Electricity + Control February 2019

TRANSFORMERS + SUBSTATIONS

same.This reduces the resistance of the windings, resulting in less wasted power being generated. The peak energy index, PEI, of a transformer is used to determine the maximum efficiency at which a transformer can operate. Due to MEPS, the PEI of a transformer has improved. As an example, consider a 1 600 kVA CRT with a standard loss de- sign (CoBk Design).The no load losses are 2 800W, while the load losses are 14 500W at 120 o C. The peak efficiency of such a transformer can be calculated as follows:

es occur during each alternating cycle. As current flows in a forward direction the current increases and as a result the magnetic flux increases till satu- ration of the core occurs; as the current decreases so does the magnetic flux. However, even though the current has decreased to zero there will still be a small amount of residual magnetic flux left within the core. This residual magnetic flux adds to the no load losses. On a standard transformer no load losses account for 16% of total losses. The losses are always present and constant no matter the load on the transformer, including at idle. Copper losses, known as load losses, are due to current flowing through the winding conductor itself. The winding material has a resistance current flowing through and that resistance produces wast- ed power (P=I 2 R). Unlike core losses, copper loss- es are load dependent, thus more current flowing produces more wasted power. Another secondary effect is that as the winding temperature increases so does the resistance of the conductor, further in- creasing the wasted power. Copper losses account for 84% of a transformer’s losses at full load. What has been done about losses? The European Minimum Energy Performance Standard (EU MEPS) was set up in 2015. It requires transformer manufacturers to comply with a mini- mum efficiency level.This level is far more stringent than previous years and aims to standardise what different manufacturers considered as high efficien- cy transformers. It must be noted that Africa has yet to implement such legislation. Although transformers are inherently more ef- ficient than most other electrical machines, there is room for improvement. The EU estimates that 25% of the network losses within the EU are due to transformer losses. To reduce core losses, the core laminate ma- terial itself must be changed for one with better ferromagnetic properties. The best choice would be cold rolled grain orientated steel with its low- er coefficient losses. The steel laminates are also thinner and provide a higher number of core lami- nates per core. This increases the cross-sectional area of the magnetic core, reducing flux density and resulting in a more efficient core. To reduce copper losses, the cross-sectional area of the conductor must be increased. By doing so, the resistance of the conductor is decreased since the amount of current flowing remains the

By reducing the losses of a transformer the efficiency of the generating system can be improved.

     

     

Po Pco + ( )

2

∗

= − 1

100

PEI EI

Po Pco Pk +

Sr

Po = No load losses Pk = Load losses Sr = Rated power Pco = Losses of ventilation

Assume that the losses due to ventilation are null and see the calculation below, which substitutes the previous values.

     

     

(

)

+ 2 2800 0

∗

PEI PEI = − 1

100

+ 2800 0 14500

1600000

PEI = 99.20%

Low voltage winding.

Now consider a CRT designed to high efficien- cy standards (AoAK Design) with the same rated power. The no load losses are 1 800 W, whilst the load losses are 13 000W.This is an improvement of 1 000 W and 1 500 W respectively. The calculation below substitutes the previous values.

     

     

(

)

+ 2 1800 0

∗

PEI PEI = − 1

100

+ 1800 0 13000

1600000

PEI = 99.4%

Although 0.2% may not sound like a vast improve- ment in efficiency, take into consideration that most transformers run 24 hours a day until the end of their 20 plus years of life expectancy. The EU plans on further reducing the no load losses by a further 10% from 1 July 2021, further improving the PEI .

Medium voltage cast resin winding.

Electricity + Control

FEBRUARY 2019

33