Basic Guide to kVA Transformer

kVA is usually a unit of transformer capacity. It is also necessary parameter information when we are doing transformer projects or quoting customers.

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What does kVA mean on a transformer?

Power transformer capacity unit KVA, that is apparent power. Most electric power loads are inductive loads and their apparent power is equal to the vector sum of active power and reactive power. Therefore, the apparent power of the inductive load is greater than the active power. If the active power selection transformer is used, the apparent power of the inductive load is greater than the capacity of the power transformer, and the load current is greater than the allowable value of the transformer, so the inductive load must be used in the apparent power selection transformer.

If the load carried by the transformer is all resistive load, the total power can be equal to the KVA value of the transformer.
KVA instead of KW? VA or KVA is the apparent power, used in AC lines, also known as capacity, is the product of voltage and current effective value, unit... But it will affect the transformer load design, so VA or KVA to calculate more accurately. And W or KW is our usual customary name, written formally.

Calculation formula of transformer capacity
Namely: β=S/Se------- formula (1)
Where S is the calculated load capacity (kVA), Se is the transformer capacity (kVA); β is the load rate (usually 80% ~ 90%)

Learn more: The Ultimate Guide to 1500kVA Transformer

How to size transformer kVA?

The selection of transformer capacity has a great impact on the comprehensive investment benefit transformer capacity is too large, there is a "big horse car" phenomenon, not only one-time investment is big, no load loss is also big. Transformer capacity selection is too small, transformer load loss increases, economic unreasonable, and technology is not feasible. The best transformer load rate (that is, the highest efficiency of the load rate), not in the rated state, but between 40% ~ 70%, the load rate is too high, the loss increases significantly; On the other hand, because the transformer capacity margin is small, the load increases slightly, it is necessary to replace the large capacity box, frequent capacity increase is bound to increase investment, affect the power supply. Selection of transformer capacity, to the existing load as the basis, appropriate consideration of load development, selection of transformer capacity can be determined in accordance with the 5-year power development plan.

Read more about Transformer Size

How to calculate transformer kVA rating?

(1) calculation of transformer capacity
First, choose the rated voltage of the transformer. The high voltage side voltage is the same as the grid-connected voltage, and the low voltage side voltage is 10% or 5% higher than the low voltage side (depending on the transformer voltage level and impedance voltage); Selection of rated capacity.

Calculate the transformer load size (calculate the maximum comprehensive load, and the active load kW value into the apparent power kVA). If there are two transformers, you can choose the capacity of each transformer according to 70% of the maximum comprehensive load, consider a transformer according to the total load, and consider the appropriate margin. Other famous parameters can be combined with transformer products. A good transformer load-bearing capacity, with a wide range of applications, is undoubtedly the first choice of the transformer.

Example: Select 35/10kV transformer. Assume a maximum load of 3500kW and a power factor of 0.8. Can choose two capacity of S=0.7×3500/0.8=3062kVA transformer, that can choose 3150kVA transformer, voltage ratio is 35kV/10.5kV. Then select the model from the product catalog.

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How to calculate 2500 kVA transformer losses?

Because transformer no-load loss NL and load loss LL both include rated active loss and reactive power active loss on the meter.
Loss NL=Po+kQo=Po+ K
(Io*Se/100)
Load loss LL=Pf+kQf=Pf+ K
(Ud*Se/100)
Po is rated no-load active power loss of transformer, namely iron loss kW;
Pf is transformer rated load active power loss, namely copper loss kW;
Qf is flux leakage power of the transformer-rated load
K is the economic equivalent of reactive power
Io is the no-load current of the transformer, %
Ud stands for transformer impedance voltage, %
Se is transformer rated capacity, kVA

There is only one known condition, Se=2500, and there are some differences in transformer loss between different models. For example, s11-2500 transformer, the no-load loss is 2.5KW, and the load loss is 23.2kW.

More detail about the 2500 kVA Transformer

25 kVA transformer load capacity in kW

Most all-electric homes in the U.S. have 200A service, though they rarely need that much at any given time. A four-bedroom 2,300-square-foot house, for example, uses about 50 kWh per day or 1,500 kWh per month. I would say it's typical.

KVA is capacity, kilovolt-ampere, generally used for transformer equipment such as 25KVA transformer, power factor calculated according to 80%100, the maximum output power is 20 kW.

Read more about the 25 kVA Transformer

100 KVA transformer load capacity in kW

1, transformer output power is the apparent power, and active power (KW) relationship is: apparent power = active power x power factor.

2, with the average power factor of 0.8 calculation,a  100 KVA transformer can provide (100 KVA × 0.8) 80 KW active power for the load.

How many kVA transformer do I need?

Suppose the required load phase current is 50 amperes, let's call it L. After these two values have been discovered or calculated, the data can be used to calculate the power demand (in kilowatts) of the load. For this purpose, the desired input voltage (V) must be multiplied by the desired current load (expressed in amperes (L)), then 1000: V*1/1000 in the above example, 150 times 50 to get 7,500, and 1000 divided by 1000 to get 7.5 kilowatts. 

35kV transformer general capacity, how much kVA is it?

1, if it is a power transformer, for example, the high voltage is 35kV, and the low voltage is 3.15-11kV. So, whether load voltage regulation or no excitation voltage regulation, according to the national standard, the maximum capacity is 20000kVA.
2. If the high voltage is 35kV and the low voltage is 400V, the maximum capacity is 2500kVA according to the above standards.

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How many kilovolt amperes are the capacity specifications of the power transformer?

The rated capacity grade of transformer in Our country is increased by multiple of R10≈1.26, such as the capacity of 100, 125, 160, 200...... KVA, etc., only capacity grades other than 30 kVA and 63,000 kVA differ from the priority number system.

Before 1967 the rated capacity grade of transformers was R8 capacity series increased by R8=8≈1.33.
R10 means 10 to the 10th power.

Learn more about Transformer Specifications

How many KVA are factory distribution transformers?

From 630-2500 kVA
The power consumption of the factory is 50 kilowatts. How many kilovolt-ampere transformers are needed?
Let's say we only have 50 households a day, 32 at most, and what if it's at night, in the summer? So a 50kVA transformer is not enough. 100 households with normal electricity and enough transformers will need at least 200kVA.

1000 kVA transformer no-load losses

The accurate value can be checked in the factory test report of the product, and the product sample can be checked in the design stage. Here's an example:
S11-m-1000/10 oil-immersed transformer, no-load active power loss 1.15KW, multiplied by 24 hours is a day of active power loss 276KWh;
No-load reactive power loss is approximate: no-load current 0.6%* rated capacity 1000., and is 6kvar, multiplied by 24 hours is 144kvarh of reactive power loss in a day.

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1000 kVA transformer impedance

For a 1000kVA transformer, the impedance is usually around 6% and will not exceed 8%.

Why transformer kVA, not kw?

Transformer load is generally mixed, including resistive, inductive, and capacitive loads. Using KW (active power) as the output capacity of the transformer may cause transformer overload. Therefore, transformers are marked with KVA(apparent power) transformer capacity.

The apparent power is numerically equal to the product of the rated voltage and current of the transformer. The transformer is certain, the load active power is certain, and the current will differ from the power factor. Therefore, the transformer with KVA marking the capacity of the transformer, as long as the rated current does not exceed the transformer will not overload operation.