WO2017089402A1

WO2017089402A1 - Hybrid power system including gensets and renewable energy resources, and method of control

Info

Publication number: WO2017089402A1
Application number: PCT/EP2016/078562
Authority: WO
Inventors: David Martini; Pietro RABONI
Original assignee: Abb Schweiz Ag
Priority date: 2015-11-26
Filing date: 2016-11-23
Publication date: 2017-06-01

Abstract

A hybrid power system (1 ) is disclosed. The system comprises a plurality of dispatchable power resources (7.i; 7.j) and a first controller (12) for controlling the dispatchable power resources. The system further comprises a power resource section (5) responsive to environment conditions and a second controller (17) for controlling the power resource* section (5) responsive to environment conditions. The second controller (17) is configured and arranged for measuring a total power (∑P_genset) generated by the plurality of dispatchable power resources (7.i; 7,j) and a total power (∑P_PV) generated by the power resource section (5) responsive to environment conditions; and for generating a power curtailment signal (PV_curtailment), when required, for reducing the total power (ΣΡ_PV) generated by the power resource section (5) responsive to environment conditions.

Description

Hybrid power system including gensets and renewable energy resources, and method of control

DESCRIPTION

FIELD OF THE INVENTION

The described embodiments relate to energy management and control systems and methods for hybrid power networks. More particularly, disclosed herein are micro- grid networks comprising a plurality of dispatchable power resources, such as fossil- fueled power generation units, and at least one power resource using renewable energy, such as, but not limited to photovoltaic panels and photovoltaic inverters.

BACKGROUND ART

Micro-grid networks are clusters of distributed power resources and loads. Micro-grids can be operated in a so-called grid-connected mode or in autonomous mode. In the first case, the micro-grid is connected to a main utility grid, i.e. an electric power distribution grid which distributes electric power over large areas. Islanded-mode operating micro-grids are not connected to a main utility grid and are designed to supply electric power to local loads only. Micro-grids can also be configured to operate alternatively connected to a main utility grid or disconnected therefrom. When the micro- grid operates disconnected from the main utility grid, but is nevertheless connectable to this latter, it is operating in an isolated mode.

Micro-grids usually comprise one or more power generation units, comprised of an electric generator driven by a prime mover, such as an internal or external combustion engine, e.g. a diesel engine or a gas turbine engine, a Stirling engine, and more in general a combustion engine converting heat power, provided e.g by combustion or waste heat recovery, into mechanical power. Power generation units will be referred to herein as "gensets". Gensets are dispatchable power resources. A dispatchable power resource is a source of electrical power that can be dispatched at request, i.e. which can be turned on or off, or can adjust its power output on request, to match the demand from an electric power distribution grid, for instance.

Gensets are powered by fossil fuel, e.g. diesel oil, natural gas or the like. Consumption of fossil fuel involves high operational costs and has a detrimental environ- mental impact, since carbon dioxide and noxious gas emissions are generated and discharged in the environment.

In an attempt to reduce operational costs and the environmental impact of micro- grids, renewable power resources have been implemented and often combined with gen- sets into hybrid micro-grids, forming hybrid power systems.

Typically, photovoltaic power resources are used in combination with gensets, to exploit solar energy which is captured by photovoltaic panels that convert the solar power into DC electric power. Inverters are used to convert the DC current into AC current that is in turn provided to the electric distribution grid. Other renewable power resources can be used such as wind turbines or the like.

Renewable power resources are typically intermittent energy resources responsive to environmental conditions. For instance, the power generated by photovoltaic installations fluctuates during the day in response to the position of the sun with respect to the photovoltaic panels. Also, weather conditions can heavily influence the amount of radiant power which can be collected by the photovoltaic panels. Similar dependency upon environmental conditions affects other plants which exploit renewable power resources, such as wind turbine installations.

The electric power demand usually also varies in an unpredictable manner, for instance depending upon the kind and number of loads connected to the micro-grid, or upon whether the micro-grid is operating in a connected mode or isolated mode.

Gensets should operate within a given operating range, to avoid too low efficiencies or damages to the gensets. For instance, most gensets, typically diesel engines, should not operate below a certain power level to avoid excessive drop of the engine efficiency increase of unburned hydrocarbons in the combustion gas. Gensets should also preferably run in a continuous manner, avoiding start-up and shut-down. When running in parallel with renewable power resources, in particular, changes in the demand from the grid can cause a reverse power operation, with electric power flowing in the genset, rather than the opposite.

When an intermittent power resource responsive to environmental conditions, such as a photovoltaic power resource, is combined with gensets into a hybrid micro- grid or power system, measures shall be taken to avoid instable operation of the power resources, reverse power scenarios or inefficient operation of the gensets, as a consequence of fluctuating power demands from the loads or fluctuating power generation from the intermittent power resource.

Similar issues arise in connection with the use of other power resources which are responsive to environmental conditions, such as for instance, wind turbines, or the like, since the environmental conditions cannot be controlled or managed.

SUMMARY OF THE INVENTION

According to a first aspect, a method is described herein, for controlling a hybrid power system comprising: a plurality of dispatchable power resources, a first controller configured for controlling the dispatchable power resources, a power resource section responsive to environment conditions, a second controller configured for controlling the power resource section responsive to environment conditions. The method comprises the following steps: measuring a total power generated by the plurality of dispatchable power resources; measuring a total power generated by the power resource section responsive to environment conditions; when required, curtailing the total power generated by the power resource section responsive to environment conditions.

The total power generated by the power resource section responsive to the environment conditions is curtailed when this is required for preventing malfunctioning or inappropriate operation of the dispatchable power resources. For instance, the power of the power resource section responsive to the environment conditions is curtailed if the total amount of power which can be theoretically generated by said section could lead to instable operation of one or more of the dispatchable power resources, or could cause one or more of said dispatchable power resources to move in a range of operation which is undesirable or forbidden. The step of curtailing the power generated by power resource section responsive to environmental conditions can comprise the steps of: comparing the total power generated by the plurality of dispatchable power resources with a reference value; generating a mismatch or error signal; and, based on the mismatch or error signal, causing curtailment of the power generated by the power resource section responsive to environmental conditions through said second controller, if needed.

According to some embodiments, the method can further comprise the following steps: for a total power generated by the hybrid power system, providing at least one power reference value, indicative of an optimal total power from the dispatchable power resources; calculating a difference between the power reference value and the measured total power generated by the plurality of dispatchable power resources; generating a power curtailment signal as a function of said difference, the curtailment signal being applied to the power resource section responsive to environment conditions, such that the total power generated by the resource section responsive to environment conditions is modulated to reduce said difference.

The power curtailment signal can be generated by the second controller.

The reference signal can be determined on the basis of an optimal tracking curve. The curve can be defined by values stored in a look-up table, or can be calculated on the basis of mathematical formulae, or in any other suitable way.

According to embodiments of the method disclosed herein, efficient and stable operation of the hybrid system is obtained, without the need for any interaction between the first controller and the second controller. Optimum operation of the dispatchable power resources is achieved by simply measuring the total power generated thereby and by acting on the power resource section responsive to environmental conditions through the second controller,

For each operating condition of the hybrid power system, i.e. for each value of total power generated by the hybrid power system, the optimal tracking curve can provide at least one, or in some ranges of operation, two power reference values. The power reference value represents a point of intersection between an iso-load curve and the optimal tracking curve. The iso-load curve is a curve representing the load applied to the hybrid power system, i.e. the total power generated by the hybrid power system. As will become clearer from the following description of exemplary embodiments of the subject matter disclosed herein, in some operating ranges two power reference values can be provided. For instance, according to preferred embodiments, in a range of hysteresis operation of at least one dispatchable power resource, two power reference values are provided for each value of total power generated by the dispatchable resource section. A first, lower power reference value can be used under certain transient conditions of the hybrid power system, e.g. if the hybrid power system is moving from a lower load to a higher load condition, i.e. if the total power generated by the hybrid power system is increasing. A second, higher power reference value can be used in the opposite situation, i.e. if the hybrid power system is moving from a higher load to a lower load condition, i.e. if the total power generated by the hybrid power system is decreasing..

Provision of two, selectively usable power reference values provides optimal operation of the hybrid power system in the range where at least one dispatchable power source is operating between a low hysteresis threshold and a high hysteresis threshold.

If the hybrid power system comprises more than two dispatchable power resources, the optimal tracking curve may have more sections where two power reference values are provided, each section corresponding to the hysteresis range of operation of a respective one of the dispatchable power resources in excess of one, i.e. the second, third and subsequent dispatchable power resources of the hybrid power system.

According to a further aspect, the present disclosure concerns a hybrid power system comprising: a plurality of dispatchable power resources; a first controller for controlling the dispatchable power resources; a power resource section responsive to environment conditions; a second controller for controlling the power resource section responsive to environment conditions; wherein the second controller is configured and arranged for measuring a total power generated by the plurality of dispatchable power resources and a total power generated by the power resource section responsive to environment conditions; and for generating a power curtailment signal, when required, for reducing the total power generated by the power resource section responsive to environment conditions.

Further advantageous features and embodiments the invention will be disclosed herein below, reference being made to the enclosed drawings and are set for the in the attached claims, which form part of the present description.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. l is a schematic of a micro-grid according to the present disclosure in a first embodiment;

Fig.2 is a schematic of a second embodiment of a micro-grid according to the present disclosure;

Fig.3 is control scheme of the photovoltaic micro-grid controller operation;

Fig.4 is a power diagram, showing the general principle of operation of a micro- grid according to the present disclosure under variable load conditions;

Fig.5 is a diagram illustrating an operation under constant load and variable solar irradiance conditions of a micro-grid according to the present disclosure;

Fig.6 is a power diagram referred to the operating conditions of Fig. 5.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

An exemplary embodiment of a hybrid power system 1 is illustrated in Fig. l . The hybrid power system 1 comprises a plurality of distributed power resources forming a micro-grid. Thus, the hybrid power system 1 will be referred to herein after also as micro-grid 1.

More specifically, the micro-grid 1 comprises a plurality of dispatchable power resources. In the exemplary embodiment of Fig. 1 the micro-grid 1 comprises a plurality of dispatchable power resources in the form of gensets.. A plurality of gensets are grouped in a power section 3, which will be referred herein as genset power section 3.

The micro-grid 1 further comprises a power resource section 5 responsive to environmental conditions, i.e. a power section comprised of at least one power resource which generates an amount of electric power that depends upon environmental conditions. In the exemplary embodiment of Fig. l the renewable power section 5 responsive to environmental conditions is a photovoltaic power section, which can comprise one or more photovoltaic generators, and will be referred to herein as photovoltaic power section 5. A photovoltaic generator generally comprises for instance one or more photovoltaic panels and an inverter, which converts DC electric power generated by the photovoltaic panel(s) into AC electric power.

In other embodiments renewable power resources exploiting a different kind of renewable power can be used, e.g. wind, wave power and the like. In this case the power resource section 5 responsive to environmental conditions would include a different kind of power generators, e.g. wind turbines.

For the sake of simplifying the present description, the genset power section 3 of the exemplary embodiment of Fig. 1 comprises only two gensets 7.i and 7.j. It shall however be understood that the number of gensets can be different, and a hybrid micro- grid with more than just two gensets can be operated on the basis of the same method disclosed herein. As noted above, a genset is in general terms a dispatchable power resource comprised of a prime mover (engine) which exploits fuel to produce heat and converts heat power into mechanical power to drive an electric generator.

The two gensets 7.i, 7.j can be identical to one another, or different from one another. For instance, the gensets 7.i and 7.j may have different power rates, or may even include different engines, e.g. a diesel engine and a gas turbine engine, respectively. The engines are labeled 8.i and 8.j, while references 9.i and 9.j designate the respective electric generators. The electric generators are electrically connected to an electric power distribution grid 10. The gensets are in data exchange relationship with and controlled by a first controller 12, which will be referred to herein below as genset micro-grid controller 12.

The electric power distribution grid 10 can provide power to local loads and/or to a main utility grid.

In the exemplary embodiment of Fig.1 the photovoltaic power section 5 comprises a plurality of photovoltaic generators 11.1, 1 1.2 1 1.n. Each photovoltaic generator 11.1 - l l .n can include a photovoltaic field ,13.1-13. n formed by photovoltaic panels, and a respective photovoltaic inverter 15.1-15.n. Each photovoltaic inverter 15.1 - 15. n has an input electrically connected to the respective photovoltaic field and an output connected to the electric power distribution grid 10.

The photovoltaic inverters 15.1 - 15. n are in data exchange relationship with a second controller 17, which will be referred herein as photovoltaic micro-grid controller 17 and are controlled thereby. The photovoltaic inverters 15.1-15. n provide information on the actual power delivered by the photovoltaic inverters 15.1-15.n to the electric power distribution grid 10. The photovoltaic inverters 15.1- 15. n are further configured for receiving input signals, such as commands or instructions from the photovoltaic micro-grid controller 17, for instance signal aimed at curtailing the amount of photovoltaic-generated power delivered by the photovoltaic inverters 15.1-15. n to the electric power distribution grid 10.

The photovoltaic micro-grid controller 17 is further in data communication with power meters 19.1 and 19.2 arranged and configured for measuring power delivered by the gensets 7.i and 7.j to the electric power distribution grid 10. In the embodiment of Fig. l a respective power meter is provided for each genset 7.i, 7.j. In other embodiments, there can be less power meters than gensets, for instance just one power meter, which provides information on the total power generated by the genset power section 3. As can be seen in Fig. l , the photovoltaic micro-grid controller 17 does not require connection to the genset micro-grid controller 12. As will become apparent from the following description, the photovoltaic power section 5 is operated under the control of the photovoltaic micro-grid controller 17, while the genset power section 3 is operated under the control of the genset micro-grid controller 12, without any need for data or signal communication between the first controller 12 and the second controller 17. This makes for instance the combination of a photovoltaic power section 5 to an existing genset power section 3 particularly easy.

One or more loads, schematically shown at 21 , are connected to the electric power distribution grid 10 and powered by electric power cumulatively generated by the genset power section 3 and by the photovoltaic power section 5.

The photovoltaic micro-grid controller 17 is configured to receive input data suitable for determining the total electric power generated by the genset power section 3, indicated as∑P_ge„set, and the total electric power generated by the photovoltaic power section 5, indicated as ΣΡρν. The sum

∑Pload = EPgenset + ΣΡΡΥ is the total amount of power provided by micro-grid 1 to the load 21. It shall be understood that, in particular if the micro-grid 1 is grid-connected, the total power generated {ΣΡΐυικί) could be partly or entirely delivered to an external electric power distribution grid or main utility grid, rather than partly or entirely used to power local loads. Also, part of the total generated power could be stored in a battery or other power storage means.

As will become clearer from the following description of exemplary operating conditions, in general terms the photovoltaic micro-grid controller 17 is configured to optimize the operating conditions of the gensets, without interaction with the genset micro-grid controller 12, but rather by modulating the electric power generated by the photovoltaic power section 5, such that under different operating condition of the micro- grid 1 optimal exploitation of renewable power is achieved, without damaging or adversely affecting the operation of the gensets 7.i, 7.j in the genset power section 3.

In some embodiments, the photovoltaic micro-grid controller 17 is configured for generating a photovoltaic power curtailment signal, based on the measured total electric power∑Phad, on the total electric power∑P_genset generated by the gensets 7.i, 7.j and on power reference values, which represent an optimal genset operating curve as a function of the total power demand from the load. The photovoltaic power curtailment signal is applied to the photovoltaic inverters of the photovoltaic power section 5 and causes, if so required, a reduction of the total power delivered by the photovoltaic power section 5. The same photovoltaic power curtailment signal can be applied to all inverters, or different photovoltaic power curtailment signals can be applied to different inverters, e.g. to clusters of inverters.

The genset micro-grid controller 12 controls the gensets 7.i, 7.j such that the total power∑P_ge„set generated by the genset power section 3 satisfies the demand from the electric power distribution grid 10. Said demand is determined by the power requested by the load (and/or deliverable to a main utility grid) and by the total power ΣΡργ delivered on the electric power distribution grid by the photovoltaic power section 5.

Fig. 3 illustrates a conceptual control scheme of the photovoltaic micro-grid controller 17. The photovoltaic micro-grid controller 17 comprises a reference generator 31 with a data input 35 and a data output 37. The data input 35 receives information on the total power∑Phad cumulatively delivered to the electric power distribution grid by the genset power section 3 and by the photovoltaic power section 5. In response to the data input, the reference generator 31 generates a power reference value∑Pgen_Set> which represents the optimal value of the total genset generated power for the instant total power∑Pioad, and consequently the optimal value of the total power generated by the photovoltaic power section. The power reference value∑Pg^_set is compared (see block 39) with the actual total genset generated power∑P_Senset which is actually generated at a given instant in time. An error signal given by the difference

ElT -∑P_genset -∑Pgenset is applied as an input of a regulator 41 , e.g. a PI regulator, whose output is a photovoltaic power curtailment signal PVcurtaiiment. If the reference value ∑Pgenset *^s higher than ∑Pgenset, the gensets 7.i, 7.j are operating below the optimal operating point for the given total power ∑P ad. The error signal is positive and determines, via regulator 41 , a photovoltaic power curtailment signal PVcurtaiiment, which will cause a reduction of the power made available by the photovoltaic power section 5 to the electric power distribution grid 10, and as a consequence an increase of the power generated by the genset power section 3. The set of ∑Pggnset ^val^ues corresponding to any ∑P cai value can be stored in the form of look-up tables, which contain the coordinates of the optimal operating points, which belong to an optimal tracking curve, or else can be calculated on the basis of formulae, which define an optimal tracking curve.

If a photovoltaic power curtailment signal PVcurtaiiment is generated, which reduces the photovoltaic power delivered by the photovoltaic power section 5 to the electric power distribution grid 10, the power shortage is detected in a manner known per se by the genset micro-grid controller 12, which acts upon at least one of the gensets 7.i, 7.j, and increases the total electric power produced by the gensets power section 3. As will be better understood from the following detailed description of exemplary operative conditions, the above approach avoids problems in the functioning of the gensets, e.g. excessive shut-down or start-up cycles, and prevents the gensets from operating below a minimum load threshold, which would damage the gensets and increase noxious emissions.

For a better understanding of various embodiments of the method disclosed herein, reference will now be made to Fig. 4, which shows a diagram where the total power generated by the genset power section 3 (∑P genset) is reported on the horizontal axis and the total power generated by the photovoltaic power section 5 (ΣΡρν) is shown on the vertical axis. The following values are further shown on the diagram of Fig. 4: p nin . _mini_mUm power which can be generated by genset 7.i. Below this power value the genset must be shut down

pm x . _maxj_mum power which can be generated by genset 7.i (i.e. maximum rated power)

pjnin. minimum power which can be generated by genset 7.j. Below this power value the genset must be shut down pmax . maximum power which can be generated by genset 7.i (i.e. maximum rated power)

pmax pmax^pmax. _maxj_mum power which can be generated by micro-grid 1 .

The gensets 7.i, 7.j shall not operate below the allowable minimum load conditions indicated as P-^nln and Pj^nm respectively. Moreover, repeated start-up and shut-down of the gensets 7.i, 7.j is detrimental and shall be avoided. Thus, two threshold power values are set, which determine when a second genset shall be started up or shut down depending upon the power demand from the load. These threshold values are indicated in Fig. 4 as follows:

PthL: is a power threshold at which the second genset is shut down if the power demand drops below said value;

Pthi i : is a power threshold value at which the second genset is started up if the power demand increases above said value.

These two threshold values determine a hysteresis operation of the second genset as follows. Assume a power demand below PthH is demanded by the load and is generated by genset 7.i. If the demand increases, the second genset 7.j is started up once the power demand reaches PthH. Conversely, assume the power demand drops from a value above PthH. The second genset 7.j will be shut down only when the poAver demand reaches PthL, lower than PthH. Note that the genset micro-grid controller 12 can operate one of the two gensets 7.i, 7.j as the first genset and the other as the second genset or vice-versa, and can also exchange the role of the two gensets, in that for a certain period of operation of the micro-grid 1 , genset 7.i is operated as the first genset, while the genset 7.j is operated as the second genset. This means that genset 7.j will undergo more turning-on and tuming-off cycles. In order to uniformly exploit the two gensets 7.i and 7.j, in a different time interval, the genset 7,j is operated as the first genset. How the genset micro-grid controller 12 handles the operation of the gensets is known to those skilled in the art and will therefore not be described in greater detail herein, unless necessary or useful for the understanding of the subject matter of the present disclosure.

In the diagram of Fig.4 the lines IL (ILj) represent "iso-load" curves, i.e. curves along which the total power∑P ad delivered on the electric power distribution grid 10 is constant. Curves IL are actually rectilinear lines parallel to the line representing the maximum admissible load

Each iso-load curve corresponds to a power value, i.e. to a value of the total load demanded by the electric distribution grid 10, and conversely to the total power cumulatively delivered by the genset power section 3 and by the photovoltaic power section 5. Herein below, therefore, a certain iso-load curve ILj will also be simply referred to as a power value ILj.

Two "forbidden regions" are further defined in the diagram of Fig.4:

- region FR1 is a region where the gensets cannot operate, since the total power delivered by a single genset would be lower than the minimum admissible power for a single genset;

- region FR2 is a region where parallel operation of two gensets is forbidden, since at least one genset would operate below minimum admissible power conditions.

In Fig. 4 an optimal tracking curve OTC is further shown. The optimal tracking curve OTC is fonned by optimal operating points and represents the optimum power generation distribution among the gensets 7.i, 7.j and the photovoltaic generators 11.1 - 1 1.n. In other terms, the coordinates of each point on the OTC represent the optimum combination between the power generated by the gensets (coordinate on the horizontal axis of the diagram of Fig. 4) and the power generated by the photovoltaic generators 11 .1— 11.n (coordinate on the vertical axis). Given a certain load, corresponding to an iso-load curve ILj, the optimal operating condition of the micro-grid 1 is given by the point of intersection between the optimal tracldng curve OTC and the iso-load curve

ILj.

Referring to the schematic of Fig.3, the reference generator 31 generates a power reference

on the basis of the total load, i.e. the total power demand applied to the micro-grid 1 , and the curve OTC. This latter can be stored in the form of a look-up table, or generated by means of suitable algorithms or can be made available to the reference generator 31 in any other manner. The power reference value ^^r _e ^e _?{_set is determined by the point of intersection between the optimal tracking curve OTC and the iso-curve which represents the instant load applied to the hybrid system 1. Thus, for each load, i.e. for each total value of power generated by the hybrid power system 1 , the optimal tracking curve provides at least one power reference value. As will become apparent from the following detailed description and as shown in Fig.4, in some situations two such values are given, i.e. the optimal tracking curve OTC has more than one intersection with a given iso-load curve ILj, and one of the power reference values will be selectively used, depending upon the operating conditions of the hybrid power system 1. The optimum point of operation can be positioned differently, depending upon whether a given operating point is achieved by reducing or by increasing the power demand from the load. When an iso-load curve ILj intersects the optimal tracking curve OTC in two points, this means that for the power corresponding to the iso-load curve ILj considered, the optimal operating point depends upon whether the that load condition is reached following an increasing power demand or a decreasing power demand.

In the exemplary embodiment of Fig. 4 the optimal tracking curve OTC is defined by a plurality of straight lines. This, however, is not mandatory. According to other embodiments, the portions of the optimal tracking curve OTC can be curved. Also, a larger number of straight line portions can form the optimal tracking curve OTC.

Operation of the micro-grid 1 under variable load condition and possible fluctuations of solar irradiance, e.g. due to variable environmental conditions, will now be described in greater detail.

A first portion of the optimal tracking curve OTC is formed by a straight line between points (a) and (b) on the horizontal axis. When the total load ΣΡΐοαΑ is between the values represented by iso-load curves ILi and IL2 which pass through points (a) and

(b), the entire electric power will be delivered by one of the two gensets 7,i, 7.j, the other remaining inoperative. Which one of the two gensets will be operative and which will be at standstill is determined by the genset micro-grid controller 12 and is outside the scope of the present disclosure. Criteria of selection of the operating genset are loiown to those skilled in the art and do not require to be described herein. When the total power required is between ILi and IL2 the photovoltaic micro-grid controller 17 will generate a photovoltaic power curtailment signal PVcurtaiimem, which curtails the photovoltaic power to zero, i.e. no electric power will be delivered by the photovoltaic generators 11.1 - 1 1.n to the electric power distribution grid 10. The next portion of the optimal tracking curve OTC is represented by a straight approximately vertical line extending between point (b) and point (c). If the load demand increases above IL2, the additional power demand, exceeding the power value corresponding to IL2 will be provided by the photovoltaic power section 5. The photovoltaic power curtailment signal PVcunaiiment will be lowered. I.e. for any power demand between IL2 (the iso-load curve IL passing through point (b)) and IL3 (the iso- load curve IL passing through point (c)) the power exceeding IL2 is provided by the photovoltaic power section 5.

If the solar radiating power is insufficient to provide the power surplus (IL3-IL2) required to achieve the iso-load curve IL3, the power gap will be filled by the gensets 7.i, 7.j. This situation is represented by way of example by point (c') in Fig. 4. If the maximum power which can be generated by the photovoltaic power section 5 is lower than (IL3-IL2), and is e.g. equal to (IL4-IL2), wherein IL4 is the iso-load curve crossing the OTC curve in point (c'), and if the total power demand is IL3, the genset which is currently operating will increase the power generated thereby, to fill the difference between IL3 and IL4. Further increase of the power demand would cause the genset- generated power to further increase along the straight line portion between point (c') and (c") rather than reaching point (c), as insufficient solar power is available.

Let's now consider that the power demand has achieved IL3 (iso-load through point (c)) and that the power generated by the photovoltaic power section 5 corresponds to the coordinate of point (c) on the vertical axis, i.e. the power generated by the photovoltaic power section is (IL3-IL2). If the power demand from the electric distribution grid 10 further increases, the micro-grid 1 , the power surplus can be supplied by the genset power section 3 only, by the photovoltaic power section 5 only, or by a combination of the two. The choice is not without consequences on the operation of the entire system. The optimal tracking curve OTC shows what happens in case of further power demand increase from point (c). Rather than further increasing the photovoltaic contribution to the total power delivered to the electric power distribution grid 10, the photovoltaic generated power is curtailed and the genset generated power is increased.

Starting from point (c), if the total power ΣΡΐοαύ increases, the reference generator 31 of the photovoltaic micro-grid controller 17 generates a lower value for the reference ∑Pge _set- By comparing the new, lower power reference value ^Pg^nset with∑P genset a higher photovoltaic power curtailment signal PVcunaiiment is generated by regulator 41 and applied to the photovoltaic power sections, such that the total photovoltaic-generated power is caused to drop. The genset micro-grid controller 12 detects a power shortage on the electric power distribution grid 10 and thus causes the genset 7.i or 7.j to generate more power. The system thus moves along line (c)-(d).

As a matter of fact, considering the iso-load curves IL immediately above IL3, they cross the optimal tracking curve OTC at points which are aligned along the descending straight curve portion (c)-(d). The reason for this trend of the optimal tracking curve OTC is the following. At point (c) only one genset is operating while the other is turned off. The second genset would be turned on upon reduction of the photovoltaic generated power, e.g. due to a cloud which temporarily reduces the solar irradiance. This transient condition would cause a sudden drop in the power available from the photovoltaic power section 5 and would trigger the start-up of the second genset. However, as soon as the cause of the solar irradiance reduction is removed, the photovoltaic generated power would suddenly increase stepwise again. The genset micro-grid controller 12 would interpret this as a reduction of the power demand on the electric power distribution grid and would shut down the second genset again. The transient and temporary solar irradiance reduction would thus cause a rapid sequence of starting up and shutting down of gensets, which is highly detrimental.

Moreover, if point (c) of the optimal tracking curve OTC was moved further vertically, the bi-univocal correspondence between load and optimal operating point would be lost, since some iso-load curves would cross the optimal tracking curve OTC in two different points, resulting in two alternative possible operating conditions.

To prevent this negative behavior of the micro-grid 1 , the optimal tracking curve OTC has a maximum in point (c) and then decreases till point (d) on iso-load curve. IL5, such that the total genset generated power is gradually increased as the power demand on the electric power distribution grid 10 increases, moving the operating point away from the forbidden region FR2.The positions of points (c) and (d) in the diagram of Fig.4 are such that the line (c)-(d) is less steep than the iso-load curve ILj. Consequently, each isolated curve between IL3 and IL5 will cross the optimal tracking curve OTC in one point only. This results in a bi-univocal relationship between optimal genset generated power and photovoltaic generated power for each load between IL3 and IL5.

For the reasons which will be explained later on, in point (d) the amount of genset generated power is equal to or slightly less than PthL. In the embodiment shown in Fig.4 the coordinate of point (d) on the horizontal axis is (Ptht-Δ), wherein Δ is a margin value (e.g. between about 0.1 % and about 1 % of Ρ«Λ). The point (d) is located outside the forbidden region FR2 and beyond the value PthL.

In the embodiment of Fig. 4, the next section (d)-(e) of the optimal tracking curve OTC is approximately parallel to the horizontal axis. Thus, a further increase in the power demand would cause the genset generated power to increase, with the photovoltaic generated power remaining constant. Point (e) is located on an iso-load curve which can be near to, but lower than the iso-load curve IL6 that corresponds to the maximum power deliverable by one genset (Ρ™^α¾).

The genset generated power at point (e) is equal to PthH or slightly higher, e.g. equal to (PUIH+Δ), wherein Δ is a margin value (e.g. between about 0.1 and aboutl % of P_IUL). As noted above, the power thresholds Pt L and PthH define the range of hysteresis operation of the second genset, i.e. the second genset is turned on when the power required by the genset micro-grid controller 12 is equal to or higher than ΡΛΗ and is turned off when the power required by the genset micro-grid controller 12 drops at or below PthL. Thus, at point (e), if the second genset is turned on, it would, not be immediately switched off in case of a step-wise drop of power demand. Therefore, once point (e) has been achieved, the photovoltaic generated power can start increasing again, if a higher power demand comes from the electric power distribution grid 10.

This is reflected by the optimal tracking curve OTC, which continues with an almost vertical and approximately straight portion from point (e) to point (g), passing through point (f). The behavior of the photovoltaic micro-grid controller 17 along this curve is thus the following: a load increase, i.e. an increasing power demand from the electric distribution grid 10, is covered by increasing the photovoltaic generated power. Starting from point (e), if the total load EPhad increases, the power reference value ∑Pgenset changes such that a reduced photovoltaic power curtailment signal PVcurtaiiment is generated. This latter causes an increase of the photovoltaic generated power, while the genset generated power remains substantially constant or may increase only slightly, e.g. between point (e) and point (f). The coordinate of point (g) on the vertical axis represents the maximum photovoltaic power which the micro-grid 1 can generate.

The total genset generated power is partly generated by the first genset 7.i and partly by the second genset 7.j. The genset micro-grid controller 12 , is in charge of detenmning how the power is shared among the two gensets 7.i, 7.j according to known control algorithms, which are outside the scope of the present disclosure and are known to those skilled in the art.

Upon reaching the maximum rated power of the photovoltaic power section 5 in point (g), which is positioned on iso-load curve ILs, further increase of the power demand from the electric power distribution grid 10 will be covered by increasing the power generated by the genset power section 3, until the maximum rated power is achieved in point (h). Point (h) is located on the iso-curve corresponding to the maximum power which can be generated by the micro-grid, i.e.

pmax _ pmax _j_ pmax

The maximum power that can be delivered by the micro-grid 1 is set equal to the maximum genset generated power, in order to take into account possible contingencies of the photovoltaic power section 5 or unavailability of solar power, e.g. at nighttime.

If the solar radiation drops while the system is operating on one of the iso-load curves between IL9 and P ^mx , e.g. due to variable weather conditions, such that the maximum photovoltaic generated power cannot be generated, and the total power demand from the electric distribution grid 10 remains constant, the genset micro-grid controller 12 will detect an increased power demand and will cause one, the other or both gensets 7.i, 7.j to increase the genset generated power. For instance, suppose the system is operating at point (g'), on iso-load curve IL7. Should the solar irradiance drop, e.g. due to a cloud, the photovoltaic generated power drops and the genset micro-grid controller 12 reacts by increasing the amount of genset generated power along iso-load curve IL7, e.g. until point (g") is achieved. A subsequent increase of the solar irradiance will move the system back to point (g'). Both gensets will remain in operating conditions outside the forbidden regions FR1, FR2. No start-up and shut down of the gensets occur, but just a modulation of the power produced thereby.

If the total solar irradiance is insufficient to move the system along the portion of the optimal tracking curve OTC from point (e) to point (g), e.g. because of adverse weather conditions, once the maximum photovoltaic generated power available under the given environmental conditions has been achieved, additional power demand from the electric distribution grid 10 will be supplied by the genset power section 3. For instance, if the maximum photovoltaic generated power under certain weather conditions corresponds to the coordinate of point (f ) on the vertical axis, starting from point (Γ) an increased power demand will cause an increase of the genset generated power along the line (f )-(f ") under the control of the genset micro-grid controller 12.

When the total load, i.e. the total power demand, from the electric distribution grid 10 decreases, starting from point (h) for instance, the first action will be taken by the genset micro-grid controller 12, which will reduce the genset generated power. This is because the optimal tracking curve OTC between (g) and (h) is substantially parallel to the horizontal axis in the diagram of Fig. 4. A reduction of the total load will thus cause saving of fuel by reducing the genset generated power, while the maximum of photovoltaic power will be exploited. The photovoltaic power curtailment signal PVcurtaiiment will not change and will remain zero.

Upon reaching the iso-load curve ILs, corresponding to point (g), the photovoltaic micro-grid controller 17 will increase the photovoltaic power curtailment signal PVcimaiiment causing the photovoltaic generated power to reduce, if the total load drops below ILs. The purpose is to avoid turning-off of one of the two gensets 7.1 , 7.2, while the total load is still relatively high. If one of the gensets was turned-off at this high power demand, a sudden drop of the solar irradiance would cause the turned-off genset to be re-started and then turned-off again, when the cause of the solar irradiance drop is removed. In order to avoid the above risk of intermittent operation of the genset power section 3, the photovoltaic generated power is reduced gradually along the curve (g)-(f), Avhile maintaining the genset generated power substantially constant.

At point (f), the optimal tracking curve OTC branches-off from the curve (e)-(f) and extends from point (f) to point (d) passing through point (f '). In other words, starting from point (f) the optimal tracking curve OTC has two different sections marked by arrows in Fig.4. One section (points (d)-(e)-(f)) is used while the power demand is increasing. The other section (points (f)-(d)) is used while the power demand is dropping. 62

Thus, if the power demand increases, the micro-grid 1 will be controlled such that the operating point moves along curve (d)-(e)-(f)-(g). Conversely, if the power demand decreases, the micro-grid 1 will be controlled such that the operating point moves along the curve (g)-(fj-(d). The portion of the optimal tracking curve OTC from point (f) to point (d) through point (f ') is less steep than the curve portion (e)-(f). The portion of

generated power thus varies less rapidly during load decrease than during load increase. Conversely, the genset generated power varies more rapidly during load decrease, i.e. along line (f)--(d), than during load increase, i.e. along line (e)-(f). Moving along curve (f)-(d), the genset power section 3 is brought back towards the threshold PthL, where one of the two gensets 7.i, 7j is turned-off and only one genset remains under operating conditions.

In the range comprised between the values Pthi. and PthH, which defines the region of hysteresis operation of the second genset 7.j, the optimal tracking curve OTC has two curve sections or branches, namely a first optimal tracking curve section (d)- (e)-(f) and a second optimal tracking curve section (f)-(d) respectively. The first optimal tracking curve section (d)-(e)-(f) is used when the total power generated by the micro- grid 1 is increasing (i.e. moving from one iso-load curve to the next from the left to the right in the diagram of Fig.4). The second optimal tracking curve section (f)-(d) is used when the total power generated by the micro-grid 1 is decreasing (i.e. moving from one iso-load curve to the next from the right to the left). As can be appreciated from the diagram of Fig.4, for each total power generated by the genset power section 3, in the range between the values PthL and PthH the optimal tracking curve OTC provides two power reference values for each iso-load curve IL. The first, lower power reference value (along curve (d)-(e)-(f)) will be used if the total power generated by the hybrid power system or micro-grid 1 is increasing. The second, higher power reference value (along curve (f)-(d)) will be used if the total power generated by the micro- grid 1 is decreasing. Since the two sections of the optimal tracking curve OTC are used selectively, depending upon whether the power demand is increasing or decreasing, a bi-univocal relationship between iso-load curves and optimal tracking curve is preserved. In fact, even though each iso-load curve between IL5 and IL9 has two points of intersection with the optimal tracking curve OTC, only one point will automatically be selected as the correct point of operation of the system, depending upon whether the power demand is increasing or decreasing. From point (d), the optimal tracking curve OTC follows the same line (d)-(c")- (c)-(c')-(b)-(a) as already described in connection with load increase, in a reverse direction, i.e. the power reference value∑Pgenset generated by the reference generator 31 and the corresponding photovoltaic power curtailment signal PVcurtaiimem generated by the photovoltaic micro-grid controller 12 are the same, irrespective of whether the load is increasing or decreasing.

Referring now to Figs. 5 and 6, with continuing reference to Fig.4, the behavior of the micro-grid 1 under constant load and variable solar irradiance will be described.

The diagram of Fig. 5 illustrates several curves as a function of time, which is shown on the horizontal axis. The curves illustrated in Fig.5 represent the following values:

CI : total photovoltaic generated power available, i.e. the total photovoltaic generated power which can be generated with the available solar irradiance. This curve is determined by the solar irradiance which is an uncontrollable parameter;

C2: the curtailed photovoltaic generated power, i.e. the actually delivered photovoltaic generated power. As explained above, this power can be lower than the maximum photovoltaic power that could be generated under a given solar irradiance condition;

C3: total genset generated power∑P gens t

The diagram of Fig. 6 further shows the value Ptm, which has been defined above, the total load Pioad and the following two values:

£p iimdown . representing the maximum photovoltaic generated power which can be exploited when only one genset is turned-on; piimup . representing the maximum photovoltaic generated power which can be exploited when two gensets are tumed-on.

These values are reported on the vertical axis of the diagram shown in Fig.6, which illustrates the same diagram of Fig.4, with the optimal tracking curve OTC and the iso- load curve representing the constant load Pioad. The two values ΣΡρ ^{1110 11} and ΣΡργ ^ηίρ are the coordinates on the vertical axis of the intersection points between the iso-load curve corresponding to Pioad and the optimal tracking curve OTC.

Referring now again to Fig.5, curve CI , which represents the total photovoltaic power which can theoretically be generated at a given solar irradiance, varies during time as a consequence of solar irradiance fluctuations. During the time interval [tl -tO] the maximum available photovoltaic generated power is comprised between Ppy"^{do n} and∑Ppy^{l lup} . Only one genset is tumed-on. Thus, the photovoltaic generated power shall be curtailed. Curve C2 shows, indeed, that the actually delivered photovoltaic generated power is equal to∑P_p ^h™^uv , i.e. lower than the maximum power which could be generated by the photovoltaic power section 5 under the given solar irradiance conditions.

At the time instant tl the solar irradiance increases and then decreases again at time instant t2 and remains constant till t3. The maximum theoretically available photovoltaic generated power is above∑P_p ^li™^up, such that the photovoltaic generated power is curtailed during the whole time interval [t3-tl] and remains equal to∑P_p ^h™^up , since only one genset is operating. During the time interval [t4-t3] the solar irradiance drops and the total photovoltaic generated power which can be obtained is less than ∑Pp™^uv . The whole available solar irradiance can thus be exploited. The photovoltaic power curtailment signal PVcurtaiimem is zero.

During the entire interval [t3-t0] the total genset generated power (curve C3) remains constant. However, at time instant t3 the total available photovoltaic generated power drops such that, in order to cover the power demand (total load Pioad from the electric power distribution grid 10) the genset generated power must increase. This is shown by curve C3, which has a stepwise increase at t3. The higher genset generated power is maintained by the genset micro-grid controller 12 until time instant t4, when the solar irradiance increases again, such that the total available photovoltaic generated power becomes equal to 2P y^ndou'ⁿ. Since the second genset is still inoperative, the maximum photovoltaic generated power which can be exploited is still∑Pp™^up and therefore the photovoltaic power curtailment signal PVcurtaiiment will curtail the total photovoltaic generated power to∑P_p™^up . The genset generated power drops at time instant t4 to the value at time instant tl . At time instant t5 the solar irradiance drops below ∑P_PV and the total photovoltaic generated power becomes so low that the genset generated power requested to cover the demand becomes higher than the threshold PthH. The genset micro-grid controller 12 thus turns the second genset on.

The photovoltaic micro-grid controller 17 can be configured such that the photovoltaic power section 5 remains inoperative for a certain time interval [t6-t5], as a response to the solar irradiance drop, until the solar irradiance increases again. At time t6 the photovoltaic micro-grid controller 12 causes the photovoltaic power section 5 to deliver photovoltaic generated power again. This power gradually increases until time instant t7, following gradual increase of the solar irradiance. The photovoltaic power generated is not curtailed, since two gensets are now operative and therefore, for the given constant Pioad, the photovoltaic generated power can increase till∑P^^{l down}. In the time interval [t7-t6] the curves CI and C2 overlap.

As the photovoltaic generated power increases, the genset generated power decreases between t6 and t7, as shown by curve C3. At the time instant t7 the available photovoltaic power (curve CI) increases beyond the upper limit ∑P^^down and consequently the excess of photovoltaic generated power is curtailed.

From the above description, at any instant in time during the time interval [t8- tO], the sum of the genset generated power and the photovoltaic generated power is equal to Pioad, which in this exemplary operating mode is supposed to remain constant. The number of operating gensets can vary depending upon the requested genset generated power and the exploited photovoltaic generated power can correspond to the total photovoltaic generated power theoretically obtainable under the given solar irradiating condition, or can be curtailed, depending upon the operating condition of the genset power section 3.

As can be understood from the above discussion, the photovoltaic micro-grid controller 17 operates on the basis of the measured total genset generated power and on data concerning the photovoltaic generated power. However, no co-action with the genset micro-grid controller 12 is needed, nor is there any interference between the photovoltaic micro-grid controller 17 and the gensets 7.i, 7.j. The photovoltaic micro- grid controller 17 only generates a photovoltaic power curtailment signal PVcurtaiiment in order to maintain the system in optimal operating conditions and only requires input data from the power meters 19.i and 19.j.

The genset micro-grid controller 12 controls the gensets based on the demand for genset generated power, which is equal to the difference between the actual demand from the electric power distribution grid 10 and the photovoltaic generated power (curtailed, as needed). The mode of operation of the genset micro-grid controller 12 can be identical to the mode of operation of a controller of a standard genset micro-grid with no photovoltaic power resource combined therewith.

The genset micro-grid controller 12 can control the power generated by the gensets based on a demand signal in any known manner. Said demand signal can for instance be a frequency signal, If the power demand from the load increases, the braking torque applied on the rotor of the electric generator of the gensets increases, which results in a reduction of the rotational speed. Said reduction can be detected by the genset micro-grid controller 12 as a frequency variation of the generated electric power. A control loop is enacted, to reduce the frequency error between a frequency set-point (e.g. 50 Hz or 60Hz) and the measured actual frequency value. The control loop causes an increase of the mechanical power generated by the prime mover(s), aimed at reducing the frequency error. The opposite happens if a decreased power demand reduces the braking torque, with a consequent frequency increase.

The above described method and system control the operation of the photovoltaic inverters 15.1 , 15.2, ... 15.n based on the total power generated by the genset power section 3 and by the photovoltaic power section 5. The photovoltaic micro-grid controller 17 generates a photovoltaic power curtailment signal PVcurtaiimem which is determined on the basis of the optimal tracking curve OTC, on the total power delivered to the load ∑P ad and on the total genset generated power ∑P_gemei. The photovoltaic power curtailment signal PVcurtaiimem varies depending upon the number of operating gensets and on the fluctuation of the solar irradiance. In order to make the control faster, according to some embodiments a solar irradiance predictor can be added to the photovoltaic micro-grid controller 17, as schematically shown at 18 in Figs. 2 and 3. Solar irradiance predictors are known to those experts in the field of photovoltaic power systems, and do not require further disclosure. The predictor signal may be used to modify the points defining the optimal tracking curve OTC on the basis of predicted weather condition variations. For instance, if a reduction of solar irradiance is improbable, due to stationary sunny weather conditions, the coordinate of point (c) on the vertical axis can be increased. In general terms, using a solar irradiance predictor 1 8 may increase the amount of solar power which can be exploited under safe conditions may increase. A predictor may for instance be used in combination with a plurality of stored data defining several optimal tracking curves OTC. The actual optimal tracking curve can be selected on the basis of an input from the predictor.

According to further embodiments, a total power meter 22 can be provided (see Fig.2), which measures the total power absorbed by the load. The measured value can be used as a correction parameter applied to the photovoltaic power curtailment signal PVcurtaiiment generated by the regulator 41 , as shown in shadow lines in Fig. 3. For instance, if power meter 22 detects a power increase, the photovoltaic power curtailment signal PVcurtaiiment can be reduced.

According to a further aspect of the present disclosure, the controller 17 can also be used to determine the threshold values PthL and Pthii of an already installed micro- grid of distributed dispatchable power resources, such as a micro-grid of diesel generators, or other gensets. As explained above, these two threshold values determine the hysteresis of the gensets. They are usually only available to the company which has installed the genset micro-grid. It may sometimes be useful to determine these values without having access to the relevant genset micro-grid controller 12. The described photovoltaic micro-grid controller 17 can be used for this purpose. According to some embodiments, for determining the threshold values PthL and PthH a constant load can be applied to the electric distribution grid 10 and the photovoltaic generated power, or power from a different resource which emulates the photovoltaic power section, is modulated, causing the switching-on and switching-off of the gensets. The difference between the total load applied and the modulated photovoltaic generated power (or the power delivered by an emulating power resource) which provokes switching-on and switching-off of the gensets corresponds to the searched threshold values.

Claims

1 . A method of controlling a hybrid power system (1 ) comprising: a plurality of dispatchable power resources (7.i; 7.j), a first controller (12) configured for controlling the dispatchable power resources (7.i; 7.j), a power resource section (5) responsive to environment conditions, a second controller (17) configured for controlling the power resource section (5) responsive to environment conditions; the method comprising the following steps:

measuring a total power (∑P_genset) generated by the plurality of dispatchable power resources (7.i; 7.j);

measuring a total power (ΣΡρν) generated by the power resource section (5) responsive to environment conditions;

when required, curtailing the total power {∑Ppv) generated by the power resource section (5) responsive to environment conditions.

2. The method of claim 1 , wherein the power resource section (5) responsive to environment conditions comprises photovoltaic generators (1 1.1 ; 1 1.2; . . . 1 1.n).

3. The method of claim 1 or 2, wherein the dispatchable power resources (7.i; 7.j) comprise combustion engines (8.i; 8.j) mechanically coupled to electric generators (9.i; 9.j).

4. The method of any one of the preceding claims, wherein the step of curtailing the power (∑Ppy) generated by power resource section (5) responsive to environmental conditions comprises the steps of:

comparing the total power (∑P_genset) generated by the plurality of dispatchable power resources(7.i; 7.j) with a reference value;

generating an error signal (Err);

based on the error signal (Err), causing curtailment of the power generated by the power resource section (5) responsive to environmental conditions through said second controller (17).

5. The method of any one of the preceding claims, further comprising the following steps:

for a total power (ΣΡΐοαά) generated by the hybrid power system (1), providing at least one power reference value

indicative of an optimal total power from the dispatchable power resources (7.i; 7.j);

calculating a difference between the power reference value (∑Pg_{g set}) and the measured total power (∑P_genset) generated by the plurality of dispatchable power resources (7,i; 7.j);

generating a power curtailment (PVcuitaiiment) signal as a function of said difference, the curtailment signal being applied to the power resource section (5) responsive to environment conditions, such that the total power generated by the power resource section (5) responsive to environment conditions is modulated to reduce said difference.

6. The method of claim 5, wherein in at least one range of total power generated by the plurality of dispatchable power resources (7.i; 7.j), a first power reference value (∑Pgenset) ^m^ ^{a secon}d power reference value (∑P₅' _set) are provided, the first power reference value being preferably lower than the second power reference value.

7. The method of claim 6, comprising the steps of using the first power reference value when the total power (∑P ad) generated by the hybrid power system (1) is increasing; and using the second power reference value when the total power generated by the hybrid power system (1) is decreasing.

8. The method of claim 6 or 7, wherein the at least one range of total power generated by the dispatchable power resources at least partly overlaps a hysteresis region (PthLj PthH) of operation of at least one of said dispatchable power resources (7.i; 7j).

9. The method of claim 8, wherein the at least one range of total power generated by the dispatchable resources (7.i; 7,j) totally overlaps the region of hysteresis of the at least one dispatchable power resource (7.i; 7.j).

10. The method of any one of claims 4 to 8, further comprising the step of modifying the power reference value (∑^_en_set) ^{on me} basis of an environmental condition predictor signal.

1 1. A hybrid power system (1) comprising:

a plurality of dispatchable power resources (7.i; 7.j); a first controller (12) for controlling the dispatchable power resources; a power resource section (5) responsive to environment conditions;

a second controller (17) for controlling the power resource section (5) responsive to environment conditions;

wherein the second controller (17) is configured and arranged for measuring a total power (∑Pgenset) generated by the plurality of dispatchable power resources (7.i; 7.j) and a total power (∑Ppv) generated by the power resource section (5) responsive to environment conditions; and for generating a power curtailment signal (PVcurtaiiment), when required, for reducing the total power (∑PPV) generated by the power resource section (5) responsive to environment conditions.

12. The hybrid power system (1) of claim 1 1 , wherein the power resource section (5) responsive to environment conditions comprises photovoltaic generators (1 1.1 ; ... l l .n).

13. The hybrid power system (1) of claim 11 or 12, wherein the dispatchable power resources (7.i; 7.j) comprise combustion engines (8.i; 8.j) mechanically coupled to electric generators (9.i; 9.j).

14. The hybrid power system of claim 11 , 12 or 13, wherein the second controller (17) is configured for: determining at least one power reference value (∑^_e ^e,{_set) _> indicative of an optimal total power from the dispatchable power resources (7,i; 7.j) for any operating condition of the hybrid power system; calculating a difference between the power reference value {^Pg^set ^tne measured total power ∑P_genset) generated by the plurality of dispatchable power resources (7.i; 7.j); and generating a power curtailment signal as a function of said difference, the curtailment signal (PVcurtaiiment) being applied to the power resource section (5) responsive to environment conditions, such that the total power (ΣΡρν) generated by the resource section responsive to environment conditions is modulated to reduce said difference.

15. The hybrid power system (1 ) of claim 14, wherein the second controller (17) is configured for determining a first power reference value ( ^^" _e* _et and a second power reference value (∑Pg^_set), in at least one range of operation of the hybrid power system (1), the first power reference value being preferably lower than the second power reference value the first power reference value being used when a total power {∑Pha<i) generated by the hybrid power system is increasing; and the second power reference value being used when the total power {∑P ad) generated by the hybrid power system (1) is decreasing.

16. The hybrid power system of claim 15, wherein the at least one range of operation at least partly, and preferably totally overlaps a range (P ; Ρ ) of hysteresis operation of at least one dispatchable power resources,

17. The hybrid power system of any one of claims 1 1 to 16, further comprising an environmental condition predictor (18).