PARALLEL INVERTERS USING VSG IN MICRO GRIDS WITH

FUZZY CONTROLLER M. Viswanadh1, G. Jayakrishna2

1. M. Tech scholar (EPS), Narayana Engineering College, Nellore, Andhra Pradesh-524004.

2. Professor (EEE), Narayana Engineering College, Nellore, Andhra Pradesh-524004.

Abstract— Virtual synchronous generator (VSG) control

is a promising communication less control procedure in a

microgrid for its inertia support feature. By using enhanced

VSG control, oscillation damping and proper transient active

power sharing are achieved by adjusting the virtual stator

reactance adjuster and also accurate reactive power sharing is

achieved based on inversed voltage droop control and

common AC bus voltage estimation. Furthermore, FLC is

used as a suitable control mechanism for controlling the

parallel inverters in microgrid with which the power

oscillations are further reduced and accurate active & reactive

power sharing are achieved. The simulation results prove that

the proposed method is superior in reduction of total harmonic

distortions (THD) of grid voltage and current.

Keywords— Distributed power generation, droop control,

micro grids, reactive power control, fuzzy controller, virtual

synchronous generator.

I. INTRODUCTION

Nowadays, inverter-interfaced distributed generators (DGs) with renewable energy sources (RES), e.g., photovoltaic and wind turbine, solar, tidal power have been developed to solve power crisis and environmental issues. They are connected to main grid via power inverters, which are often connected in parallel to provide system redundancy and reliability. They often form microgrid before connecting to main grid. The control strategies of microgrids are preferred to be in a communication less manner because of its decentralized feature. However in a hierarchical microgrid control structure, communication is required for the secondary and tertiary control, it is still recommended to realize the basic functions of a microgrid in the primary control level without communication [2], [3]. Droop control is a widely used communication-less control method in a microgrid. By drooping the frequency against the active power (P– ω cling) and output voltage in opposition to reactive power (Q– V droop), load sharing among DGs may be achieved in an autonomic way, which resembles the power sharing among parallel synchronous generators (SGs) [4], [5]. ]. In the references [6]-[8], it's proposed that P– V and Q– ω controls are suitable for low voltage (LV) microgrid in the light of the resistive line impedance aspect. In any way, the P– ω and Q– V droop controls are still valid in LV microgrid by including inductive virtual impedance [2], [3], [9].

However, as the greater part of DG control techniques and conventional droop control gives barely any inertia support to the microgrid, in this way droop control-based microgrid is usually inertia less and sensitive to fault. To give inertia

support for the system, control techniques that provide virtual inertia are proposed in recent works, as an example, virtual synchronous generator (VSG) [10], synchronverter [20]. Although their name and control scheme are differ from each other the principles are similar in the way that all of them mimic the transient characteristics of the synchronous generator by emulating its fundamental swing equation. All these techniques are known as VSG control schemes in this paper. To share load in parallel operation, droop control characteristics are also emulated in VSG control schemes. In this case the VSG control scheme inherits the advantages of droop control and outperforms the latter in terms of transient frequency stability owing to its lower df/dt rate.

When basic VSG is control applied in a microgrid, few issues have been observed, like oscillations in active power during a disturbance, improper transient active power sharing during load transition and errors in reactive power sharing. Active power oscillations during a disturbance is introduced by the notable a part of the swing condition; since it is an inherent feature for a real SG as well as VSG. It is not a major issue for a SGs as they have huge over-load limits, but the over-load capacities of inverter-interfaced DGs are not correctly excessive to ride through these oscillations. Nevertheless, these oscillations can be damped by properly increasing the damping proportion or using alternating moment of inertia. Using smaller inertia may moreover incite to reduced oscillations; but, it is not encouraged as providing large amount of inertia is an advantage of VSG control from other control techniques.

The inaccurate reactive power sharing is a well known problem in both basic Q-V droop control, and P-V droop control. In Q-V or P-V droop controls, output voltage is regulated according to active and reactive power sharing, but output voltage of each is not equal due to unequal line voltage drop. A solution is to eliminate the output impedance mismatch of DGs, but this technique cannot guarantee accurate reactive power sharing if active power is not shared according to power rating ration.

In this paper, a communication-less approach is proposed based on inversed voltage droop control (V-Q droop control) and common AC bus voltage estimation. By applying the proposed method, reactive power sharing is immune to line impedance mismatch and active power sharing change. The idea of ac bus voltage as a common reference shares some similarities with the approaches presented in [21],[39] and [40]. In microgrid applications using the measured bus voltage directly may not be feasible if DGs are not in the proximity of AC bus. In these paper bus voltage is estimated based on available local measurements, thus there should be no installation difficulty in the field applications.

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Volume 7, Issue X, October/2018

ISSN NO:2236-6124

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II. BASIC VSG CONTROL SCHEME

Fig. 1 shows the structure of a DG using the basic

VSG control [14]. The primary source of the DG could be

photovoltaic panels, gas engine, wind turbine or other

appropriated distributed energy resources (DERs). The energy

storage is designed for emulating the kinetic energy stored in

rotating mass of a SG, in order to supply or absorb the

insufficient/surplus power generated by primary source during

transient state [13]. This paper focuses on the control scheme

of the inverter. In the block "Swing Equation Function" in Fig.

1(a), ωm is solved from the swing equation (1) by an iterative

system.

𝑃𝑖𝑛 − 𝑃𝑜𝑢𝑡 = 𝐽𝜔𝑚𝑑𝜔𝑚

𝑑𝑥+ 𝐷(𝜔𝑚 − 𝜔𝑔) (1)

The block "Governer Model" in Fig. 1(a) is a ω– P

droop controller as shown in Fig. 1(b). In some previous

studies [12]– [14], a first order lag unit is used to emulate the

mechanical delay in the governer of a actual SG. However, on

this paper, this delay is removed as it degrades the dynamic

performance of DG .

The block "Q Droop" in Fig. 1(a) is a V– Q droop

controller as shown in Fig. 1(c), which differs from the

conventional Q-V droop controller in reversed input and

output. It is simple that inner current or voltage loop is not

adopted in this control scheme, in order to make filter inductor

Lf contribute to the output impedance and be considered as the

stator inductance of the VSG. This stator inductance results

more inductive output impedance, which is important for

active and reactive power decoupling in a low voltage

microgrid in which line resistance is dominant.

Fig. 1. Block diagram of (a) the basic VSG control, (b) the

“Governor Model” block and (c) the “Q Droop” block.

Eventually, output voltage is still regulated indirectly

through the V– Q droop controller and the PI controller of

reactive power. In order to decrease the effect from ripples in

the measured output power, a 20Hz first order low-pass filter

is connected for Qout as shown in Fig. 1(a). As the output

current is measured after the LC filter stage, the reactive

power consumed by using the LC filter is not included in Qout.

Thus, no particular inertial procedure is required for the

reactive power PI controller.

In a microgrid, in order to share active and reactive

powers according to ratings of DGs with communication,

𝑘𝑝∗ = (𝑘𝑝𝜔𝑜) 𝑆𝑏𝑎𝑠𝑒⁄ , 𝑘𝑞

∗ = (𝑘𝑞𝐸𝑜) 𝑆𝑏𝑎𝑠𝑒⁄ , 𝑃𝑜∗ = 𝑃𝑜 𝑆𝑏𝑎𝑠𝑒⁄ and

𝑄𝑜∗ = 𝑄𝑜 𝑆𝑏𝑎𝑠𝑒⁄ must be designed for every DG in default [2].

In this paper to simplify the clarification for the case of

different power ratings, per unit values are calculated based

respective power ratings of each DG.

Fig. 2. Structure of a microgrid in islanded mode.

III. IMPROVEMENT OF REACTIVE POWER

SHARING

Fig. 3 shows the principles of ω– P and V– Q droop controls in the "Governor Model" and "Q Droop" blocks shown in Fig.1 for the case of Sbase1: Sbase2=2 : 1. As discussed in Section II, kp*, kq*, P0*andQ0* are designed equally. In view of the predefined linear droop characteristics the desired power sharing Pin1: Pin2 = 2 : 1 can be obtained because the governor input is ωm , and ωm1=ωm2 is ensured in steady state.

Following the same principle, to share the reactive power according to power rating ratio, an equal voltage reference is required. However, for the V– Q droop in baisc VSG control shown in Fig. 1(c), the voltage reference is the inverter output voltage, which may be a different value for each DG even in steady state due to the line voltage drop. As most of previous studies are based on Q–V droop, in which the output voltage Vout i , should be regulated based on measured reactive power Qout i , the basic idea to address this issue is to equalize Vout i by equalizing the output output impedance [26],[[27] or to compensate the line voltage drop [28]. Both methods need great effort in design process and complex computations in DG control law, whereas the resulted reactive power sharing is still influenced by active power sharing.

As the voltage does not need to be controlled directly in a V–Q droop control scheme shown in Fig. 1(a), the reference voltage can be chosen other than inverter output voltage. If the common ac bus voltage Vbus is used instead of inverter output voltage Vout i, equal reactive power reference Qref 1 = Qref 2 can be guaranteed, as it is illustrated in Fig. 3.

Therefore, accurate reactive power sharing Qout1 = Qout2 should be obtained through the using of reactive power PI controller.Moreover, unlike output voltage, bus voltage is not

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ISSN NO:2236-6124

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Fig. 3. Principles of ω–P and V–Q droop control.

influenced by line voltage drop, which is determined by both

active and reactive power. Therefore, reactive power sharing

according to the bus voltage is independent from active power.

Therefore, a bus voltage estimation method using local

measurements is proposed in this paper.

(a)

Fig. 4. Block diagram of (a) the proposed enhanced

VSG control, (b) the “Stator Reactance Adjuster” block

and (c) the “Vbus Estimator” block.

IV. PROPOSED ENHANCED VSG CONTROL

SCHEME WITH FLC

The proposed advanced VSG control scheme is

shown in Fig. 4. Compared to the basic VSG control, three

noteworthy modifications are made, i.e., the stator reactance

adjuster and the AC bus voltage estimator, as shown in Figs.

4(b) and 4(c), respectively and a fuzzy logic controller (FLC)

instead of PI controller. The function of stator reactance

adjuster is to change the output reactance of the each DG

freely. It is functioning as a virtual impedance controller. The

virtual stator inductor is realized by multiplying output current

by virtual stator inductor in stationary frame. It will be more

precision if inductor current through Lf is utilized. However,

this builds the number of current sensors, which is redundant.

As the current flowing into Cf at fundamental frequency is not

as much as few percent of the inductor current, utilizing output

current rather than inductor current does not influence the

execution of the control scheme. The tuning of virtual stator

inductor Lls is recommended to set total output reactance for

the two DGs in same huge per unit value. This approach

builds active power damping ratio and shares transient load

without oscillations. The target value is proposed to be 0.7 pu

since it is a typical value for the total direct axis transient

reactance Xd of a real SG.

𝑋𝑖2 =

𝑆𝑏𝑎𝑠𝑒𝑖𝜔𝑚𝑖(𝐿𝑙𝑠𝑖+𝐿𝑓+𝐿𝑙𝑖𝑛𝑒𝑖)

𝐸02 = 0.7 𝑝. 𝑢 (2)

The Lfi and Zlinei (Rlinei+jLlinei) are considered as known parameters. As the size of microgrid is usually small, the line distance is effectively to be measured or fed by the designer. Regardless of the possibility that it is not the situation, a few online measurement or clever tuning strategies are available for Zlinei

With the proposed plan of stator reactance adjustment, oscillation in a VSG-control-based microgrid ought to be almost eliminated during a loading transition in islanded mode. Particularly, transition from grid connected mode to islanded mode can likewise be considered as a loading transition; therefore, the oscillations during an islanding situation ought to likewise be eliminated with the proposed control methodology. With respect to different unsettling influences in islanded mode, e.g., change of active power set estimation of DG(s), connection/separation of DG(s), and so on, oscillations can't be eliminated, however can even now be damped by the increased total output reactance. .

The guideline of transport bus voltage estimator in figure 4.3 is similar to that of stator reactance adjuster in figure 4.2. By calculating the line voltage drop in stationary frame using measured output current and line impedance information, the bus voltage can be evaluated from the difference of output voltage and computed line voltage drop. Since the RMS value of estimated transport bus voltage Vbus for every DG ought to be approximately equal, accurate reactive power sharing can be obtained by utilizing estimated bus voltages as the input references of "Q Droop" rather than respective output voltages of DGs. Despite the fact that the rule of presented transport bus voltage estimator is not new, the possibility of using this estimator to acknowledge communication less accurate reactive power sharing can be considered as a commitment in the present work. In any case, if there's an estimation error in

�̂�𝑏𝑢𝑠∗ , it'll cause a reactive power sharing error. Supposing

�̂�𝑏𝑢𝑠1∗ = 𝑉𝑏𝑢𝑠

∗ + ∆�̂�1∗ and �̂�𝑏𝑢𝑠2

∗ = 𝑉𝑏𝑢𝑠∗ + ∆�̂�2

∗,

𝑄𝑜𝑢𝑡1∗ − 𝑄𝑜𝑢𝑡2

∗ = −𝑘𝑞∗ (∆�̂�1

∗ − ∆�̂�2∗) (3)

That is to explicit, the reactive power sharing error caused

by estimation errors is determined by the V-Q droop gain

𝑘𝑞∗ .The design of 𝑘𝑞

∗ is well known trade-off between voltage

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deviation and reactive power control accuracy. The "Governor

Model" block goes about as a ω-P droop controller. The "Q

Droop" block goes about as a V– Q droop controller, which

contrasts from the conventional Q– V droop controller in the

inversed input and output. With a specific end goal to make

the channel inductor Lf contribute to the output impedance, the

inner current and voltage loops are not received in this control

conspire. Lf is considered as the stator inductance of the VSG.

This stator inductance brings about more inductive output

impedance, which is particularly important for active and

reactive power decoupling in a low voltage microgrid in which

line resistance is predominant. The output voltage is as yet

managed indirectly by the V– Q droop controller and the FLC

controller of reactive power. Keeping in mind the end goal to

decrease the impact from ripples in measured output control, a

20Hz first order low-pass filter is connected for Qout. As the

output current is measured after the LC filter stage, the

reactive power consumed by the LC channel is excluded in

Qout. Subsequently, no particular inertial process is required

for the reactive power of FLC controller.

V. FUZZY LOGIC CONTROLLER

The fuzzy logic is consider as a logical system the

provides a model for human reasoning modes that

approximate rather than exact. The fuzzy logic is based on

fuzzy set theory in which particular object or variable has a

degree of membership in a given set which may be

anywhere in the range of 0 to 1. During the past several

years, fuzzy logic control has emerged as one of the most

active area for research in application of fuzzy logic theory

especially in wide range of industrial process which lacks

quantitative data regarding the input-output relation. The

implementation of fuzzy logic technique to real application

requires three steps:

I. Fuzzification: Converts crisp data into fuzzy data

II. Fuzzy Inference process: Combine member -ship

functions with control rules to derive fuzzy output

III. Defuzzification: Use different methods to calculate

output and put them into lookup table. Pick up the

output from look up table based on current input

The triangular membership function is utilized as a part of

this control network and the variable input data sources and

outputs are for the most part divided into seven fuzzy subsets

as NB(Negative Big), NM (Negative Medium), NS(Negative

Small),Z (Zero), PS(Positive Small), PS (Positive Medium)

and PB(Positive Big). Due to be 7-ruled input variables,

therefore 49 principles can be developed and these guidelines

depend on control of slope climbing calculation, as appeared

in the table 5.1.

Here we using mamdani's technique with Max-Min fuzzy

curves. In this system the information scaling factor has been

designed with the end goal that information values are

between - 1 and +1.

Table.1 Fuzzy Rules

Fig. 5 Simulation circuit of microgrid with two DGs using

VSG

VI. SIMULATION RESULTS

A microgrid shown in Fig.5 is analyzed. As it is shown in Fig.5, impedances of output filters and lines of every DG changes in per unit values. The arrangement of sequence of operation is shown in Table 2. Events of islanding from grid, loading transition, and intentional active power sharing changes are simulated at 21 s, 24 s, and 27 s, separately.

As it is shown in Fig. 6(a), while the microgrid is islanded at 21 s, and when load 2 is connected at 24 s, oscillation can be observed in active power while the basic VSG control is applied for both DGs. This oscillation is almost eliminated by applying the proposed enhanced VSG control scheme with FLC in Fig. 7(a). As the disturbance at 27 s is expedited with the aid of change of active power set value of DG1, which is a not a loading transition, active power oscillations can't be eliminated for this case. However, the proposed VSG control increases the damping ratio; in this manner, the overshoots in Fig. 7(a) are smaller than that in Fig. 6(a). In the meantime, the oscillation periods end up being longer, because of the damped natural frequencies are decreased.

Moreover in case of basic VSG control, reactive power is

not shared properly in islanded mode and is not controlled at

set value in grid connected mode, because of the voltage drop

through line impedance as shown in 6(b). Likewise, reactive

power control is not free from active power, as a difference in

set value of active power at 27 s 7(b).

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Table.2. Simulation sequence

Time Grid P*01 P*02 Load

t < 21 s connected 1 pu 1 pu Load 1

21 s ≤ t ≤ 24 s Disconnected - - -

24 s ≤ t ≤ 27 s - - - Load

1+2

27 s ≤ t ≤ 30 s - - 0.6 pu -

(a) Active powers of load1&2

(b) Reactive powers of load1&2

(c) Voltage magnitudes

(d) Frequencies

(e) Load voltage THD

(f) Load current THD

Fig.6. simulation results Active power, reactive power,

voltage, frequency and THD of load voltage & current

when both DGs are controlled by VSG control without

FLC

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(a) Active powers of load1&2

(b) Reactive powers of load1&2

(c) Voltage magnitudes

(d) Frequencies

(e) Load voltage THD

(f) Load current THD

Fig.7. simulation results (a) Active power, (b) reactive

power, (c) voltage, (d) frequency and (e) THD of load

voltage & (f) current when both DGs are controlled by

enhanced VSG control with FLC

Fig.6(e) shows the FFT analysis of voltage when VSG

control without FLC is applied for a 50Hz system. The total

harmonic distortion (THD) in this case is 5.23%.The proposed

enhanced VSG control with FLC reduces this THD to 3.61%

as shown in Fig.7(e).

Fig.6(f) shows the FFT analysis of load current VSG

control without FLC is applied, the total harmonic distortion

(THD) in this case is 2.60%. The proposed enhanced VSG

control with FLC reduced this THD to 1.85% as shown in

Fig.7(f).

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VII. CONCLUSION In this paper, an enhanced VSG control with FLC is

proposed as a novel communication-less control method in a

microgrid. A stator reactance adjuster is developed, in order to

increase the active power damping and to properly share

transient active power. A novel communication-less reactive

power control strategy based on inversed voltage droop

control (V–Q droop control) and common ac bus voltage

estimation is also proposed to achieve accurate reactive power

sharing, which is immune to active power sharing changes and

line impedance mismatch. Comparing PI controller and fuzzy

controller, it is very clear that the total harmonic distortion

(THD) has been decreased from 5.23% to 3.61% for grid

voltage and it has been decreased from 2.60% to 1.85% for

grid current. Simulation results shows that the proposed

enhanced VSG control achieves desirable transient and steady-

state performances, and keeps the inertia support feature of

VSG control. As a result, the proposed enhanced VSG control

with FLC is a preferable choice for the control system of DGs

in microgrids.

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ISSN NO:2236-6124

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