Method of providing low voltage ride through (LVRT) capability for electrolysis cell plants

Abstract

An electrolytic cell plant is described. The electrolysis cell plant includes a first DC link having at least one DC link capacitor and an AC / DC power converter. An electrolytic cell is electrically connected to the first DC link and adapted to receive a load current. During normal operation of the AC power network, the controller is adapted to maintain a pre-event DC link voltage and supply a pre-event load current to the electrolysis cell. In response to a detected low voltage event, the controller is adapted to: (a) initially supply a pre-event load current to the electrolysis cell to maintain normal operation of the electrolysis cell, (b) monitor a DC link voltage, e.g., a voltage in a first DC link, and (c) if the monitored DC link voltage falls below a first voltage threshold that is less than the pre-event DC link voltage, determine that the DC link voltage falls below a second voltage threshold that is less than the pre-event DC link voltage. The load current supplied to the electrolytic cell is optionally reduced by ramping down the load current at a suitable ramping rate.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of providing low voltage ride through (LVRT) capability for an electrolyser plant. For example, the electrolyser plant can be used to generate hydrogen and oxygen. BACKGROUND

[0002] It is known to use electrolyser plants to generate hydrogen and oxygen. An electrolyser plant can have one or more electrolyser cells. Each electrolyser cell typically has one or more stacks, where each stack has one or more electrolyser units that generate hydrogen and oxygen when a direct current (DC) voltage is applied across the stack. Some electrolyser cells will comprise multiple stacks. For example, known types of electrolyser units use proton exchange membrane or polymer electrolyte membrane (PEM) technology, alkaline water (ALK) technology or solid oxide electrolysis (SOES) technology.

[0003] The electrolyser plant can be connected to an alternating current (AC) power network, such as a supply grid. Alternatively, the AC power network can be a microgrid powered by one or more renewable energy sources, such as wind turbines, which are used, for example, to generate so-called “green” hydrogen. For example, the AC power network can occasionally experience a low voltage event in which the network voltage suddenly drops due to an unexpected fault or network failure. If the electrolyser plant needs to stop operating, such a low voltage event can interrupt the production of hydrogen. It will be appreciated that sudden shutdown of a large electrolyser plant with a significant electrical load should be avoided as much as possible, as this can cause instability in the AC power network. The grid operator can need to respond to the sudden loss of electrical load by reducing the amount of power supplied to the AC power network by power generators. This problem can become more significant if future electrolyser plant loads increase from around 5-10 MW to over 100 MW, and possibly up to 1 GW. A sudden loss of electrical load in the 100 MW to 1 GW range will cause a sudden increase in the frequency (or “network frequency”) of the AC power network. Therefore, the operator of the supply grid must provide compensation in order to keep the network frequency within certain limits that are typically set by the respective grid code. This involves frequency restoration reserves (or ancillary services) that must be covered by having spare power reserves from other power generators or plants, which can be activated for a specific cost and are available. Therefore, mitigating such sudden losses of electrical load can result in lower operating costs and higher system efficiency.

[0004] A sudden shutdown can also damage the electrolyser plant - for example, degrade the polymer membranes of the electrolyser units. Restarting the electrolyser plant after any shutdown typically takes a long time, possibly up to an hour.

[0005] Accordingly, there is a need to provide an electrolysis plant with improved low voltage ride through capability. Such capability can maintain the electrolysis plant in normal operation, or operating at reduced capacity, if the AC power network experiences a low voltage event. An electrolysis plant that can continue to operate during a low voltage event and does not immediately require shut down will require less frequent restoration reserves. SUMMARY

[0006] The present invention provides a method of providing low voltage ride through (LVRT) capability for an electrolysis plant.

[0007] The electrolysis plant comprises:

[0008] a first DC link having at least one DC link capacitor;

[0009] an AC / DC power converter comprising:

[0010] at least one AC terminal electrically connectable to an AC power network, and

[0011] at least two DC terminals electrically connected to the first DC link; and

[0012] an electrolysis cell electrically connected to the first DC link, optionally by means of a DC / DC power converter and a second DC link, and adapted to receive a load current.

[0013] The first method comprises:

[0014] maintaining a pre-event DC link voltage (e.g. a voltage in the first DC link or the second DC link, if present) and supplying a pre-event load current to the electrolysis cell during normal operation of the AC power network; and

[0015] in response to a detected low voltage event:

[0016] initially continuing to supply the pre-event load current to the electrolysis cell to maintain normal operation of the electrolysis cell,

[0017] monitoring the DC link voltage, and

[0018] reducing the load current supplied to the electrolysis cell, optionally by ramping the load current down at a suitable ramp rate, if the monitored DC link voltage falls below a first voltage threshold that is less than the pre-event DC link voltage.

[0019] The present invention also provides an electrolysis plant comprising:

[0020] a first DC link having at least one DC link capacitor;

[0021] An AC / DC power converter, comprising:

[0022] at least one AC terminal electrically connectable to an AC power network, and

[0023] at least two DC terminals electrically connected to the first DC link;

[0024] an electrolysis cell electrically connected to the first DC link, optionally by means of a DC / DC power converter and a second DC link, and adapted to receive a load current; and

[0025] a controller adapted to:

[0026] maintain a pre-event DC link voltage (e.g. a voltage in the first DC link or the second DC link, if present) and supply a pre-event load current to the electrolysis cell during normal operation of the AC power network; and

[0027] in response to a detected low voltage event:

[0028] initially continue to supply the pre-event load current to the electrolysis cell to maintain normal operation of the electrolysis cell,

[0029] monitor the DC link voltage, and

[0030] if the monitored DC link voltage falls below a first voltage threshold that is less than the pre-event DC link voltage, reduce the load current supplied to the electrolysis cell, optionally by ramping the load current down at a suitable ramp rate.

[0031] An electrolysis cell plant can comprise two or more electrolysis cells. The electrolysis cells can be electrically connected to the first DC link in parallel, optionally by means of respective DC / DC power converters and second DC links. In the latter case, it will be readily appreciated that each electrolysis cell will have its own second DC link - i.e. the electrolysis cell plant will have one first DC link but multiple second DC links. Each electrolysis cell is adapted to receive a load current.

[0032] Each optional DC / DC power converter can comprise at least two DC input terminals electrically connected to the first DC link, and at least two DC output terminals electrically connected to the respective electrolysis cell by a second DC link. Each DC / DC power converter can have any suitable topology. If the electrolysis cell plant comprises two or more electrolysis cells, the DC input terminals of the DC / DC power converters can be electrically connected to the first DC link in parallel. Each second DC link can comprise at least one DC link capacitor.

[0033] If each electrolyzer is directly electrically connected to the first DC link, the load current is the current in the first DC link. If the electrolyzer plant comprises two or more electrolyzers, they can be directly electrically connected in parallel to the first DC link. The monitored DC link voltage is the voltage in the first DC link.

[0034] If each electrolyzer is electrically connected to the first DC link by means of a respective DC / DC power converter, the load current is the current at the DC output terminals of each DC / DC power converter - i.e. the current in each second DC link. The monitored DC link voltage can be the voltage in the first DC link, or the voltage in one or more of the second DC links.

[0035] Electrically connecting each electrolyzer of the electrolyzer plant to the first DC link by means of a DC / DC power converter allows for greater flexibility in controlling the load current. For example, the load current supplied to each electrolyzer through each second DC link can be controlled by controlling the respective DC / DC power converter. If each electrolyzer is directly electrically connected to the first DC link, i.e. such that the load current is the current in the first DC link, the load current can be reduced by reducing the voltage in the first DC link. The voltage in the first DC link can be controlled by controlling the AC / DC power converter. But in some cases, e.g. due to the risk of high inflow current when a low voltage event ends, there can be a limit to the extent to which the voltage in the first DC link can be reduced. This in turn can limit the extent to which the DC link current can be reduced. In particular, in some cases, it can not be possible to reduce the DC link current to zero or substantially zero before the DC link voltage reaches a limit voltage threshold and operation of the electrolyzer(s) has to be stopped. When operation of the electrolyzer(s) is stopped, the electrolyzer plant shuts down and needs to be restarted after the low voltage event has ended. This means that if each electrolyzer is electrically connected to the first DC link by means of a respective DC / DC converter such that the load current in the respective second DC link can be reduced to zero or substantially zero, the electrolyzer plant can have to shut down faster than would otherwise be the case.

[0036] The AC / DC power converter can have any suitable topology. The AC / DC power converter can be capable of bidirectional power flow, such that it can operate as an active rectifier to supply power from the AC power network to the first DC link, and as an inverter to supply power from the first DC link to the AC power network. In another arrangement, the AC / DC power converter can only operate as an active rectifier to supply power from the AC power network to the first DC link.

[0037] The AC / DC power converter can comprise three AC terminals, which can be electrically connected to a three-phase AC power network by means of a transformer.

[0038] Each electrolyzer can comprise one or more stacks, each stack comprising one or more electrolyzer cells. Each electrolyzer cell can use any suitable electrolyzer technology, for example a proton exchange membrane or polymer electrolyte membrane (PEM) technology, wherein each electrolyzer cell comprises a polymer membrane. A DC voltage can be applied to each electrolyzer. Water reacts at an anode of each electrolyzer cell to produce oxygen gas and positively charged hydrogen ions (protons), which selectively move across the polymer membrane to a cathode. At the cathode of each electrolyzer cell, the hydrogen ions combine with electrons flowing through an external circuit to form hydrogen gas. Each electrolyzer can comprise two DC terminals. The DC terminals of each electrolyzer can be electrically connected to the first DC link or the DC output terminals of the respective DC / DC power converter by a respective second DC link.

[0039] During normal operation of the electrolyzer plant, energy is stored in each DC link capacitor. Each DC link capacitor can therefore be considered an energy storage device. Each DC link capacitor can supply that stored energy to the electrolyzer(s) at the start of a low voltage event. The amount of energy stored in the DC link capacitor(s) and any optional additional energy storage device(s) - see below - can be estimated or determined. The amount of stored energy will decrease during the low voltage event - i.e. when that stored energy is supplied to the electrolyzer(s). The amount of energy received from the AC power network during the low voltage event can be estimated or determined. The low voltage event reduces the network voltage, and therefore the amount of available energy that can be supplied from the AC power network during the low voltage event will decrease correspondingly. In other words, the amount of energy supplied from the AC power network to the power converter during the low voltage event will be lower than the amount of energy supplied before the start of the low voltage event. The reduction in the level of the network voltage can therefore also be considered in terms of the reduction in the amount of energy available for supply from the supply grid or microgrid, for example.

[0040] If the total amount of energy available for supply to the electrolyzer(s) during the low voltage event falls below a threshold value, the load current can start to decrease. This alternative method for controlling the load current can be used instead of monitoring the DC link voltage and starting to decrease the load current supplied to the electrolyzer(s) if the monitored DC link voltage falls below a first voltage threshold.

[0041] The first voltage threshold can be an absolute value, or it can be a certain percentage of the pre-event (or "nominal") DC link voltage. For example, the first voltage threshold can be in the range of about 80% to about 95% of the pre-event DC link voltage, and more preferably in the range of about 90% to about 95% of the pre-event DC link voltage. A typical electrolyzer can operate in the range from about 800 Vdc up to 1500 Vdc, depending on the load and electrolyzer type and design.

[0042] The electrolyzer plant can include an additional energy storage device, which is optionally electrically connected to the first DC link by means of a power converter. The additional energy storage device can be optionally electrically connected to the one or more second DC links, if present, by means of a power converter. The power converter can have any suitable topology (e.g., a DC / DC power converter).

[0043] The electrolyzer plant can include two or more additional energy storage devices. If the electrolyzer plant includes two or more additional energy storage devices, they are preferably controlled in a coordinated manner. For example, the additional energy storage devices can supply energy simultaneously or sequentially. Each energy storage device can be a battery, a supercapacitor, a flywheel, etc., and can be controlled to supply energy to the DC link during a low voltage event. When the electrolyzer plant is in normal operation, each energy storage device can be charged from the DC link.

[0044] Each additional energy storage device can be optionally electrically connected to the first DC link by means of a power converter. If the electrolyzer plant includes two or more electrolyzers, each electrolyzer is electrically connected to the first DC link by means of a DC / DC converter and a second DC link, then each additional energy storage device can be electrically connected in parallel to two or more of the second DC links - i.e., such that each additional energy storage device can supply energy to two or more of the electrolyzers simultaneously. In an alternative arrangement, each additional energy storage device can be electrically connected to only the respective second DC link - i.e., such that each additional energy storage device can supply energy to only one of the electrolyzers. If the two or more additional energy storage devices are controlled in a coordinated manner, they can be connected in parallel to a common power converter having any suitable topology (e.g., a DC / DC power converter). The common power converter can include at least two DC output terminals, which are electrically connected to the first DC link or to one or more of the second DC links, if present.

[0045] At the start of a low voltage event, energy will continue to be supplied to each electrolyzer. This energy will include the energy stored in the DC link capacitor(s), and can also include the energy stored in any additional energy storage device(s). If the network voltage drops to zero or substantially zero, no energy is supplied to the first DC link by the AC / DC power converter, and all of the energy required to operate the electrolyzer(s) must be supplied by the DC link capacitor(s) and any additional energy storage device(s). However, if the network voltage does not drop to zero or substantially zero, some energy will also be supplied to the first DC link by the AC / DC power converter, and can be supplied to the electrolyzer(s). For a first period of time after a low voltage event has been detected, the pre-event current can be substantially maintained, and the electrolyzer(s) can continue normal operation, and hydrogen and oxygen will continue to be generated at normal production rates.

[0046] In response to the detected low voltage event, the method can also include initiating the supply of energy from the additional energy storage device (or from two or more additional energy storage devices) to the first DC link (or to one or more second DC links) if the monitored DC link drops below a second voltage threshold that is less than the pre-event DC link voltage and greater than a first voltage threshold. Alternatively, the additional energy storage device(s) can initiate the supply of energy to the first DC link (or to one or more second DC links) upon detection of the low voltage event, i.e., without waiting for the monitored DC link voltage to drop below the second voltage threshold. In either case, the supply of energy from the additional energy storage device(s) is intended to substantially maintain the monitored DC link voltage at pre-event levels. Prior to initiating the supply of energy, each additional energy storage device and its optional power converter can first be activated, meaning that it is allowed to initiate the supply of energy if necessary (e.g., release a pulse without relying on the DC link voltage). Each additional energy storage device and its optional power converter can be activated when the low voltage event is detected, or at any time thereafter. Unless the low voltage event has a very short duration, or the reduction in network voltage is very small, the supply of energy from the additional energy storage device(s) will typically be the only way to avoid curtailment or stopping of operation of the electrolyzer(s). This is because the amount of energy stored in the DC link capacitor(s) will typically be quite small.

[0047] The second voltage threshold can be an absolute value, or it can be a certain percentage of the pre-event (or "nominal") DC link voltage. For example, the second voltage threshold can be in the range of about 95% to about 99% of the pre-event DC link voltage, and typically greater than the first voltage threshold. The rate of change of the DC link voltage can be monitored and used to detect or verify that a low voltage event has occurred. It can be possible to determine that the DC link voltage has dropped below the second voltage threshold for a period of time, for example 1 ms.

[0048] If the monitored DC link voltage drops below the first voltage threshold, the load current begins to decrease. For example, the load current can begin to ramp down at a suitable ramp rate. Alternatively, the load current supplied to the electrolyzer(s) can begin to decrease if the stored energy available for supply from the additional energy storage device(s) drops below a threshold. The load current can be decreased by controlling the AC / DC power converter and / or each optional DC / DC power converter electrically connected between the first DC link and the respective electrolyzer. Thus, when the load current begins to decrease, the above-mentioned first time period (i.e., during which the load current is substantially maintained at the pre-event level) will end. For the second time period, for example, during which the load current gradually decreases towards zero or substantially zero, the operation of the electrolyzer(s) is curtailed or constrained. During this second time period, the electrolyzer plant continues to operate, but generates hydrogen and oxygen at a gradually decreasing production rate.

[0049] Decreasing the load current can passively curtail the operation of the electrolyzer(s). There can be no need for an active control mechanism. In some cases, the load current will begin to decrease a short time after the low voltage event is detected - i.e., the above-mentioned first time period can be very short. This can depend, for example, on the reduction in network voltage during the low voltage event and the availability of stored energy.

[0050] If the low voltage event ends before the monitored DC link voltage drops below the first voltage threshold (or before the amount of energy available for supply to the one or more electrolyzers drops below a threshold), there is no need to curtail the operation of the one or more electrolyzers, and the load current remains substantially constant during the low voltage event - i.e., the load current is substantially maintained at the pre-event level. The DC link voltage can also remain substantially constant. When the low voltage event ends, any additional energy storage device(s) can be controlled to cease supplying energy. In this case, the above-mentioned first time period ends when the low voltage event ends and the second time period does not begin at all.

[0051] If the low voltage event ends after the load current has started to decrease but before the load current has reached zero or substantially zero, i.e. before the electrolysis plant has been shut down, the decreased load current can be increased back to the pre-event load current, i.e. to the level before the low voltage event was detected. For example, the load current can start to ramp up with a suitable ramp rate. The load current can be increased by controlling the AC / DC power converter electrically connected between the first DC link and the respective electrolysis cell and / or each optional DC / DC power converter. When the low voltage event ends, or when the pre-event load current is reached, i.e. after the decreased load current has been increased back to its level before the low voltage event was detected, any additional energy storage device(s) can be controlled to stop supplying energy. In this case, the above-mentioned second time period ends when the low voltage event ends, and the operation of the electrolysis cell(s) remains reduced or constrained for a third time period during which the load current is gradually increased back to the pre-event load current, for example. During the third time period, the electrolysis plant continues to operate and to generate hydrogen and oxygen at a gradually increasing production rate. The third time period ends when the load current reaches the pre-event load current and the electrolysis plant can operate normally again.

[0052] If the load current reaches zero or substantially zero, the operation of the electrolysis cell(s) is stopped. When the operation of the electrolysis cell(s) is stopped, the electrolysis plant is shut down and will need to be restarted when the low voltage event has ended. As mentioned above, in some cases the electrolysis plant can also need to be shut down before the load current reaches zero or substantially zero. In this case, the above-mentioned second time period ends when the operation of the electrolysis cell(s) is stopped, i.e. when the electrolysis plant is shut down.

[0053] The duration of the first time period can depend on the amount of energy stored in the DC link capacitor(s) and any additional energy storage device(s), and the amount of energy continuously supplied by the AC power network during the low voltage event. The duration of the second time period and the third time period can depend on the respective ramp rates used to decrease and increase the load current. A maximum duration can be determined and used to control the transition between different operating conditions of the electrolysis plant, for example when the operation of the electrolysis cell(s) has to be first reduced and then completely stopped.

[0054] The second method comprises:

[0055] supplying a pre-event load current to the electrolysis cell during normal operation of the AC power network; and

[0056] In response to the detected low voltage event, the load current supplied to the electrolyzer is immediately reduced to a minimum current value, optionally by ramping the load current down at a suitable ramp rate.

[0057] The minimum current value can correspond to a polarization level required to maintain the electrolyzer in a minimum hydrogen generation operation, e.g., in which hydrogen and oxygen are generated at a minimum production rate.

[0058] For example, the minimum current value can be in the range of about 1 to about 250 A.

[0059] The maximum ramp rate for reducing the load current can be in the range of 1 per unit (pu) current per 1 ms to 1 pu current per 10 ms, where 1 pu current represents the pre-event (or “nominal”) load current of the electrolyzer. For example, the pre-event load current can be in the range of 100 A to 10 kA.

[0060] When the low voltage event ends, the load current can be increased back to the pre-event load current, optionally by ramping the load current up at a suitable ramp rate as described above. If the load current cannot be maintained at the minimum current value, it can be ramped down to zero or substantially zero, and operation of the electrolyzer can be stopped. When the low voltage event ends, the electrolyzer plant can be shut down and will need to be restarted.

[0061] The present invention provides a set of technical solutions as follows:

[0062] Technical Solution 1. A method of providing low voltage ride through (LVRT) capability for an electrolyzer plant, the electrolyzer plant comprising:

[0063] a first DC link having at least one DC link capacitor;

[0064] an AC / DC power converter comprising:

[0065] at least one AC terminal electrically connectable to an AC power network, and

[0066] at least two DC terminals electrically connected to the first DC link; and

[0067] an electrolyzer electrically connected to the first DC link, optionally by means of a DC / DC power converter and a second DC link, and adapted to receive a load current;

[0068] The method comprises:

[0069] maintaining a pre-event DC link voltage and supplying a pre-event load current to the electrolyzer during normal operation of the AC power network; and

[0070] in response to a detected low voltage event:

[0071] continuing to supply the pre-event load current to the electrolysis cell to maintain normal operation of the electrolysis cell,

[0072] monitoring the DC link voltage, and

[0073] reducing the load current supplied to the electrolysis cell, optionally by ramping the load current down at a suitable ramp rate, if the monitored DC link voltage falls below a first voltage threshold that is less than the pre-event DC link voltage.

[0074] Solution 2. The method according to solution 1, wherein the monitored DC link voltage is the voltage in the first DC link.

[0075] Solution 3. The method according to solution 1, wherein the electrolysis cell is electrically connected to the first DC link by a DC / DC power converter and a second DC link, and wherein the load current is the current at the DC output terminals of the DC / DC power converter.

[0076] Solution 4. The method according to solution 3, wherein the monitored DC link voltage is the voltage in the second DC link.

[0077] Solution 5. The method according to solution 1, wherein the electrolysis cell plant comprises an additional energy storage device, which is electrically connected to the first DC link or the second DC link, optionally by means of a power converter, and wherein in response to the detected low voltage event, the method further comprises:

[0078] starting to supply energy from the additional energy storage device to the first DC link or the second DC link if the monitored DC link falls below a second voltage threshold that is less than the pre-event DC link voltage and greater than the first voltage threshold.

[0079] Solution 6. The method according to solution 5, wherein the load current supplied to the electrolysis cell is reduced if the stored energy available from the additional energy storage device falls below a threshold.

[0080] Solution 7. The method according to solution 5, wherein the additional energy storage device is controlled to stop supplying energy to the first DC link or the second DC link when the low voltage event ends.

[0081] Technical solution 8. The method according to technical solution 5, wherein, when the low voltage event ends, the load current is increased back to the pre-event load current, optionally by ramping the load current up with a suitable ramp rate.

[0082] Technical solution 9. The method according to technical solution 8, wherein, when the pre-event load current is reached, the additional energy storage device is controlled to stop supplying energy to the first DC link or the second DC link.

[0083] Technical solution 10. The method according to technical solution 1, wherein, if the load current falls below a current threshold, operation of the electrolyzer is stopped.

[0084] Technical solution 11. The method according to technical solution 10, wherein the current threshold is zero or substantially zero.

[0085] Technical solution 12. An electrolyzer plant, comprising:

[0086] a first DC link having at least one DC link capacitor;

[0087] an AC / DC power converter comprising:

[0088] at least one AC terminal electrically connectable to an AC power network, and

[0089] at least two DC terminals electrically connected to the first DC link;

[0090] an electrolyzer electrically connected to the first DC link, optionally by means of a DC / DC power converter and a second DC link, and adapted to receive a load current; and

[0091] a controller adapted to:

[0092] maintain a pre-event DC link voltage and supply a pre-event load current to the electrolyzer during normal operation of the AC power network; and

[0093] in response to a detected low voltage event:

[0094] initially continue to supply the pre-event load current to the electrolyzer to maintain normal operation of the electrolyzer,

[0095] monitor the DC link voltage, and

[0096] If the monitored DC link voltage drops below a first voltage threshold that is less than the pre-event DC link voltage, the load current supplied to the electrolyzer is optionally reduced by ramping the load current down at a suitable ramp rate.

[0097] Solution 13. The electrolyzer plant according to solution 12, wherein the electrolyzer is electrically connected to the first DC link by means of a DC / DC power converter and a second DC link.

[0098] Solution 14. The electrolyzer plant according to solution 12, further comprising two or more electrolyzers, wherein each electrolyzer is electrically connected to the first DC link in parallel, optionally by means of a respective DC / DC power converter and a second DC link.

[0099] Solution 15. The electrolyzer plant according to solution 12, further comprising one or more additional energy storage devices, wherein each additional energy storage device is electrically connected to the first DC link or to one or more second DC links, optionally by means of a power converter.

[0100] Solution 16. A method of providing low voltage ride through, LVRT, capability for an electrolyzer plant, the electrolyzer plant comprising:

[0101] a first DC link having at least one DC link capacitor;

[0102] an AC / DC power converter comprising:

[0103] at least one AC terminal electrically connectable to an AC power network, and

[0104] at least two DC terminals electrically connected to the first DC link; and

[0105] an electrolyzer electrically connected to the first DC link, optionally by means of a DC / DC power converter and a second DC link, and adapted to receive a load current;

[0106] the method comprising:

[0107] during normal operation of the AC power network, supplying a pre-event load current to the electrolyzer; and

[0108] in response to a detected low voltage event, immediately reducing the load current supplied to the electrolyzer to a minimum current value, optionally by ramping the load current down at a suitable ramp rate. BRIEF DESCRIPTION OF DRAWINGS

[0109] Figures 1 to 7is a schematic diagram of different potlines according to the present invention;

[0110] Figure 8 is a schematic diagram of a method according to the present invention providing low voltage ride through (LVRT) capability for a potline, wherein the load current is ramped down and ramped up back to pre-event level;

[0111] Figure 9 is a schematic diagram of a method according to the present invention providing low voltage ride through (LVRT) capability for a potline, wherein the load current is ramped down and ramped up back to pre-event level;

[0112] Figure 10 is a schematic diagram of a method according to the present invention providing low voltage ride through (LVRT) capability for a potline, wherein the load current is ramped down to zero and the potline is shut down;

[0113] Figure 11 is a schematic diagram showing the change of the DC link voltage; and

[0114] Figure 12 is a schematic diagram of an alternative method according to the present invention providing low voltage ride through (LVRT) capability for a potline, wherein the load current is immediately ramped down to a minimum current value. DETAILED DESCRIPTION

[0115] REFERENCE Figures 1 to 7 The potlines 1A, 1B,..., 1G are electrically connected to an AC power network 2 by transformers 4. The AC power network 2 can be, for example, a three-phase supply grid or a microgrid.

[0116] The potline 1A comprises an AC / DC power converter 6. The AC / DC power converter 6 comprises AC terminals electrically connected to the transformers 4 and DC terminals.

[0117] The potline 1A comprises a DC link 8 electrically connected to the DC terminals of the AC / DC power converter 6. The DC link 8 comprises at least one DC link capacitor C1.

[0118] The potline 1A comprises an electrolysis cell 10 adapted to receive a load current (e.g. the current in the DC link 8). As mentioned above, the electrolysis cell 10 can comprise one or more stacks, wherein each stack comprises one or more electrolysis cell units using any suitable electrolysis cell technology.

[0119] The electrolysis cell 10 has DC terminals electrically connected to the DC link 8.

[0120] The potline 1A comprises a controller 12. The controller 12 controls the operation of the AC / DC power converter 8.

[0121] The controller 12 receives measurements 14 of current and voltage at the AC terminals of the AC / DC power converter. The measurements can be provided by suitable sensors (not shown).

[0122] The controller 12 also receives measurements 16 of current and voltage at the DC link 8. The measurements can be provided by suitable sensors (not shown).

[0123] Figure 2 The electrolyser plant IB shown in Fig. 1 B comprises a plurality of electrolyser cells 10a, 10b,..., 10n adapted to receive a load current (e.g. the current in the DC link 8). As mentioned above, each electrolyser cell 10a, 10b,..., 10n can comprise one or more stacks, where each stack comprises one or more electrolyser units using any suitable electrolyser technology.

[0124] Each electrolyser cell 10a, 10b,..., 10n has DC terminals electrically connected to the DC link 8. As shown, the electrolyser cells 10a, 10b,..., 10n are electrically connected in parallel to the DC link 8.

[0125] In all other respects, the electrolyser plant IB is identical to the electrolyser plant IA shown in Fig. 1 A. Figure 1

[0126] Figure 3 The electrolyser plant IC shown in Fig. 1 C comprises an electrolyser cell 10 adapted to receive a load current (e.g. the current in the second DC link 18). As mentioned above, the electrolyser cell 10 can comprise one or more stacks, where each stack comprises one or more electrolyser units using any suitable electrolyser technology.

[0127] The electrolyser cell 10 has DC terminals electrically connected to the first DC link 8 by a DC / DC power converter 20. In particular, the DC / DC power converter 20 has a first DC terminal electrically connected to the first DC link 8 and a second DC terminal electrically connected to the second DC link 18. The second DC link 18 comprises at least one DC link capacitor C2.

[0128] The controller 12 also controls the DC / DC power converter 20.

[0129] The controller 12 receives measurements 22 of current and voltage at the second DC link 18. The measurements can be provided by suitable sensors (not shown).

[0130] In all other respects, the electrolyser plant IC is identical to the electrolyser plant IA shown in Fig. 1 A. Figure 1

[0131] Figure 4 ​​The electrolysis plant 1D shown in FIG. 1D includes a plurality of electrolysis cells 10a, 10b,..., 10n adapted to receive a load current (e.g., the current in the respective second DC link 18a, 18b,..., 18n). As described above, each electrolysis cell 10a, 10b,..., 10n can include one or more stacks, where each stack includes one or more electrolysis cell units using any suitable electrolysis cell technology.

[0132] Each electrolysis cell 10a, 10b,..., 10n has DC terminals that are electrically connected to the first DC link 8 through a respective DC / DC power converter 20a, 20b,..., 20n. In particular, each DC / DC power converter 20a, 20b,..., 20n has a first DC terminal electrically connected to the first DC link 8 and a second DC terminal electrically connected to the respective second DC link 18a, 18b,..., 18n. As shown, the DC / DC power converters 20a, 20b,..., 20n are electrically connected to the first DC link 8 in parallel.

[0133] The controller 22 controls the DC / DC power converters 20a, 20b,..., 20n.

[0134] The controller 12 receives measurements 22a, 22b,..., 22n of the current and voltage at each second DC link 18a, 18b,..., 18n.

[0135] In all other respects, the electrolysis plant 1D is the same as the electrolysis plant 1A shown in FIG. 1A. Figure 2

[0136] Figure 5 The electrolysis plant 1E shown in FIG. 1E includes an additional energy storage device 24. As described above, the additional energy storage device 24 can be a battery, a supercapacitor, a flywheel, etc., and can be controlled to supply energy to the DC link 8. When the electrolysis plant 1E is in normal operation, the energy storage device 24 can be charged from the DC link 8. A DC / DC power converter 26 has a first DC terminal electrically connected to the DC link 8 and a second DC terminal electrically connected to the energy storage device 24.

[0137] The controller 12 also controls the DC / DC power converter 26.

[0138] In all other respects, the electrolysis plant 1E is the same as the electrolysis plant 1A shown in FIG. 1A. Figure 1

[0139] Figure 6 ​​The electrolyzer plant 1F shown in FIG. 1 includes an additional energy storage device 24. As described above, the additional energy storage device 24 may be a battery, an ultracapacitor, a flywheel, etc., and may be controlled to supply energy to the first DC link 8. When the electrolyzer plant 1E is operating normally, the energy storage device 24 may be charged from the first DC link 8. (In an alternative arrangement, the energy storage device 24 may be controlled to supply energy to the second DC link 18.)

[0140] The DC / DC power converter 26 has a first DC terminal electrically connected to the first DC link 8 and a second DC terminal electrically connected to the energy storage device 24. (In an alternative arrangement, the first DC terminal of the DC / DC power converter 26 may be electrically connected to the second DC link 18.)

[0141] In all other respects, the electrolyser plant 1F is Figure 3 The electrolyzer plant shown in FIG1 is identical to that shown in FIG1C.

[0142] Figure 7 The electrolyser plant 1G shown in FIG includes an additional energy storage device 24. As described above, the additional energy storage device 24 may be a battery, a supercapacitor, a flywheel, etc., and may be controlled to supply energy to the first DC link 8. When the electrolyser plant 1E is in normal operation, the energy storage device 24 may be charged from the first DC link 8.

[0143] DC / DC power converter 26 has a first DC terminal electrically connected to first DC link 8 and a second DC terminal electrically connected to energy storage device 24. (In an alternative arrangement, the first DC terminal of DC / DC power converter 26 may be electrically connected to one or more of second DC links 18a, 18b, ..., 18n.)

[0144] In all other respects, the electrolyser plant 1G is Figure 4 The same as the electrolysis cell plant 1D shown in FIG.

[0145] The remainder of the description will focus on Figure 6 The method shown provides LVRT capability for electrolyzer plant 1F. However, it will be readily understood that corresponding methods can be applied to Figures 1 to 5 and Figure 7 The other electrolysis cell plants 1A, 1B, ..., 1E and 1G are shown in FIG.

[0146] During normal operation of the AC power network 2, the controller 12 will control the electrolyzer plant 1F to maintain the pre-event (or "nominal") DC link voltage. This can be the voltage in the first DC link 8 and / or the second DC link 18. The controller 12 will also control the electrolyzer plant 1F to supply the pre-event load current to the electrolyzer 10. The load current is the current in the second DC link 18.

[0147] In response to the detected low voltage event, the controller 12 will initially control the AC / DC power converter 6 and / or the DC / DC power converter 20 to continue to supply the pre-event load current to the electrolysis cell 10 to maintain normal operation of the electrolysis cell.

[0148] During the low voltage event, the DC link voltage is monitored, for example by at least one voltage sensor (not shown). The at least one voltage sensor can measure the voltage in the first DC link 8 and / or the second DC link voltage 18. If the monitored DC link voltage falls below a first voltage threshold that is less than the pre-event DC link voltage, the controller 12 will control the AC / DC power converter 6 and / or the DC / DC power converter 20 to reduce the load current supplied to the electrolysis cell 10, optionally by ramping the load current down at a suitable ramp rate. The first voltage threshold can be an absolute value, or it can be a certain percentage of the pre-event DC link voltage. For example, the first voltage threshold can be in the range of about 80% to about 95% of the pre-event DC link voltage, more preferably in the range of about 90% to about 95%.

[0149] At the start of the low voltage event, energy will continue to be supplied to each electrolysis cell 10. This energy will include the energy stored in the DC link capacitors CI, C2, and can also include energy stored in the additional energy storage device 24 - see below. If the network voltage falls to zero or substantially zero, no energy is supplied to the first DC link 8 by the AC / DC power converter 6, and all of the energy required to operate the electrolysis cell 10 must be supplied by the DC link capacitors CI, C2 and the additional energy storage device 24. However, if the network voltage does not fall to zero or substantially zero, some energy will also be supplied to the first DC link 8 by the AC / DC power converter 6, and can be supplied to the electrolysis cell 10. For a period of time after the low voltage event has been detected, the pre-event current can be substantially maintained, and the electrolysis cell 10 can continue to operate normally, and hydrogen and oxygen will continue to be generated at the normal production rate.

[0150] In response to the detected low voltage event, if the monitored DC link falls below a second voltage threshold that is less than the pre-event DC link voltage and greater than the first voltage threshold, energy can be supplied to the first DC link 6 from the additional energy storage device 24. The second voltage threshold can be an absolute value, or it can be a certain percentage of the pre-event (or "nominal") DC link voltage. For example, the second voltage threshold can be in the range of about 95% to about 99% of the pre-event DC link voltage, and greater than the first voltage threshold.

[0151] Alternatively, upon detection of a low voltage event, the additional energy storage device 24 can start supplying energy to the first DC link 8, i.e. without waiting for the monitored DC link voltage to drop below the second voltage threshold. The controller 12 will control the DC / DC power converter 26 to control the amount of energy supplied by the additional energy storage device 24 to the first DC link 8. The DC / DC power converter 26 can be controlled by the controller 12 so that the DC link voltage does not increase above its pre-event level.

[0152] If the monitored DC link voltage drops below the first voltage threshold, the load current starts to decrease. For example, the load current is ramped down with a suitable ramp rate. The controller 12 reduces the load current by controlling the DC / DC power converter 20 and / or the AC / DC power converter 6 electrically connected between the first DC link 8 and the second DC link 18. Thus, when the load current starts to decrease, the first time period will end during which the load current is substantially maintained at the pre-event level. For a second time period, e.g. during which the load current is gradually decreased towards zero or substantially zero, the operation of the electrolyzer 10 is curtailed or constrained. During this second time period, the electrolyzer plant 1 F continues to operate but generates hydrogen and oxygen at a gradually decreasing production rate.

[0153] Reducing the load current will passively curtail the operation of the electrolyzer 10. There is no need for an active control mechanism.

[0154] If the low voltage event ends before the monitored DC link voltage drops below the first voltage threshold, there is no need to curtail the operation of the electrolyzer 10 and the load current is substantially maintained constant, i.e. the load current is substantially maintained at the pre-event level during the low voltage event. The DC link voltage can also be substantially maintained constant. This is illustrated in Figure 8 the example. In this example, the low voltage event starts at time t0 and ends at time ti. More particularly, it can be seen that at time t0, the network voltage VAC drops to zero and returns to its pre-event level at time ti. During a first time period (labeled “T1”), the load current I L is substantially maintained constant at its pre-event level. When the low voltage event ends, i.e. at time ti, the energy storage device 24 can be controlled to stop supplying energy. As a non-limiting example, for a particular low voltage event, the first time period (labeled “T1”) can be in the range of about 1 ms to about 20 ms, depending on the pre-event current level and without any additional energy storage. If the additional energy storage device 24 is used, the first time period can be in the range of about 1 ms to about 250 ms or even longer, e.g. up to about 500 ms.

[0155] If the low voltage event ends after the load current has started to decrease but before the load current has reached or substantially reached zero, i.e. before the electrolysis cell plant 1F has been shut down, the decreasing load current can increase back to the load current before the event, i.e. back to the level before the low voltage event was detected. For example, the load current can start to ramp up with a suitable ramp rate. The controller 12 increases the load current by controlling the DC / DC power converter 20 and / or the AC / DC power converter 6 electrically connected between the first DC link 6 and the second DC link 18. This is illustrated in Figure 9 . In this example, the low voltage event starts at time t0 and ends at time t2. More particularly, it can be seen that at time t0, the network voltage V AC drops to zero and returns to its pre-event level at time t2. During a first time period (labelled “T1”), the load current I L substantially remains constant at its pre-event level. At time t1, the monitored DC link voltage drops below the first voltage threshold and, during a second time period (labelled “T2”), the load current I L ramps down. When the low voltage event ends at time t2, the load current I L has not reached or substantially reached zero and, during a third time period (labelled “T3”), the load current I L ramps up until it reaches its pre-event level at time t3. When the low voltage event ends, i.e. at time t2, or when the load current I L has reached its pre-event level, i.e. at time t3, the energy storage device 24 can be controlled to stop supplying energy. As a non-limiting example, for a particular low voltage event, the first time period (labelled “T1”) can be in the range of about 1 ms to about 20 ms, the second time period (labelled “T2”) can be in the range of about 1 ms to about 250 ms, and the third time period (labelled “T3”) can be in the range of about 1 ms to about 1 s.

[0156] During the second and third time periods, the electrolysis cell plant 1F continues to operate and generates hydrogen and oxygen at a reduced production rate.

[0157] If the load current reaches zero or substantially zero, the operation of the electrolysis cell 10 is stopped. When the operation of the electrolysis cell 10 is stopped, the electrolysis cell plant 1F is shut down and will need to be restarted when the low voltage event has ended. This is illustrated in Figure 10 . In this example, the low voltage event starts at time t0 and ends at time t3. More particularly, it can be seen that at time t0, the network voltage V AC drops to zero and returns to its pre-event level at time t3. During a first time period (labelled “T1”), the load current IL At time t1, the monitored DC link voltage drops below the first voltage threshold, and during a second time period (labeled "t2"), the load current I L The voltage of the electrolyzer 10 is ramped down until it reaches zero at time t2. At time t2, the electrolyzer 10 is stopped and the electrolyzer plant 1F is shut down. When the operation of the electrolyzer 10 is stopped, that is, at time t2, the energy storage device 24 can be controlled to stop supplying energy. As a non-limiting example, for a specific low voltage event, the first time period (labeled as "T1") can be in the range of about 1 ms to about 20 ms, and the second time period (labeled as "T2") can be in the range of about 1 ms to about 500 ms.

[0158] Suitable ramp rates over the second time period may be in the range of 1 pu of current per 1 ms to 1 pu of current per 500 ms, where 1 pu of current represents the pre-event (or "nominal") load current of one electrolyser of the electrolyser plant and may be in the range of 100 A to 10 kA, for example.

[0159] Figures 8 to 10 The duration of the first time period (labeled "T1") in may depend on the amount of energy stored in the DC link capacitors C1, C2 and the additional energy storage device 24. It may also depend on the amount of energy that continues to be supplied by the AC power network 2 during the low voltage event. Figures 8 to 10 In the example shown in , the network voltage drops to zero and therefore no energy is available to be supplied to the first DC link 8 via the AC / DC power converter 6. The duration of the second and third time periods (labeled "T2" and "T3") may depend on the respective ramp rates for reducing and increasing the load current. A maximum duration may be determined and used to control transitions between different operating conditions of the electrolyzer plant 1F, for example, when operation of the electrolyzer 10 must first be reduced and then completely stopped.

[0160] Figure 11 The first and second voltage thresholds (respectively labeled “V DC1 ” and “V DC2 ”). When the low voltage event starts at time t0, the DC link voltage V DC When the DC link voltage V DC At time t a Reaching the second threshold V DC2 , the additional energy storage device 24 will start to supply energy to the first DC link 8. This supports the DC link voltage, and the load current can be kept substantially at the pre-event level until the DC link voltage V DC starts to decrease gradually and at time t bReaching the first threshold V DC1 At time t b , the load current will begin to ramp down as described above.

[0161] exist Figure 12 In the alternative method shown in FIG, as soon as a low voltage event is detected (ie, at time t0), the pre-event load current for the electrolyzer 10 is ramped down to the minimum current value I required to maintain electrolyzer operation. LMIN . Minimum current value I LMIN It may correspond to the polarization level required to keep the electrolyzer 10 operating with minimal hydrogen generation.

[0162] For example, the minimum current value I LMIN It can be in the range of about 1 to about 250A.

[0163] The maximum ramp rate for the load current may be in the range of 1 pu current per 1 ms to 1 pu current per 10 ms, where 1 pu current represents the pre-event (or "nominal") load current of the electrolyser 10 and may be in the range of 100 A to 10 kA, for example.

Claims

1. A method of providing low voltage ride through, LVRT, capability for an electrolyzer plant, the electrolyzer plant comprising: a first DC link having at least one DC link capacitor; an AC / DC power converter comprising: at least one AC terminal electrically connectable to an AC power network, and at least two DC terminals electrically connected to the first DC link; and an electrolyzer electrically connected to the first DC link, optionally by means of a DC / DC power converter and a second DC link, and adapted to receive a load current; the method comprising: maintaining a pre-event DC link voltage and supplying a pre-event load current to the electrolyzer during normal operation of the AC power network; and in response to a detected low voltage event: initially continuing to supply the pre-event load current to the electrolyzer to maintain normal operation of the electrolyzer, monitoring the DC link voltage, and reducing the load current supplied to the electrolyzer, optionally by ramping the load current down at a suitable ramp rate, if the monitored DC link voltage falls below a first voltage threshold that is less than the pre-event DC link voltage.

2. The method of claim 1, wherein, The monitored DC link voltage is the voltage in the first DC link.

3. The method of claim 1, wherein, The electrolyzer is electrically connected to the first DC link by means of a DC / DC power converter and a second DC link, and wherein the load current is the current at a DC output terminal of the DC / DC power converter.

4. The method of claim 3, wherein, The monitored DC link voltage is the voltage in the second DC link.

5. The method of claim 1, wherein, The electrolyzer plant comprises an additional energy storage device electrically connected to the first DC link or the second DC link, optionally by means of a power converter, and wherein in response to the detected low voltage event, the method further comprises: starting to supply energy from the additional energy storage device to the first DC link or the second DC link if the monitored DC link falls below a second voltage threshold that is less than the pre-event DC link voltage and greater than the first voltage threshold.

6. The method of claim 5, wherein, reducing the load current supplied to the electrolyzer if the stored energy available from the additional energy storage device falls below a threshold value.

7. The method of claim 5, wherein, controlling the additional energy storage device to stop supplying energy to the first DC link or the second DC link when the low voltage event ends.

8. The method of claim 5, wherein, increasing the load current back to the pre-event load current, optionally by ramping the load current up at a suitable ramp rate, when the low voltage event ends.

9. The method of claim 8, wherein, controlling the additional energy storage device to stop supplying energy to the first DC link or the second DC link when the pre-event load current is reached.

10. The method of claim 1, wherein, stopping operation of the electrolyzer if the load current falls below a current threshold.

Images