Transient analysis of integrated solar/diesel hybrid power system using MATLAB Simulink

Transcription

1 Transient analysis of integrated solar/diesel hybrid power system using ATLAB Simulink Takyin Taky Chan School of Electrical Engineering Victoria University PO Box C, elbourne 81, Australia. Abstract This paper presents atlab simulation the dynamic behavior of small autonomous power system with solar and diesel power sources. The solar power system and the diesel generator operate in parallel. It is more cost effective than a diesel generator acting alone. For the effective integration of the solar power into the power system, a method for controlling the Inverter s operation is proposed. The power demand from the network is monitored and control signals adjust the angle and magnitude of the Inverter s voltage. This simulation is to investigate the behavior of the step change in different load. Consequently, develop an advanced digital controller to provide a fast and stable dynamic response of the hybrid power system. 1. Introduction The analysis of an integrated hybrid power system and associated control system could be time consuming and expensive without a good modeling and simulation study. Therefore, simulation plays an important role in the design and analysis of power system, power converter and their controller. Simulink, a window-oriented dynamic modeling software package based on atlab numerical workspace is specifically designed for simulating dynamic system [1]. atlab is a software package for high performance matrix computation. In atlab Simulink environment, the models are entered as block diagrams with a graphical interface and a set of blocks with signal interconnections could be masked as a subsystem. The library blocks make the construction of dynamical systems easy. Therefore Simulink can be conveniently used for studies of dynamic systems. This paper presents the modeling and simulation study of a hybrid solar/diesel power system, a DC to AC power converter and the diesel generator operate in parallel, which can be analyzed the load flow and load sharing for the power system, since it is more cost effective than a diesel powered generator acting alone. Due to the diesel generator is a commercial product, it has own speed control and cannot be accessed externally. The control of such an integrated system proves to be very challenging. So this simulation is to investigate the behavior of the step change in different load. Consequently, develop an advanced digital controller to provide a fast and stable dynamic response of the hybrid power system. 2. odel of System Components The appropriate models used to simulate the solar/diesel electric power system are general and able to give accurate results. In order to examine the dynamic behavior of the whole autonomous system, models for the diesel engine generator set and the solar fed inverter are given in this section. 2.1 odel of a Diesel Engine To simulate the complete dynamics of a diesel engine system, a very large order model will be required. However for most studies on speed dynamics of internal combustion engines, it is sufficient to use a lower order model. Similar approaches have been adopted in diesel engine simulation studies [2, 3, 4]. 1,2 1 τs + 1 Actuator Load S e θ Engine DieselEngineSystem Σ Flywheel Fig.1 Block diagram of a typical diesel engine system α S ξ n

2 The mathematical model of a typical small sized diesel driven prime mover is shown in Fig.1. where the input to the plant is the control signal to the actuator, the output of the model is velocity,.35 α.2 seconds, θ =.4,.1 α 2 in velocity per second,.1 ξ.1, and load = or 1. The dead time θ in Fig.1 represents the almost pure dead time associated with an engine. This dead time is the result of having several cylinders. Not all cylinder will be in a position to accept more fuel at a given instant. The dead time θ is largely made up of the time required for all cylinders to come into position to be filled with more or less fuel. This parameter is fixed in this model, and is set to.4 seconds. A self-tuning PID controller has been developed for a small sized diesel engine and generator set very successfully [5]. 2.2 Synchronous Generator The equations of synchronous generator are obtained from Park equations [6]. The most important is that stator transients are neglected compared to the rotor. In the model presented the synchronous machine rotor consists of three windings. A field and a damping winding on the direct axis are in order to take into account the transient and sub-transient behavior respectively. A damping winding on the quadrature axis also has transient and sub-transient time constant. The stator equations are in p.u.: 2.3 odel of Inverter An inverter consists of inductors, capacitors, transformer and power electronics components such as IGBTs, OSFETs. Each H-bridge switch of the inverter is operated by a sinusoidal pulse wih modulation (PW) signal. The power electronic devices switch the signal to transform the sources. The function of the inverter is bi-directional to change a DC source and a symmetrical sinusoidal AC source. The DC input assumes a constant source (large capacity batteries). The power transfer between the inverter DC source and AC source is achieved by controlling the phase angle (δ) and magnitude of the inverter output voltage. Also a controller is used to supervise the entire system. At normal condition, the inverter generates in parallel of output voltage and should be kept synchronized with power system. 2.4 Load flow The diesel generator and the converter are connected in parallel to supply the load. The diesel and converter voltage sources are separated by a link inductor. The bidirectional power flow can be explained with reference to the single equivalent circuit shown in Fig.2. I I L Vd = Ed Rs Id + q Iq (1) Vq = Eq Rs Iq d Id (2) I C Load The differential equations corresponding to the rotor winding dynamics are in p.u.: de q Ef + ( d d) Id E q = (3) de d ( q q) Iq E d = (4) de q E q + ( d d) Id E q = (5) de d E d ( q q) Iq E d = (6) The electromagnetic torque equation is in p.u.: T D e = E d Id + E q Iq ( d d) Id Iq (7) V V C δ DieselGenerator Converter Fig.2. Single equivalent circuit for a hybrid solar/diesel power system The active power (P) and reactive power (Q) as follows: VVC P = sinδ (8) V Q = ( V VC cosδ ) (9) 1 PL δ = sin (1) VVC

3 Fig.3. The overall Simulink block diagram of hybrid power system Where δ is the phase angle between the two voltage sources, the phase angle with respect to the diesel voltage is varied for power flow. It can be seen that the power supplied by the inverter from the batteries or supplied to the batteries can be controlled by controlling the phase angle δ. The converter voltage Vc is separately controlled by the modulation index of PW pulses. Typical operation of a hybrid power system is load demand dependent. In the low loads, the diesel generator is off. Power from the batteries and solar is transferred to the load through the inverter. In medium loads, the diesel generator will be supplying the load directly. Excess power from the diesel generator is used to charge the batteries through the bi-directional inverter in the charge mode. Hence the diesel generator operates in optimum 8%-1% loading capacity. During peak loads, the diesel generator runs in parallel with the inverter which converts DC power from batteries to AC power. 3. Simulation odel atlab Simulink is used to model the system and apportion the electrical production between the inverter and diesel engine generator. In general, the Simulink model can be used to study the performance of any hybrid power system. Using Simulink for other renewable energy sources, dynamic operation and control system strategies can be easily incorporated into the existing hybrid electric power system model to study the overall performance of the system. Simulation are performed the inverter and diesel engine generator load flow sharing and dynamic transient response. The results of the simulation are used to design the comprehensive control system and predict the power system impacts of integrating a solar and diesel electric power system. A model of an inverter and diesel generator power system was built using atlab Simulink. The Simulink model was developed so that it can be used to study the performance of load flow power system. Using the S-function and SimPowerSystem in Simulink, blocks representing system components can be easily incorporated into the existing hybrid power system model. Simulink also allows the dynamic operation and the control strategy to be incorporated into the hybrid power system model to study the dynamic performance of the system. The overall block diagram of the current system is shown in Fig. 3. The model consists of two large subsystems contained in blocks as shown below.

4 Simulation model of 1kVA power converter to the normal, the converter current still maintains as shown in Fig. 8b. Since the 5kW load is withdrawn from the power system, the converter also reduces power but the diesel generator power still maintains almost same level shown in Fig. 9. Fig. 4. Voltage regulator subsystem Simulation model of 8kVA diesel generator set Fig. 5. Governor & diesel engine subsystem Fig. 6. Voltage & speed control subsystem The synchronous machine block can be operated in generator mode. The equivalent circuit of the model is represented in the rotor reference frame (qd frame). All rotor parameters and electrical quantities are viewed from the stator and the model parameters are preset. Fig. 7. (a) Generator power output (p.u.), (b) Excitation Voltage (p.u.), (c) Engine speed / frequency (p.u.). 4. Simulation and Results The simulation results are presented in this section. The main voltage of hybrid power system is normally line voltage 4 volt (single phase 23 volt rms). It is assumed that the DC system at the power converter side has a constant DC source. The diesel generator supply a 7kW resistive load after.5 second at the engine stabilized. The diesel generator is held constantly single phase 22 volt. While the diesel generator frequency stabilizes at 5Hz, the converter and diesel generator synchronize at 7.1 second and the converter current is phase shifted 31 o accordingly. Then a 5kW resistive load steps up change on the loading at the 1 second as shown in Figs. 8, 9, 1. The main current of power converter increases during the step change load and needs approximately six cycles to reach the steady state. Similarly, when the voltage is back Fig. 8. (a) Vab inverter PW output, (b) Vab inverter voltage, (c) Ia inverter current, (d) odulation Index.

5 Fig. 9. (a) inverter output power, (b) diesel generator output power, (c) load consumption. Systems, IEEE Transactions on Industry Applications, Vol., No., 2. [5] D. W. Augustine and K.S. P. Kumar, A method for self-tuning a PID controller for control of small to medium sized diesel engines, IEEE International Conference on System Engineering, P Aug [6] Paul C. Krause, Analysis of Electric achinery, cgraw Hill, [7] Ashari., Nayar C.V., Islam S., Steady-state performance of a grid interactive voltage source inverter, IEEE Power Engineering Society Summer eeting, 21. Vol. 1, P July 21 [8] Nayar C.V., Ashari., Keerthipala, W.W.L., A grid-interactive photovoltaic uninterruptible power supply system using battery storage and a back up diesel generator, IEEE Transaction on Energy Conversion, Vol. 15, Issue 3, Ps , Sept Conclusion Software simulation of solar diesel hybrid power system has been developed and tested by several simulation runs. This result represents an aid to evaluate the transient behavior of power flow in the system. Investigation has been devoted to study of dynamic behavior in the normal and step change in load condition. Therefore, oscillation in power and frequency occur with periods which are imposed by the source with the largest time constant. These oscillations may be excessive for the operation of the power system. Oscillation and instabilities can be avoided by using an inverter with advanced digital control. Reference: [1] ATLAB Simulink, dynamic system simulation software. The aths Works Inc., [2] S. Roy, O. P. alik and G. S. Hope, An adaptive control scheme for speed control of diesel driven power plants, IEEE Transactions on Energy Conversion, Vol.6., No.4, [3] S. Roy, O. P. alik and G. S. Hope, A k-step predictive scheme for speed control of diesel driven power plants, IEEE Transactions on Industry Applications, Vol. 29, No. 2, [4] B. Kuang, Y. Wang and Y.L. Tan, An H Controller Design for Diesel Engine