GB2492342A

GB2492342A - Power converting apparatus connecting AC source and DC source with load.

Info

Publication number: GB2492342A
Application number: GB1110932.9A
Authority: GB
Inventors: Fei Kong
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-06-27
Filing date: 2011-06-27
Publication date: 2013-01-02
Also published as: GB201110932D0

Abstract

The present invention provides a series-connected inverter (230) for connection of a load to one or more power supplies. In one arrangement the present invention provides an electrical connection apparatus for connecting a DC power source (10) and an AC power source (240) to a load arranged to operate on AC power, the apparatus including an inverter connected in series between said power sources and said load. In some embodiments DC source compensates AC source, if AC power is zero only DC source supplies load. The AC power may be converted to DC by a converter (260) acting to buck AC voltage. Four switches are included in the buck converter arranged to supply AC voltage in phase with that of the AC source. The AC source may be the grid and the DC source may be a photovoltaic array.

Description

ELECTRICAL CONNECTION APPARATUS

Field of the Invention

The present invention relates to electrical connection apparatuses, which are commonly referred to as inverters, although they may contain additional components as well as a true inverter. It is particularly, but not exclusively concerned with electrical circuits for use in coupling photo-voltaic power sources to loads either in conjunction with, or separate from, an AC grid supply.

Background of the Invention

During time of peak demand, the voltage of a grid can be 10% or more lower than the standard voltage (called voltage sag). This is particularly true in countries where the grid supply is not well established, monitored or supported. In the case of such voltage sag, local loads connected to the grid do not work properly, and if the sag is too great (typically over is 10%) the local loads may be damaged.

The existing solution to this problem is to use an AC voltage regulator to make the grid voltage stable. One design of such a regulator is discussed in MR. Ahmed and M. Alam, Design of a switch mode AC voltage regulator with improved performance," Power Electronics, 2006. I!CPE 2006. India International Conference on, IEEE, 2006, p. 203-207.

This solution takes the voltage from the grid and adds the voltage from the PV panel in order to step up the voltage prior to supply to the load(s). However, this solution decreases the efficiency of the supply from the grid. Typically such regulators have a maximum efficiency of only 65%.

This decreased overall efficiency has implications for the user of the electricity as their biil will increase due to the greater power required to power a set load. Furthermore, this decreased efficiency will have a knock-on effect on the grid itself as, for a set overall load on the grid, the inefficiencies caused by the use of the regulators will create greater demand on the grid, which may in turn lead to further voltage sag.

The present invention aims to address the inefficiency problems associated with the existing approaches to dealing with voltage sag on a grid and to provide devices and methods which allow for improved connection to a grid and maintenance of the voltage required by a load.

Summary of the Invention

Accordingly, at its broadest, a first aspect of the present invention provides an electrical connection apparatus which includes a series-connected inverter.

A first aspect of the present invention preferably provides an electrical connection apparatus for connecting a DC power source and an AC power source to a load arranged to operate on AC power, the apparatus including: an inverter connected in series between said power sources and said load.

The connection of the inverter in series may allow the connection apparatus to provide a connection between the AC power source and the load which has improved efficiency in operation and in maintaining the voltage required by the load.

Typically the AC power source will be an electricity grid. The load may be any domestic or commercial user of electricity.

The electrical connection apparatus may be arranged to operate such that during certain periods of operation, either the DC power source or the AC power source are not connected to the load.

Further, the electrical connection apparatus may be arranged such that under certain conditions, the inverter is bypassed and the AC power source is connected directly to the load. The electrical connection apparatus may include a In one arrangement, the inverter is arranged to use the power from the DC power source to compensate the power from the AC power source to provide a substantially constant voltage to said load.

This can allow the apparatus to compensate for voltage sag in the AC power source whilst maintaining the efficiency of the apparatus. This can also allow the apparatus to compensate for voltage sag without placing further demands on the AC power source.

In some arrangements, the inverter is further arranged to upload energy from the DC power source to said AC power source. This can allow excess power from the DC source to be fed back into the AC supply.

In some arrangements the inverter is arranged to supply power to said load from said DC power source alone when the power supplied from said AC power source is zero. This allows the apparatus to operate in a "stand-alone" mode when, for example, the AC power supply is cut off.

Preferably the apparatus further includes a transformer arranged to isolate said AC power source from said DC power source. This prevents DC power being fed to the AC power source, and may also be used to prevent DC current/power from being fed to the load.

In a preferred arrangement, said DC power source includes a photo-voltaic source such as a solar panel. When the DC power source includes a photo-voltaic source, the apparatus may further include a maximum power point tracking ("MPPT") device connected between said DC power source and said inverter. The DC power source may alternatively or additionally include a back up battery.

The apparatus may further include apparatus for adjusting the voltage supplied from said AC power source prior to supply to said inverter. In a preferred arrangement, this apparatus for adjusting the voltage is an inverter arranged to operate as an AC buck converter. An AC buck converter is a device which steps up (or steps down) AC voltage in order to regulate an AC voltage output.

In one mode of operation, the inverter which is arranged to operate as an AC buck converter includes four switches which are controlled to adjust the voltage output of said inverter arranged to operate as an AC buck converter so that it is either in phase with or out of phase with said AC power source.

Said switches may be controlled to operate so that said voltage output is in phase with said AC power source when the voltage of said AC power source is below a predetermined level and to operate so that said voltage output is out of phase with said AC power source when the voltage of said AC power source is above a predetermined level.

In another arrangement, the apparatus further includes apparatus for converting power from said AC power source to DC power prior to supply to said inverter.

The electrical connection apparatus may include a controller which arranged to control the electrical connection apparatus and the various components thereof such that the electrical connection apparatus operates as set out above and which is arranged to switch the operation of the electrical connection apparatus between different modes of operation.

These modes of operation may include some or all of the above optional features, and the switching may include switching from an arrangement in which one of the above described optional or preferred features is operational (e.g. connected to the inverter) to an alternative arrangement in which a different set of said features (which may include some or all of the previously operational features) is operational.

The controller may have associated with it one or more inputs which allow the controller to determine current operating parameters of the connection apparatus and the controller may be arranged to determine the preferred mode of operation from the signals received by said inputs. The electrical connection apparatus may include one or more switches which are operated by the controller to change between modes of operation of the apparatus and which cause components within the apparatus to be connected or disconnected from one of the power supplies or from the load.

A second aspect of the present invention provides a power supply device having a DC power source, an electrical connection apparatus according to the above first aspect, including some, all or none of the optional or preferred features of that apparatus, and connectors for connecting an AC power source to said device.

Preferably the DC power source includes a photo-voltaic source such as a solar panel. The DC power source may alternatively or additionally include a battery.

Further aspects of the present invention include methods of operating an electricaL connection apparatus and a power supply device.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a circuit illustrating an electrical connection apparatus according to a first embodiment of the present invention; Figure 2 shows the switching operation of the circuit of Figure 1 in different modes of operation; Figure 3 shows the results of a simulation of the operation of the circuit of Figure 1; Figure 4 shows, in outline, a control system used with the circuit of Figure 1; s Figure 5 shows a circuit illustrating an electrical connection apparatus according to a second embodiment of the present invention; Figure 6 shows a detail of one part of the circuit of Figure 5; Figure 7 shows a circuit of an electrical connection apparatus according to the second embodiment of the present invention; io Figure 8 shows a circuit block diagram, with control functions, of an electrical connection apparatus according to a third embodiment of the present invention; Figure 9 shows an outline circuit configuration of the circuit of Figure 8 in one mode of operation; Figure 10 shows the control circuit for the circuit of Figure 8 in another mode of operation; and Figure 11 shows an outline circuit diagram of the circuit of Figure 8 in another mode of operation.

Figure 12 shows the parallel connected mode circuit of Figure 8 in another mode of operation

Detailed Description

A first embodiment of the present invention is shown in outline in Figure 1.

In the embodiment shown in Figure 1, a photo-voltaic (RPVI) panel 10 which is a DC power source is connected to a full bridge inverter 20. The full bridge inverter 20 can also perform the function of a maximum power point tracking device (MPPT) as known in the art.

A 50Hz line frequency transformer 30 is arranged to isolate the PV panel from the grid network 40 which is an AC power source. The transformer 30 can also stop any DC current injection to the load 50. In the transformer 30, the secondary:primary turns ratio is set as 1:1

B

as the transformer is being used for isolation purposes only. The output from the line frequency transformer 30 output is low pass filtered by a filter 35 with a cut off frequency of 1KHz. This shapes the output voltage from the inverter 20 to a pure sinusoid waveform.

In this embodiment, the output of the inverter 20 is used to compensate the voltage from the AC grid source 40 and is then supplied to the resistive load 50.

In one mode of operation, the inverter 20 of Figure 1 is operated under AC mode as an "AC buck converter". The use of an iriverter in this manner in the present embodiment is similar to the operation of DC buck converters, which is known in the art. However, in contrast to the known buck converters, the inverter of the present embodiment operates in an AC-AC io arrangement. Hence the term "AC buck converter' (or simply "buck converter") will be used to describe this mode of operation.

Figure 2 shows the four stages in the operation of the inverter 20 with an LCR load 51. It can be operated just by four switches Si-S4. The buck converter operates on the same basic H-bridge inverter 20 which can adapt between PV operation and AC operation. The output IS voltage can of the converter in this mode can vary from 0 to full grid voltage.

In the AC buck converter operation illustrated in Figure 2, there is no DC link stage, and consequently no DC capacitor is required. This results in a purer AC output devoid of any high frequency distortion and DC ripple problems.

Figure 2(a) shows the operation of the inverter as an AC mode buck converter in the case of voltage sag. Switches Si and 54 operate at high frequency, and switches S2 and S3 operate at low frequency. In this case, S3 is always off, and S2 is always on. Si and 54 are switched alternately between ON and OFF.

In the positive cycle of the AC source 40, when Si is on, the current goes through SI into the inductor L and passes through the primary winding of the transformer 30 to 52, and goes back to the source, as shown by the bold line in Figure 2(a)(i). When Si is off and S4 is on, the energy stored in the inductor Land primary winding of the transformer 30 will be discharged through 54 as shown by the bold line in Figure 2(a)(2).

In the negative cycle of the AC source 40, with Si on and S4 off, the current passes through 52, the primary winding of the transformer 30, and the inductor L, and through S4 back to the source, as shown in Figure 2(a)(3). When Si is off and S4 is an, the energy stored in primary winding of the transformer 30 and the inductor L will be discharged through S4 and 52, in the opposite direction to the current in the positive cycle as shown in Figure 2(a)(4).

The voltage across the LCR load 51 when the inverter 20 is operating as an AC buck converter is made up of the output voltage of the buck converter (the voltage across the secondary winding of the transformer 30) plus the grid AC source voltage.

Figure 2(b) shows the operation of the inverter 20 as an AC mode buck converter when operating in voltage swell mode. The difference with the operation described above in relation to Figure 2(a) is that 84 remains always ON and Si is always OFF whilst switches 53 and 82 operate at high frequency and are switched alternately ON and OFF.

The four stages of the switching cycle are shown in Figures 2(b)(1)-(4) starting with the positive cycle of the AC source 40 with S2 off and S3 on (Figure 2(b)(1)) and moving through positive cycle, 52 on, S3 off (Figure 2(b)(2)), negative cycle, S2 off, 53 on (Figure 2(b)(3)) and negative cycle, S2 on, 83 off (Figure 2(b)(4)).

The output voltage of the inverter 20 in this mode of operation will be out of phase with the load voltage in order to compensate it. Accordingly, the load voltage is equal to the grid AC voltage minus the buck converter output voltage because the voltage across the secondary winding is opposite to that of the AC source.

The output voltage of the inverter 20 can be calculated using the same equation as for a conventional buck converter, V = x D where D is the duty cycle of switch Si in voltage sag mode, and the duty cycle of switch 53 in swell mode.

Figure 3 shows the output waveform of the inverter 20 operating as an AC-AC buck converter. The first waveform shows the current through the local LCR load 51. The second waveform shows the variation of three different voltages with the AC buck converter phase change, the red waveform shows the grid voltage; the black waveform shows the load voltage (when t=0.25 s the converter changes the phase from in phase to out of phase with the grid, so the load voltage becomes smaller than the grid voltage); the blue waveform shows the AC buck converter output voltage (when t=0.25s and the phase of the converter has an 180° shift, with a setting time of 0.OOis).

Figure 3 demonstrates that the proposed method results in an output voltage that has low harmonics. The inverter 20 operates under voltage sag condition before t=0.25s and the load

B

voltage is equal to the inverter output voltage plus grid voltage. Under the grid voltage swell condition (after t=025s) the load voltage is equal to the grid voltage minus the inverter voltage, and the converter takes the reactive power from grid.

Figure 4 shows the inverter 20 with a line frequency isolating transformer 30 arranged between the grid 40 and the inverter. In the case of no power input from the PV panel 10, then the control system 100 will switch the inverter to AC mode and use the inverter as an AC buck converter to step down (or step up) the AC voltage from the grid 40. When operating as an AC buck converter, the circuit uses the common full bridge circuit of the inverter, but the control strategy changes from the sinusoidal pulse width modulation (SPWM) control to an AC buck converter control strategy, as described in more detail below.

When power input is detected from the PV panel 10, the inverter 20 automatically switches back to the PV mode.

When the grid voltage is at the rated value, the transformer primary winding will become a short circuit and the inverter 20 will enter by-pass mode in which the AC supply to the load will be completed supplied by the grid and by-pass the inverter 20 through the secondary winding of the transformer.

The operation of the inverter 20 of this embodiment can be compared to the known use of AC voltage regulators in similar circumstances. The AC voltage regulators normally use an AC to DC rectifier followed by a DC to AC inverter. In such regulators, power is lost in both stages. In the embodiment shown1 using an inverter 20 as an AC mode buck converter there is on a single-stage of conversion and losses can be reduced.

By using an inverter 20 as in the present embodiment operating as an AC buck converter can also reduce the total harmonic distortion (Tl-1D) of the output AC voltage. The THD of the AC mode buck converter is 4.93% by Matlab simulation. The traditional AC-DC-DC-AC converter THD is 5.3%.

High frequency iso/a ted inverter A second embodiment of the present invention is shown in Figure 5. In this embodiment the connection circuit is a high frequency isolated inverter, and has three parts: a buck converter arranged to carry out MPPT functions, a DC/DC converter 120 which includes a full bridge phase shift converter 122, and a DC/AC inverter 130. The DC/DC converter 120 uses a high frequency isolating transformer 125 to make the size of the circuit smaller and more efficient.

A filter 135 is attached to DC/AC inverter 130 in order to filter out the noise from the grid 140 and the AC inverter, as the grid is series connected with the AC inverter before the filter.

The MPPT system 110 is arranged to get the maximum power from the PV panel in the manner known in the art.

The phase shift full bridge DC/DC converter 120, using soft switching technology, can provide a high efficiency energy conversion.

The regular PV panel 10 output voltage at MPP (Maximum Power Point) is around 36 V and the maximum output current is around.3.5 A for a 125W single panel. To supply a 1KW load operating at 240V, the maximum load current is 1 KW/240V = 4.2A Therefore, in order to supply that maximum current, the circuit is designed to to step down the output voltage from the PV panel 10 to ensure the inverter 130 can supply enough current to the load 150. To achieve this the winding ratio of transformer 125 is 6:5. A boost converter is applied for MPPT function, as shown in Figure 5.

When the inverter is working under AC mode, the grid voltage Vac through the full bridge rectifier 122 is rectified to 340V DC. The DC voltage is then stepped down by a DC buck converter 124 as shown in Figure 6. The buck converter output voltage can then be connected in parallel with the MPPT system output as shown in Figure 6.

In the embodiment shown in Figure 7, the connection apparatus includes a high frequency isolating transformer 125. During night time operation, the inverter 130 switches to AC mode and the AC voltage from the grid 140 is rectified to 340V DC voltage after coming through the full bridge inverter 132.

During day time operation, the DC voltage from the PV array 10 is directly connected with the full bridge phase shift converter 122. Finally by using the same bridge inverter 130, the DC voltage is converted into an AC voltage and connected in series with the grid supply 140.

The operation of the connection apparatus according to this embodiment can be compared favourably with the AC voltage regulator disclosed in Ahmed & Alam reference referred to above. In contrast to the regulator of that reference, the connection apparatus of this embodiment can be operated as an AC buck converter which is connected in series between the grid network 140 and the local load 150.

In Ahmed & Alam a method is described in which the regulator is connected in parallel with the load, so the regulator has to take all of the load power rating. However, an AC buck converter connected in series as in the present embodiment, it just takes the power rating of the converter to compensate power.

Moreover, the connection apparatus of the present embodiment operating as an AC buck converter can generate opposite current as the input current, so in series mode it can compensate both voltage sag and swell for both condition. The Ahmed & Alam regulator only io has the same current direction as the input current.

DC inverter The inverters described in the above embodiments are series connected in AC circuits. The inverter of the third embodiment described below with reference to Figures 8 to 11 is a DC voltage series connected inverter which can achieve higher efficiency and provide additional functions. Moreover, in a DC series connected inverter, it is not necessary to worry about the phase angle of the various supplies.

In general terms, as shown in Figure 8, the AC voltage supply 240 is passed through a rectifier, then a Power Factor Corrector (PFC) 260 and, after the PFC, an isolated DC to DC converter 270. The voltage from PV panel 10 will pass through a MPPT system 280 to provide the maximum power point tracking functionality. The voltage output from the MPPT system is connected in series with the voltage coming from DC/DC converter 270. The combined voltage is input to the single-phase inverter 230 which converts the voltage from DC to AC. In the output of the inverter 230 is connected to the local load 250, and the grid supply 240.

The connection apparatus and the inverter of the third embodiment has three different working modes: the "series connected mode"; the "parallel connected mode"; and the "stand-alone mode". The following describes the operation of the apparatus in each of these different modes and how the circuit/controller may switch from each mode to another one.

Series connected mode In the series connected mode, the DC voltage generated from the PV panel 10 is connected in series with the DC output voltage from the PFC 260. The power to load 250 is provided in two parts. The first is from the grid 240 via PFC 260 and the DC/DC isolating converter 270 which includes a phase shift full bridge (PSFB) converter 275. The second is from the PV panel 10. Figure 9 shows a simplified circuit diagram of this arrangement. The inverter 230 is connected in series between the grid 240 and the load 250 so that the inverter DC current is equal to PFC DC output current, and the percentage of power consumed is equal to the ration of DC output voltage from each of these sources. In this embodiment, the DC bus voltage is limited to 500V, and the PV panel maximum output current is BA, so the maximum load power is 4KW. The DC voltage supplied from the MPPT system is varied depending on the power demands of the load.

In order to stablise the DC link voltage, the controller is arranged to adjust the DC/DC isolating converter output voltage. In the circuit shown in Figure 9, the PID controller 202 is used to adjust the duty cycle of the PSFB converter 275. If the MPPT output voltage is increased, the RID controller 202 reduces the voltage of the PSEB converter 275 to keep the DC link voltage constant.

Conversely, when there is no sun shining on the PV array 10, the MPPT output voltage will become zero. The current will bypass the PV array 10 through the body diode of the MOSFET 204 and the diode D in the MPPT boost converter 280. The PSFB output voltage is directed to the inverter 230, to keep the local load voltage constant.

In this arrangement, if the load consumes a power higher than the maximum power output from PV panel 10, the rest of the output power will be provided from the grid 250. For example, if the maximum power output from the PV panel is 200W and the load power is 600W, the remaining 400W of power is provided from the grid.

A minimum level of power consumed by the local load in such an arrangement is 400W. At 400W consumption the MPPT system output voltage is typically 250V, which is ten times the MPPT input voltage (it is known that the efficiency of a continuous boost converter will drop dramatically if the voltage ratio is greater than ten).

The maximum voltage of the DC bus is 500V, and the maximum power output from the solar panel is 200W, so when load is consuming 400W, 200W will be supplied from the grid and the other 200W will be supplied from the PV panel. If the.output voltage from PV panel is required to be more than 250V the efficiency of the system will be lower as indicated above.

If the circuit is arranged to power a household, the minimum level of 400W is normally present as the combined power requirements of a fridge, freezer, television and computer will amount to more than 400W.

However, in the case that the power consumed by the load is less than 400W, the inverter will switch to the parallel connected mode (as described below) in order to upload all the power to the gird. The load voltage will be regulated by the PFC 260 and PSFB converter circuit 275.

Parallel connected mode If the local load consumes less power than the minimum power required for the system operation, the controller is arranged to switch the inverter into the parallel connected mode.

is In this mode, the circuit works as a standard parallel connected inverter. The control circuit diagram for the operation in the parallel connected mode is shown in Figure 10. Figure 12 shows a simplified circuit diagram of this arrangement.

The FV panel output voltage is generally around 25 V at the maximum power point, and the minimum DC link voltage at grid side inverter is 424V with a modulation ratio of 0.8. The modulation ration 0.8 is the amplitude of the sine wave signal compared with amplitude of the triangle waveform for SPWM modulation. In the parallel connected mode, the circuit comprises two boost converters which are connected in cascade to achieve the voltage amplitude ratio. One of these cascade connected boost converters is operated to provide the MPPT function and the other is used to boost the voltage. In series connected mode only one boost converter 280 was operational, which provided the MPPT function.

In the control circuit shown in Figure 10, the inner loop control 101 is a current control loop, and the outer loop is a voltage control loop 102. The inner loop control 101 is arranged to limit the power injected into the grid 240, and the RMS current into the grid is set by the maximum power from the grid divided by the grid voltage 240V.

The outer loop 102 operates to control the voltage, and specifically to set the DC link voltage to 450V. The current shape control is provided by the second inner loop. The error signal comes into the P1 controller then the correction signal goes into the SPWM generator to control the inverter output.

Stand alone mode If there is no power from the grid, the inverter disconnects from grid and works in stand-atone mode.

Figure 11 shows the circuit structure of the circuit working stand-alone mode. This circuit uses the parallel connection MPPT method. In this mode the circuit works as a voltage io source inverter, and is effectively the same as the operation of the inverter working under the series connected mode described above.

The output voltage of the inverter is set at 240V. In this system two MPPT control systems are provided: MPPT 1 and MPPT2. In the parallel connected mode, MPPT2 is enabled and MPPTI is disabled whereas in the stand-alone mode the MPPT 1 is enabled, and MPPT2 works just like an normal boost converter to boost the output of the PV panel 10 up the DC link voltage.

Night Time Mode During the night time or other intervals in which no power comes from the PV panel, the circuit will switch to night time mode. In this mode the power comes from the grid via the PFC 260 and DC/DC converter system 270. In the night time if the grid voltage is not stable then the circuit will try to compensate the load voltage, in order to keep the voltage constant, as well as maintaining the load at a high power factor.

If the grid voltage is at the standard level (e.g. 240V in the UK), then the inverter will go into bypass mode so that the load is directly connected with the grid. This means that the inverter is not active and so no power losses arise from the operation of the inverter.

The above embodiment illustrates a circuit which provides a DC voltage series connected photovoltaic inverter. This circuit utilizes an inverter circuit with a DC voltage connected in series to achieve extra functions than the previously described series connected inverters.

The circuit uses a PFC circuit for power factor correction, and a DC/DC converter for isolation from the grid with the load.

The control system for the whole circuit of this embodiment is shown in Figure 8. The control system 100 (or an operator of the control system) can configure the circuit as follows: 1) In the series connected mode, the series connection is the inverter connected in series between the grid 240 and the load 250. In this mode the switch 33 operates in MPPT mode, SI, S2, S4 are off, and S5-S8 are operated in SPWM mode. Grid switch Sg is on. The controller will also switch to the voltage control mode.

2) When the load power is less than the system minimum power (400W), the load power calculation system will send a control signal to current/voltage control block 1010 and the controller will switch to parallel connected mode. This will turn on the relay 1020 to connect the inverter in parallel with the grid 240. At the same time the grid will disconnect with the PFC system 260, the PV panel output power will come through the MPPT system and boost converter, the current will pass through the diode D3, Dl, to the full bridge inverter. If the load power calculation system detects that the load power is higher than the minimum then the system will switch back to series mode.

3) When the grid has a fault, the grid check system 1030 will send an control signal to the current/voltage control system 1010, the PLL block 1040 will take the internal sinusoid signal for the SPWM generator 1050 and also the MPPT control 1060 will change the control switch from S3 to Si, and the switches 53 and S4 will be operated in boost mode. When the power consumed by the load is higher than the PV panel generator, the switch 52 will operate as a boost converter to get the additional power from the back up battery 1070. When the supply from the grid 240 returns to normal, the switch Sg and relay 1020 which connect the grid to the circuit are both off. When grid check system 1030 recognizes that the grid 240 is back to the normal, then the PLL system 1040 will work into the normal state, and generate the sinusoid signal for the SPWM generator 1050, and the system will switch back either parallel mode or series mode, depending on the previous state.

4) When the weather is overcast or at nighttime, the PV panel 10 will stop working. The MPPT system will therefore also stop working, switches Si, 53, 32, 84 are therefore turned off. The current is supplied from the DC bus, and will come through full bridge inverter and pass through the body diode of the 54, and D5 back to the DC bus. This mode will only operates when there is grid sag or swell. If the grid in the normal state, the switch Sg will remain off, relay 1020 will remain on, and the local load will bypass the inverter and directly connect with the grid. When the MPPT system senses that the PV panel is producing again, the inverter will work in the series mode.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference,

Claims

<claim-text>CLAIMS1. An electrical connection apparatus for connecting a DC power source and an AC power source to a load arranged to operate on AC power, the apparatus including: an inverter connected in series between said power sources and said load.</claim-text> <claim-text>2. An electrical connection apparatus according to claim I wherein the inverter is arranged to use the power from the DC power source to compensate the power from the AC power source to provide a substantially constant voltage to said load.</claim-text> <claim-text>3. An electrical connection apparatus according to claim 1 or claim 2 wherein the inverter is further arranged to upload energy from the DC power source to said AC power io source.</claim-text> <claim-text>4. An electrical connection apparatus according to any one of the preceding claims wherein the inverter is arranged to supply power to said load from said DC power source alone when the power supplied from said AC power source is zero.</claim-text> <claim-text>5. An electrical connection apparatus according to any one of the preceding claims further including a transformer arranged to isolate said AC power source from said DC power source.</claim-text> <claim-text>6. An electrical connection apparatus according to any one of the preceding claims wherein said DC power source includes a photo-voltaic source, the apparatus further including a maximum power point tracking device connected between said DC power source and said inverter.</claim-text> <claim-text>7. An electrical connection apparatus according to any one of the preceding claims wherein said DC power source includes a back up battery.</claim-text> <claim-text>8. An electrical connection apparatus according to any one of the preceding claims further including apparatus for adjusting the voltage supplied from said AC power source prior to supply to said inverter.</claim-text> <claim-text>9. An electrical connection apparatus according to claim 8 wherein the apparatus for adjusting the voltage is an inverter arranged to operate as an AC buck converter.</claim-text> <claim-text>10. An electrical connection apparatus according to claim 9 wherein said inverter arranged to operate as an AC buck converter includes four switches which are controlled to adjust the voltage output of said inverter arranged to operate as an AC buck converter so that it is either in phase with or out of phase with said AC power source.</claim-text> <claim-text>11. An electrical connection apparatus according to claim 10 wherein said switches are controlled to operate so that said voltage output is in phase with said AC power source when the voltage of said AC power source is below a predetermined level and to operate so that said voltage output is out of phase with said AC power source when the voltage of said AC power source is above a predetermined level.</claim-text> <claim-text>12. An electrical connection apparatus according to any one of claims ito 7, further including apparatus for converting power from said AC power source to DC power prior to supply to said inverter.</claim-text> <claim-text>13. A power supply device having a DC power source1 an electrical connection apparatus according to any one of the preceding claims and connectors for connecting an AC power source to said device.</claim-text> <claim-text>14. A power supply device according to claim 13 wherein the DC power source includes a photo-voltaic source.</claim-text> <claim-text>15. A power supply device according to claim 13 or claim 14 wherein the DC power source includes a battery.</claim-text>