Enhancement Of Available Transfer Capability In Deregulated Powersystem Using Series Facts Device (TCSC)

Enhancement Of Available Transfer Capability In Deregulated Powersystem Using Series Facts Device (TCSC)

N.Sambasivarao

Associate Professor and Head NRI institute of Technology,agiripalli

Dr.J.Amarnath Professor,Department of EEE, JNTU,Hyderabad

Dr.V.Purnachandrarao

Professor,Department of EEE, DVR & Dr.HS MIC college of Technology

Abstract The deregulated power system offers more benefits to the customers so that it is quite popular in now a days..The Increased power demand has forced the power system to operate very closer to its stability limits .Total Transfer Capability (TTC) forms the basis for Available Transfer Capability (ATC) . ATC of a transmission system is a measure of unutilized capability of a system at a given time. The Computation of ATC is very important to transmission system security and market forecasting This paper focuses on the evaluation of impact of Thyristor Controlled Series Capacitor (TC SC) as FACTS device on ATC and its en hancement .The optimal location of FACTS devices were determined based on Sensitivity methods. The Reduction of Total Sys tem Reactive Power Losses Method was used to determine the suitable location of TCSC and SVC for ATC enhancement. The effectiveness of proposed method is demonstrated on modified IEEE-9 bus system and 24 bus rts system using power world simulator 11.0.

Index Terms Deregulated powersystem,Available Transfer capability, Thyristor Controlled Series Capacitor (TCSC), Reactive power loss,PowerTransfercapability,

1 INTRODUCTION

.

The introduction of deregulation has brought several new entities in the electricity market place, while on the other hand redefining the scope of activities of many of the existing play- ers. With the ongoing expansion and growth of the electric utility industry, numerous changes are continuously being introduced.

Improved utilization of the existing power system is pro- vided through the application of advanced control technolo- gies. Power electronics based equipment, or Flexible AC Transmission Systems (FACTS), provide proven technical so- lutions to address these new operating challenges being pre- sented today. Thyristor Controlled Series Capacitor (TCSC), Thyristor Controlled Phase Angle Regulator (TCPAR), Unified Power Flow Controller (UPFC) and Static VAR Compensator (SVC) are some of the commonly used FACTS controllers. In many deregulated markets, the power transaction between buyer and seller is allowed based on calculation of ATC. Low ATC signifies that the network is unable to accommodate fur- ther transaction and hence does not promote free competition. FACTS controllers like Thyrister Controlled Series Capaci- tor(TCSC) can help to improve ATC by allowing more power transactions.

Fig.1.Typical structure of a deregulated electricity system

2. POWER TRANSFER CAPABILITY CONCEPTS

Power system transfer capability indicates how much power transfers can be increased without compromising system security between the areas. Power system trans- fer capability calculation provides an vital information to Independent system operator for proper planning and operation of the bulk power market, accurately. For overcoming an undue risk of system overloads, equip- ment damage, or blackouts, estimates of transfer capa- bilities must be updated regularly to avoid the combined effect of power transfers. In deregulation, this is neces- sary as the market for electric power becomes more competitive.

Transfer capability is the measure of the ability of interconnected electric systems to reliably move or transfer power from one area to another over all trans- mission lines (or paths) between those areas under spe- cified system conditions. The units of transfer capability are in megawatts (MW). These are explained in detail below:

1. 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.

2. Available transfer capability (ATC) is the measure of the transfer capability remaining in the physical transmission network for future commercial activity over and above already committed uses.

   ATC can be expressed as:

   ATC = TTC TRM CBM (if there is no existing trans- mission commitment)

3. Transmission Reliability Margin (TRM) is defined as that amount of transmission transfer capability ne- cessary to ensure that the interconnected transmission network is secure under a reasonable range of uncer- tainties in system conditions.

2. FACTS CONTROLLERS

2.1 Thyristor Controlled Series Compensator(TCSC):

Thyristor-controlled series capacitor (TCSC) is a capacitive reactance compensator, which consists of a series capacitor bank shunted by a thyristor controlled reactor in order to pro- vide a smoothly variable series capacitive reactance.

Even though a TCSC in the normal operating range is main- ly capacitive, but it can also be used in an inductive mode. The power flow over a transmission line can be increased by con- trolled series compensation with minimum risk of subsyn- chronous resonance (SSR).

TCSC is a second generation FACTS controller, which con- trols the impedance of the line in which it is connected by va- rying the firing angle of the thyristors. A TCSC module com- prises a series fixed capacitor that is connected in parallel to a thyristor controlled reactor (TCR) i. e. Shown in Fig.2.1.

Fig.2 TCSC Module

A TCR includes a pair of anti-parallel thyristors that are con- nected in series with an inductor. In a TCSC, a metal oxide varistor (MOV) along with a bypass breaker is connected in parallel to the fixed capacitor for overvoltage protection. A complete compensation system may be made up of several of these modules.

1. MODELING OF FACTS DEVICES

   For enhancing of transfer capability, the static models of these controllers are considered. It is assumed that the time constants in FACTS device is very small and hence this ap- proximation is justified.

2. Analysis of Transmission Lines and its Power Flows and Loss

   Let the complex voltages at bus i and bus j be denoted as Vii and Vjj respectively as shown in figure 3.3.

   Fig. 3 Model of a Transmission line

   The complex power flowing from bus i to bus j can be ex- pressed as:

   The Active & Reactive power flow form bus i to bus j is

   Where

   Similarly, Real & Reactive Power flows from bus j to bus i can be,

   The active and reactive Power loss in the line can be calcu

   lated as,

3. Power Injection Model Of Thyristor Controlled Series Compensator (TCSC)

Thyristor controlled series compensators (TCSC) are con- nected in series with the lines. The effect of a TCSC on the network is as a controllable reactance in the related transmis- sion line which compensates the inductive reactance of the line results in reducing the transfer reactance between the buses. This leads to an increase in the maximum power that can be transferred on that line in addition to a reduction in the effec- tive reactive power losses. The series capacitors also contribute to an improvement in the voltage profiles.

Figure 3.1 shows a model of a transmission line with a TCSC connected between buses i and j. The transmission line is represented by its lumped -equivalent parameters con- nected between the two buses. During the steady state, the TCSC cn be considered as a static reactance -jXC. This control- lable reactance, XC is directly used as the control variable to be implemented in the power flow equation.

Fig. 4 Model of a TCSC

Line flows

Without TCSC (-j Xc )

With TCSC(- j Xc)

Where

These equations are used to model the TCSC to Enhance the Power Transfer Capability.

3.4. OPTIMAL LOCATION BASED ON SENSITIVITY AP- PROACH FOR TCSC AND TCPAR DEVICES

The static conditions are considered here for the placement of FACTS devices in the power system. The objectives for de-

vice placement may be one of the following:

1. Reduction in the real power loss of a particular line

2. Reduction in the total system real power loss

3. Reduction in the total system reactive power loss

4. Maximum relief of congestion in the system

1. Reduction of total system VAR power loss

   Here, a method based on the sensitivity of the total sys- tem reactive power loss (QL) with respect to the control va- riables of the FACTS device For each of device considered, Net line series reactance (Xij) for a TCSC placed between buses i and j,

   The reactive power loss sensitivity factors with respect to these control variables may be given as Loss sensitivity with respect to control parameter Xij of TCSC placed between buses i and j,

   These factors can be computed for a base case power flow so- lution. Consider a line connected between buses i and j and having a net series impedance of Xij, that includes the reac- tance of a TCSC, if present.

   The loss sensitivities with respect to Xij can be computed as:

2. Selection of optimal placement of FACTS devices

Using the loss sensitivities as computed in the previous sec- tion, the criteria for deciding device location might be stated as TCSC must be placed in the line having the most positive loss sensitivity index aij .

4. CASE STUDIES:

Case studies are conducted on a 2 number of systems IEEE 9, IEEE 24 rts. For each system, the enhancements of ATC and voltage profile at buses are determined with FACTS device placed in optimal location. The following subsections describe the models and study conducted in detail.

The calculation of ATC is carried out by using Power world simulator to compute the power flow of each transfer case. The optimal power flow under normal and at contingency conditions has been carried out.

The static models of the FACTS controllers as given inabove Sections are considered. TCSC is represented as a static reac- tance .The optimal locations for placing each of these devices are determined by sensitivity analysis.

4.1 Case Study -1-IEEE 9 Bus system

4.1.1 System Model

This system consists of 9 buses, 9 line sections, 3 genera- tor buses and 3 load buses

Fig.5 Single line diagram of IEEE 9 bus System

1. Sensitivity Index

   Line No(k)	From-To Bus	TCSC(aij)
   1	1-4	-0.5771
   2	8-2	-2.765
   3	3-6	-0.721
   4	4-5	-0.095
   5	9-4	-0.258
   6	5-6	-0.334
   7	6-7	-0.0785
   8	7-8	-0.58
   9	8-9	-0.711

   Table 4.1: VAR loss sensitivity index of IEEE-9 Bus System

   For this system, from Table 4.1, three cases have been considered A TCSC is placed in lines 4(4-5) and 7(6-7), operat- ed with an inductive reactance of 75% and 20% of the line reactance.

2. ATC Enhancement

   From-To Bus	Without FACTS	With TCSC	% En- hance- ment
   1-4	170.48	170.48	N.E
   2-8	124	124	N.E
   3-5	112.99	121.63	7.103
   3-8	135.65	135.96	0.228

   Table 4.2 Available Transfer Capability for an IEEE 9-Bus System [without Contingency]

   From-To Bus	Without FACTS	With TCSC	% Enhance- ment
   1-4	169.95	170.05	0.0588
   2-8	87	87	N.E
   3-5	166.45	166.51	0.036
   3-8	166.45	166.51	0.036

   From-To Bus	Without FACTS	With TCSC	% Enhance- ment
   1-4	169.95	170.05	0.0588
   2-8	87	87	N.E
   3-5	166.45	166.51	0.036
   3-8	166.45	166.51	0.036

   It is observed from the Table 4.2, the enhancement of ATC is of 7.1.03% with TCSC .

   Table 4.3: Available Transfer Capability for an IEEE 9-Bus System [ with Contingency at line8(7-8)]

   It is observed from the Table 4.3, the percentage enhance- ment of ATC is of 0.036% with TCSC.

3. Voltage Profiles

Fig 3.5 Comparision of voltages with and without FACTS under normal and contingency condition at

bus 5

It is also observed from the Fig 3.5 that with TCSC the voltage at bus 5 is increased from 0.97585p.u to 0.97746 p.u .

4.2.2. Sensitivity Index

1. Case Study -2-IEEE 24 rts Bus system

   1. System Model

      Lin e No(k)	From- To Bus i-j	Loss sensi- tivities TCSC(aij)
      1	1-2	-0.01232
      2	1-3	-0.0474
      3	1-5	-0.2897
      4	2-4	-0.154
      5	2-6	-0.3001
      6	3-9	-0.0756
      7	3-24	-4.898
      8	4-9	-0.1096
      9	5-10	-0.06604
      10	6-10	-1.0578
      11	7-8	-1.1189
      12	8-9	-0.1122
      13	8-10	-0.116
      14	9-11	-1.164
      15	9-12	-0.01225
      16	10-11	-2.922
      17	10-12	-3.176
      18	11-13	-0.877
      19	11-14	-3.322
      20	12-13	-0.368
      21	12-23	-4.986
      22	13-23	-4.72
      23	14-16	-13.67 /td>
      24	15-16	-1.3055
      25	15-21	-4.47
      26	15-21	-4.47
      27	15-24	-4.575
      28	16-17	-9.865
      29	16-19	-1.409
      30	17-18	-3.43
      31	17-22	-1.76
      32	18-21	-0.328
      33	18-21	-0.328
      34	19-20	-0.214
      35	19-20	-0.214
      36	20-23	-1.0022
      37	20-23	-1.0022
      38	21-22	-2.218

      Lin e No(k)	From- To Bus i-j	Loss sensi- tivities TCSC(aij)
      1	1-2	-0.01232
      2	1-3	-0.0474
      3	1-5	-0.2897
      4	2-4	-0.154
      5	2-6	-0.3001
      6	3-9	-0.0756
      7	3-24	-4.898
      8	4-9	-0.1096
      9	5-10	-0.06604
      10	6-10	-1.0578
      11	7-8	-1.1189
      12	8-9	-0.1122
      13	8-10	-0.116
      14	9-11	-1.164
      15	9-12	-0.01225
      16	10-11	-2.922
      17	10-12	-3.176
      18	11-13	-0.877
      19	11-14	-3.322
      20	12-13	-0.368
      21	12-23	-4.986
      22	13-23	-4.72
      23	14-16	-13.67
      24	15-16	-1.3055
      25	15-21	-4.47
      26	15-21	-4.47
      27	15-24	-4.575
      28	16-17	-9.865
      29	16-19	-1.409
      30	17-18	-3.43
      31	17-22	-1.76
      32	18-21	-0.328
      33	18-21	-0.328
      34	19-20	-0.214
      35	19-20	-0.214
      36	20-23	-1.0022
      37	20-23	-1.0022
      38	21-22	-2.218

      This system consists of 24 buses, 38 line sections, 11 generator buses and 17 load buses .

      Fig.3.6. Single line diagram of IEEE 24 rts bus System

      Table 4.3: VAR loss sensitivity index of IEEE-24 rtsBus system

      For this system, from Table 4.3, three cases have been considered .

      1. A TCSC is placed in lines 1(1-2) and 15(9-12), op- erated with an inductive reactance of 75% and 20% of the line reactance;

4.2.3 ATC Enhancement

Table 4.4: Available Transfer Capability for an IEEE 24 rts Bus System [without Contingency]

It is observed from the Table 4.4, the enhancement of ATC is of 2.209% with TCSC.

Table 4.5: Available Transfer Capability for an IEEE 24 rts Bus System [with Contingency at line10 (6-10)]

It is observed from the Table 4.5, the percentage en- hancement of ATC is of 8.475% TCSC.

Fig 3.7 Comparision of voltages with and without FACTS under normal and contingency condition at bus 24

It is observed from the Fig 3.7 TCSC the voltage at bus 24 is increased from 0.97302p.u to 0.97892 .

4.3. CONCLUSIONS :

It is observed that the enhancement of Available Transfer Capability comparison between various buses of an IEEE-9, 24rts, bus systems. Also observed that the enhancement of ATC using TCSC FACTS device by placing optimally with sensitivity approach.

It is observed that the enhancement of ATC is not only for IEEE test systems but also for Indian utility system with and without FACTS devices under normal and contingency condi- tions.

Also observed the voltages with and without FACTS devices while enhancing the Available Transfer Capability(ATC).

ACKNOWLEDGEMENTS

I author , very grateful to Dr.J.AMARNATH Professor De- partment of EEE,JNTU college of Engineering,Hyderabad Without his assistantship the work could not be completed.

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Author Biography

N.Sambasiva Rao received the B.Tech degree in Electrical & Electronics Engi-

neering and M. Tech in Electrical Power Engineering from JNTU Hyderabad, India. He has 12 years expe- rience in teaching. He is perusing his Ph.D from JNTU, Hyderabad, India. He published a 7 research papers in various International Journals and 2 re- search papers in National Conferences. He is the Member of International Association of Engineers (IAENG) and Life member of ISTE.

He is currently working as Associate Professor and Head of the department in

Electrical & Electronics Engineering at NRI Institute of Technology, Agiripalli, India. He got Best Achiever award of Andhra Pradesh By NCERT, New Del- hi, India. His Areas of interest include Electrical Machines, control Systems and power System Protection.