Method for protecting a micro-grid against a voltage drop or drop current

Abstract

The invention relates to a method for protecting a micro-grid from a voltage drop or a voltage dip occurring in a main grid connected to the micro-grid, comprising the following steps: a. Supplying power (105) to the micro-grid by means of the main grid; b. Determining that a voltage drop or a voltage dip is occurring in the main network by carrying out the following steps: i. Measuring (110) the instantaneous output voltage; ii. Comparing (130) the measured instantaneous output voltage with a reference instantaneous output voltage; iii. Incrementing (140) the counter when the measured instantaneous output voltage differs from the reference instantaneous output voltage by more than a voltage threshold; and iv. Determining (150) that a voltage drop or a voltage dip is occurring in the main grid while the counter remains a value exceeding the counter threshold; and c. Disconnecting the micro-grid from the main grid when it has been determined that a voltage drop or a voltage sag occurs in the main grid.

Description

Technical Field

[0001] The present invention relates to the field of microgrids for power generation and distribution. More specifically, the present invention relates to a method for protecting a microgrid from voltage drops or sags occurring in a main grid connected to the microgrid. Background Art

[0002] An island microgrid is typically a local power grid designed to generate and distribute electrical power in an area far from large power generation centers. An isolated area is, for example, an island, a mountainous area, or a desert area. However, the microgrid principle is also applicable to buildings, neighborhoods, campuses, or other entities connected to the main grid or a wide distribution grid that wish to manage their energy generation or storage differently and / or increase their resilience capabilities.

[0003] An islandable microgrid consists of various types of energy resources that are spatially distributed and can be disconnected from the main grid. Such a microgrid is set up as an autonomous island for energy supply. These distributed resources can include renewable energy resources, such as photovoltaic cells, solar panels, and wind turbines. They can also include engine-generator energy resources, such as fuel or gas consuming engines or turbines. And they can include energy storage facilities for local storage of energy, which can include chemical type storage devices such as batteries or mechanical type storage devices such as flywheels.

[0004] The main advantage of island microgrids is that they are able to operate autonomously in nature, i.e. in island mode, without being connected to the public grid, and are located near the consumption area (load). However, they are able to be interconnected to the upstream grid. Thus, the losses inherent to weak distribution networks are reduced.

[0005] Voltage interruptions, such as voltage sags or voltage drops, are one of the most common power quality problems in electrical networks, such as the main grid. This disturbance is mainly caused by faults in the components of the grid. According to IEEE 1159, a voltage sag is a decrease in the root mean square (RMS) value within the range of 0.1 and 0.9 of the nominal voltage. If the remaining voltage is less than 0.1 of the nominal voltage, it is considered an interruption. This phenomenon may cause sensitive electronic equipment to trip, which may have great consequences in a microgrid. In some microgrids, sensitive equipment can be protected by an uninterruptible power supply (UPS).

[0006] In a three-phase power network, voltage sags can occur within a single phase or between phases.

[0007] In the new energy landscape, more and more renewable energy resources are interconnected in microgrids. Namely, battery energy storage systems (BESS), which are charged to store excess energy from PV production and discharged when electricity rates are high (during peak hours). In addition, in the event of a power outage (blackout), BESS is able to be used as a backup resource within the microgrid.

[0008] On the one hand, the three-phase UPS technology used in microgrids (industrial sites or data centers) is based on a double conversion topology. The double conversion or AC-DC-AC topology allows the decoupling between the main grid and the loads of the microgrid. The power supplied to the load is therefore independent of the grid voltage of the main grid. On the other hand, the topology of the BESS is connected in parallel with the load. In order to be able to protect the load from grid disturbances, it is necessary to isolate the load from the grid and then be able to supply power to the load from the BESS.

[0009] In the event of a grid disturbance, disconnecting a microgrid from the main grid requires fast and reliable decisions. Most electronic loads are able to handle up to 20ms without power supply, so the transition from grid connection to islanding should be completed within this limit. However, if the detection is too sensitive, it may lead to wrong decisions and the microgrid will be disconnected from the grid too frequently.

[0010] Several devices have been developed and used to detect mains losses in different applications, namely: protection relays for low or medium voltage installations; islanding in inverter-based generation systems; or loss of mains detection systems in UPS.

[0011] Most protection elements today use voltage and current phasor measurements, i.e. they operate on positive, negative or zero network components. These elements require a data window of about one cycle to measure the phasors accurately enough for effective protection.

[0012] The anti-islanding function in the inverter will typically detect a mains loss within 150ms. In addition, the requirements for anti-islanding detection for inverter applications are 2 seconds in the United States and 0.2 seconds in Japan.

[0013] A Type 3 UPS is able to detect a primary loss and quickly switch to backup mode in less than 20ms. Typically, the UPS is using RMS values ​​or half-cycle RMS values. Some UPS are able to use more advanced algorithms that are able to deliver in less than 6ms. However, for UPS applications, a false positive detection will not penalize the operation because it is considered to be a load from the grid side. In a microgrid, if the protection is set to sensitive, there will be a risk of unwanted islanding delivery. Summary of the invention

[0014] The object of the present invention is a method of detecting a voltage drop or voltage sag occurring in a main grid for microgrid applications.

[0015] This object is achieved by a method for protecting a microgrid from voltage drops or voltage dips occurring in a main grid connected to the microgrid, the main grid having at least two lines, the method being characterized by comprising the following steps:

[0016] a) Supply power to the microgrid through the main grid;

[0017] b) determining that a voltage drop or voltage sag is occurring in the mains grid by performing the following steps on at least two lines of the mains grid:

[0018] i) measure the instantaneous output voltage between two lines of the main grid;

[0019] ii) comparing the measured instantaneous output voltage between the two lines with a reference instantaneous output voltage of the two lines;

[0020] iii) incrementing a counter when the measured instantaneous output voltage differs from the reference instantaneous output voltage by more than a voltage threshold; and

[0021] iv) determining that a voltage drop or voltage sag is occurring in the main power grid when the counter maintains a value exceeding a counter threshold; and

[0022] c) disconnecting the microgrid from the main grid by opening a switch connecting the main grid to the microgrid when a voltage drop or voltage sag occurring in the main grid has been determined.

[0023] Optional features of the protection method are set out below, which may be employed individually or in combination.

[0024] The counter is incremented by a value corresponding to the absolute value of the difference between the measured instantaneous output voltage and the reference instantaneous output voltage signals.

[0025] The main power grid is a three-phase power grid including three phase lines and one neutral line, and the method includes generating a reference instantaneous output voltage signal by means of a double decoupled synchronous frame phase locked loop.

[0026] The main grid is a three-phase grid having three phase conductors and one neutral conductor, steps bi to b.iv being performed for each pair of conductors consisting of a phase-to-phase and / or phase-to-neutral pair.

[0027] Disconnecting the microgrid from the main grid when a voltage drop or voltage sag has been determined to have occurred in the main grid is achieved by opening a thyristor-based static switch connecting the main grid to the microgrid.

[0028] Steps bi to b.iv are repeated periodically.

[0029] According to another aspect, the invention relates to an electrical controller for protecting a microgrid from voltage drops or voltage sags occurring in a main grid connected to the microgrid, the electrical controller comprising:

[0030] at least one instantaneous output voltage measurement unit configured to measure the instantaneous output voltage between two lines of the main grid;

[0031] At least one reference instantaneous output voltage signal generating unit, configured to generate reference instantaneous output voltage signals for two lines;

[0032] a comparison unit configured to compare the measured instantaneous output voltage between the two lines with a reference instantaneous output voltage signal of the two lines;

[0033] a counter incrementing unit configured to increment the counter when the measured instantaneous output voltage differs from the reference instantaneous output voltage signal by more than a voltage threshold;

[0034] a counter evaluation unit configured to determine that a voltage drop or voltage sag has occurred in the mains network when the counter holds a value that exceeds a counter threshold; and

[0035] a switch configured to be arranged between the main grid and the microgrid and configured to be opened by the electrical controller when the counter evaluation unit determines a voltage drop or a voltage sag,

[0036] The electrical controller is configured to perform the steps of the protection method as described above.

[0037] According to an embodiment of the electric controller, the switch is a fast disconnect switch, such as a thyristor based static switch, a hybrid circuit breaker, a solid state circuit breaker.

[0038] According to an embodiment of the electric controller, the reference instantaneous output voltage signal generating unit comprises a double decoupled synchronous frame phase locked loop.

[0039] According to another aspect, the invention also relates to a microgrid comprising an electrical controller as described above.

[0040] Advantageously, the method of the invention is capable of instantaneously detecting a dip or sag occurring in a main grid connected to a microgrid, as well as a short interruption in the main grid. In other words, the method is capable of detecting a loss of the main grid. Advantageously, the detection is reliable in less than 3ms. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Features of the preferred embodiments described below will be apparent when read in conjunction with the accompanying drawings, in which:

[0042] Figure 1is a schematic representation of a microgrid including an electrical controller configured to implement protection of the microgrid from voltage drops or voltage sags occurring in a main grid connected to the microgrid, according to a preferred embodiment;

[0043] Figure 2 is a flow chart depicting a method for protecting a microgrid from a voltage drop or voltage sag occurring in a main grid connected to the microgrid according to a preferred embodiment;

[0044] Figure 3 is a graph depicting a reference instantaneous output voltage and a measured instantaneous output voltage measured between two wires of the mains grid;

[0045] Figure 4 is a graph showing different voltage immunity standards;

[0046] Figure 5 is a graph showing an example of an allowable voltage boundary curve; and

[0047] Figure 6 is a flow chart of supplementary steps of a method for protecting a microgrid from voltage sag or voltage dip. DETAILED DESCRIPTION

[0048] The present application relates to a method for protecting a microgrid from voltage drops or voltage sags occurring in a main grid connected to the microgrid, to an electrical controller for protecting a microgrid from voltage drops or voltage sags occurring in a main grid connected to the microgrid, and to a microgrid comprising such an electrical controller. The voltage drops or voltage sags occurring in the main grid can be referred to as main losses.

[0049] The present application also relates to a microgrid comprising an electrical controller configured to implement the protection method.

[0050] Figure 1 A microgrid 10 according to an embodiment is schematically shown.

[0051] The microgrid 10 is connected to a main grid 12 .

[0052] The main grid 12 may be a single-phase power grid comprising a phase line and a neutral line. Alternatively, the main grid may be a three-phase power grid comprising three phase lines and a neutral line. In either case, the main grid comprises at least two lines.

[0053] According to an embodiment, the main grid is a three-phase and neutral grid and therefore comprises four conductor lines as three phase lines and a neutral line.

[0054] According to a variant, the mains network is single-phase and comprises a phase conductor and a neutral conductor.

[0055] The microgrid 10 generally corresponds to a local grid intended to generate and distribute electrical energy. The local grid can be located, for example, in an isolated area far from a large power generation center. The isolated area is, for example, an island, a mountainous area or a desert area. The microgrid principle also applies when a building, a neighborhood, a campus or other entity connected to a broad distribution grid intends to manage its energy generation differently and increase its resilience capabilities.

[0056] The main focus of microgrids is that they can operate autonomously (in island mode, without connection to the public network) and be located close to the consumption area (load), thus limiting the losses inherent in long-distance distribution networks.

[0057] The microgrid 10 includes an electrical controller 20 for protecting the microgrid from voltage drops or voltage sags occurring in the main grid. The microgrid can include a plurality of electrical conductors 22, one or more power sources 24, and one or more electrical loads 26. The power sources 24 and the electrical loads 26 are connected by the electrical conductors 22. The electrical loads are intended to at least partially consume the power delivered by the power sources.

[0058] The power source 24 may include a photovoltaic system 241, wind turbines 242, 243, a generator 244, and a battery energy storage system (BESS) 245. The generator may be an inverter-based generator connected in parallel. The battery energy storage system is configured to store excess energy from, for example, the production of a photovoltaic system and / or a wind turbine (or multiple), and can be used to store energy provided by the main grid when electricity prices are low. The battery energy storage system is also configured to release stored energy when the microgrid is disconnected from the main grid due to a voltage drop or voltage sag or when the microgrid is disconnected from the main grid when the main grid provides a high electricity price.

[0059] Inverter-based generators are able to support transients during disconnection by switching their mode from grid-connected to grid-forming mode.

[0060] In the event of a grid disturbance in the main grid 12, disconnecting the microgrid 10 from the main grid requires fast and reliable decisions. Most electronic loads of the microgrid are able to handle up to 20 milliseconds without power supply, so the transition from grid connection mode to grid forming or islanding mode should be completed within this limit. However, if the decision is too sensitive, it may lead to false detection and the microgrid will be disconnected from the grid unnecessarily.

[0061] The electric controller 20 according to the present application comprises:

[0062] at least one instantaneous output voltage measurement unit 30 configured to measure an instantaneous output voltage MOV between two lines of the main grid;

[0063] At least one reference instantaneous output voltage signal generating unit 32, configured to generate reference instantaneous output voltage ROV signals of two lines;

[0064] A comparison unit 34 configured to compare the measured instantaneous output voltage MOV between the two lines with a reference instantaneous output voltage ROV signal of the two lines;

[0065] A counter incrementing unit 36 ​​configured to increment the counter when the measured instantaneous output voltage MOV differs from the reference instantaneous output voltage ROV signal by more than a given voltage threshold;

[0066] a counter evaluation unit 38 configured to determine that a voltage drop or voltage sag has occurred in the mains grid when the counter holds a value that exceeds a counter threshold; and

[0067] A switch 40 , configured to be arranged between the main grid 12 and the microgrid 10 , and configured to be opened by the electrical controller 20 when the counter evaluation unit 38 determines a voltage drop or a voltage sag.

[0068] The electronic controller 20 is configured to perform the steps of the method according to the present application.

[0069] The switch 40 may be a thyristor-based static switch, a silicon controlled rectifier, a solid-state circuit breaker, or a hybrid circuit breaker. Preferably, the switch 40 is a fast disconnect switch.

[0070] The reference instantaneous output voltage signal generating unit 32 may include a double decoupled synchronous frame phase locked loop.

[0071] The instantaneous output voltage measuring unit 30 can be a voltage sensor connected to two lines of the main grid and configured to sense a voltage between the two lines.

[0072] According to an embodiment, Figure 1 As shown, the electrical controller 20 comprises a central processing unit 42 and a memory 44. The electrical controller also comprises an interface 46 configured to connect to an instantaneous output voltage measurement unit and / or a voltage sensor.

[0073] The central processing unit 42, the memory 44 and the interface 46 are adapted to communicate via an internal communication bus (not shown) and form a programmable device.

[0074] The reference instantaneous output voltage signal generating unit 32, the comparison unit 34, the counter incrementing unit 36 ​​and / or the counter evaluation unit 38 can be implemented by a programmable device formed by a central processing unit 42, a memory 44 and an interface 46. Thus, the central processing unit 42 may be configured to retrieve a signal from the instantaneous output voltage measuring unit 30 (e.g. from a voltage sensor) via the interface 46.

[0075] The central processing unit 42 may be configured to store a value representative of the measured voltage MOV in the memory 44 .

[0076] The central processing unit 42 may also be configured to calculate the signal from the reference instantaneous output voltage signal generating unit 32. The central processing unit 42 may be configured to store a value representing the reference instantaneous output voltage ROV signal in the memory 44.

[0077] The electrical controller 20 may be capable of being configured to compare a measured instantaneous output voltage MOV between the two lines with a reference instantaneous output voltage ROV signal of the two lines, configured to increment a counter when the measured instantaneous output voltage MOV differs from the reference instantaneous output voltage signal ROV by more than a voltage threshold, and / or configured to determine that the counter holds a value that exceeds a counter threshold.

[0078] Figure 2 is a flow chart of a method 100 for protecting a microgrid from a voltage drop or voltage sag occurring in a main grid connected to the microgrid, according to an embodiment.

[0079] The method 100 according to the present application comprises the following steps: Figure 2 As shown:

[0080] a. Powering the microgrid via the main grid 105;

[0081] b. Determine that a voltage drop or voltage sag is occurring in the mains grid by performing the following steps on at least two lines of the mains grid:

[0082] i. Measure the instantaneous output voltage MOV between the two lines of the 110 main grid;

[0083] ii. comparing the measured instantaneous output voltage MOV between the two lines with the reference instantaneous output voltage ROV of the two lines 130;

[0084] iii. When the measured instantaneous output voltage MOV differs from the reference instantaneous output voltage ROV by more than the voltage threshold, the counter is incremented by 140; and

[0085] iv. When the counter maintains a value exceeding the counter threshold Clim, it is determined 150 that a voltage drop or voltage sag is occurring in the main grid; and

[0086] c. Disconnecting the microgrid from the main grid by opening a switch connecting the main grid to the microgrid when a voltage drop or voltage sag occurring in the main grid has been determined 160 .

[0087] The method according to the present application ensures normal operation of the electrical loads of the microgrid.The method according to the present application may also be configured to avoid premature detection.

[0088] Disconnection of a microgrid from the main grid when a voltage drop or voltage sag occurs in the main grid can also be referred to as islanding the microgrid.

[0089] According to an embodiment, the method comprises generating 115 a reference instantaneous output voltage ROV signal by means of a double decoupled synchronous frame phase locked loop.

[0090] For a three-phase mains network, the measured voltage is divided into six channels, each corresponding to the instantaneous output voltage measured between two wires of the mains network, considering the three phases marked A, B, C and the neutral marked N, the channels are: V AB 、V AC 、V BC 、V AN 、V BN 、V CN .

[0091] In the case of a single-phase mains grid, the measured voltage MOV corresponds to a single channel, corresponding to the instantaneous output voltage measured between the phase and neutral conductors of the mains grid.

[0092] The method is applied to each two-wire channel (also called a couple), and the instantaneous output voltage measured between the two wires is referred to as MOV in the following.

[0093] In other words, in the case where the main network is a three-phase network with three phase lines and one neutral line, steps b.i to b.iv are performed for each pair of lines consisting of a phase-to-phase and / or phase-to-neutral pair. In the case where the main network is a three-phase network, steps b.i to b.iv are performed for six pairs of lines: three pairs consisting of two phase lines and three pairs consisting of a phase line and a neutral line.

[0094] According to the illustrated embodiment, the method further comprises a step 120 of calculating a voltage threshold.

[0095] The voltage threshold calculation 120 includes a formatting step 122, wherein the formatting step 122 includes scaling the voltage measurement between predetermined boundary values ​​(eg, Figure 3 As shown), and taking the absolute value of the voltage measurement for both the measured instantaneous output voltage and the reference instantaneous output voltage ROV.

[0096] In addition, step 120 includes determining the amount of the product based on one or more criteria (see e.g. Figure 5 According to the specification of the tripping curve of the instantaneous output voltage ROV, the voltage lower limit LL and the counter threshold Clim are calculated 124 from the reference instantaneous output voltage ROV. In particular, the counter threshold Clim is determined to fit the detection time to the tripping curve. The counter threshold Clim can be calculated and stored in advance.

[0097] For example, the lower voltage limit LL is calculated as a percentage of the absolute value of the reference instantaneous output voltage ROV.

[0098] Optionally, for optimization purposes, step 120 further comprises adapting 126 a lower limit around the zero margin. For example, if the reference instantaneous output voltage ROV is less than the lower boundary value ROV-low, the lower limit LL takes a predetermined value.

[0099] Then, as an embodiment of comparing 130 the measured instantaneous output voltage MOV between the two lines with the reference instantaneous output voltage ROV of the two lines, the method includes comparing the measured instantaneous output voltage MOV (in this embodiment, an absolute value) with the calculated lower limit LL, and if the measured instantaneous output voltage MOV is lower than the lower limit LL, the counter is incremented (step 140). Otherwise, the comparison 130 is followed by continuing the process.

[0100] It should be noted that comparison 130 is equivalent to comparing the difference between the reference instantaneous output voltage ROV and the measured instantaneous output voltage MOV with the difference between the reference instantaneous output voltage ROV and the lower limit. In other words, comparison 130 allows determining whether the measured instantaneous output voltage MOV differs from the reference instantaneous output voltage ROV by more than a voltage threshold.

[0101] The counter may be incremented (step 140) by a value corresponding to the absolute value of the difference between the measured instantaneous output voltage MOV and the reference instantaneous output voltage ROV signals.

[0102] Disconnecting the microgrid from the main grid when a voltage drop or voltage sag occurring in the main grid has been identified can be achieved by opening the thyristor-based static switch connecting the main grid to the microgrid.

[0103] Steps bi to b.iv may be repeated periodically, preferably every 0.5 microseconds to 1 millisecond.

[0104] Preferably, the voltage threshold may be set to a value between 0 and 90% of the nominal voltage.

[0105] The counter threshold Clim may be set to a value between 1 millisecond and 20 milliseconds.

[0106] Figure 3 A diagram depicting a reference instantaneous output voltage ROV and a measured instantaneous output voltage MOV measured between two lines of the mains grid is shown. These two lines may also be referred to as channels.

[0107] Based on the reference instantaneous output voltage ROV and the voltage threshold, the upper limit UL and the lower limit LL are calculated. When the measured instantaneous output voltage MOV is higher than the upper limit UL (when the voltage is negative) or lower than the lower limit LL (when the voltage is positive), the measured instantaneous output voltage MOV differs from the reference instantaneous output voltage ROV by more than the voltage threshold.

[0108] exist Figure 3 In the measurement, the instantaneous output voltage of MOV is measured at the first instant C Start Below the lower limit LL. At the first instant C Start Initially, the counter is incremented, preferably by a value corresponding to the absolute value of the difference between the measured instantaneous output voltage MOV and the reference instantaneous output voltage ROV. The counter is further incremented until the counter reaches the second instantaneous output voltage C End Exceeds the counter threshold, where C End =C Start +Clim. At the second instant C End , a voltage drop or voltage sag occurring in the mains grid is detected.

[0109] The upper limit UL and / or the lower limit LL may have a fixed part and a linear part to handle calculations near zero crossings.

[0110] Figure 4 is a graph showing different voltage immunity standards. Most computerized devices such as personal computers and servers follow the ITIC (Information Technology Industry Council) voltage immunity curve IVIC (for impulse voltage insulation level). Figure 4 As depicted in , the voltage sag or dip is tolerable within 10 milliseconds. Considering the delay of 6.6 milliseconds to open the static switch, the detection of the voltage sag or dip is preferably detected within 3.3 milliseconds. The detection within 3.3 milliseconds ensures the normal operation of the electrical loads of the microgrid.

[0111] Figure 5 is a graph showing an example of an allowable voltage boundary curve VBC. The allowable voltage boundary curve VBC may be compiled from several standards, preferably including the ITIC standard. The several standards may include: ITIC, UPS Class 3, IEEE Type 1, IEEE Type 2, IEEE Type 3, IC61000-4-11 Class 2, IC61000-4-11 Class 3, IC61000-4-34 Class 2, IC61000-4-34 Class 3. Thus, the allowable voltage boundary curve VBC may be selected to be within the limits of the non-interruptible functional area of ​​the selected several standards. Figure 4 The non-interruptible functional area is depicted in .

[0112] The method according to the present application can be configured by defining an admissible voltage boundary curve VBC. Thus, the method according to the present application allows compiling a trip curve from several standards depending on the needs of the customer and / or depending on the application. The boundary curve VBC is used to calculate the lower limit LL and the counter threshold Clim.

[0113] As in Figure 5 In the example given in , in order to comply with the selected standard, the microgrid can tolerate a voltage between 0-0.72 times the nominal voltage for 10ms, then tolerate a voltage between 0.72-0.8 times the nominal voltage for 100ms, and then tolerate a voltage between 0.8-0.9 times the nominal voltage for 2 seconds. Therefore, the voltage threshold and the counter threshold can be selected so that a voltage between 0-0.72 times the nominal voltage for 10ms, a voltage between 0.72-0.8 times the nominal voltage for 100ms, and a voltage between 0.8-0.9 times the nominal voltage for 2 seconds does not lead to a determination that a voltage drop or voltage sag is occurring in the main grid. The voltage threshold and the counter threshold can be calculated and defined in advance so that the detection time fits the allowable voltage boundary curve VBC.

[0114] When considering the off time of the switch, such as the off time of 6.6 milliseconds for a thyristor-based static switch, an effective detection area VDA is selected that is spaced from the allowable voltage boundary curve VBC by the off time of the switch, such as 6.6 milliseconds in this case. When, instead of a thyristor-based static switch, a fast static switch with an off time of 10 microseconds is used to separate the main grid from the microgrid, an effective detection area VDA that is spaced 10 microseconds from the allowable voltage boundary curve VBC can be selected. Such a fast static switch can be particularly beneficial depending on the grid dynamics and disturbances and customer selection.

[0115] Steps a. to c. of the method according to the present application achieve disconnection of the microgrid from the main grid in less than one current cycle. Steps a. to c. of the method can ensure so-called rapid detection.

[0116] The method may additionally comprise steps based on RMS-based calculations and / or phasor-based calculations as known from the prior art, preferably to ensure so-called slow detection. Analogously to steps a. to c. of the method, RMS-based calculations and / or phasor-based calculations can be performed for each pair of lines consisting of a phase-to-phase and / or phase-to-neutral pair. RMS-based calculations and / or phasor-based calculations enable disconnection of the microgrid from the main grid in a time higher than one current cycle.

[0117] According to a variant, Figure 6As shown in the flow chart of , the method may additionally include further tests to improve the detection of voltage drops or voltage sags occurring in the main grid connected to the microgrid.

[0118] The method then comprises a further selectivity 200 to assess whether the voltage drop occurs on the main grid side or on the microgrid side, comprising an instantaneous current measurement 210 , wherein the instantaneous current is measured by a unit suitable for measuring the instantaneous current flowing through the microgrid 10 in the switch 40 .

[0119] The instantaneous measured current Ic is compared with the current threshold I TH For comparison 220 , the current threshold can generally be configured to be one to ten times the nominal current of the microgrid 10 .

[0120] If the instantaneous measured current Ic is higher than the current threshold I TH , then step 220 is followed by step 230 of loss of the main interlock, and then the microgrid is prohibited from being disconnected from the main grid.

[0121] Furthermore, the method comprises a switch voltage measurement 240 comprising an upstream voltage measurement to obtain a measured upstream voltage Vsu and a downstream voltage measurement to obtain a measured downstream voltage Vsd, followed by a comparison 250 between the difference between the measured upstream voltage Vsu and the measured downstream voltage and a given voltage difference threshold VTH.

[0122] If the difference between the measured upstream voltage Vsu and the measured downstream voltage is higher than a given voltage difference threshold V TH , then step 250 is followed by step 230 of the loss of the main interlock, and then the microgrid is prohibited from being disconnected from the main grid.

[0123] Voltage difference threshold V TH For example comprised between 0.1 and 10 Volts.

[0124] According to this variant, if the counter holds a value exceeding the counter threshold (as for Figure 2 explained) and if the instantaneous measured current Ic is not higher than the current threshold I TH And if the difference between the measured upstream voltage Vsu and the measured downstream voltage is not higher than a given voltage difference threshold VST, a main loss is detected and the microgrid is disconnected from the main grid.

[0125] Advantageously, voltage dip detection due to disturbance events (eg short circuits) occurring inside the microgrid can be eliminated. Thus, voltage dip or voltage sag detection occurring in the main grid is improved.

[0126] The algorithm was also deployed into a digital signal processor (DSP) and tested using hardware in the loop.

[0127] The method, the electrical controller and the microgrid according to the present application allow detecting voltage drops or voltage dips occurring in the main grid, ie main losses, which can save critical loads of less than 10 ms even if the main grid is a three-phase grid and is a polluted and / or unbalanced network.

[0128] The method, electrical controller and microgrid according to the present application are able to provide the desired results despite disturbances in the main grid voltage, such as harmonics, unbalance, phase shift. This is due to the fact that the reference signal is generated using a double decoupled synchronous frame phase locked loop capable of tracking a positive sequence.

Claims

1. A method for protecting a microgrid from a voltage drop or voltage sag occurring in a main grid (12) connected to the microgrid (10), the main grid having at least two lines, the method being characterized by comprising the following steps: a. Powering the microgrid through the main grid (105); b. Determine that a voltage drop or voltage sag is occurring in the mains grid by performing the following steps on at least two lines of the mains grid: i. Measuring the instantaneous output voltage between two lines of the main grid (110); ii. comparing the measured instantaneous output voltage between the two lines with a reference instantaneous output voltage of the two lines (130); iii. When the measured instantaneous output voltage differs from the reference instantaneous output voltage by more than the voltage threshold, incrementing a counter (140); and iv. when the counter maintains a value exceeding the counter threshold, determining (150) that a voltage drop or voltage sag is occurring in the main grid; and c. Disconnecting the microgrid from the main grid by opening a switch connecting the main grid to the microgrid when a voltage drop or voltage sag occurring in the main grid has been determined (160).

2. The method according to claim 1, wherein: The counter is incremented (140) by a value corresponding to the absolute value of the difference between the measured instantaneous output voltage and the reference instantaneous output voltage signal.

3. The method according to any one of claims 1 or 2, the main power grid being a three-phase power grid comprising three phase lines and one neutral line, the method comprising generating (115) a reference instantaneous output voltage signal by means of a double decoupled synchronous frame phase locked loop.

4. The method according to any one of claims 1 to 3, the main grid being a three-phase grid having three phase lines and one neutral line, wherein steps bi to b.iv are performed for each pair of lines consisting of a phase-to-phase and / or phase-to-neutral pair.

5. The method according to any one of claims 1 to 4, wherein: Disconnecting the microgrid from the main grid when a voltage drop or voltage sag has been determined to have occurred in the main grid is accomplished by opening a thyristor-based static switch connecting the main grid to the microgrid (160).

6. The method according to any one of claims 1 to 5, wherein: Steps bi to b.iv are repeated periodically.

7. An electrical controller (20) for protecting a microgrid from voltage drops or voltage sags occurring in a main grid connected to the microgrid, characterized in that The electric controller comprises: at least one instantaneous output voltage measurement unit (30) configured to measure the instantaneous output voltage between two lines of the main grid; At least one reference instantaneous output voltage signal generating unit (32) configured to generate reference instantaneous output voltage signals for two lines; A comparison unit (34) configured to compare the measured instantaneous output voltage between the two lines with a reference instantaneous output voltage signal of the two lines; A counter incrementing unit (36) configured to increment the counter when the measured instantaneous output voltage differs from the reference instantaneous output voltage signal by more than a voltage threshold; a counter evaluation unit (38) configured to determine that a voltage drop or voltage sag has occurred in the mains network when the counter holds a value exceeding a counter threshold; and A switch (40) configured to be arranged between the main grid (12) and the microgrid (10), and is configured to be disconnected by the electrical controller (20) when the counter evaluation unit determines a voltage drop or a voltage sag, The electronic controller (20) is configured to perform the steps of the method according to any one of claims 1 to 6.

8. The electric controller according to claim 7, wherein: The switch (40) is a fast disconnect switch, such as a thyristor-based static switch, a hybrid circuit breaker, or a solid-state circuit breaker.

9. An electrical controller according to any one of claims 7 or 8, wherein: The reference instantaneous output voltage signal generating unit (32) comprises a double decoupled synchronous frame phase locked loop.

10. A microgrid (10) comprising an electrical controller (20) according to any one of claims 7 to 9.

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