Current Transformers, the CT burden and commercial solar

Tuesday, July 6, 2021
Training
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LGC metering, what accuracy?  

With LGC there is a requirement for a certain accuracy of metering and in this presentation we will be looking at a 1000 kW commercial solar system installed in Melbourne, Victoria, Australia.

A renewable energy system this size can potentially produce 1,314 GWH per year without taking photovoltaic, solar panel degradation into consideration.

This means that the LGC metering required has a maximum allowable error of 1% and this means that the system is eligible to claim 100% of the metered electricity generation (DLEG) for LGCs. And that is where the CT burden comes in!

Why is this important??  

The general formula to calculate LGC’s is below:

𝐿𝐺𝐶 = 𝑇𝐿𝐸𝐺 − (𝐹𝑆𝐿 + 𝐴𝑈𝑋 + (𝐷𝐿𝐸𝐺 𝑥 (1 − 𝑀𝐿𝐹))) where:

  • TLEG = Total power generated by the power station
  • FSL   = Fossil fuel component of the power generated by the power station
  • AUX  = Auxiliary power consumed during the production or maintenance
  • MLF  = Marginal loss factor
  • DLEG = Eligible generation = TLEG – AUX (assuming FSL = 0)

This means that anything that is considered a load must be taken into consideration when calculating the actual output of the system that is usable, in other words, can service legitimate site loads.

How does this affect the selection of the CT?  To determine which CT is appropriate for a particular application, it is important to understand the following characteristics that are used to classify current transformers which include: ratio, polarity and accuracy classification.

CT ratio, for example, is the ratio of primary current input to secondary current output at full load. Let’s say we have a CT with a ratio of 300:5 (300 primary amps at full load) and this will produce 5 amps of secondary current when 300 amps flow through the primary. If the primary current changes the secondary current output will change proportionally.

CT burden  

The total resistance of the secondary circuit of a CT is known as a burden and this the sum of the resistances of the CT secondary winding, connecting wires (lead resistance) and the resistance of the actual relay/meter.

Burden of the current transformer is expressed in VA and the total VA burden should be taken into account when a CT is used for measuring or protection purposes. While designing the protection system or measurement system the VA burden of all the measuring instruments and protection relay must be taken into account for building a reliable measurement and protective system.

How do you calculate the CT burden ? 

The total VA burden of the CT can be calculated by adding the VA burden of the following: 

  • The VA burden of the protection relay and measuring instruments
  • Of the leads connected in between the CT and the relay/measuring meter
  • The secondary resistance of the current transformer


For the equipment used and the calculations please check out our video.


Conclusion

✅ Calculating the CT burden is important for accurate LGC calculations

✅ The total resistance of the secondary circuit of a CT is known as a burden 

✅ Have to look at the temperature coefficient and resistance of copper


If you’d like to see what Greenwood Solutions gets up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:

https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial


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