Grid Connected Wind System

In Chapter 8, the control strategy for a PMSG based VSWT integrated at a low voltage distributed grid network in the presence of voltage sag at the point of common coupling (PCC) is presented. The objective of this control strategy is to protect the wind power system (WPS) from any detrimental effect of balanced or unbalanced voltage sag at PCC and remains on line to support grid stability. The control strategy is further enhanced by introducing the non linear and robust sliding mode controller (SMC).

The literature contains some previous work on the active power control of power electronic converters (or inverters) and PMSG-based WTs when connected to a grid [80] and [57],[81]-[87]. In, [80] different techniques for creating current references under unbalanced grid voltage have been studied. However, the paper does not consider the impact on the DC side during abnormal conditions. In addition the peak current is not limited to the pre-disturbed level. Similarly, the approaches in [57], [81]-[84] fail to take grid disturbances into consideration. The dual vector current controller (DVCC) is developed in [85], which uses two current controllers for positive and negative SRFs. Unfortunately, this system leads to a complex control structure and can potentially produce asymmetrical phase currents.

A current controller with positive-sequence grid voltage is used in the positive synchronous reference frame (SRF) [86], but does not completely solve the issue of voltage imbalances. Controlling each phase separately using the peak detection method (PDM) is also proposed as a compensation scheme for unbalanced voltage [87], but such a strategy is hampered by PDM’s sensitivity to low frequency fluctuations and harmonics. The feed-forward power compensation technique is implemented in [88]-[91]. In [88], sinusoidal balanced currents are obtained at the expense of DLV oscillation and a large DC link capacitor. The control to suppress the rise in grid current due to unbalanced voltage sag has not been emphasized in [88]-[91], which could damage the power electronic components of the converter or the oversized converters can suppress the effect. In addition, the control strategy requires a real time calculation for compensating power, which is complex to implement and creates a further delay in addition to the time required for sequence extraction.

For balanced voltage sag, additional sources and sinks are used to compensate for the voltage sag at the PCC. Such solutions include a braking chopper, a crowbar, storage and even a STATCOM [92]-[94], but all serve to further increase the total cost of the WPS.

In Chapter 9, the performance of the control strategy proposed in Chapter 8 is enhanced by using non-linear sliding mode controller. The WES is a non-linear system, which requires a non-linear and robust controller to perform satisfactorily. The classical PI

linear controller is used commonly in WES, which performs well under small disturbances, but may not be effective in the presence of large disturbances, such as transient disturbances [95]. This is because PI controllers are designed based on linearized small signal models. These models become invalid under large disturbances [96].

A nonlinear controller may outperform a linear controller under certain circumstances. Some of these controllers have been used in PEC such as adaptive control [97], hysteresis control [98], intelligent control [99], and modern control theory based controllers such as state feedback controllers [100] and self-tuning controllers [101]. However, most of the nonlinear controls are application dependent and difficult to design and implement, whereas sliding mode control (SMC) is an exception, as it is straightforward to design and easy to implement.

The non-linear SMC has been implemented to enhance the control performance by suppressing the transient effect and bringing robustness to the system [102]-[105]. The SMC is implemented for an ideal grid without any disturbances and the results are compared with PI based vector current control (VCC) [102]. In [103] the voltage, sag effect has been eliminated using the SMC approach however unbalanced conditions are not considered, which are common and responsible for distorted imbalanced output current. The WT is supposed to inject the distorted free balanced sinusoidal currents to the grid according to IEEE 1547 and IEEE 519, which is the primarily focus in [91], [104]–[107]. However, the rise in current amplitude due to the voltage sag is not emphasized. The hard switch limit would further distort the current. The SMC based direct power control (DPC) is implemented for the fast dynamics during an unbalanced voltage sag [104],[105]. However, the SMC control does not guarantee zero steady state error for all disturbances because the integral function is missing in the sliding surface. In [108], the integral function is considered. The SMC based direct power control is normally implemented in a two coordinate stationary reference frame, which has sinusoidal functions. Therefore, to achieve satisfactory performance the compensator must be of high order. In addition, the closed loop bandwidth must be adequately larger

than the frequency of the reference command. Thus, the control design is not straightforward.

This thesis proposes the vector current control with feed-forward of negative sequence voltage (VCCF), including the current limiting strategy under the SMC approach at the NSC, and SMC based DC link voltage regulation at the GSC. The linear controller based voltage feed-forward strategy without considering the NSC’s injected current amplitude has been studied in [109].