Coordinated Control of Multifunctional Inverters for Voltage Support and Harmonic Compensation In a Grid-Connected Microgrid

In this paper, a coordinated harmonic compensation and voltage support scheme is presented for distributed generations’ (DGs’) interface inverters in a resistive grid-connected microgrid. Voltage support is performed by reactive power compensation which can mitigate the over/under voltage problem; furthermore, the active power curtailment is proposed in order to mitigate the overvoltage problem when the reactive power compensation is not sufficient. Harmonic compensation is achieved by using virtual admittances in selected harmonic frequencies. Reactive power and harmonic compensation currents are injected with regards to the limited capacity of the interface inverter. If necessary, the reference powers of the grid-tied inverters are changed. Voltage support and harmonic compensation can be achieved based on local or central (communication-based) measurement schemes. The effect of communication delay is also investigated in this study. Experimental and simulation results are obtained in order to demonstrate the effectiveness of the proposed method. Coordinated Control of Multifunctional Inverters for Voltage Support and Harmonic Compensation In a Grid-Connected Microgrid Keyword: distributed generation; harmonics compensation; DG interfacing inverter; reactive power compensation.

To fully comprehend the voltage support of the DG interfacing inverters, reactive power compensation is conventionally proposed in distribution systems [2]- [7], [11], 12] and MGs [8]- [10] in order to mitigate the over/under voltage problems. Overvoltage can be created because of high penetration of Wind Turbine (WT) and Photovoltaic (PV) systems [2]. In [2] and [3], the voltage rise problem caused by high penetration of DGs in an LV distribution system has been studied and a droop-based reactive power control is proposed for Current Controlled Mode (CCM) of VSIs. In these papers, the reactive power compensation by DGs interfacing inverters is considered. In [4] and [5], the voltage rise mitigation based on active power curtailment in distributed systems has been proposed. The power curtailment algorithms of [4] and [5] are based on local measurement and communication system, respectively. In [6], different reactive power methods for DG units are studied. The reactive power control approaches in [6] can be classified into distributed and central controller based schemes. The reactive power control of DG interfacing inverters is also studied in [7]. In this paper, only a grid-connected interfacing inverter is considered whereas in grid-tied MGs more DG units can exist while a coordinated control of the units is required. In [8] and [9], voltage and frequency support functions of utility scale PV systems have been proposed. In [8], the performance of a PV system in voltage sag/ swell compensation is evaluated based on the small signal modeling of utility scale PV and power system. In addition, the frequency support is added in [9]. In [10] and [11], the voltage support by Voltage Control Mode (VCM) VSIs in a grid-connected MG has been discussed, while, while it is obvious that, PV and WT systems are integrated as CCM VSIs.
On the other hand, use of DGs interface inverters for compensation of harmonics has been proposed in [12]- [22]. The harmonic compensation can be achieved based on communication systems and a central controller [12]- [17] or local measurement [18]- [22].
In the communication-based compensation, the data which is used for harmonic compensation is obtained by central controller or measurement whereas in the local compensation; the compensation is achieved without any need for communication systems.
In central controller based methods, the compensation can be performed more exactly and more effectively whereas the reliability of the system is decreased and the cost and complexity of the system are increased. The method of [15] is based on central measurement of Point of Common Coupling (PCC) voltage in a grid-connected MG. In [15], the harmonic compensation is achieved by central measurement and using VCM VSIs with decentralized controller. In other words, inverters are controlled locally based on the information received from a remote bus. However, in grid-connected MGs, the interfacing VSIs of PV and WT units operate in CCM and their remaining capacity of them (which can be dedicated to harmonic compensation) changes during a day because of the maximum power point variation. The limited capacity of the inverters is not considered in [15] and the communication system failure can deactivate the harmonics compensation.
Using capacitive virtual impedance which can compensate the harmonic voltage drops of lines and filter impedances has been recommended as a local compensation for VCM VSIs in [18]- [20]. Using virtual admittance is recommended for the harmonic compensation by CCM VSIs in distribution system [21] and VCM VSIs in a grid-connected MG [22]. In [21] and [22], the control of grid-tied VCM VSI is done by using a fixed value of virtual admittance and local measurement.
In the present paper, a coordinated harmonic compensation and voltage support method are proposed for CCM VSIs in a grid-tied MG. A weak MG with high resistance is chosen as a case study. A voltage support approach with local and central measurement of PCC voltage is proposed in this paper. The voltage support algorithm can mitigate under/over voltage problems of the grid-tied MG by using reactive power control with considering the limited capacity of the inverters; Furthermore, a power curtailment algorithm is proposed in order to mitigate voltage rise problem when the reactive power compensation is not sufficient. The voltage support algorithm is flexible in using central or local measurement of voltage. In addition, a harmonic mitigation method is proposed based on the virtual admittance which has also the flexibility for changing from central measurement to local measurement when the communication system failure happens or the delay of the communication system is too high.
The main contributions of the paper are listed below: • A coordinated control of CCM VSIs for harmonic compensation and reactive power sharing/support.
• Considering the limited capacity of the inverter in harmonic and reactive power compensation.
• Voltage rise mitigation by using reactive power control and power curtailment of the DG interfacing inverters.
• Flexibility for choosing the local or remote (central) measurement • Considering the effect of communication system delay on power quality improvement.
Rest of the paper is presented as follows; Section 2 is focused on the general scheme of the system under study. Section 3 describes the details of the control system. Afterwards, simulation and experimental results are presented in Section 4. Finally, the paper is concluded in Section 5.

Control approach for DG interface inverter
Fig . 2 shows the control block of DG interfacing inverter. As mentioned before, the DG interfacing inverters are connected to the MG by using an LCL filter. Harmonic resonance problem can inherently endanger the stability of the system due to the use of LCL filter [23]. Different active and passive methods have been proposed by researchers to solve the resonance problem [24], [25]. In this paper, capacitor current feedback is used for damping the resonance (see K D in Fig. 2). In this control scheme, multi-loop control scheme is used for controlling the injected current of the VSI. The reference power is tracked by the outer loop whereas the stability improvement and resonance damping are carried out through the inner loop [24].
In the outer control loop, Proportional Resonant (PR) controllers which are tuned at fundamental, 5 th , 7 th and 11 th harmonics orders are used. The output voltage of LCL filter controlling its voltage provides a better voltage quality at PCC [16].
As depicted in Fig In Fig.2, P ref is the active power that can be injected to the grid. The reference active and reactive powers (P * and Q * ) and the reference harmonic compensation current (I com,αβ ) are generated by voltage support and harmonics compensation blocks, respectively. After calculation of the reference of active and reactive powers (P * and Q * ), the fundamental current reference in dq frame (I ref,dq ) is generated by (1) [26]. Generally DC voltage feedback is used in order to fix the DC bus voltage [26]. Since in this paper, the focus is on the inverter control, a constant DC voltage source is assumed; thus, the DC voltage feedback does not work. In order to prevent of any misunderstanding, the feedback is not considered in this equation: Division of the DC bus voltage which is applied before PWM block can isolate the tuning of the controller against the DC voltage changes. However, during the startup, since the amount of DC voltage is low, the division to DC voltage can cause a large gain which can lead to instability. In this paper, the measured DC voltage passes through a saturation block whose minimum and maximum values are defined as 0.9 V DC,N and 1.1 V DC,N (where

A. Voltage support function of the VSI interfacing inverter
Reactive power control is conventionally used for voltage regulation in power systems.
Incorporation of the free capacity of DGs inverters is considered as a potential for reactive power compensation and voltage support. Although IEEE 1547 standard [27] forbids the use of reactive power capability of inverter-based DGs for this purpose, this policy would be changed by increasing penetration of DG systems [28].
International Electronical Committee (IEC) 61850-90-7 recommends advanced reactive functions and object models for power converter based Distributed Energy Resources (DERs) [29]. According to this standard, the DG units shall respond to the voltage variation in order to enhance the voltage profile of power system [30]. According to the standard and the method presented in [3], the reactive power compensation is implemented in this paper.
For achieving this aim, the RMS of fundamental component of the measured voltage is compared to its reference value. Afterward, according to the voltage and the remaining capacity, the reference reactive power of the inverter is generated. The following criteria are utilized to compensate over/under voltage problems by the inverter according to IEC where V 1 =0.9V r , V 2 =0.95V r , V 3 =1.025V r and V 4 =1.05V r . V r represents the rated voltage of the grid which is equal to 230V in this paper. The threshold values of V 1 =0.9V r and V 4 =1.05V r are determined according to IEEE 1159 standard as minimum and maximum allowable voltages, respectively [31]. According to IEEE 1547 standard, the maximum allowable voltage fluctuation caused by DG is set at 5%. Furthermore, it is compatible with U.S. grid code in which the normal operating range of a PV system is defined in the range of 0.95-1.05 p.u. while in extreme conditions the voltage range of 0.88-1.1 p.u. is defined [5]. However, the maximum value is more restricted than the EN 50160 European standard [32] in which the allowable voltage is in the range of 0.9-1.1 p.u. It should be mentioned that in the above equation, Q max represents the free capacity of the inverter for reactive power compensation which can be written as:  The method presented in [3] is only based on local measurements and the voltage rise mitigation is achieved by measuring the voltage of each DG. In this approach, since the different voltages may be measured by each DG in a distribution system, unfair reactive power compensation occurs between DGs. However, in the present paper, when central measurement is used, due to the fact that the PCC voltage information is sent to each DG, the same voltage is measured by DGs and the reactive compensation efforts of DGs are proportional to their capacities. If the communication system fails, the local control can undertake compensation; hence, the reactive power compensation will be active even a communication failure happens.
In [33], power curtailment approach has been recommended for PV systems in order to overcome the over frequency problem when the consumption is less than the production of DG units in an inductive islanded microgrid. However, in resistive systems, the voltage amplitude is depended on active power; hence, the power curtailment may mitigate the voltage rise problem [4], [5]. The presented method in [4] is where K is reduction coefficient which is defined as where V 5 =1.05V r and V 6 =1.1V r . This voltage rise mitigation approach is depicted in Fig. 4.    Fig 2; (a) G Hh calculation block; (b) overall scheme power and the total power of all MG DGs are used as a gain (S i /ƩS j ). By using the gain, the DGs with higher rated power contributes more in harmonic compensation.
Note that the H h * can be determined based on the sensitivity of the load to harmonics or grid codes and standards. standard [34]. The maximum injectable harmonic current can be calculated as follows: As shown in Fig. 3(b), the compensation current at h harmonic (I com,hdq ) is calculated by multiplying the virtual admittance with corresponding voltage harmonic. Afterward, the compensation current in the dq frame is transformed to the αβ (stationary) frame. Finally, the compensation currents at different frequencies are added together to create the total compensation current (I com,αβ ).

Simulation and experimental results
The system shown in Fig. 1, with two inverter-based DGs is used as the case study. Each DG is connected to the PCC by using an LCL filters and a distribution line. PCC voltage measurement is implemented and fundamental and harmonics contents of PCC voltage are transferred to each DG by a one-way communication system. The parameters of power and control systems of the case study are listed in Tables1 and 2, respectively. As it can be found in Table.1, the lines impedances of DG 1 and DG 2 are considered different in order to evaluate the proposed algorithm better; Furthermore, it is assumed that the capacities of the inverters are 10% more than their available maximum powers which can be delivered in order to provide some rooms for reactive power and harmonics compensation.
For evaluation of voltage support and harmonic compensation functions, one simulation scenario and two experimental scenarios are studied.

A. First Scenario (simulation): High consumption and low power injection:
In order to evaluate the effectiveness of the method for compensation in voltage drop conditions, the DGs generated powers are assumed to be low (300 W); and a combination of nonlinear and linear loads is considered. The following steps are taken into account: • Step I (t<4s): Without any power quality compensation    [13], [15] and [16]. Fig. 9 shows the PCC voltage before and after compensation (in Steps II and III).  Table 1. The diagram of the system is similar to the first scenario without any local load in order to evaluate the effectiveness of the method in voltage rise mitigation. The following steps are considered: • Step A: integration of DGs without voltage support The test is implemented when the grid voltage is approximately 231 V. Active and reactive powers which are delivered by DGs and amplitudes of PCC and output voltage of DGs are depicted in Fig. 11. As depicted in Fig. 11 (a) higher than DG 1 . This is attributed to the higher voltage of DG 2 in respect to DG 1 due to the higher line resistance of DG 2 . In this step, due to the occurrence of power curtailment, the available reactive power increases according to Eq. 2; furthermore, it shows that if local measurement is used, the power curtailment is unfair between DGs due to different voltage measurement of DGs.
After reactivation of the communication system in Step E, the power curtailment does not happen because the reactive power compensation is enough to recover the PCC voltage as depicted in Fig. 11(a) and the PCC voltage amplitude is under the 1.05V r which is defined as threshold of power curtailment algorithm according to Fig .4. It shows that using the PCC voltage measurement can provide a real evaluation of the voltage rise problem in PCC and can prevent the unnecessary compensation as reported in Step D and is used in [4].
In order to shows the effect of grid voltage on the voltage rise of PCC, another experimental test is performed with the grid voltage of 233 V. Fig. 12 shows the performance of the proposed method in voltage rise mitigation. Fig. 12(a) shows the PCC, DG 1 and DG 2 output voltages. The voltage amplitudes in Fig. 12  Step D and using local measurement same as the methods proposed in [4], since the voltage rise in this case is more severe than the previous one, the power curtailments of DGs are also higher. It also shows that an unfair power curtailment happens because of different local measurements of DGs output voltages. After activation of the one-way communication system in Step E, a fair power curtailment happens for DG 1 and DG 2 .

Third Scenario: High generation and medium demand (with nonlinear load)
In this case, a nonlinear load is connected to the PCC bus in order to experimentally validate the harmonic compensation and reactive power support when the load is not as high as the first scenario and only nonlinear load is connected in PCC; the parameters of the nonlinear load are also listed in Table 1.
For evaluation of harmonic compensation, it is assumed that firstly the communication system is working properly with t=5ms delay. After that, the communication system fails and the local measurement is activated instead of the remote one. Fig. 13 (a) shows the voltage waveform of PCC before compensation while the measured harmonic content is depicted in Fig. 13 (b). As shown in Fig. 13 (b), the THD and 5 th order harmonic exceeds 5% and 3%, respectively, which are defined as allowable values according to IEEE 519 standard [35].
Fig.14 shows the PCC voltage waveform and its harmonics contents after compensation considering t=5ms delay time. As depicted in Fig. 14 (b), the harmonic compensation is achieved and the amounts of harmonics contents are below the limits of IEEE 519 standard. Fig. 15 shows the PCC voltage waveform and its harmonic content after communication failure. As observed, by using local measurement, in the case of remote (at PCC) measurement, the voltage harmonic compensation is achieved; however, the compensation was more effective.  Table 3. Also, it can be observed that the local measurement based compensation (the results shown in Fig. 15) can be more effective than the remote measurement case when the delay of communication system is too high (for instance 50ms). For evaluation of reactive power control in this scenario, three following steps are implemented: • Step a: without reactive power compensation  Fig. 16(b). As can be seen in Fig.   16(a), when the local measurement is available (in Step b), the DG units contribute in voltage rise mitigation by absorbing reactive power. The reactive power compensation is stopped in Step c where the remote measurement is used, because the measured voltage of PCC shows that the amplitude is lower than 1.025 V 1 (below the limit as defined in (2)) and thus, the voltage rise mitigation is not necessarily needed; whereas in Step b the local measurements have shown the necessity of compensation.

Conclusions
In