A Novel Compensation System for Standalone Wind-Energy Induction Generator Scheme A. M. Sharaf
A. Gastli
Electrical Engineering Department, UAE University, Box 17555-Al-Ain, UAE Email: A.Sharaf @uaeu.ac.ae
Electrical & Electronics Engineering Department Sultan Qaboos University, Box 33, Muscat-123, Oman Email:
[email protected]
Abstract 2 This paper presents a low cost voltage stabilization scheme for standalone wind energy utilization scheme using an induction generator. The voltage stabilization regulator ensures near-stable voltage at load terminals under wind gusting and load excursions. The scheme comprises a fixed large capacitor for self-excitation and a small capacitor controlled by a pulse width modulated control strategy enacted during terminal voltage variations due to wind speed gusting andor local load variations. Simulation results validated the concept of pulse width modulated capacitor switching that extends the voltage stability margin and prevents loss of excitation and voltage collapse.
The wind driven SEIG scheme is shown in Fig. 1 with the added two stage capacitor pulse width switched compensation unit, comprising a fixed capacitor (C,) for self excitation in addition to an "on-off' pulse width switched capacitor C1. The induction generator is modeled in the dqO stationary frame with the same state equations as those of the induction motor. The only difference is the machine terminal voltage model; instead of being applied from a voltage source it is generated by the combination of the machine current and impedance, the self-excitation capacitor impedance and current, and the load impedance and current, as described by the following set of equations [51. (1) V& = x I , =,,z , xI,
Keywords: Voltage stabilization, wind scheme, induction generator, switched capacitor.
1
SEIG System Modeling
,z
v,
Introduction
=[vds
14 = [ih
The emerging need to utilize renewable energy sources (Wind, Solar, PV, Biogas, Hybrid, ...) is motivated by economic and environmental concerns and considerations. The increasing reliance on fossil fuels and the world increasing rate of depletion of these resources is causing focus and shift to energy conservation, fuel replacement and energy displacement of conventional sources to renewable clean, environmentally safe and friendly counterparts. Photovoltaics and wind generation schemes are considered the economic choice for small and medium scales remote area electrical energy generation. This paper focuses on the low-cost wind energy driven self-excited squirrel-cage induction generator in a typical standalone scheme capacity [l-41 usually ranging from 2kw to 500kW. Typical applications include electricity supply to remote (isolated villages, heating, water pumping, ventilation and air conditioning. This paper presents a low cost technique for voltage stabilization of the variable speed wind driven self-excited induction generator (SEIG) while ensuring near-stabilized "allowable" voltage fluctuations.
OIT
(2)
hr" A q r " 3
(3)
vqs
iqs
0
where V ~ ,S V ~ = S dq components of id,
stator voltage, V
, iqs = dq components of stator current, A
dq components of secondary flux, wb Equation (1) can also be written as follows h r " , &r"=
-
Rr + 1s"P 0 -Rr"
0
where
0-7803-5957-7/00/$10.00 Q 2000 IEEE
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0 Rs
P 0
+ 1s"P 0
-Rr"
-Rr" +P M -U)
O
-
~r
Rr', -M+ "P
-
I_;
(4)
and load excursions respectively. Notice that the simulation results validate the concept of PWM-capacitor switching and additional reactive compensation that can extend the voltage stability margin and prevent loss of excietion and voltage collapse. Due to frequent capacitor switching, the interactions of the switched capacitor and the non-linear magnetizing inductance induction generator cause minor and acceptable voltage fluctuations at load terminals. Fig. 6 illustrates this phenomenon, where the mutual inductance and the magnetizing current are plotted against time for the case of voltage control with the same load excursions presented in Fig. 5. These fluctuations are inevitable but can be reduced by reducing the value of the switched capacitor C1 and increasing the value of the fixed selfexcitation capacitor C,.
+
1 hiqr M ies Ls = L M , L r = L + M,irdq= -Lr L,
Note that the load impedance is combined with the excitation capacitance C,. In general the induction generator operates with highly saturated fluxes. Thus, the mutual inductance of the induction generator is not constant but varies nonlinearly with the magnetizing current. This mutual inductance dependency on the magnetizing current is modeled using an Artificial Neural Network (ANN) model [5] having the magnetizing current as input and the corresponding mutual inductance as output. Details of the induction machine, self-excitation capacitor and load impedance are given in appendix. Fig. 2 depicts the single loop proportional plus integral plus derivative (PID) voltage stabilization loop. The controller parameters and settings are given in the appendix.
3
4
Conclusions
The paper presents a simple low cost voltage stabilization technique for standalone wind energy driven squirrel cage induction generator scheme. The scheme is utilizing a simple small pulse width switched capacitor bank to complementhpplement the required fixed selfexcited capacitor larger bank. The software simulation results validate the “on-off’ capacitor switching method as a tool for complementary variable reactive power compensation. The dynamics of the system were also improved
Simulation Results
The full-unified energy scheme was modeled with Matlab/Simulink software and the effectiveness of the proposed pulse width switched control scheme in voltage stabilization was assessed for two cases: a) No-voltage stabilization with only self-excited fixed capacitor C,. b) With voltage stabilization regulator controlling the “on-off’ switching of additional switched capacitor stage ( C , ) in combination with a fixed self-excited capacitor (C,). Figs. 3-a and 3-b show the system dynamic states without the switched capacitor stage CI, and with only the self-excited capacitor C, for both speed and load excursions. Fig. 3-a shows the results of the speed excursion of 35%.Note that for -5% speed decrease, the voltage decreases tremendously causing voltage collapse. For +5% speed increase the voltage goes very high, which can cause damage to the load equipment if they are not conceived to operate under such a continuous high voltage. Fig. 3-b shows the system .response to load excursions of f30%. Note that by increasing the load by +30% (equivalent to decreasing the load impedance 30%), the voltage drops to a’ very low level. Above 30% increase of the load there is a voltage collapse. On the other hand when the load decreases, the voltage rises and may go above the limit of the load equipments. Therefore, the capacitor value should be changed to adjust the level of the voltage within the desired range. Figs. 4 and 5 show the same system dynamic states with the voltage stabilization scheme of Fig. 2, for speed
References K. Natarajan, A.M. Sharaf, S . Sivakumarand and S . Nagnarhan, “Modeling and Control Design for Wind Energy Conversion Scheme using Self-Excited Induction Generator”, IEEE Trans. On E.C., Vol. 2, No. 3, pp. 506-512, Sept. 1987. S.S. Murthy, B.P. Sigh, C. Nagamani and K.V.V. Satyanarayana, “Studies on the use of Conventional Induction Motors as Self-Excited Induction Generators”, IEEE Trans. On E.C., Vol. 3, No. 4, pp. 842-848, Dec. 1987. N.H. Malik and A.G. Al-Bhraini, “Influence of the Terminal Capacitor on the Performance Characteristics of Self-Excited Induction Generator”, IEE, Proc. C, 1990,137, pp. 168-173. S.P. Singh, Bhim Singh and M.P. Jab, “Performance Characteristics and Optimum Utilization of a Cage Machine as a Capacitor excited Induction Generator”, IEEE Trans. On E.C., Vol. 5, No. 4, pp. 679-685, Dec. 1990.
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[5] A. Gastli, M. Akherraz, M. Gammal, “Matlab/Simulink/A” Based Modeling and . Simulation of A Stand-Alone Self-Excited Induction Generator”, Proc. of the International Conference on Communication, Computer and Power, ICCCP’98, Dec. 7-10 1998, Muscat, Sultanate of Oman, pp. 9398.
w1=
]
’
0.0710 = [-0.65601
Controller Parameters Kp=5 : proportional gain KI=O. 1 : integral gain KD= Kl/lOOO : derivative gain K,= 1ms/lOV=O.ImsN Tdw=O. 1sec : threshold
Induction Machine data & Mutual Inductance Model
.
-0.1176 0.5232
w Z= [0.1881 0.045631 , b, = 0.17249
Appendix
Phases P=4 fo=SOHz Rs=1.37R Rp3.39Q Xs=4.18 R XI= x, M=O. 173H I=1OA v=220v C,,,=65pF
[
&V: reference phase voltage
: 3, Y-connected : number of poles : rated primary frequency
V,,=220/ C1=35pF
: stator resistance : rotor resistance : stator leakage reactance : rotor leakage reactance : maximum mutual inductance : rated line current : rated line voltage : self-excitation capacitor
: controlled capacitor
Load Parameters The load is described by its impedance (2,) and power factor ( P o . This normally includes the line impedance also. For a light load the following impedance and power factor are taken: ZL=19OQ and PF=O.8. Simulation 81Disturbance Parameters
The mutual inductance is modeled with an Artificial Neural Network (ANN) n-”k [61, which Can Simulate the actual Variation Of the mutual inductance M as a function of induction machine’s magnetizing current. The ANN model is made of a two Neuron layers: TanSigmoidal and Linear Neuron layers. The weights and biases of the Neuron layers are:
The system is first run at no-load. At 1Jsec a light load is connected to the generator terminals. All plus dismbances at 2sec and finish at 3sec and all minus at 3sec and finish at 4sec. The total disturbances simulation h e is fixed to 5sec. The capacitors C,,, and C1 where chosen such that at no-load there is a voltage buildup and at light load the system doesn’t collapse. IG
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odoff
_ -
-
Turbine Self-excitation capacitor
:a::&+
-
Fig. 1 Proposed self-excited standalone wind energy induction generator scheme
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f5% speed excursion
f30% load excursion
Time (a) wind gusting I
I
I
I
I
l
0.5
1
1.5
0
I
I
I
I
lexcuisioni
2.5
2 ,
+30% load excursion I
II 1
3
3.5
I
I
I
4
4.5
5
Time
Fig. 5 Results of load +30% excursion with PWM voltage control (Cm=65pF,c1=35@)
I
4.5
4
z
3.5
e
3
i?
5 2.5 P :B
2
0
m
1.5
'0
1
0.5
1.5
2
2.5
3
3.5
4
1
4.5
Time
0.5
(b) load excursions 0
Fig. 3 Results of wind velocity and load disturbances without PWM voltage control (Cm=75pF) 1.4
1
1
1
f5% speed excursion Lidhtloh 'i l
1
l
i l l
1
0.8
0
I
,
0.5
1
'
1.5
2
2.5
3
I
3.5
I
4
i
4.5
2 3 4 Time, [sec]
5
0
1
2 3 Time, [sec]
4
5
Fig. 6 Variation of the mutual inductance and magnetizing current during voltage control with load excursion (Fig. 5)
I .2
ov
1
I
5
Time
Fig. 4 Results of 3 5 % speed excursion with PWM voltage control (Cm=65pF,c1=35pF)
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