Abstract

Due to the regulation of Brazil’s electricity sector, the unavailability of power transmission equipment, such as power transformers, imply in the application of very high fines to equipment owner, whether the unavailability have been caused in a planned manner or not.

In this context, a backup unit for a single-phase transformer bank has not been sufficient. This because of the long time to switchover from one phase to the backup phase and also to restore the original phase, rendering unfeasible temporary changes for maintenance purposes and resulting in a long time for system recovery in case of unexpected failure in one of the transformers.

To speed up this switchover process, Furnas adopted a system with transfer busbar and motorized disconnecting switches associated to a digital control system allowing both automatic and manual switchover.

This article describes the topology and the digital control system used in the switching system implemented in Rio Verde substation, on two 230-138kV banks. The field experience with the implementation and operation of this system over the last two years will be presented, describing the practical results achieved.

Due to the regulation of Brazil’s electricity sector, the unavailability of power transmission equipment, such as power transformers, imply in the application of very high fines to equipment owner, whether the unavailability have been caused in a planned manner or not.

In this context, a backup unit for a single-phase transformer bank has not been sufficient. This because of the long time to switchover from one phase to the backup phase and also to restore the original phase – a process that takes approximately 16 hours – rendering unfeasible temporary changes for maintenance purposes and resulting in a long time for system recovery in case of unexpected failure in one of the transformers.

To speed up this switchover process, Furnas adopted a system with transfer busbar and motorized disconnecting switches associated to a digital control system allowing both automatic and manual switchover.

Currently, Rio Verde substation – Goiás is equipped with two 230-138kV single-phase transformer banks, as well as a 13.8kV tertiary delta, as shown in Figure 1. A third bank will be built in the future.

Figure 1 – Control System Overview

Switching command and control system is primarily intended to remove the large amount of cables resulting when a conventional centralized system is used, where the information of each disconnecting switch is transmitted to the control room via cabling several meters away from the substation yard.

To meet this requirement, a control room system was developed featuring a modular and decentralized architecture, based on IEDs (Intelligent Electronic Devices) models DM1 and DM3 supplied by Treetech, installed in the common cubicle of each transformer close to the disconnecting switches. Theses IEDs are input/output modules specifically developed for substation yard conditions, such as high temperatures and electromagnetic interferences. They receive the open/close status of disconnecting switches and send open/close commands, being connected to a remote terminal unit installed in the control room via optical fiber (one pair for each transformer bank). Figure 2 shows this system architecture.

Figure 2 – Decentralized System Architecture

The central control unit performs all interlocking logic as well as switching sequences, both to switchover to the backup phase during failure and to return to the original phase, always meeting the pre-conditions established, such as the bank must be isolated in the 230, 138 and 13.8kV sides. These sequences may be performed automatically, where all switchover operation is made without the need for operator intervention, and in manual mode, where each disconnecting switch open and close operation is supervised step by step by the operator, according to figures 3 and 4.

Before initiating the switchover process, the operator must switchover all outlets of the defective phase to the backup phase. This is a manual process and is part of switchover pre-conditions. Therefore, to indicate that outlet switchover has been already made, just inform the control software checking the item “Outlet transferred” (Figure 4).

Figure 3 – Switchover process from original phase to backup phase

Figure 4 – Step by step switching sequence: operator clicks the item indicated by the arrow, and a pop-up window displays asking operator confirmation. When the operation is completed, the arrow jumps to the next item.

The type of architecture described above allows substantial cost cutting by reducing the amount of control wires, streamlining design and reducing installation time, whereas increases reliability and minimizes future maintenance.

A future upgrade is expected for the digital control system. All hardware part is already prepared to integrate this implementation to substation supervision system via standard protocols (IEC 60870-5-101 or DNP3.0).

As reported by the field team, switchover from a phase to the backup phase, which previously took several hours to occur, is completed in approximately 30 minutes, as a maximum, with this system in the step by step manual mode, where operator visually checks and confirms the operation of each disconnecting switch. The same process is carried out in 10 minutes in the automatic mode.

We may also note a decrease in the amount of operation errors during phase switchover, as the control system only allows executing commands in the disconnecting switches when all pre-conditions are met.