Rotor Side - Rectifier control strategy

The Synchronverter technology will also be used to control the rectifier side of the back-to-back converters connecting the WECS to the grid. This will also be a PWM controlled three-phase converter for which a typical power part is depicted in Figure 3.3. The three-phase generator to the far left of the figure will for this thesis constitute the PMSG driven by the wind turbine. The method of using the Synchronverter control technology to operate a rectifier was first proposed in [31], and builds on the idea from [27]. Instead of operating the converter as a synchronous generator, as is the case for the inverter side explained in Section 3.1, the converter will now be operated as a synchronousmotor, and it is therefore yet again a way of implementing a virtual synchronous machine.

From Figure 3.3 it can be seen that also for the rectifier sideLs andRswill represent the induct-ance and resistinduct-ance of the windings of the imaginary synchronous motor, meaning the voltage at the converter terminals constitute the back-EMFeof the VSM. The mathematical model used for the Synchronverter controlled rectifier is derived in Section 2.3.2, and as previously explained it bears much resemblance to the model used for the synchronous generator, except that the inductor currents are defined in the opposite direction, flowing into the machine.

The core of the controller will therefore be exactly the same as for the inverter, leading to negative power and electrical torque. This is due to the fact that with the mathematical model used, the power is defined positive for a generating unit, i.e. currents flowing out of the machine, and negative for a consuming unit such as the motor, based on the direction of the currents. Based on these definitions the swing equation in the controller core can also be kept unchanged using

Figure 3.3: Power part of a typical three-phase rectifier[29].

(3.1a), as the redefined signs of the electrical and mechanical torques will ultimately lead to the desired swing equation representing a motor in (2.22).

Also for the rotor side, the control system will generate the reference for the PWM signals used to control the physical IGBTs in the power part. The objective of the rotor side converter is to maintain the DC link voltage at the reference level, while sending the correct amount of power to the grid side as demanded by the grid side converter. This should be achieved for the entire operating range of the wind turbine, and the converter frequency should follow the variable frequency of the PMSG. The control topology of the rotor side controller in the Laplace domain is depicted in Figure 3.4.

As can be seen, the inputs to the controller are the DC voltage referenceV_DC,ref, the reactive power referenceQ_ref, the converter currenti_c,rotor, the mechanical rotation of the turbine shaft ωm, and during the synchronisation process, the voltage of the PMSGvpmsgis required. Qref

is usually set to zero to obtain unity power factor. i_c,rotor,v_pmsgand ω_m are obtained through measurements coming from sensors.

3.2.1 DC Voltage Control and Frequency Droop

The control structure depicted in Figure 3.4 also consists of two control channels; an upper channel for the DC voltage control and a lower channel for the reactive power control. In addition, the upper channel has an outer and an inner loop denoted the DC voltage loop and the power loop respectively for which the power loop has an additional inner frequency loop.

The DC voltage control loop is the upper loop in Figure 3.4 and is governed by the swing equation using the mechanical torque reference, electrical torque and the frequency drooping.

As seen from Figure 3.4, the DC voltage deviation is passed through a Proportional/Integral (PI) controller to create the mechanical torque reference for the virtual synchronous machine. The PI controller consist of a proportional gain and an integral gain, having the expressionK_p,dc+^Ki,dc_s . The electrical torque is calculated using (3.1c).

The drooping torque is created by comparing the frequency of the converterωto the electrical

Calc la i n f

S i ch A PWM

gene a i n

2 1 P le Pai

Figure 3.4: Synchronverter control topology for rectifier, modified from [31].

frequency of the PMSG, ω_e, giving ∆ω. Note that the electrical frequency of the PMSG is obtained using the mechanical speed of the shaft and the number of pole pairs in the PMSG.

∆ω is then multiplied with the frequency droop coefficientD_pm.D_pmis here representing the mechanical friction coefficient of a real synchronous motor[29], and will in this thesis have the same definition asDp, i.e. calculated using (3.3).

The deviation betweenT_m,ref + ∆T andT_eis sent through an integrator with the gain _J¹

m. This in turn controls the speed of the converterω. As for the grid side converter, the speedωof the VSM yields, by integration, the angle of the control signaleused for the PWM signals.J_mis the virtual inertia of the synchronous motor.

3.2.2 Reactive Power Control

The reactive power loop is the lower loop in Figure 3.4 and consists of the reactive power reference and actual reactive power. The reactive power reference is set by the operator and is for the rectifier often set to zero to obtain unity power factor. The actual reactive power is calculated by use of (3.1e). The reactive power deviation is fed through an integrator with the gain _K¹

qm, creating the virtualMfif.

As for the grid side converter, the reactive power loop ultimately controls the amplitude of the control signaleby adjustingM_fi_f. Therefore, with the angle controlled by the DC voltage loop and the amplitude coming from the reactive power loop, the control signal for the PWM signals can be calculated based on (3.1b), completing the Synchronverter control technique for rectifiers.

3.2.3 Self Synchronisation

Before the PWM signals are released to the IGBTs the controller needs to synchronise with the grid frequency and voltage governed by the PMSG. During this period the rectifier works as an uncontrolled three-phase diode rectifier. The synchronisation is important to limit high inrush

currents to the rectifier, and the procedure used here is modified from the procedure proposed in [52]. The synchronisation follows to a great extent the method in Section 3.1.4. Both the torque reference and the reactive power reference is set to zero, and a virtual current,i_s, is introduced as feedback to the controller. The virtual current is necessary during the synchronisation because the rectifier is operating in an uncontrolled mode[52], i.e. the currenti_c,rotorcannot be controlled by the controller as no signals are sent to the IGBTs.

The virtual currentiscan be calculated as

i_s = v_pmsg−e Lsync·s+Rsync

(3.15) however based on the previously defined direction of currents for the rectifier control system, the virtual current used as the feedback will be−i_s, resulting in the virtual current in (3.16).

is,f eedback = e−v_pmsg

L_sync·s+R_sync (3.16)

When synchronisation is achieved the PWM signals can be released to the IGBTs, the mechanical reference torque can be set to the output from the PI controller, and switch A in Figure 3.4 can be thrown into position 2 so that the real currenti_c,rotor can be fed into the controller as feedback instead of the virtual current.