When a rail freight company needed to increase corridor capacity on an electrified traction network supplied by Eskom, a robust solution was required to control network voltage unbalance.
As South Africa’s licenced transmission service network provider, Eskom is responsible for delivering a reliable supply of electricity. In addition to ongoing operations and maintenance, this involves adapting and strengthening the transmission system in line with changing customer needs.
In 2011 a project was launched to enhance the stability and security of power supply from Eskom’s Garona Substation to the traction network of Transnet Freight Rail (TFR - formerly Spoornet). TFR needed to increase the single phase load and frequency of train operations on its network in the Northern Cape province. A sophisticated dynamic reactive power compensation solution was deemed necessary to manage the network impact of this customer connection upgrade. Following a competitive global tender, the technically challenging project was awarded to RXHK (then part of RXPE).
All traction supply networks have to deal with complex heavy loads. Train consumption of electric power varies significantly depending on track conditions, acceleration, braking and train length and loading. The 25kV single phase supply needs to be obtained from the three-phase grid supplying the power. The unbalance this causes can create problems which have ramifications for the entire supply grid and the other grid customers.
Studies showed that TFR’s plans to increase train frequency and network load would cause an excessive level of voltage unbalance at the Garona Substation’s 275kV network connection point. There was a critical need to control the negative and positive phase sequence components of the network voltage, with the negative phase sequence taking priority.
Eskom contracted RXPE to design, supply, install and commission a reactive compensation system including a coupling transformer on an EPC basis. The initial expectation was that a +20Mvar inductive / -45Mvar capacitive Static Var Compensator would be deployed for dynamic voltage control at the Garona Substation. However, RXHK’s engineering team proposed the high power ±45Mvar Maxivar™ STATCOM as an alternative to meet the project’s requirements and deliver additional value.
The core objective of this project was to control the continuously changing pattern of unbalance from the single phase traction load. The Maxivar™ STATCOM enabled this goal to be met cost-effectively as well as offering wider benefits.
Performance related advantages of STATCOMs over traditional SVC solutions are significant. They range from faster response time and a wider operating range to lower harmonics and stronger voltage fluctuation and flicker mitigation. Overall losses are lower and the equipment benefits from built-in redundancy as well as a smaller footprint. There is also more potential to upgrade the technology if future network modifications are required.
The Maxivar™ STATCOM underwent extensive Factory Acceptance Testing witnessed and verified by Eskom representatives. It met project requirements, providing dynamic reactive compensation support, voltage control and three phase unbalance control.
After the solution’s successful installation and commissioning, Eskom sought to evaluate its behavior under weak grid conditions including physical network reconfiguration to test the weak grid performance. A comprehensive RTDS Simulator test provided the insights needed to further enhance and maintain the security of the transmission system.
The Maxivar™ STATCOM has been in operation since August 2013 and has significantly improved the voltage quality and performance of the 275kV network due to its control actions.
The ongoing success of this project underlines the superior capability, reliability and benefits of voltage source converter STATCOMs versus SVC solutions.
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