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: ANTI-ISLANDING FOR A POWER ELECTRONIC DEVICE
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 electrical power transmission networks. More specifically, the present subject matter relates to actively detecting islanding condition and anti-islands a power electronic device connected to an electric grid and an electric load.
BACKGROUND [0002] Islanding is a condition in which a power electronic device, such as an inverter, connected to a main electric grid continues to feed the connected electric loads even though the electric grid supply is disconnected. If islanding condition is not detected or addressed within the critical clearing time, it may result in a safety implication for workers and even adversely affect the performance of the network as well. An active and a passive method may be utilized to detect islanding of the power electronic device by analyzing or by adding disturbances in output of the power electronic device. On detecting abrupt changes in output of the power electronic device, an anti-islanding phenomenon is performed. Since these methods add disturbances in the output of the power electronic device, it may be understood, that detecting changes in the output of the power electronic device may result in delay in estimation of the islanding condition and subsequently delay in performing anti-islanding as well.
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 provides an illustration depicting an electrical power
transmission network having a power electronic device, as per one
example;
[0005] FIG. 2 provides a block diagram of an example power electronic
device, as per one example;
[0006] FIG. 3 provides a block diagram of an example synchronizing
loop, as per one example;
[0007] FIG. 4 provides illustrative graphs depicting detection of
islanding, as per one example; and
[0008] FIG. 5 provides a flow diagram depicting an example method
for detecting islanding of a power electronic device and severing its electric
connection from rest of the network, as per one example.
[0009] It may be noted that 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
[0010] As may be understood, a power electronic device, such as
inverter, is used to convert Direct Current (DC) electrical power from power generators, such as Photovoltaic (PV) system, into an Alternating Current (AC) electrical power that household appliances and other electrical loads use. Such power electronic device, which are part of a power system, convert and may deliver power from multiple power generation sources to the electric loads and to an electric grid as well (in our example, from PV panels to electric grid). Under normal operating conditions, i.e., when the power electronic device is connected to the electric grid, power electronic device uses electric grid signal as a reference and generates an output

signal that is synchronized with the electric grid. However, in event of power
failure, such as caused by lightning strike or any grid failure event, the
electrical connection of the electric grid with the power electronic device
may be severed, interrupting the flow of electric grid signal to the power
electronic device. In such a case, the power electronic device still receiving
energy from the power generation sources or from other auxiliary grids and
converting, and delivering it to the connected electric loads, which results in
a degradation of the quality of electricity supplied to the electric loads, such
as commercial high load equipment. A power electronic device operating in
this configuration may said to be islanded from the electric grid.
[0011] As per Institute of Electrical and Electronics Engineers (IEEE)
standard 1547, such power electronic device has to turn themselves off or trip their connection in an islanding configuration, shortly after detection of a loss of connection, i.e., typically within 2 seconds. The disconnection of power electric device from the electric grid is referred to as anti-islanding, in which the power electronic device automatically detects an islanding condition and quickly acts to disconnect itself from the electric grid by mechanical or electrically disabling its power transmission connection that powers the locally connected loads.
[0012] In general, there are two types of -islanding detection
techniques, i.e., a passive technique and an active technique. Passive techniques use output of the power electronic device to detect islanding while active techniques introduce external disturbances in the output of the power electronic device to detect islanding and then performs tripping. The above described methods and other methods present in the art have been implemented to detect the islanding problem, however these methods result in complex real time calculation which may eventually leads to delay in detection of the islanding phenomenon and lapsing of the critical clearing time. Each of the available techniques are associated with analyzing output voltage or frequency of the power electronic device during islanding and accordingly disconnect the power electronic device from the utility grid.

[0013] Approaches for implementing anti-islanding, are described. In an example, a power electronic device, such as an inverter, is coupled with an auxiliary grid, such as Photovoltaic (PV) systems, to convert the received DC power into AC power to feed connected electric load and a coupled electric grid as well. While power electronic device is operating in a grid-connected mode, the power electronic device is synced with the electric grid to provide appropriate power to the connected electric load devices and appropriately feed the electric grid in case of excess power available. For example, grid-connected mode may be considered as a mode in which the power electronic device considers electric grid signals as a reference and accordingly sync itself with the electric grid and power other locally connected loads. On the other hand, while grid is disconnected from the power electronic device, the power electronic device may be said to be working in an islanded mode. As mentioned above, occurrence of islanded mode of operation may be dangerous to workers, who may not realize that some part of the electric system is still powered. [0014] In operation, present invention poses techniques for continuously monitoring for an islanding condition and disconnects the power electronic device from the electric loads as early as possible upon detecting an islanding condition. In an example, a small value of frequency, i.e., delta (∆) frequency is injected into a synchronizing signal at least once in each control loop of a synchronizing loop. In an example, the injection of the delta (∆) frequency corresponds to shifting the synchronizing signal frequency from its normal operating frequency to check stability of the synchronizing loop. The synchronizing signal is generated by the synchronizing loop, such as a phase locked loop, in order to synchronize power electronic device with the electric grid. In grid-connected mode, when the electric grid is connected to the power electronic device, the injection of delta (∆) frequency may not affect the synchronizing signal frequency because the synchronizing loop, i.e., the phase locked loop (PLL) is a feedback circuit which appropriately balances the change in the frequency

and in addition, the stiff nature of the connected electric grid also helps in balancing the change in the frequency.
[0015] While operating in an islanded mode, when the electric grid is disconnected from the power electronic device, the injected delta (∆) frequency may repetitively add in the angular frequency of the synchronizing signal during each control loop of the synchronizing loop. During islanding, the feedback based synchronizing loop repetitively inject delta (∆) amount of frequency in synchronizing signal frequency which results in a deviation in the frequency of the synchronizing signal. The change in the frequency of the synchronizing signal may then be compared with a predefined threshold frequency change.
[0016] As per the IEEE 1547 standard, the range of change in operating frequency of electric grid is +/- 2Hz, so the value of delta (∆) is such calculated that it doesn’t create disturbances in normal operating condition, i.e., when electric grid is connected. Continuing further, based on the comparison, power electronic device’s operation in the islanded mode is determined. For example, if the change in the frequency of the synchronizing signal is greater than the predefined threshold frequency change, may be representative of the fact that the power electronic device being operating in the islanded mode. On determining the power electronic device to be operating in an islanded mode, a trip signal is generated to direct the switching device, such as a circuit breaker, to isolate power electronic device from the connected electric loads and electric grid.
[0017] FIG. 1 provides a block diagram of an example electric power transmission network 100 depicting connection of a power electronic device to an electric grid and its local electric load, as per an example. The power transmission network 100 comprises an auxiliary grid 102, such as a Photovoltaic (PV) system, a micro hydro and fuel cell system, which powers the connected electric grid 104. The electric grid 104 is further connected with an electric load 106, such as a commercial high-end equipment in factories. It may be noted that the power transmission network 100 as depicted is only illustrative, it may have other electrical devices without deviating from the scope of the invention.

[0018] The power transmission network 100 further comprises a power
electronic device 108. The power electronic device 108 is electrically connected with the auxiliary grid 102 at one end and with the electric grid 104 at the other end, either directly or through other connecting means. Examples of such connecting means include electric transformer, such as transformer 110. In an example, to interface the power electronic device 108 with the electrical grid 104, a stepping up operation is required which is generally achieved by transformer 110. The transformer 110 also provide an electrical isolation which is important from the safety perspective. In one example, the power electronic device 108 acts as a coupling element between the auxiliary grid 102 and the electric grid 104. The power electronic device 108, during operation, may receive the electrical power stored or conserved by auxiliary grid 102 in the form of DC electrical power and convert this DC electrical power into AC electrical power which is to be used for powering connected electric load 106 and may even feed the electric grid 104 as well in event of excess power available. During conversion, the power electronic device 108 utilizes electric grid 104 signals as a reference to generate output AC electrical power which is in sync with the requirement of the electric grid 104 and with the electric load 106 as well. In an example, the power electronic device 108 uses a synchronizing loop, such as phase locked loop, to synchronize its output signal with the electric grid 104.
[0019] The power electronic device 108 further includes an inverter
module 112 and a Digital Signal Processing (DSP) control module 114 (referred to as DSP control module 114). The inverter module 112 and the DSP control module 114 may be implemented as either software installed within the power electronic device 108, or as hardware in the form of electronic circuitry integrated within the circuitry of the power electronic device 108. The present example is described considering that initially the electric grid 104 is connected with the power electronic device 108 and then due to some electrical fault, such as lightning or grid failure, the electric grid 104 is disconnected from the power electronic device 108. The power electronic device proposed in the present subject matter is capable of detecting such electrical fault which results in disconnection of electric grids, such as electric grid 104, within a critical clearing time, when the electric grid 104 is disconnected from the power transmission network 100.

[0020] In operation, the inverter module 112 injects a delta (∆) frequency into a synchronizing signal at least once in each control loop of the synchronizing loop. The value of delta (∆) frequency and manner in which the frequency is injected, is further explained in conjunction with other figures. Once the delta (∆) frequency is injected into the synchronizing signal, the power electronic device 108 starts monitoring the angular frequency of the synchronizing signal, whether its frequency is operating within the threshold limits or not. In an example, the DSP control module 114 may monitor the angular frequency of the synchronizing signal.
[0021] To such an end, the DSP control module 114 may monitors and determine a change in the frequency of the synchronizing signal. Once change in the frequency of the synchronizing signal is determined, the DSP control module 114 further compares the change in the frequency of the synchronizing signal with a threshold frequency change. Based on the comparison, the DSP control module 114 assess whether the power electronic device 108 is operating in islanded mode or grid-connected mode. In an example, the threshold frequency change may be considered as the maximum change in the electric grid 104 frequency which the power electronic device 108 may handle and continue to operate in the grid-connected mode under normal operation. As also described previously, as per the IEEE 1547 standard the change in the frequency of the electric grid 104 would be in the range of +/- 2Hz (or 12.56 radian/second in terms of angular frequency) from the nominal frequency of the electric grid 104. [0022] To such an end, the DSP control module 114 may compare the change in the frequency of the synchronizing signal with the threshold frequency change of the electric grid 104. If the change in the frequency of the synchronizing signal is less than the threshold frequency change, the DSP control module 114 may indicate that the power electronic device 108 is operating in a grid-connected mode. However, if the change in the frequency of the synchronizing signal is greater than the threshold frequency change, the DSP control module 114 may accordingly indicate that the power electronic device 108 is operating in an islanded mode.
[0023] On determining the power electronic device 108 being operating in the islanded mode, the DSP control module 114 may further generate a

trip signal for a switching device, such as a circuit breaker, which may be embedded into the power electronic device 108 or may be used as connection means for connecting power electronic device 108 with the electric grid 104. Based on the trip signal, the switching device may get activated to isolate the power electronic device 108 from the rest of the network. In an example, the switching device may disconnect the electrical connection between the power electronic device 108 and the connected electric load 106 and from the electric grid 104 as well. These and other examples are further described in conjunction with FIG. 2.
[0024] FIG. 2 provides a block diagram of a power electronic device
108, as per one example. The power electronic device 108 includes processor(s) 202, interface(s) 204 and a memory(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. Among other capabilities the processor(s) 202 are adapted to fetch and execute processor-readable instructions stored in the memory 206 to implement one or more functionalities.
[0025] The interface(s) 204 may include a variety of software and
hardware enabled interfaces. The interface(s) 204 may enable the communication and connectivity between the power electronic device 108 and other components of the power transmission network 100. Examples of such components include, but is not limited to, auxiliary grid 102, electric grid 106, electric load 108 and transformer 110. The interface(s) 204 may facilitate multiple communications within a wide variety of protocols and may also enable communication with one or more computer enabled terminals or similar network components.
[0026] The memory(s) 206 may be coupled to the processor(s) 202.
The memory(s) 206 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.
[0027] The power electronic device 108 may further include a
synchronizing loop 208 and a switching device 210. The synchronizing loop
208 generates a synchronizing signal for synchronizing power electronic
device 108 with the electric grid 104. The synchronizing loop 208 may be
implemented as either software installed within the power electronic device
108, or as hardware in the form of electronic circuitry integrated within the
circuitry of the power electronic device 108. In an example, the
synchronizing loop may be a phase locked loop (PLL) which utilizes electric grid
104 signals as a reference and generates synchronizing signal which drives output
of the power electronic device 108 as per its characteristics. In one example, the
synchronizing signal corresponds to a DC signal which represents 3-phase AC
signal of the electric grid 104 in DC form. Further, the switching device 210 allows
isolation of the power electronic device 108 by disconnecting mechanical or
electrical connection of the power electronic device 108 from rest of the network.
[0028] The power electronic device 108 may further include one or
more modules, representing as module(s) 212 and a data 216. The module(s) 212 may be implemented as a combination of hardware and programming (for example, programmable instructions) to implement a variety of functionalities of the module(s) 212. 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) 212 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 power electronic device 108 or indirectly (for example, through networked means). In example implemented as a hardware, the module(s) 212 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 processor-readable storage medium may store instructions that, when executed by the processing resource, implement module(s) 212. In other examples, module(s) 212 may be implemented by electronic circuitry.
[0029] The data 216 includes data that is either stored or generated as
a result of functionalities implemented by any of the module(s) 212. It may be further noted that information stored and available in data 216 may be utilized for detecting operating mode of the power electronic device 108 and then accordingly severe connection of the power electronic device 108 with rest of the network. In an example, the module(s) 212 include inverter module 112, DSP control module 114 and other module(s) 214. The other module(s) 214 may implement functionalities that supplement applications or functions performed by the power electronic device 108 or any of the module(s) 212. The data 216 on the other hand may include a synchronizing signal angular frequency 218 (referred to as synchronizing signal frequency 218), a delta angular frequency 220 (referred to as delta frequency 220), a change in synchronizing signal angular frequency 222 (referred to as change in synchronizing signal frequency 222), a threshold angular frequency change 224 (referred to as threshold frequency change 224) and other data 226. In addition, the power electronic device 108 may further include other component(s) 228. Such other component(s) 228 may include a variety of other electrical components that enable functionalities of managing and controlling the operation of the power transmission network 100. Examples of such other component(s) 228 include, but is not limited to, relays, switches, contactors, and isolators.
[0030] The power electronic device 108 detects occurrence of an
islanded mode of operation and then anti-islands itself from the power transmission network, such as power transmission network 100. The operation of the power electronic device 108 with respect to the injection of the delta frequency 220 is further described in conjunction with FIG. 3. FIG. 3 provides an example of a synchronizing loop with injection of a delta frequency 220 in its synchronizing signal. It may be noted that the

components depicted in the example synchronizing loop are only indicative and may be pertinent to the present example. The structure and working of the components may differ slightly depending on the implementation. [0031] Returning to the present example, a delta frequency 220 is injected into the synchronizing signal of a synchronizing loop 208 and the power electronic device 108 accordingly keeps track of the deviation in frequency of the synchronizing signal and generates a trip signal to direct switching device to isolate itself from rest of the network. In an example, the injection of a delta frequency 220 corresponds to adding a delta frequency 220 into the synchronizing signal frequency 218. The synchronizing loop 208 present in the power electronic device 108, utilizes signals from electric grid 104 as a reference to sync output signal of the power electronic device 108with the electric grid 104. In an example, synchronizing loop 208 may be implemented as a phase locked loop, which generates a synchronizing signal to drive output signal of the power electronic device 108 according to the changes in the signal of the electric grid 104.
[0032] The synchronizing loop 208 includes a transformation circuit 302. The transformation circuit 302 converts 3-phase AC signal (VA, VB, VC) of the electric grid 104 which are separated in phase by an angle of 120 degree from each other into two component DC signal, i.e., VD and VQ. In an example, the conversion of 3-phase AC signal of the electric grid 104 into DC signal, i.e., VD and VQ is a two-stage transformation process, i.e., VA, VB, VC initially converted into Vα, Vβ reference frame using Clark’s transformation and then to VD, VQ reference frame using Park’s transformation. In one example, the VD and VQ represents equivalent value w.r.t to the AC signal into two DC components, i.e., VD and VQ. The synchronizing loop 208 may further includes a PI Controller 304 to properly track the electric grid 104 frequency. In an example, by properly designing or selecting gain of the PI controller 304, the electric grid 104 frequency and phase may be tracked, and a synchronizing signal is generated having a frequency component representing frequency of the electric grid 104.

[0033] Continuing with the present example, the synchronizing loop 208 further includes an integrator 306. The integrator 306 transforms frequency component of the synchronizing signal into phase (θ) component. Since the synchronizing loop 208 works as a phase locked loop, therefore the frequency component of the synchronizing signal has to be converted into phase (θ) component.
[0034] In operation, the 3-phase AC signal (VA, VB, VC) of the electric grid 104 is utilized as a reference to generate a synchronizing signal which drives output of the power electronic device 108. Initially, the 3-phase AC signal (VA, VB, VC) is transformed into DQ reference using transformation circuit 302. In an example, the 3-phase AC signal VA, VB, VC is transformed into two DC components, i.e., VD and VQ components. In an example, VD and VQ represents amplitude and phase angle of the electric grid 104, respectively. In another example, the feedback phase angle (θ) may also be utilized during transformation and it represents phase angle of the feedbacked synchronizing signal. The VQ component after subtracting from a reference signal 308 having a zero value is passed through the PI controller 304 to give an output frequency at its output. In an example, the output frequency represents frequency of power electronic device output. [0035] Once the output frequency is obtained, a delta frequency 220 and a fundamental frequency w_ref of the electric grid 104 is injected into the frequency difference to obtain final frequency w_final of the synchronizing signal. In an example, the injection of the delta frequency 220 corresponds to shifting the synchronizing signal frequency from its normal operating frequency to check stability of the synchronizing loop. The value of the delta frequency 220 may be determined based on the equations1 -3, as indicated below:


is the switching frequency of the synchronizing loop.
[0036] In one example, equation (1) is used to obtain the sampling time of the synchronizing loop, so that we may calculate a sampling rate (M) of the synchronizing loop. In one example, equation (1) follows Nyquist criteria for determining sampling time. The sampling rate (M) represents the number of samples required for a change in the frequency of the electric grid 104. The value of the sampling rate (M) may be determined based on the following equation:

where is the time period of the one cycle of the electric grid
signal and it is represented in ms (millisecond);
is represented in μs; and n represents number of cycles.
[0037] Once the sampling rate (M) is obtained, the value of delta frequency (Δ) for injection is calculated by the below indicated equation:

where Δ represents the frequency value to be added for a change in frequency of the electric grid 104.
[0038] During normal operating conditions, when the electric grid 104 is connected to the power electronic device 108, the injection of delta frequency (Δ) 220 may not affect the stability of the synchronizing loop 208 because every time the delta frequency (Δ) 220 is added, it is adjusted by the connected electric grid 104 signals, since the synchronizing loop 208 is a feedback loop. However, during islanded mode, when the electric grid 104 is not connected to the power electronic device 208, the signal from the

electric grid 104 may not be present to compensate the increased frequency of the synchronizing signal which may result in deviation of the frequency components out of the threshold frequency range of the electric grid 104 as per the IEEE 1547 standard.
[0039] In an example, the w_final represents synchronizing signal
frequency 218. Once the synchronizing signal frequency 218 is obtained, the DSP control module 114 may monitors and determine a change in the synchronizing signal frequency 218 and stored it as a change in synchronizing signal frequency 222. Once change in synchronizing signal frequency 222 is determined, the DSP control module 114 further compares the change in synchronizing signal frequency 222 with a threshold frequency change 224. Based on the comparison, the DSP control module 114 may determine the operating mode of the power electronic device 108. If the change in synchronizing signal frequency 222 signal is less than the threshold frequency change 224, the DSP control module 114 may indicate that the power electronic device 108 is operating in a grid-connected mode. However, if the change in synchronizing signal frequency 222 is greater than the threshold frequency change 224 of the electric grid 104, the DSP control module 114 may accordingly indicate that the power electronic device 108 is operating in an islanded mode.
[0040] On determining the power electronic device 108 being operating
in the islanded mode, the DSP control module 114 may further generate a trip signal for the switching device 210, which may be embedded into the power electronic device 108. Based on the trip signal, the switching device 210 may get activated to isolate the power electronic device 108 from the rest of the network. In an example, the switching device 210 may disconnect the power electronic device 108 from the connected electric load 106 and from the electric grid 104.
[0041] FIG. 4 provides illustrative graphs depicting disconnection of the
power electronic device on detecting islanded mode, as per an example. It may be noted that the waveforms thus depicted are only indicative and may

be pertinent to the present example. The waveforms may differ slightly depending on the implementation.
[0042] Returning to the present example, the power electronic device
108 may injects a delta frequency into the synchronizing signal of a synchronizing loop 208 and accordingly detects change in the synchronizing signal frequency 218 and generates a trip signal to direct switching device 210 to isolate itself from the rest of the network. In FIG. 4, 402 represents voltage and current signals from the electric grid 104 and the 404 represents synchronizing signal frequency 218. It may be noted from FIG. 4 that, as soon as the electric grid 104 disconnects from the power electronic device 108, the voltage and current signals 402 trips to zero and as a result no input is present at the input of the synchronizing loop 208. Since the signal from the electric grid 104 is not present, the synchronizing signal frequency 404 starts deviating abruptly and the DSP control module 114 on determining the change in synchronizing signal frequency 222 greater than the threshold frequency change 224, generates a trip signal to direct switching device 210 to disconnect electrical connection between the power electronic device 108 and rest of the network.
[0043] FIG. 5 illustrates a flowchart of a method 500 for detecting
occurrence of electrical fault within a monitored zone of a power transmission line in a power transmission system, in accordance with one implementation of the present subject matter. The order in which the methods are 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 methods, or an alternative method. Furthermore, method 500 may be implemented by processing resource through any suitable hardware, non-transitory machine-readable instructions, or combination thereof.
[0044] At block 502, a delta frequency is injected into a synchronizing
signal at least once in each control loop of a synchronizing loop. In an example, injection of the delta frequency 220 may shift the operating

frequency of the synchronizing signal to destabilize it. In one example, the inverter module 112 of the power electronic device 108 calculates value of delta frequency 220 according to the above discussed equations 1-3 and injects the calculated value into the synchronizing signal.
[0045] At block 504, the synchronizing signal which synchronizes the
power electronic device 108 with the electric grid 104 may be monitored. For example, the DSP control module 114 may monitor the synchronizing signal generated by the synchronizing loop 208. The synchronizing signal corresponds to a DC signal which represents frequency of the electric grid 104. The value of the monitored synchronizing signal frequency may be stored in the power electronic device 108 as synchronizing signal frequency 218.
[0046] At block 506, a change in the value of the synchronizing signal
frequency is determined. For example, the DSP control module 114 may determine whether the synchronizing signal frequency 218 has changed or not. It may be noted that the synchronizing signal frequency 218 may change in response to the injection of the delta frequency 220. The injection of the delta frequency 218 once in each control loop of the synchronizing loop 208 deviates the synchronizing signal frequency 218. Then this change in the value of the monitored synchronizing signal frequency 218 may be stored in the power electronic device 108 as change in synchronizing signal frequency 222.
[0047] At block 508, an operating mode of the power electronic device
108 is determined based on the comparison of the change in synchronizing signal frequency with the threshold frequency change. For example, the DSP control module 114 may compare the change in synchronizing signal frequency 222 with the threshold frequency change 224. In an example, the threshold frequency change 222 may corresponds to the maximum change in the electric grid 104 frequency up to which electric grid 104 may be treated as connected. For example, if the change in synchronizing signal frequency 222 is less than the threshold frequency change 224 then

operating mode of the power electronic device 108 is considered as grid-
connected mode. However, if the change in synchronizing signal frequency
222 is greater than the threshold frequency change 224 then operating
mode of the power electronic device 108 is considered as islanded mode.
[0048] At block 510, a trip signal is generated and transmitted in
response to determining the power electronic device 108 being operating in the islanded mode. For example, the DSP control module 114 may generate the trip signal for directing a switching device 210 to isolate the power electronic device 108 from the rest of the network. In an example, the switching device 210 on receiving trip signal may disconnect the electrical connection between the power electronic device 108 from the connected electric load 206 and the electric grid 104.
[0049] 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 power electronic device to perform anti-islanding, coupled to an
auxiliary grid, and an electric grid, wherein the power electronic device
comprises:
a processor;
a phase locked loop for generating a synchronizing signal;
an inverter module to:
inject a delta frequency into the synchronizing signal at least once in each control loop of a synchronizing loop;
a DSP control module to:
monitor the synchronizing signal;
determine a change in a frequency of the synchronizing signal during each control loop of the synchronizing loop;
compare the change in the frequency of the synchronizing signal with a threshold frequency change;
based on the comparison, determine an operating mode of the power electronic device;
generate and transmit a trip signal for a switching device in response to determining that the power electronic device being operating in an islanded mode;
2. The power electronic device as claimed in claim 1, wherein the power electronic device is used to convert DC electric power from the auxiliary grids into AC power to feed electric loads.
3. The power electronic device as claimed in claim 1, wherein the synchronizing signal is a DC signal representing a 3-phase AC electric grid signal.

4. The power electronic device as claimed in claim 2, wherein the synchronizing signal is generated by the synchronizing loop for synchronizing power electronic device with the electric grid.
5. The power electronic device as claimed in claim 1, wherein the operating mode is one of a grid-connected mode and the islanded mode.
6. The power electronic device as claimed in claim 1, wherein during grid-connected mode, power electronic device continues to feed electric loads, and feed electric grid in case of excess power available.
7. The power electronic device as claimed in claim 1, wherein during islanded mode, the switching device on receiving the trip signal disconnects electrical connection of the power electronic device from rest of the network.
8. A method to perform anti-islanding of a power electronic device, wherein the method comprises:
injecting a delta frequency into a synchronizing signal at least once in each control loop of a synchronizing loop;
monitoring the synchronizing signal;
determining a change in a frequency of the synchronizing signal during each control loop of the synchronizing loop;
determining an operating mode of the power electronic device based on a comparison between the change in the frequency of the synchronizing signal and a threshold frequency;
generating and transmitting a trip signal for a switching device in response to determining the power electronic device being operating in an islanded mode;
9. The method as claimed in claim 8, wherein the synchronizing signal
is a DC signal representing a 3-phase AC electric grid signal.

10. The method as claimed in claim 8, wherein the power electronic
device operates in one of a grid-connected mode and an islanded mode.
11. The method as claimed in claim 8, wherein during islanded mode,
the switching device on receiving the trip signal disconnects mechanical or
electrical connection between the power electronic device and rest of the
network.
12. The method as claimed in claim 8, wherein during grid-connected mode, power electronic device continues powering connected electric loads, and feed electric grid in case of excess generated power.
13. A non-transitory computer-readable medium comprising computer-readable instructions, which when executed by a power electronic device, causing a processor to:
inject a delta frequency into the synchronizing signal at least once in each control loop of a synchronizing loop;
monitor the synchronizing signal;
determine a change in a frequency of the synchronizing signal during each control loop of the synchronizing loop;
compare the change in the frequency of the synchronizing signal with a threshold frequency change;
based on the comparison, determine operating mode of the power electronic device;
generate and transmit a trip signal for a switching device in response to determine the power electronic device being operating in an islanded mode;

14. The non-transitory computer-readable medium as claimed in claim 13, wherein the synchronizing signal is generated by a synchronizing loop for synchronizing power electronic device with an electric grid.
15. The non-transitory computer-readable medium as claimed in claim 13, wherein the switching device on receiving the trip signal is to disconnect mechanical or electrical connection between the power electronic device and rest of the network.