FR3099309A1 - Protection system for a micro-grid and associated micro-grid - Google Patents

Abstract

The invention relates to a system (6) for protecting an electricity distribution micro-grid, said micro-grid comprising an electricity generation device (1), a transformer (4) configured to raise an electrical voltage. from the electricity production device (1), a three-phase electrical network (5) configured to convey electricity to at least one load (ch1, ch2, ch3) of electricity consumption, said system (6) of protection comprising a bank (7) of three coils (B1, B2, B3) intended to be connected to the three-phase electrical network (5), said bank (7) being configured to compensate for a capacitive effect between each line of the three-phase network (5) and earth for the purpose of canceling capacitive currents on the low voltage side of the transformer (4) and delivered by the power generation device (1). Figure for abstract: FIGURE 1

Description

Protection system of a micro-grid and associated micro-grid

GENERAL TECHNICAL AREA

The invention relates to the protection of a network isolated from a main network in the presence of an earth fault and relates more specifically to microgrids .

Electrical networks are segmented into several parts of different voltage levels. A distinction is made between High Voltage B (HTB) networks, with a voltage typically between 63 and 400 kV, High Voltage A (HTA) networks with a voltage typically equal to 20 kV, and Low Voltage (LV) networks with a voltage typically equal to 400 V .

The electrical networks, for example HTA, of certain isolated places such as islands, rural areas or mountainous areas can be isolated from the main network (corresponding in France to the HTB network). They can be isolated from it permanently, as in the case of an island not electrically connected to the mainland, or temporarily, for example following an incident on the electricity supply lines during difficult weather situations.

In the event of isolation, local energy resources sometimes associated with storage can be used to maintain a power supply in the isolated network.

These so-called microgrid type power supply networks are energized by sources interfaced by power electronics, for example an alternating voltage inverter which connects an energy storage element to the three-phase electrical network.

Thus, a battery and inverter assembly is connected to a three-phase MV electrical network which makes it possible to supply a set of loads. The battery and inverter assembly then becomes the voltage source, and replaces the public network (the HTB network for example).

Electrical networks, and particularly underground MV networks, have a capacitive effect with respect to earth and induce capacitive currents. In other words, they provide reactive power.

A known problem with MV networks and that they can be subject to "faults" and more particularly to phase-earth faults, known as "earth faults". These faults can result, for example, from an overhead conductor falling to the ground, or from contact between a tree and a section of overhead line.

In the case of micro-grids, i.e. a MV network connected to a battery/inverter assembly, these earth faults induce an increase in the capacitive currents delivered by the inverter which can be detrimental to operation. of the MV three-phase electrical network.

The invention proposes to overcome at least one of these drawbacks.

To this end, the invention proposes, according to a first aspect, a system for protecting an electricity distribution micro-network, said micro-network comprising an electricity production device, a transformer configured to raise a voltage electrical output from the electricity generating device, a three-phase electrical network configured to convey electricity to at least one electrical consumption load, said protection system comprising a bank of three coils intended to be connected to the three-phase electrical network , said bench being configured to compensate for a capacitive effect between each line of the three-phase network and the ground in order to cancel capacitive currents on the low voltage side of the transformer and delivered by the electricity production device.

The invention, according to the first aspect, is advantageously completed by the following characteristics, taken alone or in any of their technically possible combination:

- the coils have variable inductances, said protection system comprising a device for adjusting the inductance values of each coil;

- the system comprises a computer in connection with the adjustment system, said computer being configured to determine an inductance value for each of the coils of the bench, said inductance value being a function of the lengths of the three-phase electrical network;

- it comprises a man-machine interface configured to control said adjustment system;

The invention proposes, according to a second aspect, a micro-network comprising an electricity production device, a transformer configured to raise an electric voltage from the electricity production device, a three-phase electricity network configured to convey electricity to at least one electricity consumption load, said micro-grid further comprising a protection system according to the invention.

The invention, according to the second aspect, is advantageously completed by the following characteristics, taken alone or in any of their technically possible combination:

- the micro-network comprises a neutral point coil connected to the three-phase electrical network, said neutral point coil being connected between the transformer and the coil bank;

- the transformer is dY or Yd, said transformer being configured to raise the voltage from the electricity production device between 15kV and 20kV;

- the electricity production device comprises an electricity storage unit connected to an inverter, said electricity production device being configured to produce electricity with a voltage equal to 400 V.

The advantages of the invention are multiple.

It avoids having to oversize the inverter of the electricity production device.

The capacitive currents delivered by the production source during an earth fault and in normal operation are canceled by the coil bank.

In the presence of an earth fault, the production sources are able to maintain the voltage, which prevents the consumer from suffering from voltage dips and power cuts.

It is always possible to choose the neutral system of the network, for example impedant, isolated, compensated, etc. and therefore to control the amplitude of the currents to the earth.

Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and which must be read in conjunction with the appended drawings in which:

- Figure 1 illustrates a micro-grid according to the invention;

- Figure 2 illustrates an electrical diagram intended to explain the principle of the invention.

In all the figures, similar elements bear identical references.

FIG. 1 illustrates a micro-grid comprising a device 1 for producing electricity that is autonomous in that it is not connected to the public grid.

The device 1 for producing electricity comprises an electricity storage unit 2 obtained by means of producing electricity (not shown) such as photovoltaic panels, a wind turbine, etc. the electricity storage unit is typically a battery. An inverter 3 is connected to the battery and makes it possible to transform a direct voltage coming from the battery into an alternating voltage.

The voltage of the electricity produced by the device 1 for producing electricity is typically 400 V at the output.

A low-voltage HTA transformer 4 (LV/HTA) located downstream of the electricity production device makes it possible to raise the voltage between 15 kV and 20 kV which is then supplied to a three-phase electrical network 5 which will make it possible to convey electricity to consumption charges ch1, ch2, ch3. This three-phase electrical network is the MV network. Of course, between the MV network and the consumption loads ch1, ch2, ch3, the micro-grid comprises, in a known manner, transformers (not shown) which again lower the voltage so that it can be used by the consumption loads. Transformer 4 is of the dY, yD, yY or dD type.

Thus, the micro-grid also includes loads ch1, ch2, ch3 of consumption typically of individual customers.

Additionally (and optionally), the micro-grid also includes a BPN neutral point coil connected to the MV network. This BPN neutral point coil is used to create a neutral point that the operator generally connects to earth, via a resistor or a coil. This assembly is used to control the amplitude of currents to earth.

In an isolated neutral system, the current to earth comes only from the capacitance of the MV conductors. BPN is not required.

In compensated neutral, the capacitive currents of the MV conductors are compensated by the compensation impedance associated with the BPN, so that the fault current is a few amperes.

In impedance neutral, the fault current is limited to 150 A for example, to which are added the capacitive currents.

If transformer 4 has a Y winding on the MV side, to control the amplitude of the fault currents it is possible to connect the neutral point of the winding to earth via a resistor. So BPN is not necessary.

In relation to FIG. 2 , the MV network naturally generates capacitive currents Ica, Icb, Icc on the MV network, which cause reactive currents ica, ib, icc to appear that the inverter 3 must provide. Indeed, a capacitive effect between each phase A, B, C and earth is observed. This effect is modeled by capacitors C1, C2, C3 connected between each phase and earth.

We note Icx (x=a, b, c) the currents on the MV network and icx the corresponding currents on the inverter side.

These currents ica, ib, icc delivered by the inverter 3 increase sharply in the event of a fault D to the ground (a phase of the three-phase electrical network is found on the ground) on the MV network, whatever the neutral regime as impedant , compensated or isolated, and even if the current is very low in the fault.

Inverter 3 is not sized to supply currents that are too high and in the event of high current inrush, it will not be able to maintain the voltage. As a result, voltage sags may appear in the microgrid. Another phenomenon is that the inverter 3 can also limit the currents it delivers, which will result in providing the voltage with a very high harmonic content. Another phenomenon is that the inverter 3 disconnects and therefore the microgrid is de-energized.

In order to protect the micro-grid, a micro-grid protection system 6 is put in place.

This protection system comprises a bank 7 of three coils B1, B2, B3, each connected to a phase A, B, C of the MV three-phase electrical network.

These coils B1, B2, B3 are used to compensate for the capacitive effect between each phase A, B, C of the MV three-phase network and earth.

Advantageously, each coil B1, B2, B3 has a variable inductance, which makes it possible to adapt the value of each inductance to the MV network and more particularly to its length. As the three phases of the three-phase electrical network have a very similar capacitive effect, the coils B1, B2, B3 have identical inductance values L so that the compensations for each capacitive effect are balanced.

To do this, each coil includes several pads which allow their inductance to be adjusted (off) by connecting the network to various places on the coil, which makes it possible to change the number of windings. Other possibilities for adjusting the coils can be put in place, in particular to be able to adjust them without turning off the device. For example a set of several coils in parallel connected via controllable switches.

In order to adjust the value of the inductance L of each coil B1, B2, B3 via the pads, the protection system comprises an adjustment device 8 connected to a computer 9 (for example a processor).

The computer 9 makes it possible to calculate the value of the inductance L of each coil B1, B2, B3 which depends on the length of the network.

A man-machine interface 10 is connected to the computer 9 and allows an operator to configure the computer 8 so that it calculates the value of the inductance L.

The protection system makes it possible to cancel the capacitive currents ica, icb and icc delivered on the LV side of transformer 4.

The value of the inductance L is preferably such that L=1/(c.ω ² ) with ω the pulsation of the network in rad/s, C the capacitance between each phase of the network and the earth.

From the value of the inductance L, the computer 9 selects the pad of the coil which makes it possible to best approach this value of inductance L.

In the event of a fault D to earth, we have ica=icb=icc=0 upstream of the transformer, i.e. the inverter 3 does not deliver any current due to the capacitive effect of the MV conductors.

Claims (8)

System (6) for protecting an electricity distribution micro-network, said micro-network comprising an electricity production device (1), a transformer (4) configured to raise an electric voltage from the production device electricity (1), a three-phase electrical network (5) configured to convey electricity to at least one electricity consumption load (ch1, ch2, ch3), said protection system (6) comprising a bench ( 7) of three coils (B1, B2, B3) intended to be connected to the three-phase electrical network (5), said bench (7) being configured to compensate for a capacitive effect between each line of the three-phase network (5) and the earth in the purpose of canceling capacitive currents on the low voltage side of the transformer (4) and delivered by the electricity generating device (1). Protection system (6) according to claim 1, in which the coils (B1, B2, B3) have variable inductances, said protection system (6) comprising a device (8) for adjusting the inductance values of each coil (B1, B2, B3). Protection system (5) according to claim 2, comprising a computer (9) in conjunction with the adjustment system (6), said computer (9) being configured to determine an inductance value for each of the coils (B1, B2 , B3) of the bench (7), said inductance value being a function of the lengths of the three-phase electrical network (5). Protection system according to claim 3, comprising a man-machine interface (10) configured to control said adjustment system (6). Micro-network comprising a device (1) for producing electricity, a transformer (4) configured to raise an electric voltage from the device (1) for producing electricity, a three-phase electric network (5) configured to convey electricity to at least one electricity consumption load (ch1, ch2, ch3), said micro-grid further comprising a protection system (6) according to one of the preceding claims. Micro-network according to claim 5, comprising a neutral point coil (BPN) connected to the three-phase electrical network (5), said neutral point coil (BPN) being connected between the transformer (4) and the coil bank. Micro-network according to one of Claims 5 to 6, in which the transformer (4) is dY or YD, the said transformer (4) being configured to raise the voltage coming from the device (1) for producing electricity between 15 kV and 20kV. Micro-grid according to one of Claims 5 to 7, in which the device (1) for producing electricity comprises an electricity storage unit (2) connected to an inverter (3), said device for producing electricity being configured to produce electricity of voltage equal to 400 V.