The present invention relates to a method for oper ating a grid forming power supply plant based on both a renewable energy, such as based on wind energy, solar en ergy, hydro energy, wave energy, and a carbon based ener gy, such as carbon based fuel. Furthermore, the present invention relates to a plant for operating a grid forming power supply plant based on both a renewable energy, such as based on wind energy, solar energy, hydro energy, wave energy, and a carbon based energy, such as carbon based fuel .

It is known to provide grid forming power plants for providing power to a grid, such as an island grid. Such plants originally operated on fossil fuels such as diesel. Previously, it has been conceived to combine the grid forming capacity of such a generator set with added power from a renewable energy source, such as wind energy. An advantage thereof is that carbon emissions are reduced during availability of such renewable energy source.

However, next to such advantages, it is the object of the present inventor to improve a relation between fos sil fuel usage and degree of stability of the grid, such as determined by grid parameters such as a voltage and frequency. Instances of sudden demand increases or de creases, as well as instances of variations of renewable power availability leads to instabilities in parameters of the grid.

In order to provide such an improvement, the present invention therefore provides a method for operating a grid forming power supply plant based on both a renewable ener gy, such as based on wind energy, solar energy, hydro en ergy, wave energy, and a carbon based energy, such as car- bon based fuel, the grid forming power supply system com prising :

- a power input connection from a renewable power supply system,

- a power input connection from a carbon fuel engine based generator set, the generator set comprising:

- the engine for converting the carbon-based en ergy into motion energy,

- a generator, such as an alternator, for con verting the motion energy into electrical energy,

- and a clutch for coupling and uncoupling of the engine with the generator,

- a power buffer, such as comprising a battery, sub system for providing short term grid forming capacity,

- a plant grid forming controller for controlling grid parameters by means of controlling steps of the meth od,

- the method comprising steps of:

- controlling the grid parameters to remain within predetermined limits based on availability of power from the renewable power supply system, the generator set and the battery,

- uncoupling of the engine with the clutch in case of sufficient power capability by the renewable power sup ply system,

- power driving the generator such that it rotates and functions as an electromotor by providing it with electrical energy during at least part of the time the generator is uncoupled from the engine.

Important advantage is provided by the combination of the power buffer with the capability that the generator remains connected to the grid when and clutched from the engine. This achieves that both the positive effect of the generator relative to the grid due to its reactive power related properties and the power delivery capabilities of the power buffer, preferably with an grid forming inverter incorporated in the power buffer, further preferably with an incorporated battery.

An advantage of a plant according to the present in vention is that because the generator is arrangeable to remain coupled to the grid while being decoupled from the engine, the generator is arrangeable to function as a pow er buffer relative to variations in the grid in that state of being decoupled from the engine and coupled to the grid. Thereby, unusable power components, such as a reac tive load, may still be produced by the generator as it is connected to the grid, thereby providing assistance to the grid forming capacity of the plant.

Also, as the generator is kept connected to the grid, the generator is controllable to take up energy from the grid or the renewable energy supply system, and there by controllable to absorb peaks in power supplied or peaks in power created in the grid due to sudden demand changes, such as decreases and increases. Controlling this is pref erably performed by controlling the amount of power sup plied to the generator while functioning as an electromo tor .

Furthermore, because the generator is kept into ro tation based on energy from the renewable energy supply system, which is then excess energy not required by the grid, a rise in demand of the grid may be provided to the grid by momentarily lowering the power towards in the gen erator. As such, the generator is used as a buffer for power. This energy does not need to be provided by the power buffer, such as comprising a battery, therefore low ering the strain on such power buffer. It is emphasized that such resulting relatively low strain on a battery has a high effect on the durability and life expectancy of such a power buffer. A lower strain means that lower tem perature variations occur, and that depletion of the al ternative power source is prevented more often. In case of such battery based embodiments of the power buffer, as further detailed below, a battery life expectancy of five to six years instead of one to two years is expected to be advantageously achievable with the present invention and/or preferred embodiments.

Furthermore, due to provided controlling of the gen erator into rotation, a startup procedure of the engine of the carbon fuel based engine based generator set is be performed while using the motion energy of the generator as being in motion to overcome part of the inertia of the engine as well as not requiring to overcome the inertia of the generator at the time of starting up the engine. This provides very quick startup time of the combination gener ator and engine. As power becomes available from the gen erator set in this quick fashion according to preferred embodiments of the present invention, a discharge load or strain on the, preferably battery based, power buffer is effected to be significantly shorter or significantly smaller or a combination thereof then would have been pos sible without such power driving of the generator into ro tation. A quick starting up of the engine according to preferred embodiments provides the advantage of limited thing the risks of fails during attempts to synchronize the generator with the grid. Also variable losses of time during synchronization are limited or eliminated. The electrical grid parameters are as such controlled to im prove stabilization thereof and increasing power quality levels . Further preferably, steps are provided to start ro tation of the generator at moments that are determined to be moments that either an increase in greater demand, or a decrease in power supply from the renewable power supply system, or a combination thereof, is expected within a predetermined time frame. This provides the advantage that provisional startups of the carbon fuel engine that would be required for providing predetermined grid parameter quality, can be omitted while maintaining grid quality by being able to start up the engine while using motion ener gy of the generator before, at the time of, or after cou pling of the engine with the generator by means of the clutch. So not only the strain on the power buffer is low ered, but also the strain on the engine due to preventing precautionary startups, is lowered as well as accompanying fuel usage.

A further advantage of the present invention is that both the charging loads and the and charging loads, such as charging peak loads and in charging peak loads are sub sequently lowered relative to the prior art.

According to a first preferred embodiment, the meth od according to the present invention the steps of power driving the generator comprise steps of power driving the generator for providing motion energy to the engine during coupling of the engine with the generator. A preferable effect thereof is that the strain on the power buffer due to slow engine startups is limited and/or and that fuel consumption by the engine is lowered because of later and/or less startups.

Further preferably, the steps of power driving the generator comprise steps of providing electrical energy originating from the renewable power supply system towards the generator. As indicated, this provides the advantage of keeping the generator in motion on excess energy from the renewable power supply system while for this purpose and no fossil fuel or carbon based fuel is used. Also as indicated, this provides a lower strain on the power buff er .

According to a further preferred embodiment, the generator is controlled to buffer energy, due to varia tions originating in the grid, from the grid. This is ad vantageous as, in an island grid towards which the inven tion is oftentimes directed, it is quite common to have heavy equipment that have occurrences of producing rela tively high levels of reactive power.

According to a further preferred embodiment, the method comprises steps of:

- monitoring at least one predetermined battery pa rameter of the power buffer when comprising a battery, such as charge level, and/or

- controlling at least one parameter such that or with the aim of keeping the at least one predetermined battery parameter within certain ranges, such as certain ranges between a minimum and maximum value thereof of the battery capabilities. This preferred embodiment provides advantages specifically directed at limiting strain on a battery as comprised in a power buffer by the actual moni toring of the at least one predetermined battery parame ter. Parameters to be comprised in the at least one prede termined battery parameter comprise level of charge, rate of discharge speed, rate of charge speed, wherein the lev el of charge is preferably maintained between 40 and 60% of maximum charge level in order to be able to switch be tween charging and discharging depending on parameters of the grid, generator with coupled engine or with uncoupled engine, and or parameters of the renewable power supply system. An advantage of such controlling of the at least one parameter is that with a relatively small battery grid power quality can be maintained while variations occur within the grid, renewable energy supply system and while being able to maintain the engine in a stopped state as much as possible or to limit the number of starts of the engine or precautionary starts of the engine that would be required without such feature.

According to a further preferred embodiment, the steps of controlling of the grid parameters comprise steps of limiting the rate of discharge of the batteries, pref erably by respective raising or lowering of the amount of power supplied to the generator. The inventor has recog nized a buffering capability of the generator by both low ering the amount of power provided to the generator and increasing the amount of power provided to the generator as well as using a reactive power capability of the gener ator, preferably while connected to the grid. As such, this buffering capability of the generator is applied within the scope of the present invention and/or embodi ments thereof to prevent high levels of strain on the bat tery both charging and discharging. In order to use this buffering capability towards raising the amount of power supplied to the generator, steps of controlling comprise steps of keeping the power towards the generator below a certain maximum threshold that is below a predefined maxi mum uptake capacity of the generator. In order to use this buffering capability towards lowering the amount of power supplied to the generator, steps of controlling comprise steps of keeping the power towards the generator above a certain minimum threshold that is above 0 or above a mini mal predetermined level, such as a minimal predetermined level to keep the generator into rotation. According to a further preferred embodiment, the method comprises steps of controlling the generator to ab sorb and/or produce reactive power, preferably contrib uting to the controlling of the grid parameters, also while the generator is uncoupled from the engine. An ad vantage thereof is that effects of the grid can be allevi ated by keeping the generator couples to the grid in a ro tary fashion. Preferably, the amount of electrical power directed to the generator is a parameter of such control.

Further preferably, steps of controlling the power buffer to charge on power from the renewable power supply system that is in excess of power from the renewable power supply system momentarily required for controlling the grid parameters are provided to favorably charge the bat tery and a carbon emission free manner.

In case of for instance relatively abrupt lowering of the amount of power available from the renewable power supply system, I preferred embodiment provides controlling steps for controlling the grid parameters comprising steps of controlling the power buffer to discharge power in or der to maintain the grid parameters within the predeter mined limits. As such, advantageously uncontrollable vari ations in the availability of renewable energy from the renewable power supply system can be buffered by the power buffer in order to prevent a requirement to start up the engine, either in reaction to a shortage or as a predeter mined preemptive start up.

During such state of providing power towards the grid from the power buffer in order to maintain the grid parameters the method preferably comprises steps of having a power buffer control controlling the grid parameters during a situation of power loss from the renewable power supply system and/or a time period of starting up the en- gine or coupling the energy with the generator by the clutch. This advantageously allows to optimize the func tioning of the power buffer towards maintaining the param eters of the grid within predetermined limits. All accord ing to one or more of the preceding claims comprising steps of controlling the power buffer by preventing reac tive power influences to and from the power buffer.

According to a further preferred embodiment, the power buffer is controlled to only providing active power in controlling grid parameters, preferably in combination with the generator controlled to be handling reactive pow er demands from the grid. An advantage thereof is that a reactive strain on the power buffer is limited.

Further preferred steps according to the present in vention provides steps of monitoring a temperature level of batteries of the power buffer and controlling the use of the batteries of the power buffer to remain within pre determined temperature parameters. Such embodiment is pre ferred in light of the advantage that limiting excess tem peratures of batteries highly curtails the degenerative effect of such high temperatures to the batteries. Further preferably steps according to the present invention pro vides steps of monitoring a battery current level of bat teries of the power buffer and controlling the use of the batteries of the power buffer to remain within predeter mined battery current levels. Such embodiment is preferred in light of the advantage that limiting the current in a battery highly curtails the degenerative effect of such high temperatures to the batteries.

According to a further preferred embodiment, the method comprises steps to keep a battery level in a prede termined buffer range, such as between 40 and 60%. Other preferred ranges considered are between 30 and 70%, be- tween 30 and 50%, between 50 and 70%, between 20 and 80%. In case of e.g. variable winds, in which relatively more when the drops and then certain when prices are expected, it may be preferable to start the foreseen period of such went parameters with a relatively high charge of 8% in or der to allow the power buffer to buffer more of such sud den drops in that time.

According to a further preferred embodiment, the method comprises steps of increasing power from the renew able power supply system towards the grid in case battery parameters indicate battery operation above predetermined limits. This is for instance achieved by limiting the amount of power towards the rotating generator as a manner of controlling variations in grid parameters. This is pos sible as the generator is kept into rotation by excess power from the renewable power supply system.

According to a further preferred embodiment, the method comprises steps of decreasing power from the renew able power supply system in case battery parameters indi cate battery operation below predetermined limits. In case for instance the battery charge rises above an upper lim it, the renewable power supply system is controlled to limit its output, such as used for charging the battery.

An advantage thereof is that the battery will be able to absorb excess energy in the system as a whole, thereby ad justing temporary rising grid parameters values by charg ing the battery.

Further preferably, the method comprises steps of keeping track of expected power availability from the re newable power supply system for a pre-determined period of time to come. This provides the advantage of, 40 predeter mined time period, predicting of grid power requirements variations in relation to the available amount of renewa- ble power and or battery power. Therefore, battery-powered can be preemptively raised or lowered to meet demands caused by variations in renewable power availability as predicted. This provides the ability to prevent either preemptive engine starts or to prevent engine starts for providing power for moments that battery reserves would be too limited to cover the demands to maintain grid parame ters during such variations.

A further aspect according to the present invention provides a plant for operating a grid forming power supply plant based on both a renewable energy, such as based on wind energy, solar energy, hydro energy, wave energy, and a carbon based energy, such as carbon based fuel, the grid forming power supply system comprising:

- a power input connection from an renewable power supply system,

- a power input connection from an carbon fuel en gine based generator set, the generator set comprising:

- the engine for converting the carbon-based en ergy into motion energy,

- a generator, such as an alternator, for con verting the motion energy into electrical energy,

- and a clutch for coupling and uncoupling of the engine with the generator,

- a power buffer, such as comprising a battery, sub system for providing short term grid forming capacity,

- a plant grid forming controller for controlling grid parameters by means of controlling steps of a method according to one or more of the preceding claims, the plant grid forming controller comprising:

- interaction means for interacting with a control unit of the renewable power supply system, - interaction means for interacting with a power buffer control unit,

- interaction means for interaction with a control unit of the generator set.

Advantages of this aspect of the present invention have been described in the above in relation to the above aspect of the present invention. Further details of the present aspects will be set out below in relation to the attached drawings .

According to a preferred embodiment, a plant accord ing to the present aspect of the invention comprises a grid tied inverter in relation to the renewable power sup ply system when the renewable energy is solar energy and in which the plant comprises a grid forming inverter in relation to the renewable power supply system when the re newable energy is wind energy.

According to a further preferred embodiment, the clutch comprises a magnetic clutch arrangement in case the engine is a high-speed engine and in which the clutch com prises a standard clutch arrangement, such as with a clutch disc of pressure plate, in case the engine is a me dium speed engine. In further preferred embodiments, en gine is a high-speed diesel engine, a medium speed diesel engine, a gas engine or a hydrogen engine.

Further preferably, the plant comprises a dumpload system for absorbing electrical energy in case of an elec trical energy surplus, either caused in the grid or cost in the renewable power supply system or carbon fuel engine based generator set. Such dumpload system is preferably comprising resistors for converting electrical energy into heat. Such dumpload system is capable of absorbing elec trical energy even faster than batteries in case of very sudden variances in the energy supply, preferably the re- newable energy supply. In case of a sudden surge in for example solar energy or wind energy, such a dumpload sys tem is arranged to very quickly absorb the surplus energy. In case such a dumpload system is kept absorbing electri cal energy for a duration of time, switching down, or off, of the dumpload system provides very quick way of raising the energy supply towards the grid. Such control strategy is usable in case of a renewable energy surplus for at least part of said duration of time.

According to a further preferred embodiment, the plant comprises a generator set that is arranged to oper ate in low load operation states in which the engine oper ates below 30 % of nominal capacity up to -10% of nominal capacity .

Further advantages, features and details of the pre sent invention will be further elucidated on the basis of a description of one or more embodiments with reference to the accompanying figures.

Fig. 1 relates to a first preferred embodiment of a plant 1 according to the present invention, comprising both required parts and optional parts, the required parts only being limited to parts as identified in the respec tive dependent and independent claims. A busbar 2 is pro vided to link the plant with a power grid, preferably an island grid that is preferably solely provided with elec trical power from this power plant.

The plant 1 comprises a plant controller 3, a, pref erably battery based, power buffer subsystem 20, generator set subsystem 30, as well as an optional dumpload system 50. The plant controller is provided with an interface controller 4 for controlling a power supply towards the busbar from a renewable power supply system comprising ei ther a wind turbine 11 or a solar energy collector 11' . The interface controller for is intended to be the outer limit of the power plant, being arranged to cooperate with controlling the devices of the renewable energy supply system, such as wind turbine controller 15 and solar con troller 15' . The busbar 2 is connected to the grid line 8 via a switch 8' . Inrush currents from starting up off electrical drives in the grid are alleviated by the rotat ing alternator connected to the busbar. With embodiments of the present invention, the electrical grid is run in a stable manner and power quality is kept at a high level as opposed to the prior art. Shorts circuitry currents are taken up during grid faults. A Scada system 5 is used for monitoring and storing of operational data relating to the renewable energy sources and used as an input for creating control signals by the plant controller 3.

Outside the plant according to the direct scope of the present invention, but herein added for clarity, the wind turbine 11 is provided with a controlling device for controlling the power parameters of the energy provided by the wind turbine 11 towards the power line 16 connected with the busbar 2 via a transformer 14, a multi-function measuring device 19 and a switch 18. Similarly, the solar collector 11' is connected with the power line 16 by means of a DC / AC converter 12', preferably also comprising a controller 15' in communication with the interface con troller for of the plant 1. Optionally, outgoing trans formers are used with respect to the wind generator and the solar collector towards power line 16 in case design parameters, such as the length of the power line 16, so indicate in line with the understanding of the skilled person .

The power buffer subsystem 20 comprises of the com bination of an energy store for storing electrical energy, such as a battery 22 consisting of battery cells 22 . A power buffer subsystem controller 21 is provided to con trol the power buffer subsystem from the plant controller 3 as well as to provide autonomous functions to the power buffer subsystem 20. The power buffer subsystem 20 also comprises an inverter 23 that is preferably a grid forming inverter able to control grid parameters, also if the pow er buffer subsystem is the only subsystem providing power to the grid via the busbar. The extent to which the power buffer subsystem solely controls the grid parameters and provides power to the grid, is determined by the plant controller 3.

Preferably, the grid forming inverter 23 is arranged to control the grid parameters as long as the grid forming inverter 23 is coupled to the busbar by means of switch 28. An advantage of such an arrangement is that the power buffer subsystem 20 is constantly on standby to provide power in case of variations in supplied power towards the grid, mostly caused by variations in winter or sunlight, or variations in consumed energy in the grid, mostly caused by switching on or switching off of large consumers in the grid. Especially advantageous in this is the coop eration or co-functioning of both the power buffer subsys tem with its grid forming inverter and the generator set, both with the alternator alone and the alternator clutched in which the engine. The specific advantage of this combi nation is that the alternator constantly provides its re active energy producing capability along with the power buffer battery capacity. This means that the strain on the batteries, both charging and discharging, as indicated elsewhere in this description, is advantageously kept low you to this capability of the alternator when co-connected with the power buffer to the busbar. Furthermore, this means that a relatively small bat tery can be used, preferably so arranged that, besides maintenance cycles of fully charging and discharging bat teries, the batteries are kept at levels such that suffi cient capacity to charge and discharge depending on varia tions in the grid exists, as also indicated in other pas sages of this description. Furthermore, by lowering the strain on the batteries, such as by lowering the number of situations in which a quick charge or discharge occurs, situations in which the temperatures of the batteries rise out of necessity occur less frequently or are prevented.

Keeping occurrences of high temperatures in the bat teries low by keeping strain law is also achieved by the arrangement of providing the possibility of quick or al most instant engine startups because the alternator is kept in rotation and situations wherein the engine is ful ly stopped. Although keeping the alternator in rotation costs electrical energy, this electrical energy is taken from a surplus of renewable energy beyond the momentary demand by the grid. These advantages are to be seen in combination with lowered carbon fuel usage due to the ca pability of the engine to be kept in the off state due to the buffering capacity of the rotating alternator because less provisional engine starts have to be performed, such as also disclosed in the above in this description. A sim ilar use of renewable energy exists when the engine is running below 30% of nominal capacity, as disclosed below, which provides the advantage of the engine remaining in a mere immediate power-up state preferably at the expense of a small amount of excess renewable energy.

The generator set subsystem 30 as included in this preferred embodiment according to the present invention generally comprises a generator embodied as an alternator 32 and an engine 34, which alternator 32 and engine 34 are connected by means of a drive shaft comprising a clutch 35 for coupling and decoupling of the alternator relative to the engine. The drive shaft is provided for both driving of the alternator by the engine and driving the engine by the alternator, either based on kinetic energy present in the alternator or by the alternator being driven by elec trical energy driving the alternator into motion.

The engine is adapted to run in several modes, com prising a normal mode, in which the engine is operated on carbon based fuel, preferably fossil fuel further prefera bly diesel fuel, in which in normal mode the engine runs on 30% to a 100% of nominal engine capacity. In this range, a regular diesel engine is able to regularly func tion under load providing motion energy towards the con sumer, in this case the alternator.

The engine of the embodiment is adapted to also function in a low load mode. This low load mode is a mode in which the engine load achieved based on fuel supply is between 0 and 30%. An advantage of this mode is that the engine uses a relative lead low amount of fuel while still running and being capable of powering up to mode one very quickly based on this state, such as by means of increas ing fuel supply towards the engine.

A further mode for operating the engine is reverse mode in which no fuel or almost no fuel is provided to wards the engine and the engine is kept in motion by the alternator, the alternator being provided with electrical energy to function as a motor. Also in this mode, the en gine is capable of being powered up towards normal mode as the engine is kept into motion as well as the alternator. Because of this kinetic energy, a powering up of the en- gine may be performed without having to overcome the iner tia of an engine and or alternator at standstill.

A further mode for operating the engine is in a stopped mode. In stopped mode, the engine is preferably clutched out from the alternator and allowed to come to a standstill. A restart of the engine from this mode takes more time than a powering up of the engine from the other modes. Therefore, in a situation in which the controllers are expect a grid power demands requiring input from the engine, a startup of the engine has to be timed a head of such demand on order to enable a full startup.

An advantage of the situation that the alternator is clutched out from the engine is that the alternator is ar- rangeable to be both in a rotary state and coupled to the busbar. This is advantageous as it provides the capability of using the alternator to provide electrical and mechani cal inertia to handle variations in grid parameters during startup of electrical devices in the grid. In other words, the alternator assists in absorbing and inrush current finding its origin in electrical devices connected to the grid, such as from large motors during startup thereof. In other words, the alternator continues to produce reactive power while connected to the busbar and disconnected from the engine. These effects have a positive lowering effect on the achieved maximum strain on the batteries. A further positive advantage of the alternator being kept connected to the busbar is that it absorbs unintended power peaks, such as from the grid or renewable energy source.

A further subsystem, albeit optional, as present in the preferred embodiment is the dumpload system 50. The dumpload system provides a very fast absorbing capacity of excess electrical power, substantially provided by the power absorption assembly 52 comprising a number of resis- tors, preferably air cooled or water cooled or a combina tion thereof. The resistors are arranged to directly ab sorb electrical energy and convert it to heat.

From the plant controller 3, communication lines or control lines lead to subsystems and metering devices in order to convey control signals and measuring signals be tween the plant controller 3 and the subsystems and meter ing devices. The main control line 3' exits the plant con troller 3 and is branched off toward control lines for the subsystems and metering devices.

For convenience of the description, the control lines are described separately from the steps in which the control lines are used in. A control line 33' is used to control the functioning of the engine and to feedback data towards the plant controller. Part of this data is data collected by measuring device 38 providing information re lating to fuel usage, frequency, KWM operation levels, whether the engine is clutched in with the alternator, and in the engine is running or stopped. A control line 35' is connecting the plant controller 3 with the clutch control ler 35. A control line 31' is connecting the plant con troller with the alternator controller 31. A dumpload con troller 51 receives information from measuring device 37 measuring alternator parameters, such as rotations per mi nute .

A control line 21' next to plant controller with the power buffer controller 21 in order to monitor the status of the batteries and provide controls towards the grid forming inverter of the power buffer.

Several multi-instrument measuring devices are con nected to the plant controller. These multi-instrument measuring devices are arranged to provide information re lating to kilowatts, kWe Balance, voltage, reactive power in KVAr or KVAr balance, frequency or variations thereof relative to a predetermined frequency such as 50 Hz, and a relation between the real power and apparent power in Cos phi. Respective multi-instrument measuring devices com prise the vice 9 connected with the plant controller 3 via control 1-9', device 19 connected to the plant controller of a control line 19', device 59 connected with the plant controller via control line 59' , device 39, connected with the plant controller via control line 59'. Devices 14', 14"provide information towards the plant controller via interface 4.

Figure 2 discloses a preferred embodiment of con trolling the plant by plant controller 3. Parts of the control of the grid parameters are performed by the power buffer controller or battery controller 21 with the, pref erably grid forming, inverter 23. The method starts in step 100. In step 102, it is determined whether the amount of power in kilowatts at the generator indicates whether the engine is required to run in normal mode or below nor mal mode, preferably equivalent to above or below to 30% threshold of nominal capacity. In case it is determined that the engine is required to run the above the bottom threshold of normal mode, in step 106, the engine is set to operate in normal mode or to keep operating in normal mode, after which the method returns to step 102 with mon itoring the engine power requirement relative to the bot tom threshold of normal mode.

In case it is determined in step 102 that the power requirement from the engine is lower than the bottom threshold of normal mode, in step 104, the engine is set to run at a level of low load mode corresponding to de mand. From running in a level of low load mode, the power requirement is monitored in step 114 as to whether demand is below 0 kW. In case such demand is not below 0 kW, as determined in step 114, the method proceeds in step 102.

In case in step 114, it is determined that the power re quirement is below 0 kW, the engine is clutched out from the alternator in step 120. Upon clutching out of the en gine, control of grid parameters is transferred to the battery controller 21 cooperating with the inverter 23 to maintain the control of the grid parameters in step 130. This is described in conjunction with figure 3. In step 135, a monitoring is performed as to whether the generator set is running. In case the generator set is not running, control is returned to the battery controller in step 130. In case in step 135, it is determined that the generator set is running, the control proceeds in step 102 with mon itoring whether the power requirement is above or below the bottom threshold of normal mode of the engine, the en gine being started based on step 180, as set out below.

Figure 3 discloses a preferred embodiment of con trolling the grid parameters by the power buffer subsys tem, the controller 21 thereof in conjunction with the in verter 23. Method starts in step 100. In step 155, it is monitored whether the battery is operating outside of pre determined limits, such as safety limits. Such limits are preferably defined as a battery charge level being between an upper limit and a lower limit depending on power re quirements and expected power supply from the renewable energy. Further predetermined limits as to the battery is cell temperature, preferably individual cell temperature. To the end of monitoring the status of battery cells, the batteries are preferably provided with temperature sensors at each cell or group of cells, as well as voltage and or current sensors at each cell or group of cells. In case the batteries are being charged from the renewable energy supply, and the temperatures exceed predetermined tempera ture limits, the dumpload system is preferably engaged or power to the batteries is limited or switched off. Another way of limiting power to the batteries is by directing more power towards the alternator to have its function as a motor in addition of the dumpload system or as a re placement thereof.

In case it is determined in step one in the 55 that the battery does not operate outside of predetermined lim its, the method proceeds in step 192 continue in step 135. In case it is determined in step 155 that the battery is operating outside of the predetermined limits, it is de termined in step 160 whether the battery operates above low operational limits. In case this determination is neg ative, the method proceeds in step 170 by sending an in struction to increase the output of the renewable energy system, such as the solar system or the went system. In case the determination of step 160 is positive, and in struction is sent to the renewable energy system to de crease its output. In step 155, I determination is made whether the battery charge is below a predetermined mini mum threshold. In case this determination is made posi tively, such that the battery charge is lower than the threshold, in step 180, the engine is started and or the engine is clutched in with the alternator, after which through step 190, the method proceeds in step 135.

Alternative implementations of control besides the above implementation according to figure 2 and 3, comprise controls as follows. Preferably, as long as the alternator is running, preferably clutched out from the engine, the power buffer converter is set to deliver 0 reactive power. This leads to a prevention or limitation of current flow in the batteries in case of reactive power demands from the grid. Therefore, producing heat in the batteries is prevented or limited. Such reactive power demands are then delivered from the alternator. It is also provided that when the alternator is operating as a motor, and thus re ceiving electrical power for the rotation thereof, it is still able to control voltage on the busbar and deliver reactive power. Thus, consumers in the grid using reactive power, by for example and electrical motor in the grid, such reactive powers are met by the alternator rotating instead of by a battery in the power buffer and/or the converter thereof.

Furthermore, controls are provided to control the maximum production allowed from alternative power sources. These controls are provided from the plant controller 3 towards the interface controller 4 that is in communica tion with controls of the renewable energy system. As such, the plant controller 3 is arranged to control the battery charge level and to control current flow charging and discharging the batteries during operation of the plant. As such a power balance is obtained by the plant controller 3 providing the overarching control. In such an embodiment, power buffer system can be arranged to compen sate for fluctuations introduced to the network by consum ers, or in wind and sun. As a matter of purpose, the cur rent flow through the batteries is minimized contributing to a minimized temperature in the batteries. All of this contributes to a maximum battery life, such as at least 5- 6 years instead of 1-2 years for a purely battery-operated system in which batteries are arranged to provide the full control of the grid parameters and in their full load of all disturbances, such as including reactive power demands and renewable energy fluctuations. Furthermore, if all battery cells are fully charged or charged to the maximum threshold as predetermined for a situation, then the output of the renewable energy is re duced, preferably by means of a control signal to the re spective system controller, lead to the alternator or the dumpload system until the battery is in a position to ab sorb energy again.

A further control setting is that the power buffer is arranged to balance the power while controlling output from the renewable energy system, and thereby minimizing a power draw from the battery and maximizing a charge to wards the battery. As such it is preferred to minimize the currents to and from the batteries by controlling the grid parameters by balancing the renewable energy system with the alternator, preferably including the absorption capac ity of an optional dumpload system.

In case it is expected that renewable energy will not be available within a predetermined period or power demands scenario, controls are arranged to charge the pow er buffer or the batteries thereof with energy from the engine. In another situation, if the power demand from the grid is so high that a combination of power from the re newable energy, power from the engine and power from the power buffer reaches its limit, the discharge of the bat tery may be above desirable levels to maintain the battery parameters within predetermined desired levels. In such case, the batteries will be discharged at a level up to a threshold for safety levels for such batteries. In such case, maximum discharge is limited or the batteries or even shut off. In the opposite situation, if the power from the grid towards the busbar is so high that battery charge reaches above safety levels for such batteries, the batteries are disconnected or the charge current is lim ited to not exceeds such levels.

Predetermined setpoints are used for controlling the renewable power system from mean values. A PID controller or a PID controller equivalent is used to a monitor fluc tuations, based upon which control signals are changed to take such fluctuations into account in order to limit the strains on batteries.

An aspect of the invention relates to a method for operating a grid forming power supply plant based on a carbon based energy, such as carbon based fuel, the grid forming power supply system comprising:

- a power input connection from a carbon fuel engine based generator set, the generator set comprising:

- the engine for converting the carbon-based en ergy into motion energy,

- a generator, such as an alternator, for con verting the motion energy into electrical energy,

- and a clutch for coupling and uncoupling of the engine with the generator,

- a power buffer, such as comprising a battery, sub system for providing short term grid forming capacity,

- a plant grid forming controller for controlling grid parameters by means of controlling steps of the meth od,

- the method comprising steps of:

- controlling the grid parameters to remain within predetermined limits based on availability of power from the generator set and the battery,

- power driving the generator such that it rotates and functions as an electromotor by providing it with electrical energy during at least part of the time the generator is uncoupled from the engine. Further preferably, an aspect of the present inven tion is a non-volatile data carrier, such as a solid-state memory or a disk, comprising code means for causing that a processing unit of a plant controller performs steps ac- cording to a method according to the present invention.

While the present invention is described in the foregoing on the basis of several preferred embodiments. Different aspects of different embodiments can be com bined, wherein all combinations which can be made by a skilled person on the basis of this document must be in cluded. These preferred embodiments are not limitative for the scope of protection of this document. The rights sought are defined in the appended claims.

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