Power Flow Control in Distributed Microgrid with Wind Energy System Megha Goyal, Student Member, IEEE and Rajesh Gupta, Senior Member, IEEE Abstract−In this paper, power flow is controlled in a distributed microgrid. The local loads in the microgrid are supplied from the distributed generator (DG) with wind energy system (WES). In DG, a voltage source inverter (VSI) is operated in a current control mode in order to inject real power and achieve load compensation. The generation of reference currents for voltage source inverter depends upon the available wind power for real power injection and load reactive and harmonic component for load compensation. Simulation studies are performed using PSCAD/EMTDC to validate the proposed power flow control and load compensation in the distributed microgrid. Index Terms− Current control, distributed generation (DG), microgrid, power flow control, wind power.
-variable speed wind turbine with permanent magnet synchronous generator (PMSG) is considered. Output voltage of the PMSG is changed according to the wind speed. AC output of the PMSG is converted into DC using uncontrolled rectifier. To interface this wind energy system with the grid voltage source inverter is considered which is operated in current control mode to inject the real power with load compensation current components to offset the harmonics, reactive and unbalance component of the load current. The reference current generation for the VSI is derived using instantaneous symmetrical component theory [6] and it depends upon the available wind power.
I. INTRODUCTION
II. SYSTEM STRUCTURE
eneration of electrical power from wind power and its interface to the local microgrid is a newer concept and gaining importance due to its utilization and environmental advantages. A wind turbine can be used in wide variety of ways to generate electrical power. There are many aspects to use electrical power generated from the wind power [1]. These include feeding local loads, interface to the existing power grid, strengthening local micro grid, etc. Besides these the wind power can also be used for power quality enhancement. In this paper, wind energy conversion system is used to supply real power to the local load supplied from the main power grid and load compensation to support reactive, harmonic and unbalanced components of the load [2]. These features are not only required in the domestic power supply system but also of importance in the industrial distribution system. The example includes process control industries, medical support systems, manufacturing plants, etc. Cluster of DGs connected to the grid can be viewed as micro-grid. In this paper, power flow from wind turbine to the grid is controlled based on the available wind power and the local load requirement [3]. The estimate of the available wind power is obtained using wind speed measurement. The wind turbine is modeled for torque control of wind turbine generator. It is shown that if the power requirement by the load is more than the available wind power then excess power can be received by the grid [4]. Wind power is important renewable technologies that require an inverter to interface with the grid [5]. In this paper-
The power system model with two wind DGs basic structure is shown in Fig.1.Microgrid is connected to the grid [7]. Both DG1 and DG2 are connected to the micro grid through VSI. Each DG has its own individual local loads.
G
Megha Goyal is a Master student in the Department of Electrical Engineering, Motilal Nehru National Institute of Technology, Allahabad-211004, India (e-mail:
[email protected]). Rajesh Gupta is an Associate Professor in the Department of Electrical Engineering, Motilal Nehru National Institute of Technology, Allahabad211004, India (e-mail:
[email protected]).
978-1-4673-0455-9/12/$31.00 ©2012 IEEE
Fig. 1. Schematic representation of Microgrid and utility system.
The active and reactive power flow is represented by P and Q, respectively. The function of the DGs with WES is to supply the real power according to the available wind power with the load compensation to offset the harmonics, reactive and unbalance component of the load currents. The VSI is operated in current control mode to track the specified reference current. Therefore, appropriate reference currents need to be generated for each phase for real power injection and load compensation. The structure of the three legs VSI supplied from the common DC link is shown in Fig. 2.
speed varies according to the applied per unit torque as input argument. III. REFERENCE CURRENT GENERATION SCHEME
Fig 2. Schematic diagram for the real power injection and load compensation scheme.
The DC link of the VSI is fed from the uncontrolled rectifier as shown in Fig.3. The variable speed wind energy system with permanent magnet synchronous generator is connected to the bridge rectifier.
In this section, the reference current generation for DG is obtained. The VSI is operated in current control mode to track the specified reference current. Both the DGs have same control strategy. Therefore description of only one DG and its compensator are given. The main aim of the DGs is to compensate the nonlinear, reactive and harmonic components of the local loads and supply the available wind power. DGs are also supplying the reactive power of the utility load. To fulfill these, zero sequence components of the source current will be zero[13,14], i.e.,
is1a + is1b + is1c = 0
(3)
For the unity power factor, the reactive power delivered from the source is zero, i.e.,
(vb − vc ) is1a + (vc − va ) is1b + (va − vb ) is1c = 0
(4)
Instantaneous real power is supplied from the source, i.e.,
vaisa + vbisb + vcisc = ps
Fig. 3. Schematic representation of proposed wind energy system.
The wind turbine converts the wind energy into the mechanical energy through a suitable turbine configuration [8]. The wind power pw extracted by the wind turbine can be defined as below [9], [10]. pw = 1/
2* ρ AVw3C p (λ , θ )
Tr =
ωr
(2)
For the torque control of the variable speed wind turbine, the generator side rotational speed (GSRS) is feedback [11,12] as shown in Fig. 3. In this paper, the generator side rotational speed ωe is fedback to the wind turbine as mechanical rotating speed ωg.
ωg =
ωe np
VSI is also injecting the real power according to the available wind power. Source will supply only the active power to the local and utility load. Both DGs will supply the reactive power of utility load. Let’s assume that both the DGs share utility load power equally, such that the following is satisfied.
PS = PG + Pl
(6)
PG1 = Plav1 − Pw1
(7)
PS1 = Plav1 + 0.5* Pl − Pw1
(8)
(1)
Where, ρ is air density, A is swept area of turbine, Vw is wind speed, Cp is power coefficient which is a function of blade pitch angle θ and tip speed ratio λ. The pitch angle θ is kept constant at optimum position in torque controlled wind turbine. The torque Tr is defined by the following equations.
Pw
(5)
(3)
Where, np is the number of pole pair in the PMSG. The operation of PMSG in normal mode implies that the machine rotor
Where, Plav1 is the average local load 1 power which can be computed by a moving average filter using instantaneous power pl1 defined as below
pl1 = va il1a + vb il1b + vc il1c
(9)
Pl is the average utility load power which can be computed by a moving average filter using instantaneous power pl defined as below
pl = vaila + vbilb + vcilc
(10)
Pw1 is available wind power for DG1, i.e., computed using (1).
Assuming, VSI will track the reference shunt current. Therefore the reference shunt current for three phases can be written as
idg 1kref = il1k +
ilk − i s1 k 2
where, k = a,b,c
(11)
Combining (3)-(11), the expression for reference current generation for the DG-1 three phases VSI is as follows.
ila i v − is1aref = il 1a + la − a ( PS 1 ) 2 2 Δ ilb ilb vb ( PS 1 ) + − is1bref = il1b + − 2 2 Δ i i v + lc − is1cref = il1c + lc − c ( PS 1 ) 2 2 Δ
active power component of the load current. This is shown in the Fig. 4(i).
idg 1aref = il 1a + idg 1bref = il 1b idg 1cref = il1c
(12)
Where, Δ in (12) is defined as below. Δ = v a2 + v b2 + v c2
(13)
and PS1 can be computed using (8). In a similar way, expression of reference current generation for the DG-2 can be calculated.
Fig. 4 (a) Wind Speed For DG1 and DG2
IV. SIMULATION RESULTS For simulation studies the system data is given in Table I and Table II. The source voltage is assumed to be 250 V (rms) per phase. A combination of nonlinear and unbalanced linear load is considered with the values given in Table II. The simulation results are obtained through the PSCAD/EMTDC software. A. Sharing of local load and utility by DGs
DG with WES is partly supplying real power to its own local loads and performing load compensation for it and also for utility loads such that the current supplied from the grid become balanced and sinusoidal with unity power factor. Balance real power requirement of the local loads and utility load is supplied from the main power grid. Both DGs with WES have different wind speed. Here DG1 wind speed is considered 8m/sec and DG2 wind speed is considered 10m/sec, as shown in Fig. 4(a). Fig. 4 (b) shows the DC link capacitor voltage which is constant for a particular speed under steady state. Both DGs have its individual local loads. DGs with WES are supplying real power according to available wind power. Both DGs have different wind speed so real power supplied by these DGs to its local load is different as shown in Fig.4(c) and Fig.4(d), respectively. The total local load real power is supplied by the grid and its DG. Reactive power of the local load and utility load are supplied from the DGs as shown in Fig. 4(e) & (f). It shows that the reactive power supplied by the grid is close to zero. Fig. 4(g) represents the three phase source current which is balanced and closed to sinusoidal. Fig. 4(h) shows the source current in a phase with the source voltage. This means that the grid is supplying only real power to the loads. The local loads are considered unbalanced and non-linear. Therefore the load current contains the harmonics and reactive power. These local load harmonics and reactive power component is supplied by the DG and the grid supplies only the
Fig. 4 (b) DC link capacitor voltage.
Fig.4(c) Active power distribution for local load 1
Fig. 4(d) Active power distribution for local load 2
Fig.4(e) Reactive power of Local load 1, supplied by DG1 to utility load, DG1 and Grid Reactive Power.
Fig.4(i) Source current and local load current. TABLE I SYSTEM PARAMETERS Parameters Source voltage(vs) System frequency(fo) Net shunt impedance (Lshk,Rshk) DG-1 Local Unbalanced load The subscripts a, b and c denote three phases. Non linear load Contains three-phase rectifier supplying load. DG-2 Local Unbalanced load The subscripts a, b and c denote three phases.
Fig.4(f) Reactive power of Local load 2, supplied by DG2 to utility load, DG2 and Grid Reactive Power.
Non linear load Contains three-phase rectifier supplying load. Balanced Utility load DC Link capacitor
Numerical value 250V (L-N) 50Hz 5mH, 1 Ω Zl1a =11.25 + j5.43 Ω Zl1b =10.00 + j7.85Ω Zl1c =10.62 + j6.59Ω 50- j3.18 Ω Zl2a =11.25 + j5.43 Ω Zl2b =10.00 + j7.85Ω Zl2c =10.62 + j6.59Ω 50 - j3.18 Ω Zla=11.25 + j5.43 Ω Zlb=11.25 + j5.43 Ω Zlc=11.25 + j5.43 Ω 2200µF
TABLE II WIND GENERATION SYSTEM Parameters Generator rated voltage Rotor radius Fig. 4(g) Three Phase Source Current.
Numerical value 300Vrms(L-N) 3.5m
Rotor area
38.5 m2
Air density
1.2 kg/m3
Power Coefficient
0.4789
V. CONCLUSIONS
Fig. 4(h) Source voltage (reduced by factor 2) and source current.
This paper presents the power flow control in a distributed microgrid. The DGs with WES are supplying active power according to the available wind speed and the remaining active power requirement of the local load is supplied from the grid. The DGs are capable of compensating the harmonics, reactive and unbalanced components of its own local loads and the utility loads. Therefore the microgrid is not putting any effect of its local loads on the grid.
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