Substation Computer as a Power Restoration Controller2

When a substation computer is used as a power restoration controller that hosts the RSA engine, it requires a distribution network model consisting of at least the fol­lowing major feeder components: sources, such as distribution substation trans­formers; switching devices, or “switches” that act to sectionalize parts of the net­work, load switches, circuit breakers, and reclosers; and loads. The RSA engine is triggered following fault detection and uses a network tracing-based algorithm to identify a valid radial post-restoration network with no current or voltage violation on any network component or network node. The outcome of the RSA is a resto­ration switching sequence generated online according to the pre-fault network condition instead of pre-programmed rules that are usually generated offline.

It is important that back-feed power restoration not overload any part of the back-feeding network, which the RSA algorithm deals with by a recursive net­work tracing-based loading aggregation method involving following steps: (1) It begins at a back-feeding source (usually a transformer); (2) it traces down all the network components it supplies until it reaches the end of the node-component graph tree structure; (3) when returning to the source, it sums up the loading cur­rent at each network component, and if applicable, compares the loading current with its corresponding limit; and (4) after the tracing returns to the source, it cal­culates the available capacity of a source.

The RSA algorithm begins with a back-feeding isolation switch search carried out on the graph tree structure of the pre-fault network with the tripped break- er/recloser as the root. The search is conducted down the tree to find the most downstream switch that passed the fault current. This switch is called the forward – feed isolation switch. The search then moves downward to the first layer of down­stream switches, referred to as back-feed isolation switches. The algorithm then applies numerous recursive steps, including the following:

• It identifies any multi-connected load nodes (also known as “T-nodes,” de­fined in the following paragraph) via network tracing

• It determines if single-path restoration can be achieved via a single source

• If single-path restoration cannot be achieved, it then continues to search for other switches in the network in order to achieve multi-path restoration.

A T-node is defined as the connection point of a lateral in a feeder. If an isolated network has T-nodes, its pre-restoration tree structure will define the isolation switch as the root and the potential back-feeding tie switches as the termination end. If both of the two downstream branches of a T-node are able to provide a backfeed, the algorithm has to choose one out of the two; otherwise, a circuit loop will be generated in the post-restoration network. If the two downstream branches are to be back-fed from the same source, the branch with higher loading capability all the way to the source is chosen for backfeeding.

A back-feeding path is defined as a set of connected circuit components from the back-feed source to the to-be-closed tie switch. If a source can provide the restoration power over a single path to an out-of-service load zone, the restoration is called a single-path restoration. Otherwise, the out-of-service load zone may have to be split into two or more load zones to be back fed, and the scenario is called multi-path restoration.

In the case of multi-path restoration, the algorithm attempts to determine the best reconfiguration. In some cases, the network must be divided into two sub­networks to restore all the possible unserved loads, moving one or more normally – open tie switches to other switching device locations. In other cases, all the un­served loads cannot be completely restored even if the tie switch locations are moved. Both single – and multi-path restorations may have to shed load if the back­feed source capacity or the feeder component loading capability is not sufficient.

The restoration algorithm produces radial post-restoration networks. Using the loading aggregation method, the algorithm performs a current violation check that ensures that the post-restoration loading currents of all the network components are less than their loading current limits. After a load flow analysis of the post­restoration network takes place, voltage violations can be checked as an integral part of the algorithm. The restoration validation check confirms the validity of the post-restoration network configuration to ensure that the network is radial and all the currents and voltages fall within the limits.

The RSA algorithm may also consider other requirements such as minimizing losses, minimizing the number of switching operations, and balancing the loading of the back-feed transformer.