Conversion System Using Doubly Fed Induction ... Department of Electrical and Computer Engineering .... frequency using full power electronic converter. The.
Variable Speed Constant Frequency Diesel Power Conversion System Using Doubly Fed Induction Generator (DFIG) Tajuddin Waris , C.V.Nayar Curtin University of Technology Department of Electrical and Computer Engineering Bentley Western Australia. Tel: +61-08-92661782
Most of the diesel generators (DG) set used in remote areas runs at constant speed to produce ac output power with constant voltage and constant frequency. In remote areas the peak demand typically occurs only for a few hours in the evening. Therefore the DG set has to be heavily over-sized in relation to the prevalent load conditions. However prolonged light load operation results in poor fuel efficiency and excessive maintenance problems. In this paper, a variable speed diesel power generation system using a DFIG is presented. The control strategy for the diesel engine enables it to operate below and above the synchronous speed according to load demand. Simulation results of the dynamic system and preliminary experimental results based on a 15kW prototype are included.
Index Terms—Diesel engines, doubly fed induction generator (DFIG), remote area power generation. I.
INTRODUCTION
The DG set is commonly used at remote areas to provide electricity. Traditional generating technology requires the DG set to operate at a fixed speed so that it can produce the 50Hz or 60Hz power expected by the consumers. Remote areas with relatively small communities generally show significant variation between the daytime peak loads and the minimum nighttime loads. An example profile is shown in Fig.1 Load profile 160 140
Load (kW)
120 100 80
The conventional approach for sizing the DG set is carried out by selecting the DG set based on the peak load. Therefore, the DG set has to be oversized according to the prevalent load conditions. Operation of the DG set at an average load as low as 30 % of the full capacity is common. However for light load operation, the fuel economy is poor due to the fact that not all fuel is burnt in combustion process. The unburnt fuel dilutes the oil in the cylinders and causes excessive wear in the cylinder walls, cylinder glazing and carbon build-up. These harmful and destructive conditions inflict severe deterioration to the engine performance and premature engine failure. To prevent this condition, manufactures insist that constant speed DG set operate with load above 50 %. [1]-[2]. In order to overcome the problems caused by lightload some schemes have been proposed. In large system, multiple generators sets running at constant speed which are brought on line or shut down as the load demand, so as to operate the DG set with high capacity all the time. However, in smaller systems, multiple DG set is not practical or economical. Conventional DG sets hybrid energy system which combines renewable energy sources is commonly applied in remote area. Unfortunately, the introduction of renewable energy sources also results in even longer period’s low load of the conventional DG set. Recently, variable speed diesel generator sets have been developed where the engine speed is adjusted to match the engine power output to the load power demand. The system uses a Permanent Magnet Generator driven by an IC engine. This system applies a full converter to realize variable speed generation. This paper describes modeling and simulation of a diesel engine driven wound rotor induction generator. Some preliminary results on a 15kW prototype laboratory system are also included.
60 40
A.
Variable Speed Diesel Generator Diesel
20 0 0:00
4:00
8:00
12:00
16:00
20:00
0:00
Time of Day
Figure 1.
Typical load pattern for remote area power supplies
978-1-4244-1668-4/08/$25.00 ©2008 IEEE
A number of sources reported that variable speed diesel generator is more efficient than constant speed diesel generator. In isolated power system, the fuel consumption can be reduced by up to 40 % in comparison with the constant speed diesel power generation, especially when the DG operates to low electrical loads. [3]-[4].
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Throttle Angle
θr
Raw Engine Torque TL Load Torque Engine Speed Diesel Engine and Torque Map _ DFIG Inertia To 1 Engine Speed +
(J
_ Td
e
)
15 kW Variable Speed Diesel Generator Speed - Load Map
P load (kW)
The minimum load for constant speed diesel engine is about 40%, this value is approximately 23 % for variable speed diesel engine. Variable speed operation enables the diesel engine to be more effectively utilized over its entire range operation. The main advantage of the operation of engine at variable speed is the ability to reduce fuel consumption and to generate more power from diesel engine without exceeding its rated torque. The best fuel consumption by considering the restriction such as rated torque and engine speed can be achieved when the DG set operates at or near to its rated torque [5]. Recently variable speed generator sets have been developed where the engine speed is adjusted to match the engine power output to the load power demand. There are two categories of variable speed DG sets have been developed. The first category is a variable speed constant frequency using full power electronic converter. The configuration is simple but applies expensive power electronic converter because of the converter have to be sized at least the same rating of the generator. The system uses a permanent magnet generator driven an IC engine [2]-[6]. The second category is a variable speed constant frequency generation system which uses a DFIG and reduced rating power converter. The power converter used is sized to only a fraction of the total generator rating, typically at 30% the machine rated power. DFIG has long been considered as a good choice for variable speed generation system both wind and diesel power generation. DFIG controlled from the rotor circuit for variable speed constant frequency is applied due to some advantages as follows [7],[8],[9]: • Easier generator torque control using rotor current control • Smaller generator capacity as the generated power can be accessed from the stator as well as from the rotor. Rotor output power is proportional to the slip speed. • Smaller capacity for the power electronics converter as converter in the rotor only controls the current in the rotor winding. This enables the control of the whole generator output, using the power electronics converter rated at 20 – 30 % of the nominal generator power. • Fewer harmonics because control is in the rotor while the stator is directly connected to the grid.
+
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1400
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Figure 3. Optimal rotational speed versus load curve.
B. Diesel Engine The diesel engine is a complex device involving many non linear factors ٛ affecting its performance. For the study of the dynamics performances of the main system, a simplified model for the diesel engine may be used in simulation. A simplified model of the diesel engine is presented in Fig. 2. [2] - [6] . In order to reduce the fuel consumption, the diesel engine has to be operated based on its fuel efficiency map. It can be provided by experimental analysis and extracting the fuel efficiency power curve. The Maximum fuel efficiency map for 15 kW variable speed diesel DG set used in this study as presented in Fig.3. In the experimental setup, the dc machine is applied to simulate the engine diesel performances. II.
MODELLING AND CONTROL OF THE DFIG.
A large number of papers describe the modeling of the DFIG. Only the most important aspect will be presented. Generally, a d-q reference frame is chosen for DFIG modeling. When modeling of DFIG in the generator convention, the current output and the real power and reactive power have a positive sign when they are fed into the load or grid. Using this convention, the following set of equation results [10]-[11]-[12]. .
ωr
+ Jg s
v ds = − R s i ds − ω s λ qs +
v qs = − Rs i qs + ω s λ ds +
dλ ds dt
dλ qs
dt dλ v dr = − Rr idr − ω r λ qr + dr dt dλ qr v qr = − Rr iqr + ω r λ dr + dt
(1)
Where:
ωB1 + ω2 B2
λds = Ls i ds + Lm i dr = Lm ims
+ Bo
600
Speed (rpm)
Drag and Windage
Drag Torque
17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 500
λ qs = Ls i qs + Lm iqr Friction
λ dr = Lr idr + Lm i ds λ qr = Lr iqr + Lm i qs
Figure 2. Simplified Diesel Engine model
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(2)
With v being the voltage, R is the resistance, i is the current, ωs and ωr are the stator and the rotor electrical angular velocity respectively, and λ is the flux linkage. The d and q the direct and quadrature axis components of the reference frame, s and r indicates stator and rotor quantities. All quantities are a function of time. The electromechanical torque is given by.
Tem
L = − n p m (i dr λ qs − iqr λ ds ) . Ls
(3)
The stator and rotor power equation are.
Ps = v ds ids + v qs i qs Qs = v qs ids − v ds iqs
(4)
Pr = v dr idr − v dr i qr Qr = v qr i dr − v dr iqr The mechanical power and stator power output are determined as follows:
Pm = Tmω r
(5)
Ps = Temω s For lossless generator, the mechanical equation can be simplified as follows.
J
dω r = Tm − Tem dt
(6)
In steady state at fixed speed,
dω r = 0 . It follows that. dt
⎛ ω − ωr Pr = Pm − Ps = −Tm ⎜⎜ s ⎝ ωs Pr = − sPs Where,
⎞ ⎟⎟ω s ⎠
(7)
⎛ ω − ωr ⎞ ⎟⎟ s = slip = ⎜⎜ s ω s ⎠ ⎝
Ps − ref = −v s Qs − ref =
Ps-ref
v ds = 0
v s λ s v s Lm − idr −ref Ls Ls
Pmq
irq-ref +
λ qs = 0
_
+
Ps
Hence the electromagnetic torque and the active power will be only determined by the q – axis rotor current. The stator resistance is small that can be neglected. The stator voltage vector is consequently leading in comparison with the stator flux factor. It means that.
(8)
The delivered active and reactive powers from the stator directly depend on the quadrature rotor current and direct rotor current respectively. The aim of the grid side converter (GSC) is to regulate the dc link voltage regardless the direction of the power flow. The converter current is controlled with the conventional vector control approach with a d-q reference frame oriented along the stator voltage vector position. The reference frame orientation angel is derived from the stator flux vector position using PLL. The control schematic for rotor side converter and grid side converter is depicted in Fig 4 and Fig 5. The reactive power component reference current could be set to zero for unity power factor operation. For decoupled control of active and reactive power, the instantaneous position of the rotor with respect to the stator is required. In conventional field oriental scheme, this is derived from an incremental or absolute encoder fitted to the machine shaft. In this system proposed, a position sensor-less method is applied for updating the sector information [13]. In a stand alone generation system, the quality of output power has to be considered. In fact, the presence on non linear load connected to the utility line can not be avoided. In this study, a control scheme is proposed where the concept of active filter can be incorporated into the GSC to address the power quality problems. For this purposed, appropriate current references must be determined for simultaneously generating powers and compensating non linear harmonics. For the harmonic extraction, the synchronous reference frame (SRF) approach will be applied [14].
By setting the stator flux vector aligned with d-axis result in.
λ ds = λ s
Lm i qr −ref Ls
irq ird-ref
Qs-ref + Qs
_
_
Pmd _
+ ird
v qs = v s
The coupling control of the stator active and reactive power can be realized by using a d-q reference frame attached to the stator flux. The stator active and reactive power references can be expressed in rotor currents as follow [11].
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Figure 4.
The schematic block diagram of the control structure of the Rotor Side Converter
Vdc-ref
Pmq
isq-ref
+
_
Vdc
+
_
isq
isd-ref +
Pmd
_
isd Figure 5.
The schematic block diagram of the control structure of the grid side converter Figure 6.
III.
Variable Speed Diesel power generation with DFIG
PROPOSED VARIBLE SPEED DIESEL GENERATOR
The proposed system prototype as presented in Fig. 6 consists of 15 kW DFIG, back to back IGBT–based PWM three phase voltage inverters with a common dc link, 20 hp diesel engine incorporated with microcontroller based governor and three phase transformer. The stator is directly connected to the constant frequency three-phase grid and the rotor is supplied by back to back IGBT–based PWM three-phase voltage inverters with a common dc link. Diesel engine is equipped with governor microcontroller based. A DSP based digital control board is employed to implement the control algorithm. The requirement for fast real-time control requires software that should be efficient in terms of execution time. This requires the implementation of Assembly Language for the DSP programming. The objective of the front end converter is to keep the dc link voltage constant regardless the magnitude and the direction of the rotor power. Unlike rotor side converter, this operates at the grid frequency. A vector control approach is used for independent control of the active and reactive power. Active and reactive power flow is controlled by adjusting the phase and amplitude of the inverter terminal voltage with respect to the grid voltage. By controlling the flow of active power, the dc bus voltage is regulated within a small band. Control of reactive power enables unity power factor operating at the GSC. In fact, the GSC can be operated at a leading power factor, if it is desired. It should be notice that the slip range is limited hence the output voltage level of the GSC is less in comparison with the stator voltage. A transformer is therefore necessary to match the voltage level between the grid and the GSC. For decoupled control of active and reactive power, the instantaneous position of the rotor with respect to the stator needs to be determined. In this study, a simple position sensor-less method for updating the rotor position will be applied as used in [13]. With a PWM converter at the rotor circuit of the DFIG, the rotor current can be controlled for the desired phase, frequency and magnitude. This enables bidirectional power flow in the rotor and the system may be operated at sub-synchronous and super-synchronous speed. .
Using vector control techniques, the active and reactive powers can be controlled independently and hence fast dynamic performance can be achieved. In order to improve the quality out put power, in this scheme the GSC can also be operated as a shunt active filter with schema algorithm as discussed in [14] The system proposed is designed to perform two operation modes either grid connected or stand alone variable speed power generation. For the grid connected mode, the frequency and magnitude voltage are maintained by the grid and the reactive power for magnetizing is available from the grid. However, for stand alone operation mode, the DG set must regulate the voltage and the frequency independently for all load condition by controlling the rotor speed and converter. For stand alone operation mode, initial reactive power needed for magnetizing the machine have to from RSC. But initially the link capacitor has no charge. In the system proposed initial reactive power for starting process is provided by the battery bank connected to the dc link via bidirectional converter. Battery bank with bidirectional converter and the RSC form the grid. Bidirectional converter connected enables to charge the battery. Moreover, the battery bank with bidirectional converter attached to the dc link can be acted as a buffer source to provide the good performance in maintaining a steady dc link voltage during extremely load changing. IV.
SIMULATION RESULTS
In order to illustrate the performance of the proposed system, the simulations have been performed using Matlab/Simulink. The system is simulated for sub synchronous and over synchronous speed range. Fig.7 presents the rotor speed response of the DG set in two mode operation both sub synchronous and over synchronous as response toward the load changing. Active powers from the stator are presented Fig.8. During the simulation stator active power Ps is positive. At sub synchronous speed stator sources both the load and the rotor converter. Meanwhile, at over synchronous speed stator and rotor fed the load. The rotor active power Pr changing during the simulation time as presented in Fig.9. At super synchronous mode, Pr is positive. It means that
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Pr from the rotor is injected to the load. Whereas, at sub synchronous mode Pr is negative indicates Pr is drawn from the stator side. Pr is zero at synchronous speed operation. Fig.10 presents Ps and net active power Pg. At sub synchronous speed Pg is less than Pr as negative Pr. Whereas, at over synchronous speed Pg is higher than Ps as power contribution for the net grid comes from the stator and rotor.
Fig.11 presents the result simulation of the dynamic reactive power. In this case Qs is set to zero for unity power factor at stator. Rotor reactive power fluctuates according to the mode operation. The amount of reactive power that can be injected into the grid depends on the available dc bus voltage and the value of inductance in the ac side. In this case the exchange processes the reactive power only at the rotor side. 1.45
1.2
1.25
1.15
1.05
1.1
ro to r s p e e d (p u )
Ps Pg
G rid a c tiv e p o w e r (p u
1.25
S ta to r a c tiv e p o w e r (p u )
1.05 0.85
1
0.95
0.65
0.9 0.85
0.45
15
10
5
0.8 10
5
0
15
20 time
40
35
30
25
20
time (second)
40
35
30
25
Figure 10.
Simulated response of net grid active power and stator active power
Figure 7. Simulated rotor speed response at sub-synchronous and oversynchronous speed 0.25 1.2
Qr Qs
0.2
1.15
0.15 (p u )
S ta to r a c tiv e p o w e r (p u )
1.1
R e a c t iv e p o w e r
1.05 1
0.95
0.1 0.05 0
-0.05
0.9
0.85
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time (second)
time (second)
Figure 8.
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25
Figure 11.
Simulated response of stator reactive power and rotor reactive power
Simulated response of stator active power 1.005
0.3
1.004 1.003
0.25
1.002
R o to r a c tiv e p o w e r (p u )
0.2
1.001
0.15 1
0.1 0.999
0.05
0.998 0.997
0
0.996
-0.05 0.995
-0.1
5
5
10
15
20
25
30
35
40
10
15
25 20 time (second)
30
35
time (second)
Figure 9.
Figure 12. Simulated response of the dc link voltage
Simulated response of rotor active power
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40
Fig. 12 illustrates the dynamic of the dc link voltage. The GSC has to maintain the dc link voltages constant during the operation. For over synchronous speed Pr is injected to the dc bus capacitor and tends to increase the dc voltage. Whereas, at over synchronous speed Pr is taken out from the dc bus capacitor and tends to decrease the dc link voltage. A change in dc link voltage can be attributed to an imbalance the active power between the ac and the dc side. V.
EXPERIMENTAL RESULTS
The laboratory setup consists of a 15 kW slip ring induction machine with its stator connected to the 415 (Volt), 50 (Hz), 3 phase load and the rotor being fed by back-to-back IGBT-based PWM converter. In order to demonstrate the application of such a system to variable speed diesel power generation, the diesel engine torque – speed characteristics is simulated by a 20 hp dc motor driven by a commercial four-quadrant thyristor drive. Some experimental results are presented for investigating the system performance of the system proposed. Fig.13 and Fig. 14 present the load current wave form and the voltage wave form for difference load. The magnitude and the frequency of the voltage load are constant for during the load changing.
Figure 15.
The out put voltage wave form and current wave form of the GSC around synchronous speed.
Igc
Figure 16.
Figure 13. Experimental result showing the load voltage and the load current. Vl = 383 (V) Il = 6 (A) R l = 120 (Ω)
Figure 14. The load voltage wave form and the load current wave form. VL = 383 (V) IL =17.5 (A) Rl = 39 (Ω)
Vgc
The output voltage wave form and the current output wave form of the GSC during sub synchronous speed.
Fig.15 shows the waveform during regeneration. Initially it was regenerating. The current out of phase with phase voltage and later synchronous speed the current in phase with the voltage it means that current is from main to the converter. Fig. 16 shows in detail the relationship between the voltage and the current wave of the GSC. Note that, during the regeneration the magnitude and frequency of the GSC is constant. Photographs of the prototype generator coupled to a variable speed motor, prototype of complete power conditioning unit and the laboratory set up are shown in Fig.17. VI. CONCLUSION In this paper, the control strategy of variable speed diesel power generation with sensor-less and active filter function has been proposed. The generation system use DFIG with corresponding back to back PWM inverter. The control strategy for the diesel engine enables to operate the system below and above the synchronous speed according to load demand. Simulation results of the dynamic system proposed has been presented. The experimental results have verified that the output voltage regulation is very good
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ACKNOWLEDGMENT The first author acknowledges Hasanuddin University for providing scholarship to pursue PhD studies in The Department of Electrical and Computing Curtin University of Technology. The second author would like to thank the Australian Research Council for funding this project and the industry partners ( Leonics Ltd, Daily Life Renewable Energy , Integrated Electric Company and Regen Power Pty Ltd) for their input. He acknowledges the contribution by Professor VT Ranganathan, Indian Institute of Science in the development of the prototype. REFERENCES [1]
[2] [3]
[4]
[5] [6]
[7]
[8]
[9]
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[11]
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[13]
[14]
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Figure 17. Photograph of the prototype generator, power conditioning unit and the laboratory setup APPENDIX SYSTEM RATING
Pnom = P= base 15 (kW) Vnom= Vbase=415 (V) p =4 Rs= 0.25 (Ω) Rr= 0.25 (Ω) Xls= 0.28 (Ω) Xlr= 0.71 (Ω) Xlm= 17.5 (Ω) f =fbase 50 (Hz)
