A centralized protection and control system using a well proven transmission class protection relay

Chee-Pinp Teoh and Graeme Lloyd, GE Grid Solutions, UK, Rich Hunt, GE Grid Solutions, USA, and Gerardo Rebollar, GE Grid Solutions, France

Substation Human Machine Interface (HMI)

Substation Human Machine Interface (HMI)
A CPC solution using an industrial computer as the hardware platform must connect to a monitor to display status and information. This provides a Human Machine Interface, but the monitor typically is not built for the rugged substation environment. This could be a problem because an MV substation may have a control room.

Built in graphical LCD & virtual annunciator windows: A numerical protection relay today is advanced and user friendly. Some even offer large seven inches graphical LCD display ordering option which can display the live status of single line diagram (SLD) on the protection relay itself. As the LCD is built in the relay, it has been designed to work in a substation. As in Figure 5, The same LCD can also be programmed to display as a virtual annunciator window.

User friendly multi-functional centralized HMI toolsuite:  If a substation HMI is a preferred solution over the relay LCD, the relay toolsuite software can offer a similar HMI display. The relay toolsuite offers complete feature such as Open Platform Communication (OPC) server integration for database management, remote monitoring, third party device monitoring. While the relay toolsuite requires to be hosted on a PC, but this typically can be hosted on the gateway computer. If the substation is unmanned, the HMI will only be required when the user visits the substation, to which the user can connect directly with a laptop.  CPC is a scalable solution, where a big substation with many bays will require more hardware. The relay toolsuite can communicate to multiple CPC relays and act as a centralized HMI and data repository. The toolsuite displays SLD with the breaker, isolator status and current, voltage measurement of all bays in the substation collected from two CPC relays on a polling basis. (Figure 6).

Each CPC unit has its own oscillography recorder and sequence of events record built in. In this case, the HMI toolsuite can act as the integrator, which it automatically downloads from each CPC unit and stored in a centralized, system-wide, sequence of event record as shown in Figure 7.

Fail Safe & Redundancy
Redundant SV sources: Since PIU can have multiple analogue inputs, cross connecting two bays’ CTs and PIUs as shown in Figure 8 is possible. Having this connection can offer redundant SVs publishing.
As an example, in the event of Bay 1 PIU failure, the Bay 2 PIU continue to publish the SVs for Bay 1 CT. The CPC unit can detect the loss of SV publishing on the faulty Bay 1 PIU and automatically switch to the SV subscription to Bay 1 SV from PIU2.
Seamless switching will allow protection and control function to operate correctly as shown in Figure9. This redundant protection scheme was not possible in the conventional protection without significant additional cost and complicated wiring. However, with the multiple analogue inputs PIU, this can offer additional security and reliability at the small cost of connecting of adding another analogue input at the PIU.The one practical concern using redundant SV sources is the CPC unit has a fixed number of subscriptions to SV streams. Every redundant SV source reduces the number of bays that can be protected by one.
Thus, a fully redundant SV source can protect half the number of bays. However, thanks to the scalable relay solution as discussed above, another pair of relays could be added to cover all bays.

Time Synchronization
For proper operation of a CPC system, it is necessary that CPC and PIUs are time synchronized to each other to ensure protection functions to operate properly. The proper method for time synchronization for process bus is to use one of the industry-specific application profiles of IEEE 1588 (or PTP, “precision time protocol”) as the protocol, over the communications network.  The CPC can be designed to act as an IEEE 1588 master clock to the process bus network. In normal operations, the master clock in the CPC device is synchronized to the substation master clocks. However, if the substation master clocks are no longer available, all PIUs communicating to the protection will be synchronized with the same relative time to the CPC device, allowing protection functions to remain in service. CPC does not require absolute time synchronization, but only relative synchronization between the PIUs and CPC.
As long as all the PIUs are synchronized to the same source, the CPC relay, the SV streams can be aligned and used for protection, including the low impedance differential for transformer and busbar. This therefore allows the application of the same CPC protection without the need of a satellite clock or absolute time synchronization.

Conclusion: A transmission class protection relay not only offers a good alternative to the industrial computer-based CPC solution, but it is a much more mature and proven solution having been used in transmission. For example, a transmission class protection was designed for a harsh environment, type tested for IEEE 1613 and IEC 61850-3. Using transmission class relay as a CPC offers many other benefits such as IEC 61850-90-5 PMU & R-goose functions, teleprotection, IEEE 1588 master clock built in, legacy communication protocols which is currently lacking on the industrial computer-based CPC.
A transmission class relay was designed to operate for 20 years life in the substation and offers peace of mind after installation. Process bus MUs and PIUs are future proof and should have facilitated flexibility and adaptability by providing all basic data for state, status, and control of the substation to the station level from the beginning.

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