Voltage Oriented Decoupled Control Scheme for DFIG’s Grid Side Converter

This paper proposes a novel voltage oriented decoupled control scheme for the DFIG’s Grid Side Converter (GSC) of a 2.3Mw, 690V, 50Hz, 6 pole doubly-fed induction generator (DFIG) based wind generation system. For Rotor Side Converter (RSC), slip and constant V/Hz control scheme along with a feedback control via PWM is selected but not explained in this paper.Based on the per-phase steady state equivalent circuit model of a DFIG, relationship between stator and rotor voltages is developed. Voltage oriented decoupled control scheme for GSC is designed in such a way that it can keep the dc link voltage constant by regulating grid reactive power when required. The space vector modulation (SVM) algorithm is explained breifly and implemented for the two-level GSC. MATLAB/SIMULINK (R2015a) software validates the proposed control scheme for GSC.


Introduction
Conventional energy sources generate energy at a constant rate while renewable fluctuate with the variability in the natural sources from which they derive energy.Such fluctuations are particularly acute for wind and solar photovoltaics.Due to the fluctuating nature of wind power, it is advantageous to operate the WTG at variable speed which reduces the physical stress on the turbine blades and drive train, and which improves system aerodynamic efficiency and torque transient behaviors [1].Conventional power generation utilizes synchronous machines, modern wind power systems use induction machines extensively in wind turbine applications.
These induction generators fall into two types: fixed speed induction generators (FSIGs) with squirrel cage rotors [2] and doubly fed induction generators (DFIGs) with wound rotors [3].The DFIG is currently the system of choice for multi-MW wind turbines as over 85% of the installed wind turbines utilize DFIGs .In the DFIG topology, the stator is directly connected to the grid through transformers and the rotor is connected to the grid through PWM power converters.The converters can control the rotor circuit current, frequency and phase angle shifts.As shown in Figure 1, the rotor of the DFIG is mechanically connected to the wind turbine through a drive train system, which may contain high and low speed shafts, bearings and a gearbox.The rotor is fed by the bi-directional voltage-source converters.Thereby, the speed and torque of the DFIG can be regulated by controlling the rotor side converter (RSC) [4]- [6].
Taking into account the only one converter within a DFIG system.This paper is organized as follows.Section II describes the DFIG mathematical modeling.To analyze the DFIG's performance, its per-phase equivalent circuit for steady state operation is analyzed and relationships between stator and rotor voltages are developed.In section III, operation of GSC with VOC and reactive power control are illustrated in detail.Brief principle and implementation of SVM algorithm for the two-level GSC is explained in section IV.Simulation results on a 2.3MW DFIG system are provided in section V and finallysection VI draws the conclusions.

DFIG's mathematical modeling
To determine DFIG's performance under steady-state operation, its per-phase equivalent model is selected and shown in Figure 2  The matrix form of the equation for this circuit is: The frequency relationship for the DFIG can be developed as follows [7]: where s f is the frequency of the stator voltage, m f is the frequency of the rotating shaft and r f is the frequency of the injected rotor voltage.The stator voltage and rotor voltage relationship, neglecting the voltage drops in the series elements can be expressed as: where the frequency of the rotor voltage is given by: The injected rotor voltage and the stator voltage has a ratio a given by: where s n and r n are the turns of the stator windings and the rotor windings, ' r V is the rotor voltage seen at the stator side and r V is the rotor voltage.Induced stator voltage can be obtained by substituting value of

Operation of GSC with VOC and reactive power control
In order to analyze GSC, Figure 1 is reduced by replacing the wind turbine, DFIG and RSC by a battery in series with a small resistance that represents the power losses in the system as shown in Figure 3 [10]- [11].

Voltage oriented control (VOC)
GSC is controlled by the voltage oriented contro l(VOC) scheme [8][9][10][11][12] as shown in Figure 3. Assuming the three-phase balanced sinusoidal grid voltages   active and reactive components of the three-phase line currents, respectively.In VOC control scheme, the q-axis grid voltage , from which the active and reactive power of the system can be calculated by: The q-axis current reference

VOC with decoupled controller
The state equation for GSC in the dqsynchronous reference frame can be expressed as [8]: where (k1 + k2 /S) is the transfer function of the PI controller.Substituting ( 7) into (6) yields: The above equation indicates that the control of the d-axis grid current is decoupled, involving only d-axis components, so is the q-axis current .The decoupled control makes the design of the PI controllers more convenient, and system is easier to be stabilized.

Space vector modulation for GSC
Space vector modulation (SVM) is one of the real-time modulation techniques and is widely used for digital control of voltage source inverter [13]- [14].

Simulation results
Consider a 2.3MW/690V GSC, controlled by the scheme with a decoupled PI controller as shown in Figure 3.The system parameters and operating conditions are given in Table 1. Figure 4 shows the waveforms of the phase-a grid voltage and the space angle .When rotates in space, and varies from zero to 2π periodically.When is equal to zero, reaches its peak value as shown at Point A in Figure 4.The transient waveforms of the inverter are illustrated in Figure 5, where the inverter initially delivers the rated active power ( ) and zero reactive power ( ) to the grid.Ignoring all the ripples (produced by current harmonics), the d-axis current is -1.41pu (rated) and the zero q-axis current is zero.The corresponding waveforms of phase-a grid voltage and current during the transient are also given in the Figure 6. Figure 6 shows the peak value of the phase-a grid current is equal to the peak value of the phase-a grid voltage i.e1.414 pu (rated).The grid current is out of phase with its voltage by 180° so both the active and reactive powers delivered to the grid can be calculated as: Where the negative sign indicates that the GSC delivers the active power to the grid.
Figure 6.Steady-state operation-I Figure 7 depicts, when the battery voltage E starts to reduce such that the active power to the grid is reduced to -0.8pu, which leads the reduction of the d-axis current from its rated value to -1.13pu ( √2 ×-0.8 ).The q-axis current remains unchanged during the transients due to the decoupled control of the active and reactive power while the magnitude of the phase-a grid current is reduced to 1.33pu, but kept out of phase with its voltage by 216.87°.Both the active and reactive powers to the grid can be calculated by:     The negative reactive power indicates that the GSC operated with leading (capacitive) power factor to sustain the grid voltage.

Conclusion
GSC is controlled by Voltage Oriented Control (VOC) scheme with a decoupled PI controller.It successfully kept the dc link voltage constant and provided reactive power to the grid in leading power factor.

Figure 3 .
Figure 3. Voltage oriented Control (VOC) with a decoupled controller

( 5 )
There are two inner current loops for the accurate control of the dq-axis currents dg i and qg i , and one outer dc voltage feedback loop for the control of dc voltage dc v .With the VOC scheme, the three-phase line currents in the abcstationary frame dqsynchronous frame, which are the

Q
is the reference for the reactive power, set to zero for unity power factor operation, and a negative value for leading power factor operation.Thed-axis current reference * dg i , which represents the active power of the system, is generated by the PI controller for dc voltage control.When the inverter operates in steady state, the dc voltage dc v of the inverter is kept constant at a value set by its reference voltage * dc v .The PI controller generates the reference current * dg i according to the operating conditions.Hence, the active power on the ac side of the GSC is equal to the dc-side power (neglecting the losses in the GSC), that is,

Figure 4 .
Figure 4. Angle of grid voltage vector for the voc scheme

Figure 5 .
Figure 5. transient waveforms of the grid-tied inverter with voltage oriented control .