One of the key characteristics of the much-discussed Smart Grid refers to electric power systems that enhance grid reliability and efficiency by automatically anticipating and responding to system disturbances. To achieve Smart Grid status at the power distribution system level, various automation technologies have been attempted in the areas of system metering, protection, and control. Within these technologies, automated power restoration is an important part of the Smart Grid puzzle.
Traditionally, electric utilities have used their trouble call systems to detect power outages. Specifically, when a fault occurs and customers experience power outages, they call and report the power outage. The distribution system control center then dispatches a maintenance crew to the field. The crew first investigates fault location and then, implements the switching scheme(s) to conduct fault isolation and power restoration. This traditional procedure for power restoration often takes several hours to complete, depending on how fast customers report the power outage and how quickly the maintenance crew can locate the fault point and conduct the power restoration.
In recent years, utilities have deployed feeder-switching devices, such as reclosers and circuit breakers, with intelligent electronic devices (IEDs) for protection and control applications. The automated capabilities of IEDs, such as measurement, monitoring, control and communications functions, make it practical to implement automated fault identification, fault isolation and power restoration. As a result, the power outage duration and the system reliability can be significantly improved.
Based on the information provided by IEDs, automated fault location identification and fault isolation are relatively easy to achieve. In contrast, automated power restoration is a challenging task, and many research efforts have been focused on tackling this application and to consider the operating constraints, load balancing and other practical concerns.
Although some of the many proposed automated power restoration algorithms aim to provide a real-time solution, most of them are only suitable for planning analysis or were developed to be executed in the distribution control centers to assist system operators in making appropriate decisions.
This article presents an online method for the automated power restoration application previously described. The developed method conducts an analysis to achieve back-feed power restoration – healthy load zones that have lost power that are restored through their boundary-tie switching devices from neighboring sources, with no reconfiguration beyond the tie devices under consideration. The back-feed restoration should not overload any part of the back-feeding network.
Requirements, Concepts and Methodologies
A restoration switching analysis (RSA) method produces a switching sequence that, when executed, will reach a valid post-restoration network that satisfies the following requirements: 1) it is radial; 2) there is no current violation at any network component; 3) there is no voltage violation at any network node.
Other optimization requirements are also considered. For example, losses can be minimized, and the back-feed transformer’s loading can be balanced.
A. Network Model
For the sake of method description, a simplified network model (depicted below) includes three types of components: 1) sources, 2) switching devices (i.e., switches that represent sectionalizers, load switches, circuit breakers and reclosers) and 3) loads. Feeder conductors are assumed to be load attributes.
Sources are assumed to have limited capacity (ampere rating) but constant voltage. Switches are assumed to have limited loading capability. (In amperes, circuit breakers and reclosers have unlimited current interruption capability.) Loads are assumed to be constant, aggregated lumps that connect to switches over zero-impedance feeder conductors. The conductors have limited current carrying capability.
B. Network Connectivity
The connectivity of the network model must be known in order to achieve successful restoration. The switching devices, loads and sources – as well as how these different components are connected – are required for the restoration method. Restoration by this method is most effective when multi-layered back-feed networks are present in the distribution system.
C. Restoration Validation Check
The restoration validation check confirms the validity of the post-restoration network configuration in order to ensure that the network is radial and all the currents and voltages are within the component limits. The restoration method produces radial post-restoration networks. Thus, any additional radiality checks are not necessary.
A current violation check is done as an integral part of the algorithm, based on the loading aggregation method described below. This check ensures that for all the network components, their post-restoration loading currents are less than their loading current limits.
D. Network Tracing-based Loading Aggregation
As stated in the introduction, back-feed power restoration should not overload any part of the back-feeding network. In the described method, this is achieved by recursive network tracing-based loading aggregation method:
1) Start from a back-feeding source (usually a transformer), and trace down all the network components it supplies until the end of the tree structure is reached;
2) When returning to the source, the tracing method sums up the loading current at each network component and if applicable, compared with its corresponding limit;
3) The available capacity of a source can be calculated after the tracing goes back to the source.
E. Single-Path and Multi-Path Restoration
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; this scenario is called multi-path restoration. Both single-path and multi-path restorations may have to shed load in cases where the back-feed source capacity or feeder components’ loading capability is not adequate.
Solution Examples
Figure 1 shows a single-path, full restoration example, where a fault at T-node, L3, must be isolated by opening a forward-feed isolation switch, (R3) and two back-feed isolation switches (R6 & R10). In this example, back-feed sources (S3 & S4) both have sufficient capacity to pick up the out-of-service load on their corresponding restoration path and each tie switch (R9 & R12) can be closed to achieve the restoration. The post-restoration circuit topology is shown in Figure 1B.
Figure 1: Single-Path Full Restoration Example
(1A) Normal Topology
(1B) Post-Restoration Topology
Figure 2 shows a multi-path full restoration example, where a fault at load node L1 must be isolated by a forward-feed isolation switch R1 (in this case no forward restoration is required) and a back-feed isolation switch R2. In this example, none of the back-feed sources S2-S5 can completely pick up all the loads that are left unserved after fault isolation.
Hence, the algorithm splits the network into two parts – as in Step 4 (above) by opening R13 and the out-of-service load restored by closing both R9 and R12 (from both S3 and S4). The post-restoration circuit topology is shown in Figure 2B.
Figure 2: Multi-Path Full Restoration Example
Figure 2A -Normal Topology
Figure 2B - Post-Restoration Topology
Figure 3 shows an extreme example where the splitting of the out-of-service load zones is still not enough. Following the fault at load L1, and its isolation by opening R1 and R2, none of the back-feed sources can pick up the out-of-service loads completely or even partially without violating the current capacity limits of those sources.
Load L5 has to be shed in order to restore power to as many out-of-service loads as possible. The post-restoration circuit topology is shown in Figure 3B. Note that the out-of-service load zone has to be split into three portions, according to the algorithm.
Figure 3: Multi-Path Partial Restoration Example
Figure 3A - Normal Topology
Figure 3B - Post-Restoration Topology
Algorithm Demonstration
During the development of the algorithm, a physical circuit with three sources, five switches and three loads was setup, and a controller application was programmed.
In the circuit of Figure 4A, because of the given source capacity (4B), a fault at load L1 results in a splitting of the out-of-service network of L2, R3 and L3, by the opening of switch R3. Both tie switch R4 and R5 close to restore power to the out-of-service loads, as shown in 4C.
Figure 4 - The Demo Circuit
Figure 4A - Physical Circuit
Figure 4B - Normal Operation
Figure 4C - Normal Operation
The fault detection and service restoration switching sequence in Figure 4D proves the effectiveness of the algorithm.
Figure 4D - Operation Log of the Controller Application
Conclusion
This article has described a deterministic algorithm that identifies a restoration strategy to restore the out-of-service load due to fault isolation while ensuring that the post-restoration network has a valid configuration. The algorithm is based on the concepts of network tracing and it supports both single-path and multi-path restoration. Applications and physical demonstration circuits today have proven that this process can produce appropriate back-feed switching strategies for any network topology.
This algorithm provides significant value to electric utilities in the Smart Grid arena. By deploying this intelligent solution with an adequate communications infrastructure, the reduction of customer outage minutes and the improvement of service reliability will be achieved. This technology is one major step in truly achieving a self-healing distribution network.
About the Authors
Zhenyuan Wang joined ABB US Corporate Research Center in Raleigh, North Carolina in 2000, where he is currently a Principal Consulting R&D Engineer. His research interests include electric power equipment condition monitoring/ assessment/diagnosis, system monitoring, and control and automation. His experience includes asset management IT applications in the electric power industry, power system transient analysis, substation/distribution automation, and data integration/warehousing/mining applications.
James Stoupis is a Principal Consulting R&D Engineer in the Power Technologies Department for ABB’s US Corporate Research Center located in Raleigh, North Carolina. Jim has been employed at USCRC for 13 years, and his research has been focused in the areas of distribution and feeder automation, wireless communications, power system protection and control, and event detection and classification.
Fahrudin Mekic is Global Product Manager -Distribution Automation for ABB and is located in Allentown, PA. He has worked at ABB since 1996, where he has focused his attention in the area of power system protection and control in
various engineering and managerial positions. Fahrudin has published several technical papers in the area of protection and reliability and is a senior member of IEEE.
