A Flexible GPS based system for synchronized phasor measurement in distribution netwo
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18-12-2010, 10:47 PM



A Flexible GPS based system for synchronized phasor measurement in distribution networks

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Sethunath.S
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.doc   A Flexible GPS based system for synchronized phasor measurement in distribution networks.doc (Size: 240 KB / Downloads: 51)
INDEX

1. Abstract 3
2. Introduction 4
3. Synchrophasors 5
4. Hardware components 7
5. Reallocation of samples 9
6. Synchronized phasor Evaluation 10
7. Applications of Phasor Measurement 15
8. Conclusion 19
9. Reference 20













1. ABSTRACT

Large-scale distributed measurement systems are the object of several applications and research. The goal of this paper is to develop, by employing Global Positioning System (GPS) receivers, measurement techniques that are suited to the continuous monitoring of the electrical quantities in distribution networks in terms of synchronized phasors. The proposed measurement procedures, differently from commercially available phasor measurement units, are based on general-purpose acquisition hardware and processing software, thus guaranteeing the possibility of being easily reconfigured and reprogrammed according to the specific requirements of different possible fields of application and to their future developments. The goal of this paper is to develop, by employing GPS receivers, measurement techniques that are suited to the continuous monitoring of the electrical quantities in distribution networks in terms of synchronized phasors.
The protection schemes for traditional distribution networks, thought to be for radial structures, should be reconsidered since the presence of distributed supply sources can modify local structures of the networks, making them no longer radial. As a consequence, a dynamic management of the protection coordination that can be based on adaptive protection schemes is necessary, in which breakers and reclosers, with remote communication capability, may be operated to automatically reconfigure the network in the presence of faults.


2.INTRODUCTION

Integrated systems for the optimal dispatching of the electric energy from renewable sources and for the control of the loads are being diffusely studied with the goal of designing automatic systems that are aimed at optimizing energy production and distribution. These distributed measurement and control systems are composed of a set of peripheral units equipped with intelligent sensors and transducers for the simultaneous acquisition of measurement data in different network points, some microprocessors for their pre-processing, a wide range of communication systems, and a central unit that is dedicated to processing the data coming from distributed remote units. It becomes important to design large-scale distributed measurement systems, which are able to carry out simultaneous measurements of electrical quantities in several monitored points on the tested system. Due to the extension of distribution systems, suitable synchronization devices should be used, so that the measurement results are not significantly affected by the eventual lack of synchronization between the remote instruments.

For the phasor estimation, commercial phasor measurement units (PMUs) are currently available for transmission systems. The proposed measurement procedures are based on general-purpose acquisition hardware and processing software, thus guaranteeing the possibility of being easily reconfigured and reprogrammed according to the specific requirements of different possible fields of application and to their future developments. In each PMU, a dedicated GPS receiver produces a timing pulse, which phase locks the sampling hardware of the instrument, and an implemented phasor algorithm estimates the phase angles of input signals at fixed data rates.


3. SYNCHROPHASORS

The concept of synchronized phasor is “a phasor calculated from data samples using a standard time signal as the reference for the measurement.” The time reference signal, to which the standard refers for the evaluation of the synchronized phasors, is the Coordinated Universal Time (UTC). To this purpose, specifies that the synchronization signal shall have a basic repetition rate of 1 pulse per second (1 PPS). The synchronizing source shall have sufficient availability, reliability, and accuracy to meet power system requirements. A synchronization source should have continuous uninterrupted availability and be accessible to all sites among which the data are to be compared. The synchrophasor representation X of a sinusoidal signal x(t) is the complex value given by

X = (Xm/v2)e^ja = Xm/v2(cos a+ j sin a)

where Xm/v2 is the root-mean-square value of the signal x(t), and a is its instantaneous phase angle relative to a cosine function at a nominal system frequency.
Practically, it is possible to also consider an absolute phase, which is representative of the synchrophasor . Let the following function u(t) represent the sinusoidal voltage at a bus in a power system network

u(t) =v2U cos(2pft + a)

where f is the actual frequency, and a is the initial phase.
The argument of the cosine function is given by

?(t) = 2pft + a.

When the actual frequency f differs from the nominal one fn, the argument of the cosine function can be rewritten as

?(t) = 2pfnt + 2p?ft + a
where ?f = f - fn.
The absolute phase is defined as

ß(t) = 2p?ft + a

In this case, if the system operates under steady-state conditions, but the frequency of the electrical quantities differs from the nominal frequency fn, PMUs show continuous phase changes. On the other hand, examining the phase a , which is considered as the representative phase of the phasor, and assuming the presence of steady-state quantities, even under off-frequency conditions, PMU devices would provide a constant phase evaluation. The phase that has real interest, in practice, is the relative phase, i.e., the phase difference between two quantities that can be measured in different nodes of an electric distribution network.
If we consider two measurement points A and B that are characterized, the absolute Phases are
ßA(t) = 2p?ft + aA and ßB(t) = 2p?ft + ?B, respectively,

the relative phase is ?ß(t) = ßA(t) - ßB(t) = aA - aB = ? a.

This shows that the same result would be obtained by considering as absolute phases.











4.HARDWARE COMPONENTS

This particular method of monitoring requires certain hardware components to be implemented. Certain components have to be installed at every node of the network where quantities have to be monitored.

1. Global Positioning System (GPS)
2. Phasor Measurement Unit (PMU)
3. Data Acquisition Board (DAQ)

4.1. Global Positioning System (GPS)

It is a constellation of satellites, which orbit the earth twice a day, transmitting precise time and position (Latitude, Longitude and Altitude) Information. The complete gps system consists of 21 satellites and 3 spares. Electrical monitoring systems require accurate time measurements. Repeated power blackouts have demonstrated to power companies the need for improved time synchronization throughout the power grid. Analyses have led many companies to place GPS-based time synchronization devices in power plants and substations. It furnishes a common-access timing pulse which is accurate to within 1 microsecond at any location on earth. A 1-microsecond error translates into 0.021° for a 60 Hz system and 0.018 ° for a 50 Hz .

4.2. Phasor Measurement Unit (PMU)

They are devices which use synchronization signals from the global positioning system (GPS) satellites and provide the phasor voltages and currents measured at a given substation. PMU is installed in every node of the network and it relays back the information to the control center where all the information is analyzed and proper steps are taken.


4.3. Data Acquisition Board (DAQ)

Data acquisition (abbreviated DAQ) is the process of sampling of real world physical conditions and conversion of the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition and data acquisition systems (abbreviated with the acronym DAS) typically involves the conversion of analog waveforms into digital values for processing. The components of data acquisition systems include:
• Sensors that convert physical parameters to electrical signals.
• Signal conditioning circuitry to convert sensor signals into a form that can be converted to digital values.
• Analog-to-digital converters, which convert conditioned sensor signals to digital values.
Data acquisition applications are controlled by software programs developed using various general purpose programming languages such as BASIC,C. COMEDI is an open source API (application program Interface) used by applications to access and control the data acquisition hardware. Using COMEDI allows the same programs to run on different operating systems, like Linux and Windows.












5.REALLOCATION OF SAMPLES














As far as the internal clock of the DAQ board is concerned, it should be noted that its accuracy (whose nominal relative value can be in the order of 10-4) can significantly affect the accuracy of the time positioning of the samples, which is important in many applications, such as time stamping. Indeed, the uncertainties affecting the sampling intervals, arising from the clock instability, can be considered to be almost totally correlated between each other. This means that, in the samples immediately acquired before the rising edge of the next PPS pulse, an uncertainty on the order of 10-4 s can be expected on the value of the time elapsed from the last pulse. This would definitely make the excellent accuracy of the time reference provided by the GPS receivers useless. Therefore, a routine has been implemented on the input signal to reallocate the samples by their correct time position. By evaluating the number N of samples between two PPS edges, the actual sampling interval Ts can be simply calculated as Ts = 1/N. By counting N, it is possible to use the actual sampling interval Ts = 1/N instead of the nominal value Ts0 = 1/fs0 in the computations performed on the data records that are acquired during the subsequent second. Indeed, the internal clock of the acquisition board could be even quite inaccurate (according to its nominal specifications), but the variations of its instability are expected to be very low. Thus, this simple technique allows each sample to be positioned more correctly in the time axis.
6.SYNCHRONIZED PHASOR EVALUATION

The proposed measurement system is designed to operate in distribution systems, where the signals can feature a nonnegligible harmonic content. By this system, it is possible to obtain the measurement of the synchrophasor that is related to the fundamental frequency component for every period of the signal. Due to the nonnegligible harmonic content, a discrete Fourier transform (DFT) is needed to extrapolate the components at the fundamental frequency. Therefore, the instrument, besides measuring the synchrophasor that is related to each period of the fundamental frequency component, could also evaluate the amplitude and the absolute phase of the harmonic components, thus allowing their value in different points of the electrical network to be compared. This information could be very useful in the attempt to understand the origin of power quality phenomena, which are, today, important issues in the management of electric distribution systems. Since one of the main goals of this paper is to make the measurement systems easily adaptable to different possible requirements of the specific practical needs, two different procedures have been implemented, with the main difference between them consisting of the choice of the observation window. They are

6.1. Fixed length window.
6.2. Variable length window

6.1FIXED LENGTH WINDOW








Synchrophasors is obtained by processing the samples acquired in a window having a fixed length Tw. The window Tw considered in the DFT is chosen as an integer multiple of the nominal period, which is Tn = 20 ms for systems that are operated at a 50-Hz nominal frequency. Here, similarly to some commercial PMUs, Tw = 5Tn = 100 ms has been chosen. As a consequence, the frequency resolution of the DFT is fw = 1/Tw = 10 Hz. If the actual frequency of the system was perfectly equal to its nominal value, the output data (the amplitude and the phase of the synchrophasor) would be directly gauged at the fifth component of the DFT. On the other hand, in the normal practice of distribution systems, the actual frequency can differ from the nominal one. Under these circumstances, the results obtained by means of the DFT are affected by spectral leakage whose effects can be reduced by applying smoothing windows, such as the Hanning one. In any case, the accuracy of the amplitude and phase evaluations can be significantly improved by means of suitable compensation algorithms. Here, a compensation technique for the phase estimation has been implemented, which is aimed at obtaining an error reduction of the phase estimation with an interpolated DFT.
Let us consider a periodic signal u(t) composed of M harmonic components. Its generic sample k can be expressed as





where Um, fm, and Fm are the amplitude, the frequency, and the phase, respectively, of the mth component.
The window length Tw = 1/fw is not generally a multiple of the signal period T = 1/f, so the frequency fm can be decomposed into two parts, i.e.,

fm = imfw + dmfw

where im is an integer value, and dm (-0.5 < dm < 0.5) is the displacement term caused by noncoherent sampling.




The ith component of the DFT of the windowed signal is given by













where W represents the DFT of the smoothing window function, and ?m = fm/fw.
By considering only the fundamental component (m = 1), the phase F1 can be evaluated as a function of the phases of the harmonic components evaluated by the DFT (denoted with the symbol F) and depends on the sign of the term f - ifw (which is i = 5 in this case due to the selected parameters)







It can be noticed that such compensation requires the knowledge of the actual frequency f of the system. To this purpose, the frequency is measured by means of a zero-crossing algorithm, which is applied to a signal that has been preliminarily digitally filtered.

6.2. Fixed length window

For Class A instruments, the measurement time interval shall be a 10-cycle time interval for 50 Hz power system or 12-cycle time interval for 60 Hz power system. Also, the rectangular window should be used for the acquisition. Both the fixed lengths of the observation window and the Hanning smoothing window used in the above-described procedure are, therefore, unsuited for these purposes. Hence we use variable length window method.










In this procedure the duration Tw of the observation window on which the analysis is performed depends on the actual system frequency and is chosen as an integer multiple M of the actual period Tp. It should be M = 10 for 50-Hz frequency systems. . The outcomes of the two procedures are more directly comparable. However, the flexibility of the proposed measurement system also allows for different choices. The synchrophasors can be evaluated for each period of the signal by simply selecting M = 1. The correct evaluation of Tp is essential to avoid the effects of leakage distortion. This task is performed again by means of a zero-crossing algorithm on a digitally filtered signal. Once the period has been accurately calculated, it is used to determine the length of the record of the original nonfiltered signal to which the DFT is applied. It should be observed that in the general-purpose acquisition systems considered here, the sampling pulses are not phase-locked with the local system frequency, and thus, a true coherent sampling cannot be performed. On the other hand, owing to the high sampling rates used, from a practical point of view, this limitation can be considered negligible since it affects the accuracy of the result less than the other uncertainty sources existing in the measurement chain.

















7.APPLICATIONS OF PHASOR MEASUREMENT

1. Adaptive relaying

Adaptive relaying is a protection philosophy which permits and seeks to make adjustments in various protection functions in order to make them more tuned to prevailing power system conditions. A dynamic management of the protection coordination that can be based on adaptive protection schemes is necessary, in which breakers and reclosers, with remote communication capability, may be operated to automatically reconfigure the network in the presence of faults. This way, the portion of the network not supplied is minimized.

2. Instability Prediction

The instability prediction can be used to adapt load shedding and/or out of step relays. We can actually monitor the progress of the transient in real time, thanks to the technique of synchronized phasor measurements.

3. State Estimation
The state estimator uses various measurements received from different substations, and, through an iterative nonlinear estimation procedure, calculates the power system state. By maintaining a continuous stream of phasor data from the substations to the control center, a state vector that can follow the system dynamics can be constructed. For the first time in history, synchronized phasor measurements have made possible the direct observation of system oscillations following system disturbances.




4. Improved control
Power system control elements use local feedback to achieve the control objective. The method was necessary to capture data during the staged testing and accurately display this data and provide comparisons to the system model.

5. Fault Recording
They can capture and display actual 60/50 Hz wave form and magnitude data on individual channels during power system fault conditions.


6. Disturbance Recording
Loss of generation, loss of load, or loss of major transmission lines may lead to a power system disturbance, possibly affecting customers and power system operations.









7. Transmission and Generation Modeling Verification
Computerized power system modeling and studies are now the normal and accepted ways of ensuring that power system parameters have been reviewed before large capital expenditures on major system changes. In years past, actual verification of computer models via field tests would have been either impractical or even impossible. The PMU class of monitoring equipment can now provide the field verification required .









8. Fault Location
A fault location algorithm based on synchronized sampling. A time domain model of a transmission line is used as a basis for the algorithm development. Samples of voltages and currents at the ends of a transmission line are taken simultaneously (synchronized) and used to calculate fault location.

















8.CONCLUSION

A flexible measurement system for measuring synchronized phasors is explained. The synchronization between the different remote units is achieved by means of high accuracy GPS receivers. The flexibility arises from the use of general-purpose acquisition hardware and virtual instruments, which allow the system to be easily upgraded and/or reconfigured according to the specific measurement needs existing and evolving in modern electric distribution systems. Two different approaches for the evaluation of the absolute phase of an electrical quantity, based on fixed and adjustable window lengths, respectively, are explained. The proposed measurement systems could be further improved simply by using more sophisticated acquisition hardware.

















9.REFERENCES

1. A. G. Phadke, “Synchronized phasor measurements in power systems,”IEEE Comput. Appl. Power, Apr. 2008,vol. 6, no. 2, pp. 10–15.
2. A. Carta, N. Locci, C. Muscas, and S. Sulis, “A flexible GPS-based system for synchronized phasor measurement in electric distribution networks,”in Proc. IEEE IMTC, Sorrento, Italy, Apr. 24–27, 2006, pp. 1547–1552
3. C. Mensah-Bonsu, U. Fernández Krekeler, G. T. Heydt, Y. Hoverson, J.Schilleci, and B. Agrawal, “Application of the global positioning system to the measurement of overhead power transmission conductor sag,” IEEE Trans. Power Delivery, Jan. 2002, vol. 17, pp. 273–278












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19-12-2010, 05:54 PM

The report and ppt of this topic is also available in this thread:
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