Load Frequency Control of a Hybrid Renewable Power System with Fuel Cell System Swati Rawat Department of Electrical and Electronics Engineering Graphic Era University Dehradun, India Email id:
[email protected]
Shailendra Singh
Kshitij Gaur
Department of Electrical and Electronics Engineering Graphic Era University Dehradun, India
[email protected]
Abstract— This paper proposes the frequency control scheme for a stand-alone hybrid power system in presence of different controllers. The hybrid isolated power system consists photovoltaic (PV) system, micro hydro power system (MHP) , diesel engine generator (DEG) and fuel cell system. Assuming that power produced from PV and MHP system remains constant during a load disturbance. A case study on the impact of FC operation in the frequency control of the integrated renewable power system has been carried out with PI and PID controllers. The hybrid system has been analyzed in MATLAB /Simulink environment with step load demand. A comparative evaluation of frequency deviation for proposed hybrid system in presence of different controlling techniques reflects the improvement in frequency stability in the presence of PID controller as compared to PI controller. Keywords- Hybrid Power System, Fuel cells System (FC); Frequency deviation, Photovoltiac, PI and PID controllers
I. INTRODUCTION For social and economical development of any country electrical energy is playing very important role . At present stage one third area of the world’s population stil facing the shortage of electricity. In such conditions, an alternative is required which can provide guarantee of cost-effectiveness and sustainablilty of energy resources. By implementation of renewable energy resources (e.g. PV, wind, hydro etc) shortage of power can be reduced. [1]. Renewable energy technologies are widely implemented in distributed generation (DG), on-site generation and grid connected power generation system. DGs are generally stalled near to utility points. Solar light directly converted into electricity using photovoltaic system [2]. The opportunity of using an isolated power system, which contains solar and MHP for low-priced electricity generation can be meet the load demand of the rural area is discussed in [3]. The research study in ref. [4] gives an ideal about electric energy efficient function and management strategy for operation of FC system and MHP system.The installation, design and practical performance analysis of hydro power system has been presented in ref. [5]. The dynamic behaviour of a hybrid system of photovoltaic (PV) s with MHP system has been discussed in ref [6]. The all important purpose of an electrical power system is to meet up the power requirement of consumers. The chief drive of
978-1-4799-6042-2/14/$31.00 ©2014 IEEE
Department of Electrical and Electronics Engineering Graphic Era University Dehradun,India
[email protected]
energy crisis in power sector is loaded mismatch. There is variance in frequency because of load fluctuation. The energy storage system provides transient stability through load deviation [7-8]. The distributed generation and their valuable advantages have reported in ref. [9], [10]. DGs are getting popularity in modern senerio due to their salient features such high fuel effiency, modular installation, less time required in construction etc. Authors of ref. [11], [12], and [13], have been focused their work on the optimal operating strategy, considerations concerned with the implementation of DGs into the distribution system. Ref.[14],[15] have proposed an operation and control policy of distributed generation in deregulated markets. Ref.[16]’ authors have focused on the potential of distributed generation to offer additional inspection and repairs such as voltage regulation, frequency regulation and reactive power control, etc. In ref. [17] has extended the small-signal stability analysis of an isolated hybrid power generation with energy storage device connected to isolated loads has been discussed in both cases first in time-domain and other in frequency domain. Hybrid operation of wind, PV, diesel and fuel cell power in remote area village power supply have presented in ref. [18], [19]. Ref. [20] described a novel control approach for active power flow in a hybrid distributed generation system. Ref.[21] has discussed the dynamics of a tiny, isolated system, containing a DEG and a wind power. The issue has demonstrated in both domains assuming a simplified model for mechanism and considering the effect of speed governor in the diesel enginepitch angle controller in wind turbine. x The Dynamic modeling of grid connected FC system has been presented in ref. [22], [23]. In this paper a hybrid renewable power system with FC (Fuel cell system) has been modeled. Contol of frequency fluctunation with respect to changes in load and intermittence nature of renewbles have been presented. Frequency of hybrid system has been controlled by using of different controller schemes. Hydrogen produced from electrolyzer used as fuel for fuel cell. This process is according to operational strategy of the monitoring arrangement. PEM (photon exchange membrane) fuel cell is practiced as an energy storage scheme.
II. SYSTEM DESCRIPTION AND MODELING TABLE I. SPECIFICATION OF THE HYBRID
this power into electrical energy. A micro-hydropower system (MHPS) has a generating capability of less than 100 kW [24].
RENEWABLE POWER SYSTEM KFC = 0.03, and TFC = 3s KDEG = 5 , and TDEG = 5s, R =.5 KE = 0.1,andTE = 0.05s M = 10, and D = 0.8
System power rating =1 MW Fuel cell power = 10 KW, DEG. Power = 30 KW, Peak PV Power = 50 KW, MHP Power = 100KW Electrolyzer Power = 10KW H2 max. volume = 500Nm3, H2 min. Volume = 60 Nm3, Intial H2 Vol. = 250Nm3 Ș ൌ 85%, Șେ ൌ 50% , HHV =3.509 Kwh/Nm3, Frequency 50 Hz
TABLE II. PARAMETERS AND RATINGS FOR MICRO HYDRO POWER SYSTEM Turbine
Francis
Selected design Head
28 m.
Rated Capacity
40-110%
Rated Power
100KW
DŝĐƌŽ,LJĚƌŽWůĂŶƚ
>ŽĂĚ
(1)
PR = 9.8H Q R Ș T Ș G
Where PR =Rated power of Generator (KW), Q R =Rated WsƌƌĂLJ
ŝĞƐĞů ŶŐŝŶĞ 'ĞŶĞƌĂƚŽƌ
ŽŽƐƚ ŽŶǀĞƌƚĞƌ
capacity in % (discharge), ȘT = % Efficiency of Turbine, ȘG =% Efficiency of generator.
ŝĚŝƌĞĐƚŝŽŶĂů ĐŽŶǀĞƌƚĞƌ ŽŽƐƚŽŶǀĞƌƚĞƌ
DŽŶŝƚŽƌŝŶŐ ^LJƐƚĞŵ
&ƵĞůĞůů
,ĞĂƚ>ŽĂĚ
ƵĐŬŽŶǀĞƌƚĞƌ
ůĞĐƚƌŽůLJnjĞƌ
,LJĚƌŽŐĞŶ^ƚŽƌĂŐĞ
KdžLJŐĞŶ
Fig.1.Stand-alone hybrid power system with energy storage as fuel cell system
The system consists PV, micro hydro plant (MHP) and diesel engine generator (DEG) as primary energy sources. The surplus energy with respect to the load requirement has been stored in hydrogen tank in form of hydrogen energy. A 10-kW electrolyzer has used to produce hydrogen from water and utilized it as a fuel in 10-kW fuel cell (FC) system to produce electrical energy. The electrolyzer and the FC system are major elements of the RE system for energy storage as and its re-use. The RE system components are connected through power converter devices. Power flow between load , input sources and storage device is controlled through power converter devices as shown in fig.1.Electrolyzer is connected to DC bus through a DC-DC buck converter similarly FC system is also connected to DC bus via boost converter respectively. Monitoring system conveys power conditioning signals for proper on-off function of fuel mobile phone arrangement. A. Micro Hydro Model A micro hydro system converts the potential energy of water into electricity by using the flow of water. This water flows in streams with different slopes giving rise to different potential for creating heads, which vary from river to river. Head and flow rate (water discharge) are the primary component which bears upon the capability of power. Electric generator converts
Fig.2.Turbine performance curve
B. PV Model A combination of more than one solar panel creats a PV system. It consists of several parts which include the photovoltaic modules, mechanical and electrical connections and. Photovoltaic systems are useful in water pumping, communication system and low power appliances in remote regions. The equivalent circuit for the solar cell consists of diode and current source connected in parallel as shown in figure 3. The equation of ideal solar cell can be represented as follows ൌ െ ୖ ቂ ቀ ቁ Ȃ ͳቃ (2)
Where: “IL ” is photocurrent (A); “IR ” is reverse saturation current (A); “V” is diode voltage (V); “Vi” is thermal voltage, “A” is diode ideality factor. Modified voltage-current characteristic equation of solar cell is given as: ൌ െ ୖ ቂ ቀ
ା୍ୖ
ቁ Ȃ ͳቃ െ
ା୍ୖ ୖK
(3)
Where RSe and RSh are series and parallel resistances connected in circuit.
Pe= Change in power consumption by electrolyzer, VH= Net H2 volume stored in tank,ୌ is initial H2 volume of the tank. η e = Efficiency of electrolyzer, η F =Efficiency of fuel cell, HHV=Higher heating value of H2. [25]
Fig.3.Equivalent circuit diagram of PV model
The generated output voltage from a PV cell is very low (around 0.5V). So to get desired output voltage and current many PV cells are connected in series and parallel combination. This combined form of PV cells is known as PV module. For high voltage several PV cells are connected in series and for high current in parallel. Usually there are of 36 or 72 cells in general PV modules. Current-voltage characteristic equation of equivalent circuit for a PV module arranged in series Ns and parallel Np can be described as: ൌ ୮ െ ୮ ୖ ቂ ቀ
ൗ౩ ା୍ ൗ౩
ቁቃ െ
൫౦ Τ ౩൯ ା୍ ୖ ୖK
(4) Where “ Np ” is cells parallel number; “ NS ” is cells series number. C .Diesel Engine Generator Model The transfer function equation of diesel power generation expressed by § 1 K · § 1 · § K deg · Pd = − ¨ + I ¸ ¨ (5) ¸¨ ¸ Δf e s ¹ ¨© Tsg s + 1 ¹¸ ¨© Tdeg + 1 ¹¸ ©R R is speed regulation. DEG automatically starts up with proper control. D. Electrolytic Hydrogen and FC System Operation There are many types of fuel cell. The proton exchange membrane fuel cell (PEMFC) can operate at air temperature, allowing rapid start-up. Its power density is high. The efficiency of PEM electrolyzers have less than other electrolyzers. The PEM fuel cell is now their low operating life span systems which are the primary drawback of such type of energy storage scheme. The electrolyzer stores extra energy in form of electrolytic hydrogen.
ηF =
PF
(6)
•
V F HHV •
ηe =
V e HHV Pe •
(7)
ELECTROLYZER
FUEL CELL
ǻVE
HYDROGEN STORAGE ǻVFC
Fig. 4.Operation of fuel cell system connected to hybrid power system
1) Transfer Function Equation of Fuel Cells Equation 9 shows The transfer function between system frequency variation and fuel cell power . οେ ൌ
ూి ଵାୱూి
Ǥο
•
•
(8)
Where Ve =Net change in H2 volume due to processing of electrolyzer, VF =Change in H2 volume due to process of fuel cell, PF= Change in fuel cell power,
(9)
2) Transfer Function Equation of Electrolyzer A part of surplus power of the system is sent to the electrolyzer to generate hydrogen. The transfer function of the electrolyzer is expressed byు ο ൌ ଵାୱ Ǥο (10) ు
E. Controlling Techniques 1) PI Controller The actuating signal in PI controller is the combination of proportional error signal and integral of the error signal. Therefore laplace transform of actuating signal represented as:
G(s) = KP +
KI s
(11)
Where KP, KI is the proportional gain and integral coefficient respectively. The optimal parameters of PI controller are determined by trial and error method such that error in frequency variation ¨F is minimum. KP=0.267, KI=0.001, these are parameters of a controller which is connected with power system transfer function. 2) PID Controller Transfer function of PID controller in the continuous time domain is represent as :
G(s) = KP + KD s +
VH = VH0 + V e − V F
•
Power generated by fuel cell
Power consumed by electrolyzer
KI s
(12)
Where KP, KI and KD is the proportional gain, integral coefficient amd derivative coefficient respectively. 3) Tuning of PID Controller PID controller is tuned through ziegler-nichols tuning technique.
TABLE III.ZIEGLER-NICHOLS TUNING FORMULA FOR CONTROLLER AND OBTAINED VALUES AFTER TUNING Controlling Parameters Z-N Criteria Obtained Parameters
KP
KI
KD
0.6KU
PU/2
PU/8
10.56
5.93
1.48
PID
The numerical value of KP that produces continuous cycling (for proportional only control) is called the ultimate gain, KU. The period of the corresponding sustained oscillation is referred to as ultimate period, PU. F. Power and Frequency Deviation οୗ ൌ ο െ οୈ (13) Where ¨PS is net power deviation. The system frequency variation ¨F is calculated by ο ܨൌ ುೄ οܲௌ (14) ଵା்ುೄ
Since an inherent time delay exists between system frequencies so the transfer function for system frequency variation to per unit power deviation can be expressed by ଵ ȟ ൌ Ǥȟܲௌ (15) ୈାୱ
ଵ
ൌ
ୗ
IV.SIMULATION RESULTS & ANALYSIS
ǡ ൌ ౌ
ౌ
Where M and D are, respectively, the equivalent inertia constant a damping constant in per unit of the hybrid power system. Consider 1 MW power generations as base value of the complete system. III. OPERATION STRATEGY
KE 1 + sTE
1 D + sM
VH 0
Simulation of system models have been carried for steady state stability analysis using step load response of system model for 40 seconds. The output of PV and MHP has taken as constants in per unit. Results with PI and PID controllers have analyzed and compared in steady state analysis. Results of simulation indicate that FC system can improve frequency stability of the system when the load raises and when the load drops. System model has been analyzed in Mat lab / Simulink software. A) During Peak Load
K DEG 1 + sTDEG
ηE H H Vs
1 H H V sη F C
Operation of plant model can be represented by a flow diagram shown in figure 6. Frequency of Power system is reciprocal to load demand. For the stability of system, frequency variation should be zero. A control scheme is developed to extract more energy from the RE sources to the electrolyzer. It utilizes the extra generated energy. Fuel cell compares its hydrogen volume of tank before operating. If it is less than its maximum volume, then electrolyzer will run and produce H2 and O2 that are stored in their tanks. This H2 is used in fuel cell. It produces power only of volume of hydrogen present in it exceeds minimum volume limit. Fuel cell does not supply the power and thereby the total generated power reduces, when frequency variation is positive. Before running the electrolyzer, hydrogen volume of tank has compared to its maximum hydrogen volume and then excess power will supply to electrolyzer. If hydrogen volume is at maximum level in electrolyzer it will not work. The monitor system sends the conditioned signal (duty ratio) for on/off operation of the electrolyzer and the FC. These conditioned signals are according the hydrogen volume level at the storage tank and pre-defined limits of volume in tank.
50.2
ΔF
−ΔPL
Without Controller With PID Controller With PI Controller
50.1
50
K FC 1 + sT FC
Frequency(Hz)
49.9
Fig.5. Mathematical block diagram of proposed hybrid system
49.8
49.7
49.6
49.5
0
5
10
15
20
25
Time(Sec)
Fig.7. Variation in frequency
Fig.6. Flow chart of methodology
30
35
40
B) During off Peak Load 0.01 50.6
0.009
50.5
0.007 50.4
0.006
Frequency(Hz)
Change in Fuel Cell Power(p.u.)
0.008
With PI Controller With PID Controller Without Controller
0.005
0.004
0.003
50.3
50.2
50.1
0.002 50
0.001
0
0
5
10
15
20
25
30
35
0
40
5
10
15
20
25
30
35
40
30
35
40
35
40
Time(Sec)
Time(Sec)
Fig.11.Variation in frequency
Fig.8. Variation in fuel cell power 250 0.01
0.009
249.98
Ch ang e in Electrolyzer p ower(p .u.)
0.008
249.94
249.92
0.007
0.006
0.005
0.004
0.003
249.9 0.002
0.001
249.88
0
249.86
0
5
10
15
20
25
Time(Sec) 0
5
10
15
20
25
30
35
40
Time(Sec)
Fig.12.Variation in electrolyzer power
Fig.9.Variation in hydrogen volume
250.01
0.03
250.009 0.025
250.008
Hydrogen Volume(Nm3)
C h a n g e I n D E G P o w e r ( p .u .)
H y d r o g e n V o lu m e ( N m 3 )
249.96
0.02
0.015
0.01
250.007
250.006 250.005 250.004 250.003 250.002
0.005
250.001
0
250 0
5
10
15
20
25
Time(Sec)
Fig.10.Variation in DEG power
30
35
40
0
5
10
15
20
25
30
Time(Sec)
Fig.13.Variation in hydrogen volume
[9]
0
Variation in DEG power(p.u.)
-0.005
-0.01
-0.015
-0.02
-0.025
-0.03
0
5
10
15
20 Time(Sec)
25
30
35
40
Fig.14.Variation in DEG power
V.CONCLUSION The work in this paper is mainly focused on frequency control of a hybrid renewable power generation with fuel cell system connected to the isolated power system. Impact of fuel cell system and different controlling techniques on system frequency has been analyzed. A model of fuel cell generator for response to frequency fluctuations has been developed. From simulation results of step response load analysis, it indicates that steady state stability of hybrid system with tuned PID controller gives better performance in comparison to PI and without controller. This flexible model of operation of fuel cell along with electrolyzer would be very useful for hybrid systems having micro hydro and solar power system. REFERENCES [1] [2]
[3]
[4]
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[7]
[8]
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