WO2009052908A9

WO2009052908A9 - Photovoltaic generator system

Info

Publication number: WO2009052908A9
Application number: PCT/EP2008/007629
Authority: WO
Inventors: Rosa Mario La
Original assignee: Rosa Mario La
Priority date: 2007-10-22
Filing date: 2008-09-15
Publication date: 2009-06-11
Also published as: WO2009052908A3; ITTO20070750A1; WO2009052908A2

Abstract

The system comprises: a static switch (16) between an A.C. current supply network mains (12) and a load (10), the switch being controllable to connect or isolate the load from the mains; a voltage booster (20, 22, 24) connectable to the output of a set of photovoltaic panels (18); a static DC/ AC converter (26), connectable to the voltage booster (20, 22, 24) for generating a voltage at 50Hz; a static switch (47) from the output of the static converter (26) to the load (10), and controllable to connect or isolate the load from the photovoltaic generator; and a controller (30) receiving voltage, current and power signals from respective sensors (32, 34, 35, 36, 38, 39, 40, 45), and which is programmed to control the switches in order to instantaneously connect and isolate the load with respect to the mains or the photovoltaic panels, depending on circumstances.

Description

"Photovoltaic generator system"

This invention is concerned with a photovoltaic generator system, to be installed as a supplement to the public electric power network, and provided with a static switching capability whereby the two supply systems (public network mains and photovoltaic system) can be kept electrically isolated, while providing a high degree of efficiency and convenience.

Photovoltaic generators of electric power are usually installed as auxiliary means to the conventional electric power network, in order to reduce the dependence of the consumer from the latter, by replacing the network during daylight hours on sunny days. In practice, the photovoltaic system, comprising a set of solar panels feeding an inverter, is connected in parallel to the network mains. However, the problem of the switchover from drawing current from the network and from the photovoltaic system has not found a satisfactory solution in the past: certain of the approaches followed to this date have favored efficiency, other approaches have favored convenience of use, but all have failed to meet both requirements.

Where the operator of the public electric network allows the consumer to return power to the network, the regulations governing the consumer's system and its connection to the network are, for reasons of security of the public network, so stringent that the photovoltaic system becomes very expensive, both in its manufacture and in its subsequent maintenance. The costs are so high that they almost generally make the system economically unviable for private residences, even taking into account the savings made when power is returned to the network.

In view of the above situation, photovoltaic systems have often been installed that are completely separate from the network. This approach has the advantage of a lower expense in purchasing and maintaining the system, because the onerous steps for ensuring the reliability in protecting the public network are no longer required. However, the prospect of selling any excess photovoltaic power to the public operator is thereby lost, and there is the even more serious dis- advantage that, when the photovoltaic power is not available (no or insufficient sunlight), the consumer has to disconnect his appliances from the photovoltaic system and then connect them to the public network, with momentary disruption of the operation of lamps, TV sets, etc.

The primary object of the invention is now to provide a photovoltaic generator system capable of automatically regulating the best exploitation of the solar- generated power, by ensuring that the consumer uses the photovoltaic generation wherever possible, having recourse to the network only when indispensable, while avoiding or minimizing any degradation of the supply service to the con- sumer's appliances.

Another object is to provide the above photovoltaic generator system so that its manufacture and maintainance costs are moderate.

In the following disclosure and claims, the terms "solar system" and "photovoltaic system" will be used interchangeably with the same meaning, and similarly for the terms "photovoltaic panel" and "solar panel".

The above objects are achieved by the invention, together with other objects and advantages such as will appear from the disclosure below, by a photovoltaic generator system having the features recited in claim i.

The dependent claims define other advantageous features of the invention.

A few preferred embodiments of the invention are disclosed below, by way of non-limiting example, with reference to the attached drawings, wherein:

Fig. i is a block circuit diagram of a first embodiment of a battery-free photovoltaic generator system according to the invention;

Fig. 2 is an example of a static disconnecting switch belonging to the system of Fig. i;

Fig. 3 is an example of a static converter belonging to the system of Fig. l;

Figs. 4, 5 and 6 are operative flowcharts of a controller in the system of Fig. i; Fig. 8 is a block circuit diagram of a second embodiment of a photovoltaic generator system provided with a storage battery, according to the invention;

Figs. 9, 10 e ii are operating flowcharts for a controller belonging to the system of Fig. 8.

In Fig. l, a load io, comprising, e.g., a set of electrical appliances Qamps, radio and TV sets, home appliances, etc.) of a residence or office is supplied with power from a public electric power network 12, e.g. at 230 V, 50 Hz, by way of a bifilar connection 14, via a static controlled switch 16, as described below.

A set of photovoltaic panels, or solar panels 18, appropriately installed as known to the person skilled in the art, e.g. on the roof of a building, are capable of generating low-voltage, DC electric power. The power is fed to a DC/ AC converter 20, known per se, which is adapted to convert the DC power into AC power at a high frequency, e.g. 2000 Hz. A high-efficiency transformer 22 transforms the AC power to boost its output voltage. The high-frequency voltage delivered by the transformer is applied to an AC/DC converter 24 (such as a conventional rectifier bridge having a not shown RC filter at its output), in order to provide a high-amplitude DC voltage, e.g. at 325 V. A controlled DC/ AC converter 26 (described below) finally converts the latter DC voltage to an AC voltage at 230 V, 50 Hz, which is delivered to load 10 by way of a bifilar con- nection 28, in parallel to the network mains, via a disconnecting switch 47: the latter, in particular, has the task of uncoupling the photovoltaic branch when the network is enabled, and vice versa.

The operation of the above described apparatus is managed by a controller 30, comprising, as known per se, a microprocessor provided with data and program memory aa well as with appropriate interfaces adapted to perform the functions described below.

Controller 30 receives signals from a number of sensors, as follows:

- a voltmeter 32 supplies controller 30 with a signal V_RE_TΈ representing the voltage available from network 12; - a voltmeter 34 and an ammeter 35, as well as a wattmeter 36, deliver to the controller 30 respective signals Vout_SOL» ¹ _SOL ^and ^Poutso_L> representing the voltage, the current and the power delivered by the photovoltaic panels 18, respectively. More specially, the controller derives from V_Out_SOL> based on the loss and conversion parameters at the maximum load current value I, the AC voltage V_S0L of the array, from which P_S0L is derivable;

- a voltmeter 38 and an ammeter 39, as well as a wattmeter 40, deliver to the controller 30 respective signals V_CAR, I_CAR e P_CAR, representing the alternate voltage applied to load 10, the current and the power drawn by the load, respec- tively;

- a voltmeter 45, connected to a single solar cell 19, delivers to the controller the instantaneous V_Ouτ generated by the single loadless cell. The controller, based on the loss and conversion parameters at the maximum load current value I, derives from this value the AC voltage V_SOL of the array;

- a voltmeter 46, connected to the output of the second inverter, provides the value V"out_SOL. More particularly, the controller, based on the loss and conversion parameters of the circuit for the effective current value I instantaneously drawn by the load, derives the AC voltage V"_S0L of the loadless cell.

Moreover, the controller generates command signals, which are applied to dis- connecting switches 16, 47 and to converter 26, respectively, via multifilar connections diagrammatically shown as dotted lines 42, 49, 44, in order to drive the operation of the respective device, as further described below.

Having now reference to Fig. 2, the static disconnecting switch 16 is preferably implemented as four solid-state switches, e.g. of type IGBT, arranged in two anti-parallel pairs 46, 48, which are respectively in sieries in the two-wire connection 14. The gates of the four IGBT switches are driven by controller 30 via connection 42, for enabling or disabling the four IGBT switches so that the network mains and the load are isolated or stably connected with each other, with a switching time of a few milliseconds from one to the other condition, as known to a person skilled in the art.

Static disconnecting switch 47, similarly to static disconnecting switch 16, has an arrangement similar to Fig. 2, but the gates of the four IGBTs are here driven by controller 30 through connection 49 to enable or disable the four IGBTs so that the solar source and the load are isolated or connected to each other with a switching time of a few milliseconds between the two conditions, as known to a person skilled in the art.

Fig. 3 shows a preferred circuit diagram for DC/ AC converter 26. A bridge of four IGBTs 50, driven by rectifier 24, is connected on its output side to static disconnecting switch 47 which in turn is connected to the load 10 through bifilar connection 28, in parallel with connection 14. The four IGBTs 50 are provided with respective parallel-connected, free-wheeling diodes 52, and their gates are controlled through connection 44. Again, controller 30 can stably disable or enable DC/ AC converter 26 and static disconnecting switch 47, in order to isolate or connect, respectively, the upstream cascade with respect to load 10

(and consequently with respect to network mains 12). More particularly, switch

30 enables the IGBTs of DC/ AD converter 26 in a programmed sequence, with on times and off times suitably chosen in order to transform the DC current into width-modulated impulses (PWM) with appropriately alternated polarities, and therefore to create a sinusoidal wave of AC current at 230 V, 50 Hz, according to an approach that will be obvious for a person skilled in the art.

Each of both blocks of static devices 47 and 16 of Fig. 2 is therefore able, under command from controller 30, to completely isolate the load from the respctive upstream generator (be it the network or the solar source), within a time interval that can be kept at 2 msec or less. Vice versa, each of both blocks of the above static devices is also able to establish connection within the same extremely short time of 2 msec.

The controller directs the supply to the solar source or to the network depending on a set of measurements that are made moment by moment. If the state when the controller is dorected to the network is labeled as state A and the state when the controller is directed to the panels (see Fig. 4) is labeled as state B, Figs. 5 and 6 show Machine Cycles 1 e 2, respectively, which are cyclically repeated in order to perform a continuous monitoring of the parameters playing a role in determining the switching to either of both states. At each cycle iteration, controller 30 measures the instantaneous values of V_S0L, V_S0L, V"_S0L, V_RETE, P_S0L, V_CAR and P_CAR. The controller further has a sinusoidal voltage reference V_RIF> ^wnose amplitude and frequency correspond to the ideal AC voltage which should be applied to the load, i.e. a voltage equal to V2 • 230 sin (2π • 50 • f)-

For Cycle 1 in Fig. 5, where the switch is directed to the network (State A), tests are performed to check whether the following conditions are true:

- V_S0Lis in tolerance with respect to V_RIF ;

- P_S0L is in tolerance with respect to the parameters of the inverter; the purpose is to prevent the inverter, in case the current is too strong flow voltage) from blowing out in the attempt to reach the required peak voltage;

' ^_SOL ^> P_CAR> i-^e- the power from the solar panels is larger than the power drawn by the load;

- V_S0L is in tolerance with respect to V_RIF .

For Cycle 2 in Fig. 6, where the switch is directed to the panels (State B), tests are performed to check whether the following conditions are true:

- V'_SO_L is in tolerance with respect to V_RIF ;

- V_RE_TJ. is in tolerance with respect to V_m? .

More particularly, as far as the tolerance is concerned, the system operates so that, when the value of the voltage considered (sinewave A in Fig. 7) is outside the rectangular area (representing the frequency and the amplitude tolerances) of reference voltage V_mF (sinewave B in Fig. 7), the switch is kept unchanged. As soon as the above conditions are no longer met, the switch is operated. As stated above, the switchovers take place in such a short transition time (< 2 msec) that they do not interrupt or disturb practically any of the usual home or commercial appliances. As known to the person skilled in the art, a supply interruption of a few milliseconds is shorter than the time constants due to the internal electrical capacities of the supply units, or, in the case of resistive loads such as lamps, is below their thermic inertia. Consequently, the switchovers made by the system according to the invention are tolerated by the appliances in common use without any disturbance to their operation.

It can therefore be seen that the invention provides an auxiliary supply system based on photovoltaic solar panels, which can be used without a need for approval by the public administrator, while maintaining operational continuity of the appliances during changeover between the network and the solar panels.

Although the system does not allow the user to sell the excess photovoltaic power generated, the lower cost of initial installation largely offsets this short- coming.

In the above described system, the power generated by the solar panels is lost if it is not sufficient for the requirements of the load. A second embodiment of the invention, shown in Fig. 8, has an improved efficiency, by supplementing the system with the power stored in a battery when the power delivered by the solar panels is insufficient.

The diagram of Fig. 8 is largely identical to the diagram of Fig. 1, and like parts bear the same reference numbers. The system of Fig. 8, however, also includes a battery 60, connected across the set of solar panels 18, in series with a battery charger 62 and with a switching unit 64, controlled by controller 30 through a command line 66.

Moreover, the current flowing into battery 60 is measured by an ammeter 68, the voltage across battery 60 is measured by a voltmeter 70, and the current and voltage signals are multiplied together in a wattmeter 72, so that the controller is provided with respective signals V_0UtBATr and P_BATr. These are used, together with the other voltage and power signals considered above with refer-

RECTlFlED SHEET (RULE 91) ISA/EP ence to the embodiment of Fig. i, for executing the machine cycle of Fig. 9. More particularly, the controller, based on the loss and conversion parameters of the circuit for the maximum value of the load current I, uses V_0Ut_BATr to derive the AC voltage V_BATT of the array, from which P_BATT is then derived.

Depending on whether the controller is in the A o B state, as shown on Figs. 10 and 11, the controller of the system of Fig. 8 executes all the operations of machine cycles 1 and 2 of Figs. 10 and 11, but also monitors the data V_BATT and P_BAΓΓ for determining when switching unit 64 should be enabled in order to feed battery 60. In practice, it can be seen that the battery is charged whenever the load requires no power (no load in operation) and whenever P_S0L < P_CAR, i.e. the power delivered by the solar panels is lower than the power required by the load (so that the load is supplied from the network, leaving the solar panels free).

A detailed description of the programming of controller 30 is omitted, both for the first and the second embodiments, because such programming will be easy for a person skilled in the art, based on the above disclosure and on the flowcharts of Figs. 5, 6, 10 and 11. A person skilled in the art will readily understand how the controller can be designed to have the necessary quickness of action to insure that, at the time of changeover, the network is not be connected with the system even momentarily, and, at the same time, that the consumer is not sub- jected to power interruptions longer than the tolerated time of a few milliseconds.

In conclusion, it can be seen from the above disclosure that the system of the invention does in fact achieve the objects set forth in the introduction, i.e. to be installable without parallel with the public network (operation as island), with switchover times so short (in the order of 2 msec) that they do not influence the operation of any appliances, even the most sensitive (such as, e.g., electronic apparatus). Moreover, by using a high-frequency transformer and an appropriate turns ratio, for any given power the current is reduced, together with any attendant problems of large losses.

Claims

l. A photovoltaic generator system comprising:

- first static AC/ AC disconnecting switch means (16), connectable between an AC current supply network mains (12) and a load (10), and electrically controllable to connect or isolate the load from the mains;

- DC voltage booster means (20, 22, 24) connectable to the output terminals of a set of solar panels (18) to raise the level of the DC voltage generated by the latter;

- first static DC/ AC converter means (26), connectable to the voltage booster means (20, 22, 24) for generating an AC voltage at 50HZ;

- second static AC/ AC disconnecting switch means (47) connectable between the output of the first static DC/ AC converter means (26) and the load (10), and electrically controllable to connect or isolate the load from the first static DC/ AC converter means (26);

- a controller (30) having inputs for receiving voltage, current and power signals from respective sensors (32, 34, 35, 36, 38, 39, 40, 45) associated with the network mains, with the solar panels (18), and with a single loadless cell (19) and having outputs (42, 44, 49) connected to respective command inputs of said first and second static AC/ AC disconnecting switch means (16, 47) and said first static DC/AC converter means (26), the controller being programmed to iteratively execute a control loop in which the following conditions respectively take place:

- said first static AC/ AC disconnecting switch means (16) are disabled, and said first static DC/ AC converter means (26) and second static AC/ AC disconnecting switch means (47) are driven to generate said AC output voltage if the voltage across the solar panels (V_S0L) is larger than the voltage applied to the load (V_CAR) and differs from a predetermined reference voltage (V_RJP) no more than a predetermined tolerance interval, and if the power delivered by the solar panels (P_S0L) is larger than the power drawn by the load (P_CAR);

- said first static AC/ AC disconnecting switch means (16) are enabled and said static converter means (20, 26, 24) and second static disconnecting switch means (47) are disabled, whereby the network is enabled.

2. The photovoltaic generator system of claim 1, characterized in that said DC voltage booster means (20, 22, 24) comprise second static DC/ AC converter means (20) capable of generating a high-frequency voltage, a voltage-raising transformer (22) and static AC/DC converter means (24), connected in cascade between the solar panels (18) and said static DC/ AC converter means (26).

3. The photovoltaic generator system of claim 1, characterized in that it further comprises the series arrangement of a battery (60), a battery charger (62) and an interface (64) connected across the solar panels, and in that said controller (30) has inputs for receiving voltage and power signals from respective sensors (68, 70, 72) associated with the battery and is programmed to control said interface (64) for enabling the charging of the battery when the power required by the load (PQ_AR) i^{s zer}°_> ^an<i f°^r cutting off said static AC/ AC disconnecting switch means (16) and enabling said first static DC/ AC converter means (26) and said second static AC/ AC disconnecting switch means (47) whenever the power required by the load (P_QAR) i^s lower than the power delivered by the solar panels (P_SOL) ^OΓ th^e power delivered by the battery (P_BATT).

4. The photovoltaic generator system of claim 3, characterized in that said controller (30) is further programmed for enabling the charging of the battery (60) when the power required by the load (P_CAR) is lower than the power delivered by the solar panels (P_SOL) or the power delivered by the battery (P_BATT).