Load Frequency Control Of An Interconnected Power System Using Thyristor Controlled Phase Shifter (TCPS)

Load Frequency Control Of An Interconnected Power System Using Thyristor Controlled Phase Shifter (TCPS)

B.Prakash Ayyappan,

M.E. Student, EEE Department, University of Department Madurai,

Alagar Kovil Road, Madurai, Tamilnadu.

U. Thiruppathi, Assistant Professor, EEE

Department, Chendhuran College of Engineering and Technology, Pudukkottai, Tamilnadu.

1. Balaganapathi,

   EEE – Head of the Department, Sivakasi Institute of Printing Technology, Sivakasi, Tamilnadu.

   Abstract—In an Interconnected power system, as a power demand varies randomly, both area frequency and tie-line power interchange also vary. Load frequency control (LFC) is of importance in electric power system operation to damp frequency and voltage oscillations originated from load variations or sudden changes in load demands. The objectives of load frequency control(LFC) is to minimize the transient deviations in these variables (area frequency and tie-line power interchange) and to ensure their steady state errors to be zeros. The control error signal in LFC is called area control error (ACE) which is a linear combination of net tie-line power error and frequency error. This paper presents the analysis of the load frequency control (LFC) of a realistic two area interconnected power system having diverse sources of power generation. A Thyristor Controlled Phase Shifter (TCPS) is used in series with AC tie line for improving the dynamic performance of the LFC system. The power system simulation is done using MATLAB Simulink and control problem is solved using MATLAB programming. An optimal integral controller with and without TCPS are obtained following a step load perturbation in either of the areas using Particle Swarm Optimization (PSO) algorithm with a strong ability to find the most optimistic results. Simulation results show that due to the presence of TCPS, the dynamic performance in terms of settling time and overshoot is greatly improved. The system with TCPS is capable of suppressing the area frequency and tie line power deviations more effectively under the occurrence of area load perturbations.

   Keywords—Load Frequency Control, Matlab Simulink, Integral Controller, Particle Swarm Optimization, Thyristor Controlled Phase Shifter.

   L

   L

   1. INTRODUCTION:

      OAD frequency control (LFC) is of importance in electric power system operation to damp frequency and voltage oscillations originated from load variations or sudden changes in load demands. In a deregulated environment load-frequency control (LFC) is very important in order to supply reliable electric power with good quality and to provide better

      conditions for the electricity trading.

      The main goal of LFC is to maintain zero steady state errors for frequency deviation and good tracking load demands in a multi-area power system, it is treated as an ancillary service

      B.Prakash Ayyappan, M.E. Student, Electrical and Electronics Department, University of Department, Anna University Madurai campus, Alagar Kovil Road, Madurai.

      U. Thiruppathi, Assistant Professor, Electrical and Electronics Department, Chendhuran College of Engineering and Technology, Pudukkottai.

      G. Balaganapathi, Head of the Department, Electrical and Electronics Engineering in Sivakasi Institute of Printing Technology, Sivakasi.

      essential for maintaining the electrical system reliability at an adequate level.

      The main goal of LFC of a power system within specified tolerance is to maintain the frequency of each area and tie- line power flow by adjusting the MW outputs of generators so as to accommodate fluctuating load demands.

      Load Frequency Control (LFC) in a multi-area interconnected power system has four principal objectives when operating in either the so-called normal or preventive operating states:

      • Matching total system generation to total system load

      • Regulating system electrical frequency error to zero

      • Distributing system generation amongst control areas so that net area tie flows match net area tie flow schedules

      • Distributing area generation amongst area generation sources so that area operating costs are minimized, subject to appropriate security and environmental constraints.

        Frequency deviation is a direct result of the imbalance between the electrical load and the power supplied by the connected generators, so it provides useful index to indicate the generation and load imbalance. LFC provides an effective mechanism for adjusting the generation to minimize the area frequency deviation and regulate tie line power flows. The concept of power system with multi-source power generation in each area is taken for the simulation of this interconnected power system. In this paper, a two area interconnected power system model comprising Hydro, Thermal with Reheat turbine and Gas units in each area as shown in Figure. TCPS is connected in series with the AC tie line for stabilizing the area frequency and tie line power deviations. The linearized models of governors, reheat turbines, Hydro turbines and Gas turbines are used for simulation of the proposed power system. The Flexible AC Transmission Systems (FACTS) devices provide more flexibility in power system operation and control. TCPS is an effective FACTS device for the tie line power flow control of an interconnected power system. The TCPS device is modeled and used in series with tie lines to improve the dynamic performance of LFC of the interconnected power systems.

        In this paper, the dynamical response of the LFC problem is improved with a practical point of view by considering the output feedback control strategy using TCPS. Practically, access to all of the state variables of a system is limited and measurement of all of them is not feasible and also costly. An

        output feedback control strategy is presented in this paper to overcome this problem. Literature survey shows that most of the researchers applied optimal control theory on thermal – thermal power systems. Some researchers have studied the LFC of thermal-thermal or hydro-thermal power systems considering TCPS but surprisingly there is hardly any literature that applies optimal output feedback control strategy for the LFC of realistic interconnected power system considering TCPS.

        The main contributions of the present work are:

      • Simulation of a realistic power system with TCPS using MATLAB Simulink tool and MATLAB coding for solving the controller design problem.

      • Improvement of the dynamic response of LFC system in a realistic two area interconnected power system considering TCPS in series with tie line.

      • Comparison of the dynamic responses of the LFC of the power system with and without TCPS.

      The recent advances in power electronics have led to the development of the flexible alternating current transmission systems (FACTS). These devices are playing an increasing and major role in the operation and control of power systems. FACTS devices are utilized both in controlling the power flow in the transmission networks and improving transient and dynamic performances of a power system. Among the different features presented by the FACTS devices the improvement of electromechanical oscillation damping is considered as one of the main tasks of FACTS devices installed in power systems.

      The Thyristor controlled phase shifter (TCPS), called also Thyristor controlled power angle regulator (TCPAR) belongs to the group of FACTS of special consideration. The main function of a TCPS is to enforce a desired distribution of power flows between parallel lines linking two areas of a power system in order to increase the overall power transfer capability between the areas. The ability of effective damping of electromechanical oscillation is also an important feature of the TCPS.

      The recent advances in power electronics have led to the development of the flexible alternating current transmission systems (FACTS). FACTS are designed to overcome the limitations of the present mechanically controlled power systems and enhance power system stability by using reliable and high-speed electronic devices. One of the promising FACTS devices is the Thyristor controlled phase shifter (TCPS). A considerable attention has been directed to analysis and realization of various TCPS schemes. However, a relatively little work in TCPS control aspects has been reported in the literature. In their control scheme the phase shift angle is determined as a nonlinear function of rotor angle and speed. However, in real life power system with a large number of generators, the rotor angle of a single generator measured with respect to the system reference will not be very meaningful.

   2. THYRISTOR CONTROLLED PHASE SHIFTER

      Schematic of interconnected power system considering TCPS in series with tie line

      A simple single machine infinite bus system with a TCPST;

      (a) single line diagram; (b) equivalent circuit

   3. TCPS CONTROL STRATEGY

      Error signal to TCPS can be any signal such as the area frequency deviation or area control error to control the TCPS phase shifter angle. If the frequency deviation of area-1 is sensed as error signal, it can be used as the control signal to the TCPS unit to control the TCPS phase shifter angle which results in controlling the tie line power flow.

      1. Power System Model of TCPS

         A single area power system with hydro, the hydro, thermal and gas power generation sources is thermal and gas power generation sources is considered for the study. considered for the study.

         Load Frequency Control is applied only to Load Frequency Control is applied only to thermal and gas power generating units thermal and gas power generating units.

         Hydro is allowed to operate at its scheduled Hydro is allowed to operate at its scheduled generation level with only speed governor generation level with only speed governor control. Integral (I) load frequency control is applied to thermal and gas power generating units. Common LFC for both thermal and gas power generating units. Individual LFC for thermal and gas power generating units.

      2. Mathematical Modelling

         Under normal operating conditions there is no mismatch between generation and load. The total generation is given by,

         PG = PGth + PGhy + PGg —- (i)

         Where

         Area thermal power generation, PGth = Kth PG Area hydro power generation, PGhy = Khy PG Area gas power generation, PGg = KgPG

         Kth + Khy + Kg = 1.0 —– (ii)

         Kth, Khy and Kg represent the share of the power generation by thermal, hydro and gas sources to the total area power generation respectively.

         For small perturbation equation (i) can be written as

         PG = PGth + PGhy + PGg —– (iii)

         Common Load Frequency Controller

         PC = KP f + KI f dt

         Individual Load Frequency Controller

         PCth = KPth f + KIth f dt

         PCg = KPg f + KIg f dt

         The optimum values of Kp and KI increase whereas KPth and KIth decrease and KIg increase with decrease in gas power generation to match the decrease in operating load.

         Individual LFC provides better response that common LFC, Performance of the system improves as gas power generation decreases.

      3. Steady State Response

         Let the step changes in loads PD1 and PD2 be simultaneously applied in areas 1 and 2 respectively. When steady state conditions are reached, the output signals of all integrating blocks will become constant and their input signals must become zero.

         Ptie 1 + b1 f1 = 0 Ptie 2 + b2 f2 = 0 f1 f2 = 0

         P tie 1 / Ptie 2 = -a12 = constant

         P tie 1 = Ptie 2 = 0 And f1 = f2 = 0

         Under steady condition change in tie line power and frequency of each area is zero.

      4. Tie Line Bias Control of Two Area Systems

      The control strategy is termed as tie line bias control and is based upon the principle that all operating pool members must contribute their share to frequency control in

      addition to taking care of their own net interchange. It is possible to divide an extended power system (say, national grid) into sub-areas in which the generators are tightly coupled together so as to form a coherent group, i.e., all the generators respond in unison to changes in load or speed changer settings. Such a coherent area is called a control area in which the frequency is assumed to be the same throughout in static as well as dynamic conditions. A control area is interconnected not only with one tie-line to one neighbouring area, but with several tie lines to neighbouring areas in the power pool.

      Area control error, ACEi The Net interchange =

      The reset control is implemented by sampled data techniques. At sampling intervals of one second, all tie-line power data are fed into the central energy control area, where they are added and compared with predetermined power. Now this error is added with biased frequency error, to give ACE results. Under normal operating condition, each control area should have the capacity to meet its own load from its own spinning generator, plus the scheduled interchange between the neighbouring areas. Under emergency condition, the energy can be drawn from the spinning reserves of all the neighbouring areas immediately due to the sudden loss of generating unit.

      The Load frequency control involves the sensing of the bus bar frequency and compares with the tie-line power frequency. The difference of the signal is fed to the integrator and it is given to speed changer which generates the reference speed for the governor. Thus, the frequency of the tie-line is maintained as constant.

      Primary Control: The speed change from synchronous speed initiates the governor control action resulting in all the participating generator turbine units taking up the change in load, and stabilizes the system frequency.

      Secondary Control: It adjusts the load reference set points of selected turbine generator units so as to give nominal value of frequency.

      In frequency bias tie-line control, the ACE comprises both the area frequency deviation and tie-line power deviation. Hence, both the deviations are brought to zero under steady state. Proper section of the frequency bias factors B1 and B2 is important form the dynamic performance considerations. The performance of this mode of integral control is examined for different settings of area frequency bias factors and the selection of B = . The basic role of Automatic Load Frequency Control(ALFC) is to maintain desired megawatt output of a generator unit and assist in controlling the frequency of the larger interconnection. Static response of an ALFC loop will inform about frequency accuracy, whereas, the dynamic response of ALFC loop will inform about the stability of the loop.

   4. TRANSFER FUNCTION OF REALISTIC POWER SYSTEM

      MODEL WITH TCPS

      The two area interconnected power system model with TCPS is shown in Figure 1. The realistic two area interconnected power system with TCPS which comprises more practical combination of generating units in each area is simulated using MATLAB Simulink. As shown in Figure 1, each area comprises Reheat thermal, Hydro and Gas generating units and the two equal areas are interconnected. The simulation of this interconnected power system in a new

      power system environment is based on the concepts of considering variety of generators with their corresponding participation rates in each area and TCPS. The governor turbine dynamic models of Reheat thermal, Hydro and Gas generating units taken for simulation are decribed in Furthermore, in the new environment, generators may or may not participate in the LFC task and participation rates are not the same for all participant generators. Let the participation factor of kth generator unit and ith area. In a given control area, the sum of participation factors is equal to 1. The system parameter values are given in Appendix. The nominal loading of each area is taken 1740MW with the power generation scheduling and generator participation factors.

      .4312

      KI9

      -K-

      KI2

      Reheat Turbine Thermal Power Plant

      PI1

      Constant

      1

      0.06s+1

      3.06s+1

      10.2s+1

      1

      0.3s+1

      .5714

      Sum12

      e Uv

      Sum2

      0

      governor

      1

      reheat

      4.9s+1

      turbine

      -1.1s+1

      KI1

      DelPd(s)

      68.5714

      11.43s+1

      Sum5

      0.2s+1

      Governor

      28.749s+1

      Reheat

      0.55s+1

      Turbine

      .2857

      KI3

      Sum4 Sum3

      Inertia & load1

      Hydro Power Plant with Mechanical Hydraulic Governor

      Product

      KI5

      0.416

      Sum6

      1

      .049s+1

      Inertia & load5

      Gas Turbine Power plant

      .6s+1 1.1s+1

      Inertia & load9

      -0.01s+1

      .239s+1

      Inertia & load10

      1

      0.2s+1

      Inertia & load11

      .1428

      KI4

      1

      s

      Sum21 Integrator

      Limited

      Scope

      0.416

      TCPS

      Scope2

      -1

      KI16

      KI6

      .4312

      KI14

      Sum14

      Tie Line

      .2721

      s

      .4048

      .1s+1

      Sum13

      Scope1

      .4167

      KI8

      -1

      KI15

      Scope4

      PI2

      1 .6s+1

      -0.01s+1 1

      Product1

      e Uv

      .4285 .4571

      Sum1

      Sum7

      .049s+1

      Inertia & load6

      1.1s+1

      Inertia & load7

      .239s+1

      Inertia & load3

      0.2s+1

      Inertia & load4

      KI20

      KI7

      DelPd(s)

      Constant1 0

      1

      4.9s+1 -1.1s+1

      .5428

      Sum9 Sum8

      68.5714

      11.43s+1

      Inertia & load2

      Sum20

      0.2s+1

      Governor1

      28.749s+1

      Reheat1

      0.55s+1

      Turbine1

      KI11

      Scope3

      0.416

      KI13

      Sum15

      Inertia & load18

      .4048

      .1s+1

      Inertia & load19

      .2721

      s

      Sum10

      -1

      KI21

      PI3

      .4167

      KI19

      Reheat Turbine Thermal Power Plant

      -1

      KI17

      Product2

      Sum11

      e Uv

      Sum17

      1

      0.06s+1

      governor2

      3.06s+1

      10.2s+1

      reheat 2

      1

      0.3s+1

      turbine2

      .5714

      KI18

      Sum16

      1

      .049s+1

      Inertia & load16

      Gas Turbine Power plant

      .6s+1 1.1s+1

      Inertia & load17

      -0.01s+1

      .239s+1

      Inertia & load14

      1

      0.2s+1

      Inertia & load15

      .4285

      KI10

      DelPd(s)

      Sum19 Sum18

      68.5714

      11.43s+1

      Inertia & load8

      0.416

      KI12

      .4312

      KI22

      TRANSFER FUNCTION BLOCK DIAGRAM OF A POWER SYSTEM HAVE THERMAL, HYDRO AND GAS POWER STATIONS

   5. SIMULATION RESULTS AND DISCUSSION

      The optimum values of the K of the output feedback controller for the simulated ower system are obtained using MATLAB code. The optimal value of K for the power system

      -3

      x 10

      8

      6

      4

      Del Ptie1 in MW

      Del Ptie1 in MW

      2

      0

      -2

      -4

      -6

      -8

      without TCPS

      with TCPS

      with TCPS in series with AC tie line.

      Dynamic responses of the system are obtained for 1% SLP in the area-1. The Eigen values of the system with open and closed secondary loop are given in Table 2. The closed loop Eigen values have negative real parts and satisfy the system stability conditions. The frequency deviation responses of area-1 and area-2 are shown in Figures. The tie line power deviation response is shown in Figure 5. It is observed that the output feedback controller considering TCPS in power system gives better dynamic responses having relatively smaller peak overshoot and lesser settling time with zero steady state error as compared to the power system without TCPS. The

      -10

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      Fig: 4 Change in tie line power and time with TCPS and without TCPS in area 1

      								without TCPS with TCPS

      								without TCPS with TCPS

      0.01

      0.005

      Del Ptie2 in MW

      Del Ptie2 in MW

      0

      -0.005

      -0.01

      -0.015

      -0.02

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      Fig: 5 Change in tie line power and time with TCPS and without TCPS in area 2

      -3

      x 10

      quantitative comparison is made in Tables 1 and 2 where percentage reduction in the overshoot (OS) of f1, f2 and becoming 35.16, 51.27 and 40, respectively and the settling time of f1, f2 and becoming 34.11, 53.29 and 74.29, respectively. The phase angle deviation of TCPS in response to 1% SLP in area-1 is shown in Figure 6 where maximum

      phase angle deviation on positive side is 1.270 and on negative

      4

      2

      Phase angle deviation in radians

      Phase angle deviation in radians

      0

      -2

      -4

      -6

      -8

      -10

      								wit	hout TCPS

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      side is 1.760.

      Fig: 6 Change in phase angle and time with TCPS and without TCPS in area 1

      0.04

      0.03

      0.02

      Del F1 in Hz

      Del F1 in Hz

      0.01

      0

      -0.01

      -0.02

      -0.03

      -0.04

      without TCPS

      with TCPS

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      5

      Phase angle deviation 2 in radians

      Phase angle deviation 2 in radians

      0

      -5

      -10

      -15

      -3

      x 10

      Fig: 1 Change in frequency and time with TCPS and without TCPS in area 1

      without TCPS

      0.02 with TCPS

      0.01

      Del F2 in Hz

      Del F2 in Hz

      0

      -0.01

      -0.02

      -0.03

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      Fig: 2 Change in frequency and time with TCPS and without TCPS in area 2

      -20

      tr>

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      Fig: 7 Change in phase angle and time with TCPS and without TCPS in area 2

      Settling Time
      Description	DelF1	DelF2	DelF3	Ptie1	Ptie2
      Without TCPS	95.6	98.3	83.4	96.2	95
      With TCPS	25.6	29.6	19.9	17.4	18.6

      0.06

      0.04

      0.02

      Del F3 in Hz

      Del F3 in Hz

      0

      -0.02

      -0.04

      -0.06

      -0.08

      -0.1

      without TCPS

      	Peak over shoot
      Description	DelF1	DelF2	DelF3	Ptie1	Ptie2
      Without TCPS	0.0155	0.007	0.008	0.00194	0.00189
      With TCPS	0.0045 2	0.01	0.056	0.0005	0.0087

      	Peak over shoot
      Description	DelF1	DelF2	DelF3	Ptie1	Ptie2
      Without TCPS	0.0155	0.007	0.008	0.00194	0.00189
      With TCPS	0.0045 2	0.01	0.056	0.0005	0.0087

      with TCPS

      0 10 20 30 40 50 60 70 80 90 100

      Time in Secs

      Fig: 3 Change in frequency and time with TCPS and without TCPS in area 3

      Gain Value
      	PI	I1	I2	I3
      Without TCPS	4.397968647	0.20254	0.00541	0.040687
      With TCPS	2.851779265	0.54159	0.95898	0.001603
      Percentage Reduction of ISE	35.156

      Gain Value
      	PI	I1	I2	I3
      Without TCPS	4.397968647	0.20254	0.00541	0.040687
      With TCPS	2.851779265	0.54159	0.95898	0.001603
      Percentage Reduction of ISE	35.156

      Overall Performance Measure of ISE

   6. CONCLUSION

This paper presents a mathematical model of a Thyristor Controlled Phase Shifter (TCPS) in a simple power system to investigate the dynamic behaviour of the system. The voltage magnitude and angle of the bus at which the TCPS is connected are expressed in terms of the control parameters of the TCPS. This allows expressing the output power of the generator in a much simpler way. The above model of TCPS is then used to investigate the damping improvement of the system. Simulation results indicated that the TCPS is capable of significantly improving the damping of the system. However, the degree of improvement depends of the size or rating of the TCPS.

APPENDIX

L

L

Pn = 2000 MW (Rated capacity of each area) P o = 1740 MW (Nominal load of each area) F = 60Hz, H=5 MWsec/MVA,

D=0.0145 pu MW/Hz,

KPS = 68.9655 Hz/pu MW, TPS = 11.49 sec, TSG = 0.06 sec, TT = 0.3 sec, T12 = 0.0433 RTH = RHY = RG = 2.4 Hz / puMW

B = 0.4312, a12 = -1, KR = 0.3, TR = 10.2 sec, TW = 1.1 sec, TRS = 4.9 sec, TRH = 28.749 sec, TGH = 0.2 sec, XG = 0.6 sec, YG = 1.1 sec

Cg = 1, bg = 0.049 sec, TF = 0.239 sec, TCR = 0.01 sec, TCD = 0.2 sec

TCPS parameters

K = 1.5 rad/Hz, T = 0.1 sec

max (positive) = 100, max (negative) = -100

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B.Prakash Ayyappan, M.E. Student, Electrical and Electronics Department, University of Department, Anna University Madurai campus, Alagar Kovil Road, Madurai. He has completed his Bachelor of Engineering in Alagappa Chettiar College of Engineering and Technology, Karaikudi, Tamilnadu.

U.Thiruppathi, Assistant Professor, Electrical and Electronics Engineering, Chendhuran College of Engineering and Technology, Pudukkottai. He has been completed his Master of engineering in

J.J College of Engineering and Technology, Trichy. and he has cmpleted his Bachelor of Engineering in Alagappa Chettiar College of Engineering and Technology, Karaikudi, Tamilnadu.

G. Balaganapathi, He is a Head of the Department, Electrical and Electronics Engineering in Sivakasi Institute of Printing Technology, Sivakasi. He has finished his Master of Engineering in Anna University Madurai Campus, Madurai and he has completed his Bachelor Degree in Arulmighu Kalasalingam College of Engineering and Technology, Madurai, Tamilnadu.