978-1-4244-9074-5/10/$26.00 ©2010 IEEE 2010 Annual IEEE India Conference (INDICON)

Effect of Automatic Generation Controller of Participating Generators Under Frequency Linked Indian Tariff System

Sandip Kumar Gupta, Deepak Kumar, T. Ghose, Member, IEEE

Abstract— The frequency linked availability based tariff in Indian tariff system which encourages the redispatching of participating generating units in real line. According to availability based tariff the generating units are paid at higher rates for the unscheduled interchange they make to restore the frequency to nominal value of 50 Hz. Hence the generating units can earn more profit through unscheduled interchange. This paper compares the effect of primary and secondary controller in automatic generation control on dynamic change in generation and profit earned by generating units. A comparative study in terms of profit earning has been made between generating units with only primary controller or free governor mode and generating units with secondary or supplementary controller along with primary controller. This work also encourages the generating units to operate the units not only on free governor mode but also with supplementary controller which is still not commonly practiced in India. .Keywords— Automatic Generation Control (AGC), Free Governor, Availability Based Tariff (ABT), Central Electricity Regulatory Commission (CERC, India) Regulation 2001; Load frequency control.

NOMENCLATURE i= Block index;

j= Day index;

NDM = Number of Days in a Month;

SG = Scheduled Generation in MW;

AG = Actual Generation in MW;

DC = Declared Capacity in MW;

RC = Running Charge;

ECR = Energy Charge Rate in Rs./MWh;

UIC = Unscheduled Interchange Charge;

RUI = Rate of Unsheduled Interchange in Rs./MWh;

CC = Capacity Charge;

FC = Fixed Cost;

NBD = Number of blocks in a day;

Sandip Kr. Gupta, Deepak Kumar, T.Ghose are with Electrical and Electronics Engineering Department, Birla Institute of Technology, Mesra, Ranchi- 835215, India (e-mail: tirtha_ghose@yahoo.com).

I. INTRODUCTION Frequency and voltage are the two fundamental indices which actually relate two balanced equations required to be maintained for normal power system operation. One of the balance equations relates total power generation and demand. Any unbalance between power generation and demand is reflected by system frequency. Therefore any act to balance power generation with load is actually bringing the frequency at stable condition. The task of AGC is to maintain the system frequency at nominal value and the net tie line power interchange from different areas at their scheduled values. The concept of conventional AGC is discussed in [1]-[2]. Several researchers have applied different control concepts, such as variable structure control, adaptive control, optimal control and fuzzy control to design the load frequency controllers for a large power system [3]-[7].In a competitive electricity market, there will be many market players, such as generating companies, distribution companies, transmission companies and those are controlled and integrated by system operator.. Detailed discussions on load frequency control issues in power system operation after deregulation are given in [8]-[9]. In India the concept of frequency linked market signal has been developed to facilitate competition and improve grid control. The commercial mechanism adopted in India named as Availability based tariff (ABT), defines a power tariff structure that allows recovery of capital and fuel cost as well as a condition based price incentive to participants which help to minimize the real power imbalance in real time operation. This paper give us an idea how the generators in power system respond when there is a momentary unbalance occurs in between power generation and load, so that the generating company makes maximum profit under ABT regime. This work develops a dynamic model of 3-bus system and demonstrates the response of units with primary and secondary controller.

II. TARIFF MECHANISM Operation under ABT is elaborated in this section as a prerequisite to develop objective function and methodology to solve the problem. The new tariff regime aims at inducing reliability at both the generation and the consumption ends either through adequate rewards or punitive measures. Recovery of fixed charges is conditional and frequency linked

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unscheduled interchange is used as an incentive/penalty for power traders which leads to a self regulating power market regime. The tariff mechanism and the calculation of charges to be recovered on monthly basis are discussed in the following section.

A. Calculation of Running Charge This charge is received only against the scheduled

generation decided by regional load dispatch center for every time block. The energy charge for a generating plant is fixed by CERC by estimating the fuel cost on committed annual energy to be delivered by the plant [10]. Total Energy charge, payable to the generating company for a month depends on the average daily scheduled generation, is calculated according to the following equation,

1(E C R )

NDM

M jj

RC SG (1)

B. Calculation of UI Charges The third part of the tariff is UI charges. This incentive or penalty depends on the grid condition. The grid frequencies, in India hardly touches 50Hz line a day therefore, utilities get a good scope to earn profit at rate of high UI charge. The difference between the actual generation and the scheduled generation is accounted through the UI charges and the rate is based on the average frequency at that time block. As per ABT regulations, the whole day is divided into 96 time blocks, each of 15 minutes duration. Average generation during each 15 minutes time block is considered as the actual generation (AG) and if the AG is more than the Scheduled Generation (SG) then the over generation is rewarded if the system frequency is less than the specified or is penalized otherwise. If AG is less than SG and at the same time frequency is healthy, the utility will be rewarded by paying the running cost on SG even though the AG is less than SG during the period. Therefore the monthly earned from UI charges, UICM, can be calculated from Eqn.2,

96

1 1( )M i

NDM NBD

ij ijj i

UIC AG SG RUI (2)

RUI is the rate payable for unscheduled interchange of power which is basically a frequency regulating price that is dependent on the average frequency prevailing during each 96 time blocks. The existing rate in Rupees/MWh as per the order No.L-1(1)2009-CERC dated 30/03/2009 is as follows.

5710;for 49.24080;for 49.2 & 49.62( 6000 301800);for 49.62 & 50.3

RUI f HzRUI f Hz f HzRUI f f Hz f Hz

(3)

This paper uses Eqn.2 to calculate money earned from dynamic change in generation with only primary controller

and with both primary and secondary controller.

III PROBLEM FORMULATION

This work investigates the effect supplementary controller of generating units in an isolated area. Fig.1 shows a two bus network where generating units and load are connected in two different buses. Fig.2 shows the block diagram of a unit with droop R. Load reference set point is considered zero here to examine the effect.

Fig 1: A two bus network

Fig 2: Non Reheat Thermal system for a single area The differential equations describing are:- d f/dt =1/TP (- f+KP ( PG- PD)) (4) d XE/dt=1/TG ( XE – (1/R) f) (5) d PG /dt= 1/TT ( PG + XE) (6) Where,

PG = Incremental generation change, XE = Incremental governor valve position change PD = Incremental load change demand

f = Incremental frequency deviation f = nominal system frequency Hi = inertia constant Di = load frequency constant (KP =1/Di, TP =2Hi/fDi) Ri =speed regulation parameter TG =Governor Time constant TT = Turbine time constant TP =Power system time constant

Fig 3: Block diagram of a transmission line

f1(s)

+

f2(s)

2 T0 1/s P12

KT/(1+sTT) KG/ (1+sTG) Kp/ (1+sTP)

1/R PD

XE

f(s)

PG

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Transmission line power flow between two buses is defined by the following equation.

P12 = 2 T0 *1/s ( f1(s) - f2(s)) (7) Where,

f1(s) = Incremental frequency deviation at bus 1. f2(s) = Incremental frequency deviation at bus 2.

T0 = Transmission line Stiffness coefficient. P12 =Transmission line power interchange between buses 1

and 2. IV CASE STUDY

A case study has been taken with 3 Gencos in which Genco1 and Genco3 are using primary as well as secondary controller whereas Genco2 is on free governor mode of operation. Therefore the effect of controller on profit earned from UI is compared. For comparing the performance of the secondary controller, the same MW rating and droop characteristics have been considered for all the units.

TABLE I Data for Generating units

System investigated consists of three bus and three generating units where a step load change is considered at bus 3, as shown in Fig 4. A dynamic modeling of the generators, load and transmission line considering GRC is developed in MATLAB SIMULINK environment. GRC is considered as 10% for all the units [11].The optimized value of gain of integral controller, Ki, is obtained through Hooke-Jeeves optimization technique. This method comprises of an iterative application of an exploratory move in the locality of the current point and a subsequent jump using the pattern move.

Fig 4: Block diagram of a 3-Bus System

V RESULTS

As it has already been mentioned that any generation change in real time above or below the scheduled power is considered as UI if the change has a role to improve the system frequency. Increment of 0.2 p.u load change at bus 3 to determine UI generated from the generating unit to compensate the frequency decrement.The responses of primary and secondary controller are observed and subsequently profit earning has been calculated from UI.

GEN-1 and GEN-3 connected at bus 1 and 3 respectively are operating with both primary and secondary controller, where as GEN-2 is operating only on free governor mode. The results of frequency deviation at all the three buses are shown in Fig 5. The corresponding change in generation of the Gencos’ 1, 2 and 3 are also shown in Fig 6.

Fig 5: Frequency Deviations at all the three buses

At transient period frequency deviation at bus 3 becomes more due to load change is considered at bus 3 but the steady state frequency will be the same for all the buses as it is shown in Fig 5. Fig 6 shows the corresponding changes in generation of all the three generating units, according to the change in frequency. Though the steady state frequency is same for all the three buses but GEN-2 generates less as it has no supplementary controller. Frequency goes back to nominal frequency due to more generation from generating unit 1 and 3, as they have supplementary controller. Generating unit 2 goes back to scheduled generation at steady state period as in Fig 6. As per the regulation of ABT, the profit is calculated on every 15 minutes. During each 15 minutes the average generations change and frequency change are calculated and subsequently profit is calculated on the RUI rate prevailing for the particular average frequency existing during each 15 minutes block. All the calculations are done on one hour run of simulation model. Profit calculation for unscheduled interchange made by Gencos’ 1, 2 and 3 for one hour period under ABT regime is calculated as shown in TABLE 1, 2 and 3 respectively. The running cost of all the generating units is considered here as Rs.1.54/kWh. Therefore the cut off frequency is 50.03 Hz for the units where RUI is equal to running cost. Therefore frequency below the 50.03 Hz will give a scope to generating unit to earn profit through UI. TABLE 1 shows the profit of Genco1 where unit keeps there generation change at 18.75 MW during the last three blocks. Generating unit 2 earns profit during transient period only because it has no change in load reference set point as no supplementary controller is present. Though both units 1 and 3 has supplementary controller even unit 3 shares more load therefore earns more profit as compared to unit 1 because the load change occurred at same bus 3.

Item Genco1 Genco2 Genco3Installed capacity(MW) 275 275 275

Normative Auxiliary consumption 10% 10% 10%

Ex-Bus Normative Capacity in (MW) 250 250 250

GRC(in p.u .MW /min) 10%

10%

10%

Fig 6: Change in generation for unit 1, 2 and 3 respectively

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TABLE 1 Profit earned by Genco-1

Time

Avg. Frq (Hz)

Change in gen.(p.u)

Average change in gen(MW) (UI)

Profit (Rs.)

First Fifteen min 49.9996 0.0748 18.70 33704

Second Fifteen min 50.0000 0.0750 18.75 33660

Third Fifteen min 50.0000 0.0750 18.75 33660

Fourth Fifteen min 50.0000 0.0750 18.75 33660

Total 74.95 134684

TABLE 2 Profit earned by Genco-2

Time

Avg. Frq.(Hz)

Change in gen.(p.u)

Average change in gen(MW)

(UI)

Profit (Rs.)

First Fifteen min 49.9994 0.000278 0.0695 125.35

Second Fifteen min 50.0000 0.0 0 00

Third Fifteen min 50.0000 0.0 0 00

Fourth Fifteen min 50.0000 0.0 0 00

Total 0.0695 125.35

TABLE 3 Profit earned by Genco-3

Time

Avg. Frq.(Hz)

Change in gen.(p.u)

Average change in gen(MW) (UI)

Profit (Rs.)

First Fifteen min. 49.9994 0.1248 31.20 56272

Second Fifteen min 50.0000 0.1250 31.25 56250

Third Fifteen min

50.0000 0.1250 31.25 56250

Fourth Fifteen min 50.0000 0.1250 31.25 56250

Total 124.95 225088

VI CONCLUSION

The significance of the work is to given an idea on the amount of profit that can be realized by the generating units in an hour through UI. Results shows that the unit operating with both primary and secondary controller earns enormous profit as compared to the unit operating on free governor mode. It is also seen that the generating unit near the load center earns more profit as it responds more to the load change. This work encourages the Gencos’ to operate with advance controllers in automatic generation control so that they make more profit under ABT regime.

VII REFERENCES

[1] O.I.Elgerd and C.Fosha,”Optimum megawatt-frequency control of multiarea electric energy systems,”IEEE Trans.Power App. Syst., vol. PAS-89, no.4, pp.556-563, Apr.1970. [2] N.Jaleeli, D.N.Ewart,and L. H. Fink ,”Understanding automatic generation control,” IEEE Trans.Power Syst.,vol.7,no.3.pp. 1106-1122, Aug.1992. [3] G.Ray, N. Yadaiah and G.D Prasad,” Design of a load frequency regulator by Schur approach,” Elect. Power Syst. Res., vol.36, pp.145-149, 1996. [4] G.A. Chown and R.C. Hartman, “Design and experience with a fuzzy logic controller for AGC,” IEEE Trans.Power Syst., vol.13, no.3, pp. 965-970, Aug.1998 [5] B.Tyagi and S.C.srivastava, “A Fuzzy Logic based load frequency controller in a competitive electricity environment,” in Proc.2003 IEEE Power Engineering Society General Meeting,Vol.2,Jul.2003,pp.560-565. [6] K.Bhattacharya, “Frequency based pricing as an alternative to frequency regulation ancillary services,” in Proc.Eleventh National Power System Conf.Bangalore, India, 2000, pp210-215. [7] G.A. Chown and B.Wigdorowitz,”A methodology for the redesign of frequency control for ac n/w,” IEEE Trans.Power Syst., vol.19, no.3.pp. 1546-1554, Aug.2004. [8] R.D.Christie and A. Bose,” Load frequency control issue in power system operation after deregulation,” IEEE Trans.Power Syst., vol.11, no.3.pp. 1191-1200, Aug.1996 [9] N.Bekhouche,” AGC before after Deregulation,” in Proc. 34th Southeastern Symp. System Theory, Mar.2002, pp.321-323. [10] Regulation No.L-7/145(160)/2008-CERC, Notified By CERC, India, Published on 19th January, 2009, [Online], Available in: http://www.cercind.org [11] Cheres.E.(2001)’The Application of Generation Rate Constraint in the Modeling o a Thermal Power System’,Ele ctric Power Components and Systems,29: 2;83-87. [12] Indian Electricity Grid Code, Notified in 2005 (Effective From April 2006) By CERC,2005,India, [Online], Available: http://www.cercind.org

VIII BIOGRAPHY

[1] Sandip Kumar Gupta received the B-Tech degree in EE from West Bengal University of Technology in 2008. He is presently doing his M.E in EE at B.I.T, Mesra. His research interest is advanced power system analysis. [2] Deepak Kumar received the B.E degree in EEE from BIT, Mesra, Ranchi, in 2007. He is presently doing his M.E in EE at B.I.T, Mesra. His research interests are electric power deregulation system and portfolio based analysis of electrical power system [3] T. Ghose received the B.E degree from BIT, Rajshahi, Bangladesh, in 1989, the M. Tech degree from Calcutta University, Calcutta, in 1993, and the Ph.D. degree from Jadavpur University in 1999. Currently, he is Professor at B.I.T, Mesra, Ranchi. His research interests are electric power distribution system, power system deregulation and soft computing techniques.