AU2021104115A4 - A system for fully distributed peer-to-peer control of prosumer-based islanded ac microgrid - Google Patents

Abstract

The present invention generally relates to a system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid. The system comprises an islanded ac microgrid configured using a plurality of differently rated DGs common loads, wherein the DGs are provided with local loads to behave it as a prosumer, wherein each DG consists of a CRES so that the prosumers provides dispatchable and reliable power to the MG; a plurality of special switches connected to each DG to operate in grid-connected or islanded mode with the MG to prioritize the local loads, wherein the switches are provided with each common load to connect and disconnect them to or from the grid; and a local controller is achieved by incorporating voltage reference generator, voltage control loop, and current control loop with a pulse with modulation (PWM) generator of each inverter. While, to make the prosumers participation fully independent and distributed a distributed secondary controller is achieved by incorporating apf-synchronization and gossip communication-based peer to peer (P2P) controller. The prosumer's DGs are synchronized with their nearest point of common coupling (PCC) instead of a common PCC to avoid the effective line impedance mismatch among the DGs, which causes a circulating current between the DGs and results in poor reactive power-sharing. 27 w -c 0~ tfl 0 -j C - r4 -jm ii U -j N UC q) Co Sb C 0 U (2

Description

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A SYSTEM FOR FULLY DISTRIBUTED PEER-TO-PEER CONTROL OF PROSUMER-BASED ISLANDED AC MICROGRID FIELD OF THE INVENTION

The present invention relates to a system for fully distributed peer-to peer control of prosumer-based islanded AC microgrid.

BACKGROUND OF THE INVENTION

Due to the decline of fossil fuels and an increased understanding of environmental issues, now peoples are moving towards the distributed generations. It mainly includes electric power generation from renewable energy sources like solar photovoltaic, wind turbines, fuel cells, natural gas, etc., an energy storage system, and an interfacing power converter. Predominantly the interfacing power converters are dc/ac inverter. The distributed generations have a less environmental impact and high-power efficiency than central power generations. However, due to complexity in integration and single machine operation and control, DGs find restrictions in large-scale applications. The microgrid is like a scaled-down version of a central power system that integrates distributed generations, energy-storing systems, critical and non-critical loads, etc. Potentially it can complete control, operations, monitoring, management, and maintenance of the distributed generations within a small distribution system. Unlike central power systems, microgrids are competent in operating in grid-connected mode and islanded mode. Hence, it can reduce power consumption, improve efficiency, and provide flexible and reliable power. Principally microgrids can be operated as master-slave control, hierarchical control, and peer-to-peer control while adopting different power converters controls like PQ control,

V/f control, and droop control. Microgrid control strategies control, monitor, optimize, and maintain the entire microgrid systems' stability. In contrast, the power converter's controls regulate and balance the distributed generations.

The most straightforward control strategy is Master-slave control, which is centralized and highly dependent on communication lines. One DG behaves like a master and others like slaves, i.e., it works on the concept of master slaves. It adopts a PQ controller in grid-connected mode and a V/f controller in the islanded mode of operation. Hence it needs a change of control methods for different modes of operation. A small failure in communication line or master unit can lead to an entire system shutdown which is a significant disadvantage of master-slave control.

Hierarchical control is relatively decentralized but highly dependent on the central controller and preferably employed in large microgrid applications. It could adopt a two-layer, or three-layer control structure based on the largeness of the microgrid. Hence it requires massive and robust communication channels. However, it still possesses the drawback of single-point failure.

The peer-to-peer (P2P) control strategy is fully distributed and or decentralized and needed gossip communication among the DGs to share the local status information. This strategy inherently adopts droop control locally, which was proposed to eliminate the need for the wired communication lines. So completely removes the drawbacks related to centralized control and single-point failure. It allows all the DGs to work independently, i.e., no master-slave concept. It enables the DGs to share the local loads as well as common loads simultaneously. Also, it welcomes the DGs into the grid with different ratings and provides extensive flexibility with the plug-and-play feature.

However, the conventional droop control offers some disadvantages; (i) The asymmetrical line impedance between the DGs raises the difference in line drops that causes inaccurate power-sharing even for identical inverters. (ii) Moreover, with the asymmetrical line impedance, the active and reactive power-sharing between the DGs leads to difficulty for separate control. Many researchers have paid their attention to resolve these issues by modifying the DGs' droop control characteristics and adding and integrating the grid status using secondary control that is mainly distributed gossip control in the case of P2P controlled islanded microgrid.

These days, the consumers consume power and take an interest in power generation mainly from renewable resources, i.e., becoming Prosumer. So, it is necessary to provide them with the liberty of flexibility and plug-and-play feature. The conventional algorithm allows P2P controlled buyers to modify their consumption based on the available price and power quality, while sellers have the freedom to choose who they sell to. In addition, a peer-to peer energy market platform based on the new concept of multi-class EMS to coordinate trading between prosumers with varying preferences. However, these studies suggest that prosumers will be forced to go through a purposeful plug-in and plug-out process by the EMS. Consequently, it may cause synchronization and power quality concerns for prosumers and the grid as a whole. So, it is essential to use a control technique for Prosumer based AC microgrids that can provide both intrinsic PLL and secondary synchronization. And, it is believed that the droop-based P2P control technique is most suited for regulating Prosumer DGs. In conventional techniques, droop-based P2P control of islanded ac microgrids have been conducted without taking prosumers into account. Further, the conventional techniques also ignored frequency monitoring at the Prosumer's terminals, instead opting for power and voltage references at each Prosumer terminal to regulate power flow between peers. So, there is a lack of studies exploring how the DGs with local loads, i.e., Prosumer, will behave while operating with respect to the pre-set references and the local PCC connecting it to the MG.

In the view of the forgoing discussion, it is clearly portrayed that there is a need to have a system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid.

SUMMARY OF THE INVENTION

The present disclosure seeks to provide a system for fully distributed gossip communication-based P2P control strategy for prosumer-based islanded ac microgrids while considering local PCC with each Prosumer instead of a single PCC.

In an embodiment, a system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid is disclosed. The system includes an islanded ac microgrid configured using a plurality of differently rated DGs common loads, wherein the DGs are provided with local loads as a prosumer, wherein each DG consists of a CRES so that the prosumers provides dispatchable and reliable power to the MG. The system further includes a plurality of special switches connected to each DG to operate in grid-connected or islanded mode with the MG to prioritize the local loads, wherein the switches are provided with each common load to connect and disconnect them to or from the grid. The system further includes a local controller in P2P controlled microgrid (MG) is achieved by incorporating voltage reference generator, voltage control loop, and current control loop with a pulse with modulation (PWM) generator of each inverter.

In an embodiment, the DGs and common loads are expected to be placed at different locations, therefore, the line impedance is considered in ohm per kilometer.

In an embodiment, the voltage reference generator consists of a droop control loop and a virtual impedance control loop, wherein the droop control loop comprises power calculation, droop equations, and a sinusoidal voltage generator.

In an embodiment, the system further comprises a LCL filter configured to suppress the harmonics content from the output of the inverter.

In an embodiment, the system further comprises a first-order low pass filter (LPF) to attenuate both switching noise and harmonics to obtain the active power and reactive power, wherein cut-off frequency of the LPF is generally selected one decade below the MG nominal frequency.

In an embodiment, inner control loops consist voltage control loop, current control loop, and PWM for generating the gating signals for the DG inverters' power switches, wherein the voltage and the current control loops are constructed using the cross-coupling loops of the filter capacitor and inductor voltage loops to guarantee fast dynamic response in any operating conditions and maintain system stability under high bandwidth and performance feed-forward compensators (current and voltage).

In an embodiment, symmetrical optimum techniques are used to tune and determine the PI controller's gains of the voltage control loop.

In an embodiment, current control loop is configured for shaping the voltage across the filter inductor and results in minimum current error under fast dynamic response while maintaining the system stability.

In an embodiment, the DGs are synchronized with the connecting PCC, and the DGs share their status information with the peers using the distributed secondary controller system.

In an embodiment, a gossip communication system among the prosumers are interfaced to collect output terminal's status information of the prosumers, wherein the Prosumer's DGs are synchronized concerning their local PCC to avoid circulating current between the DGs due to line impedance mismatch.

An object of the present disclosure is to facilitate hassle-free parallel operation of unequally rated prosumer-based DGs in an islanded ac MG.

Another object of the present disclosure is to provide fully plug-in and plug-out enabled participation of prosumers. Another object of the present disclosure is to promote no circulating current flow among the DGs by incorporating ap-synchronization with each DG.

Yet another object of the present invention is to develop expeditious and cost-effective distinctive system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid.

To further clarify advantages and features of the present disclosure, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings.

BRIEF DESCRIPTION OF FIGURES

These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 illustrates a block diagram of a system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid in accordance with an embodiment of the present disclosure;

Figure 2 illustrates microgrid configuration in accordance with an embodiment of the present disclosure;

Figure 3 illustrates complete layout diagram of the proposed control strategy of a prosumer-based DG in accordance with an embodiment of the present disclosure; and

Figure 4 illustrates equivalent circuit of a prosumer connected to MG busbar in accordance with an embodiment of the present disclosure.

Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have necessarily been drawn to scale. For example, the flow charts illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present disclosure. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

Reference throughout this specification to "an aspect", "another aspect" or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrase "in an embodiment", "in another embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises...a" does not, without more constraints, preclude the existence of other devices or other sub systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.

Embodiments of the present disclosure will be described below in detail with reference to the accompanying drawings.

Referring to Figure 1, a block diagram of a system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid in accordance with an embodiment of the present disclosure. The system 100 includes an islanded ac microgrid 102 configured using a plurality of differently rated

DGs 104 common loads, wherein the DGs 104 are provided with local loads as a prosumer, wherein each DG consists of a CRES so that the prosumers provide dispatchable and reliable power to the MG.

In an embodiment, a plurality of special switches 106 are connected to each DG to operate in grid-connected or islanded mode with the MG to prioritize the local loads, wherein the switches 106 are provided with each common load to connect and disconnect them to or from the grid.

In an embodiment, a local controller 108 in P2P controlled microgrid 102 (MG) is achieved by incorporating voltage reference generator, voltage control loop, and current control loop with a pulse with modulation (PWM) generator of each inverter.

In an embodiment, the DGs 104 and common loads are expected to be placed at different locations, therefore, the line impedance is considered in ohm per kilometer.

In an embodiment, the voltage reference generator consists of a droop control loop and a virtual impedance control loop, wherein the droop control loop comprises power calculation, droop equations, and a sinusoidal voltage generator.

In an embodiment, a LCL filter 110 is configured to suppress the harmonics content from the output of the inverter.

In an embodiment, a first-order low pass filter (LPF) 112 is used to attenuate both switching noise and harmonics to obtain the active power and reactive power, wherein cut-off frequency of the LPF 110 is generally selected one decade below the MG nominal frequency.

In an embodiment, inner control loops consists voltage control loop, current control loop, and PWM for generating the gating signals for the DG inverters' power switches 106, wherein the voltage and the current control loops are constructed using the cross-coupling loops of the filter capacitor and inductor voltage loops to guarantee fast dynamic response in any operating conditions and maintain system stability under high bandwidth and performance feed-forward compensators (current and voltage).

In an embodiment, symmetrical optimum techniques are used to tune and determine the PI controller's gains controller 108 of the voltage control loop.

In an embodiment, current control loop is configured for shaping the voltage across the filter inductor and results in minimum current error under fast dynamic response while maintaining the system stability.

In an embodiment, the DGs are synchronized with the connecting PCC, and the DGs share their status information with the peers using the distributed secondary controller 108 system.

In an embodiment, a gossip communication system among the prosumers is interfaced to collect output terminal's status information of the prosumers, wherein the Prosumer's DGs are synchronized concerning their local PCC to avoid circulating current between the DGs due to line impedance mismatch.

Figure 2 illustrates microgrid configuration in accordance with an embodiment of the present disclosure. An islanded ac microgrid 102 is formed using 'N' differently rated DGs (DG1, DG2, DG3...DGn) and 'm' common loads (cL1, cL2...cLm). The DGs are provided with their local loads (L1, L2, L3...Ln) so that they can act as a prosumer. Each DG consists of a CRES so that the prosumers can provide dispatchable and reliable power to the MG. The circuit configuration is shown in Figure 2. To prioritize the local loads' special switches 106 (SW1, SW2, SW3.... SWn) provided to each DG to operate in grid-connected or islanded mode with the MG. The switches 106 (S1, S2 Sm) are provided with each common load to connect and disconnect them to or from the grid. The DGs and common loads are expected to be placed at different locations; therefore, the line impedance(z) has been considered in ohm per kilometer(Q/km).

Figure 3 illustrates complete layout diagram of the proposed control strategy of a prosumer-based DG in accordance with an embodiment of the present disclosure. Figure 3 shows the proposed P2P control strategy's entire control architecture, which employs communication less droop-based-local controller 108 and gossip communication-based distributed secondary controller 108.

The local control scheme in P2P controlled microgrid 102 (MG) is achieved by incorporating voltage reference generator, voltage control loop, and current control loop with a pulse with modulation (PWM) generator of each inverter. The voltage reference generator consists of a droop control loop and a virtual impedance control loop. Further, the droop control loop comprises power calculation, droop equations, and a sinusoidal voltage generator. As shown in Figure 3, Li, Cf, and Lg are the LCL filter parameters, and Z is the line impedance. The LCL filter 110 is used to suppress the harmonics content from the output of the inverter. LCL filters' parameters are calculated based on the study carried out in the literature. The further sub-sections describe the implementation and designing of the local control scheme.

The output voltage (Vabc) and current (Iac) is transformed into a dqo rotating reference frame using Park transformation that results in d-axis and q-axis components of Vabc and Iac. The instantaneous active power (p) and reactive power(q) can be calculated as,

p =3(vdid +Vgi) (1)

q= 3(Vqid - Vdiq) (2)

Further, it passes through a first-order low pass filter (LPF) 112 to attenuate both switching noise and harmonics to obtain the active power (P) and reactive power (Q). The cut-off frequency (ac) of the LPF is generally selected one decade below the MG nominal frequency.

Droop control plays a requisite role in the P2P control strategy. In earlies, the droop control was developed for synchronous generators to control the system's voltage and frequency by regulating active ad reactive power. Later, its application extended to power inverter based DGs.

Figure 4 illustrates equivalent circuit of a prosumer connected to MG busbar in accordance with an embodiment of the present disclosure. As shown in Fig.3, the DG with output voltage ELS volts connected to the local load as well as the MG busbar bearing terminal voltage of VOvolts through the line impedances Z, and ZT respectively. The ZT is the line impedance offered by the MG to the prosumer while Z, is the wireline impedance tendered by the connecting wires of the local load. The Z, is highly resistive and tiny compared to theZT. So, the presence of the local load can impact the effective line impedance (Zef) experienced by the DG as follows,

Zef = Ref+jXef=ZefL= ZTI Zi (3)

Similarly, during consumer operation of the Prosumer, the effective line impedance of the MG can vary as follows

ZTef = ZT + Zi (4)

The complex power (S) delivered by the DG and flows through the effective line is represented as follows

S~Pj S=P +jQ . _ _ _* = V. (ELS-VLO) =V.I* Ze5f

EV V2 - L(O -) )- L(-O) Zef Zef 2 V = (cos(O-)+jsin(O - ))- - (cosO -jsin0) (5) Zef Zef

It is assumed that the line impedance is inductive dominated so Ref 0 and 0 90°. Therefore, the above equation can be simplified to follows.

P ~ sin 5 (6) Xef

Q - (E cos 5 - V) (7) Xef

In practice, the 5 is too small and hence sin 5 ~ Sand cosS5 1. Thus, the above equations can be rewritten as follows. ~EV~ = Ev 8 P (8) Xef

Q =-v(E Xef - V) (9)

These two equations establish approximately direct relations between power angle (6 ) and active power (P) while voltage difference (E- V) and reactive power (Q). Since the angular frequency a = , hence the active power can be controlled by controlling the frequency, while the reactive power can be controlled by controlling the voltage difference. Thus, the frequency and voltage amplitude of the grid canvary by adjusting P and Q independently. This conclusion formulates the basis for the P - a and Q - E droop control method. Decisively, the P - a and Q - E droop characteristics are modelled as equations (10-11) and shown in Fig.4.

Oiref = Onom - KoP (10)

Eref = Enom - KQ (11)

Wherewref and onom are the reference and nominal angular frequency in rad/sec. Vref and Vnm are the reference and nominal voltage in volts. Similarly,Km and Ky are the P - a and Q - E droop coefficients in rad/kW and Volts/kVar, respectively.

In the prosumer scenario, where the DGs directly connected to its local loads, the line impedance become resistive dominated; in such case the P - a and Q - E droop characteristics are changed to equations (12) and (13) and defined as reverse droop characteristics shown in Fig.5.

Oiref = Onom + KQ (12)

Eref = Enom - KP (13)

However, these control algorithms outcomes improper power-sharing as the DGs are connected to a strong grid and primarily aimed to share the loads in proportion to their nominal capacity. Therefore, to achieve accurate load sharing among the DGs, the droop coefficients are calculated according to their nominal power capacity.

K, = ", (14) Pmax

Kv = Av (15) Qmax

Where A, and A, are the maximum permissible deviation in frequency and voltage, respectively. Pmax and Qmax are the nominal active power and reactive power in kW and kVar, respectively. The K, and Kv calculated by considering 1% deviation in frequency and 5% deviation in voltage amplitude. It is worth mentioning that for accurate power-sharing among the parallel operated DGs, the droop characteristics of each DG-inverters should satisfy the following relations

K,1.Pmaxi K2-Pmax2 ... ... . = Kn.Pmaxn ... = Aa (16)

Similarly

Kvi- Qmaxi KV2- Qmax2 -- ... ... ... = Kvn- Qmaxn = Av (17)

The droop equations output the oref and Eref which used to generate a

three-phase sinusoidal voltage Eref sin(ot). As we know that oref = Hence, integration of it results in time-varying ot. After processing it through a mod function of 2rc the synchronizing function 'e' is obtained, a necessary component in the abc/dqo dqo/abc transformation. Further, a mathematical realization (shown in Fig.2) used to generate the sinusoidal reference voltage Eref.abc.

In the case of Prosumer, the DGs are connected to local loads as well as the grid line. The local load line offers low impedance in parallel with the grid line. So, the effective line impedance between the DGs becomes more resistive dominated and uneven. Therefore, the droop characteristics may change to equations (12) and (13) and lead to power-sharing inconsistencies between the parallel operated DGs in the MG. So, to avoid this problem, a virtual voltage drops (Vzd,Vzq) is inserted with the reference sinusoidal generator output (Ed, Eq) by considering the virtual impedance (Z, = R, +jL,) without introducing any power loss in the circuit. The designed virtual impedance loop makes the effective line impedance inductive for low frequency components and resistive for high-frequency components. It ensures accurate power-sharing among the parallel operated DGs. The virtual impedance loop is realized in the dqo reference frame using the following equations (18)- (19),

Vzd = jaofLqz1. + Rvz. Iohd (18)

Vvzq = jwLz-.Iofd+ Rvz. Iohq (19)

Where Vvzdq is the virtual voltage drops caused due to coupling of virtual resistance Rvz and virtual inductance Lvz . The o is the angular frequency in rad/sec, and it has been imported from the P -a droop control loop. Also,Iofdq and Iohdq are the fundamental and higher-order harmonic components of the DG inverter's output current (Iabc). With the use of an LPF and equation (21), the fundamental and higher-order components have been extracted as follows,

'ofdq . (l =dqp ) (20)

'ohdq Idq - Iof dq (21)

Finally, the suitable values of Lvz and Roz for each DG inverters have been approximated using the works of literature that mainly. It is found that the equivalent output impedance of the parallel DGs (DG2 and DG3) becomes more inductive and resistive for the impedance angle 89.40 and 1.12 0 respectively at the system frequency that is shown in Fig.6. Similarly, we can find it out for DG1 also.

The inner control loops are mainly responsible for determining the operating states of the DG inverters. It mainly contains the voltage control loop, current control loop, and PWM for generating the gating signals for the DG-inverters' power switches 106. The voltage and the current control loops are constructed using the cross-coupling loops of the filter capacitor and inductor voltage loops. To guarantee the fast dynamic response in any operating conditions and maintain system stability under high bandwidth and performance feed-forward compensators (current and voltage).

The dq-components of the sinusoidal reference voltage Eref.abc is used as the reference to the voltage control loop, and it has been compared with the dq-components of DG inverter output voltage Vabc and they were processed through a PI controller to achieve zero steady-state error. After that, the current controller's reference signal is obtained with the filter capacitor's voltage coupling loop and feed-forward compensators. The dynamics of the voltage control loop can be realized as follows

Idref kv(Vdref - Vd) + k f (Vref- Vd)dt - CfVq + H'Id (22) 'qref k(vqrf 4+~fVrf Vq)dt + aJCf Vd +H'Iq (3 Igr; k(Voref - V) +ky f (Vorer Vf t+wCV ( 23)

Where k, ky are the proportional and the integral coefficients of the PI controller andH' is the current feed-forward gain. Several methods of tunning the PI controllers' gains of inner control loops are extensively discussed in the literature. The symmetrical optimum techniques or modulus optimum can be applied for tunning the voltage controller loop. However, in this work, the symmetrical optimum techniques are used to tune and determine the PI controller's gains controller of the voltage control loop. The equations (24) and (25) are used to calculate the kv and ky for the voltage control loop.

c fsw)

k= where a = 2+1 (24)

ky = a 2 .2.Ta where Ta (25) 2fsw

Here w is the system frequency and its equal to 2nf while fSw and ( is the switching frequency and damping ratio of the system.

The current control loop is responsible for shaping the voltage across the filter inductor and results in minimum current error under fast dynamic response while maintaining the system stability. The current reference components obtained from the voltage control loop has been compared with the dq-components of the DG-inverter output current iacand processed through a PI controller to achieve zero steady-state error. After that, with the use of filter inductor voltage-coupling loops and feed-forward compensators, the reference signals (Ud and uq) for the PWM is obtained. The dynamics of the current control loop can be realized as follows,

Ud k=k(Idref- d)+ k f(Idref- id)dt - (fiLfiq+ HvVd (26)

Uq k=k (Iqref- iq)+ k f(Iqref- iq)dt + LLfid+ HVq (27)

Where kip, ki are the proportional and the integral coefficients of the PI controller and Hv is the voltage feed-forward gain. The modulus optimum technique is used to tune the current control loop, even though this technique is used primarily in drive applications. The equations (28) and (29) are used to calculate the ki and ki of the current control loops.

k P T.Rf 2.Ta where T = Rf (28)

k Lf a)Rf (29)

It is worth mentioning that Lf is the sum of Li and L. in case LCL filters.

In this part of the control architecture, the DGs are synchronized with the connecting PCC, and the DGs share their status information with the peers using the DGC system. This communication system could be wired or wireless. The implementation of this controller can be described as follows.

To integrate the inverter of a prosumer with the MG without any difficulty, and to avoid the circulating current flow between the inverters, it is necessary to synchronize the Prosumer's terminal voltage and frequency with the voltage and frequency available at the PCC where a particular prosumer is ready to be connected. The research works mainly considered only one PCC to synchronize the DG-inverters, which is unfortunate because it creates the central controller's necessity. Due to line impedance and loads presence in the MG, the voltage and frequency are not the same throughout the line. So instead of considering a single PCC for integrating the prosumer inverters, distributed PCCs are considered, and the concept of ap1 synchronization makes it possible too. As shown in Fig.2, this synchronization is processed using the apl-components of theVajJc and

Vabc.If both the voltages are synchronized, then we can assume the frequency difference as,

(Vgp.V-Vga.V)= 0 (30)

being (x)the average value of the variable (x)over the grid frequency. While the voltage amplitude difference can be assumed as,

Vg+V2 - y+V2 (31)

Thus, we can easily configure the distributed phase lock loop (PLL)to obtain the synchronization correction signals by cascading the equation (28) and (29) with a 1st order LPF and a PI controller individually. syn (Osyn = (Vgg. V - Vp)( ).(k" '+ w ) (32) z2 _ yz +V2).( 'C ).(k + ) (33)

Where, oc is the cut-off frequency of the 1st order LPFs and kndn, k and ks", '" are the proportional and integral gains of the PI controllers, respectively. While the osyn and Vsyn are the frequency and voltage amplitude correction signals of the distributed PLL sent to theP- w and Q- E droop control loops to integrate and add over the phase and voltage amplitude of the system, respectively. Note it is worth mentioning that the distributed PLL associated with the DG to be started first during a black start should be kept in off condition to avoid any malfunctioning.

The works mainly presented distributed P2P control approach by employing a gossip communication algorithm to collect the voltage amplitude and frequency status information (vi and wi) from the different DGs sending end PCCs. Unfortunately, this research works directly or indirectly based on centralized control, making it partly distributed. Thus, to make it fully distributed, a distributed P2P controller is employed with each DG-inverter control. Hence it makes the prosumers fully distributed P2P operated. As shown in Fig.2 the Vi and og from each prosumer terminals (PCC1, PCC2, PCCn) has been collected, and the average value of the status information calculated as follows,

V^"g - 21v=1 Vi ( 34)

oJAvg = j i (35)

Where 'N" indicates, no. of Prosumer-based DGs are in operation in the MG. Further, this average status information has been compared with the pre-set value of voltage amplitude (Vset)and frequency (set). The error status information is obtained, which further processed through a PI controller and a limiter to obtain the error correction status information (Svi and a 1 ) and sent to the associate DG's P - a and Q - E droop control loop respectively to make required correction in the DG-inverter's output voltage and frequency.

SVi = (Vse - 1 V ) .(kv' + (36)

(5w =Oset - i wY)i(k-)'+ L-)(37)

Where k',k' and ky',k ' are the proportional and integral coefficients of the respective PI controllers. In this fashion, each DG-inverter will contribute to maintaining the stability of the entire MG.

The drawings and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any component(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or component of any or all the claims.

Claims (10)

WE CLAIM

1. A system for fully distributed peer-to-peer control of prosumer-based islanded AC microgrid, the system comprises:

an islanded ac microgrid configured using a plurality of differently rated DGs and common loads, wherein the DGs are provided with local loads to behave it as a prosumer, wherein each DG consists of a CRES so that the prosumers provide dispatchable and reliable power to the microgrid (MG); a plurality of special switches connected to each DG to operate in grid connected or islanded mode with the MG to prioritize the local loads, wherein the switches are provided with each common load to connect and disconnect them to or from the grid; and a local controller in P2P controlled microgrid (MG) is achieved by incorporating voltage reference generator, voltage control loop, and current control loop with a pulse with modulation (PWM) generator of each inverter; and a fully plug-in and plug-out enabled participation of prosumers.

2. The system as claimed in claim 1, wherein the DGs and common loads are expected to be placed at different locations, therefore, the line impedance is considered in ohm per kilometer.

3. The system as claimed in claim 1, wherein the voltage reference generator consists of a droop control loop and a virtual impedance control loop, wherein the droop control loop comprises power calculation, droop equations, and a sinusoidal voltage generator.

4. The system as claimed in claim 1, comprises a LCL filter configured to suppress the harmonics content from the output of the inverter.

5. The system as claimed in claim 1, comprises a first-order low pass filter (LPF) to attenuate both switching noise and harmonics to obtain the active power and reactive power, wherein cut-off frequency of the LPF is generally selected one decade below the MG nominal frequency.

6. The system as claimed in claim 1, wherein inner control loops consists voltage control loop, current control loop, and PWM for generating the gating signals for the DG-inverters' power switches, wherein the voltage and the current control loops are constructed using the cross-coupling loops of the filter capacitor and inductor voltage loops to guarantee fast dynamic response in any operating conditions and maintain system stability under high bandwidth and performance feed-forward compensators (current and voltage).

7. The system as claimed in claim 1, wherein symmetrical optimum techniques are used to tune and determine the PI controller's gains controller of the voltage control loop.

8. The system as claimed in claim 4, wherein current control loop is configured for shaping the voltage across the filter inductor and results in minimum current error under fast dynamic response while maintaining the system stability.

9. The system as claimed in claim 1, wherein it contains a distributed secondary controller and wherein a ap-synchronization is incorporated with each DGs to synchronize with the connecting PCC, and the DGs share their status information with the peers using the distributed secondary controller system.

10. The system as claimed in claim 1, wherein a gossip communication system among the prosumers are interfaced to collect output terminal's status information of the prosumers, wherein the Prosumer's DGs are synchronized concerning their local PCC to avoid circulating current between the DGs due to line impedance mismatch. Further, a gossip communication based distributed P2P controller used to restore voltage and frequency deviations caused by the load's variations thus over all MG stability is maintained.