10-11-2012, 05:33 PM
A Practical Comprehensive Approach to PMU Placement for Full Observability
A Practical Comprehensive Approach to PMU Placement.pdf (Size: 676.75 KB / Downloads: 36)
Introduction
PMU Overview & History
The phasor measurement unit (PMU) has the potential to revolutionize the way electric power systems are monitored and controlled. This device has the ability to measure current, voltage, and calculate the angle between the two. Phase angles from buses around the system can then be calculated in real time. This is possible because of two important advantages over traditional meters – time stamping and synchronization. The algorithms behind phasor measurement date back to the development of Symmetrical Component Distance Relays (SCDR) in the 1970’s. The major breakthrough of SCDR was its ability to calculate symmetric positive sequence voltage and current using a recursive Discrete Fourier Transform. The sampling process is described in Figure 1.1. The recursive algorithm continually updates the sample data array by including the newest sample and removing the oldest sample to produce a constant phasor [22][23]. Phasor XN
Phasor
The advent of the Global Positioning System (GPS) in the 1980’s was the second breakthrough that enabled the modern PMU. Researchers at Virginia Tech’s Power Systems Laboratory in the mid-1980’s were able to use the pulses from the GPS satellites to time stamp and synchronize the phasor data with an accuracy of 1.0 μs. With the addition of effective communication and data collection systems, voltage and current phasors from different locations could be compared in real-time. Figure 1.2 shows the functional block diagram of a PMU [22].
Figure 1.2 PMU Functional Block Diagram
As Section 1.3 will show, PMUs have come out of their academic infancy with commercial viability. They are now commercially produced by all major IED providers in the power industry, including ABB, GE, Siemens, Arbiter, UCS, Macrodyne, SEL, and Seifang. To aid the maturing of the industry, an important standard has been developed by the IEEE. The IEEE SYNCHROPHASOR [14] standard, c37.118-2005, was developed from an earlier version, the IEEE 1344-1995 [13]. It ensures PMUs from different manufacturers operate well together. Initial cost of PMUs in the early 90’s was about $20k. The price has since dropped to $3k for the simplest units. However, installation costs remain high, between $10k-50k depending on the utility and location [8].
Applications
Monitoring real-time angle differences has many potential applications in power systems. Simply placing PMUs in various substations can help prevent blackouts by real-time monitoring by system operators. System operators can be warned of potential problems more quickly during critical situations, where seconds can make all the difference in detecting and dealing with dangerous cascading events. Operators neighboring a highly stressed system would also be more alert to potential dangers originating outside of their control area. If a cascading problem were to arise, PMUs would be very useful in determining where and how to perform system separation to limit the effect of the system disturbance [18].
State estimation is the application in which PMUs could first have a significant impact. Incorporating data from a limited number of PMUs into existing state estimators that are fed by traditional SCADA systems has been shown to be both beneficial and relatively easy. The synchronized phasor data can improve bad data detection and provide better initialization for iterative state estimation algorithms, and the data itself can be used in the estimator alongside other metering data [18]. An even greater impact would be to replace all the traditional SCADA data with data input solely from PMUs. Current estimates can be referred to as “static state estimates” because it takes seconds to minutes for data to be collected and the state calculated. But since voltage and current are directly measured with PMUs, the state estimation solution becomes linear and much quicker—leading some to refer to such a system as “state measurement” rather than “state estimation”[22].
One application that is gaining attention in today’s deregulated market is the improvements that PMUs offer for real-time congestion management. Currently system tie lines and transfer corridor loading levels are compared to a predetermined Nominal Transfer Capability (NTC) which is set to the transfer level allowable before thermal, voltage, or stability limits are reached. The NTC is calculated offline beforehand using transfer levels, load levels, and a generation dispatch that may not fully represent the present system flows. PMUs allow for both more accurate measurement of transfer path loading and the computation of Real-time Transfer Capability (RTC). Real-time data
acquisition and quick RTC calculation would provide the system operator an accurate transfer capability on a moment to moment basis and in many instances lead to more economic system operation [17][18].
Adaptive protection is a concept that has been around for decades but has yet to be widely implemented in transmission systems. Presently, protective relays operate on fixed settings. These settings may have been set many years ago and have no way to adapt to the system operator’s preference to operate on one side or the other of the security/dependability spectrum. With appropriate communications, PMUs would allow for detecting system conditions and either change protection settings themselves, or wait for the operator to remotely change settings based on real-time data from PMUs. This could be particularly effective in reducing the harm caused by cascading blackouts. When the system conditions are particularly stressed, the protection settings should be set to be more secure so that one event doesn’t trip a line, which then overloads and trips another line, and so on. Line fault location can also be performed if PMUs are located on each end of the line to measure both current phasors [18][24].
Another class of applications uses PMUs’ time synchronized data, but doesn’t rely on real-time monitoring. There are already many PMUs installed around the world for the purpose of postmortem analysis. Previously, it was very difficult to recreate a timeline of events without an accurate and uniform timestamp. The GPS time pulses make it much easier to see what happened when and where, even across systems with different SCADA systems and state estimator time delays. PMUs can also improve system models when the data is analyzed offline. Time synchronized recording of how a generator or other systems react after a series of actions can be used to verify/improve existing models or create new ones. Measuring the current phasors from both ends of a transmission line is also useful in deriving the line’s π model [18].
State of PMU deployment
This section presents several PMU initiatives from around the world with the goal of indicating the level of deployment, the intended applications, and how these applications affected the PMU placement. After the 2003 Northeast Blackout, DOE’s Pacific Northwest Laboratory set up the North American SyncroPhasor Initiative
(NASPI), formerly the Eastern Interconnect Phasor Project (EIPP), as a working group to encourage PMU installation and monitor the PMU data across the Eastern Interconnect. As of 2006, there were 35 working PMUs delivering online data to a central location. In addition there were 11 more installed but not activated, and another 75 planned in the North American Eastern Interconnect. The stated goals of the deployment were postmortem analysis and general monitoring of system heath. With these goals in mind, the Equipment Placement Task Team (EPTT) targeted the PMUs at points of congestion, generator sites of 1500MW or greater, major load centers, and voltage sensitive areas [15].