FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
1. Title of the invention: PARALLEL OPERATION OF THREE-PHASE INVERTERS
2. Applicant(s)
NAME NATIONALITY ADDRESS
RAYCHEM RPG PVT. LTD Indian RPG House 463, Dr. Annie Besant Road, Mumbai, Maharashtra 400 030, India
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it
is to be performed.

TECHNICAL FIELD [0001] The present subject matter relates, in general, to microgrids. More specifically, the present subject matter relates to approaches for parallel operation of three-phase inverters in islanded microgrids.
BACKGROUND [0002] As is generally understood, a microgrid is a localized group of electrical sources and loads. For example, the electrical sources in the microgrid may include, but are not limited to, conventional power sources, Renewable Energy Sources (RES), Distributed Energy Resources (DERs), storage devices, or a combination thereof. The microgrid may be tied to a macro grid (or traditional power grid) for synchronous operation. However, in cases where a fault may have occurred in the macro grid, the microgrid may be isolated for islanded operation thereof. The operation of the electrical sources may need to be managed and coordinated for reliably supplying the loads in the microgrid. In this regard, control and protection of the electrical sources is crucial in the microgrid.
BRIEF DESCRIPTION OF DRAWINGS [0003] The features, aspects, and advantages of the present subject matter will be better understood with regards to the following description and accompanying figures. The use of the same reference number in different figures indicate similar or identical features and components.
[0004] FIG. 1 illustrates an isolated grid having electrical protection device, as per an example;
[0005] FIG. 2 illustrates a block diagram of an electrical protection device, as per an example;
[0006] FIG. 3 illustrates a high-level block diagram of an electrical protection device, as per an example;
[0007] FIG. 4 illustrates a block diagram for power measurement calculation, as per an example; [0008] FIG. 5 illustrates a block diagram of DQ module, as per an example;

[0009] FIG. 6 illustrates a block diagram of a control module, as per an
example; and
[0010] FIG. 7 illustrates a flow diagram depicting a method for facilitating
parallel operation of a three-phase inverter in an isolated grid, as per an example.
[0011] Throughout the drawings, identical reference numbers designate
similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.
DETAILED DESCRIPTION
[0012] With localization of electrical sources owing to Distributed Energy
Resources (DERs), microgrid may be formed for locally feeding loads connected to the microgrid. Such microgrid enables transfer of local energy to the loads with minimal losses due to distribution over short distances. As would be understood, the microgrid may include electrical sources, distribution systems, control and switchgears, and loads. Examples of such electrical sources include, but are not limited to, conventional power generators, Cogeneration systems, renewable energy sources, Distributed Energy resources (DERs), storage systems, or a combination thereof.
[0013] As would be understood, the microgrid may be coupled to a traditional
electrical power grid, such as an electrical transmission or electrical distribution system. Additionally, the microgrid may be capable of operating in cases of fault that may arise in the traditional electrical power grid. In such cases, the microgrid may be isolated from the traditional electrical power grid. Such operation in isolated mode ensures continuity of power flow with high degree of sophistication in the isolated microgrid. During the isolated operation of the microgrid, power from electrical sources within the microgrid may be provided to loads within the microgrid. In certain cases, where the electrical sources in the microgrid may be DC sources, the electrical sources may be connected to inverters for conversion of power. It may be noted that the inverters may convert DC power to AC power, wherein the AC power may then be supplied to the loads within the microgrid.

[0014] In this regard, inverters connected to the microgrid convert DC power
from DC source to AC power. In certain cases, inverters connected to the microgrid may operate in parallel to effectuate load sharing between the inverters. In this regard, output power of a three-phase inverter may be regulated based on operating voltage of the microgrid and load connected to the three-phase inverter. Further, the output power of the three-phase inverter may be regulated to achieve an output voltage equal to the operating voltage at each phase thereof, and output current based on the load connected to each phase of the three-phase inverter. For example, three-phase controls such as droop control, master-slave control, and the like, may be employed for regulating output power of the three-phase inverter to a desired output power. In this regard, a reference power may be generated for each phase of the three-phase inverter to regulate the output power at each of the phase. In an example, voltage and frequency between the three-phase inverter and loads connected thereto may be changed for regulating the output power of the three-phase inverter.
[0015] However, regulation of the output power of the three-phase inverter
using the three-phase control is limited to implementations where similar loading
condition are present at each phase of the three-phase inverter. In other words,
different reference power may be generated for each phase of the three-phase
inverter in cases where the three-phase inverter is connected to an unbalanced
load. Further, regulation of the output power at each phase of the three-phase
inverter, based on different power references, may result in unbalanced voltage
at each phase of the three-phase inverter. Such unbalanced voltage may further
result in generation of sequence quantities and hamper quality of the output
power of the three-phase inverter. Moreover, the unbalanced voltage may
prevent parallel operation of the three-phase inverter within the microgrid.
Subsequently, such unbalanced voltage may be undesirable for the loads
connected to the three-phase inverter, as well as the three-phase inverter.
[0016] Approaches for facilitating parallel operation of a three-phase inverter
in an isolated grid, are described. In an example, the present subject matter may facilitate the parallel operation of the three-phase inverter in cases where unbalanced load may be present at phases of the three-phase inverter. The

present subject matter may be implemented in the isolated grid, wherein the isolated grid may be an islanded microgrid. It may be noted that such isolated grid (referred to as microgrid) may be a part of a traditional electrical power grid. Further, the microgrid may be islanded due to a fault on the traditional power distribution grid.
[0017] The three-phase inverter connected to the isolated grid may be
coupled to an electrical protection device for protection thereof. The electrical protection device may further enable regulation of output power of the three-phase inverter, based on a desired output power, for parallel operation of the three-phase inverter in the microgrid. The three-phase inverter may include, but are not limited to, switching mechanisms, such as thyristors, diodes, and the like, voltage source, connecting wires, and switching loads. It may be noted that the switching devices of the three-phase inverter are triggered in a defined manner in order to generate three-phase AC output power from DC input voltage provided by the voltage source, and further regulate the three-phase AC output power to the desired output power. Further, the three-phase AC output power may be fed to load connected to respective phase of the three-phase inverter.
[0018] In operation, the electrical protection device may obtain current and
voltage measurements of the three-phase inverter. It may be noted that the current and voltage measurements may correspond to three-phase AC power generated by the three-phase inverter.
[0019] Thereafter, the electrical protection device may calculate power
measurements for the three-phase inverter. In one example, the power measurements may include an active power measurement and a reactive power measurement. Continuing further, an active power coefficient and a reactive power coefficient may be determined for the three-phase inverter. In this regard, the active power coefficient and the reactive power coefficient may be droop coefficient for active power and droop coefficient for reactive power, respectively. As would be understood, droop control enables control of output power of a system by changing line frequency and line voltage of the system. To such an end, the active power droop coefficient may be calculated based on an active

power-frequency (P-f) plot, whereas the reactive power coefficient may be calculated based on a reactive power-voltage (Q-V) plot.
[0020] Once the active power measurement, the reactive power
measurement, the active power coefficient and the reactive power coefficient are determined, a reference voltage for each phase of the three-phase inverter may be calculated. In one example, a maximum voltage measurement may be calculated based on the reactive power measurement and the reactive power coefficient. Additionally, an angle measurement may be calculated based on the active power measurement and the active power coefficient. Such maximum voltage measurement and the angle measurement may be used to calculate the reference voltage for each phase of the three-phase inverter.
[0021] The electrical protection device may further use the reference voltages
for regulating output power at each phase of the three-phase inverter. In one example, a reference voltage corresponding to a phase of the three-phase inverter may be provided to, for example, a control module associated with the phase under consideration. The control module may then determine a desired output power for the phase, based on load connected to the phase and the reference voltage. Thereafter, switching devices associated with the phase may be triggered in a manner to achieve the desired output power at the phase. In a similar manner, output power at all phases of the three-phase inverter may be regulated.
[0022] As would be understood, the various examples of the present subject
matter provide a variety of technical advantages. For example, the present disclosure enables parallel operation of three-phase inverter in the microgrid. In this regard, droop control is employed to regulate output power of the three-phase inverter for parallel operation thereof. Pursuant to present subject matter, droop control is used in per-phase manner to calculate a constant magnitude of reference voltage for the phases of the three-phase inverter, even during unbalanced load at the phases thereof. Such constant magnitude may enable regulation of output power of the three-phase inverter to feed the unbalanced load as well as operate in parallel in the microgrid. To such an end, parallel operation of the three-phase inverter in the microgrid enable load sharing. Furthermore,

droop control may enable regulation of output power for parallel operation in independent manner, i.e. without employing communication link for communication between inverter(s) connected in the microgrid.
[0023] These and other aspects are further described in conjunction with the
accompanying figures FIGS. 1-7. The above examples are further described in conjunction with appended figures. It should be noted that the description and figures merely illustrate the principles of the present subject matter. It will thus be appreciated that various arrangements that embody the principles of the present subject matter, although not explicitly described or shown herein, may be devised from the description and are included within its scope. Moreover, all statements herein reciting principles, aspects, and examples of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components.
[0024] FIG. 1 illustrates an isolated grid 100 having an electrical protection
device, as per an example. Although not depicted, the isolated grid 100 (referred to as a microgrid 100) may further include additional electrical components such as lightning arrestors, transformers, measuring equipment (for example, current transformers and potential transformers), insulators, switching stations, and constructional structures (for example, poles and towers). The microgrid 100 may be an islanded for isolated operation. The microgrid 100 is shown to include a DC electrical source 102 and a three-phase inverter 104. The three-phase inverter 104 may convert DC power from the DC electrical source 102 into three-phase AC power for supplying load 106. Further, the three-phase AC power generated by the three-phase inverter 104 may be fed to the phase lines 108-1, 108-2 and 108-3 (collectively referred to as phases 108). In an example, the phase line 108-1 may correspond to a first phase or a-phase, the phase line 108-2 may correspond to a second phase or b-phase, and the phase line 108-3 may correspond to a third phase or c-phase. In an example, the phases 108 of the three-phase inverter 104 may form Δ-connection within the microgrid 100.

[0025] It may be noted that the microgrid 100 depicted to have the three-
phase inverter 104 is only illustrative. In other implementations, the microgrid 100 may include a plurality of inverters, such as three-phase inverters, single-phase inverters, or a combination thereof. Moreover, the plurality of inverters may be connected to a plurality of DC electrical sources.
[0026] Returning to the present example, the three-phase inverter 104 is
further coupled to an electrical protection device 110. In an example, the electrical
protection device 110 may be implemented in conjunction with control and
protection switch (CPS) and/or motor protection system (MPS) associated with
the three-phase inverter 104. Moreover, the electrical protection device 110 may
be in electrical communication with the three-phase inverter 104 either directly or
through other connecting means. Further, the electrical protection device 110
may be triggered when the microgrid 100 operates in islanded mode.
[0027] In operation, the electrical protection device 110 obtains current and
voltage measurements from the three-phase inverter. In an example, the obtained current and voltage measurement may be obtained from local or remote measuring devices connected to the three-phase inverter 104. Thereafter, the electrical protection device 110 may calculate power measurements based on the obtained current and voltage measurements. In this regard, the calculated power measurements may include an active power measurement and a reactive power measurement.
[0028] Continuing further, the electrical protection device 110 determines an
active power coefficient and a reactive power coefficient pertaining to the three-phase inverter 104. In one example, the active power coefficient and the reactive power coefficient may be droop coefficients. In such as case, the droop coefficient for active power and the droop coefficient for reactive power may be determined based on power system regulation standards, P-f plot and Q-V plot.
[0029] The electrical protection device 110 may then calculate a reference
voltage for each phase of the three-phase inverter 104. In this regard, a maximum voltage measurement may be calculated based on the reactive power measurement, the reactive power coefficient, and a nominal voltage for the microgrid 100. Moreover, an angle measurement may be calculated based on the

active power measurement, the active power coefficient, and a nominal angle for the microgrid 100. Based on the maximum voltage measurement and the angle measurement, a reference voltage may be calculated for each phase of the three-phase inverter 104.
[0030] Further, the electrical protection device 110 regulates output power at
the phases of the three-phase inverter 104 based on the reference voltages. In one example, a reference voltage for a phase of the three-phase inverter 104 may be provided to, for example, a control module associated with the phase. In an example, the control module may be a PI controller. The regulated output power may then be fed to the load 106. These and other examples for regulation of output power of three-phase inverter 104 for parallel operation in the islanded microgrid 100 are further described in conjunction with FIG. 2.
[0031] FIG. 2 illustrates a block diagram of an electrical protection device 110,
as per an example. The electrical protection device 110 includes processor(s) 202, memory(s) 204 and interface(s) 206. The processor(s) 202 may be a single processing unit or may include a number of units, all of which could include multiple computing units. The processor(s) 202 may be implemented as one or more microprocessor, microcomputers, digital signal processors, central processing units, state machines, logic circuitries, and/or any devices that manipulate signals based on operational instructions. The processor(s) 202 may be adapted to fetch and execute processor-readable instructions stored in the memory(s) 204 to implement one or more functionalities. The processors(s) 202 may be operable to extract, receive, process, share and store information based on instructions that drive the electrical protection device 110.
[0032] The memory(s) 204 may be coupled to the processor(s) 202. The
memory(s) 204 may include any computer-readable medium known in the art
including, for example, volatile memory, such as Static Random-Access Memory
(SRAM) and Dynamic Random-Access Memory (DRAM), and/or non-volatile
memory, such as Read Only Memory (ROM), Erasable Programmable ROMs
(EPROMs), flash memories, hard disks, optical disks, and magnetic tapes.
[0033] The interface(s) 206 may include a variety of software and hardware
enabled interfaces. The interface(s) 206 may facilitate multiple communications

within a wide variety of protocols and may also enable communication with one
or more electronic protection devices within the microgrid, electronic protection
devices associated with other microgrids, switchgear equipment, and the like.
[0034] The electrical protection device 110 may further include other
component(s) 208. The other component(s) 208 may include a variety of other electrical components that enable functionalities of monitoring, controlling and protecting the three-phase inverter (such as the three-phase inverter 104) or the electrical protection device 110. Example of such other component(s) 208 include, but is not limited to, switching device(s), fuse(s), housing, sensor(s), power source(s), and controller(s).
[0035] The electrical protection device 110 further includes module(s) 210.
The module(s) 210 may be implemented as a combination of hardware and
programming (for example, programmable instructions) to implement a variety of
functionalities of the module(s) 210. In examples described herein, such
combinations of hardware and programming may be implemented in several
different ways. For example, the programming for the module(s) 210 may be
executable instructions. Such instructions in turn may be stored on a non-
transitory machine-readable storage medium which may be coupled either
directly with the electrical protection device 110 or indirectly (for example, through
networked means). In an example, the module(s) 210 may include a processing
resource (for example, either a single processor or a combination of multiple
processors), to execute such instructions. In the present examples, the non-
transitory machine-readable storage medium may store instructions that, when
executed by the processing resource, implement module(s) 210. In other
examples, module(s) 210 may be implemented as electronic circuitry.
[0036] The module(s) 210 include a processing module 212, a control module
214 and other module(s) 216. The other module(s) 216 may further implement functionalities that supplement applications or functions performed by the electrical protection device 110 or any of the module(s) 210. In an example, the other module(s) 216 may include, but are not limited to, DQ module, a modulation module (for example, pulse width modulator (PWM), support vector machine

(SVM), and discontinuous pulse width modulator (DPWM)), and comparison module (for example, triangle comparison module).
[0037] The data 218 includes data that is either stored or generated as a result of functionalities implemented by any of the module(s) 210. It may be further noted that information stored and available in the data 218 may be utilized for facilitating operation of the electrical protection device 110. The data 218 may include line measurements 220, power measurements 222, power coefficients 224, reference voltage 226 and other data 228.
[0038] In operation, the electrical protection device 110 may be connected to the three-phase inverter 104. The three-phase inverter 104 may convert DC power from a DC electrical source to three-phase AC output power (referred to as output power). Moreover, the output power at three phases may be out of phase with each other by 120⁰.
[0039] Further, the processing module 212 of the electrical protection device 110 may obtain current and voltage measurements from the three-phase inverter 104. The obtained current and voltage measurements may correspond to three-phase AC output power generated by the three-phase inverter 104. The obtained current and voltage measurements may be stored as line measurements 220. [0040] Continuing further, the processing module 212 may calculate power measurements 222 based on the line measurements 220. In an example, the line measurements 220 may be converted to line-to-neutral measurements for such calculation. Further, the power measurements 222 may include an active power measurement and a reactive power measurement. In an example, the active power measurement may be calculated by calculating a mid-value from mean active power of the three phases of the three-phase inverter 104, and the reactive power measurement may be calculated by calculating a mid-value from mean reactive power of the three phases of the three-phase inverter 104. [0041] Thereafter, the processing module 212 determines an active power coefficient and a reactive power coefficient pertaining to the three-phase inverter 104. In an example, the active power coefficient and the reactive power coefficient may be droop coefficients. In this regard, the active power coefficient may be determined based on an active power-frequency (P-f) plot and the

reactive power coefficient may be determined based on a reactive power-voltage
(Q-V) plot. In one example, the active power-frequency (P-f) plot and the reactive
power-voltage (Q-V) plot may be plotted based on standard values defined by a
standard for interconnecting Distributed Resources with Electric Power Systems,
such as IEEE standards. The determined active power coefficient and the
reactive power coefficient may be stored as the power coefficients 224.
[0042] Furthermore, the processing module 212 calculates reference voltage
226 for each phase of the three-phase inverter 104. In this regard, a maximum
voltage measurement may be calculated based on the reactive power
measurement, the reactive power coefficient, and a nominal voltage of the
microgrid 100. Moreover, an angle measurement may be calculated based on the
active power measurement, the active power coefficient, and a nominal angle of
the microgrid. In an example, sinusoidal voltages, such as a sine voltage and a
cosine voltage may be calculated based on the maximum voltage measurement
and the angle measurement. In an example, such sine voltage may correspond
to a reference voltage for one of the three phases. Further, reference voltage for
the other two phases of the three-phase inverter 104 may be calculated by
shifting the sine voltage out of phase by 120° and 240°. Mathematically, the
reference voltage for the other two phases of the three-phase inverter 104 may
be calculated based on the sine voltage and the cosine voltage. Such reference
voltage for each of the three phases may be stored as the reference voltage 226.
[0043] To such an end, the reference voltage 226 thus calculated (referred to
as AC reference voltages) for the phases of the three-phase inverter may be transformed into DC reference quantities. In this regard, the processing module 212 may provide the reference voltage 226 to the DQ module for such transformation. In an example, the DC reference quantities may reduce ripple in regulation of the output power of the three-phase inverter 104
[0044] Thereafter, the processing module 212 regulates output power of the
three-phase inverter 104 based on the reference voltage 226. In this regard, the processing module 212 may provide the reference voltage 226 or the DC reference quantities to the control module 214 for regulating the output power of the three-phase inverter 104 to a desired output power. To such an end, the

regulation of the output power of the three-phase inverter 104 is performed using per-phase droop control. In this regard, active power of a phase of the output power may be regulated by changing angle between the phase and the load connected to the phase, and reactive power of the phase of the output power may be regulated by changing voltage between the phase and the load connected to the phase. The three-phase inverter 104 may operate in parallel within islanded microgrid while supplying required power to unbalanced load connected thereto. Moreover, the droop control enables parallel operation of the three-phase inverter in independent manner, i.e. without requiring communication with other inverter(s) connected in the microgrid.
[0045] FIG. 3 illustrates a block diagram of an electrical protection device 110,
as per an example. As mentioned previously, the electrical protection device 110 facilitates parallel operation of a three-phase inverter (such as the three-phase inverter 104). In this regard, the electrical protection device 110 causes to regulate output power of the three-phase inverter 104 such that voltage across the phases of the three-phase inverter 104 is constant while supplying power to unbalanced load connected to the phases of the three-phase inverter 104. Some blocks of the electrical protection device 110 illustrated in the FIG. 3 is discussed in conjunction with FIGS. 4-6.
[0046] In operation, a processing module (such as the processing module
212) of the electrical protection device 110 obtains current and voltage measurements from the three-phase inverter 104. The obtained current and voltage measurements may correspond to output power generated by the three-phase inverter 104. In an example, the obtained measurements may be line-to-line measurements corresponding to the output power of the three-phase inverter 104. Further, the processing module 212 calculates power measurements from the obtained current and voltage measurements. The calculation of the power measurements is explained in conjunction with FIG. 4.
[0047] FIG. 4 illustrates a block diagram 400 for power measurement
calculation, as per an example. In this regard, an obtained voltage measurement
V and an obtained current measurement I from the three-phase inverter
grid grid
104 may be used for calculating the power measurements. As shown, the

obtained voltage measurement V grid may be converted to per-phase voltage
measurements 402, i.e. line-to-neutral voltage measurements. Such per-phase voltage measurements 402 may include a first phase voltage VG_a , a second
phase voltage V G_b , and a third phase voltage V G_c .
[0048] Thereafter, the processing module 212 calculates per-phase power measurements 404 using the per-phase voltage measurements 402 and the obtained current measurement I grid. In this regard, per-phase active power and
per-phase reactive power may be calculated. Further, the active power for a phase may be calculated as:

[0049] Additionally, the reactive power for the phase may be calculated as:

[0050] wherein PG is active power for the phase, QG is reactive power for the phase, VG is output voltage measurement at the phase (source voltage), VL is load voltage corresponding to load connected to the phase, X is impedance between the source and the load, and δ is angle difference between the source and the load.
[0051] It may thus be noted that if δ = 0 then Pg = 0. In other words, if angle difference between the source and the load is zero, then no active power will flow between the source and the load. Thus, the active power is dependent on angle difference, i.e. δ. Moreover, the reactive power is dependent on voltage magnitude. Subsequently, it may be noted that controlling voltage and frequency between the source and the load may enable regulation of active and reactive power between the source and the load. For example, the active and the reactive power may be regulated in a manner to maintain uniform voltage magnitude at output of the three-phase inverter 104 (or the source) while supplying unbalanced load connected to the three-phase inverter 104.
[0052] Returning to the present example, the processing module 212 may calculate per-phase active power (depicted as Pa, Pb and Pc) corresponding to three phases and per-phase reactive power (depicted as Qa, Qb and Qc) corresponding to the three phases. Thereafter, the processing module 212 may

determine a mean of each of the per-phase active power Pa, Pb and Pc, and the per-phase reactive power and at 406. Furthermore, the processing
module 212 may determine an active power measurement (depicted as Pmid) and a reactive power measurement (depicted as Qmid) at 408. In this regard, a mid-value of the mean per-phase active power Pa, Pb and Pc is calculated, wherein the mid-value corresponds to the active power measurement. In a similar manner, the processing module 212 may calculate a mid-value of the mean per-phase reactive power Qa, Qb and Qc, wherein the mid-value corresponds to the reactive power measurement.
[0053] Returning to FIG. 3, the processing module 212 determines an active
power coefficient pertaining to the three-phase inverter 104 at 302. Moreover, the processing module 212 determines a reactive power coefficient pertaining to the three-phase inverter 104 at 304. In this regard, the active power coefficient at 302 may be determined based on an active power-frequency (P-f) plot, whereas the reactive power coefficient at 304 may be determined based on a reactive power-voltage (Q-V) plot. Such P-f and Q-V plot may be plotted using mathematical tools, such as Jacobian matrix, Monte Carlo sampling, and the like. Further, the P-f and Q-V plot may be plotted based on pre-defined standards for regulation. In an example, such regulation standards may be defined by IEEE 1547 standard. As shown, the active power coefficient may be represented as ‘m’ and the reactive power coefficient may be represented as ‘n’.
[0054] Continuing further, the processing module 212 integrates the active
power coefficient (m) and the active power measurement at 306, for
example, by way of multiplication. It may be noted that the active power measurement may be calculated as described in conjunction with FIG. 4.
Thereafter, the processing module 212 calculates an angle measurement at 308 based on the active power coefficient (m), the active power measurement and It may be noted that the is an angle between the source and
the load when 0 kW of power is supplied to the load. In an example, the angle measurement δ is calculated as:


wherein and w is 8. In an example, 8 may vary between 0° to
10⁰.
[0055] Further, the angle measurement Δ is calculated from the w as:

wherein Δ is the angle measurement in radians.
[0056] In a similar manner, the processing module 212 integrates the reactive power coefficient (n) and the reactive power measurement at 310, for
example, by way of multiplication. It may be noted that the reactive power measurement may be calculated as described in conjunction with FIG. 4.
Thereafter, the processing module 212 calculates a maximum voltage measurement at 312, based on the reactive power coefficient (n), the reactive power measurement and It may be noted that the is a
nominal voltage of the isolated microgrid (such as the microgrid 100). In an example, the is calculated as:

[0057] Continuing further, the processing module 212 calculates reference voltage based on the angle measurement Δ and the maximum voltage measurement Vm, at 314. In this regard, sinusoidal voltages (Vsin and Vcos) may be calculated using the angle measurement Δ and the maximum voltage measurement Vm. Further, the sinusoidal voltages may be calculated as:

[0058] In an example, the Vsin may correspond to a reference voltage for a phase of the three-phase inverter 104. Subsequently, reference voltages for other phases, such as phases are calculated by shifting the out of phase by ±120°. To such an end, the reference voltage for the first phase may be . Further, the reference voltage for the second phase may be and the reference
voltage for the third phase may be [0059] Thereafter, the processing module 212 may provide the reference voltages, to DQ modules 316-1, 316-2 and 316-3,
respectively. The DQ module 316-1, 316-2 and 316-3 (collectively referred to as DQ modules 316) may transform AC reference voltages into corresponding DC

reference quantities. The operation of a DQ module 316 is further explained in conjunction with FIG. 5.
[0060] FIG. 5 illustrates a block diagram 500 for a DQ module 502, as per an
example. As would be understood, the DQ module 502 transforms AC quantity to DC quantity. Pursuant to present example, the DQ module 502 may transform an AC reference voltage, such as to a corresponding DC reference quantity.
For example, the may be the reference voltage for a first phase, or phase ‘a’.
Advantageously, the DC reference quantity enables steady controlling of the output power of a three-phase inverter (such as the three-phase inverter 104). In an example, matrices may be used to transform the AC reference voltage into DC reference quantity. Subsequently, VD and VQ may be calculated corresponding to the The VD and VQ may be fed to a filter, such as a low
pass filter 504-1 and 504-2, respectively, to remove any noise in the DC reference quantity. To such an end, DC reference quantities and is
determined for controlling the voltage and frequency of phase a, in order to regulate output power at phase ‘a’. It may be noted that DC reference quantity may also be calculated for a second phase or phase ‘b’ and a third phase or phase ‘c’.
[0061] Returning to the FIG. 3, the the DQ modules 316 may calculate DC
reference quantity for the first, second and third phases. Thereafter, the processing module 212 may provide the DC reference quantity for the first phase to a first control module 318-1, the DC reference quantity for the second phase to a second control module 318-2, and the DC reference quantity for the third phase to a third control module 318-3. The first control module 318-1, the second control module 318-2, and the third control module 318-3 (collectively referred to as control modules 318) may then determine a desired output power corresponding to each phase. In an example, the control modules 318 may be one of: a PI controller, a PR controller, a PID controller. The operation of the control module 318 is further explained in conjunction with FIG. 6.
[0062] FIG. 6 illustrates a block diagram 600 of a control module 602, as per
an example. In an example, a processing module (such as the processing module 212) may provide a DC reference quantity (such as the DC reference quantities

corresponding to a first phase or phase ‘a’ to the control module 602. Further, the processing module 212 may also provide DC feedback voltage from a corresponding DQ module associated with
the first phase, such as the DQ module 502. The DC feedback voltage and may be based on instantaneous or actual output power at the first
phase of a three-phase inverter (such as the three-phase inverter 104). Thereafter, a Proportional Integral (PI) controller may calculate a desired output power for the first phase of the three-phase inverter 104. In one example, the PI controller 604-1 may compare the In an example, the PI
controller 604-1 determines a resulting error based on the comparison, wherein the PI controller 604-1 may multiply the error with a constant to get an output. Similarly, a PI controller 604-2 may compare to get an
output. Further, the output of the PI controller 604-1 is provided to a limiter 606-1 and the output of the PI controller 604-2 may be provided to a limiter 606-2. The limiters 606-1 and 606-2 may define constraints for the value of the output generated by the PI controllers 604-1 and 604-2, respectively. Further, the output of the PI controllers 604-1 and 604-2 may be fed to an alpha-beta (αβ) transform module 608. Herein, the alpha-beta (αβ) transform module 608 may transform the DQ output of the PI controllers 604-1 and 604-2 into AC quantity Va. Moreover, Va may correspond to voltage and angle for desired output power at the first phase.
[0063] Returning to the FIG. 3, the control module 318-1 may determine voltage Va for the first phase, the control module 318-2 may determine voltage Vb for the second phase, and the control module 318-3 may determine voltage Vc for the third phase. Thereafter, the processing module 212 may feed the voltages Va, Vb and Vc to a comparison module, such as a triangle comparison module. The comparison module may compare the voltages Va, Vb and Vc with corresponding output voltage. Further, a modulation module 320 may determine a duty cycle for switching of switching devices in the three-phase inverter 104 based on the comparison performed by the comparison module. Further, the modulation module 320 may switch the switching devices of the three-phase inverter 104 by providing, for example switching pulses. In an example, 12

switching devices may be provided within the three-phase inverter 104. Subsequently, the modulation module 320 may generate switching pulses S1, S2, …, S12 for each of the 12 switching devices. Examples of the modulation module 320 may include, but are not limited to, pulse width modulator (PWM), support vector machine (SVM) and discontinuous pulse width modulator (DPWM). In a manner as described above, desired output power may be achieved at each phase of the three-phase inverter 104, and thus desired reactive power may be generated by the three-phase inverter 104. Moreover, based on the angle of the voltages Va, Vb and Vc phase angle may also be changed, thereby achieving desired active power at the output of the three-phase inverter 104. In an example, the output of the three-phase inverter 104 may have constant voltage magnitude at all the phases and varying power to feed unbalanced load and operate in parallel manner at the same time.
[0064] FIG. 7 illustrates a flow diagram depicting a method 700 for facilitating parallel operation of a three-phase inverter in an isolated grid, as per an example. The order in which the method 700 is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the aforementioned method 700, or an alternative method. Furthermore, method 700 may be implemented by a server (such as the server 104) through any suitable hardware, non-transitory machine-readable program instructions, or combination thereof, or through logical circuitry. [0064] It may also be understood that method 700 may be performed by programmed and/or configured network devices present within a communication network, with such electrical protection device 110 as depicted in FIG. 1 Furthermore, in certain circumstances, program instructions stored in a non-transitory computer readable medium when executed may implement the method 700 through the respective devices, as will be readily understood. The non-transitory computer readable medium may include, for example, digital memories, magnetic storage media, such as one or more magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. Although, the method 700 are described below with reference to the electrical protection device 110, as described above, other suitable systems for the

execution of the method can be utilized. Additionally, implementation of the method 700 is not limited to such examples.
[0065] At block 702, current and voltage measurements from a three-phase inverter may be obtained. In an example, the current and voltage measurements may be line-to-line measurements corresponding to output power of the three-phase inverter (such as the three-phase inverter 104).
[0066] At block 704, power measurements for the three-phase inverter may be calculated. In an example, the power measurements may include an active power measurement and a reactive power measurement. The active power measurement and the reactive power measurement may be calculated by calculating a mid-value from mean active power and a mid-value from mean reactive power at the three phases of the three-phase inverter 104. [0067] At block 706, an active power coefficient and a reactive power coefficient pertaining to the three-phase inverter are determined. The active power coefficient may be droop coefficient of active power and the reactive power coefficient may be droop coefficient of reactive power. Such droop coefficients may be determined based on power system regulation standards. In an example, the power system regulation standards may be defined by IEEE. [0068] At block 708, a reference voltage for each phase of the three-phase inverter may be calculated. The reference voltage may be calculated based on the active power measurement, the active power coefficient, the reactive power measurement and the reactive power coefficient. In an example, a maximum voltage measurement may be calculated based on the reactive power measurement, the reactive power coefficient, and a nominal voltage. Moreover, an angle measurement may be calculated based on the active power measurement, the active power coefficient, and a nominal angle. Thereafter, sinusoidal voltages, such as a sine voltage and a cosine voltage may be calculated based on the maximum voltage measurement and the angle measurement. In an example, such sine voltage may correspond to a reference voltage for one of the three phases. Further, reference voltage for the other two phases of the three-phase inverter 104 may be calculated by shifting the sine voltage out of phase by 120⁰ and 240⁰.

[0070] In certain cases, such reference voltages calculated using sinusoidal voltages (referred to as AC reference voltages) may be transformed to DC reference quantities. In this regard, DQ transform may be used to transform the AC reference voltages to DC reference quantities.
[0070] At block 710, an output power at each phase of the three-phase inverter may be caused to regulate, for parallel operation. The regulation may be performed based on the reference voltages. In this regard, the reference voltage for a phase may be provided to a control module associated with the phase, wherein the control module may determine a desired output power for the phase. To such an end, the regulation of the output power of the three-phase inverter 104 is performed using per-phase droop control. In this regard, active power of the phase of the output power may be regulated by changing angle between the phase and a load connected to the phase, and reactive power of the phase of the output power may be regulated by changing voltage between the phase and the load connected to the phase.
[0071] Although implementations of present subject matter have been described in language specific to structural features and/or methods, it is to be noted that the present subject matter is not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained in the context of a few implementations for the present subject matter.

I/We Claim:
1. A method for facilitating parallel operation of a three-phase inverter in an
isolated grid, the method comprising:
obtaining, current and voltage measurements from the three-phase inverter;
calculating, power measurements for the three-phase inverter based on the obtained current and voltage measurements, wherein the power measurements comprise an active power measurement and a reactive power measurement for the three-phase inverter;
determining, an active power coefficient and a reactive power coefficient pertaining to the three-phase inverter;
calculating, a reference voltage for each phase of the three-phase inverter based on the active power measurement, the active power coefficient, the reactive power measurement and the reactive power coefficient; and
causing to regulate an output power at each phase for parallel operation of the three-phase inverter, based on the reference voltage.
2. The method as claimed in claim 1, wherein the active power coefficient
and the reactive power coefficients are droop coefficient of active power and
droop coefficient of reactive power, respectively.
3. The method as claimed in claim 1, the method further comprising:
calculating, mean active power and mean reactive power for each phase
of the three-phase inverter, based on the obtained current and voltage measurement;
determining, the active power measurement, wherein the active power measurement corresponds to a mid-value of the mean active power for the phases for the three-phase inverter; and
determining, the reactive power measurement, wherein the reactive power measurement corresponds to a mid-value of the mean reactive power for the phases for the three-phase inverter.

4. The method as claimed in claim 1, the method further comprising:
calculating, a maximum voltage measurement based on the reactive
power measurement and the reactive power coefficient;
calculating, an angle measurement based on the active power measurement and the active power coefficient; and
calculating, AC reference voltage for each phase of the three-phase inverter based on the angle measurement and the maximum voltage measurement.
5. The method as claimed in claim 4, the method further comprising:
transforming, the AC reference voltage for each phase of the three-phase
inverter to DC reference quantity; and
calculating, a desired output power for each phase of the three-phase inverter based on the DC reference quantity for each phase.
6. The method as claimed in claim 5, wherein Direct-Quadrature (DQ)
transformation is used for transforming the AC reference voltage for each phase
of the three-phase inverter to corresponding DC reference quantity.
7. The method as claimed in claim 5, the method further comprising:
transforming, the desired output power for each phase, calculated based
on the DC reference quantity, into AC measurements;
comparing, the AC measurements for each phase with instantaneous measurements; and
causing to regulate the output power at each phase of the three-phase inverter for parallel operation, based on the comparison.
8. An electrical protection device for facilitating parallel operation of a three-
phase inverter in an isolated grid, the electrical protection device comprising:
a control module;
an output interface; and

a processing module to:
obtain current and voltage measurements from the three-phase inverter;
calculate power measurements for the three-phase inverter based on the obtained current and voltage measurements, wherein the power measurements comprise an active power measurement and a reactive power measurement for the three-phase inverter;
determine an active power coefficient and a reactive power coefficient pertaining to the three-phase inverter;
calculate an AC reference voltage based on an angle measurement and a maximum voltage measurement;
transform the AC reference voltage into DC reference quantity;
receive, from the control module, a desired output power for each phase of the three-phase inverter, wherein the control module calculates the desired output power for each phase based on corresponding DC reference quantity; and
cause to regulate an output power at each phase for parallel operation of the three-phase inverter, based on the desired output power.
9. The electrical protection device as claimed in claim 8, wherein the control module is one of: Proportional and Integrator (PI) controller, Proportional, Integrator and Differentiator (PID) controller, and Proportional and Resonant (PR) controller.
10. The electrical protection device as claimed in claim 8, wherein the active power coefficient and the reactive power coefficient are droop coefficients that are determined based on a standard for interconnecting Distributed Resources with Electric Power Systems.
11. The electrical protection device as claimed in claim 8, wherein the angle measurement is calculated based on a nominal angle measurement, the active power measurement and the active power coefficient.

12. The electrical protection device as claimed in claim 8, wherein the maximum voltage measurement is calculated based on a nominal voltage measurement, the reactive power measurement and the reactive power coefficient.
13. The electrical protection device as claimed in claim 8, wherein the electrical protection device is connected with the three-phase inverter tied to the isolated grid, in order to enable parallel operation of the three-phase inverter without communication links.
14. A non-transient computer readable medium containing program instruction which when executed cause an electrical protection device to facilitate parallel operation of a three-phase inverter in an isolated grid, is to:
obtain current and voltage measurements for each phase of the three-phase inverter;
calculate power measurements for the three-phase inverter based on the obtained current and voltage measurements, wherein the power measurements comprise an active power measurement and a reactive power measurement for the three-phase inverter;
determine an active power coefficient and a reactive power coefficient pertaining to the three-phase inverter;
calculate an AC reference voltage based on an angle measurement and a maximum voltage measurement;
transform the AC reference voltage for each phase of the three-phase inverter into DC reference quantity;
receive, from the control module, a desired output power for each phase of the three-phase inverter, wherein the control module calculates the desired output power for each phase based on corresponding DC reference quantity; and
cause to regulate an output power at each phase for parallel operation of the three-phase inverter, based on the desired output power.

15. The non-transient computer readable medium as claimed in claim 14, wherein the non-transient computer readable medium is to:
transform the AC reference voltage for each phase of the three-phase inverter into DC reference quantity, using DQ transform;
provide the DC reference quantity to a PI controller, wherein the PI controller calculates the desired output power for each phase;
transform the desired output power for each phase into AC measurements, using alpha-beta (αβ) transform; and
cause to regulate the output power at each phase of the three-phase inverter, using one of: pulse width modulator (PWM), support vector machine (SVM) and discontinuous pulse width modulator (DPWM).