WO2008095988A1

WO2008095988A1 - A grid-tie

Info

Publication number: WO2008095988A1
Application number: PCT/EP2008/051521
Authority: WO
Inventors: Mark Truswell
Original assignee: Semplice Energy Limited
Priority date: 2007-02-07
Filing date: 2008-02-07
Publication date: 2008-08-14
Also published as: GB0702351D0; GB2446432A

Abstract

A grid-tie for connecting the output of a wind generator to an inverter comprises an input for connection to the output of a wind generator (1), an output for connection to an inverter (6), and a back-up power supply (11) for, dependent on the voltage at the input of the grid-tie, selectively outputting a voltage for powering at least one control circuit of the inverter (6). The back-up supply can continue to power the control circuit(s) of the inverter (6) in a prolonged period of little or no wind so that the inverter does not switch off. The back-up power supply is preferably adapted to, dependent on the voltage at the input of the grid-tie, selectively supply the voltage to the input of the grid-tie. The back-up power supply is thus able to power the control circuit(s) of a wind generator (1) having its output connected to the input of the grid tie, so that the control circuit(s) of the wind generator do not switch off even in a prolonged period of little or no wind. Some types of wind generator are prone to stalling if their control circuits loose power, in which case they may be very hard to re-start. Ensuring that the control circuit(s) of the wind generator are continuously power eliminates this problem.

Description

A Grid-Tie

The present invention relates to a grid-tie, in particular to a grid-tie for use in feeding the electricity generated by a wind generator into a mains electrical supply.

There is currently much interest in generating electricity by "renewable" means. In particular, there is interest in generating electricity by means of a wind generator, which generates electricity from the kinetic energy of the wind at the location of the wind generator. Since a wind generator can only generate electricity when the wind speed exceeds some minimum value, one method of using a wind generator is to arrange for the electrical output of the wind turbine to charge a battery. Electrical items are powered by the battery, which is kept charged by the wind generator. The residual charge in the battery is used at times when, owing to low wind speed, the wind generator is not producing electricity. One example of a wind generator intended to operate in this way is the Airdolphin ZlOOO wind generator produced by Zephyr Corporation of Tokyo, Japan.

In some cases the electrical power generated by a wind generator can at times be greater than the power needs of the items powered by the wind generator. There is therefore interest in feeding excess electrical power generated by a wind generator or other power source into the mains electrical supply (or "grid"). Since a wind generator typically provides a DC electrical output whereas a main grid is typically an AC mains grid, it is known to provide the output electrical power of a wind generator to an inverter, which converts the DC output of the wind generator into an AC current suitable for feeding into the mains grid. The inverter is generally provided with a "grid-tie", which is an electrical circuit that senses the AC voltage characteristics of the target mains grid and matches the inverters output to these so it can push the electricity into the mains grid.

Feeding power from a wind generator into a mains grid can, however, give rise to a number of problems. One problem, which arises in the case of a wind generator intended to operate by charging a battery, is that the load characteristics of a mains grid are quite different from those of a battery charger. In the case of a wind generator intended to charge a battery, such as the Airdolphin ZlOOO wind generator, the internal electronic circuit(s) of the wind generator are arranged to provide an output to a battery charger, and are thus designed to operate with a substantially constant load equal to the electrical load created by a typical battery. However, if the output of the wind generator is connected to the mains grid by an inverter it is possible to have zero load.

The internal battery charging electronic circuits of the Airdolphin ZlOOO wind generator are designed to "understand" that a battery may become over-charged, potentially leading to damage to the battery, if the output voltage from the wind generator becomes too high. The generator's internal electronic circuits are therefore arranged to put the wind generator into "stall" mode if the output voltage exceeds a certain value, thereby stopping the blades and reducing or stopping the electrical output from the wind generator. If an Airdolphin ZlOOO wind generator is connected to the mains grid by a grid-tie and an inverter, the different load characteristics of the mains grid compared to a battery can lead to unnecessary interruptions in power generation - if there should be a situation of zero electrical load present on the generator's output, the output voltage will rise and the wind generator is put into its stall mode and stops generating electricity.

A further problem is that the utility grid-tie guidelines in the UK and elsewhere specify many parameters that must be satisfied before connection to the mains grid is allowed, and the commercially available grid-tie inverters that are used must be certified to this standard (G83 in the UK). One of the required features of these inverters is that they have to wait for a specific time (3 minutes in the UK) before they can start to output power to the mains grid. An inverter does not act as a load on the input side until it can connect to the grid on the output side so, during the period of three minutes before the inverter can connect to the mains grid, the wind generator does not see a load. As explained above, the Airdolphin ZlOOO wind generator's output voltage can "run-away" with no load (i.e. no battery or inverter) present on its output, and this "run-away" can occur during the period of three minutes before the inverter can connect to the mains grid. If run-away should occur, once the output voltage exceeds 32V the generator's internal electronic circuits will put the generator into stall mode (on the false assumption that the generator is supplying power to a battery that has become overcharged) and the generator will stop producing power.

A further problem is that both a wind generator and a grid-tie inverter require a DC supply voltage in order for their electronics to operate. If no battery is present (or the battery is flat) and the wind generator is not already producing electricity then there is no DC supply and both the wind generator's and the inverter's electronic circuits will switch off. Once the turbine starts to produce power again, it is necessary to wait for three minutes before the inverter will connect to the grid, to comply with the above- mentioned UK regulations. This leads to a low efficiency, since power generated in the three minute period cannot be fed to the mains grid and is lost

It also means that a very strong wind will be required to restart the wind generator, since the wind generator may have put itself into stall mode, effectively applying a brake to the turbine blades. In this instance the system may not start to provide electricity to the grid until there have been consistently high winds to allow the wind generator to provide power consistently for over three minutes without stalling and the inverter has been able to connect.

Furthermore, if the wind generator's and the inverter's electronic circuits switch off owing to lack of a DC supply voltage, data communications from the wind generator and inverter are interrupted, since the wind generator and inverter will have switched off and will not be producing data.

Although the above problems have been described with specific reference to an Airdolphin ZlOOO wind generator, they apply equally to any wind generator whose internal electronic circuits are arranged to provide an output suitable for charging a battery. The present invention provides a grid-tie for connecting the output of a wind generator to an inverter, the grid-tie comprising: an input for connection to the output of a wind generator; an output for connection to an inverter; and a back-up power supply for, dependent on the voltage at the input of the grid-tie, selectively outputting a voltage for powering at least one control circuit of the inverter.

In a conventional grid-tie a prolonged period of no wind leads to the inverter's electronic circuits switching off, as explained above, thus incurring the prescribed regulatory delay of three minutes when the wind-generator restarts generation before power can be fed into the mains grid. In a grid-tie of the invention, however, the backup supply can continue to power the control circuit(s) of an inverter connected to the output of the grid tie, so that the inverter does not switch off even in a prolonged period of little or no wind and the prescribed regulatory delay of three minutes is avoided.

The back-up power supply is preferably adapted to, dependent on the voltage at the input of the grid-tie, selectively supply the voltage to the input of the grid-tie. In general, the control circuit(s) of a wind generator are powered by a voltage derived from the output of the wind generator. When the wind generator is producing little or no electricity the back-up power supply is able to power the control circuit(s) of the wind generator (which, in use will have its output connected to the input of the grid tie), so that the control circuit(s) of the wind generator do not switch off even in a prolonged period of little or no wind. The grid-tie can thus provide a constant source of DC supply to the control circuits of the wind generator and the inverter, whatever the duration of a period of little or no wind.

The grid-tie preferably further comprises: a battery selectively connectable to the input of the grid tie; and control means responsive to the voltage at the input of the grid-tie for either connecting the battery to the input of the grid tie and disabling the back-up power supply or isolating the battery from the input of the grid tie and enabling the back-up power supply to supply the voltage. When the wind generator outputs electrical power, some of the electrical power charges up the battery. When the wind drops and the power output from the wind generator falls, the power stored in the battery is initially used to provide an output to the mains grid. In order to prevent exhaustion of the battery in a prolonged still period, the control means is arranged to disconnect the battery from the input of the grid tie when the voltage at the input of the grid tie falls below a pre-set value.

The invention also provides the combination of a grid-tie as defined above and an inverter coupled to the output of the grid-tie, and further comprising means for, when the back-up power supply is enabled, disabling the inverter from providing an AC output.

Other features of the invention are set out in the other dependent claims.

Preferred features of the invention will now be described with reference to the accompanying figure, in which:

Figure 1 is a block circuit diagram showing a grid tie according to one embodiment of the present invention.

Figure 1 shows a grid-tie of the invention connecting a wind generator 1 to an AC mains grid line 2. The wind generator 1 is of the type intended to charge a battery, and includes a generator 3 for generating electricity from the kinetic energy of the wind, a battery charging circuit 4 for producing an electrical output having a current and voltage suitable for charging a battery, and a controller 5 for controlling the operation of the wind generator.

The wind generator 1 may be, for example, an Airdolphin ZlOOO wind generator. The output voltage of the Airdolphin ZlOOO wind generator is between 24V DC and 32V DC (the Airdolphin ZlOOO wind generator is arranged to stall if the output voltage exceeds 32V to prevent damage to a battery being charged, although greater output voltages up to 60V can occur for a few seconds). In order to feed the output of the wind generator 1 into the mains grid 2, the DC electrical output of the wind generator 1 must be converted to a suitable voltage and AC frequency for connection to the mains grid 2. This conversion is effected by an inverter 6 coupled between the output of the wind generator 1 and the mains grid 2. In the case of connection of an Airdolphin ZlOOO wind generator to the UK mains grid, the choice of suitable commercially available inverters is currently limited to only two (and currently only one of these is approved for government grants in the UK).

The grid-tie of the invention comprises a buffer battery 8 that is selectively connectable, dependent on the voltage at the output of the wind generator 1, to a link 7 that connects the output of the wind generator 1 to the input of the inverter 6. In the embodiment of figure 1 the selective connection of the buffer battery is effected by a first switch 9 that is controlled by the output of a voltage sensing means 10 that senses the voltage at the output of the wind generator 1. The switch 9 may for example be a DC relay and the voltage sensing means 10 may for example be a voltage sensing relay. (The relay may be implemented as a mechanical relay, as an IC circuit, or as a combination of an IC circuit and a mechanical relay.)

In principle it would be possible to use just use a very large capacity battery and technically achieve grid-tie through an inverter. However, such a solution would fail during periods of extended wind absence, and could potentially fail at other times dependant upon the configuration of the grid-tie inverter.

The grid-tie of the invention thus comprises, in addition to the buffer battery 8, a backup power supply 11 for, when the wind generator 1 is generating little or no electrical power, producing a DC voltage that may be used to power the generator controller 5 and the control circuit of the inverter 6. The back-up power supply 11 is selectively enabled in dependence on the output voltage generated by the wind generator, and is enabled if the output voltage generated by the wind generator becomes low. The back-up power supply 11 is preferably powered from the mains grid, and in the embodiment of figure 1 the back-up power supply 11 is enabled by means of a second switch 12 placed between the mains grid 2 and the input to the back-up power supply and that is controlled by the voltage sensing means 10 - so that, when the second switch is closed to connect the input to the back-up power supply to the mains grid 2, the back-up power supply generates an output DC voltage. In the embodiment of figure 1 the back-up power supply 11 is a mains-powered transformer. (The transformer 11 is provided with smoothing/rectifying circuits (not shown) to obtain a DC output.) The second switch 12 may be for example an AC relay.

The voltage sensing means 10 is arranged to close the first switch 9 and to open the second switch 12 if the output voltage from the wind generator exceeds a pre-set threshold voltage, so that the buffer battery is connected to the link 7 and the back-up power supply 11 is disabled. If the output voltage from the wind generator falls below the pre-set threshold voltage, the voltage sensing means 10 is arranged to open the first switch 10 and to close the second switch 12, so that the buffer battery is disconnected from the link 7 and the back-up power supply 11 is enabled. The threshold voltage will depend on the normal output voltage range of the wind generator 1, and is preferably set at or near the lower end of the normal output voltage range of the wind generator 1 ; in the case of an Airdolphin ZlOOO wind generator for which the normal output voltage range is 24V - 32V, a suitable value for the threshold voltage would be 24V.

Operation of the grid-tie of figure 1 is as follows:

When the wind generator is generating electrical power, the output voltage produced by the wind generator is greater than the threshold voltage of the voltage sensing relay 10, so that the first switch 9 is closed to connect the buffer battery to the link 7 and the second switch 12 is open to disable the back-up transformer 11. The output power from the wind generator 1 is partly fed to the mains grid 2 via the inverter 6 and is partly fed to the buffer battery 8. In the specific example of an Airdolphin ZlOOO wind generator and an SMA 110OL V WindyBoy inverter, the Airdolphin ZlOOO wind generator can generate up to 3.2 kW of electricity whereas the SMA inverter can only convert 1.1 kW. The buffer battery 8 absorbs the part of the wind generator output that exceeds the capacity of the inverter to convert to mains supply. If the wind falls and the power output of the wind generator 1 falls below the capacity of the inverter 6, excess current that was stored in the buffer battery 8 is released. The voltage at the link 7 is thus initially maintained above the threshold voltage of the voltage sensing relay 10 by the buffer battery. The voltage supplied by the battery 8 powers the control circuit(s) of the inverter 6 and the generator controller 5, so that the inverter 6 and the wind generator 1 do not switch off.

In principle, it would be possible for the inverter 6 continues to supply an AC current to the mains grid 2 from the voltage supplied by the battery 8. It may however be preferable the inverter 6 does not completely drain the buffer battery in the absence of wind, to allow the buffer battery to keep the control circuit(s) of the inverter 6 and the generator controller 5 "live" for as long as possible using just the stored "green" energy in the buffer battery 8 (by "green energy" is mean electricity provided by the wind generator from the wind rather than electricity from the utility mains supply 2.) The inverter 6 may thus be configured not to provide an AC output to the mains grid when the power output of the wind generator 1 falls below the capacity of the inverter 6.

In the case of a prolonged still period, the battery will eventually become unable to maintain the voltage at the link 7 above the threshold voltage of the voltage-sensing relay 10. When the voltage falls below the threshold voltage of the voltage-sensing relay 10, the voltage sensing relay 10 opens the first switch to disconnect the buffer battery from the link 7, thereby preventing the buffer battery from completely discharging.

Further, when the voltage falls below the threshold voltage of the voltage sensing relay 10, the voltage sensing relay 10 closes the second switch 12 to enable the back-up transformer 11. The back-up transformer 11 starts to supply a DC voltage to the input of the inverter 6 and to the output of the wind generator 1 via the link 7. This DC voltage can power the generator controller 5 and the control circuit of the inverter 6 (typically, the power for a control circuit of an inverter is derived from the input voltage to the inverter, and the power for a control circuit of a wind generator is derived from the output power produced by the wind generator). According to the invention, therefore, the wind generator 1 and the inverter 6 do not shut down, even in prolonged periods of no wind. The generator controller 5 and the control circuit of the inverter 6 continue to receive power (assuming there is no interruption in the mains supply) regardless of the duration of the period of no wind. The inverter 6 remains in a quiescent state (in which it does not generate an ac output), and the wind generator 1 remains in a standby mode. Since the generator controller 5 and the control circuit of the inverter 6 continue to be powered, any data signals that the wind generator 1 and/or inverter 6 are arranged to send will continue to be sent.

It will be appreciated that the DC voltage supplied by the back-up transformer should be less than the threshold voltage of the voltage-sensing relay 10, to prevent the DC voltage supplied by the back-up transformer from switching the voltage sensing relay. In an embodiment in which the threshold voltage of the voltage-sensing relay 10 is 24V, a suitable voltage supplied by the back-up transformer would be, for example, 23V.

It will also be appreciated that the inverter 6 is preferably configured so that it does not try to generate an AC voltage from the DC voltage supplied by the back-up transformer.

When the wind starts to blow again and the wind generator 1 again generates electrical power, the voltage-sensing relay 10 will switch once the output voltage from the wind generator 1 reaches the threshold voltage. When this happens the first switch 9 closes to connect the buffer battery to the link 7 and the second switch 12 opens to disable the back-up transformer 11.

Furthermore, when the wind starts to blow again and the wind generator 1 again generates electrical power, the fact that inverter 6 has not switched off because the inverter's control circuits have been powered continuously means that the regulatory requirement for a delay (of three minutes in the UK) before the inverter 6 can supply power to the mains grid does not apply. The inverter may supply power to the mains grid as soon as it has powered up from its quiescent state (which typically takes only a few seconds). The capacity of the buffer battery 8 is determined by two conflicting requirements. If the capacity is too large the battery will take priority over the inverter as a load. However, if the capacity is too small the battery will not last long during calm spells and the backup transformer will be activating more frequently.

One embodiment of the invention suitable for use with the Airdolphin ZlOOO wind generator has the following details:

Wind generator 1 - Airdolphin ZlOOO wind generator; Inverter 6 - SMA 1 IOOLV WindyBoy Inverter; SMA inverter configuration parameters: o Set to KONST mode (the inverter adjusts its load characteristics to try to keep the input voltage at a constant value up to its maximum potential input of 1.IkW); o Target DC input constant voltage set to 24v DC; o Grid-tie start-up input voltage set to 24v DC; Capacity of buffer battery: between 5 kWh and 55 kWh; Threshold voltage of voltage sensing relay 10: 24 V; First switch 9 - 30 amp DC switching relay; Second switch 12 - 10 amp AC switching relay; Back-up power supply - Regulated transformer to provide a low current output at a substantially constant voltage of 23V DC.

Claims

CLAIMS:

1. A grid-tie for connecting the output of a wind generator to an inverter, the grid- tie comprising: an input for connection to the output of a wind generator; an output for connection to an inverter; and a back-up power supply for, dependent on the voltage at the input of the grid-tie, selectively outputting a voltage for powering at least one control circuit of the inverter.

2. A grid-tie as claimed in claim 1 wherein the back-up power supply is adapted to, dependent on the voltage at the input of the grid-tie, selectively supplying the voltage to the input of the grid-tie.

3. A grid-tie as claimed in claim 1 or 2 and further comprising: a battery selectively connectable to the input of the grid tie; and control means responsive to the voltage at the input of the grid-tie for either connecting the battery to the input of the grid tie and disabling the back-up power supply or isolating the battery from the input of the grid tie and enabling the back-up power supply to supply the voltage.

4. A grid-tie as claimed in claim 3 wherein the control means comprises a voltage- sensing device for sensing the voltage at the input of the grid-tie.

5. A grid-tie as claimed in claim 3 and comprising a first switch for selectively connecting the battery to the input of the grid tie.

6. A grid-tie as claimed in claim 5 when dependent from claim 4 wherein the first switch is controlled by the voltage-sensing device.

7. A grid-tie as claimed in claim 3, 4, 5 or 6 wherein the back-up power supply is selectively enableable by a second switch.

8. A grid-tie as claimed in claim 7 when dependent from claim 4 wherein the second switch is controlled by the voltage sensing device.

9. The combination of a grid-tie as claimed in any preceding claim and an inverter coupled to the output of the grid-tie, and further comprising means for, when the backup power supply is enabled, disabling the inverter from providing an output to the mains grid.

10. The combination of a grid-tie as claimed in any preceding claim and an inverter coupled to the output of the grid-tie, wherein the inverter is configured to, when the back-up power supply is enabled, provide no AC output.