Combined Cycle Gas Turbine and Generator ...

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1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016). Combined Cycle Gas Turbine and Generator.
t 1s IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)

Combined Cycle Gas Turbine and Generator Matching Performance Jitendra Nath Rail and Ajendra Singh2

Department of Electrical Engineering; Delhi Technological University; Delhi; India E-mail: [email protected];[email protected] Abstract-Due to ever-increasing demand for power there

arise

a

need for

installation

of

hi gher

The generator voltage is controlled by the exciter whose current (excitation current) is controlled by the automatic voltage regulator which produces signals according to the measured value of the generated voltage and current of the generator. The governor of the turbine of the generator adjusts the output of the turbine according to the power and the frequency at which it is produced by the generator by comparing it with a reference frequency and power. The output of the turbine is controlled by the varying fuel flow by the governor. In this paper we have analyzed the behavior of a synchronous generator driven by single shaft combined cycle gas turbine under no load; under load and the effects of excitation control and governor control using MATLAB/Simulink for matching performance.

efficiency

g enerating stations. This p roblem of hi gher efficiency plants can be resolved by the combined cycle power plants which p rovide a hi gh installation capacity and hi gher efficiencies; but the installation of newer g enerating stations affects the stability of g rid. The behavior of combined cycle power plants is still a top ic of research as none of the models develop ed yet have been successfully able describe its overall behavior.

The following pap er studies the behavior of combined cycle g enerator under various conditions using MATLAB/Simulink model.

Keywords-Combined Cycle Gas Turbine; Synchronous Generator; Simulink Model; Stability I.

INTRODUCTION

Due to economy and power transmission maximization the present power systems consist of interconnected generating units supplying energy to a common grid [2]. The addition of new units or modifications in existing units requires a careful examination of the system stability as it affects the stability of grid [6-8; 14]. Thus it is required that the operating characteristics of various types generating stations are known so that the stability of the interconnection can be enhanced. The combined cycle gas turbines (CCGT) have a characteristic feature of high efficiency; quick start-up and low emission of greenhouse gases which make them an attractive choice for expansion of power generation [11-13]. As gas turbine operate at high speeds therefore the generator driven by these types of turbines are of cylindrical rotor type of construction. Fig. 1 shows the control equipment of synchronous generator used for controlling the output of the generator.

II.

GENERATOR AND

CCGT

Synchronous Generator [3]: When in running generator the armature (stator in case of large generators) windings of the generator are open-circuited the generator is said to be operating on no load. The equations describing the operation under such conditions are as follows: eA eB ec

= wMJiJ sinwt . , = wM JiJ sin(wt-2 * pi! 3) = wM JiJ

sin(wt- 4 * pi! 3)

(1)

Where eA; eB and ec are the internal voltage (or Excitation EMF) of the generator; Mf is the mutual inductance between the stator and the rotor windings; w is speed of the rotor in rad/sec and if the field is current. When the generator is connected to a load armature currents starts to flow and there is a voltage drop in the armature windings due armature resistance and an armature reaction which opposes the field flux. The terminal voltage of the generator under these conditions is given as: V

a

=

-R I - (L s

a

s

d' .. t +M )....!.E. dt +JiIElcos(OJ +8) s

I

(2)

Where Rs is the stator resistance; Ls is the self­ inductance of the armature winding; Ms is the mutual inductance between the adjacent pair of windings; ia is the current in the armature winding of phase 'a' and 8 is the load angle.

Fig. I: Block Diagram of a Power Generation Unit 978-1-4673-8587-9/16/$31.00 ©2016 IEEE

CYLINDRICAL ROTOR SYNCHRONOUS

[1]

t 1s IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)

IV.

When the system operating under normal conditions i.e. under equilibriwn; is disturbed the mechanical torque provided by the turbine is not equal to the electrical torque due to the load; the system under these conditions is described by what is known as the swing equation given as follows: H d28 (3) Pm - Pe Pa 180 f dt"2 Where H is the inertia constant; f is the frequency; Pm is the mechanical power and Pe is the electrical power. =

A.

A.

IMPLEMENTATION

Generator on no Load

The operation of generator on no load is shown in the graphs of Fig. 3. This is the case when generator supplied by mechanical power suddenly goes on no load. It can be seen that the voltage and current waveform have a sub­ transient and transient period total of about 1 sec. The cause of the high magnitude of the waveforms during the begirming portion is due damper winding reactance and the synchronous transient reactance which causes the reactance of the machine to be lower than the normal value.

=

CCGT (Combined Cycle Gas Turbine)

The power output of the gas turbine is controlled by varying the fuel flow rate which controlled by the governor of the turbine. An increase in fuel flow causes an increase in the power output and vice versa; but an increase in the fuel flow also causes an increase in the temperature of the turbine which is controlled by increasing the air flow in the turbine [1; 15]. III.

SIMULlNK MODEL

The model consists of cylindrical rotor synchronous generator model connected to single shaft CCGT block which provides mechanical energy to the generator. The gas turbine model is based on the Rowen Model [9; 16]. The CCGT block [1; 10] controls the output of the generator on the basis of the speed of the generator provided as an input to the block. There is a separate block for the excitation system consisting of Vref; input voltage that voltage at the terminal of the generator and automatic voltage regulator in conjunction with the power system stabilizer block Vstab attached to it for better stabilization of the generated voltage [5]. The input excitation voltage is provided to the generator by the block as a signal Vf to the generator. The generator model is connected to short length transmission line as depicted by the block consisting of R (resistance) and L (inductance) of the line. A block for the transformer is also provided for stepping up the generated voltage of llkV of the generator (under normal conditions) to a voltage of 132kV for connection to the grid. A block representing the grid is also present which represents infmite power at 132kV. A block representing a load of 5MW is connected to the transmission line which is connected to a block which is for measurement of output and provides the voltage and current across the load. Various scopes have been connected for the graphical display of speed; mechanical power; stator current; voltages; load angle; electrical power; field current; active and reactive power and excitation voltage in continuous mode (represented by powergui).

Fig. 2: CCGT Generator Simulaton

It can be seen that when the steady state is attained (i.e. after 1 sec) the voltage waveform is of constant magnitude of about llkV and the current is of negligible magnitude (small current is present due to the conductance present between the wires of transmission line). The magnitude of the induced voltages in the generator terminals are according to equation (1). Power which is a product of voltage and current is in the range of kW during transient and sub-transient period and then becomes negligible as current becomes near zero. The Excitation System Implements an IEEE type 1 synchronous machine voltage regulator [5] combined to an Exciter. k ) ----------------� ( I"; � �� ; e� o M� - � "h � (ma- s� - O US Ch' ," Y"� �s� Implements a 3-phase synchronous machine modelled in the dq rotor reference frame.

Stlltor windings are connected in wye to lin internal neutrlll point.

---1

J

Parameters L...:A dv": ",":: :ce'-L" :: loa " "d" F"" d Configuration IO::.:. W "'---____ .::: Nominal power, line-to-line voltage , frequency [Pn(VA) vn(vrms) fn(HZ)]: (37500000 11000 50]

Reactances [Xd xd' Xd" xq xq' xq" Xl] (pu): (1.81,0.3,0.23,1.76,0.65,0.25, O.IS)

d axis time const2lnts: q axis time constants:

[ Open-circuit

·1

= ========== == I Sh Ort-'ircU I; '1



Time constants [Tdo' Tdo" Tq' Tq"] (s):

�1

(8.0,0.03,0.3371,0.0295]

Stator resistance Rs (pu): 0.003

Inertia coeficient, friction factor, pole pairs [H(S) F{pu) pO]: (18.5 0 2]

Initial conditions [dw(%) th{deg) ia,ib,ic(pu) pha,phb,phc(deg) Vf(pu)]: (0 -61.1356 0.800573 0.800573 0.800573 -26.4399 -146.44 93.5601 1.79267) o Simulate saturation

Fig. 3: Parameter Box of Synchronous Generator in MATLAB

[2]

t 1s IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)

The excitation control of the field winding during no load is as shown in Fig. 5. The excitation is controlled by the AVR and PSS systems when the machine is suddenly goes at no load. The variation in speed is due sudden changes in electromagnetic torque (and hence power) and according to swing equation (equation (3)) the speed varies as shown in Fig. 6. The speed of the generator rotor is controlled by the governing system of the turbine .

Fig. 6: Speed (w) Variation · • • 1"".,.

Fig. 4: Power; Current and Voltage at no Load

/

Fig. 7: Line Currents in the Armature (Stator) Winding

:,---7----t----+---�---t_--_j

B.

." ,

Generator on Load

The response obtained when the generator is suddenly connected to load are shown as follows (both steady state and transient state):

Fig. 5: Excitation Control of Voltage at no Load

[3]

t 1s IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)

1. Currents: When generator is suddenly connected to the bus the waveform of currents are shown in Fig. 7. The Generator first operates in sub-transient and transient state for about 2 to 3 seconds and then attains steady state. Under steady state the terminal voltages have magnitude according to the equation (3) and the developed internal emf; E or no load voltage according to equation (1) which produce steady state currents as can be seen after 2 seconds time in the plot. 2. Transmission Line Currents: The current waveform in the b-phase of the transmission line is shown in figure 8. The magnitude of the transmission line current directly follows form the line current developed in the generator. The current cycle at the beginning is more negative because of the dc offset voltage present at the beginnings which die out eventually according to the current decay rate in an inductive circuit. 3. Excitation Control and Field Control: The excitation control of the generator is shown in figure 9. The excitation is to be reduced at the beginning due to sudden rise in the voltages and currents in the transient phase of generator operation. The sudden increase in the excitation of the rotor above the operating value is due to the transfer function response of the AVR and PSS control system. The waveform of the field current is shown in figure 10 which directly follows the excitation voltage control of the excitation voltage. 4. Speed of the Rotor: When the generator is loaded there is an increase in an electromagnetic torque which opposes the mechanical torque which causes a reduction in the speed (frequency of generation) of the generator. The speed of the generator is readjusted by the governor system of the turbine. The effect on the speed of the rotor on loading the generator is shown in figure 11. The speed characteristics of the rotor can also be explained by the swing equation [equation (3)]. 5. Input Mechanical Power: As the speed of the generator drops on application of electrical load the power input from the turbine also drops as shown in Fig. 12. The balance in electrical power and mechanical power is re-obtained using the turbine governor as explained before. 6. Load Angle: The load angle of the generator on disturbance is adjusted by the governing system of the generator. It can be seen from figure 13 that the delta comes back to a stable value after the disturbance showing that the system is stable. The stability of the result is in accordance with equation

(3) (Swing Equation) which show that the mechanical and electrical power balance each other at constant load angle. 7. Active Power and Reactive Power Flow: The active and reactive power flow is shown in figure 14. It can be seen that when there is a reduction in the load angle the active power also reduces as the sin of the load angle reduces. 8. Turbine Governor Control of Temperature: When the mechanical power from the generator is to balance the electrical power the mechanical power from the turbine is increased by increasing the fuel flow which causes an increase in the temperature of the turbine that is controlled by increasing the airflow as shown in figure 15. 1b3

Three·Phase Parallel RL Branch

Time(sec) Fig. 8: Current in the Transmission Line of Phase b :'\

" "

I:

I, I I

I

I I

I I I I

I

I I I

I

I

I I II �I

\

, I I , , , , , ,

"

/"---'--

:,

...

----

.

--.

----

//

I

\j

., ,I II ,I '\:

Fig. 9: Excitation Control of the Field Winding

[4]

.-

t 1s IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)

V.

The model setup of CCGT coupled generator was developed and its behavior under various conditions was analyzed. From the implementation results it can be concluded thatThe generated emf of the generator (internal emf) is directly related to the excitation provided to the rotor. (as seen in generator at no load). Effect of sudden changes in load on speed was studied and it is concluded that for system to be able to operate properly the governor system should be such that it is able to balance electrical and mechanical torque after a disturbance has occurred in the system. The changes in the load angle were observed when the generator was suddenly loaded; it showed that the setup is stable for small disturbances as the load angle was able to attain a constant value after some disturbance was applied to the system. The dependence of real power on the load angle delta was observed that as with the decrease in delta active power generated also decreases. The voltages during transient and sub-transient periods are much higher than those during the steady state conditions so the insulation material of the generators should be such that it is able to withstand very high voltages for about 3 to 4 seconds.The current during the transient period is also very high so the thermal properties of the windings and insulators should be such that it is able to withstand the heat generated during that period. From the speed-time graph it can inferred that the rotor inertia should be large so that there is less oscillation about the steady state speed also the strength of the rotor should be such that it does not break due to coupling torque during the period of oscillations in speed. The maximum power input from the turbine i.e. mechanical power; input from the turbine to the generator is limited by the maximum temperature rise in the turbine which rises with the increase in the rate of fuel flow and is controlled by the air flow rate in the turbine.

"J '.

Fig. 10: Field Current on Excitation Control

!i\

'I I .-1 I I .. , I I �"'-f

- - -_.

I

:

'

I . , . � .. -II

CONCLUSION

!

V

.�.

Fig. I I: Rotor Speed Variation

.6.-__ &0_

.

:'

J

�.

Fig. 13: Load angle (0) Variations versus Time

Fig. 12: Change in Mechanical Power (Pm)

[5]

t 1s IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)

A.

Voltages and Currents across 5 MW Load

From the simulations model it can be seen that the generator is supplying a load of 5 MW. Figure 16 shows the voltage and current across that load; both transient and steady state result. REFERENCES

o·"'v 0.73

0.729

[I]

Rai J.N.; Hasan Naimul; Arora B.B.; Garai Rajesh; Kapoor Rahul; Ibraheem "Performance Analysis Of CCGT Power Plant Using Matlab/Simulink Based Simulation" IJoART Volume 2; Issue 5; May 2013 Issn: 2278-7763

[2]

Rai IN.; Hasan Naimul; Gupta Rishabh K.; Kapoor Rahul; Garai Rajesh "Enhancement Of Voltage Profile Of Transmission Line By Using Static Var Compensator-An Overview" IJERT Vol. 2 Issue 5; May - 2013 Issn: 2278-0181

[3]

Krause; P.C.; Section 12.5.

[4]

Kamwa; 1.; et al.; "Experience with Computer-Aided Graphical Analysis of Sudden-Short-Circuit Oscillograms of Large Synchronous Machines; " lEE� Transactions on Energy Conversion; Vol. 10; No. 3; September 1995.

[5]

"Recommended Practice for Excitation System Models for Power System Stability Studies; "IEE� Standard 421.5-1992; August; 1992.

[6]

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[7]

S. A1-Zubaidy; F.S. Bhinder; "Towards optimizing the efficiency of electrical power generation; " Energy Conversion Engineering Conference; vol.3; pp. 1857-1862; Aug. 1996.

[8]

K. Kunitomi; A. Kurita; H. Okamoto; Y. Tada; S. Ihara; P. Pourbeik; W. W. Price; A. B. Leirbukt and J. J. Sanchez-Gasca; "Modeling frequency dependency of gas turbine output; " Proc. IEEE/Power Eng. Soc. Winter Meeting; Jan. 200I.

[9]

W. 1. Rowen; "Simplified mathematical representations of heavy­ duty gas turbines; " Trans. Amer. Soc. Mech. Eng.; vol. 105; pp. 865-869; Oct. 1983.

[10]

F. P. de Mello and D. J. Ahner; "Dynamic models for combined cycle plants in power system studies; " IEEE Trans. Power Syst.; vol. 9; pp. 1698-1708; Aug. 1994.

0.728 0.727

o·'r

0.' ., •.. .. , ., •..

Fig. 14: Active and Reactive Power Output k;\,I(Cv,,,,,1II1 ill

Phu:. ...

C); �

MW L.......oJ

Analysis of Electric Machinery;

McGraw-Hill; 1986;

[II] John H Horlock; "Advanced gas turbine cycles; " Pergamon Press; 2003. 0.'

, ..

Ilmo(soc)

[12] Rolf Kehlhofer; Rukes Bert; Hannemann Frank and Stirnimann Franz. "Combined-cycle gas & steam turbine power plants; " PennWell; 2009.

2.'

Fig. 15: Voltage Variation and Current Variation of Phase C in 5MW Load

[13] F. Drbal Lawrence; G. Boston Patricia; L. Westra Kayla; Black& Veatch. Power Plant Engineering. p. 241 Springer. 1996. [14] G. Lalor; M. O'Malley; "Frequency control on an island power system with increasing proportions of combined cycle gas turbines; " IEEE Power Tech Conference Proceedings; Bologna; vol.4; pp. 7; June 2003. [15] J. N. Rai; Naimul Hasan; B.B. Arora; Rajesh Garai; Rishabh K. Gupta; Rahul Kapoor "Study the Effect of Temperature Control on the Performance of the Output of Combined Cycle Gas Turbine; " International Journal of Theoretical and Applied Mechanics. ISSN 0973-6085 Volume 8; Number I pp. 15-23 2013. [16] William 1. Rowen; "Simplified mathematical representations of single shaft gas turbines in mechanical drive service." Turbomachinery International 33.5 (1992).

Fig. 16: Temperature Exhaust (C); Air Flow (pu) and Fuel Flow (pu)

[6]