1 Abstract—Bad data detection and identification is one of the essential functions performed by state estimators. Capability of a state estimator to detect bad data is directly related to the measurement configuration. Bad data associated with critical measurements can not be detected. Transforming critical measurements into redundant measurements requires adding extra measurements at strategic locations. In this paper, phasor measurement units (PMU) are used for this purpose. It is shown that with few extra PMUs, bad data detection and identification capability of a given system can be drastically improved. Description of the placement algorithm is given and case studies carried out on different size test systems are presented. Index Terms—State Estimation, Critical Measurements, Bad Data Processing, Phasor Measurement Units, Meter Placement. I. INTRODUCTION tate estimation has become one of the essential energy management system (EMS) functions used in modern control centers. Its main purpose is to maintain a reliable real-time database, which will in turn be used by all other EMS functions. In doing that state estimators must detect, identify and eliminate errors associated with the measurements and network parameters. In state estimators that use weighted least squares (WLS) method, this is accomplished by a post-estimation bad data processing method. The key issue in detecting and identifying bad data is measurement redundancy. Redundancy can be roughly defined as a measure of excess measurements over the minimum number that is required for estimating the system state. Given enough redundancy, state estimation should be able to detect, identify and remove bad data. However, existing measurement configurations may not always yield such desired level of redundancy. As a result of low local redundancy, systems may contain several critical measurements. These are measurements whose removal from the system will make the system unobservable. Bad data associated with critical measurements can not be detected or identified. Critical measurements can be identified by well- documented procedures, which are based on topological [1] or numerical methods [2]. Once identified, they can be Jian Chen is with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843 USA (e-mail: jianchen@ee.tamu.edu). Ali Abur is with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843 USA (e-mail: abur@ee.tamu.edu). transformed into redundant measurements by adding few measurements at strategic locations. In recent years, GPS synchronized phasor measurement units (PMU) have started populating power systems. While the primary purpose of their installation is not related to state estimation, widespread placement of PMUs presents an opportunity to improve state estimation. In addition to the conventional measurements, a PMU placed at a given bus can provide the voltage phasor at the bus and the phasor currents on several or all lines incident to that bus [3]. It is simple to incorporate synchronized phasor measurements into state estimators along with conventional measurements and this can be shown to improve state estimation performance [4]. In this paper, a PMU placement algorithm for transforming existing critical measurements into redundant ones will be presented. The main goal is to improve bad data processing capability of state estimators by taking advantage of the PMU technology. A feasible numerical method is developed to find an optimal strategy of PMU placement to achieve this objective. II. APPROACH As mentioned above, the critical measurement is a non- redundant measurement, whose elimination from the measurement set will result in an unobservable system. For example, consider removal of a critical measurement, which results in a system split into three observable islands as shown in Figure.1. Observable island Observable island Unobservable branches Observable island Unobservable branches Unobservable branches Bus i Bus j i j θ θ - Figure.1 Observable islands after removal of critical measurement Let us now assume that there are several PMUs installed in the system and by using the measurements of these PMUs, the voltage angle of some buses can be obtained. If among these buses, there are two buses, which belong to different Improved Bad Data Processing via Strategic Placement of PMUs Jian Chen, Student Member, IEEE and Ali Abur, Fellow, IEEE S