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.
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:652
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
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:653
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
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:654
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).
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:655
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
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:656
(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).
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:657
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.
REFERENCES
[1] R. H. Lasseter, “Microgrids,” in Proc. IEEE Power Eng.
Soc. Winter Meeting, New York, NY, USA, 2002, pp. 305–
308.
[2] J. M. Guerrero, J. C. Vasquez, J. Matas, L. G. de Vicuña,
and M. Castilla,“Hierarchical control of droop-controlled AC
and DC microgrids—A general approach toward
standardization,” IEEE Trans. Ind. Electron.,vol. 58, no. 1, pp.
158–172, Jan. 2011.
[3] A. Bidram and A. Davoudi, “Hierarchical structure of
microgrids control system,” IEEE Trans. Smart Grid, vol. 3,
no. 4, pp. 1963–1976, Dec. 2012.
[4] M. C. Chandorkar, D. M. Divan, and R. Adapa, “Control
of parallel connected inverters in standalone AC supply
systems,” IEEE Trans. Ind.Appl., vol. 29, no. 1, pp. 136–143,
Jan./Feb. 1993.
[5] H. Bevrani, M. Watanabe, and Y. Mitani, Power System
Monitoring and Control. Hoboken, NJ, USA: Wiley, 2014.
[6] J. M. Guerrero, J. Matas, L. G. De Vicuña, M. Castilla, and
J. Miret, “Decentralized control for parallel operation of
distributed generation inverters using resistive output
impedance,” IEEE Trans. Ind. Electron.,
vol. 54, no. 2, pp. 994–1004, Apr. 2007.
[7] T. L. Vandoorn, B. Meersman, L. Degroote, B. Renders,
and L. Vandevelde, “A control strategy for islanded
microgrids with DC-link voltage control,” IEEE Trans. Power
Del., vol. 26, no. 2, pp. 703–713,
Apr. 2011.
[8] T. L. Vandoorn, B. Meersman, J. D. M. De Kooning, and
L. Vandevelde, “Analogy between conventional grid control
and islanded microgrid control based on a global DC-link
voltage droop,” IEEE Trans. Power Del., vol. 27, no. 3, pp.
1405–1414, Jul. 2012.
[9] J. C. Vasquez, J. M. Guerrero, M. Savaghebi, J. Eloy-
Garcia, and R. Teodorescu, “Modeling, analysis, and design of
stationary-referenceframe droop-controlled parallel three-
phase voltage source inverters,” IEEE Trans. Ind. Electron.,
vol. 60, no. 4, pp. 1271–1280, Apr. 2013.
[10] J. Driesen and K. Visscher “Virtual synchronous
generators,” in Proc. IEEE Power Energy Soc. Gen.
Meeting—Convers. Del. Elect. Energy 21st Century,
Pittsburgh, PA, USA, 2008, pp. 1–3.
[11] L. M. A. Torres, L. A. C. Lopes, T. L. A. Moran, and C.
J. R. Espinoza, “Self-tuning virtual synchronous machine: A
control strategy for energy storage systems to support dynamic
frequency control,” IEEE Trans.Energy Convers., vol. 29, no.
4, pp. 833–840, Dec. 2014.
[12] Y. Hirase et al., “Virtual synchronous generator control
with double decoupled synchronous reference frame for
single-phase inverter,” IEEJ J. Ind. Appl., vol. 4, no. 3, pp.
143–151, May 2015.
[13] K. Sakimoto, Y. Miura, and T. Ise, “Stabilization of a
power system with a distributed generator by a virtual
synchronous generator function,” in Proc. 8th IEEE Int. Conf.
Power Electron. ECCE Asia, Shilla Jeju, Korea, 2011, pp.
1498–1505.
International Journal of Research
Volume 7, Issue X, October/2018
ISSN NO:2236-6124
Page No:658