T-41 Progress Report

Implications of the Smart Grid Initiative on Distribution Engineering

July, 2009

Daniel Haughton

1. Objectives

The primary objectives investigated are to find an appropriate method to accomplish Tasks 2 (reconfiguration) and 3 (sensors-based restoration) of the project. Tasks 2 and 3 are described as follows (as taken from the project proposal):

Task 2: Reconfiguration of legacy radial distribution systems to networked structures suitable for the Smart Grid. It is acknowledged that a move toward a meshed distribution system, similar to that existing in the transmission levels of the system provides advantages of increased reliability for dispersed end users [6]. While networked distribution system architectures are common in central business districts, this design is not used in lighter urban, suburban, and rural areas. To increase reliability of supply and maximization of existing distribution systems, a systematic study quantifying the levels and locations of infrastructural improvements (i.e., selective conversion from radial to meshed networks or the use of other doubly fed designs) is integrated into the proposed research. The concept of a meshed distribution system, with multiple distributed generation (DG) sources (renewables included), is shown in Figure 1. The task on reconfiguration of legacy radial topology of distribution systems to meshed networks will take into account the maximization of existing distribution system infrastructure and a cost to benefit analysis of selecting optimum locations for achieving the reconfiguration. The intent is to develop a framework for designing the reliable distribution system architecture that is most conducive for integrating sensors based controls and renewable sources of generation.

Task 3: Improvement of reliability through sensors-based supervised and fully automated restoration in distribution systems. This task builds on PSerc projects T-31 and S-30. The intent is to develop the logic for restoring segments of the distribution system after failure or removal of a component in the system. It is known that for a system to be highly reliable and fault tolerant, in addition to multiple redundant paths, it is paramount to have smart strategies such as fault detection, isolation and reconfiguration (FDIR) to manage redundancy. The existence of FDIR in current networked distribution system is limited to local protection schemes, which usually do not communicate with each other. Thus, in order to selectively convert existing networked distribution systems into smart distribution systems, we propose to add FDIR mechanisms and other smart control concepts. The methods used shall also replicate transmission system concepts that use system restoration logic to automatically restore the distribution system. Using circuit matrix concepts, the optimization of the reliability at system buses shall be performed based on the reliability of individual components. Some of these concepts have been applied in special environments (e.g., space power systems) and specialized hyper reliability applications. It is possible to base designs on a 0.99999 reliability concept. The tasks planned under this venture will not include studies on protection engineering; rather, the effort will be focused on optimal location of sensors in a smart distribution system and the processing of information from the sensors to achieve automated and supervised restoration of the distribution system.

The approach taken addresses both Task 2 and Task 3. Fig. 1 shows the general approach. This is an automated (but perhaps operator assisted) reconfiguration and restoration algorithm.

Fig. 1 An automated restoration and reconfiguration technique: (Left) generalized approach (Right) the relationship of the restore – reconfigure algorithm

2. Method used

Various methods are available for system reconfiguration. The investigations are to model a generic distribution system with multiple circuit breakers, loads and feeder interconnections. The distribution system is modeled as a binary connection matrix indicating breaker, bus, load and line status. This model will be investigated for its ability to reconfigure the distribution system with the objective of achieving goals such as maximizing the load restored, minimizing the number of de-energized buses or minimizing the interruption time.

The problem addressed is the restoration of a distribution system – and the reconfiguration of that system to accomplish certain objectives. The method used is based on the binary bus connection matrix, B. This matrix and its powers (i.e. Bn) can be used to trace connectivity of a networked system. A system status table (SST table) is constructed as shown in Fig. 2. The data from the SST is used in an algorithm shown in Fig. 3.

3. Example

To illustrate this concept, a sample distribution system was used. The one-line diagram is in Fig. 4. In this example, two radial feeders extend from the same distribution substation. There are four breakers located on the feeders that allow for either radial or networked operation of the feeders. The breaker status table is constructed based on the configuration of the breakers. For example, all four breakers being closed correspond to state [1, 1, 1, 1]. The bus connection matrix, B, is constructed and used to verify energized and de-energized buses and line segments. According to the B-matrix powers, the power Bn provides the number of connected buses through the available lines. The load status table, as indicated in Fig. 3, is simply a subset of the bus status table but with includes only those buses with loads.

BREAKER
STATUS
TABLE / BUS
STATUS
TABLE / LINE
STATUS
TABLE / LOAD
STATUS
TABLE
Fig. 2 The system status table (SST)

With the system configured, the algorithm can be used to detect changes in the system, such as breaker(s) openings post-fault to clear lines. The algorithm used then selects a target state based on some pre-defined philosophy such as maximum load served. The available states are established based on the breaker status table while all other states are eliminated. From available states, an optimum is determined. If no optimum state can be found (no load can be feasibly served), the worst ‘available’ state is [0, 0, 0, 0], or all breakers open. As indicated in Fig. 3, if the source is unavailable no calculation is required.

4. Work in the immediate future

The immediate attention in this project is focused on:

  • Implement prioritization of loads
  • Explore system configurations and the concomitant response / performance of those systems
  • Examine reliability of alternative network configurations
  • How to migrate radial systems to networked systems – development of a roadmap
  • Establish a full working model of the automated restoration / reconfiguration algorithm as applied to a distribution system
  • Examine meshed distribution systems as described in Task 2 of the original proposal
  • Examine FDIR topics as described in Task 3 of the original proposal
  • Generate an example suitable for publication at the IEEE PES T&D Expo (due August 2009).

Fig. 3 Flow chart of restoration and reconfiguration algorithm

Fig. 4 One line diagram of the distribution system used in the example