WO2020030673A1

WO2020030673A1 - An active method of islanding detection

Info

Publication number: WO2020030673A1
Application number: PCT/EP2019/071174
Authority: WO
Inventors: Rohit Kumar GNANASEKAR; Shrinath KANNAN; Krishna Vasudevan
Original assignee: Maschinenfabrik Reinhausen Gmbh; INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2018-08-10
Filing date: 2019-08-07
Publication date: 2020-02-13

Abstract

A method of detecting islanding in a micro grid system having one or more distributed generators (DG) connected to a load and the grid at point of common coupling (PCC) is disclosed. The method injects into a control loop a step change in active power reference at predetermined time intervals. The ratio of direct axis voltage at the PCC and the grid current at PCC is monitored continuously for each step change. Islanding is detected when the ratio remains unchanged for a predetermined number of consecutive step changes. In multiple inverter scenario the pattern of injection is synchronized in each inverter with the help of GPS signal. The method does not affect the power quality and also does not have a prerequisite to determine the nature and characteristics of the RLC load.

Description

AN ACTIVE METHOD OF ISLANDING DETECTION

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] None

FIELD OF THE INVENTION

[0002] The disclosure relates generally to microgrids and in particular to islanding detection in a microgrid having multiple distributed generators connected with a common utility load.

DESCRIPTION OF THE RELATED ART

[0003] Grid connected inverters are designed to operate in synchronism with the grid for active power injection, typically at unity power factor (UPF). Whenever the grid power fails, for example, because of line tripping due to faults or load shedding, the disconnected portion of the network becomes an island cut off from the grid. If there are distributed generation sources such as solar inverters, wind energy systems, etc. in the islanded portion of the network, their continued operation or otherwise needs to be considered carefully. While continued operation of the sources within the islanded grid may be possible with appropriate control strategy, safety and power quality related issues may arise, which need to be addressed. Switching the inverter OFF is the other option, but enough care has to be taken to avoid nuisance trips. Therefore, a reliable method of islanding detection becomes crucial whether the goal is to switch OFF or continue the inverter operation. In addition, non-detection of islanding occurrence will delay restoration of utility service and cause re -tripping due to out of phase closure. Several algorithms are currently used to detect this islanding event. Conventional methods include active and passive methods. Passive methods monitor certain features of the system such as voltage, frequency, phase, THD, spectral density, unbalance factor etc. The underlying criterion is that upon islanding these parameters of the system will exceed a certain threshold limit. This in turn is used as an indication of islanding occurrence. These methods may be simple to implement and perform well when multiple distributed generators (DG) are interfaced in the potential island but may not perform well when the operating region of system is within set thresholds. Active methods disturb the system by introducing some kind of perturbation to the system operating conditions. The repercussions of these intentional injections can be observed in the system parameters that can be used as an indication of islanding occurrence.

[0004] “Islanding Detection Based on Measuring Impedance at the Point of Common Coupling”, PegahY azdkhasti (2015) proposes an islanding detection method based on computing the frequency dependent impedance at the point of common coupling. The patent US9671442B2 detects a grid event based on the comparison of grid impedance at the one or more frequencies to known expected grid impedance at the one or more frequencies. A system that synchronizes the signals using a GPS clock is disclosed in the patent US9444257B2. US20170155279A1 discloses a communication platform that may communicate with other devices to form a microgrid. US20170179720A1 proposes a remedial action when islanding is detected. The invention discloses novel systems and methods of detecting islanding in a microgrid.

SUMMARY OF THE INVENTION

[0005] In various embodiments a method of detecting islanding in a system is disclosed. The system includes a node connecting a grid, one or more distributed generators (DG) and a load at a point of common coupling (PCC). Each DG includes a power source that is interfaced with a control loop that includes an inverter control, a voltage source inverter (VSI) and a filter The method includes injecting into each inverter control loop at a predetermined time t„, a step change in active power reference. The ratio of direct axis voltage at the PCC to the direct axis grid current at PCC is calculated. Further a step change in active power reference is injected into each inverter control loop at time t_n+i· The ratio of direct axis voltage at the PCC to the grid current at PCC at time t_n+i is calculated. The calculated ratio at time t„ and t„_+i is compared. The above mentioned steps are repeated in an arbitrary injecting pattern at arbitrary for time t_n+2, t_n+3,.... successively. In various embodiments when the ratio of direct axis voltage signal V_pcc-d and the grid current I_gd at the PCC remains unchanged for more than a predetermined number of consecutive step changes, the islanding of the system is detected. The predetermined number is for example equal to two or more.

[0006] In various embodiments islanding is detected when at least 3 consecutive ratios are unchanged.

[0007] In some embodiments the average power injected into each inverter control loop is constant.

[0008] In one embodiment the signals injected into the grid from the one or more DG sources are co-phasal.

[0009] In some embodiments the injecting pattern in the one or more DG sources are synchronously triggered using timing signals received via GPS. In various embodiments the load is one or more of resistive, inductive or capacitive components (RLC) or a combination thereof. [0010] In various embodiments the filter is selected from L, LC, LCL or any other filter that filters the harmonics produced by the inverter.

[0011] In some embodiments the power source is selected from a wind energy source, a solar power array, a fuel cell, a biomass reactor driven generator or a combination thereof.

[0012] The invention in various embodiments proposes a method of synchronously injecting active power variation into a grid in a power system that includes two or more distributed generators (DG) is disclosed. The method includes attaching a GPS receiver to each DG. Signals that contain phase and timing information are received from a GPS satellite at each receiver. A time reference signal is generated from the received GPS signal for the two or more DGs. At a predetermined time, active power variation sequences that are in phase with corresponding sequences from each DG source are injected.

[0013] In various embodiments an islanding detection system for an electrical grid that provides ac power to a load is disclosed. The islanding detection system comprises one or more distributed generators (DG), each DG includes a power source that is interfaced with a control loop that includes an inverter control, voltage source inverter, and a filter. The system further includes a node that connects the grid the one or more distributed generators and loads at a point of common coupling An injecting module that is configured to synchronously inject a step change in active power reference in each inverter control in a predetermined pattern at predetermined time intervals and a detection module that is adapted to acquire a direct axis voltage signal V_pcc-d at the PCC and a grid current I_gd at the PCC is also included. In some embodiments the detection module is adapted to monitor the ratio of the voltage signal V_pcc-d to current signal I_gd each time a step change is injected in each inverter control. In various embodiments the detection module is configured to detect islanding as having occurred when the ratio of the voltage signal V_pcc_d to current signal I_gj remains unchanged for a predetermined number of consecutive step changes, and to trigger an action. The predetermined number is two or more.

[0014] In various embodiments the injecting module receives a timing signal from a GPS receiver that acts as a trigger to effect injection.

[0015] In one embodiment the injection from the one or more DG sources is synchronously triggered using timing signals received via GPS.

[0016] In some embodiments the signals injected by the one or more DG sources are co-phasal.

[0017] In some embodiments, the action triggered by the system includes switching off the inverter and its control. In some other embodiments the action triggered includes seamless transfer to control for islanded mode of operation.

[0018] In various embodiments the filter is selected from an L, LC, or LCL filter.

[0019] In some embodiments the islanding detection system includes one or more of a distributed electricity generating source selected from a wind energy source, a solar power array, a fuel cell, a biomass reactor driven generator or a combination thereof.

[0020] Further embodiments of the islanding system follow readily from the various embodiments of the method for detecting i slanding or combinations thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention has other advantages and features which will be more readily apparent from the following detailed description of the disclosure and the appended claims, when taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an island detection system.

FIG. 2 shows the method of islanding detection in a microgrid system.

FIG. 3 shows the method of injecting co-phasal signals from two or more distributed generators in a power system.

DET AILED DESCRIPTION

[0022] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0023] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0024] The invention in its various embodiments proposes systems and methods of detecting the occurrence of islanding in a microgrid system that includes one or more distributed generators (DG) connected with a common utility load. The method combines output power variation and an estimation of ratio of direct axis voltage signal and the direct axis grid current at the point of common coupling (PCC) to determine the occurrence of islanding. Also, the method uses GPS signals to inject co-phasal power signals for islanding detection.

[0025] In various embodiments an islanding detection system 100 as shown in FIG. 1 in a microgrid is disclosed. The system includes an electrical grid 110 that provides ac power supply to a load 170, one or more distributed generators (DG) 120 connected to the grid 110 and the load 170 at a point of common coupling (PCC) 140. Each distributed generator 120 includes a power source 126 that is interfaced with a voltage source inverter (VSI) 122 and a filter 124. An inverter control loop that includes the inverter control 130, VSI 122, and a filter 124 regulates the operation of the inverter. The system further comprises an injection module 150 and a detection module 160 attached to each inverter. The injection module 150 includes a GPS receiver 154 and a step input block 152 that introduces a step change in the active power reference according to a predetermined pattern. The GPS receiver 154 is configured to synchronously inject a step change P_lnj in active power reference, P* in each of the inverter control in a predetermined pattern at predetermined time intervals . The detection module 160 is attached at the PCC and includes a transform block 162 and a computation block 164. The transform block 162 converts the three-phase signal received at the PCC to synchronous reference frame that is aligned to the grid voltage phasor. The computation block 164 is configured to acquire the direct axis voltage signal at the PCC and the grid current at the PCC and to compute the ratio of the voltage signal and current signal. The detection module 160 is adapted to monitor the ratio of the voltage signal V_pcc-d and current signal I_gd each time a step change is injected by the injection module 150 in each of the inverter. In various embodiments when the ratio of direct axis voltage signal V_pcc-d and the grid current I_gd at the PCC remains unchanged for more than two consecutive step changes the islanding of the system is detected and the system triggers an action. In some embodiments the action triggered by the system includes switching off the inverter and its control. In some other embodiments the system may seamlessly transfer to islanded mode of operation.

[0026] In various embodiments the GPS receiver 154 sends timing signals to the step input block 152 in the injection module. The step input block 152 according to a predefined pattern is triggered by the timing signals and may inject a step change in the active power reference signal.

[0027] In some embodiments the injecting pattern from the one or more DG sources are synchronous. The injecting pattern is predefined and is triggered using timing signals received from the GPS receiver attached to each inverter. In some embodiments in multiple inverter scenarios the signals injected into the grid from the one or more DG sources are co-phasal. In various embodiments the magnitude of step increment or decrement can be made small to maintain terminal voltage variation to predetermined limit.

[0028] In various embodiments the power source 126 of the distributed generator system may include one or more of renewable energy sources that may include a photovoltaic (PV) panel, a wind turbine, fuel cell, biomass or may include a combination of two or more making it a hybri d distributed generator system.

[0029] In some embodiments the filter that connects the inverter to the grid may include any filter that removes the harmonics produced by the inverter. The filter is selected from L, LC, LCL or any other filter that removes harmonics.

[0030] In various embodiments a power grid incorporating the islanding detection system is disclosed. The power grid may include a solar distributed generator system. The power grid includes a grid supplying ac power to the load and a photovoltaic or solar distributed generator system and a node connecting the grid, the photovoltaic distributed generator system and the load at a PCC. The photovoltaic distributed generator system includes a photovoltaic panel as power source. The photovoltaic panel is interfaced with a voltage source inverter (VSI) and a filter. An inverter control system that includes the inverter control, VSI, and the filter regulates the operation of the inverter. The system further comprises an injection module to inject a step change in the reference power in a predefined pattern and a detection module adapted to monitor the ratio of the voltage signal V_pcc_d and current signal I_gd each time a step change is injected by the injection module. The system detects islanding whenever the ratio of direct axis voltage signal Vpcc-d and the grid current I_gd at the PCC remains unchanged for more than two consecutive step changes and the system triggers an action.

[0031] The power grid may also include a wind distributed generator system. The power grid may include a grid supplying ac power to the load and a wind distributed generator system and a node connecting the grid, the wind distributed generator system and the load at a PCC. The wind distributed generator system includes a wind turbine as power source. The wind turbine is interfaced with a voltage source inverter (VSI) and a filter of which one embodiment is an LCL filter. An inverter control system that includes the inverter control, VSI, and the said filter regulates the operation of the inverter. The system further comprises an injection module to inject a step change in the reference power in a predefined pattern and a detection module adapted to monitor the ratio of the voltage signal V_pcc-d and current signal I_gd each time a step change is injected by the injection module. The system detects islanding whenever the ratio of direct axis voltage signal V_pcc-d and the grid current I_gd at the PCC remains unchanged for more than two consecutive step changes and the system triggers an action.

[0032] In one embodiment the invention disclosed herein is a method 200 of detecting islanding in a microgrid system. The system includes a grid supplying ac power to a load, one or more distributed generators (DG) connected to the grid and the load at a point of common coupling (PCC). Each DG includes a power source and is interfaced with a control loop that includes an inverter control, a VSI and a filter. The system further comprises an injection module to inject a step change and a detection module to detect islanding. The method 200 in various embodiments includes in step 202 injecting at time t, a step change in active power reference into the grid by each inverter control loop. The step change in active power reference is injected in predetermined time intervals set in a pre-defined pattern in the injection module. In step 203, for the step change in active power reference at time t„, the ratio of direct axis voltage at the PCC and the grid current at PCC is calculated at the detection block. In step 204 at time t_n+l a step change in active power reference is injected into the grid by each inverter control loop. The injecting module then proceeds to inject a step change in active power reference in arbitrary patterns successively at time t_n+2, t_n+3, t_n+4.... in step 204. In one embodiment the ratio of the direct axis voltage at the PCC and the direct axis grid current at PCC for the step change introduced in step 204 is calculated in step 205. The detection module in step 206 outputs the ratios at time t_n, t_n+l, t_n+2, t_n+3. In one embodiment 210 the detection module monitors if the calculated ratio at time, t_n and time t_n+l is unchanged. If the ratios remain unchanged, then islanding is detected and is confirmed by the detection module in step 215. In another embodiment in step 220 the detection module monitors if the ratio of direct axis voltage signal and the grid current at the PCC remains unchanged at time t„, t_n+l, t_n+2. If the three consecutive ratios remain unchanged, then islanding is confirmed in step 215. In various embodiments when the ratio remains unchanged for more than two consecutive step changes, then islanding is detected and necessary action is triggered. In some embodiments three consecutive unchanged ratios are considered to detect islanding and in various other embodiments the islanding can be confirmed on detection of four or more unchanged ratios.

[0033] In various embodiments average power injected into each inverter control loop is constant. Hence, the deviation from actual intended power requirement instantaneously will be small. In some embodiments the signals injected into the grid from the one or more DG sources are time synchronized. In some embodiments the timing signals that are required to synchronously trigger the signals from the DG sources are received from a GPS receiver attached to each DG.

[0034] In one embodiment the load connected to each DG is an RLC load connected in parallel to each DG. The load may also include purely resistive, capacitive or inductive components or a combination of two or more components.

[0035] In various embodiments a method 300 of synchronously injecting active power variation into a grid in a power system that includes multiple distributed generators (DG) systems is disclosed. The method as shown in FIG. 3 includes attaching GPS receiver to each of the DG in step 301. In step 302 the GPS receivers receive carrier signals from the satellite that contains timing and phase information. A reference time signal that is common to all the DGs is generated in step 303 from the received GPS signal. The reference time signal is phase accurate to each DG source. In step 304 active power variation signals that are in phase at all the DG sources are injected into the grid at a predetermined time instances.

[0036] In various embodiments the method is implemented in injecting co-phasal active power variation signals into the grid. The disclosed method does not have a prerequisite to determine the nature and characteristics of the RLC load. The performance of the method will not fail for weak grid scenario. The voltage variation and the frequency of variation are set such that the variation do not induce flicker effects. The current signal is not distorted and hence does not create any additional harmonic content in the current to affect the power quality. In multiple inverter scenario the pattern of injection is synchronized in each inverter with the help of GPS signal and hence does not involve communication requirement between inverters for implementing islanding detection in networks having multiple DG sources.

[0037] While the above is a complete description of the embodiments of the invention, various alternatives, modifications, and equivalents may be used. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention as described above. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope. Therefore, the above description and the examples to follow should not be taken as limiting the scope of the invention which is defined by the appended claims.

Claims

WE CLAIM:

1. A method (200) for detecting islanding in a system comprising a node connecting a grid, one or more distributed generators (DG) and a load at a point of common coupling (PCC), each DG comprising a power source interfaced with a control loop comprising an inverter control, a voltage source inverter (VSI) and a filter , the method comprising:

a) injecting (202) into each inverter control loop at a predetermined time t„, a step change in active power reference;

b) calculating (203) the ratio of direct axis voltage at the PCC to the grid current at PCC;

c) injecting (204) into each inverter control loop at time t_n+i a step change in active power reference;

d) calculating (205) the ratio of direct axis voltage at the PCC to the grid current at PCC at time t_n+i;

e) comparing (210) the calculated ratio at time t„ and t_n+i ; f) repeating steps c) to e), wherein an interval t_n+i-t_n is arbitrary, the successive times forming an arbitrary injecting pattern; and

g) detecting (214) islanding when a predetermined number of consecutive ratios are unchanged.

2. The method of claim 1, wherein islanding is detected when at least 3 consecutive ratios are unchanged.

3. The method of claim 1 , wherein the average power injected into each inverter control loop is constant.

4. The method of claim 1, wherein the signals injected into the grid from the one or more DG sources are co-phasal.

5. The method of claim 4, wherein injecting in the one or more DG sources is synchronously triggered using timing signals received via GPS.

6. The method of claim 1 , wherein the load comprises one or more of resistive, inductive or capacitive components (RLC) or a combination thereof.

7. The method of claim 1, wherein the filter is selected from an L, LC, or LCL filter.

8. The method of claim 1, wherein the power source is selected from a wind energy source, a solar power array, a fuel cell, a biomass reactor driven generator or a combination thereof.

9. A method (300) of synchronously injecting active power variation into a grid, in a power system comprising two or more distributed generators (DG) comprising:

attaching (301) a GPS receiver to each DG;

receiving (302) at each receiver, signals from a GPS satellite wherein the signal comprises phase and timing information;

generating (303) a time reference signal from the received GPS signal for the two or more DGs; and

injecting (304) at a predetermined time, active power variation sequences in phase with each DG source.

10. An islanding detection system (100) for an electrical grid (110) providing ac power to a load (170), the islanding detection system (100) comprising: one or more distributed generators (DG) (120) each DG comprising a power source (126) interfaced with a control loop (132) comprising an inverter control (130), voltage source inverter (122), and a filter (124); a node (140) for connecting the grid (110), the one or more distributed generators (120) and the load (170), and having a point of common coupling (PCC) (140);

an injecting module (150) configured to synchronously inject a step change in active power reference in each inverter control (130) in a predetermined pattern at predetermined time intervals;

a detection module (160), adapted to acquire a direct axis voltage signal V_pcc-d at the PCC (140) and a grid current I_gd at the PCC and to monitor the ratio of the voltage signal V_pcc-d to current signal I_gd each time a step change is injected in each inverter control (130); wherein

the detection module (160) is configured to detect islanding and trigger an action when the ratio of the voltage signal V_pcc-d to current signal I_gd remains unchanged for a predetermined number of consecutive step changes.

11. The system of claim 10, wherein the injecting module receives a timing signal from a GPS receiver that acts as a trigger to effect injection.

12. The system of claim 11, wherein the injecting in the one or more DG sources is synchronously triggered using timing signals received via GPS.

13. The system of claim 10, wherein the signals inj ected into the grid from the one or more DG sources are co-phasal.

14. The system of claim 10, wherein the action triggered comprises one of switching off the inverter and its control or seamless transfer to islanded mode of operation.

15. The system of claim 10, wherein the filter is selected from an L, LC, or LCL filter.

16. The system of claim 10, wherein the power source is selected from a wind energy source, a solar power array, a fuel cell, a biomass reactor driven generator or a combination thereof.