11-Electrical - IJEEER - AVAILABLE - Gopi Krishna - Paid

International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol.2, Issue 3 Sep 2012 121- 138 Š TJPRC Pvt. Ltd.,

AVAILABLE TRANSFER CAPABILITY COMPUTATIONS IN DEREGULATED POWER SYSTEM WITH THE OPTIMAL LOCATION OF UNIFIED POWER FLOW CONTROLLER 1

1

P. GOPI KRISHNA & 2 T. GOWRI MANOHAR

Associate Professor, Department of EEE, Jagan’s College of Engineering & Technology, Nellore, India 2

Associate Professor , Department of EEE, S. V. University College of Engineering, Tirupathi, India

ABSTRACT This paper proposes a usage of Unified Power Flow Controller (UPFC) in the computations of Available Transfer capability (ATC) in the deregulated environment. The computation of ATC at different buses plays a significant role on day to day operation and maintenance of any deregulated power system and it has been a continuous task for the engineers at various load dispatch centers. In this paper, four case scenarios were presented in order to identify the effect of UPFC on ATC, at all buses of IEEE 9-bus test system. The results with and without UPFC, and with and without contingencies were obtained using repeated power flow. The optimal location of UPFC is identified based on power losses at various buses and the corresponding ATC values. The detailed analyses of results using proposed test system shows a way to the power engineer for knowing better ATC values and as well as optimal location of UPFC. In this way the results in the paper are the advancement in engineering research apart from the existing methods of usage of conventional methods in the planning of power system.

KEYWORDS : Available Transfer Capability, Repeated Power Flow, UPFC, FACTS, Deregulated Power System etc.,

INTRODUCTION All over world the economical growth and enhancement of the technological achievements initiates the electrical power system continuously expanding its size and enhancing its complexity in many aspects. Therefore the governments have been changing their rules and regulations by allowing the private sectors into the power generation, transmission and distribution called as Deregulated Power System [1]. The continuous monitoring and maintenance of security under different circumstances in the deregulated power system is becoming a complex task for the power system engineer in day to day operation, maintenance and planning of deregulated power system [2].

Flexible Alternating Current Transmission System (FACTS) is a technology introduced by Electrical Power Research Institute (EPRI) in the

1980s. Its principle role is to increase the

transmission capacity of the ac lines and to control power flow over designated transmission lines .FACTS technologies involve conversion and switching of power electronics in the range of a few tens

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few hundred megawatts . New solid state self commutating devices such as MOSFETs, IGBTs, GTOs and also other suitable power electronic devices are used as controlled switches in FACTS devices .The universal and most flexible FACTS device is the Unified Power Flow Controller (UPFC).

Gyugyi proposed the Unified Power Flow Controller (UPFC) concept in 1991. The UPFC [3]was devised for the real time control and dynamic compensation of ac transmission systems, providing multifunctional flexibility required to solve many of the problems facing the delivery industry.

Within the framework of traditional power transmission

concepts, the UPFC is able to control,

simultaneously or selectively, all the parameters affecting power flow in the transmission line (i.e., voltage, impedance and phase angle), and this unique capability is signed by the adjective “unified” in its name. Alternatively, it can independently control both the real and reactive power flows in the line.

Available Transfer Capability (ATC)[5] is a measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses.

As a measure bridging the technical characteristics of how interconnected transmission networks perform to the commercial requirements associated with transmission service requests, ATC must satisfy certain principles balancing both technical and commercial issues. ATC must accurately reflect the physical realities of the transmission network, while not being so complicated that it unduly constrains commerce.

The term “transfer capability” refers to the amount of electric power that can be passed through a transmission network from one place to another. The concept of transfer capability is useful for several reasons. A system, which can accommodate large inter-area transfers, is generally more robust and flexible than a system with limited ability to accommodate inter-area transfers. Thus, transfer capability can be used as a rough indicator of relative system security. Transfer capability is also useful for comparing the relative merits of planned transmission improvements. A transmission expansion that increases transfer capability between two areas of the grid might be more beneficial for increasing both reliability and economic efficiency than an alternate improvement that provides a lesser increase in transfer capability. Along similar lines, transfer capability can be used as a surrogate for more specific circuit modeling to capture the gross effects of multi-area commerce and provide an indication of the amount of inexpensive power likely to be available to generation deficient or high cost regions. Transfer capability computations facilitate energy markets by providing a quantitative basis for assessing transmission reservations.

ATC DEFINITION AND DETERMINATION Available Transfer Capability (ATC) [5]is a measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses.

Mathematically, ATC is defined as the Total Transfer Capability (TTC) less the Transmission Reliability Margin (TRM), less the sum of existing transmission commitments (which includes retail customer service) and the Capacity Benefit Margin (CBM).

ATC can be expressed as: ATC = TTC – TRM – Existing Transmission Commitments (including CBM)

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

123

The ATC between two areas provides an indication of the amount of additional electric power that can be transferred from one area to another for a specific time frame for a specific set of conditions.

ATC can be a very dynamic quantity because it is a function of variable and interdependent parameters.

These parameters are highly dependent upon the conditions of the network. Consequently, ATC calculations may need to be periodically updated. Because of the influence of conditions throughout the network, the accuracy of the ATC calculation is highly dependent on the completeness and accuracy of available network data.

Total Transfer Capability (TTC) is defined as the amount of electric power that can be transferred over the interconnected transmission network in a reliable manner while meeting all of a specific set of defined pre- and post-contingency system conditions.

Transmission Reliability Margin (TRM) is defined as that amount of transmission transfer capability necessary to ensure that the interconnected transmission network is secure under a reasonable range of uncertainties in system conditions.

Capacity Benefit Margin (CBM) is defined as that amount of transmission transfer capability reserved by load serving entities to ensure access to generation from interconnected systems to meet generation reliability requirements.

REPEATED POWER FLOW (RPF)

The Repeated Power Flow (RPF) method [1], which solves power flow equations repeatedly at a succession of points, along specified load increments with a constant power factor, is used for computing Total Transfer Capability (TTC). For this method the Newton Raphson’s method in polar coordinates is used.

Newton Raphson’s Power Flow Solution: The complex power at bus ‘i’ is

Pi − jQi = Vi * I i

(1)

Substituting from the previous equation for IL n

Pi − jQi = Vi * ∠ − δ i ∑ Yij V j ∠(θ ij + δ j ) j =1

(2)

Separating the real and imaginary parts n

Pi = ∑ Vi Yij V j cos(θ ij − δ i + δ j ) j =1

(3)

n

Qi = ∑ Vi Yij V j sin(θ ij − δ i + δ j ) j =1

(4)

These equations constitute a set of non linear algebraic equations in terms of the independent variables, voltage magnitudes per unit, and phase angle in radians. Expanding the two equations in Taylor series about the initial estimator and neglecting all the higher order terms results in the set of linear equations in the form of jacobian matrix.

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 ∂P2K  ∂δ 2 ∆P2K   .    .   .

K .    K   ∂Pn   ∆Pn  ∂δ 2  ∆Q K  =  ∂Q K  2  2  .   ∂δ 2    . .   ∆Q2K   .    K  ∂Qn  ∂δ  2

..... .....

.....

..... .....

∂P2 ∂δ n . . ∂Pn ∂δ n ∂Q2 ∂δ n . . ∂Qn ∂δ n

∂P2K ∂ V2 . .

K ∂Pn ∂ V2 ∂Q2 ∂ V2 . .

K ∂Qn ∂ V2

K

∂P2 ∂ Vn . . ∂Pn ∂ V2 ∂Q2 ∂ V2 . . ∂Qn ∂ V2

..... .....

.....

..... .....

     ∆δ 2 K   .     .    ∆δ K    nK  (5)   ∆ V2   .     .  K  ∆ V     n    

In the above equation, bus1 is assumed to be the slack bus. The jacobian matrix gives the linearized relationship between small changes in the voltage angle

∆δ i

K

and voltage magnitude

∆ Vi

(K )

with

small changes ‘i’ real and reactive powers. Elements of the jacobian matrix are the partial derivatives of

∆δ i and ∆ Vi (3) and (4), evaluated at K

(K )

. In short form it can be written as

J 2   ∆δ    J 4  ∆ V 

 ∆P   J 1  ∆Q  =  J    3

(6)

The diagonal elements of J1, J2, J3, J4 are found using

∂Pi ∂Qi ∂Q i ∂Pi ∂δ i , ∂ Vi , ∂δ i & ∂ Vi

(7)

Similarly for off-diagonal elements in the place of i, j is used in the denominator terms of the above diagonal elements. The terms

∆Pi

(K )

and are the difference between the scheduled and calculated

values, known as the power residuals, given by:

∆Pi

(K )

∆Qi

(K )

= Pi

( sch)

= Qi

− Pi

( sch)

(K )

(8)

− Qi

(K)

(9)

The new estimates of bus voltages are

Vi

δi

( K +1)

( K +1)

= Vi

= δi

(K )

(K )

+ ∆ Vi

+ ∆δ i

(K )

(10)

(K )

(11)

UNIFIED POWER FLOW CONTROLLER (UPFC)

The UPFC[6] consists of two switching converters A & B as shown in “Figure 1””, which in the implementations considered are voltage source inverters using gate turn-off (GTO) thyristor valves, as illustrated in the “Figure 1”. These back to back converters labeled “Inverter 1 and “Inverter 2” are

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

125

operated from a common dc link provided by a dc storage capacitor. This arrangement functions as an ac to ac power converter in which the real power can freely flow in either direction between the ac terminals of the two inverters and each inverter can independently generate (or absorb) reactive power at its own ac output terminal.

Figure 1: Basic circuit arrangement of unified power flow controller

OPERATION OF UPFC Inverter 2 provides [7] the main function of the UPFC by injecting an ac voltage Vpq with controllable magnitude Vpq

(0≤Vpq≤Vpqmax) and phase angle ρ(0≤ρ≤360), at the power

frequency, in series with the line via an insertion transformer. The injected voltage is considered essentially as a synchronous voltage source. The transmission line current, flows through this voltage source resulting in real and reactive power exchange between it and the ac system. The real power exchanged at the ac terminal (i.e., at the terminal of insertion transformer) is converted by the inverter into dc power that appears at the dc link as positive or negative real power demanded. The reactive power exchanged at the ac terminal is generated internally by the inverter.

The basic function of inverter 1 is to supply or absorb the real power demanded by Inverter 2 at the common dc link. This dc link power is converted back to ac and coupled to the transmission line via a shunt-connected transformer. Inverter 1 can also generate or absorb controllable reactive power, if it is desired, and there by it can provide independent shunt reactive compensation for the line. It is important to note that where as there is a closed “direct” path for the real power negotiated by the action of series voltage injection through Inverters 1 and 2 back to the line, the corresponding reactive power exchanged is supplied or absorbed locally by inverter 2 and therefore it does not flow through the line.

Thus, Inverter 1 can be operated at a unity power factor or be controlled to have a reactive power exchange with the line independently of the reactive power exchanged by the Inverter 2. This means there is no continuous reactive power flow through UPFC.

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TEST SYSTEMS AND RESULTS Test System-1

The IEEE 9-bus shown in “Figure 2” is considered as test system-1 to estimate the power flows and losses at different buses. To consider the deregulated environment the test system-1 has been divided into two areas and are connected to each other through two tie-lines, in that bus-1 is the slack bus, all other generator’s active power are kept constant except for the slack bus generator. So that the power increased in the load would be drawn from the slack bus. The flow limits in all transmission lines are assumed to be infinity.

Figure 2. IEEE 9-bus system without UPFC The ATC computations with the optimal location of UPFC in deregulated power system were achieved through three cases.

Case 1: Power Flow and Losses without UPFC Case 1.1: The first test is done on base loads (Bus 5: 90+

j30MVA; Bus 7: 100+j35MVA; Bus 9:

125+j50 MVA) without incorporating the UPFC between the buses. Power transfer from one bus to another bus and corresponding power losses are presented in Table-I. Case 1.2: Increasing the load at bus-5 The load applied at bus-5 is 200MW, 66.66MVAR. The power flow and power losses at different buses are shown in Table-II. Case 1.3: Increasing the load at bus-7

127

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

The load applied at bus-7 is 300MW, 105MVAR. The power flow and power losses at different buses are shown in Table-III. Case 1.4: Increasing the load at bus-9 The load applied at bus-9 is 325MW, 130MVAR. The power flow and power losses at different buses are shown in Table-IV. TABLE-I :At Base Load Power Flow and Losses without UPFC

FROM

TO

POWER FLOW

POWER LOSSES

BUS

BUS

MW

MVAR

MW

MVAR

4

5

32.17

11.66

0.29

-32.12

5

6

-58.12

13.78

1.39

-1.35

6

7

25.49

-14.06

0.08

-42.00

7

8

-75.31

-7.31

0.48

1.01

8

9

87.21

-44.88

2.41

-51.46

9

4

43.85

9.25

0.25

-35.54

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P. Gopi Krishna & T. Gowri Manohar

TABLE-II Power Flow and Losses for Variation of Load at bus-5 without UPFC

FROM BUS

POWER FLOW

POWER LOSSES

MW

MVAR

MW

MVAR

TO BUS

4

5

74.65

8.59

0.97

-28.32

5

6

-16.31

6.90

0.14

-6.66

6

7

68.54

6.46

0.65

-35.20

7

8

-124.24

-25.59

1.45

9.38

8

9

37.31

-46.98

0.53

-59.83

9

4

-91.25

-38.36

0.84

-30.40

TABLE-III : Power Flow and Losses for Variation of Load at bus-7 without UPFC

FROM

TO

POWER FLOW

POWER LOSSES

BUS

BUS

MW

MVAR

MW

MVAR

4

5

113.95

9.89

2.16

-21.20

5

6

21.79

1.09

0.19

-6.22

6

7

106.60

29.92

2.93

-23.82

7

8

-167.35

-41.54

1.78

22.12

8

9

-7.28

-43.19

0.08

-60.07

9

4

-133.40

-33.53

1.78

-21.68

129

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

TABLE-IV: Power Flow and Losses for Variation of Load at bus-9 without UPFC

POWER FLOW FROM BUS

POWER LOSSES

TO BUS MW

MVAR

MW

MVAR

4

5

41.98

4.87

0.36

-30.69

5

6

-48.38

5.56

0.94

-3.16

6

7

35.69

-10.77

0.16

-40.69

7

8

-63.95

-4.89

0.35

-0.01

8

9

98.70

-21.8

3.12

-44.02

9

4

-120.81

-63.61

1.79

-20.02

PROPOSED TEST SYSTEM-2 Second test is done on base loads by incorporating the UPFC between the various buses i.e., 45, 5-6, 6-7, 7-8, 8-9, and 9-4 as shown in “Figure 3”. Suppose if we incorporate UPFC between 4-5

buses, then two additions busses are added to the original bus system i.e., Bus 10 and Bus 11. Bus 10 is added to bus 4 by removing the connection with 5 bus and bus 5 is connected to bus11.

Figure 3: Location of UPFC at different busses

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P. Gopi Krishna & T. Gowri Manohar

CASE 2: POWER FLOW AND LOSSES WITH UPFC\ Case 2.1: Base Load

By incorporating the UPFC, the power flow and power losses are shown in the below Table-V. Case 2.2: Increasing the load at bus-5with UPFC The load applied at bus-5 is 200MW, 66.66MVAR. The power flow and power losses at different buses are shown in Table-VI. Case 2.3: Increasing the load at bus-7with UPFC The load applied at bus-7 is 300MW, 105MVAR. The power flow and power losses at different buses are shown in Table-VII.

TABLE-V At Base Load Power Flow and Losses with UPFC

FROM BUS

POWER FLOW

POWER LOSSES

MW

MVAR

MW

MVAR

TO BUS

4

10

82.79

-94.37

2.79

-76.57

11

5

40.00

2.00

1.04

-76.71

5

10

82.77

-96.08

2.77

-78.28

11

6

40.00

2.00

1.04

-69.91

6

10

82.90

-85.92

2.90

-68.12

11

7

40.00

2.00

1.04

-72.04

7

10

82.94

-83.77

2.94

-65.97

11

8

40.00

2.00

1.04

-70.09

8

10

82.93

-84.29

2.93

-66.49

11

9

40.00

2.00

1.04

-77.55

9

10

83.31

-66.51

3.31

-48.71

11

4

40.00

2.00

1.04

-76.04

131

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

TABLE-VI Power Flow and Losses for Variation of Load at bus-5 with UPFC

POWER FLOW FROM BUS

POWER LOSSES

TO BUS MW

MVAR

MW

MVAR

4

5

82.62

-92.39

2.80

-74.64

5

6

83.37

-63.26

3.41

-45.47

6

7

82.96

-82.65

2.91

-65.85

7

8

83.06

-76.98

3.06

-59.18

8

9

82.94

-83.65

2.94

-65.85

9

4

83.35

-65.07

2.94

-47.26

TABLE-VII Power Flow and Losses for Variation of Load at bus-7 with UPFC

FROM BUS

POWER FLOW

POWER LOSSES

MW

MVAR

MW

MVAR

TO BUS

4

5

82.83

-91.20

2.83

-73.40

5

6

82.75

-97.30

2.75

-79.5

6

7

82.91

-85.27

2.91

-67.47

7

8

61.32

-11.03

4.69

2.97

8

9

83.08

-76.28

3.08

-58.48

9

4

84.04

-45.45

4.11

-27.66

Case 2.4: Increasing the load at bus-9with UPFC The load applied at bus-9 is 325MW, 130MVAR. The power flow and power losses at different buses are shown in Table-VIII.

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P. Gopi Krishna & T. Gowri Manohar

TABLE-VIII Power Flow and Losses for Variation of Load at bus-7 with UPFC

FROM

TO

POWER FLOW

POWER LOSSES

BUS

BUS

MW

MVAR

MW

MVAR

4

5

82.92

-83.67

2.92

-67.27

5

6

82.90

-85.96

2.90

-68.16

6

7

82.93

-84.29

2.93

-66.49

7

8

82.97

-81.91

2.97

-64.11

8

9

82.94

-83.67

2.94

-65.87

9

4

39.13

-17.25

1.94

-11.22

Case 3: Comparison of results with and without UPFC In this case, the results obtained by case 1 and case2 at base loads are compared to identify the optimal location of UPFC. The result of this case is shown in Table IX. At branch 6-7 the power flow with UPFC is 82.90MW,-85.92 MVAR and the power flow without UPFC is 25.49 MW and -14.06 MVAR. The power flow with UPFC is greater than the Power flow without UPFC. The power flow is improved up to 57.41MW and -71.86 MVAR. Therefore the best optimal location for UPFC is at branch 6-7.

Case 3.1: Enhancing the load at bus-5 The load at bus-5 is gradually increased to identify the power flow with and without UPFC at branch 6-7 and corresponding results shown in Table X. Case 3.2: Enhancing the load at bus-7 The load at bus-7 is gradually increased to identify the power flow with and without UPFC at branch 6-7 and corresponding results shown in Table XI. At branch 6-7 the power flow with UPFC is 82.90MW, -85.92MVAR and the power flow without UPFC is 25.49MW,-14.06MVAR. The power flow with UPFC is more than without UPFC. The power flow is improved up to 57.41MW,-71.86MVAR. However by comparing the results in Table XI, for the enhancement of load bus-7, the branch 6-7 is not the best optimal location for UPFC.

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Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

TABLE-IX : Comparison of Results at Base Loads with and without UPFC BUSES

WITH UPFC

WITH OUT UPFC

DIFFERENCE

FROM

TO

MW

MVAR

MW

MVAR

MW

MVAR

4

5

82.79

-94.37

32.17

11.66

50.62

-82.71

5

6

82.77

-96.08

-58.12

13.78

24.62

-82.3

6

7

82.90

-85.92

25.49

-14.06

57.41

-71.86

7

8

82.94

-83.77

-75.31

-7.31

7.63

-76.46

8

9

82.93

-84.29

87.21

-44.8

-4.28

-39.49

9

4

83.31

-66.51

43.85

9.25

39.46

-57.26

TABLE-X : Power Flow at Branch 6-7 by Enhancing the Load at Bus-5

Load

With UPFC

Without UPFC

Difference

90+j30MVA

82.90

25.49

57.41

100+j33.33MVA

82.91

24.10

58.91

200+j66.66MVA

83.05

9.36

73.6

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P. Gopi Krishna & T. Gowri Manohar

TABLE-XI: Power Flow at Branch 6-7 by Enhancing the Load at Bus-7

Load

With UPFC

Without UPFC

Difference

100+j35MVA

82.90

25.49

57.41

200+j70MVA

82.91

68.54

14.37

300+j105MVA

82.91

106.6

-23.69

Case 3.3: Enhancing the load at bus-9 The load at bus-9 is gradually increased to identify the power flow with and without UPFC at branch 6-7 and corresponding results shown in Table XII. By observing the results in Table XII, the power flow at branch 6-7 is improved when the load at bus-9 is increased. Therefore in this case, the best optimal location for UPFC is branch 6-7.

The comparative power flows with and without UPFC at base load and at bus-9 is shown in Figure 4” and “Figure 5”respectively.

TABLE-XII : Power Flow at Branch 6-7 by Enhancing the Load at Bus-9

Load

With UPFC

Without UPFC

Difference

125+j50MVA

82.90

25.49

57.41

225+j90MVA

82.92

35.69

47.23

325+j130MVA

82.93

44.86

38.07

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

Figure 4: Power Flow at Base Loads

Figure 5: Power Flow for 325MW at Bus-9 Case 4: Case Scenario for Computation of ATC Case 4.1: Enhancing the load at bus-7 Load is increased to 200 MW at bus-7; Base load=225MW-163MW=62MW TTC for UPFC =82.79MW; ATC= TTC-Base load =82.79-62=20.79MW

135

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P. Gopi Krishna & T. Gowri Manohar

TABLE-XIII : ATC Calculation for 200 MW at Bus-7

TTC FROM BUS

TO BUS

4

ATC=TTC-BASE LOAD

WITH UPFC

WITHOUT UPFC

WITH UPFC

WITHOUT UPFC

5

82.79

74.65

20.79

12.65

5

6

82.74

-16.31

20.74

-45.69

6

7

82.91

68.54

20.91

6.54

7

8

83.85

-124.24

21.85

62.24

8

9

82.99

37.31

20.99

-24.69

9

4

83.57

-91.25

21.57

29.25

Figure 6: ATC for the Load of 200 MW at Bus-7

Case 4.2: Enhancing the load at bus-9 Load is increased to 225MW at bus-9; Base load=125-63=62MW Total transfer capability=98.70MW;

Available transfer capability= 98.70-62=36.7MW

Available Transfer Capability Computations in Deregulated Power System with The Optimal Location of Unified Power Flow Controller

137

Figure 7: ATC for the Load of 325 MW at Bus-9

TABLE-XIV : ATC Calculation for 325 MW at Bus-9 TTC FROM BUS

TO BUS

4

ATC=TTC-BASE LOAD

WITH UPFC

WITHOUT UPFC

WITH UPFC

WITHOUT UPFC

5

82.92

50.95

20.92

-11.05

5

6

82.90

-39.52

22.90

-22.48

6

7

82.93

44.86

20.93

-17.14

7

8

82.97

-53.42

20.97

-8.58

8

9

82.94

109.33

20.94

47.33

9

4

39.13

-184.56

-22.87

122.56

The comparative ATCs with and without UPFC at bus-7 and bus-9 is shown in Table-XIII and Table-XIV, the corresponding comparative graphs shown in “Figure 6” and “Figure 7” respectively. All results were obtained using FORTRAN software.

CONCLUSIONS The detailed analyses of results in this paper are useful to identify appropriate usage of costly UPFC based on power loss and amount of available transfer capability. The optimal location of UPFC based on power loss is in between buses 6-7, but based on ATC its location in between buses 5-6. The results from this paper gives the readymade ATC values for IEEE 9-bus system, further the by using the

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proposed analysis used in this paper similar results would be obtained related to any number of bus system. Further the analysis may be extended using intelligent techniques for the continuous monitoring ATC values in on-line environment.

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