[ieee 2013 international conference on circuits, power and computing technologies (iccpct) -...

2013 International Conference on Circuits, Power and Computing Technologies [ICCPCT-2013]

An Optimal P MU Placement Technique for the Topological Observability of aPart of the NER

Grid of In dia Dr. A. K. Sinha

Department of Electrical Engineering Ramesh Kumar

Department of Electrical Engineering Chaynika Saikia

Department of Electrical Engineering

National Institute ofTechnology Silchar

Assam, INDIA Pin: 788010


Assam, INDIA Pin: 788010 National Institute ofTechnology Silchar

Assam, INDIA Pin: 788010 chaynika. [email protected], ashokesinha200 [email protected], [email protected]

Satyabrata Rudrapaul Department of Electrical Engineering


Assam, INDIA Pin: 788010 aj ay. [email protected]

Abstract- This paper considers the analysis of network

observability and optimum PMU placement problem under

normal operation conditions. The aim of the optimum PMU

placement (OPP) problem is to provide minimum number of

PMU installations to ensure full observability of the particular

power system. The approach used in this paper is based on the

topological observability of the system. The technique used is

based on integer linear programming. The proposed technique

has been firstly verified on IEEE 14- bus and New England 39-bus system. It has then been implemented on a part of the 77- bus

North Eastern grid of India. The part considered in the paper

comprises of the states- Nagaland, Manipur, South Assam,

Mizoram, Tripura, Meghalaya and Palatana. Lines wh ich are

currently under construction and under outage have not been

considered.

Index Terms- Optimum PMU placement problem, Integer

Linear Programming, North Eastern Grid of India, topological

observability.

I. INTRODUCTTON

S YNCHRONIZED phasor measurement unit (PMU) were fIrst

introduced in the early 1990s, and since then it has been

widely used with many applications [1]. The applications

include state estimation, power system monitoring, power

system protection and power system control. The conventional

state estimators use a set of measurements which inc1ude bus

voltages, real and reactive power flows, power injections in

order to estimate the bus voltage phasors of the system. These

measurements were obtained through Supervisory Control and

Data Acquisition (SCADA) systems which could only provide

the bus voltage magnitudes but unable to provide the bus

phase angle measurements in real time. With the employment

of PMUs, this problem can be diminished. The Remote

Terminal Units (RTUs) can therefore be replaced by PMUs

since the data gathered by the RTUs are unsynchronized and

do not contain information about the dynamic stability of the

system.

978-1-4673-4922-2/13/$31.00 ©20 13 IEEE 142

Bristi Saikia Department of Electrical Engineering

National Institute ofTechnology Silchar Assam, INDIA Pin: 788010

saikia. [email protected]

The measurements obtained by the PMUs are time

synchronized via Global Positioning System (GPS) with an

accuracy of 1 microsecond. The phasor measured at the same

instant of time provide snapshot of the power system network

and by comparing the snapshot with two consecutive time

instants, it can monitor the steady state as weH as the dynamic

state of the system.

The PMUs are relatively costly and hence to minimize the

cost, the PMUs should be strategically placed in the power

system network. The objective is therefore to optimally place

the PMUs in the system in order to achieve fuIl network

observability.

Different authors have implemented different techniques for the optimal placement of the PMUs. A binary search algorithm has been developed in [2] in order to attain full network observability under normal operating conditions as weIl as single branch outage. A BPSO based methodology for the placement of PMUs for complete observability of a power system has been shown by the authors in [3]. A Tabu search method for meter placement for the purpose of state estimation has been introduced in [4]. In [5]-[6], the OPP optimization is solved using PSA T, a MATLAB based toolbox, and DeFS method is compared with other methods. Even though this algorithm is computationally faster, but the solution is not optimum, because the optimization criterion is stiff and unitary. In [7], the spanning trees of the power system graph are used to [md the optimal locations of PMUs based on the concept of depth of unobservability [8]. OPP has been dealt with in [9] as the first attempt, using a dual search algorithm by combination of a modifIed bisecting search and a simulated annealing method. Bacteria Foraging (BF) technique has also been used by the authors in [lO] for the OPP problem.

Though PMU has not yet been deployed in India, a novel attempt has been made to introduce it here and fInd a strategic method for the OPP problem and implement it on a part of the North Eastern Grid of India.

The rest of the paper has been organized as folIows. In section 11, the observability rules while employing PMUs are introduced. In section III, the proposed method has been explained. Section IV briefly describes the system under


which the work has been done. The simulation results have been presented in section V. Section VI provides concluding remarks along with the purpose of the work being done.

11. OBSERVABIUTY RULES

Power system observability holds importance for finding out the real time state estimation. The network is said to be observable if sufficient measurements are observable, such that all the states of the system, as weil as the current and voltage phasors at all the buses can be estimated. There are three classes of algorithms used for this purpose, namely,

numerical, topological and hybrid techniques. For a network to be numerically observable, its gain or measurement Jacobian matrix should be of full rank [11]. A network is said to be topologically observable if it contains at least one spanning measurement tree of fuU rank [12]. Since numerical observability technique involves calculation of huge matrices, topological approach is more preferable. Thus, this approach has been used in the work. The topological observability techniques focus on finding a spanning tree of full rank with the placement of the PMUs for Wide Area Monitoring (W AM) purpose.

While formulating the OPP problem, there are certain rules and assumptions which are to be followed [13].

The first rule implies that if the current and the voltage phasors at one end of the branch are known, by using Ohm's law, the voltage at the other end of the bus can be calculated.

The second rule implies that if the voltage phasors at both ends of the branch are known, the branch current can be calculated.

III. FORMULATION OF THE OPP PROBLEM

In order to minimize the total cost of the PMU installation,

while ensuring that the system is observable, the problem can

be reduced to a binary decision variable problem, defined as,

X' = p, if PMU is in bus - i ! t O,otherwise (1)

If the PMUs are to be placed in an N-bus system and the cost

of installation of a PMU is taken to be the same in all the

buses and that value is taken as 1 p.u., the OPP problem can

be formulated as follows:

N

Minimize L Xi i=l

T such that, X= [XI X2 ... Xn]

Xi D {O,l} F(X) � 1

Here, F(X) is a vector of functions representing the

constraints. The constraint formulation depends upon the type

of measurement considered and also varies as per the system.

The ILP technique used here has been explained in details in

[14].

In this work, only the voltage/current phasor measurements

143

have been considered. The constraint formulation has been

explained for a 7- bus system as shown in Fig 1.

Fig. I. The sampie 7-bus system

Since it is assumed that a PMU placed at a bus can provide the

phasor values of all the branch currents connected to that bus,

apart from the bus voltage phasors, the phasor voltages at all

the neighboring bus can be computed. Thus a matrix M can be

obtained having the elements as given below,

m . . = fl, if i = j or if bus i and j are connected !] t 0, otherwise

Thus, the product of the matrices M and X gives the

constraint function F(X). Elements of this matrix will always

be greater than or equal to one, which means that there must

be at least one PMU at one of the buses of the constraint

function. The constraints for a 7 bus system can be given as

folIows: F\: XI+X2� 1 F2: XI+X2+X3+X6+X7� 1 F3: X2+X3+X4+X6� 1

F4: X3+X4+X5+X7� 1

Fs: X4+X5� 1

F6: X2+X3+X6� 1

F7: X2+X4+X7 � 1

IV. NORTH EASTERN GRID FOR INDIA

(2)

The North Eastern Region (NER) comprises of seven states viz. Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura. NHPC, NEEPCO & OTPCL are the Central Generating Companies presently operating in the region with installed capacity of 105 MW,II30 MW & 363 MW respectively. Depending on the network connectivity, NER Grid is broadly classified as folIows: 1. 400/220 kV Network 0/ POWERGRID as backbone o/NER Grid 2. 220/ 132 kV Network 0/ Assam System 3. 220/ 132 kV Network 0/ Arunachal Pradesh System 4. 400 / 220 / 132 kV Network comprising 0/ Nagaland, Manipur, South Assam, Mizoram, Tripura, Meghalaya and Palatana.

The fourth system has been considered as the practical system on which the ILP method has been implemented. This network consists of six states. The connectivity of this


400/2201132 kV system with all the states has been explained in [15].

V. SIMULATION

A. IEEE 14-bus system

The IEEE 14-bus system has 14 buses as shown in Fig 2. Hence the vector X contains 14 binary decision variables. The complete ILP problem can be formulated for the 14-bus system as follows:

subject to:

14

Minimize I Xi i=1

F\: X/+X2+Xj � 1

F2: X/+X2+X3+X4+Xj � 1

F3: X2+X3+X4 � 1

F4: X2+X3+X4+Xj+X7+X9 � 1

Fs: X/+X2+X4+Xj+X6� 1

F6: Xj+X6+XI/+XI2+X/3 � 1

F7: X4+X7+X8+X9 � 1

Fg: X7+X8 � 1

F9: X4+X7+X9+X/O+XI4 � 1

F\O: X9+X/O+XI/ � 1

Fll: X6+X/O+XI/ � 1 F 12: X6+X I2+X /3 � 1

F\3: X6+XI2+X/3+XI4� 1

F\4: X9+X/3+XI4 � 1

The locations found by the ILP method where the PMUs

can be placed are at buses 2, 6, 7 and 9.

B. New England 39-bus system

The 39-bus system is as shown in Fig 3. The OPP formulation can be done for this system in the same manner as shown for IEEE 14- bus system. There are 13 PMU locations obtained for this system after simulation. The places where the PMUs are to be located are the buses 2, 6, 9, 10, 13, 14, 17, 19, 20, 22, 23, 25 and 28.

C. 400/220/ 132 kV Practical Network

The practical network consists of 77 buses comprising of Nagaland, Manipur, South Assam, Mizoram, Tripura, Meghalaya and Palatana. The OPP technique has been implemented, in a similar way as shown in the previous two cases, on this network without considering the lines which are currently under construction and under long outage. The buses where the PMUs are to be located have been highlighted in dark blue as shown in Fig 4. A total of 25 PMUs are required to be placed for complete observability of the system. The places where the PMUs are to be located are the buses 3, 7, 9, 13, 17, 20, 24, 25, 26, 27, 3 1, 34, 37, 41, 44, 47, 48, 50, 53, 59, 61, 65, 70, 71 and 72.

144

The number of PMUs required for each of the systems is given in Table I.

TABLE I No OF PMUS REQUIRED

SYSTEM No. of PMUs

IEEE 14-bus system 4

New England 39-bus system 13 400/220/132 kV Practical system 25

GI

C3

7

l-+-I--+-'-3

f\; G2

Fig. 2. IEEE 14- bus system

@ 30 --L

37 26 29

2

.,L-,---..L 23

Fig. 3. New England 39-bus system


rml (TR)

-+-.. " I

- -400kV - -220kV - -132kV - -66kV - - PMU placed bus

J,a:-... ,,----�

Fig. 4. 400/220/ 132 kV network of the NER Grid and its 77- bus structure

145


VI. CONCLUS10N

The practical implementation of the technique used for the optirnal location of PMUs has been successfully shown in this paper for a part of the practical network of the NER Grid of India. The entire network can be observed by placing the PMUs in few of the locations. The synchrophasor technology has not yet been introduced in India. However, it is recommended that PMUs should very soon be adopted in the grid with the view of irnproving the security, monitoring and protection of the system, thus making the Grid more reliable.

ACKNOWLEDGMENT

The authors would like to thank the Electrical Engineering

Department of National Institute of Technology, Silchar for

believing in them and allowing them to work in the Research

Lab of the college.

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on Power Systems, Vol. 23, No. 3, pp. 1433-1440, August 2008. [3] S. Chakrabarti, E. Kyriakides and G. K. Venayagamoorthy, "PMU

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Engineering Conference, Sydney, Australia, December 2008. [4] H. Mori and Y. Sone, 'Tabu search based meter placement for

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[5] M. Farsadi, H. Golahmadi, and H. Shojaei, "Phasor measurement unit (PMU) allocation in power system with different algorithms," in int. Conf on Electrical and Electronics Engineering, 2009, pp. 396-400.

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procedures 0/ NER grid, 8th revision, Nov. 2012.

Dr. A. K. Sinha had received his B.E. and M. Tech. from VNlT, Nagpur in 1978 and 1981. Dr Sinha had received his Ph. D. from lIT, Kharagpur, India in 1990. His field of work is Multiprocessor based digital relays, Power Quality Monitoring and Signal processing.

Chaynika Saikia was born on 10th July, 1991. She is currently pursuing her B. Tech in Electrical Engineering from National Institute of Technology, Silchar, 1ndia. Her areas of interest are Power System Monitoring and Control.

Satyabrata Rudra Paul is currently in the final year of B. Tech (Electrical Engineering) in National Institute of Technlogy, Silchar, 1ndia. His areas of interest include Power System monitoring, protection and control.

Bristi Saikia was born on 17th April, 1991. She is presently pursuing her B. Tech in Electrical Engineering from National Institute of Technology, Silchar, India. Her areas of interest include Power System protection and Electric Vehicles.

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