Distribution System Topology Detection Using Consumer Load and Line Flow Measurements Raffi Avo Sevlian and Ram Rajagopal
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1 Distribution System Topology Detection Using Consumer Load and Line Flow Measurements Raffi Avo Sevlian and Ram Rajagopal Abstract—This work presents a topology detection method for breaker status indicators, and assume knowledge of the combining home smart meter information and sparse line flow system line parameters. The measurement types are focused measurements. The problem is formulated as a spanning tree on substation SCADA and load measurements in a very simple detection problem over a graph given partial nodal and edge flow information in a deterministic and stochastic setting. In the network. In [9] the authors introduce the use of high fre- deterministic case of known nodal power consumption and edge quency micro-Phasor Measurement Unit (µPMU) data in the flows we provide sensor placement criterion which guarantees topology detection task. They propose a method of comparing correct identification of all spanning trees. We then present a simulation and measured µPMU data for each topology. This detection method which is polynomial in complexity to the size work assumes high frequency voltage magnitude and phase of the graph. In the stochastic case where loads are given by forecasts derived from delayed smart meter data, we provide measurements are available in each bus. In [10], [11] and a combinatorial Maximum a Posteriori (MAP) detector and [12] the authors develop a voltage time series approach to a polynomial complexity approximate MAP detector which is identifying topology changes relying on voltage data at each shown to work near optimum in low noise regime numerical home. However they rely on long time captures, so their cases and moderately well in higher noise regime. method is more in line of network discovery not real time IEEE Transactions on Control of Network Systems topology detection. In [13] the authors present a general state estimator based method that is used in topology detection, similar to [8]. I. INTRODUCTION The need for advanced controls in the distribution system is an emerging topic in power system and controls com- munities. Proposed computational models for problems such as dispatching of distributed energy resources [1], [2] or coordinated voltage control [3], [4], [5], [6] assume known system topology and network parameters. In reality customer level feeders are not known with an accuracy equivalent to that of the transmission system. Enabling improved management and control requires signif- icantly improved estimation of the system state. This can be illustrated in the IEEE 123 Bus System shown in Figure I. The network not only has end nodes which represent residential transformers (blue rectangle), but various switching devices (green rectangles) and four feeders (red circles). In this system, Fig. 1. IEEE 123 Distribution system with commonly occurring sensing and actuating technologies, illustrated in clockwise order: Switching device, estimating the system state requires the determination of the smart meters, substations and line sensors. arXiv:1503.07224v5 [cs.SY] 15 Sep 2017 voltage phasor at every node and the status of all discrete devices that can connect and disconnect loads. In such a setting The contributions of this work differs significantly from a Generalized State Estimator (GSE) [7] is used to determine prior work in the following ways. Fist, we assume the the f0; 1g of each discrete device as well as the voltage at following information is available: 1) widely available load each bus. measurements from smart meters; 2) line flows on a fraction Some previous work has presented solutions to this issue,1 of the lines obtained from either line sensing or substation which differ from the contributions of this work. In [8], a SCADA. The line measurements are typically available in real traditional GSE is employed to identify the correct topology time, while smart metering data is delayed by multiple hours in a distribution system. The work presents a traditional requiring some forecasting if real time topology detection is weighted least square state estimator and use dummy variables required. Second, the detection and sensor placement problems are developed in both the deterministic case of combining R. Sevlian is with the Department of Electrical Engineering and the Stanford Sustainable Systems Lab, Department of Civil and Environmental historical load and line data and the stochastic case of combin- Engineering, Stanford University, CA, 94305. Email: [email protected]. ing line measurements with load forecasts. Additionally, our R. Rajagopal is with the Stanford Sustainable Systems Lab, Department model assumes a lossless network, which although introducing of Civil and Environmental Engineering, Stanford University, CA, 94305. R. Rajagopal is supported by the Powell Foundation Fellowship. Email: some error is much smaller than the typical load forecast. We [email protected]. show this solution lends itself well to a very robust data driven 2 approach where many of the line parameters are not known, D. Measurement Model or when AMI connectivity information may be in error. This For any edge e of the original distribution system, we denote robustness under large uncertainty makes this a very practical by s the power flow on it to all active downstream loads. and useful method for utilities. The sensor placement M ⊂ E, is a subset of edges of the The paper is organized as follows. Section II, III formulates network. We assume that the magnitude and direction of power the problem of topology detection. Sections IV and V solve the flow is measured. Additionally, we assume that the power flow detection and sensor placement problems in the deterministic measurements are error free. This assumption can be made and stochastic cases respectively. Finally, numerical demon- since any instrumentation error will be much smaller than the strations are given in Section VI, with additional details in the pseudo-measurement errors in practice. Appendix. Given a topology defined by w, the set of all measurements is s where the kth is given by II. PROBLEM FORMULATION X Consider a power distribution network where multiple feed- sk(w; x) = xj: (2) ers can supply energy to all consumers, and must be operated j:vj 2Vk(w) in a radial structure at all times. In network reconfiguration, The set V (w) is the subset of nodes for a particular topology sets of breakers and tie switches can reconfigure themselves k downstream of kth flow measurement under switch state w. such that all loads are connected and no feeders are connected. The task of recovering the network topology is to detect the switch statuses given all available information. E. Topology Detection A. DC Power Flow The detection and placement problem is solved in two scenarios: (1) deterministic case, where loads and flows are We use a DC power flow approximation to the actual AC known perfectly, (2) stochastic case, where loads are known flow in the distribution system [14]. The model is normally with uncertainty due to forecasting error. used in approximating the voltage magnitude and phase in In the deterministic case, a simple detector will return all the network, but since our detection problem relies on power topologies which satisfy the load and flow information as flows, this is equivalent to using a lossless network flow follows: representation. K In a usual representation, the distribution system is modeled w^ = fw 2 f0; 1g : sobs = s(w; x)g: (3) as a graph G(V; E) where vertices, v 2 V represent nodes (transformers) and the edges e 2 E represent the distribution In the stochastic case, a MAP detector can be written as lines. The signed incidence matrix is B 2 {−1; 0; +1gjV |×|Ej, w^ 2 arg max Pr (w j s; ^x) : (4) K where each undirected edge has a pre specified direction: ek = wi2f0;1g (vn; vm) on which to assign columns of B as follows: These naive methods are inefficient and provides no guaran- 8 tee on unique detection or sensor placement. For both detector >+1 if vi is the originating node of edge ej < types the following general questions are explored. Bi;j = −1 if vi is the terminal node of edge ej (1) :>0 else: 1) (Correctness): How to guarantee that this method will return a unique and correct spanning tree? Given the set of net injections in the network, y the flow 2) (Efficiency): How to search for the correct configuration constraints can be represented as: Bf = y. This can be without evaluating all 2K configuration since this can extended to a complex load case but is out of the scope of be inefficient? this work. 3) (Sensor placement): Where to place line sensing to minimize missed detections in both deterministic and B. Load Model stochastic settings? Each load vn in the system has a consumption xn. We The following sections show how this problem can be assume that the loads are single phase real power quantities reduced to a spanning tree detection problem, and how it can and the forecast errors are independent random variables: be solved in an efficient matter and provide some guarantees 2 2 n ∼ N(0; σn) and xn ∼ N(^xn; σn). Given the single global on sensor placement for correct status recovery. source of energy, we have the following y = [1T x − x]T . C. Switching Model Network Configuration III. MODEL REDUCTION Each switch has a status wi 2 f0; 1g, and w = We show that the general distribution system with switching fw1; : : : ; wK g. The switching is constrained so that all loads devices under a lossless power flow can be reduced to an island must be connected to some feeder and there can exist no graph which simplifies the structure of the valid configurations. path between various feeders. This ensures that each feeder The detection problem is then cast as a spanning tree detection is connected to some set of loads in a radial configuration, problem with nodal and edge measurements on the island and that no loads are in outage.