Under unbalanced conditions, the most important things are that the current controller is capable of providing a high-quality waveform within a shorter transient response. Once an unbalanced condition occurs in the grid, the current controller must inject the desired positive and negative sequence currents to the grid. During the past few years, several interesting methods have been proposed to deal with abnormal conditions using various techniques. The most popular are voltage oriental control (VOC) which is based on the synchronous reference frame and the stationary reference frame. Other control schemes such as the direct power control method, predictive control, and hysteresis current control have also been proposed.
Chapter 2: Literature Review
A vector control approach was proposed for the first time for a grid rectifier under abnormal grid conditions to derive positive and negative current sequences in 1996 by Rioual et al.[109]. In this work, only a PI current controller was implemented in the synchronous reference frame to control the positive sequence in case of abnormal grid conditions in order to track positive and negative sequence components. Nevertheless, one of the main drawbacks of this type of the control is that the negative sequence voltage during unbalance conditions cannot work properly with this control due to the limitation of the bandwidth. Therefore, in 1999, a combination of two sequence components with the DC link voltage controller in the dq synchronous reference frame has been proposed [110]. The proposed method has a dual current control scheme for the positive and negative sequence currents. The technique is able to reduce the steady state error compared with single current control. However, this method is used when operating the grid-connected inverter only in steady-state conditions and it cannot be applied under unbalanced supply conditions due to limitations in the dynamics of the system.
In another subsequent method, one PI controller was used with resonant (R) instead of dual current controllers allowing a rectifier to generate a dc output under unbalanced grid conditions at double frequency [111]. However, eliminating a steady state error at twice of the fundamental frequency was one of the main critical issues.
In addition, vector control technique has been used in a distributed generation (DG) inverters under abnormal grid operation conditions [112]. Optimum power quality characteristics were achieved, under unbalanced grid voltages. This research focused on many important aspects, such as DC voltage optimization and maximum power exchange, and also a combination of the different methods called a hybrid strategy was described.
Control of the three-phase grid-connected inverter during unbalanced grid conditions using direct power control based on instantaneous active and reactive power has also been proposed.
Chapter 2: Literature Review
On the other hand, vector control method during unbalanced gird condition needs double reference synchronization frames for the positive and negative sequences transformation modules for voltage and current. However, PI controller has some drawbacks such as:
Four PI controllers should be implemented for the positive and negative sequences based on the double synchronous reference frame which will increase the complexity and computational load
In the PI controller, grid synchronization can be achieved with two PLLs using one for the positive sequence components of the voltage and another one of the negative voltage sequence.
As an alternative method to PI current control based on the double synchronous reference frame, proportional-resonant controllers can extend the operation on stationary reference α-β rotating frame to control the current injected by the grid-connected power converter under abnormal grid conditions [116]. In this control scheme, the output currents are converted to the stationary reference frames that results in two AC current signals to be controlled [112, 117]. This type of control is based on frequency adaptive filters. These filters give the same performance for both positive and negative frequencies [62], [118]. In addition, the multiple d- q rotations become unnecessary.
It can be said that there are many advantages of using PR controller including:
We do not require additional controllers for the positive and negative sequence currents. Synchronization can be achieved with one PLL using the positive sequence component of the voltage.
Only one PR controller can regulate positive and negative sequences (+ω and –ω) Only the positive sequence angle needs to be synchronized.
Based on current control approaches, the current injected into the grid by the three-phase inverter should follow different strategies in unbalance grid conditions at the point of common coupling (PCC). In the literature, different control approaches are used to meet the requirement of control under abnormal conditions and to enhance the system’s response [79]. The first approach is instantaneous unity power factor control (IUPFC). This method of control is essential to be used if the injected current vector is required to follow the voltage vector. A second approach is average unity power factor control (AUPFC). In this strategy, the harmonics can be reduced, resulting in a lower THD.
Chapter 2: Literature Review
A further method is instantaneous active reactive control (IARC). This method is more useful in control active and reactive power based on p-q theory. In this type of method, both active and reactive power is controlled instantaneously since the current vector is aligned with voltage vector as well as any orthogonal components. However, the voltage vector has oscillations at twice the fundamental grid frequency that increases the harmonics into the grid [29].
To overcome these drawbacks another strategy has been proposed, called the positive and negative sequence control (PNSC). This strategy can deal with the current control under unbalanced conditions. Here, both positive and negative sequence current control is applied in the system. This method can be used in the synchronous reference frame using dual PI controllers. However, one of the issues can be that when the reactive power not considered, the oscillation term of q will not appear.
In the balance positive sequence control (BPSC) method a balanced sinusoidal waveform is injected by applying positive sequence control to the grid during unbalanced conditions. There are many advantages of this strategy, such as improving the power quality of the system by allowing the current waveform to be symmetrical during unbalanced conditions. In addition, it is a simple method since only the positive sequence angle needs to be synchronized. This method can be easily applied to stationary reference frame using PR controller while needs double synchronous reference frame using dual PI controllers. This method is the only one can obtain the balanced current among other strategies [29].