Patent Description:
It should be noted that reference to the prior art herein is not to be taken as an acknowledgement that such prior art constitutes common general knowledge in the art.

Electricity or power is an essential part of modern life. In residences, in businesses, in institutions and in other locations, consumers use electricity in a variety of ways. Utilities deliver power generated by power plants through a network of transmission and distribution lines. This network is hereinafter referred to as the "power transmission and distribution grid," "the electric grid," "the grid" or "power grid.

Renewable energy is a practical and environmentally conscious alternative to traditional utility production. One of the more desirable renewable sources is solar power. Solar equipment consumes no fossil fuels and generates no air pollutants. The use of solar power is generally regarded as environmentally safe. Utilities in most countries are required (or voluntarily do so) for public policy reasons to credit or actually buy excess power generated by a solar generating system from a consumer. In addition to these benefits, solar systems can provide customers with significant cost savings in the long run. As an incentive to install solar systems, government entities may provide rebates or tax deductions to customers who purchase and install solar systems.

Such programs have met with limited success for various reasons most particularly the inability of some types of energy users to curtail energy use and the lack of real-time information regarding the immediate cost of energy usage.

The use of power generated from renewable energy resources is rapidly increasing, attention is being focused on systems and methods in which such power is produced, transmitted, delivered, and consumed. The technology used in developing renewable energy resources, electricity grids and current energy infrastructure suffer from many limitations.

Furthermore, as the demand for such power increases, and regulatory requirements for use of "green" resources become more prominent there is a growing need to provide a useful alternative or at least an improvement on what has previously been done.

In recent years, concerns have been raised that high demand for electricity is taxing the capacity of existing electricity generating plants.

Furthermore, concerns regarding the availability and environmental safety of fossil and nuclear fuel are being raised. As a result of the above factors, the price of electricity has been on a path of steady increase. Likewise, the electrical utility industry has for some time laboured under the problem of supplying cost effective power to comply with system peak-demand period requirements.

Renewable energy systems have been used with gaining popularity to resolve at least partially the peak-demand issue of the power grid. For example, a solar power system may convert generated DC electricity from solar panels into AC electricity and be used to power electrical appliances. The generated DC power is also converted to AC power by an inverter so that power grid companies may purchase AC power produced.

These systems are typically being developed for the home or business which remains connected to the main electricity grid, so any electricity that your system generates above what you use is fed back into the grid. When you require more electricity than you are producing, your system imports it from the grid automatically. Your electricity bill is calculated as the difference between the amount of electricity you export from your renewable energy system and the amount you import from the grid - you only pay for the electricity you use that is over and above what your renewable energy system produces.

At present, feed-in regulations or tariffs for renewable energy exist in over <NUM> countries, states or provinces internationally, all involving the payment of a premium for the electricity fed into the grid from a variety of renewable energy sources. These feed in tariffs (FiT) are typically applied in two forms. A first form is a gross FiT - whereby all electricity generated from a renewable source is purchased from the generator at a generous price, with the generator buying-back any electricity they need to use from the grid. The second form of FiT is a net FiT - whereby only unused or surplus electricity is purchased from the generator.

In order to recover some of the expenses out laid in converting to a grid fed renewable energy system users are looking for ways to maximise the FiT benefit. Presently, consumers are maximising their financial benefit by improving the energy efficiency of their home to export more electricity to the grid. This could be achieved by reducing standby power consumption, switching to controlled load tariffs and minimising the use of energy intensive appliances such as air-conditioners. The fact that the FiT payment levels are performance-based puts the incentive on producers to maximise the overall output and efficiency of their system.

The problems with current domestic solar grid feed systems is that they are creating problems on the energy network regarding power quality and voltage spikes/abnormalities which requires expensive equipment to rectify it.

"<NPL> provides as a case study the solar panel installation of the Colorado Convention Center in Denver, Colorado. The inverter energy system supplying power to the site is controlled by monitoring the power flows into or out of mains power supply using a meter, setting a rate limit of <NUM> kW for the power flow, and reducing output from the inverters if the power drops below <NUM> kW.

Furthermore domestic solar grid feed systems are reducing the amount of power that can be sold by the generators and distributors, yet the actual amount of energy produced by the generators has to remain constant due to the fluctuating nature of small scale solar energy production. This means that the power generation companies and distributors still have the same or higher network costs, yet their income from selling and distributing the power has reduced. Thus there have been increases seen in electricity bills for items such as service charges.

Clearly it would be advantageous if a renewable energy generation load compensation system, method and apparatus could be devised that helped to at least ameliorate some of the shortcomings described above. In particular, it would be beneficial for a renewable energy generation load compensation system to improve on these deficiencies in renewable energy generation for the purpose of compensating the load from the utility grid, or to at least provide a useful alternative.

The invention provides a method of controlling an inverter energy string as set out in claim <NUM>, an inverter as set out in claim <NUM> and an inverter energy system as set out in claim <NUM>. Certain preferred features of the invention are set out in the dependent claims.

The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only.

The following description, given by way of example only, is described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments.

The following describes various systems implementing load compensation. The present invention as defined in claims <NUM> and <NUM> specifically pertains to a load compensation device installed within an inverter (<NUM>) during manufacture. To the extent that described approaches are incompatible with the approaches set out in the claims such approaches are provided for context but shall not be considered part of the claimed subject matter.

Described embodiments relate generally to methods, systems and apparatus for an energy generation load compensation and computer readable storage configured to control the performance of such methods, systems and apparatus. The energy load compensation system is typically used for Inverter Energy Systems, such as renewable energy solar photovoltaic fed grid installations, for the purpose of compensating the load from the mains supply grid and the described embodiments are particularly suited to such purposes. Embodiments are not, however, limited to such use. For example, the Inverter Energy System at site may be an array of photovoltaic panels and inverter(s) as discussed, wind turbine(s) and inverter(s), hydroelectric turbine(s) and inverter(s), or any other energy source and inverter combination.

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels <NUM> composed of a number of solar cells containing a photovoltaic material. An Inverter Energy System in the form of a PV system <NUM> comprises a number of solar strings <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. Each solar string (or series) comprises one or more photovoltaic (PV) panels <NUM> as a renewable energy source, a DC/AC power converter or inverter <NUM>, electrical interconnections, and associated switches and contactors <NUM>, <NUM>. The electricity generated can be either stored, used directly (island/standalone plant), or fed into the electricity grid <NUM>, or combined with one or many site renewable energy generators to feed into a small grid.

Renewable energy is energy that comes from resources which are continually replenished such as sunlight, wind, rain, tides, waves and geothermal heat. Therefore the present invention is not limited to any particular renewable energy. For example, in addition to the solar systems, wind turbines have also been employed to provide clean or renewable energy. The wind turbine generates an AC power from the kinetic energy of the wind through a system comprising a rotator, a gearbox and a generator. The AC power is rectified into a DC power and is further converted into AC power with the same frequency as the AC power from the power grid. Likewise, hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water.

The following description will be described with reference to solar energy and the use of photovoltaic panels however, the production of renewable energy is not limited to only such use. Likewise, isolation referred to in the following paragraphs refers to both electrical and mechanical isolation.

Therefore isolation for both the mains grid and the renewable energy supply may incorporate both mechanical and electrical isolation in order to protect both the main and the renewable energy supplies and their associated components.

<FIG> and <FIG> show a schematic diagram and a block diagram of a renewable energy load compensation system <NUM>.

<FIG> shows a single line drawing of the mains power supply <NUM> and <FIG> shows a block diagram of the main components in the load compensation system <NUM>. <FIG> shows a schematic diagram of a three phase mains power supply <NUM>. In order to provide the compensation for the load or sub circuits <NUM> on the mains power supply <NUM> the present invention incorporates a controllable switch in the form of load compensation device <NUM> and associated circuitry. Reference to a "load compensation device" includes reference to Consumption Export Controller, Load Compensation Device or Export Control Device, Network Protection Relay and Reverse Power Relay.

A mains power supply <NUM> provides mains electricity in the form of general-purpose alternating-current (AC) electric power supply. Worldwide, many different mains power systems are found for the operation of household and light commercial electrical appliances and lighting. The main differences between the systems are primarily characterised by their voltage, frequency, plugs and sockets (receptacles or outlets), and earthing system (grounding).

Auxiliary power supply to the site comprises an Inverter Energy System in the form of the PV system <NUM>. The PV system <NUM> includes five inverter energy strings, each inverter energy string comprising a supply of renewable energy <NUM> connected to an inverter <NUM>, in the form of five solar strings <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

The mains power or grid supply <NUM> is fed via transmission lines <NUM> to a site. The site may be a domestic residence or dwelling, factory or shop.

An electricity meter or energy meter <NUM> is a device that measures the amount of electric energy consumed by the site. Incorporating renewable energy generating equipment at the site, means a customer may be generating more electricity than required for his own use, the surplus may be exported back to the mains power grid <NUM>. Customers that generate back into the "grid" usually must have special equipment and safety devices to protect the grid components (as well as the customer's own) in case of faults (electrical short circuits) or maintenance of the grid (say voltage potential on a downed line going into an exporting customers facility).

Power export metering <NUM> provides metering which is capable of separately measuring imported and exported energy as used or required.

Typically these meters <NUM> are a bi-directional import/export meter which can measure both how much electricity is used at site, and how much electricity gets fed back into the grid <NUM> from the solar power system <NUM>. A main switch <NUM> isolates the main power grid from the site sub circuits <NUM>. Likewise, the renewable energy main switch <NUM> isolates the PV system <NUM> from the mains power supply <NUM> and the sub circuits <NUM> of the site.

The renewable energy load compensation device <NUM> is designed to continuously measure and monitor both forward and reverse direction and value of power flow in the mains power supply <NUM> to and from the site. The power flow value and direction is measured via a current transformer <NUM> and/or voltage transformer <NUM>. Alternatively the power flow can be measured directly by hard reference connection to the power lines from the main switch <NUM> to the site sub circuits <NUM>. The current and/or voltage transformer <NUM> may similarly be a current and/or voltage transducer(s).

When power is in the forward direction energy flows from the grid supply <NUM> to the site and is consumed by the site sub circuits <NUM> (consumption). When power is in the forward direction, the load compensation device <NUM> will first connect solar string <NUM> with the inverter <NUM> and solar array <NUM> to feed the sub-circuits <NUM>.

If further forward power or consumption is generated by sub circuits <NUM>, for example when the load from the sub circuits <NUM> increase due to increased power use, load compensation device <NUM> will energise further solar strings <NUM>, <NUM>, <NUM>, <NUM> to compensate for the increased of load and additional power use from the mains power supply <NUM>. Likewise, when forward power or load decreases, for example when the load from the sub circuits <NUM> decrease due to decreased power use, the load compensation device <NUM> will isolate the further solar strings <NUM>, <NUM>, <NUM>, <NUM> to prevent over generation from the Inverter Energy System <NUM>. If no load is sensed by the load compensation device <NUM>, and the system is configured for zero power export, the first solar string <NUM> will also be isolated. Configuration of the load compensation device for zero power export includes setting a power consumption lower limit at or slightly over <NUM> kW. This means that the load compensation device <NUM> will control the PV system <NUM> not to export any net energy to the mains power supply. The lower power consumption limit may be set slightly over 0kW, for example <NUM>. 1kW, to take into account a margin of error in measuring the flow of power, thereby to err on the side of caution not to export any energy into the mains power supply <NUM>.

As an example of the above, the power consumption lower limit is set to <NUM> kW, the load by the sub circuits is a constant 11kW(hypothetical) and the inverters <NUM> are rated at 5kW. When it is dark (at night) there will be little or no power generation from the PV system <NUM>, so all inverters <NUM> will be connected (contactors closed). As the sun rises power output from the PV system <NUM> will rise. Just before the power output from the PV system matches the 11kW load (when the consumption lower limit of <NUM>. 1kW is reached), the load compensation device <NUM> will isolate one of the inverters <NUM> (contactor K5 open) to isolate solar string <NUM> and thus step the power generation from the PV system <NUM> down. If the power from the PV system <NUM> need to be stepped down again then the next solar string will be isolated. If all inverters are producing <NUM> kW, for example when it is sunny at midday, then only two solar strings (<NUM>, <NUM>) would be connected (<NUM> kW from the PV system <NUM> consumed by the 11KW site load) and the remaining three solar strings (<NUM>, <NUM>, <NUM>) isolated. As the sun goes down and the inverters <NUM> produce less and less, the solar strings <NUM>, <NUM>, <NUM> are sequentially brought back online without going over the <NUM>. 1kW power consumption lower limit.

As is shown in <FIG>, when configured for export of energy to the mains power supply <NUM>, a first solar string <NUM> does not include contactor K1. If further forward power or consumption is generated by sub circuits <NUM> load compensation device <NUM> will energise further solar strings <NUM>, <NUM>, <NUM>, <NUM> to compensate for the usage of load from the mains power supply <NUM>. Likewise, when forward power or load decreases the load compensation device <NUM> will isolate the further solar strings <NUM>, <NUM>, <NUM>, <NUM> to prevent over generation from the renewable energy source <NUM>. If no load is sensed by the load compensation device <NUM>, the first solar string <NUM> will also be isolated.

In order to isolate and energise the solar strings <NUM>, <NUM>, <NUM>, <NUM>, <NUM> the renewable energy load compensation system <NUM> of <FIG> uses contactors <NUM> (K1 to K5) to selectively energise or isolate each solar string. A contactor <NUM> is an electrically controlled switch used for switching a power circuit. The contactor <NUM> is controlled by an energising means or circuit <NUM>, with separate circuits <NUM> used to energise each of the K1 to K5 contactors <NUM>. The generator (renewable energy) services board <NUM> also incorporates string isolation switches <NUM> which open should an error condition or protection device energise due to an over or under voltage, current or frequency event occur. The switch <NUM> will isolate and protect each solar string <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to protect the inverters <NUM> and solar array <NUM>. These string isolation switches <NUM> are circuit breakers, also referred to as AC isolators for the inverter, which will only activate on an over current load/fault.

<FIG> shows a detailed schematic drawing of the load compensation system <NUM> for a three phase mains power supply <NUM>. With any multi-phase system the components are typically the same for each phase and similar to that of a single phase system.

In <FIG> the load compensation device <NUM> shows the sensing of the load using current sensing <NUM> in the three phases. The monitoring point <NUM> for current sensing <NUM> is on the load side, of the main switch <NUM>. Current transformers (CT) <NUM>, <NUM>, <NUM> are used for measurement of electric currents in each of the phases. Voltage reference measurements are taken at the monitoring point <NUM> by voltage monitoring reference lines <NUM>, <NUM> and <NUM>.

As discussed above the sensing of the load on the mains supply <NUM> can be performed at the monitoring point <NUM> by either current transformers (or voltage transformers (VT) (also known as potential transformers (PT), which are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit.

<FIG> also shows a number of circuit breakers <NUM>, <NUM>, <NUM> used to protect the respective circuits. The circuit breakers <NUM>, <NUM>, <NUM> are typically an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Circuit breaker <NUM> provides a power input <NUM> to be fed to the load compensation device <NUM> to provide AC power when the renewable energy source <NUM> is connected. Further protection devices <NUM>, <NUM>, and <NUM> (are used to isolate and protect the respective components from the system <NUM>. The protection devices <NUM>, <NUM> and <NUM> are isolation switches.

<FIG> shows the three phase system of <FIG> with inverters <NUM> on each phase. This provides the added advantage of being able to compensate the load on a single phase without affecting the other two phases. Therefore any one or more of the phases of a multiple phase system can be implemented to compensate for the load on that phase.

<FIG> show the operation of respective parts of the renewable energy load compensation system <NUM>.

<FIG> illustrates the connection of each solar string <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the renewable energy source <NUM> with string <NUM> connected for export of energy to the mains power supply <NUM> in line with the agreed feed in tariff for the system <NUM>.

<FIG> shows the system <NUM> including a Zero Export contactor <NUM> and a Full Export contactor <NUM>. The contactors <NUM>, <NUM> are controlled by the load export device <NUM>.

The Zero Export contactor <NUM> will be opened by the device <NUM> when a fault condition is detected, including when export is detected at measuring point <NUM>.

The Full Export contactor <NUM> is a protection device for the export solar string <NUM>. The Full Export contactor <NUM> will be opened by the device <NUM> when a fault condition is detected, except for power export. The Full Export contactor <NUM> does not trip when export is detected at <NUM>, as the device <NUM> still allows export of an agreed feed-in by solar string <NUM>. The first solar string <NUM> is connected to the generator supply main switch <NUM> at a connection so that isolation of the further solar strings (<NUM> to <NUM>) by contactor <NUM> does not isolate the first solar string <NUM> from the mains supply generator supply switch <NUM>.

<FIG> shows a similar schematic to <FIG>, however all inverters <NUM> are connected through the energising of contactors <NUM>, and therefore there is no export of renewable energy to the mains power supply <NUM> if the lower consumption limit of the load compensation system <NUM> is set at <NUM> kW or more.

<FIG> shows the switching of the coils <NUM> to control the contactors <NUM> (K1 to K5). For example, when consumption is generated by sub circuits <NUM> as sensed by the voltage or current sensing <NUM>, <NUM> the load compensation device <NUM> will energise solar strings <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to compensate for the usage of load from the mains power supply <NUM>.

<FIG> (voltage sensing <NUM>) and <FIG> (current sensing <NUM>) show the two forms of sensing the site load on the mains power supply <NUM>. In <FIG> the voltage <NUM>, <NUM>, <NUM> on each phase of a three phase system is sensed and returned to the load compensation device <NUM>. The voltage sensing <NUM> can be connected or isolated from the system <NUM> by circuit breaker <NUM>. <FIG> shows current sensing <NUM> using CT's <NUM>, <NUM>, <NUM> on each phase of a three phase system.

<FIG> and <FIG> show flow charts of the control process and timing for the energy load compensation system <NUM>. <FIG> shows only the energising and de-energising of the K1 contactor. <FIG> and <FIG> shows the control process wherein the load compensation device <NUM> is programmed for <NUM> export and limited consumption. That is to say the load compensation device <NUM> is programmed with a lower consumption limit of <NUM> kW and an upper consumption limit of X kW.

The process starts at step <NUM> at which <NUM> kW of energy is being consumed as measured at measuring point <NUM>. If <NUM> kW of energy is consumed for greater than <NUM> seconds, contact or K1 remains de-energised. At step <NUM> if consumption is greater than X kW for a preset time period of <NUM> seconds then contactor K1 energises at step <NUM> to keep consumption within the X kW upper consumption rate limit. At step <NUM> if consumption decreases to <NUM> kW or greater, meaning export, for more than <NUM> seconds then K1 de-energises at step <NUM> and remains de-energised if no consumption is sensed at step <NUM>. At step <NUM>, K1 remains energised as consumption is greater than X kW for a preset period of greater than <NUM> seconds. Step <NUM>, K1 remains de-energised, however if load becomes greater than X kW for the preset time period then K1 is energised at step <NUM>.

<FIG> shows the flow chart for the energising and de-energising of all strings <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. Steps <NUM> to <NUM> are the same as <FIG> and each string as represented by steps <NUM>, <NUM>, <NUM> and <NUM> are replicated with the exception that if energy continues above X kW then each contactor K1 to K5 are energised respectively. Likewise as energy decreases below X kW contactors K5 to K1 are de-energised. Alternatively and an advantage of the present system comprising a programmable microprocessor in the load compensation device <NUM>, allows the order of energising to be changed and any order may be programmed into the load compensation device <NUM>.

A designer of the system choses X taking into account a number of factors, including the design kW size of each solar string <NUM> to <NUM>. Purely as an example, if the solar strings included a solar array <NUM> and inverter <NUM> capable of producing 10kW (assuming <NUM>% efficiency), then X may be selected to be <NUM>. This is to prevent a solar string from being brought online only to be isolated again (also known as on-off cycling) because bringing the solar string online causes the system to export energy (over the lower export limit of <NUM> kW for the system). When consumption is higher than the set rate of <NUM> kW for <NUM> seconds, K1 is energized, which brings <NUM> kW from the PV string <NUM> online. A PV string is "online" when its associated contactor <NUM> is closed so that power from its inverter is supplied. The consumption (assuming the site load from sub-circuits <NUM> remain constant) will then fall below the upper consumption limit of <NUM> kW to <NUM>. 5Kw (<NUM> kW - <NUM> kW = <NUM> kW). ) K1 remains energized unless the consumption changes to export (exporting in reverse direction of power flow below the lower consumption limit of <NUM> kW) or the consumption rises above <NUM> kW again. The consumption can change to export, because the load from consumer circuits <NUM> fall, or the energy power supply from PV string <NUM> increases, or a combination of these two factors. Similarly, the consumption may rise above <NUM>. 5kW again if the load from consumer circuits <NUM> increase or the renewable energy power supply from PV string <NUM> decrease, or a combination of these two factors. If the consumption rises above <NUM>. 5kW again for more than <NUM> seconds, K2 is energised to also bring PV string <NUM> online.

The 10kW solar string example given above assume <NUM>% efficiency of the solar string. In a real world solar energy fed example of the system <NUM>, the X value may be chosen to be in the region of <NUM>% of the design rating of the solar string. That is because solar strings are never <NUM>% efficient and rarely go over <NUM>% of their rating. The <NUM>% of rated solar string power output as value for X allows the X value to be more than the real world power output from the solar string most of the time, if not all of the time. For a solar string having a 10kW rated inverter, the X value may thus be chosen as <NUM>. 7kW as the solar string will rarely produce more than <NUM>. A similar design methodology may be applied for other renewable energy generation strings such as wind turbines. In other examples of the system <NUM> where the power generation from each string remains reasonable constant (hydro energy for example) the X value can be set marginally above the known constant power out of the power generation string.

The above is an example of configuring the load compensation device <NUM> as a limited consumption device or <NUM> export device where the upper limit is set as <NUM> kW and the lower limited for <NUM> export is set as 0kW. The load compensation device <NUM> thus controls the PV system <NUM> so that the consumption is maintained in a set range between an upper consumption limit and a lower consumption limit where possible.

To configure the load compensation device <NUM> as a limited export device, for example, an upper export limit is set as export of X kW (reverse direction power flow). Up to X kW can thus be exported from the PV system to the mains supply <NUM>. If export goes above X kW for a set amount of time the load compensation device <NUM> will take one of the PV strings offline (isolate the PV string by opening associated contactor <NUM>) to bring the export rate below the set export limit.

It will be appreciated that the example above relies on isolation of the PV strings by switches K1 - K5 to step the power output from the PV system <NUM> up or down as controlled by the load compensation device <NUM>. The output from the PV system <NUM> may similarly be ramped/stepped up or down by electronically controlling the power output at each of the inverters <NUM> by binary or digital control.

The inverters <NUM> may be controlled to have power at the respective inverters <NUM> ramp up and down in parallel. Alternatively the power output from just one of the inverters <NUM> may be ramped up or down, followed by ramping up or down the power output from the other inverters one after the other, to match the required power output from the PV system <NUM> for the required load profile (to maintain consumption/export within the consumption/export limits).

Electronic control of the inverters <NUM> allow the inverters <NUM> to remain connected to the site circuits <NUM> (no isolation) but have a power output variable by the controller <NUM> between <NUM>% and <NUM>% power output. Rather than selective isolation of the solar strings to manage the power output from the PV system <NUM>, the power output form the inverters <NUM> are controlled between <NUM>% power output and <NUM>% power output. Control of the power output form the inverters is referred to as electronic control. Electronic control of the inverters <NUM> provide for quicker power ramp up and ramp down time when compared to stepped isolation of the solar strings. The power from the PV system <NUM> can also be controlled in smaller increments/steps by electronic control of inverter <NUM> output as compared to isolation of the inverters <NUM>. Electronic control of the inverters <NUM>, <NUM> can provide almost infinite control to throttle the power output (up or down) from the PV system <NUM>.

An example where a single inverter <NUM> is controlled to ramp up or down during load compensation is described with reference to <FIG> and <FIG>.

<FIG> illustrates a system with the energy load compensation system used as a master and slave installation. Master/slave is a model of communication where one device or process, the master <NUM> has unidirectional control over one or more other devices, slaves <NUM>, <NUM>, <NUM>. The master load compensation device <NUM> has control over each slave <NUM>, <NUM> and <NUM>. In each dwelling <NUM> a slave load compensation device <NUM>, <NUM>, <NUM> is controlled by respective contactors <NUM> (S1 to S3) which are energised and de-energised by the master load compensation device <NUM>. Each dwelling has a renewable energy source <NUM> (PV array), inverter <NUM>, sub-circuits <NUM>, and slave isolation switch <NUM>. The master load compensation device <NUM> is in control of a mini-grid of grid connected renewable energy installations as well as other loads <NUM>. Like the load compensation device <NUM> the master load compensation device <NUM> has been designed to continuously measure and monitor both forward and reverse direction of power flow in the mains power supply <NUM>.

For example if export of renewable energy is connected, when power is in the forward direction (consumption) the master load compensation device <NUM> will connect first slave <NUM> with the inverter <NUM> and solar array <NUM> designed and sized to the agreed feed in tariff to export renewable energy fed to the grid. If further forward power or consumption is generated by sub circuits <NUM> the master load compensation device <NUM> will energise further slaves <NUM>, <NUM> to compensate for the usage of load from the mains power supply <NUM>. Likewise, when forward power or load decreases the master load compensation device <NUM> will isolate the further slaves <NUM>, <NUM> to prevent over generation from the renewable energy source <NUM>. If no load is sensed by the master load compensation device <NUM> the first slave <NUM> will also be isolated. It should be identified by a person skilled in the art that any number of slaves may be implemented and the order in which each slave is energised may also be changed, and that the present disclosure only illustrates three slaves by way of example only.

The power output from inverters <NUM> of the slave units may similarly be electronically controlled by binary/digital control to ramp power up or down as required by the master or slave controllers <NUM>, <NUM>, <NUM>, <NUM>.

An example use of the master slave configuration is in a caravan park with rental caravans <NUM> and permanent sites with privately metered installations <NUM> and other amenities <NUM> (other loads) such as toilets etc. As illustrated in <FIG> the master load compensation device <NUM> may monitor and control the rental sites as slaves.

Another optional addition to the present system is the use of a data network (not shown) to monitor and control the transmission of data around the system <NUM>. By way of example and as illustrated in <FIG> the master controller <NUM> controlling each slave load compensation device <NUM>, <NUM>, <NUM> in respective unit installations <NUM>. A data network is an electronic communications process that allows for the orderly transmission and reception of data; in this present system this includes load sensed on the mains power supply <NUM> and control signals to the respective installations <NUM>. The data network could be either a private data network or a public data network designed to transfer data between various installations <NUM>.

The present system has been illustrated as a new installation in which the load compensation device <NUM> and <NUM> are installed in various new installations with associated components. The present invention concerns an installation in which the load compensation device is installed within an inverter (<NUM>) during manufacture (<FIG>). An alternative in which a modulator device is installed off the inverter (<FIG>) is also described below but this approach is not covered by the claimed invention.

In <FIG> an inverter <NUM> is shown comprising the load compensation device <NUM> which can measure and monitor the load on the mains power supply <NUM> by voltage and current sensing <NUM>, <NUM> and adjust the power output at the inverter <NUM>. All of the components are installed in the inverter <NUM> during manufacture. The present embodiment has been designed so that the conventional inverter <NUM> can be replaced by inverter <NUM> in accordance with another embodiment of the present invention.

<FIG> also show single line diagrams of the present embodiment in which inverter <NUM> is installed as well as showing the voltage (<FIG>) and current sensing (<FIG>) and inputs into the inverter <NUM> from the voltage and current sensing circuits.

<FIG> shows a further arrangement (not covered by the claimed invention) in which the load compensation system <NUM> is installed as a modulator device <NUM> off the inverter <NUM>. In this arrangement the device <NUM> is a separate device with the same capabilities as the previous load compensation device <NUM>, but supplied separately from the inverter <NUM>. In this arrangement the device <NUM> is easily used as a retro-fit of an existing renewable energy installation. Retrofit projects typically replace or add equipment to existing installations to be able to continuously measure and monitor both forward and reverse direction of power flow in the mains power supply <NUM>. The modulator device <NUM> provides a variable output to the mains supply <NUM> and to the loads <NUM> and can measure and monitor the load on the mains power supply <NUM> by voltage and current sensing <NUM>, <NUM> and adjust the power output and/or reactive power output at the modulator <NUM> in line with the load sensed.

<FIG> show block diagrams of the respective load compensation devices <NUM>, <NUM>, <NUM>. In <FIG> shows the load compensation device <NUM> including a microprocessor <NUM>, a display <NUM>, voltage and current sensing input terminals <NUM> with inputs from the voltage and current sensing <NUM>, <NUM>. Control relays <NUM> and output terminals <NUM> which connect the control signal outputs <NUM> are also provided in load compensation device <NUM>.

<FIG> illustrates a block diagram of inverter <NUM> including microprocessor <NUM>, a display <NUM>, voltage and current sensing input terminals <NUM> with inputs from the voltage and current sensing <NUM>, <NUM>. Inputs from the renewable energy source <NUM> (PV arrays) are fed to input terminals <NUM> and then to both the microprocessor <NUM> and the electronic control board/regulator <NUM> of the inverter <NUM>. Outputs <NUM> and control relays including output terminals are also provided at <NUM>.

<FIG> illustrates the modulator <NUM> which includes input from the inverter <NUM> and renewable energy source and all the remaining componentry of the load compensation device <NUM> including a microprocessor <NUM>, a display <NUM>, voltage and current sensing input terminals <NUM> with inputs from the voltage and current sensing <NUM>, <NUM>. The electronic control board/regulator <NUM> of the load compensation device <NUM> and outputs <NUM> and control relays including output terminals are also provided at <NUM>.

<FIG> and <FIG> do not include export of energy to the mains power supply <NUM>. The K1 contactor may be programmed so that a feed in tariff and export are provided.

The inverters <NUM> as illustrated in <FIG> show a polyphase inverter <NUM>. Alternatively the system may also be implemented using single phase inverters <NUM> on each phase. This provides the added advantage of being able to compensate the load on a single phase without affecting the other two phases. Therefore any one or more of the phases of a multiple phase system can be implemented to compensate for the load on that phase.

Alternatively the inverter <NUM> can include a micro-inverter <NUM> as shown in <FIG> for each panel of a solar array <NUM>. A micro-inverter <NUM> converts direct current (DC) electricity from one or two solar panels <NUM> of a solar array to alternating current (AC).

The output from several micro-inverters <NUM> is combined and often fed to the electrical grid. Micro-inverters <NUM> contrast with conventional string or central inverter devices, which are connected to multiple solar panels.

Micro-inverters <NUM> have several advantages over conventional central inverters. The main advantage being small amounts of shading, debris or snow lines on any one solar panel, or even a complete panel failure, does not disproportionately reduce the output of the entire array. Each micro-inverter harvests optimum power by performing maximum power point tracking for its connected panel.

By way of example only, a micro-inverter <NUM> system can be implemented as shown in <FIG> where each micro-inverter <NUM> is treated as a slave and is controlled by a master load compensation device <NUM>. Likewise each micro-inverter <NUM> may have its own load compensation device <NUM> for each photovoltaic module <NUM>. This would be similar to the embodiment described with reference to <FIG> in which a micro-inverter <NUM> would replace inverter <NUM>.

<FIG> shows another embodiment of the energy load compensation system <NUM> including load compensation device <NUM>. The load compensation device <NUM> of <FIG> includes switches SW1, SW2, SW3, SW4, SW5 and SW6 for switching the contactors K1, K2, K3, K4, K5 and K6, respectively. The switches SW1 to SW6 may also be used for digital control of the power output from the inverters <NUM> as shown in <FIG>.

<FIG> shows an embodiment of the compensation system <NUM> where the first four switches SW1, SW2, SW3, and SW4 can be used for <NUM> bit digital control to vary the power output from the inverters <NUM>. Each switch SW1, SW2, SW3 and SW4 is connected to a respective digital control input of the inverters <NUM>. Each switch can be either on or off, giving a <NUM> increments (<NUM><NUM>) of controlled power output between <NUM>% and <NUM>%. For example, the inverters <NUM> can be throttled to <NUM>% power output when all of the switches SW1 to SW4 are opened (<NUM> bit binary <NUM>). Similarly the inverters may have full power output when all of the switches SW1 to SW4 are closed (<NUM> bit binary <NUM>). The table below shows one example of the <NUM> inverter throttle positions for the different on/off combinations of the switches SW1 to SW4.

The inverters <NUM> are controlled in parallel, meaning when one inverter is throttled all the inverters are throttled as they are all linked together to the switches SW1 to SW4.

The load compensation device <NUM> of <FIG> and <FIG> also includes a serial communications port <NUM>. The serial port <NUM> can be connected via communications link to each of the inverters <NUM> to electronically control variable power output from the inverters <NUM>. <FIG> shows the communications port <NUM> connected to the inverters <NUM> via a data cable <NUM>. The inverters may, for example, be controlled using the RS485 standard.

The serial port <NUM> provides precise control of the power output from the PV system <NUM>. The load compensation device <NUM> can be configured for the power output from one inverter <NUM> to be controlled separately from the power output of any of the other inverters <NUM> via the serial port. For example, the power output from one inverter <NUM> may be at <NUM>%, the power output of another inverter being ramped up and down between <NUM>% and <NUM>% to follow the load profile of the load (keeping the power flow at the measuring point <NUM> at or close to the lower consumption limit) and the power output of the remainder of the inverters set at <NUM>%. Alternatively, the load compensation device <NUM> can be configured for the % power output of all of the inverters <NUM> one to be simultaneously controlled to be the same. For example, the power output of all of the inverters <NUM> may be ramped up and down between <NUM>% and <NUM>% simultaneously (all inverters has the same % output at any given time) for the power output of the PV system <NUM> to follow the load profile of the load.

The load compensation device <NUM> of <FIG> shows the sensing of the load using current sensing <NUM> in the three phases at monitoring point <NUM>. The inverters <NUM> can be electronically controlled via the serial port <NUM> to vary the power output in each phase supplied by the inverter <NUM>. If the load compensation device <NUM> measures the phases to be unbalanced at monitoring point <NUM>, one or more of the inverters <NUM> can be controlled by the device <NUM> to balance the three phase load to the mains supply <NUM>. In particular, each of the phases of power supply from an inverter <NUM> is individually controlled via the serial port <NUM> to balance the three phase load. The device <NUM> is thus operable to control the inverters <NUM> to balance the three-phase load on site. Each phase independently follows the load, the inverters <NUM> are controlled so that phase with higher load receives more power generation support from the inverters <NUM>, resulting in a balance of phases measured at the monitoring point <NUM>.

The device of <FIG> and <FIG> includes a Master Control function. The Master Control Function includes two master control relays MC1 and MC2 controlled by the microprocessor <NUM> of the device <NUM>. The master control relay MC1 is operable to control zero export contactor KM1 (<FIG>) or K1 (<FIG>), which corresponds to Zero Export contactor <NUM> of <FIG>. When contactor KM1/K1 is opened by MC1, all of the generation from the solar power system <NUM> is isolated. The microprocessor <NUM> is programmed so that zero export contactor KM1/K1 will be opened by relay MC1 of the device <NUM> when a fault condition is detected, including when export is detected at measuring point <NUM>.

<FIG> shows an example of the system <NUM> configured for power export from the first solar string <NUM>. The microprocessor <NUM> is programmed so that master control relay MC1 of the device <NUM> is configured to trip (open) the contactor KM1 (<FIG>) upon any of the protection events, including export. The microprocessor <NUM> is also programmed so that master control relay MC2 of the device <NUM> is configured to trip (open) the contactor KM2 (<FIG>) upon any of the protection events, other export. Because the system <NUM> is configured to export power from the solar string <NUM>, the device will not trip contactor KM2 if export (reverse flow of power) is detected at monitoring point <NUM>. The first solar string <NUM> is connected to the mains supply <NUM> at a connection so that isolation of the further solar strings (<NUM> to <NUM>) by contactor KM1 does not isolate the first solar string <NUM> from the mains supply <NUM>.

<FIG> shows an example of the system <NUM> configured for full export of power from the whole of the PV system <NUM>. It will be appreciated that all of the solar strings (including inverters <NUM>) of the PV system <NUM> of <FIG> are connected to the mains supply via contactor KM1. The master control relay MC2 of the device <NUM> is configured to trip (open) the contactor KM1 upon any of the protection events, other than export protection. Because the system <NUM> is configured to export power from all of the solar strings, the MC2 will not trip contactor KM1 if export (reverse flow of power) is detected at monitoring point <NUM>.

Power output from the PV system <NUM> may be throttled, up or down as required, either by isolation (<FIG>) of the PV strings using the contactors K1 to K6, or by electronic control (<FIG>) of the power output from the inverters <NUM>.

The system <NUM> may also use a combination of power generation string isolation and electronic control of the power output by the inverters <NUM> of the strings by the load compensation device <NUM>. <FIG> shows the combination of PV string isolation control and digital inverter power output control. The PV strings are taken offline (disconnected from the load) by mechanical isolation via the contactors K1 to K6 and the PV strings which are online (connected to the load) have their power output regulated by electronic control from the load compensation device <NUM> via the serial port <NUM>. A system <NUM> wherein the device <NUM> is configured for combined isolation and electronic control is useful to isolate an inverter <NUM> independently as needed or required. Being able to digitally control output from the inverters <NUM> also assist in preventing mechanical isolation of the PV strings.

The systems <NUM> of <FIG> include generator current transformers <NUM> which are operable to measure the generation by the PV system <NUM>. The generator current transformers <NUM> are connected to inputs <NUM> of the device <NUM>. The device <NUM> monitors the power generation of the PV system <NUM>.

The device <NUM> of <FIG> may be incorporated into and/or form part of the inverter <NUM> of <FIG>. Similarly, the device <NUM> of <FIG> may be incorporated into and/or form part of the modulator of <FIG>.

The present invention also allows the consumer to continuously measure, control, and monitor both forward and reverse direction of power flow in the mains power supply. With the use of a data network connecting a control system having a computer readable program stored on the computer the renewable energy load compensation system can be automated or manually controlled by the consumer with the use of a computer.

Also and as is sometimes the case, when the renewable energy load compensation system is not connected for export of energy to the mains supply the inverters may be configured to whatever value desired by the consumer.

For example, a site may be configured for <NUM> kW reverse power on one phase with <NUM> kW on a second phase and <NUM> kW on the third phase. For all of the above options the circuitry has been designed to be energised to connect. Therefore if any fault or failure in the system occurs all items are protected on de-energise. The present system provides a number of additional programmed protection devices such as over voltage protection, under voltage protection, over frequency protection, under frequency protection, differential frequency protection between the phases, phase failure protection, voltage vector shift (VVS) protection, rate of change of frequency (RoCoF) protection, anti-islanding protection and reverse power flow protection. All of the above are programmable into the load protection device in order to further protect and isolate the system should any one of the faults occur. This also applies to all mechanical isolation devices within the renewable energy load compensation systems which are designed to be energised to connect and therefore on failure or faults will be de-energised to protect both the mains power supply and the renewable energy power supply and associated components.

The device <NUM> is configured for the following protection functions for the different protection events set out in the paragraphs below with reference to the Zero Export device of <FIG>:.

With reference to <FIG>, Full Export contactor <NUM> (KM2) will open at the same time as the Zero Export Contactor <NUM> (KM1) opens for the fault conditions described above with reference to <FIG>. Full Export contactor <NUM> (KM2) will not, however, open upon the condition being export of power through monitoring point <NUM>, whereas the Zero Export Contactor <NUM> (KM1) will open on the condition being export of power.

It is also envisaged that a data logger to log and graph each phase of the monitoring point and each phase of the generation point over a predetermined time scale. A data logger or data recorder is an electronic device that records data over time or in relation to location either with a built in instrument or sensor or via external instruments and sensors. The data logger can be based on a digital processor (or computer). The data logger may be a small, battery powered, portable, and equipped with a microprocessor, internal or removable memory for data storage, and sensors. The data logger may interface with a personal computer and utilize software to activate the data logger and view and analyse the collected data.

Data is typically collected in one minute intervals. The collected data can be processed to provide information such as one or a combination of; graphs in amps, kVA, kWatts, total of kWatt hrs for Consumption, Export, Generation, Total usage for the monitored site, Voltage values, Power Factor, Peak Demand with value and time for each day, in digital control the % value of power output of inverters and connected relays.

Likewise the data logger may have a local interface device (keypad, LCD) and can be used as a stand-alone device. The ability of the present invention to monitor power flow at a point and adjust the current output of the inverter is particularly important when considering the real and reactive power flow to the load and as shown in <FIG> (inverter <NUM>) and <FIG> (modulator <NUM>) the present system also extends to the monitoring of the reactive power or power factor at the same point or any other relative point and the ability to adjust the supply of reactive power, power factor or phase position of the current output to suit the load. The device <NUM> of <FIG> may similarly monitor and control reactive power relative to set power rate limits as is described for active power and control the inverters <NUM> to assist in power factor correction.

Typically, any practical load will have resistance, inductance, and capacitance therefore both real and reactive power will flow to the load. The ratio between real power and apparent power in a circuit is called the power factor. It is a practical measure of the efficiency of a power distribution system.

The power factor is unity (one) when the voltage and current are in phase. It is zero when the current leads or lags the voltage by <NUM> degrees. Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle of current with respect to voltage.

The renewable energy load compensation system of <FIG> and <FIG> which incorporates the monitoring of the power factor, the power factor would typically be between - <NUM> to + <NUM> monitored at the monitoring point and supported by the output of the inverter <NUM> or modulator <NUM>. Similarly, the device <NUM> of <FIG> and <FIG> monitors the power factor at the monitoring point <NUM> and electronically controls the output of the inverters <NUM> (via communications port <NUM>) to correct the power factor.

Each device <NUM> of Figures is designed to control and monitor either single phase, two phase or three phase PV systems <NUM>.

<FIG> shows a single phase energy load compensation system <NUM> wherein power output from all the inverters <NUM> are connected to a single phase and digitally controlled.

<FIG> shows a two phase energy load compensation system <NUM> wherein power output from the inverters <NUM> are connected to one of the two phases and power output from the inverters <NUM> are connected to the other of the two phases. The inverters <NUM> and <NUM> can be digitally controlled by the device <NUM> to balance the phases and correct the power factor.

<FIG> shows a three phase energy load compensation system <NUM> wherein power output from the inverters <NUM> is connected to the first phase, the power output from the inverters <NUM> is connected to the second phase and the power output from the inverters <NUM> is connected to the third phase. The inverters <NUM>, <NUM> and <NUM> can be digitally controlled by the device <NUM> to balance the phases and correct the power factor.

By using renewable energy systems to power your home or business you are reducing greenhouse gas emissions and your electricity bills. The present invention provides the added advantage of being able to allow additional solar generation in an installation to be used for the purpose of compensating the load from the grid only.

Furthermore the present invention monitors and protects the grid by way of isolation from unwanted exporting of over generated renewable energy. The load compensation device in accordance with the present invention allows for the continuous measuring and monitoring of forward and reverse direction of power supply in the mains power supply of an installation. The present invention manages and limits the export of power to the mains supply by way of isolation or digital control of the inverter power output. Likewise both the renewable energy source and componentry and the mains power componentry are energised to connect therefore any fault or failure in the system all items are protected on de-energise.

Upon failure of an inverter the present invention extends to the fitting of a new inverter with the load compensation circuitry installed within the inverter.

The ability of the present invention to monitor power flow at a point and adjust the current output of the inverter is particularly important when considering the real and reactive power flow to the load. A number of advantages exist in the ability to monitor the reactive power or power factor at the same point or any other relative point and the ability to adjust the supply of reactive power, power factor or phase position of the current output to suit the load. Apart from power factor correction, current loads and minimising voltage rises the present invention also allows the network or retailer who supplies the power the ability to minimise voltage rise issues on the network created by the renewable energy site, minimise power quality issues transposed to the renewable energy site and also minimise taking needed reactive power from the grid. For the networks, suppling reactive power is an expense and is not something they can properly measure and charge for.

The present invention provides a consumer with the ability to manage the output of their renewable energy supply using the AC side and the load on the mains supply.

It will be realized that the foregoing has been given by way of illustrative example only and that all other modifications and variations as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth.

Claim 1:
A method of controlling an inverter energy string supplying power to a site, the inverter energy string including an inverter (<NUM>) and a renewable energy source (<NUM>) and being connected to a mains power supply (<NUM>) and to a site load, the method performed at a load compensation device (<NUM>) having:
- a microprocessor (<NUM>);
- voltage and current sensing input terminals (<NUM>) with inputs for voltage and current sensing (<NUM>, <NUM>);
- inputs (<NUM>) for the renewable energy source (<NUM>); and
- output terminals (<NUM>) and control relays (<NUM>) ;
wherein the load compensation device is configured to monitor the load on the mains power supply using the voltage and/or current sensing inputs and to adjust the power output at the inverter (<NUM>), the inverter (<NUM>) having a control board (<NUM>);
wherein the load compensation device (<NUM>) is installed within the inverter (<NUM>) during manufacture;
the method including:
monitoring, by the load compensation device (<NUM>), the forward or reverse power flow into or out of the mains power supply (<NUM>) at a monitoring point at the site by means of the voltage and/or current sensing (<NUM>, <NUM>);
setting a rate limit for power flow into or out of the mains power supply (<NUM>); and
varying the supply of power to the load from the inverter energy string by controlling the inverter (<NUM>), by the load compensation device (<NUM>), so that the power flow into or out of the mains power supply (<NUM>) is controlled to be within the rate limit.