Phasor Measurement Unit

IntroductionAs the electric power grid continues to expand and as transmission lines are pushed to their operating limits, the dynamic operation of the power system has become more of a concern and has become more difficult to accurately model. In addition, the ability to effect real-time system control is developing into the need to prevent wide scale cascading outages.For decades, control centers have estimated the state of the power system (the positive sequence voltage and angle at each network node) from measurements of the power flows through the power grid. It is very desirable to be able to measure the system state directly and/or augment existing estimators with additional information.Alternating Current (AC) quantities have been analyzed for over 100 years using a construct developed by Charles Proteus Steinmetz in 1893, known as a phasor. On the power system, phasors were used for analyzing AC quantities assuming a constant frequency. A relatively new variant of this technique that synchronizes the calculation of a phasor to absolute time has been developed2, known as synchronized phasor measurement or synchrophasors. In order to uniformly create and disseminate these synchronized measurements, several aspects of phasor creation had to be codified.Phasor Phasor is a quantity with magnitude and phase (with respect to a reference) that is used to represent a sinusoidal signal (figure below). Here the phase or phase angle is the distance between the signals sinusoidal peak and a specified reference and is expressed using an angular measure. Here, the reference is a fixed point in time (such as time = 0). The phasor magnitude is related to the amplitude of the sinusoidal signal.

Phasor technologyPhasor technology is considered to be one of the most important measurement technologies in the future of power systems due to its unique ability to sample analog voltage and current waveform data in synchronism with a GPS-clock and compute the corresponding 60 Hz phasor component (i.e. complex numbers representing the magnitude and phase angle of a 60 Hz sinusoidal waveform) from widely dispersed locations (see figure). This synchronized sampling process of the different waveforms provides a common reference for the phasor calculations at all the different locations.

The phase angle differences between two sets of phasor measurements (i.e. d1-d2) are independent of the reference. Typically, one of the phasor measurements is chosen as the reference and the difference between all the other phase angle measurements (also known as the absolute phase angle) and this common reference angle is computed and referred to as the relative phase angles with respect to the chosen reference (see figure below).

The time synchronization is important Through the use of integral GPS receiver-clocks, PMUs sample synchronously at selected locations throughout the power system. This provides a system-wide snapshot of the electrical system. The GPS not only provides time tagging for all the measurements but also ensures that all phase angle measurements are synchronized to the same time as well.

Figure 6 shows how time skews in the measurement and analysis process can induce errors in the phase angle difference computations. Here, the phase angle difference between two sets of measurements was approximately 10 degrees. However, when one of these two phase angle signals was skewed by 1 second and then the phase angle difference was computed, then the resulting answer incorrectly indicated a phase angle difference of approximately 5 degrees between those two data sets. Phasor calculations demand greater than 1 millisecond accuracy.

Phasor network elements are The simplest form of phasor network consists of TWO nodes; one Phasor Measurement Unit (PMU) at node 1 that communicates with one Phasor Data Concentrator (PDC) at node 2. Typically, many PMUs located at various key substations gather data in real-time and they are connected to a PDC at the utility center where the data is aggregated. A personal computer, connected to the output of the PDC provides the users with software, such as RTDMS that calculates and displays locally measured frequencies, primary voltages, currents, MWs and MVARs for system operators. Additionally, many PDCs belonging to different utilities can also be connected to a common central PDC (a.k.a., SuperPDC) to aggregate data across the utilities to provide an Interconnection-wide snapshot.

Definition of Phasor Monitoring Unit (PMU)A PMU is an electronic device that uses state-of-the-art digital signal processors that can measure 50/60Hz AC waveforms (voltages and currents) typically at a rate of 48 samples per cycle (2880 samples per second). The analog AC waveforms are digitized by an Analog to Digital converter for each phase. A phase-lock oscillator along with a Global Positioning System (GPS) reference source provides the needed high-speed synchronized sampling with 1 microsecond accuracy. Additionally, digital signal processing techniques are used to compute the voltage and current phasors

Line frequencies are also calculated by the PMU at each site. This method of phasor measurement yields a high degree of resolution and accuracy. The resultant time tagged phasors can be transmitted to a local or remote receiver at rates up to 60 samples per second.

PMUs come in different sizes. Some of the larger ones can measure up to 10 phasors plus frequency while others only measure from one to three phasors plus frequency. The approximate cost of the larger PMUs can range in the $30 to $40 thousand of dollars while the smaller ones cost considerably less.

PMU installation and connectionInstallation of a typical 10 Phasor PMU is a simple process. A phasor will be either a 3 phase voltage or a 3 phase current. Each phasor will, therefore, require 3 separate electrical connections (one for each phase). We are talking about 6 wires per phasor 2 for each phase of either voltage or current. The PMU will also measure the line frequency from a specific voltage phasor (typically a major bus assigned by the user).

Typically an electrical engineer designs the installation and interconnection of a PMU at a substation or at a generation plant. Substation personnel will bolt equipment rack to the floor of the substation following established seismic mounting requirements. Then the PMU along with a modem and other support equipment will be mounted on the equipment rack. They will also install the Global Positioning Satellite (GPS) antenna on the roof of the substation per manufacturer instructions. The antenna signal cable will be connected to the antenna and brought directly to the PMU. Substation personnel will also install shunts in all Current Transformer (CT) secondary circuits that are to be measured. Potential Transformer (PT) connections will not require the installation of any additional equipment other than terminal blocks and fuses. They will have to run wires from the CT shunts and the PTs to either an interface cabinet or directly to the input connections of the PMU (Figure 10). Note: Each phasor (either Voltage or Current) will require three connections one for each phase.

In addition to the CT and PT connections the PMU will also require the following connections:- Power connection typically from station batteries.- Station ground connection.- Global Positioning Satellite (GPS) antenna connection.- Communication circuit connection (Modem if using 4-wire connection or Ethernet for network connection).

After all the connections are made, the PMU is configured and tested. This task is typically performed by a substation Test Technician.

The utilitys IT department will play a key role will the phasor data connections phase of the PMU installation. After the entire input channel configuration and testing is completed, the PMU is connected to the utilitys Phasor Data Concentrator (PDC) via 4-wire Modem or Ethernet connection depending on the bandwidth needs. They will also need to evaluate the need to install additional communication equipment in order to provide the necessary circuit connections between the PDC at the master site and the PC workstations at the client sites.

Phasor Data Concentrator (PDC)A PDC forms a node in a system where phasor data from a number of PMUs or PDCs is correlated and fed out as a single stream to other applications. The PDC correlates phasor data by time-tag to create a system wide measurement set. The PDC provides additional functions as well. It performs various quality checks on the phasor data and inserts appropriate flags into the correlated data stream. It checks disturbance flags and records files of data for analysis. It also monitors the overall measurement system and provides a display and record of performance. It can provide a number of specialized outputs, such as a direct interface to a SCADA or EMS system.

Super Phasor Data Concentrator (SuperPDC)Within a point-to-point phasor network architecture, the SuperPDC is simply a central PDC that collects and correlates phasor data from all remote PDCs and PMUs and makes it available to a visualization software package as described above (Figure 11). Typically, the SuperPDC is also connected to a central database for long-term archiving of the collected data

A SuperPDC should have the capability to do both if it is fast enough. The obvious problem of locally storing ALL the data would be the need to employ large disk drives and have a system in place to regularly transfer full disk phasor data to DVD for permanent storage. A rate of 30 or 60 samples per second fills up a disk drive very quickly.

Kind of delays which can be expected in the real time streamA PDC receives data streams from PMUs and other PDCs and correlates it in real-time into a single data stream that is transmitted to a PC via an Ethernet port. They propagation delays associated with communication links from a PMU and PDC depend on the medium and the physical distance separating these components. For a typical PMU with 10-12 phasors, the associated delays for various communication mediums are summarized below.Associated Delays with Various Communication LinksCommunication LinkAssociated Delay one way (milliseconds)

Fiber-optic cables (50 Mbps 1 Gbps)100-150

Digital microwave links100-150

Power line (PLC) (upto 4 Mbps)150-350

Telephone lines (upto 56 kbps)200-300

Satellite link500-700

In addition, the fixed delay associated with processing, concentrating, multiplexing, and transducers, and is independent of the communication is 75 ms.

Finally, PDCs also have a maximum wait-time, typically of 1-4 seconds, to allow for all the PMU data to come in before aggregated data is outputted by the PDC. If the data from all the PMUs reach the PDC within this wait-time, it outputs the aggregated data right away. However, in the extreme case that the data from one of the PMUs is indefinitely delayed, then the PDC will wait upto its pre-defined wait-time (i.e., 1-4 seconds) before the data is outputted by the PDC. Hence, the PDC can also introduce an additional delay equal to its wait-time if one of the PMU channels stops transmitting data to the PDC. In such circumstances, if there are additional PDCs downstream in the point-to-point phasor network architecture (such as the SuperPDC), then they too will introduce a secondary delay equal to their wait-time.

Phasor applications such as Real-Time Dynamics Monitoring System (RTDMS) are designed to directly integrate with the PDCs over the utilitys high speed Local Area Network (100 Mbps) and display the data and calculated individual engineering units such as MW, MVAR, etc. within 1 second of receiving the data from the PDC (Note: The Web based versions of the RTDMS Clients, such as those deployed across the Eastern Interconnection as part of the EIPP project, integrate over secure internet connections rather than Local Area Networks and can longer to display the information within its visualization screens).

Bottom Line: The total delay from when the data is captured by the PMU to it being visualized within the RTDMS screens is typically a few seconds.

Type of data quality problems does one typically encounter with such a systemThere are two main types of problems associated with data validation: First, there is a data loss problem associated with network problems such as bandwidth limitations, collisions, misrouting, maintenance outages, and equipment breakdown to name a few. There is not much one can do about this other than to report it to IT for review and repair. The other problem is measurement data validation which has to do with obtaining inaccurate information such as incorrect resistor shunts as well as incorrect CT and PT rations. This also includes larger inaccuracies, such as 120, due to incorrect wiring errors or labeling inconsistencies in what is called Phase A, Phase B and Phase C across utilities. The latter type will take time to correct and may have to involve station technicians at a particular site and even sometimes the engineering department. If this data is also available through the SCADA system the two values should be compared in order to achieve some degree of validation.

Finally, each PMU vendor utilizes their own proprietary phasor computation algorithms as well as pre and post processing filters, each with their own unique design characteristics. These and other factors can result in adding constant offsets to the phase angle measurements which may be more significant at off-nominal operating frequencies. Figure 12 compares the phase angle offsets of different PMUs over a wide frequency range about the normal 60Hz operating frequency. As long as all the PMUs within the phasor network are provided by the same vendor, this is not an issue; otherwise these offset errors should be corrected.

Synchronized phasor measurement technology is relatively new, and consequently several research groups around the world are actively developing applications of this technology. It seems clear that many of these applications can be conveniently grouped such as Power System Real Time Monitoring, Advanced network protection, Advanced control schemes.Fig.2. PMU layout with GPS time stamped signal

Fig. 2 shows PMUs geographically dispersed to form a Wide Area Monitoring System (WAMS) in which the PMUs deliver GPS time-tagged measurements to a Phasor Data Concentrator (PDC). The PDC sorts the incoming phasor measurements before signal processing converts PMU data into actionable information that can be presented to an operator in the form of a Human Machine Interface (HMI). This HMI provides an operator with critical information about the state of the power system.

PMU WORKING PRINCIPLE

Fig.2 : Basic block diagram of PMU

The Voltages and currents in their analog form are derived from the potential and current transformer secondaries which are then fed to Anti-Aliasing filter. The surge isolation stages have omitted in this block diagram although they must be included in the practical measurement system meeting theIEEE Surge Withstand Capability standard, C37.9 1

1- Anti-aliasing filterAs per Nyquist-Shannon sampling theorem, to reconstruct a signal after sampling, sampling frequency must greater than twice the maximum frequency of the signal to be sampled. If lower sampling rates are used then the original signals information may not be completely recoverable from the sampled signal and they may appear as aliases. So to avoid this effect, Anti-aliasing filter is used which restricts the bandwidth of the signal to approximately satisfy the sampling theorem, for the fixed sampling.

2-GPS

Global positioning system is a satellite based navigation system which gives information about location and time irrespective of weather conditions. It consists of a network of 24 satellites orbiting in 6 geo-synchronous orbits such that at any given instant 4 satellites are visible from any point on the earth surface. GPS provides accurate information about location and time (could be local time or Universal Time Coordinate) of the GPS receivers location. Along with it satellite is capable of transmitting common access one pulse per second with an identifier, which is accurate about 1s at any location on earth. This pulse combined with time-tag is crucial for the application considered here.

3- Phase locked oscillator

Usually in PMU, the pulse signals from the satellite are phase locked with the sampling clock. This job is accomplished by phase lock oscillator. PLO divides the one pulse per second signal from GPS into required number of pulses per second for sampling. At present in most systems, this is 12 per cycle of fundamental frequency [4]. The sampling instant could be identified, as the pulse number within one second interval is identified by the GPS time tag.

When the phasors are time tagged then they are referred as synchrophasors. New IEEE Standard C37.118-2005 [10] (In 2011, it was divided into two parts with one describing the standards of phasor measurements and the other describing standards of communication channel) describes the exact format of the time tagging and the measurement of synchrophasors.

Fig.3 : Synchrophasor definition and angle convention

Using the synchronized clocks as a reference, a PMU creates the phasor representation (complex number) of a constant sinusoidal signal as shown in fig.3. The time tag, representing the reporting instant, forms the reference for the phasor representation of the measured sinusoidal signal. The relationship between the time tag and the phasor representation is such that the phase angle of the phasor is equal to the angular separation between the time tag and the peak of the sinusoid. For instance, in Fig.3.a the peak of the signal coincides with the time tag producing an angle measurement of 0, whereas in Fig.3.b the signal crosses zero at the time tag producing an angle measurement of -90 as per the synchrophasor standards.

If the phasors are determined with respect to arbitrary time signals the phase angle by itself has no particular significance. So when all PMUs in the power system use the same time reference, their measurements are comparable and the phase angle differences between the phasors are accurate.

4 -A/D Converter

The Analog to digital converter digitizes the analog signal, from the AAF,at sampling instants defined by the sampling time signals from PLO. These digitized samples are then fed to the phasor microprocessor.

5- Phasor microprocessorIt is programmed to calculate the positive sequence components from the digitized sampled data by using an recursive algorithm which is usually Discrete Fourier Transform (DFT) as described in [11]. This calculated phasor is time-tagged. All the measured data are transmitted to the remote location through a proper communication channel using modems.

6- PMU utilization in power systemThe phasor measurement units installed at various buses in the power system network provides with a pool of time-tagged phasor measurement data at various nodes in the network. These data are gathered by the device called phasor data concentrator which synchronizes the measurement taken at every time instant independent of when the data was received. Then these time synchronized datas are fed to the advanced application software for the analysis of power system. Based on this analysis, system control, protection and various other functions are guided. Phasor data concentrator has been detailed discussed in [12].

OPTIMUM PLACEMENT OF PMU

Observability of the system is necessary for better monitoring, control, operation and protection of the system. In simple context, the system is said to be observable if all the state vectors of the system can be estimated or measured directly or indirectly to the required accuracy. The concept of system observability is defined in two ways [13] namely, Numerical observability and Topological observability. It is obvious, that installing PMU at every bus will prove to be uneconomical. So, Measurements that can used for observability estimation are listed in [13] as of three types namely,

(i) Direct measurements: They are the voltage and incident current phasor values at the buses where PMUs are located.(ii) pseudo measurements: These are the current and voltage phasor values calculated from direct measurement data through ohms law.(iii) Extended measurement: They are the current and voltage phasors which are calculated at the zero injection bus. This extended measurement is defined only in two cases. First,in the case of a bus where all the incident current are known except one. Hence, the unknown current value can be inferred from the kirchoffs law. Second, in the case of a bus where all the incident bus voltages are known, hence the voltage of that bus can be inferred from the kirchoffs law.

Optimal placement of PMUs in the system topology fulfilling the objective of complete observability of the system is necessary. This objective has been a subject of research in many papers[13],[14],[15],[16]. The task of analyzing power system observability is equivalent to the task of building measurement spanning tree. According to the network topology, identified by the system adjacency matrix, pseudo and extended measurements are obtained from the direct measurements. In turn, by using this calculated measurements, new pseudo and extended measurements at adjacent buses or nodes are calculated to we fail to find new measurements.

As a result of this, a measurement spanning tree will be built and if it included all of the buses in the system then the system is said to be observable in the topological sense. Though this technique is basic one, several methods have been adopted in above cited research papers in determining the optimal placement of PMUs in the system for its complete observability such as binary search algorithm, incomplete observability, observability factor analysis, hybrid genetic algorithm and simulated annealing, to name a few.

APPLICATIONS

Phasor measurement unit encompasses high potential to be effectively utilized by the power system utilities to advance the overall existing technology. Considering its accuracy and speed in the measurement of the phasors and other parameters, it will materialize as a fundamental component in smart grids. A survey on its applications is presented below

1- Early applicationsDuring early times at the introduction of commercial PMUs, due to low availability and high cost of communication channels, post-event monitoring were the only application of PMUs. At this stage they were essentially used as digital system disturbance recorders. The frequency and phasor measurement data provided by PMUs showed interesting information and expanded their application scope in the power system.2- State estimationVoltage and phase angles at different buses describe the state of the system. Usually in the traditional state estimation, the line flow measurements of real and reactive power were used to estimate the voltage and phase angle at all the buses in the system. In other words, the state of the system was just inferred from the unsynchronized power flow measurement. But with the advent of PMU, state measurement instead of state estimation, is made possible. In other words, state of the system is obtained through direct measurements from PMUs rather than estimating from various available data in the system3- Power system protectionSynchronized phasor measurements helps in advancing the power system protection techniques. PMUs enable the early fault detection in the system, allowing for the quick isolation of the faulted segment preventing the power outages. Synchronised phasor measurement provides us with time track of the state variables and several of their derivatives over an observational interval, so it makes simpler to predict the instabilities or outcome of a power swing or transients or any other disturbance using relatively good and simplified models. With these predictions, appropriate protection decisions can be taken by the operator. This leads to the concept of adaptive relaying, where the protection functions changes according to the changing power system conditions. Time tracking and prediction of state variables is posturized in fig.5.mathematical model based calculations are used in the control functions. To the extent that the assumed model is not valid under the prevailing conditions, control function may not be effective.

Fig.5 : Time tracking and prediction of state variables

With synchronous phasor measurements, it is possible for direct feedback of system parameters from remote locations to the controllers. Experience shows that improved control performance is achieved with PMUs based controllers, than the model based controllers. Also the frequency data are representative of the transient stability, electromechanical oscillation and certain overload phenomenon, which certainly helps in advanced control of power system. PMUs enable wide area measurement, protection and control in the whole area of regional transmission networks and local distribution grids. Power quality can be increased by precise analysis and automated correction of power sources based on the feedback from the PMUs. Various applications of PMU are detailed in [17],[18],[19].

RELIABILITY ISSUES WITH PMU

The problems associated with PMUs include clocking system failures and inaccuracies, communication channel problems, instrument transformer problems, filter instabilities and inaccuracies, calibration errors, component failures, software errors, etc.. These problems affect the phasor measurement applications of monitoring, controlling and protecting power system. The reliability of the PMUs must be ensured for its effective application in the power system.

CONCLUSIONThe above presented a brief review on the phasor measurement unit and its application in the power system. For detailed analysis, appropriate references were cited. Currently, researches are carried out in integrating more functional components into the existing PMU and extend its application capabilities in the power system. Also, a number of pilot projects are being carried out in many countries and utilities to install PMUs and analyze its possible advantages in their existing systems. There is no exaggeration in saying that PMU is one of those technologies which has revolutionized the way of power system operation in the recent decades. PMU proves to be a promising component for the Smart girds for its advanced measurement, monitoring, control, protection, reliability and automated decision supporting objectives. In the coming years, we can witness the extensive use of PMU world over.

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