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Why Transformer Rated In kVA, Not in KW?

Transformer Always Rated In kVA instead of KW

As the name suggest, transformer only transfer the power from one circuit to another without changing the value of power and frequency. In other words, It can only step up or step down the value of current and voltage while the power and frequency would remain same. A general date on transformer nameplate are printed for further details, such as rating in VA, single phase / three phase (power or distribution transformer), step up / step down, connection etc.

There are two type of losses in a transformer;

1. Copper Losses

2. Iron Losses or Core Losses or Insulation Losses

Let’s explain in more details to get the idea that why a transformer rated in VA instead of kW?

When manufactures design a transformer, they have no idea which kind of load will be connected to the transformer. The load may be resistive (R), inductive (L), capacitve (C) or mixed load (R, L and C). Its mean, there would be different power factor (p.f) at the secondary (load) side on different kind of connected loads depends on R, L and C. This way, they go for VA instead of W in case of Transformer.

Lets clear the rating of transformer in VA instead of W  with solved example.

Losses of transformer will remain same as  long as the magnitude of current / voltage is same. No matter what power factor of the load current / voltage is

Example 

Suppose for a single phase step up transformer

Transformer rating in kVA = 11kVA

Primary Voltages  = 110V

Primary Current = 100 A

Secondary Voltages = 220V

Secondary Current = 50 A.

Equivalent resistant on Secondary = 5Ω

Iron losses = 30W

In first scenario, If we connect a resistive load to the secondary of the transformer at unity power factor θ = 1,

Then total losses of transformer would be copper losses + iron losses, i.e.

I²R + Iron losses

Putting the values,

(502 x 5 ) + 30W = 12.53kW

i.e. losses on primary and secondary of transfer is still same. (See below example for secondary losses as well)

The transformer output will be:

P = V x I x Cos θ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 1 = 11kW.

Now rating of transformer

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

Now, In second scenario, connect a capacitive or inductive load to the secondary of the transformer at power factor θ = 0.6.

Again, total losses of transformer would be copper losses + iron losses, i.e.

I²R + Iron losses

Putting the values,

(502 x 5 ) + 30W = 12.53kW

Hence proved that losses in both of primary and secondary is same.

But The transformer output will be:

 

P = V x I x Cos θ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 0.6 = 6.6kW.

Now rating of transformer

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

Now, In second scenario, connect a capacitive or inductive load to the secondary of the transformer at power factor θ = 0.6.

Again, total losses of transformer would be copper losses + iron losses, i.e.

I²R + Iron losses

Putting the values,

(502 x 5 ) + 30W = 12.53kW

Hence proved that losses in both of primary and secondary is same.

But The transformer output will be:

P = V x I x Cos θ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 0.6 = 6.6kW.

Now rating of transformer

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

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Its mean, 11kVA transformer rating means it can handle of 11kVA. It is our turn to transform and utilize the 11kVA as 11kW (we can do it by improving the power factor to 1 in case of pure resistive load) which is not predictable and even very hard to get in case of inductive and capacitive loads where power factor would have different values.

From the above example, it is clear that the rating of transformer is same (11kVA) but different output in power (11kW and 6.6kW) due to different power factor values after connecting different kind of load which is not predictable for transformer manufactures where the losses are same in both cases.

So these are the exact reason for transformer rating in kVA instead of kWA.

Distributed pumping solutions represent a new paradigm in chilled water air conditioning

Chilled water systems with modulating valves are common air conditioning systems in today’s commercial and residential buildings.

However, these systems face challenges with balancing and poor dynamic flow regulation, which leads to severe energy loss, inadequate climate control, and an often uncomfortable environment.

Focus on the heart of the HVAC system

All chilled water distribution systems require pumps for moving the chilled water, and all buildings have several terminal units such as AHUs, FAHUs, FCUs with different needs.

There are several reasons why chilled water loops can get out of balance, such as improper commissioning, components deteriorating over time, aging of the building, and changes to other parts of the system installation.

An imbalanced water loop can lead to a low Delta T, causing the chillers to work outside the best efficiency point and over pumping the loop.

This leads to excessive energy consumption and can result in an uncomfortable environment.

As a solution to these challenges, distributed pumping solutions are growing in popularity.

Replacing valves with pumps on each floor of the building, instead of centralizing them in the basement, provides continuous automatic balancing, reducing pump energy consumption and providing a more consistent and comfortable indoor climate.

Holistic solution to imbalanced water loops 

Distributed pumping solutions are a paradigm shift away from centralised pumps in distribution networks towards decentralised pumps throughout the building.

By replacing balancing and motorised valves with pumps, the system is equipped only with components that generate pressure only when and where it is needed.

This reduces the time spent on balancing the system, as once the correctly sized pumps are selected, there are no valves needed to 

balance the system.

Additionally, the main pumps can be downsized as distributed pumps generate the needed pressure individually, saving pump energy that way as well.

Distributed pumping solutions can be applied to existing chilled water systems that need refurbishment, or to new commercial buildings planned with chilled water air conditioning.

How distributed pumping works

Distributed pumping systems consist of five key components: primary pumps, distributed pumps, primary pump controller, check valves, and sensors located throughout the building.

The primary pump controller uses a control algorithm to manage the primary pumps, which are variable speed pumps that are regulated by sensor measurements from the decoupled line to avoid over or under pumping the system. Dedicated distributed pumps are installed with a non-return valve at each air handling unit (AHU) or a branch containing multiple FCUs.

The distributed pumps measure the air temperature using the AHU air duct sensor and will automatically regulate the speed to achieve the desired temperature.

Interfaces with the building management system (BMS), if installed, and other control options can be discussed during the design process, ensuring seamless integration based on the sequence of operations.

For multiples FCUs in the branch, the pumps’ in-built differential pressure control enables perfect proportional pressure control, so even the further FCU is adequately fed with flow and pressure to create perfect indoor climate

A well-balanced loop system creates a well-balanced indoor climate

Distributed pumping solutions represent a new paradigm in chilled water air conditioning.

By providing consistent, accurate load balancing, distributed pumping solutions save energy and provide optimal comfort for people in the building.

They are also fast and easy to commission, reducing the initial investment and the time spent on system balancing.

For all these reasons and more, distributed pumping is becoming widely spread in commercial building projects around the world.