The design of new substations has the advantage of starting with a blank sheet of paper. The new substation will typically have many IEDs for different functions, and the majority of operational data for the SCADA system will come from these IEDs. The IEDs will be integrated with digital two-way communications. The small amount of direct input/output (hardwired) can be acquired using programmable logic controllers (PLC). Typically, there are no conventional remote terminal units (RTU) in new substations. The RTU functionality is addressed using IEDs and PLCs and an integration network using digital communications.
In existing substations there are several alternative approaches, depending on whether the substation has a conventional RTU installed. The utility has three choices for their existing conventional substation RTUs: integrate RTU with IEDs; integrate RTU as another substation IED; and retire RTU and use IEDs and PLCs, as with a new substation. First, many utilities have integrated IEDs with existing conventional RTUs, provided the RTUs support communications with downstream devices and support IED communication protocols. This integration approach works well for the operational data path, but it does not support the nonoperational and remote access data paths. The latter two data paths must be done outside of the conventional RTU. Second, if the utility desires to keep their conventional RTU, the preferred approach is to integrate the RTU in the substation integration architecture as another IED. In this way, the RTU can be easily retired when the RTU hardwired direct input/output transitions to come primarily from the IEDs. Third, the RTUs may be old and difficult to support, and the substation automation project might be a good time to retire these older RTUs. The hardwired direct input/output from these RTUs would then come from the IEDs and PLCs, as with a new substation.
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FIGURE 7.1 SA system functional architecture diagram.
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