Many utilities use direct transfer trip (DTT) for disconnecting/reconnecting distributed generation sites – e.g., solar PV. Let us review a real-life application that a U.S. power and energy company deployed to support DTT.
The main function of DTT is to remove the distributed generation site during faults on the grid. A circuit typically starts at the substation breaker feeding through downline reclosers and ultimately to the utility point of circuit connections (PCC). If the circuit breaker in the substation or downline reclosers open for a fault or operational reasons, the PCC will receive a DTT signal form these devices to disconnect the distributed generation (DG) site. This function will ensure that the circuit is deenergized should the utility need to perform maintenance work on the circuit. The communication of the DTT signal is of utmost importance and if not available could lead to a situation were the DG site will not receive a DTT signal and energize a circuit that should be deenergized for maintenance work. For this reason, if the communication is interrupted, the DTT system will operate in a fail-safe mode and disconnect the DG site.
Figure 1. Need for Direct Transfer Tripping
In the first case, a utility could have leased copper circuits from telecommunications companies. However, these systems have become very unreliable since telecom companies struggle to support copper circuits. Additionally, many DG sites get tripped offline for non-fault events (e.g., communication failures). Siemens set out to develop a new solution based on modern technologies where DG sites can remain online to support the grid by replacing unreliable copper communication circuits for DTT.
Siemens began to explore cellular and fiber as a means of developing a new and economical DTT since most telecommunications providers have IP-based systems that Siemens could immediately utilize. Cellular seemed to be the best option since getting fiber to remote rural areas for DTT was not deemed feasible. As a result, Siemens looked at developing private mobile networks and sending encrypted data through these networks, as shown in figure 2.
Figure 2. DTT Cellular Communication Setup
The new DTT system included two digital communications systems (cellular and fiber) running redundantly, where both had to fail before initiating a trip to the solar site.
Siemens wanted to deploy flexibility in the design, especially as DG increases across the United States. The cellular capability makes for quick deployment and can be followed up with adding fiber for speed and redundancy. The IEC61850 “GOOSE” messaging protocol allowed for control with the use of small data packets, keeping the cellular data plan costs minimal.
Siemens’s redundant communications path DTT system can tolerate communications failures as indicated in figure 3. Cellular or fiber can be interrupted at any site and the system can seamlessly perform DTT functionality. Only in the very unlikely event that both communications systems permanently fail at a location will the DG site be disconnected. If the system is initially run using only cellular communications, temporary interruptions can be expected and the intelligent DTT system will activate a back-up protection function to ride though these short communications interruptions.
Figure 3. Redundant Communication for a Self-Healing Grid
The intelligent DTT provides a reliable, easy, and fast to deploy solution maintaining the security of a communicating DTT system.
To learn more about Siemens’s vision of the grid of the future, we encourage you to join an upcoming webinar hosted by Informa/T&D World and featuring a case study of Central Virginia Electric Cooperative.