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Depletion of fossil fuel resources and the adverse effects of their usage on environment is what mainly triggered the interest in renewable energy sources. Being a clean and vastly available energy source, wind stands as the most promising alternative to conventional schemes for electricity generation. Various options are available for wind energy conversion however this work focuses mainly on the direct driven Permanent magnet synchronous machine (PMSG) based variable speed wind turbine (VSWT). Power electronic interface in VSWT effectively decouples the grid frequency from the rotor frequency, reduces mechanical loading and hence optimizes system performance. Large air gaps of PMSG results in reduced flux linkages; this in turn encourages its availability in multi-pole design. The low rotational speed of a multi-pole PMSG successfully eliminates the requirement of a gear box.
In general PMSG based VSWT are connected to the power grid via full-size frequency converter. Among the two types of converter topologies used in PMSG based VSWT, the focus is laid on the one comprising of generator side rectifier, DC-link, and grid side inverter. Reliability of the system is jeopardized when due to equipment fault or for maintenance purpose, the full size converter goes out of service. Another reason that makes the choice of single full-size converter less favorable is its reduced efficiency at lower power levels and harmonics injected by it to the grid. Hence to improve system reliability, efficiency, and harmonic reduction, multi-modular converter design offers a promising solution.
In this topology, a single full size converter is replaced by a number of parallel connected frequency converters of smaller ratings. The use of a PMSG that has multiple three phase winding sets, that are both electrically and magnetically independent, helps in resolving issue of circulating currents which would exists if multi-modular converter structure was connected to a PMSG with single set of three phase winding.
This study is focused on development of a detailed control strategy of autonomous operation of parallel connected multiple frequency converters working with a multi-phase PMSG. As shown in Fig.1, the multi-modular converter system is connected to a SG with multiple three phase winding sets. Each converter unit has a controller operating independently of the other. Effectiveness of proposed control scheme is verified through simulation using MATLAB.
Fig. 1. Multi-phase PMSG machine connected to the grid using multi-modular converters.
Autonomous operation dictates that each of the parallel connected converter modules will have an independent controller. Each converter module has controller constituting of a machine-side and line-side control unit. Control structure of both these sub-controllers is discussed in this section. In addition to it model for a multi-phase SG is also developed.
Model of multi-phase SG:
Model of PMSG available in MATLAB has a set of three phase winding whereas our system configuration required a multi-phase PMSG machine. In order to simplify the dynamic model of SG, rotor reference frame is used. The model for the multi-phase PMSG was constructed making use of the voltage, flux linkage and motion equation used for developing the model for six-phase PMSG are as under:-
Fig. 2. Model of PMSG in rotor reference frame (i) d- axis model (ii) q-axis model
Equation (1), (2), (5) and (6) are voltage and flux linkage equation corresponding to 1st set of three phase windings and (3), (4), (7) and (8) correspond to the 2nd set of three phase winding. For a PMSG the field winding is replaced by permanent magnets which can be visualized as a fixed magnitude field current source If.
Substitution of equation for rotor flux and d-q axis flux linkages into voltage equations results in the following four equations:
Rotor mechanical speed (wm) and electromagnetic torque (Te) of multiphase PMSG are given by (14) and (15). Parameter Tm, J , F and Np stand for mechanical input torque , moment of inertia, friction factor and number of pole pairs respectively
Based on these equations, model of multi-phase PMSG machine is developed in MATLAB.
Fig. 3. Model of multi-winding PMSG developed in MATLAB .
Controller for individual machine side converter unit:
Beside wind regime, the amount of energy obtained from a wind turbine, also depends on the control strategy used. The synchronous machine is controlled by the generator side converter and its controller has a nested-loop structure. The outer slower loop is responsible for torque control of the PMSG and the fast inner control loop controls stator d- or q-axis currents.
Control strategies conventionally used for PMSG include, 1) constant stator voltage control, 2) unity power factor control, and 3) maximum torque control ,. By controlling the stator voltage in the constant stator voltage control scheme, the risk of over voltage and converter saturation at high speeds is eliminated. The main drawback of using constant stator voltage control is that it requires a higher power rating converter which is due to the reactive power demand of generator ,.
Contrary to the stator voltage control , in unity power factor control, the d and q axis components of the stator current are controlled such that the reactive power of the stator is fully compensated. This control scheme has the benefit of generator operation at unity power factor however the lack of direct control over stator voltage may cause stator voltage to exceed the rated value in case of an over speed .
Fig. 4. Reference frame of permanent magnet synchronous generator
In the maximum torque control, the stator current is controlled such that the d-axis current component is set to zero, which makes the stator current to have only the q-axis component . In Fig. 4 stator current and magnet flux space vector of permanent magnet generator in rotating d-q reference frame is shown. The stator current space vector is denoted by (is) and the permanent magnet flux (Î»rf) is aligned with the d-axis. In (16) the stator current vector is expressed in terms of d and q-axis components
With d-axis current component equal to zero the electromagnetic torque (Te) is given by (17) and it is the maximum possible torque provided by generator.
Fig. 5. Block diagram of sub-controller for machine-side
Since in maximum torque control the reactive power is not controlled , this may result in excessive power rating of converter or generator unit . Structure of a conventional maximum torque control strategy implemented in rotor-flux- reference frame is presented in Fig.5. Measured quantities include rotor position (determined by using an encoder mounted on the rotor) and stator currents. The optimal power reference is determined through the MPPT controller , which sets the q-component reference of the stator current. The d-component reference of the stator current is set to zero.
In order to maintain a constant switching frequency within the converter, both direct and quadrature axis currents are controlled indirectly through a current regulated voltage-source PWM rectifier . By comparing the d and q-axis current reference signals with the measured generator stator d- and q-axis currents, the voltage reference signals for the converter are obtained. Carrier phase shifted (CPS) PWM scheme is used for creating gating signals for the power converter.
Controller for grid side converter unit:
Va Various control strategies have been developed for control of the grid side converter. These strategies can be can be classified on the basis of the reference frame used for their implementation . This paper designed grid side controller using synchronous reference frame. The three-phase electrical quantities are transformed into dq quantities using reference frame transformation. The frame rotates synchronously with grid voltage vector and the transformation angle (Î¸g) is detected by using Phase Locked Loop (PLL) .
Fig. 6. Classification of grid side control strategies.
Grid-side converter regulates the flow of active and reactive power to the grid. The control strategy has nested loop structure with independent control of active and reactive currents. The inner loop controls the grid current and the outer loop is responsible for regulating DC-link voltage and reactive power flow . To enable successful power transfer from turbine to grid it is important to have a very stable DC link voltage. Closed loop control of the DC bus generates reference for active power whereas grid management service decides the reference for the reactive power. When no reactive power compensation is required then grid operator sets Qref as zero as a result the system operates at unity power factor and maximum active power is transmitted into grid.
If grid side resistance and inductance is represented as Rg and Lg respectively and inverter d and q axis voltages are vid and viq then grid d and q axis voltage components vgd and vgq are represented by (18) and (19) as under :-
With control implemented in a reference frame that is rotating synchronously with grid voltage, the dq- components of vg are given by the following two equations.
It is possible to express active and reactive power in terms of grid voltage and currents. Looking at (22) and (23) it is evident that active and reactive power can be controlled by controlling direct and quadrature current components of grid current .
Fig. 7. Block diagram of sub-controller for (ii) Grid -side
The control structure for the grid side converter controller is shown in Fig. 7. As seen the controller contains cascaded loops using PI regulators. Resultant of the current controllers output, voltage feed-forward and cross coupling terms generates voltage reference signal for the PWM generator. Similar to rectifier controller, Carrier based PWM scheme is used for generation of gating signals for inverter unit.
In order to test the control strategy, a WECS connected to stiff grid through a six phase PMSG and two converter modules operating in parallel is implemented in MATLAB. Parameters of the test system are listed in Table.1 under appendix A. The simulation results obtained are given in Fig. 8 up to Fig. 16.
Fig. 8: Total active power reference
Fig. 9: Active power reference for each Converter module
Fig. 10: Active power delivered through each channel towards the grid
Fig. 11: Reactive power delivered through each Channel to the grid under unity power factor (UPF) operation
Fig. 12: Current flowing towards each converter module from PMSG
Fig. 13: DC link voltage reference for each channel
Fig. 14: DC link voltage for each channel
Fig. 15: Total electromagnetic torque (Te) reference.
Fig. 16: Total electromagnetic torque (Te)
The simulation results are for the WECS comprising of 6-Phase PMSG and two set of rectifier and inverter units connected to a stiff grid. The results given are for the case of unity power factor operation that is when no reactive power compensation is required and the Qref is set as zero VAR by grid management services. This is verified by Fig. 9. Both machine side and grid side converter units used CPS-PWM for gating signal generation. Electromagnetic torque of the PMSG is controlled through q-axis component of stator current. Using vector control technique, active power and reactive power Q are decoupled and are controlled by means of d- and q-axis component of grid current. DC-link voltage is maintained is maintained by grid-side .
Both converter units have carrier phaseâ€™s shifted by 180 Â°. The power reference signal for each converter module is determined through MPPT is shown in Fig .9. Both converter modules followed the active power reference signal closely, as seen in Fig. 10. Current flowing in each of the parallel module from PMSG is plotted in Fig. 12. Total electromagnetic torque reference is shown in Fig.15. The electromagnetic torque developed by PMSG is plotted under Fig. 16, value of it is found to be 1.69 M N.m which corresponds accurately to the total reference power of 4MW when wm is 2.365 rad/sec. The DC link voltage reference was set to 975 V for both converter units and the measured DC link voltage stays close to the reference value as seen in Fig.14.
Power electronic interface in VSWT offers clear advantage over Fixed Speed Wind Turbines (FSWT) as possibility of variable speed operation is accompanied by improved power production and reduction in mechanical / structural load. The concept of using single full size power converter in VSWT is common practice; however with the growing trend of Multi-MW-turbine installation, the idea of modular- converter units is gaining popularity.
Modular converter units are gaining popularity with the growth of Multi-MW-turbine installation. This study explores control strategy for autonomous operation of controllers working in conjunction with modular converter system. Main challenges faced in designing such independent controllers is to ensure that without any means of communication, the parallel connected modules continue to share power equally even in case of a channel failure or system unbalance.
The control structure should enable interleaving of operational units under all conditions so as to enhance system harmonic performance. Such an autonomous controller will improve system reliability/redundancy and will help in acquiring true benefit from modular converter system. Control strategy devised was simulated for the base case using MATLAB and revealed proper functioning of the parallel converter modules.