However, Transformers are mostly used for the purpose of power distribution, that’s why common standard KVA transformers are common to use.

Two types of transformer KVA ratings are mentioned below.

For three Phase Delta Wye transformer 408 to 120/208 common standard KVA ratings are 15 KVA, 30 KVA, 45 KVA, 75 KVA, 112.5 KVA, 225 KVA, 300 KVA, and 500 KVA.

For a Single-phase transformer with the size of 277- or 480-Volt transformer common standards are 5 KVA, 7.5 KVA, 10 KVA, 15 KVA, 25 KVA, 37.5 KVA, 50 KVA, 75 KVA, and 100 KVA.

Please note, Besides the mentioned standard ratings there are other sizes of transformer available. However, the most common size is mentioned above.

Transformer KVA Rating Formula

The transformer KVA rating formula is derived from the power rating of the transformer. In this article, we will include a formula for both single-phase and three-phase transformers. So, let’s start with the formula of single-phase Transformer.

Single Phase Transformer

Single-phase Transformer Power Rating P = V x I

While : P is transformer power in VA, V is transformer voltage, I Transformer current.

Note that transformer power is fixed for low and high voltage sides. As you know, transformer affects voltage and current only. In the formula, we use voltage and current of the same side (low or high voltage).

Now, KVA Rating for Single Phase Transformer: P = (V x I)/1000

Hence putting the value of Current and voltage we will get the required rating of a single-phase transformer.

KVA Rating Formula for Three-phase Transformer:

The power rating of the three-phase transformer:

P = √3. V x I

Hence the rating of Three Phase transformer in KVA is given below

P = (√3. V x I)/1000

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How to Increase Transformer Rating?

Increasing transformer rating is possible by increasing the cooling method. Adding new fans or oil pumps increases the transformer rating up to 66%, Some transformers have more than one rated power on the nameplate depending on the cooling stage.

Most people think that rating depends upon the power rating formula. I.e. From the formula of power, they can change the rating. But this will need some extra work.

By increasing the value of current or voltage different types of losses such as eddy current losses, Hysteresis losses occur. Similarly, the same situation occurs for increasing the frequency. And the transformer becomes hot. Hence to increase the rating of the transformer we must try to cool down the transformer.

And for this purpose, we will need additional cooling fans to install on the transformer. Also, an automatic system will be required which automatically starts cooling the fan when the core temperature exceeds the specified temperature limit.The more the transformer is cooled, the more power will be transferred from supply to load side.

Transformer Rating Example

We already discussed the KVA rating formula for both single-phase and three-phase transformers. Now let take an example to get a better understanding of it. Here we will calculate the KVA rating of both single and three-phase transformers.

Single Phase Transformer Rating calculation

For a single-phase transformer the rating formula is given by: P = (V x I)/1000

Assume the voltage and current values as

Voltage (V)= 120 V

Current (I)= 50A

Now putting the values in the rating formula of the single-phase transformer to find the KVA rating of the transformer.

P = (V x I)/1000

(120 V x 50 A) / 1,000 =6 KVA

Hence the rating of a single-phase transformer with 12o Volt and 50 A current is 6 KVA.

  • 3 Phase Transformer Rating calculation formula

To calculate the rating of the transformer we will need to know about the primary and secondary voltages as well as current. Besides this, we will also need to assume a power factor value. Consider the following values on the nameplate of the 100KVA transformer.

Primary Voltages or High Voltages (H.V) is 11000 V = 11kV.

Primary Current on the High Voltage side is 5.25 Amperes.

Secondary voltages or Low Voltages (L.V) is 415 Volts

Secondary Current (Current on Low voltages side) is 139.1 Amperes.

Now using the 3 Phase transformer KVA rating formula= P = (√3. V x I) /100

Putting the values of either primary or secondary size we get the rating of the transformer.

P = (√3. 11000 x 5.25)/1000 =100KVA

Transformer Load Capacity

Transformers are passive devices used for changing the level of voltage or current. Transformer load capacity can be defined as the maximum amount of voltage or current that a transformer can withstand to safely operate the load.

The process of stepping up or stepping down current or voltage depends upon the number of turns in the primary and secondary sides of the transformer. Hence these windings determined the voltage and current ratings of the transformer.

This is the reason that each transformer has a nameplate, that contains this information. And from which we can decide the load capacity of the transformer. Common transformers in the market with load capacity are 15 kVA, 30 kVA, 45 kVA, 75 kVA, 112.5 kVA, 150 kVA, 225 kVA, 300 kVA, 500 kVA, 750 kVA, and 1,000 kVA. Besides this, there are transformers other than mentioned capacity. However, they are not very common.

Transformers Parallel Operation

In some cases, we use transformer parallel operation to feed large loads. In this case both transformers should have some identical properties. Let’s dive into details about parallel operation of transformers.

What is Parallel Operation of Transformer?

Parallel operation of transformer means to connect two or more transformers in parallel, to operate as one larger transformer unit, so that we can feed larger loads. And increase the reliability of the power system.

The parallel operation of the transformers can be understood as multiple transformers connected in parallel combinations.

Let’s define this phrase in a little more conceptual and engineering terms. When we connect two devices in a parallel combination, it means to connect them in such a way that their line (source) and the neutral wire are shared from a common supply.

The parallel operation of the transformer means that all the transformers in that combinations share their line (source) and the neutral wire at the supply as well as at the output of the combination.

As we know that a transformer has an input side from which it takes a high voltage or a low voltage, steps it down or steps it up respectively and an output terminal from which the stepped-down or stepped-up voltage can be accessed.

In the case of parallel operation of the transformer, the job remains the same which is to step down or up the voltage according to the need but here we use multiple transformers in such a way that each one of them contribute in doing the same job but efficiently.

Why is Parallel Operation of Transformers Necessary?

The parallel operation of a transformer is the most common practice used in the combination to perform the job of a transformer. The main reason for using the parallel combination of the transformers is :

  • They can supply larger loads than a single transformer.
  • And the reliability of the power system increased. In case of one transformer fault the others will continue supplying the loads.

When the load increases the rated load value of the transformer, the parallel combination becomes a very useful solution to the problem. Another solution to the problem is to replace the overloaded single transformer with a bigger unit of larger load rating value.

But this is not economical. As engineers, the economical factor of the device matters more than the solution of the problem. So, here is the actual solution i.e., the parallel combination of the transformer. This combination is actually the most economical solution as well as the most efficient solution to the problem.

Say the number of consumers that are fed by a single transformer increase in such a way that the kVA need at the consumer end has increased more than what our transformer can support.

The above solution explained is the best for this problem but if a new problem occurs that is the single transformer goes out of order (malfunction) due to increased load.

The power supply through that single transformer will be completely terminated. But in the case of our parallel combination of the transformer, the power supply to the consumers is not terminated completely.

But in some cases, the voltage level at the consumer end gets dropped which might affect the consumer device. If there is a need to get a break for maintenance purposes, we don’t have to turn the whole power system off instead we can maintain the first transformer then the second, and then the third one individually while those transformers that aren’t being maintained can continue to supply power to the load.

So, to be very specific on the reasons for the parallel combination we can say the parallel combination facilitates us as we can have:

  • Maximize the efficiency of the power system.
  • No interruption of power.
  • Reliable power system.
  • Flexible power system.

Every system has certain advantages and disadvantages of itself. So does the parallel combination of the transformers have. Let’s see the positive image of this system first.

Advantages of Transformers Parallel Operation? :

  • Maximum Efficiency:The parallel combination allows us to turn on/off one or more transformers according to our needs. Say 3 transformers are connected in parallel while the load needs can be fulfilled by 2 transformers. In such a case we can switch off one transformer. When the load increases and cannot be managed by transformers’ combination, the third transformer can be turned on. At the full load, the efficiency of the parallel combination of the transformer is maximum.
  • Increasing reliability:The parallel combination of the transformer is very much liked due to its own mechanism to deal with the faults that occurred in itself. Say, one of the transformers from the combination goes out of order, the power supply will not be stopped, and the reason is that the functioning of the whole system is not dependent on one single unit. In such situations, other transformers supply power without any kind of interruption.
  • Flexibility:The parallel combination is a flexible system as it allows to turn on or off any number of transformers.
  • Improved power quality:The quality of power is much more maximized when a parallel combination of transformers is used.
  • Transportation and installation in case of one large transformer is more difficult than in case of more than one smaller transformer in parallel operation
  • Its much cheaper to get one transformer to be standby for more than one smaller transformers in parallel.

So, connecting the same loads on more than one transformer in parallel operation is much economic and will increase reliability.

Disadvantages of Transformers Parallel Operation?

  • The bus rating (ratings of the line and neutral wire) could be too high.
  • The circulating current comes into the business that can cause losses. The core losses are one of the biggest drawbacks of this system.
  • The current rating for short-circuit situations is increased. And to cater to this situation, we need protections devices known as current limiters.
  • In the most important consumer end, that is the industry, the parallel combination is not very much liked due to the lower impedance of the system that needs other devices to correct its power factor.
  • The impedance of the system gets lower and that is the very reason for high short circuit currents.
  • Due to lower impedance, the protection and control devices are difficult to be matched. So, the system overall becomes complex.

Condition for parallel operation of transformers

The conditions that must match in case of using the parallel combination of the transformers are known as the Mandatory conditions. When these conditions are not met, the system will not work at all.

The conditions that can be compromised are the Convenient conditions. If these conditions are not met the system may work but the optimal efficiency is not guaranteed.

Mandatory conditions:

  • Same frequency.
  • Same polarity.
  • Same phase angle.
  • Same phase sequence.

Convenient Conditions:

  • Same voltage ratio.
  • Same turn ratio.
  • Same kVA rating.
  • Same tap changer position.
  • Same percentage impedance.

Which is Better, Two Parallel Transformers or One Large Unit?

Two or more transformers allows us to turn on or off the number of transformers in the combination according to the load, but in the single unit we cannot have this option, as the power supply to the load is dependent upon the whole of the single unit.

If one of the transformers in the combination goes out of order, other transformers continue to supply the load until it is repaired, but if the single unit goes out of order the whole consumer end will go dark.

If the load increases and a single transformer is being used, the transformer needs to be replaced with a higher-rated transformer. This solution is very expensive and takes time meanwhile the consumer’s power supply is interrupted.

While if the parallel combination is used, and the load increases we can just add one more transformer to the system without the interruption of the power to the consumer. Its easier to schedule preventive maintenance in case of parallel operation.

Also, its more economic to set one small standby transformer in case of parallel operation. Besides, transportation of large unit is harder than small ones.

Read my other articles:

Allowed Combinations of Parallel Transformers

What are the allowed combinations of the parallel combined transformers? Say we have multiple transformers connected in the parallel combinations.

Transformer 1Other transformers
1: Low Voltage primary: Star High voltage primary: Delta1: Low Voltage primary: Star High voltage primary: Delta
2: Low Voltage primary: Star High voltage primary: Star2: Low Voltage primary: Star High voltage primary: Star
3: Low Voltage primary: Delta High voltage primary: Delta3: Low Voltage primary: Delta High voltage primary: Delta
4: Low Voltage primary: Delta High voltage primary: Star4: Low Voltage primary: Star High voltage primary: Delta
5: Low Voltage primary: Star High voltage primary: Delta5: Low Voltage primary: Star High voltage primary: Delta
5: Low Voltage primary: Star High voltage primary: Star5: Low Voltage primary: Delta High voltage primary: Delta
6: Low Voltage primary: Delta High voltage primary: Delta6: Low Voltage primary: Star High voltage primary: Star
7: Low Voltage primary: Star High voltage primary: Delta7: Low Voltage primary: Delta High voltage primary: Star