Research Paper on Inverter Control Systems: Ensuring Quality Power Supply

1. Introduction

The need for a renewable source of power that uses inverters led to the creation of a more sophisticated power system called inverter control systems. The new scheme guarantees the users high-quality power that can support the function of many distributed generators. It can also supply energy to various loads depending on the ability of energy source (Yoon et al., 2018). According to the result of the research conducted on consistency, synchronization, and optimization, the new power system in hybrid microgrids shows that droop control system has multiple benefits in either in sparse communications or plug (Zhang et al. 2017). It has various qualities of synchronous generators that make it superior to other systems (Hosseinzadeh and Salmasi, 2015). For that matter, it is mostly applicable in microgrid control (Li et al., 2017) because it can guarantee accurate power-sharing. It is not easy to enhance the accuracy in which microgrids that are in island-mode share their energy (Sun et al., 2014). The P-V control scheme is the appropriate mechanism that can be used to accurately share power by the fine-tuning coefficient of droop or simultaneously adjusting the droop gain. Usually try to moderate droop gain since a high level of droop gain destabilizes the power system (Vasquez et al., 2009). According to the research conducted by Vasquez, et al. (2009) power control loop can overcome errors that make the system unstable while the result of the study conducted by He, Li and Blaabjerg (2015) indicates various techniques that can be employed in adding high power signal. Since the method requires the generation of top signals and complicated processing for it to work in a microgrid, it is not the appropriate method to use. Therefore Lee et al. (2013) introduced Q-V dot droop, although it could also not allow accurate power-sharing when local loads are used primarily reactive power. As a result, there was a need to incorporate a reactive power disturbance in the equation of p-f droop to eliminate errors that inhibit reactive power-sharing. It merely controls active power to influence power-sharing in microgrids (He and Li, 2012).

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Virtual impedance is widely used nowadays in ensuring that there is a solution to the problem of reactive power-sharing. It means incorporating output impedance regulating section in the exterior part of the inverter switch. The virtual impedance suppresses power supply of the line impedance. Its use only increases the cost; thus, there is a need to ensure that the current-sharing device is eliminated completely. According to He et al., (2018) there is another type of coupled virtual impedance which senses power supply in microgrid and cut sections where there is common coupling by simply controlling it. The result of the research conducted by Dong et al. (2018) showed that virtual impedance is also essential for solving power allocation issue and its consequent choral clampdown in droop control. For it to work, users must have more information about line impedance. Gupta and Swarnkar (2018) created a new topography capable of improving power-sharing quality, and since the modification of the closed-loop device influences output impedance of the power source, virtual impedance is a very critical parameter in the droop control system (Guerrero et al., 2007). As a result, the circulating current flowing through two micro-sources reduces, thus enhancing power-sharing precision. Another way that can be used in improving power-sharing precision is the use of adaptive virtual impedance, which requires the addition of voltage drop within different droop generators. Because the application of this technique uses centralized control mechanism and it relies on communication reliability network, it can be used in enhancing distribution of both active and reactive power (Micallef et al., 2014). The mechanism is capable of achieving a positive result, although it is more complicated than any other method. Other researchers adopted the use of multi-agency theory, which demands the use of communication between micro-sources that are next to one another (Anand et al, 2013).

2. Mechanism of Improving Droop Controller Based on Virtual Impedance

Islanded microgrid usually suppresses circulating current and also prevents accurate power-sharing. Circulating current can only pass through inverters resulting in overheating of various electronic, inhibit efficient transmission of power and destabilizes the system. The most appropriate action to take is to find ways in which circulating current is suppressed when creating DGs and microgrid. This problem can be eliminated by applying multi-loop control, which has droop control, voltage, and current control to improve the consistency and efficiency of parallel inverters (Zhang et al., 2015). The solution to this problem does not require new invention but require the improvement of the conventional method. Its improvement requires the addition of a model that solves the problem of change in output voltage (Mahmood, Michaelson, and Jiang, 2015). To eliminate the frequency variation problem, it is essential to use phased angle droop control. The improvement of power-sharing accuracy, various proposals have been made. It includes the use of Q-U dot droop control while droop control with voltage drop compensator is identified to regulate voltage frequency variation. The proposal ensures that there is compensation of voltages lost as a result of incompatible line impedance (Wang, Liu, and Zhang, 2017). The application of these techniques is capable of enhancing the precision of power-sharing of inverters connected parallel without taking into account local load. In the evaluation, it is essential to use one of the inverters as the point of reference to allow for the determination of discrepancy in feeder impedances. The reduction of the effect of line impedance, it is important to introduce external inductance to the inverter to improve the appearance of the design (Majumder et al., 2009). The use of external conductors with higher values usually increases the cost of investment and loss of power. For that reason, there is a need to introduce new design composing of various parameters such as closed-loop controller and transmission line. The application of this design is incapable of distributing power equally and also suppresses circulating current. To alter output characteristics, it is necessary to feedback output current to reference voltage by using virtual impedance (Chen et al., 2016). The efficiency of virtual impedance is relative when there is an alteration of the load, and the drift of parameters and inoculation of harmonic overcomes the destruction of circulating current. The elimination of this problem requires the implementation of adaptive virtual impedance to ensure the system is stable and also enhance power-sharing ability (Shi et al., 2014). Because external inductance is capable of ensuring there is high energy in the system and virtual inductance controls system flexibility, they are all used together.

3. Circulating Current

The flow of circulating current through various inverters with various power capacities in a parallel circuit, the description of the inverter is as shown below.

Where a1: a2... an = P1 rated: P2 rated... Pn rated

In this case, because it is a multi-inverter system, it is essential to select an inverter that has the least power capacity. It must be used as a reference inverter making a1 1.i0i to be its output current while i unit is reflected as shown below:

iunit=in=1i0iin=1aiThe result of the research conducted by Xiao, Luo, Shuai, and Jin (2016) on circulating current where there is a parallel connection of inverters shown below in figure 2.

Fig 1: Connection of parallel Inverters

The grid voltage is represented by U0<d0 while the output voltage is U0i<di i=1,2. Within the same figure equivalent, feeder impedance is represented by ZeqLi while LG reflects external inductance. Within the same system, there is also equivalent impedance of the load which is abbreviated as Zlocal i and line impedance abbreviated as. Since the calculation of ZeqLi is clear, the flow of circulating current can be obtained as illustrated below:

iH1= -iH2= a1.i01+i0a1+a2-io1=1a1+a2(a1.io2-a2-io1)

=1a1+a2(a1.UO2<d0-U0<d0ZeqL2-a2.U0i<di -U0<d0ZeqLi)Both the proportion of equivalent feeder and the output voltage affect the strength of circulating current. For that reason, it is possible to eliminate circulating current since a1ZeqL1=a2ZeqL2 when U0i=U02, d1 = d2. Because the use of external inductance makes it more complex and difficult to understand, it is, therefore, essential to use the equivalent feeder.

4.Control Mechanism

The mechanism which is used for control purposes comprises of various components as illustrated below:

Fig 2: Control mechanism of inverters

The control mechanisms have resistors represented by rf, capacitors by Lf and inductors by Cf. 1i represent the current that flows inside the filter while uo and io are used to show output voltage and the amount of current flowing through the inverter.iL Stands for the transmission line, while LG represents external inductor. Figure 2 also has compensation voltages meant for a similar line, which is reflected by Ucom while the voltage of adaptive virtual impedance control is depicted by udqV. Other abbreviations show reference signals, and they include Udq and old. These initials are for both voltage control and current control.

4.1 Equivalent Feeder

For microgrid that consumes high voltages, there is a need to use external inductor. It is usually used in the output side to increase the amount of energy leaving the inverter and also to simplify the configuration of the inverter. Other important elements that must be available include transmission line and the load. These two elements cause a mismatch of the line impedance, thus ensuring that there is a high voltage difference and also intensify circulating current in all the inverters that are in parallel connection. The examination of the system is simplified by adding an equivalent feeder to replace external inductance, load, and even the transmission line. This is done for all the inverters used in the system. Under the condition where n inverter work in parallel connection, both active and reactive power for the rest of inverters are shown as indicated below:

PGi=PLocal L+ PLiQGi=QLocal L+ PLiTo effectively calcul...