ISSN (PRINT) :2320 – 8945, Volume -1, Issue -5, 2013
39
Real and Reactive Power Control of a Grid in Wind Energy Conversion System Based on Permanent Magnet
Synchronous Generator
Md Abul Hasnat & Vishal Shende
Department of Electrical Engineering, Motilal Nehru National Institute of Technology , Allahabad -211004, India
E-mail : [email protected], [email protected]
Abstract – This paper proposed the configuration of kilo- watt wind energy conversion system (WECS) using permanent magnet synchronous generator (PMSG). The converter consists of a diode rectifier, a buck converter and a pulse-width-modulated (PWM) current source inverter (CSI). The topology using a dc chopper to bridge the diode rectifier and the PWM CSI introduces an additional control freedom to manipulate the dc-link current, and therefore is able to operate the system in the full range. Using the grid voltage orientation method, the active power is controlled by d-axis current whereas the reactive power is controlled by q-axis current. The phase angle of utility voltage is detected using software PLL (Phased Locked Loop) in d-q synchronous reference frame. Proposed scheme gives a low cost and high quality power conversion solution for variable speed WECS.
Keywords – Current source inverter, Diode rectifier, PMSG, WECS, DC-DC Converter
I. INTRODUCTION
Wind power is the most fast growing energy source in the world. According to the technological development the wind energy cost reduced down to the range of 4cents/kWh recently, which is competitive against conventional energy sources. The total capacity of wind power is about 25,000MW and the average power rating of a unit is 1.2MW in 2001. Many number of on-shore or off-shore wind farms are being built on behalf of the policy of good tariff for electricity from natural renewable energy source, wind. More than 75%
of large wind turbines constructed in 2001 are variable speed machine with grid connection [5] There are three basic configurations of the current source Converters for WECS. The first one uses PWM voltage source rectifier (VSR) and PWM current source inverter (CSI) [2], which gives most freedoms for the control objectives, but the price of the switching devices is relatively high
and the control scheme design is complex; the second one employs diode rectifier and thyristor inverter, of which the technology is well established and the reliability is proven [3], but the poor grid waveforms, lack of reactive power control, and extra investment on the compensation system will counteract more or less its low cost advantage in transferred to the grid can be controlled but with limited range [4]. In this paper, we implemented simple ac-dc-ac converter and proposed modular control strategy for grid-connected wind power generation system. Line side inverter maintains the dc- link voltage constant and the power factor of line side can be adjusted. Input current reference of boost chopper is decided for the maximum power point tracking of the turbine without any information of neither wind speed nor generator rpm. As the proposed control algorithm does not require any speed sensor for wind speed or generator rpm, construction and installation are simple, cheap, and reliable. Proposed algorithm applied to the output power converter of 30kW HAWT/VAWT. Maximum power extraction is implemented by voltage source rectifier (VSR) connected to PM generator. Previous research on maximum wind power extraction methods [6-8] can be summarized as turbine-generator speed control, direct power control and wind speed sensorless control. In this paper, the method of tracking turbine-generator speed for maximum power is implemented at the rectifier side.
The rectifier controller is also optimized for the generator operation, either to help adjust generator terminal voltage or to minimize generator winding loss.
Although control schemes may vary with the configuration of generators and converters, most VSC systems keep dc link voltage constant at all operating conditions [9-11] when the converter is connected to the grid. Similarly, in CSC, the dc link current can be
ISSN (PRINT) :2320 – 8945, Volume -1, Issue -5, 2013
40 maintained at the highest current for all operating conditions. However, since the wind generation system operates in a wide power range, maintaining high dc link current at a low power level causes a large amount of short-through states in current source converter. This will lead to high conduction loss and additional switching loss. A minimum dc link current for variable speed operation is proposed in this paper to reduce the converter power loss. Fig. 1 shows the proposed configuration for wind power generation. The system consists of a permanent magnet synchronous generator, a full power back-to-back current source converter, and a grid-connected transformer. The PMSG is specially designed for direct drive wind application with low operating speed. PWM VSR (generator side) and PWM CSI (grid side) are connected via a dc link choke. Filter capacitors are parallel connected at both sides to assist current commutation as well as to filter switching harmonics. A dual- purpose step-up transformer is connected between CSI and grid, acting as line filter and converter isolation.
II. WIND ENERGY CONVERSION SYSTEM The amount of power harnessed from the wind of velocity v is as follow.
= A (1) where,
=wind power [W].
=Air density [kg/m3].
A=swept area [m2].
=Power coefficient of wind turbine.
v=wind speed [m/s].
Consequently, the output energy is determined by the power coefficient of wind turbine if the swept area, air density, and wind velocity is constant. The power coefficient depends on the aerodynamic characteristics of blades. Fig. 1 represents the relation between generator speed and output power according to wind speed change ( < < < < ).
Fig.1 Relationship between the power and the rotational speed
III. POWERFLOWANALYSIS
The power flow of WECS is generally unidirectional from the wind turbine unit to the grid.
Low cost and highly reliable Diode rectifier is hence a possible choice to act as the Generator side converter to transfer the AC power to DC power. The configuration of a diode rectifier and a PWM CSI applied for a PMSG WECS is shown in Fig. 1. However, the operating Range of this system is rather limited as will be demonstrated in the following DC current analysis for both generator and grid sides [3]
PMSG
Grid side Control Diode
Rectifier DC link Inverter Grid
Fig.2 A wind energy conversion system using diode rectifier and PWM CSI With a chopper The converter topology using a dc chopper to boost the dc-link current is shown in Fig. 1 When the active device in the chopper is turned on, the real power flows from the generator to the grid through the diode rectifier, switching device CS, dc-link inductor and the PWM CSI. When the device is off, the current path interconnecting the PWM CSI and diode rectifier is cut.
A free-wheeling diode across the dc link is hereby necessary to resume the current path for the dc inductor.
To maintain the current in the generator stator winding, a dc capacitor Cdc is placed at the output of the diode rectifier bridge.
The electrical torque, generator speed and generated power can be calculated as follows,
=1.5P (2) - = (3) = (4)
For proper generator operation the output of diode rectifier can be simplified as a constant DC current.
With the help of DC voltage and commutation angle of diode rectifier we can control the generator
= - (5)
=1- < ) (6)
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41 Where
𝑉𝑑𝑐 =DC voltage of rectifier.
𝛿=commutation angle of thyrister.
𝜔𝑔=angular speed of PMSG.
𝐿𝑔 =synchronous inductance of the PMSG.
𝐸𝐿𝐿 =induced electromotive force.
With the loss being neglected, the DC output power of the diode rectifier equals to the active power 𝑃𝑔 generated by PMSG.
IV. CONTROLLINGOFDC-DCCONVERTER For maximum power point tracking (MPPT) in low- power wind energy converters, there are two common control strategies, namely power signal feedback (PSF) control and hill-climb searching (HCS) control [14].
PSF control assures operation at the MPP using pre- calculated look-up table, which is a relationship of two system parameters. HCS control is based on directly altering the DC–DC converter duty ratio, DC-side voltage or other parameter, according to the result of the comparison of successive wind turbine generator output power measurements.
WIND MPPT
SPEED
REGULATOR PWM
Wg Wg*
D CHOPPER GATING
SIGNAL
Fig.3 Chopper control
At a certain wind speed, the control strategy gives an optimum DC-side voltage, DC-side current or DC–
DC converter duty ratio as a reference to force the system to track the MPP, as shown in Fig. 4.Fig. 4 shows the control scheme that controlling DC-side voltage and current, respectively, to track MPP. The determination of these optimum values is not the concern of the paper and therefore is not given here. The effects on the torque ripple of controlling DC-side voltage, DC-side current or even directly adjusting duty ratio may be different and is also subject to the power electronic topology used[6].
PI CONTROLLER
DC SIDE VOLTAGE OPTIMUM DC
VOLTAGE REFERENCE
BOOST DRIVING SIGNAL
Fig 4a Controlling DC-side voltage for MPPT using a PI Controller
PI CONTROLLER
DC SIDE CURRENT OPTIMUM DC
CURRENT REFERENCE
BOOST DRIVING SIGNAL
Fig. 4b Controlling DC-side current for MPPT using a PI controller
DC LINK VOLTAGE OPTIMUM DC
VOLTAGE
REFERENCE BOOST
DRIVING SIGNAL
D=1-Vlink/Vdc
Fig.
4c Controlling DC-side voltage for MPPT using feed forward control
V. INVERTERSIDECONTROL
The inverter control is developed based on grid voltage oriented synchronous frame. The grid voltage 𝑣𝑠 has only d component 𝑉𝑑𝑠 while q component 𝑉𝑞𝑠 = 0 if the d-axis is aligned to the voltage vector. The active and reactive power can therefore be independently controlled by regulating dq reference currents.
DC CURRENT CALCULATE
DC CURRENT REGULATION
CARTESIAN TO POLAR
CSI SVM
Q REGULATOR Pg
Q Qref
Idc Idc*
INVERTER GATING PULSE
Fig.5 Inverter control
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42 Grid-inverter model was developed for the control scheme analysis. Grid is assumed to be stiff grid and can be simplified as a voltage source with small source impedance (𝐿0 𝑎𝑛𝑑 𝐶0). 𝐿0 represents the sum of line impedance and leakage inductance of transformer, while 𝑅0 is referred to the transformer and line loss. The dq- axis steady state voltages on the capacitors are as following [3]
𝑣𝑑𝑐𝑜=𝑅0𝑖𝑑𝑠-𝜔𝑠𝐿0𝑖𝑞𝑠+𝑉𝑑𝑠 (7)
𝑣𝑞𝑐𝑜=𝑅0𝑖𝑞𝑠+𝜔𝑠𝐿0𝑖𝑑𝑠 (8)
Similarly the current in capacitors is thus derived using capacitor voltages in steady-state
𝑖𝑑𝑐𝑜=-𝜔𝑠𝐶0𝑣𝑞𝑐𝑜 (9)
𝑖𝑞𝑐𝑜=𝜔𝑠𝐶0𝑣𝑑𝑐𝑜 (10)
Therefore in steady state condition, inverter ac reference Current is the sum of the transformer current and capacitor Current
𝑖 𝑤𝑜=𝑖 𝑐𝑜+𝑖 𝑠= (−𝜔𝑠𝐶0𝑣𝑞𝑐𝑜+𝑖𝑑𝑠)+j(𝜔𝑠𝐶0𝑣𝑑𝑐𝑜+𝑖𝑞𝑠) (11)
Suppose the output is unity power factor (𝑖𝑞𝑠=0) then 𝑖𝑤0 can be simplified
𝑖 𝑤𝑜=(1-𝜔𝑠2𝐿0𝐶0)𝑖𝑑𝑠+j[𝜔𝑠𝐶0(𝑅0𝑖𝑑𝑠+𝑉𝑑𝑠)] (12)
For grid side unity power factor operation, the relationship between dc link current and the output can be derived in (12) by setting 𝑖𝑞𝑠 to 0 in (11)
𝑚𝑎𝑖𝐼𝑑𝑐= (1 − 𝜔𝑠2𝐿0𝐶0)2𝐼𝑑𝑠2+𝜔𝑠2𝐶𝑜2(𝑅0𝑖𝑑𝑠+ 𝑉𝑑𝑠)2 (13)
Obviously the dc link current is determined by both sides: grid side LC filter and generator side LC filter.
However, the LC filter selected at grid side is usually
larger than that of generator side to get better THD, which results that, the dc link current is mainly determined by grid side, as shown in (12). Equation (12) also shows that dc link current 𝐼𝑑𝑐 varies inversely as modulation index changes provided that the system active power flow is constant. Therefore the minimum dc link current can be found by asserting modulation index to be 1. The dc current calculation block in Fig. 2 receives the rectifier power and calculates the minimum dc link current with the assumption of lossless system and 𝑚𝑎𝑖 = 1. The grid side real power and reactive power can be calculated in (14) and (15). But in general, reactive power can be adjusted through proper control of q-component current
=1.5 (14)
=1.5 (15) It can be observed that inverter side controller has no direct Active power regulation. The power difference at the two sides of the dc link choke will lead to the variation of dc link current. Therefore, dc link current regulator is inserted to adjust the power difference, which comprises the rectifier loss, dc link conduction loss, energy stored in dc link and inverter loss, etc. the loss is treated as disturbance to the control system and compensated by the dc link current regulator. Power feed forward together with dc link current regulation controls the converter active power flow. In practice, 𝑅0 is unknown and there might be deviation of system parameters (e.g. 𝐶0 𝑜𝑟𝐿0 ). Output reactive power regulator is implemented to allow minor adjustment of reactive current. This minor regulation ensures better dynamic performance and desired PF control insensitive to system parameters.
VI. SIMULATIONANDEXPERIMENTRESULTS Simulation Results
The power topology and proposed control strategy have been simulated in Matlab/Simulink for a low power prototype system. Major system parameters are listed in Table I. Simulation results are shown. The torque from wind turbine drives the generator to slowly speed up until it reaches to 50% of the reference speed.
Once the speed is over threshold, both rectifier and inverter start operating to build up dc link current and capacitor voltage at the grid side. During this period, the grid side inverter (with filter capacitor) is disconnected from the grid. Once the capacitor voltage is synchronized to the grid voltage, the inverter is connected and bridges the generation system to the grid.
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43 TABLE 1: SYSTEM PARAMETERS FOR
SIMULATION
In the simulation, the system is driven by a model of wind turbine. The maximum power point tracking is implemented in the turbine model by giving an optimized reference speed to the control system. In order to simulate the transient response of proposed control system, wind speed has a step change from 12m/s to 8m/s at 2s. As a result, the reference speed for MPPT is changed accordingly as shown in Fig. 6(a).
The rotor speed follows the reference speed very well in steady-state.
The transient of real power is shown in Fig. 6(b). In Fig. 6(c) the reactive power is maintained at zero for grid side and unity power factor is kept regardless of the real power level. It can be observed that although wind speed is decreased by 1/3, the active power is reduced to almost 1/3. This large power variation makes the minimum dc link current control more significant for system efficiency. As shown in Fig. 7, the dc current is decreased from 12A to 8A due to the system power variation. The dc link current is minimized at steady state when inverter modulation index is automatically adjusted to 1, During transient, the modulation index is less than 1 to ensure fast response. Fig 8 shows the waveforms of current for unity power factor operation under different conditions It should be noted that the turbine-generator inertia is reduced to shorten the simulation time. Therefore, the speed response as shown in Fig. 3 is much faster than in the real system. The grid side active power has a large transient at time 2s. This is due to the fast response of speed regulator, which extracts the kinetic energy from generator and rapidly slows down the rotor speed. Several methods can be done to limit the power flick, including ramp or power limit.
(a) Generator speed (pu)
(b) Grid Active Power (W)
(c) Grid Reactive Power
Fig. 6.Power flow control to a step change in wind speed
Fig. 7 DC link current control
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44 Fig. 8 current waveforms of grid
VII. CONCLUSIONS
In this paper, a novel control scheme for PWM CSC in direct-drive wind energy system was proposed.
Control scheme was developed for independent active and reactive power control while maintaining the maximum converter efficiency and extracting the maximum power. The proposed scheme decouples the active power and reactive power control of grid side.
The dc link current is minimized in steady state to reduce the devices switching loss and conduction loss for achieving maximum efficiency. Simulation and experimental Results obtained verified the proposed control strategy.
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