Substation gets protection upgrade

November 13th, 2015, Published in Articles: Energize

 

Fordsburg substation, as one of three major bulk intake substations for City Power from the 275 kV Eskom network, supplies a large portion of southern Johannesburg and surrounding areas. It has an installed capacity of 1000 MVA with 4 x 250 MVA 275/88 kV transformers, which give a firm supply of 750 MVA at 88 kV. The 88 kV protection relays and SCADA system have been in service since 1995.

At the start of this project in 2013 the system had been in service for 18 years. The relays were now obsolete with limited functionality. The biggest limitation is that they inhibit the fault investigation process after trips occurred. The relays did not provide adequate fault recording and fault event analysis data which made it difficult to analyse complex system faults and resulted in longer delays in supply restoration.

Fig. 1: New protection relays installed on the capacitor banks.

Fig. 1: New protection relays installed on the capacitor banks.

The project was initiated by City Power in order to replace the ageing 88 kV protection relays with new numerical relays, which provide high accuracy fault recording and time stamped sequence of events (SOE) reports. The SCADA system, mimic board and HMI were also upgraded with new equipment as part of the same project. The 88 kV gas insulated switchgear (GIS) consists of the following bays:

  • 4 x 88 kV transformer incomers (250 MVA each)
  • 4 x 88 kV overhead line interconnectors
  • 12 x 88 kV radial cable feeders
  • 2 x 88 kV bus sections
  • 2 x 88 kV bus couplers
  • 2 x 88 kV filter banks
  • 4 x 88 kV busbar voltage transformers (VTs)
  • 4 x 88 kV busbar earthing bays
  • 1 x 88 kV bus-zone for 29 bays and 4 zones

Challenges

The first major challenge was to decommission and upgrade the automatic voltage regulators (AVR) relays on the four transformers. The inputs and outputs of each transformer’s AVR was wired to a PLC located in Transformer 2 panel. This PLC controlled the entire master follower scheme. The new
REG-DA relays which were to be installed, were not compatible with the old PLC scheme. There was also a great deal of bus wiring between the AVR panels due to the use of the PLC. Due to operational concerns, the existing AVR scheme could not be switched out of service as the transformers would be forced to run at fixed tap. It was decided to decommission the AVRs in reverse sequence to facilitate the removal of the wiring while keeping the other transformers in service. The REG-DA has a delta cos-phi programme for voltage regulation using circulating current and using this programme, it was possible to decommission one transformer AVR at a time and operate with the remaining transformers on the master follower scheme under PLC control purely using circulating current. This scheme worked perfectly and one by one, all the transformer AVRs were changed to REG-DA relays using delta cos-phi regulation.

Fig. 2: Substation busbars.

Fig. 2: Substation busbars.

The second major challenge was to decommission an existing bus-zone and commission a new four zone low impedance bus-zone. The new panel had to be installed in the same position as the old panel and all the existing cables had to be replaced. Isolation of DC and tripping circuits had to be managed to ensure no false trips. The CT circuits had to be temporarily shorted out in a temporary junction box after each bay was switched out in turn. Only once this was done could the old panel be removed and the new panel could be installed. Once again each bay had to be switched off in turn to disconnect the shorted CT circuits and connect them on the new bus-zone. On the HOLEC GIS switchgear access to the CTs for primary injection was not possible, which meant that the bus-zone could only be proved for stability using load current. All stability tests were done with a live system and doing various transfer operations. Through faults on the system was also another way of confirming that the bus-zone is stable.

Howard Nkuna, City Power’s acting project manager, says that the team was working in a live network environment, which required steps to be taken to ensure safety by complying with relevant regulations, and keep interruptions on the network to a minimum so as to ensure a successful project implementation with minimal outages.
Fordsburg substation has numerous N-1 contingency lines feeding other substations, such as Bree, Johnware, Braamfontein and Orlando, with Orlando substation feeding other substations as well. So it was essential to monitor the load on the lines, especially when one line was taken out of service, to avoid overloading and unplanned outages, he said.

Innovative solutions

The substation has two 88 kV capacitor banks for power factor correction. The old relays used a PLC to control the switching of the capacitor bank depending on the VAR requirements. At the start of this project it was found that the PLC was no longer operational. The PLC was completely removed and all the control logics were implemented in the new protection relays installed on the capacitor banks. Measurement of the four incoming transformer Mvars and power factor are sent via remote analogs using IEC 61850 protocol between the transformer and capacitor bank relays. Automation logics in the capacitor bank relays summate these values and at certain Mvar set points open and close the capacitor bank breakers for var control.

Fig. 3: Tebogo Mohlapi, Howard Nkuna and Deon de Villiers in the newly refurbished substation.

Fig. 3: Tebogo Mohlapi, Howard Nkuna and Deon de Villiers in the newly refurbished substation.

A second innovative solution was to implement local control bits on the protection relays to simulate digital signals (alarms and trips) to the HMI and SCADA system. This system worked very well and saved a great deal of time during the commissioning of the SCADA system. The protection alarms and trips could be tested on load and at any time without the need to inject them again as this was done during the protection commissioning.

The existing inter-tripping scheme between Fordsburg, Mayfair and Orlando substations on the four interconnector lines using an aged audio pilot carrier system were upgraded and changed to SEL-2505 fibre optic interface units. These units are much more reliable and also solved existing problems which caused incorrect operations for line faults. Nkuna says that the project was tackled carefully, with the methodical coordination of City Power and CONCO project teams. In the end, the project has been successfully implemented and meets the project specification and City Power’s quality standards.

Contact Dorothy Mabatle, CONCO, Tel 011 729-1481, dorothy.mabatle@concogrp.com

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