TECHNICAL FIELD

The present invention generally relates to energy storage systems and to energy storage units thereof.

BACKGROUND

Energy storage systems (ESSs) are systems for storing electrical energy in bulk. The energy storage system is typically connected, or connectable, to an energy distribution grid for delivering power to the grid when demanded.

The energy storage system typically comprises energy storage units, which are series-connected to a voltage level of up to tens of kilo-volt or even more, in medium voltage (MV) or high voltage (HV) systems. A common voltage rating of the energy storage (ES) unit as such is within low voltage (LV) range. In order to meet power and/or energy requirements, commonly the ES units are also connected in parallel. The series and parallel connected ES units form the ESS.

To protect the ES units from safety related events, e.g. thermal runaway, or overvoltage, disconnecting the ES unit from the rest of the ESS is required. For the ESS operating at considerable voltage levels, for instance in an MVsystem, such disconnection is not trivial since voltage stress could occur. Typical mechanical switches used today are bulky and costly in order to be applicable for the high voltage and current.

SUMMARY

In view of the foregoing, a concern of the present invention is to reduce the bulkiness and structural complexity of the prior art switches.

To address at least this concern, an energy storage unit and a method in accordance with the independent claims are provided. Preferred embodiments are defined by the dependent claims.

According to a first aspect of the present invention, there is provided an energy storage unit comprising a first unit terminal and a second unit terminal, an energy storage element having a first element terminal and a second element terminal, wherein the second element terminal is connected with the second unit terminal, and a protection circuit comprising a first pyrotechnic switch connected between the first unit terminal and the first element terminal and a second pyrotechnic switch connected between the first unit terminal and the second unit terminal. Thereby, a more compact switch solution has been obtained.

The first pyrotechnic switch may have a primary state where it is closed and a secondary state where it is open, and the second pyrotechnic switch may have a primary state where it is open and a secondary state where it is closed. Thereby, in the first state the energy storage element is connected to the first and second unit terminals and in the second state the energy storage element is disconnected from the first unit terminal and the second pyrotechnic switch is connected to the first and second unit terminals, thereby bypassing the energy storage element.

The energy storage unit may comprise a control unit connected with the first and second pyrotechnic switches, wherein the control unit is arranged to switch the first and second pyrotechnic switches from the primary states to the secondary states.

The control unit may be arranged to perform the switching with an adjustable time delay between the switchings.

According to a second aspect of the present invention there is provided an energy storage system comprising several energy storage units according to above, where the energy storage units are connected in series.

The energy storage system may comprise several energy storage groups connected in parallel, wherein each energy storage group comprises several series connected energy storage units.

The energy storage system may be connected to an AC/DC converter, which, in turn, is connected to an energy distribution grid.

According to a third aspect of the invention there is provided a method of controlling an energy storage unit having a first unit terminal a second unit terminal, the method comprising:

- providing the energy storage unit with an energy storage element having a first element terminal and a second element terminal, and a protection circuit comprising a first pyrotechnic switch and a second pyrotechnic switch; - connecting the second element terminal with the second unit terminal;

- connecting the first pyrotechnic switch between the first unit terminal and the first element terminal;

- connecting the second pyrotechnic switch between the first unit terminal and the second unit terminal; and

- controlling switching of the first and second pyrotechnic switches.

The first pyrotechnical switch may have a primary state where it is closed, and the second pyrotechnic switch may have a primary state where it is open. Then, said controlling switching of the first and second switches may comprise: controlling the second pyrotechnic switch to change state to a secondary state where it is closed; and after a time delay, controlling the first switch to change state to a secondary state where it is open. As an alternative, the switching sequence may be the opposite, i.e. switching the first pyrotechnic switch to the secondary state and then, after an adjustable time delay, switching the second pyrotechnic switch to the secondary state. The time delay may be adjustable.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic view of an energy storage system according to an embodiment of the present invention.

Figure 2 is a schematic view of a part of the system shown in figure 1.

Figure 3 is a flowchart of a method according to an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is a schematic view of an ESS according to an embodiment of the present invention. The ESS 1 may be connected or connectable via, for example, an AC/DC converter 4 to the grid 5. The ESS 1 comprises several energy storage units (ESU) 3 connected in series, according to an embodiment of the present invention. The series connected ESUs 3 may be referred to as forming an ESU group 2. The ESS 1 may comprise two or more ESU groups 2 connected in parallel. The ESU groups 2 are all connected to a DC link 16. The DC side of the AC/DC converter 4 is connected to the DC link 16 as well.

Each ESU 3 comprises a first unit terminal 6, a second unit terminal 7, and an energy storage element (ESE) 8. Each of the first and second unit terminals 6, 7 may be connected in series with another ESU 3 or with the DC link 16 depending on where in the ESU 2 group the ESU 3 is located. The ESE 8 comprises a first element terminal 9 and a second element terminal 10. The second element terminal 10 is connected with the second unit terminal 7. An output voltage of the ESU 3, i.e. the voltage between the first and second unit terminals 6, 7 may be within LV range or within MV range. An output of the ESS 1 may be considerably high, and for example above 1.5 kV.

The ESU further comprises a protection circuit, which comprises a first pyrotechnic switch 11 connected between the first unit terminal 6 and the first element terminal 9, and a second pyrotechnic switch 12 connected between the first unit terminal 6 and the second unit terminal 7. The first pyrotechnic switch 11 may have a primary state where it is closed and a secondary state where it is open, and the second pyrotechnic switch 12 may have a primary state where it is open and a secondary state where it is closed. In other words, during normal operation the first pyrotechnic switch 11 is closed while the second pyrotechnic switch 12 is open.

As understood by the person skilled in the art a pyrotechnic switch is a switch, which utilizes a pyrotechnic charge to open or close the contacts inside the switch. The pyrotechnic charge stores the energy for opening or closing operation in form of chemical compound and releases the energy with an electric triggering signal. As contrast, a type of switch conventionally used in the present environment contains electromagnetic switches that open or close the contacts with energy from the electromagnetic element, such as a coil. Since the energy density and power density of the pyrotechnic charge is much higher than that of an electromagnetic element, a pyrotechnic switch has a significantly smaller volume and a good current making capacity. However, the pyrotechnic charge is an element that can be used only once, and therefore the pyrotechnic switch has to be replaced after being switched. For an energy storage system fault, one-time operation is acceptable for some severe and non-repeating system faults. Therefore, the pyrotechnic switch is advantageous for energy storage system protection.

Therefore, the first and second pyrotechnic switches 11, 12 provide advantages of reducing the cost and the physical volume of the ESU 3 in comparison with prior art solutions.

The energy storage unit 3 may further comprise a control unit 14 connected with the first and second pyrotechnic switches 11, 12, wherein the control unit 14 is arranged to control the states of the first and second pyrotechnic switches 11, 12. More particularly, in order to switch the first and second pyrotechnic switches 11, 12 from the primary state to the secondary state, the control unit 14 triggers the pyrotechnic actuator within each of the first and second pyrotechnic switches 11, 12. The control unit 14 may be arranged internally of the ESU 3, and the control unit 14, in turn, may be connected to an external controller 15, either by wire or wireless.

Referring to Fig. 3, an embodiment of a method of controlling the ESU 3 may for example comprise the following operations. Initially, in box 31, the ESU 3 is arranged by providing it with the ESE 8 having the first and second element terminals 9, 10, and, in box 32, by providing the ESE 8 with the protection circuit comprising the first pyrotechnic switch 11 and the second pyrotechnic switch 12. Assuming that the energy storage system 1 is connected to the grid 5 and feeds the grid with energy, the pyrotechnic switches of the ESU 3 are in their primary states. This means that the ESE 8 delivers power via its element terminals 9, 10 and through the first pyrotechnic switch 11 to the unit terminals 6, 7 of the ESU 3. The method further may comprise connecting the second element terminal 10 with the second unit terminal 7, box 33; connecting the first pyrotechnic switch 11 between the first unit terminal 6 and the first element terminal 9, box 34; and connecting the second pyrotechnic switch 12 between the first unit terminal 6 and the second unit terminal 7, box 35. Furthermore, in box 36, the method may comprise controlling switching of the first and second pyrotechnic switches 11, 12.

When a need for disconnecting the ESE 8 occurs the control unit 14 first switches the second pyrotechnic switch 12 to the secondary state, i.e. the control unit 14 closes the second pyrotechnic switch 12. Thereby, the ESE 8 becomes bypassed by the obtained direct connection between the first and second unit terminals 6, 7 through the second pyrotechnic switch 12. Then, after a short time delay, the control unit 14 switches the first pyrotechnical switch 11 to its secondary state, i.e. the control unit 14 opens the first pyrotechnic switch 11. Thereby, the ESE 8 becomes finally disconnected from the first and second unit terminals 6, 7, and, consequently, from the ESS 1 as a whole. This bypassing/disconnection of an ESE 8 may be desirable for different reasons, such as, for example, short circuit failures, overvoltage, overtemperature, aging, etc.

The operation of the control unit 14 to perform the switching of the first and second pyrotechnical switches 11, 12 may, in turn, be controlled by control signals sent to the control unit 14 from the external controller 15. The time delay between closing the second pyrotechnic switch 12 and opening the first pyrotechnic switch 11 may be adjustable. For example, the time delay may be in the order of 1-10 ms. However, other suitable time delays may be used as well. The time delay may be set by the control unit 14. The adjustment of the time delay may be done by an operator providing input to the control unit 14, directly or via the external controller 15.

As an alternative, which may be used in some applications, the order of the switching of the first and second pyrotechnic switches 11, 12 will be the opposite of the above-described order. This means that first the control unit 14 will switch the first pyrotechnical switch 11 to the open state, and then, after a time delay, the control unit 14 will switch the second pyrotechnical switch 12 to the closed state.

In addition, the control unit 14 may monitor the status of the pyrotechnic switches 11, 12 and send information about their status to the external controller When the first and second pyrotechnic switches have been switched to the secondary state, the AC/DC converter may perform a control of the voltage on the DC link to either keep the same DC voltage or reduce the DC voltage.