Analysis of the effect of capacitor switching transient on a grid substation (case study)

Switching of breaker switched capacitor

Main objective of this study is to analyze the effect of switching of the breaker switched capacitor (BSC) banks to the power quality and also to identify solutions for existing power quality issues with capacitor bank energizing.

100 Mvar installed at Pannipitya substation in Sri Lanka which is owned by Ceylon Electricity Board (CEB) was failed after putting into commercial operation. Because of that the 10 Mvar capacitor bank installed at Thulhiriya substation and 20 Mvar capacitor bank installed at Athurugiriya substation were switched off and kept de-energized even without any failure since all the three capacitor banks came to Sri Lankan network under same project with same equipment.

This results in under utilization of available resources with an existing requirement. This scenario made the motivation for this study.

Scope of work

The study is mainly based on a selected Grid Substation in Sri Lanka’s power network which is the Thulhiriya Grid Substation (GSS) as a case study.

The study includes following steps:

1. Analyze the present need of capacitor banks for the selected substation by collecting data through data logger and the log sheet data available at the substation.
2. Collecting the major equipment data in the substation and model the selected substation using the PSCAD simulation software.
3. Run the modeled simulation for transients and harmonics in various scenarios and loadings.
4. Analyze the obtained results for the selected substation and identify solutions for the existing power quality issues with the capacitor banks.

The case study

Failure of capacitor banks at Pannipitiya substation

During year 2000 it was identified that the low power factor of the Colombo power system as a serious problem to keep the operating voltage of the 132 kV in national grid. Not only in Colombo but also studies reveals that to boost the voltage and the power factor capacitor banks should be installed at Pannipitiya, Athurugiriya and Thulhiriya Substation.

So as a result of that, under the transmission and Substation development project:

• 2, 100 Mvar capacitor bank was installed at Panninpitiya GSS,
• 20 Mvar capacitor bank was installed at Athurugiriya and
• 10 Mvar capacitor bank was installed at Thulhiriya GSS.

However the studies revealed that the cause for the Pannipitiya capacitor bank failure is due to manual switching of entire 100 Mvar within 3 min and thus creating excessive voltage rise within short period of time.

The motivation for this study is the switched off capacitor banks due to the Pannipitiya capacitor banks failure. So as a case study Thulhiriya substation and the capacitor bank are selected.

Thulhiriya substation details

Thulhiriya Substation is a 132/ 33 kV switching station which is fed by two number of 132 kV incomers from Polpitiya/ Athurugiriya 132 kV line.

The substation is equipped with three numbers of 31.5 MVA transformers and 10 number of 33 kV feeder bays. Two capacitor banks are available each of size 5 Mvar bank feeder and filter feeder.

132 kV side of the substation is AIS (Air Insulated Switchgear) while 33 kV side of the substation is a GIS (Gas Insulated Switchgear).