GB2507039A - Surplus power detection and diversion in co-generation system - Google Patents

Abstract

A system for detecting and diverting surplus power in a co-generation system to prevent the export of power to the grid network comprises a local power generator or micro-generator 10, a local energy load or store 14 and a connection to the grid network 12. A system manager and load controller 20 scan for excess power returning to the grid by adjusting the power to the load 14 and measuring the change in magnitude of energy flowing through the connection to the grid 12 using a sensor 18. If the change in energy flow through the connection 12 is in the opposite direction as the change in power diverted to the load 14, power to local loads should be increased. Optionally, if the change is in the same direction, power to local loads should be decreased. The need to measure the direction of energy flow in the connection 12 is overcome. A boost function may be provided in the system manager 20 interface to provide maximum power to the load 14 notwithstanding power being imported from or exported to the grid.

Description

DESCRIPTION

SURPLUS POWER DETECTION SYSTEM

This invention relates to a surplus power detection system. In one embodiment, the invention provides a system to automatically detect and divert surplus power in a micro-generation system.

All power generators can use their own power for free but the random io nature of renewable energy sources means that most power generators of this type will also use power from the Grid when their own power is not sufficient for their immediate needs. Similarly, at other times it will often be the case that such power generators will produce surplus power which they can then sell back to the Grid. The power generator can save money if the surplus power can be used locally to replace other, more expensive power. This can be done by storing the surplus power for use at a later time when local power is not sufficient or by using it for other purposes. A typical example of the latter would be the use of surplus power to heat water.

To achieve a financial return, the money saved must cover the cost of supplying and installing a power storage or other system to use the surplus power and cover the loss of income from selling surplus power to the Grid. In a large installation, the value of surplus power that can be used locally is sufficient to cover these costs. Indeed, suitable systems are already available.

Small installations, such as in a normal home using solar panels and/or a wind turbine, which do not produce as much surplus power, struggle to provide a financial return on the high cost of existing systems designed to use the surplus power. Smaller installations need a lower cost system to provide better returns.

At present, the meters used in most small installations cannot easily measure the amount of power exported to the Grid. Therefore, the income that is received for power exported is based on the amount of power that they generate locally, regardless of how much they actually export. This means that for a small installation to achieve a financial return the money saved only has to cover the cost of supplying and installing a power diversion system and not any loss of income. However, even without this benefit, a financial return can still be achieved if the cost of the system to use the surplus power is sufficiently low.

Currently, the existing approach to finding out the amount of surplus power is to measure the power used locally and the power generated locally.

By subtracting one from the other, the surplus power is calculated and diverted to another, local use such as an immersion heater in a tank of water. Such a io system requires one or more, accurate, power flow sensors and a sophisticated computing element leading to a high cost plus the costs of installation by an appropriate, highly skilled expert. Figure 1 shows an example of such a prior art system. Local power generators 10 provide power as does the connection 12 to the general power grid. An energy store 14, which can be used to store surplus power, is provided and the various components are connected together via a network 16.

It is therefore an object of the invention to improve upon the known art.

According to the present invention, there is provided a surplus power detection system for use in a power arrangement comprising a local power generator, a connection to a general power grid, a local energy load and a network connecting together the local power generator, the connection to the general power grid and the local energy load, wherein the surplus power detection system comprises a sensor connected to the connection to the general power grid and arranged to measure the magnitude of the energy flow in the connection to the general power grid, a load controller connected to the local energy load and arranged to control the flow of power to the local energy load, and a system manager connected to the sensor and the load controller and arranged to change the power diverted to the local energy load by a defined amount, detect whether the change in the energy flow in the connection to the general power grid is equal or opposite to the change in the power diverted to the local energy load, and increase the power diverted to local energy usage if the change in the energy flow in the connection to the general power grid is opposite to the change in the power diverted to the local energy load.

Owing to the invention, it is possible to provide a surplus power detection system that will enable small, micro-generation systems to easily benefit from the local use of any surplus power generated. This will be achieved from a low cost, automatic system which will provide a better financial return from the micro-generation system. The improved system reduces costs dramatically, by as much as half, with a dedicated unit that only io needs a single, low cost current sensor, no general purpose computing element and has a low cost of installation that uses the existing infrastructure.

The invention provides an adaptive control technique to identify the direction of power flow to the Grid. Optionally, the use of phase control to match the load to surplus power generated can be used.

By breaking the surplus power detection system down into modular components, the improved system can still be realised at low cost but in a form allowing the system to be part of a more comprehensive, modern building management control system. All sensor and load control elements will be capable of communication with a system manager either by wire, for lower product cost, or by wireless for lower installed cost. Communication will be bi-directional for appropriate modules. This approach allows, for example, initially having a sensor and load control elements to realise the new concept of low cost sensing of energy flow to and from the Grid. At a later date, the energy flow measurement can be obtained directly from a smart meter thus reducing costs further. The modules can also be used to create or as part of a complete building control system.

The system manager is arranged to track local power consumption.

This is achieved by periodically changing the power diverted to the local energy load by a defined amount, detecting whether the change in the energy flow in the connection to the general power grid is equal or opposite to the change in the power diverted to the local energy load, and decreasing or increasing the power diverted to local energy usage if the change in the energy flow in the connection to the general power grid is respectively equal or opposite to the change in the power diverted to the local energy load.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-Figure 1 is a schematic diagram of a prior art surplus power storage system, Figure 2 is a schematic diagram of surplus power storage system according to embodiment of the invention, io Figure 3 is a schematic diagram of the exterior of a main unit of the surplus power storage system of Figure 2, Figure 4 is a schematic diagram of the components of a system manager of the surplus power storage system of Figure 2, Figure 5 is a diagram of two graphs showing power increments to be used dependent on time delays between sensor measurements in different scenarios, and Figure 6 is a process diagram of the method of operating the surplus power storage system.

Figure 2 shows the improved surplus power detection system that is for use in a power arrangement such as that provided in a home that has local power generation. Local power generators 10 provide power, as does the connection 12 to the general power grid. An energy load 14 is provided, which is an immersion heater. A network 16 connects the various components together. The surplus power detection system, in the preferred embodiment, is dedicated to automatically diverting the maximum amount of surplus, locally generated power to the energy load. In the power arrangement, the surplus power detection system achieves this by turning on an immersion heater when the surplus power detection system senses that power is being exported to the Grid.

The surplus power detection system comprises three elements, a sensing element 18, a load control element and a system manager element, both of the latter two being provided in a main unit 20. The sensing element provides the ability to measure the magnitude of a current flow in real time, the load control element provides the ability to control a load on the system in real time and the system manager element provides the central intelligence, control and reporting that makes all the elements into a complete system.

The component parts of the surplus power detection system need to communicate. However the cost of providing this ability by wireless is relatively high compared to having a simple, physical connection between the components. The components thus need to be able to operate with a physical io connection or a wireless connection when the latter results in a significant reduction in the cost of installation.

Although the use of the IP (Internet Protocol) over WiFi is becoming generally available, it may not be best suited to control and monitoring of loads. Many loads will have issues with providing the power required to maintain an IP link, access to a wired cable for the link or be too cost sensitive to provide an IP standard Wi-Fi link. In addition, the central processing power required to maintain such links can become prohibitive as the quantity of links increases.

The surplus power detection system will provide a system manager element that forms an interface between low cost wireless solutions for peripheral loads (for example, EnOcean or ZigBee) and the need to provide IF based identification and communication to enable real time control and communication with wider area systems.

The surplus power detection system uses an adaptive algorithm to provide the ability to measure total energy flow into or out of a site without requiring a special purpose component. Similarly, as the system manager can have access to all the data on each peripheral load, it is ideally placed to report back to a central monitoring point and to react to instructions from that point for control of detailed elements of the building based on centrally determined criteria such as energy tariffs.

By monitoring the effect on Grid power of a change in the load provided by an energy load, the components of the surplus power detection system optimise, or adapt, the amount of power diverted to the energy load in such a way as to track and utilise the surplus, locally generated power. The system manager changes the power diverted to the energy load by a small increment (e). This will change the amount of power being drawn from or exported to the Grid by the same amount (e).

If prior to the change power was being drawn from the Grid, and there is an increase in the amount diverted to the energy load, then the amount of power being drawn from the Grid will increase by the same amount. However, if prior to the change power was being exported to the Grid and there is an io increase in the amount diverted to the energy load, then the amount of power being exported to the Grid will decrease by the same amount. Similarly, if prior to the change power was being drawn from the Grid, and there is a decrease in the amount diverted to the energy load, then the amount of power being drawn from the Grid will decrease by the same amount. And, if prior to the change power was being exported to the Grid and there is a decrease in the amount diverted to the energy load, then the amount of power being exported to the Grid will increase by the same amount. Thus:

A: = e if power is being drawn from the Grid, and B: e59 = -e if power is being exported to the Grid.

Using this information, the system manager diverts less power for local use while A is true and diverts more power for local use when B is true. The surplus power detection system thus tracks the amount of excess local power being produced by continuously evaluating which of A or B is true and will settle around the optimum point where the maximum power is being diverted to the local energy load without reaching the point where power is drawn from the Grid. In the simplest implementation, the load that is placed on the system will also be the location for storage of surplus power, but this does not have to be the case. For example, a light bulb could be turned on to determine whether there is surplus power available, but any surplus could then be diverted to an immersion heater or a battery store.

With the adaptive control, the surplus power detection system only needs a single sensor to monitor the amount of current flowing to/from the Grid. Adaptive control needs a measurement that is consistent but not highly accurate. There is no need to determine power or power factor or to use sensors that measure current and voltage or for accurate calibration of the sensor.

A simple sensor can be used that clips around the cable from the main io Grid meter. Its reading can be transmitted to the main unit (which comprises the load controller and the system manager) over a licence free wireless connection. The sensor can be self-powered or use a long life battery thus requiring no connection to the mains supply. Costs are reduced with only one, simple sensor. A wireless sensor costs more but this is offset by the saving in installation costs. A self-powered sensor will require minimal maintenance but may not be worthwhile if the cost is higher than using a battery especially if the system manager component can detect and warn of impending battery expiry.

The surplus power detection system requires a known load to be turned on and off in order to track surplus energy. Typically this load will be an energy store such as an immersion heater. The optimum solution will come down to the one with the lowest cost, taking into account not just the system cost but also the cost of installation.

The preferred solution is to use phase control. Metal oxide semiconductor field effect transistors (MOSFET5) or similar devices such as an Insulated-gate bipolar transistor (IGBT) are the best devices for implementing phase control. By controlling the gate drive, the rate of rise or fall of load current can be controlled providing a low cost method of ensuring that the amount of EMI produced is within regulatory limits.

As these devices can turn the load current off as well as on it also means that the load can be turned on at the zero current point at the beginning of each cycle and then off part way through the cycle. This avoids any issues with loads that have a current inrush, such as a tungsten lighting load or a transformer load. The speed of response of these devices is also fast enough that it is possible to use their turn off capability to protect the system manager and circuit in the event of a sudden overload or short circuit load. Additionally the circuit can sense if there is no load connected.

Basic installation for using stored hot water is straightforward and low cost. It is assumed that a local isolating switch for the energy store is already properly installed. For example, a typical installation would have an existing immersion heater with its own nearby, fused, double pole isolating switch. The installer should be a competent person as required for a domestic electrical installation. No plumbing knowledge is required as the existing immersion heater is used and so it is not necessary to drain the hot water cylinder. Only standard tools and consumables are required and the only complication is ensuring the wireless signal is available at the desired location. Otherwise, the installation should be capable of being carried out in a morning.

The following steps are for a typical installation of the surplus power storage system into an existing situation comprising a hot water cylinder and immersion heater: clip the current sensor around the Grid meter phase conductor, connect the sensor to its self-powered transmitter unit, temporarily connect the new main unit to a mains supply, identify a suitable location for the main unit where it can be wall mounted near the existing energy store isolating switch and ventilation and the wireless signal are both available, check the appropriate indicator to ensure that the wireless connection works at this location, disconnect the main unit and isolate the immersion heater circuit, wall-mount the main unit at the selected location, disconnect the immersion heater from its isolating switch, connect the mains input terminals in the main unit to the isolating switch, connect the mains output terminals in the main unit to the special immersion heater using the existing, heat resistant, flexible cable, reconnect the immersion heater circuit, turn on the isolating switch and check for correct operation. The system starts normal, automatic operation as soon as power is applied.

The key control input to the surplus power detection system is the magnitude of the current flowing to or from the Grid. The first part of the system is a current sensor mounted by the Grid meter. In a preferred embodiment, the sensor is a current transformer that measures the magnitude of the current flowing through the cable to which it is clipped. The current sensor can be self-powered; it gets the very small amount of energy it needs from the current transformer it uses to sense the current flow. Alternatively it can use a battery giving several years of use.

The values measured by the sensor are transmitted directly or over a wireless link to the other part of the new system, the main unit, on a regular basis, for example every six seconds. The sensor does not need to transmit io continuously. This minimises the energy it requires to operate and the main unit gets no benefit from continuous readings. However if a large current increase is detected due to, for example, turning on a kettle, then the change could be sent sooner than the regular schedule to minimise any time lag in response. The design of the main unit means that there is no need to know the direction of the current flow, only its magnitude.

The mains electricity supply is the power input to the load controller.

The power is mainly used by the load controller to control the amount of power going to the energy load but a small amount is used to power the system manager. It is purely a power input and is not used for control purposes. The main unit has a set of three terminals (phase, neutral and earth) to connect it to the mains electricity supply to the energy store. It is effectively inserted into the supply to the energy store which is now connected to one of the outputs of the main unit.

The main unit has a push button 22, as shown in Figure 3. Alternate presses of the button toggle the system manager between normal automatic mode and a boost mode. It is the only user operated control on the main unit.

The operation of this control can be overridden by a demand satisfied control input. Using the control is also a very effective and simple self-test facility, meaning that if all is well, then pushing the button so the unit enters boost mode will immediately cause an indicator to come on. In automatic mode, the system manager gradually turns on the output to the energy store by an amount required to utilise the locally generated power that would otherwise be exported to the Grid. It will not use power drawn from the Grid.

In the boost mode, the system manager turns the output full on, so that the energy store draws the maximum amount of power that it can from the mains supply, even if this requires extra power to be taken from the Grid. If the energy store is an existing immersion heater, this provides what was previously the normal operation of the heater. For a more sophisticated form of energy store it provides a means of initially charging the store. In boost mode the indicators operate as usual, for example they will still show if local power is io available.

The main unit has two volt free terminals for connecting an external, mains electricity voltage control signal, the demand satisfied input. When mains voltage is present, the output of the main unit is effectively shut down. It is used to prevent further power being diverted to a full energy store, for example water temperature is at maximum, even if there is locally generated power available. Any energy store will suffer losses, so a small amount of power can be allowed to go to the energy store even when it is shut down.

This allows the rest of the main unit to continue to operate normally. The output will continue to have very small increments of power sent to it in order that the system manager can monitor whether or not surplus, locally generated power is available.

The main unit has a set of two larger terminals (neutral and earth) and multiple smaller (phase) terminals, one for each connected load. The output from the logic controller will energise the phase output terminals locked to the mains frequency in such a way as to avoid electromagnetic interference. The output stage controls the amount of power sent to the energy store by energising the output terminals as required to provide the required energy load to match the amount of excess power available from local generation.

Overcurrent protection can be achieved by using switching devices large enough to be protected by a fast acting fuse. However, to achieve this the device uprating and the number of devices required is likely to be too expensive. The driver stage of a MOSFET can be designed to rapidly turn off the device in the presence of fault conditions allowing a lower rated and therefore lower cost device to be used. The load waveform must only alter in complete mains cycles, i.e. half cycles are identical, so no dc voltages are produced that could cause galvanic effects.

The main unit is also provided with indicators 24 for user feedback. The main purpose of the indicators is to feed back the state of the system to the user. The feedback required by a user can comprise system on (the system has power), sensing (the wireless link is active), exporting (energy is being exported to the Grid), storing (excess, locally generated energy is being io diverted to the store), store boost (the store is being charged whether or not excess energy is available), store full (the store cannot accept any more energy) and system error (an error condition has been detected). The system error indication could be expanded into indicators for different faults.

The terminology for each indicator can vary by market and use. For example, if the energy store is a water heater then the word heating may be preferred to storing, the latter being more appropriate for a battery energy store. Thus a replaceable method of labelling the indicators is preferred. As users are prone to losing manuals, all key safety and use instructions should be on the main unit.

LEDs are an ideal means of indication. A single indicator could be used for each element. However, for further cost reduction it is possible to use multi-mode indication. This could be multi-coloured LEDs but this is not likely to be as cost effective as firmware driven multi-mode indication. This is achieved by each indicator being a single, low cost LED that can have four possible modes, off, slow pulse, on solid, and fast flash. If more than one mode could be valid then the order in which the modes will be displayed is fast flash on solid slow flash -÷ off. For example, if an indicator could be on solid or fast flash then the fast flash will be displayed. On initial power up, all indicators light on solid for six seconds as a self-test.

The main unit can provide error indication. Some potential error states include: no wireless connection, unstable environment, no load, output overload, an invalid machine state which can be stored in local flash for later diagnosis. The main error indication required will be that there is no wireless connection as this is essential to commissioning the device. As these are error states, it is required that they are clearly identifiable as such to the user. Two standard methods for achieving this are to use a red colour or a fast flash both S of which impart a degree of urgency and should only ever occur if user action of some sort is expected.

One option would be to use a separate red LED to indicate fault states.

But this would only have three possible error indication modes and it is likely that the number of error states that would be useful to the user may exceed io three. Therefore, use of a separate fault LED is not the ideal solution. The fast flash mode should be used for warnings and errors. Various combinations of indication are possible: Option 1 This, preferred option maximises the use of full off or full on indications.

Oil Slow Pulse On Solid Fast Flash 1-Supply No Supply Wireless Activity* No Wireless No Radio Link Activity 2-Divert No Diversion Excess Being Internal Error IDivrtcd 3-Full Energy Load On Boost No Energy Load No Load/Overload Availabic Availabic 4-Export No Exportrng Excess Exporting Unstable State * For more sophisticated feedback this could be a short off period, say a half second, corresponding to the actual sensing transmission being received.

Thus in typical use the indicator would be on solid and momentarily turn off every few seconds.

This would give the following typical indications: State Supply Store Full Export No Mains Supply Auto Mode No excess local powei Auto Mode Local powet as ailable c= Local eneig load Auto Mode -Local power available> Local energy load Boost Osernde On -No local powei) Boost Osenide On -Local powei asailable <= Local eneigy load Boost Osenide On -Localposkei asailable > Local eneigy load Option 2 0f1 Slow Pulse On Solid Fast Flash 4 1-Supply No Supply Unit On No load 2-Status Boost Auto Mode Demand Satisfied Internal Error 3-local Power fmrn Grid Using Local Powei Excess Expoiting No Radio Link Power 4-Output Output Off Output Modulating Output On Overload This would give the following typical indications: State Supply Status Local Output No Mains Supply c. C: a a Auto Mode No excess local posser Auto Mode I ocal posser asailable <=1 ocal eneigy load Auto Mode Local powet asailable > Local eneigy load Boost Overnde On -No local poss ci available Boost Os crude On -Local poss ci as ailable <= Local eneigy load Boost Osenide On -Local powei asailable > Local energy load Option 3 s Boost (above) E Override and Output On (below) Demand Satisfied (above) E Override and Output Off (below) Using Local Power (above) E Using local power and Output Modulating (below) Excess Being Exported (above) E Using local power and Output On (below) 011 Slow Pulse S On Solid * Fast Flash 1-Supply No Supply I mit On No Load 2-Srnrns O%eriide Auto lntennl l-rioi 3-Local Po er from (iiid Stoirng No Radio Link Power 4-Output Output Ott Output Modulating Output On Oxeiload This would give the following typical indications: State Supply 5tatt Local Output No Mains Supply C: Aulo Mode -No excess local power C: C Auto Mode I oal poser a\ailahle <= I oal eneigy load Aulo Mode -Local power available > Local energy load: : : Boost Override On -No Storage (no local power) 4: Boost Ovcrridc On -local power availahle c= Local encrgy load c.4 Boosi Ovcnide On -Local power available > Local energy load It is also possible to produce a low cost, low power version of the main: S unit (1Kw instead of 3Kw load). To cut costs only two indicators would be used and there would be no demand satisfied control input Option 1 Off Slow Pulse S On Solid S Fast Flash No Supply Boost On No WuCILSS 2 Using kxat Lncrgy Hxccss FuLigy OR Option 2 Oil Slow Pulse S On Solid S Fast Flash S 1-Supply No Supply Ou No WnLlcss 2-Si this Using bc il Lneigy Excuss Lueigy Boost Excess energy is a key indicator for the low power solution as it indicates a larger device is needed. This would give the following typical indications: State Supply Status NoMispp1y Auto Mode -No excess local power Auto Mode -Local power available <= Local energy load Auto M)de -Local power available > Local energy load Boost Override On -No locai power Boost Override On -Local power available <= Local energy load C: Boost Override On -local power available> local energy load C: The system manager 26, shown in detail in Figure 4, is responsible for decoding the various inputs to the system and producing the various intermediate values and the output to drive the load controller output stage. No complex mathematical calculations need to be carried out during normal operation. The system manager, in a preferred embodiment, is a firmware implementation of logic and is provided with a logic controller 28. It has two io key functions, to ensure the system remains stable and to carry out the adaptive control described above. System stability has to be able to deal with shocks to the system. A local power system is subject to sudden and unpredictable changes in load, for example an electric kettle or other appliance being turned on as well as sudden changes in generation, for example the is passing of a cloud.

The system manager 26 is provided with a DC power supply 30, which is powered from a mains connection 32. The sensor 18 is connected to the system manager 26 by a sensor connection 34. On the output side, the system manager 26 is connected to one or more local loads through the output connections 36. The system manager 26 is also provided with a wireless system 38 for the sensor and a Wi-H connection 40 (optional). The logic controller 28 of the system manager 26 is also connected to the push button 22 and the LED indicators 24.

The surplus power detection system will be in one of three states: stable tracking or unstable. Generally, the load on the electrical system and the power generated do not fluctuate wildly. In a stable state, the Grid current will not change significantly. In this state, applying a change to the output load will result in a predictable change to the Grid current allowing the system manager to determine if power is being exported. In the stable state, the core process causes a known change in output power and then monitors the corresponding change in Grid power to identify if power is being used from or exported to the Grid.

io The sensor is used to measure the magnitude of the current flowing through the Grid connection. It is possible that the lowest cost sensors are not reliable when sensing low magnitudes of current, i.e. the current that is flowing when there is a change from using Grid power to exporting power to the Grid.

Similarly, other connected equipment such as an inverter can cause unexpected changes in current flow around this point. If this is the case then the system manager will have to be able to identify such a situation and vary the size of the incremental change accordingly. As the controller operation requires incremental changes and the output stage needs a mapping of power to phase angle, the system will operate on the basis of switching the output in known increments rather than being continuously variable.

A simple sensing system cannot reliably detect if the power flow to/from the Grid changes as a result of a sudden change, for example if the system suffers a step change from exporting 1KW to importing 1KW a simple, magnitude sensor will not register a change. For example, if the power flow changes from -1 to +1, then before and after the change, the sensor detects magnitudes of 1 and 1, indicating no change. Therefore, when in the stable state, the system must periodically check that there has not been an undetected change. The system manager can do this by running the core process. This also provides a side benefit as this ensures there is always a Grid power flow to allow the sensor to be self-powered.

When the surplus power detection system is running, the core process is to track the amount of surplus power being generated locally. Using the adaptive control algorithm, this involves changing the local energy load and sensing the effect it has on Grid power. When this occurs, either too much power can be diverted to local use leading to the unnecessary use of Grid power or too little can be diverted to local use leading to the unnecessary export of power that could be used locally. Both conditions will potentially reduce the overall efficiency of the system.

Therefore, to increase the system efficiency, running the adaptive algorithm process needs to be carried out quickly but infrequently. A fully optimised system would tend to settle at the point where there is no Grid io current. If the stable state is biased towards allowing a small export current or using accelerator techniques (see below) then the system efficiency can be improved. It is anticipated that the loss in system efficiency can be kept to less than 2% of the overall power diverted to local use.

One method of optimising the core process will be to sense the grid current twice within a short period of say less than half a second and repeat the process at much longer intervals. This can be achieved by the sensor sending two readings of Grid current to the system manager. The readings will be taken a fixed, short time interval apart. This interval will be chosen to allow enough time for the system manager to receive the first reading and immediately change the output to the load as required by the tracking algorithm so that the second reading will occur after the load has been changed. Even allowing for a delay in transmitting the readings by wireless this whole process can be completed in less than 200mS. The sensor then waits, say 10 seconds, before repeating the process. In this way, the sensor power requirements are dramatically reduced allowing it to be powered by a long life battery or by energy harvesting. Any extra energy used to perform tracking will only occur at the most for 200mS every few seconds thus achieving high system efficiency.

When there is a sudden change to the system there will be a large, one-off, unexpected change to the Grid current. This will usually be followed quite quickly by a return to a stable state. Based on the recent history, the system manager will react to the change in such a way as to track the amount of local power available and divert the surplus to local use. Various techniques can be used to accelerate this process.

At its simplest level, the system manager could always change the output load by single small increments. However, if the Grid current is being monitored at intervals of several seconds, a large period could elapse before the change is fully tracked. If the change has been caused by adding a large load to the electrical system, such as a kettle, then during this time the system could be drawing power from the Grid. To minimise the unwanted use of Grid power the rate at which changes are tracked can be accelerated by changing io the output load in multiples of an increment at a time during large changes.

The number of increments can be determined by looking at the magnitude of the change and using a binary chop approach or simply doubling the incremental change while the system is in the tracking state. In this way the rate of increase in diversion of power to the energy store can be slower than the rate of decrease thus biasing the system to less use of Grid power and increasing system efficiency, as shown in Figure 5.

An alternative could be a controller determined cycle. The sensor element could default to taking a set of two readings and then wait a few seconds until enough energy has been harvested until it can start again or, if battery powered, take a set of readings say every three seconds. If two way communications can be established between the sensor element and the system manager then the system manager could tell the sensor element when to take the next reading based on its knowledge of the immediate past history of Grid power flow. During stable conditions the interval between each set of two readings could be increased significantly to several tens of seconds.

Alternatively, the sensor element could default to taking but not sending a single reading every few seconds as determined by the time it has harvested enough energy or, if battery powered, take but not send a reading every three seconds. It would then take and send a set of two readings at a pre-set, longer interval, say every twenty seconds, but would do this every few seconds whenever it detects a significant change in Grid current flow.

If the system is subject to continuous shocks, for example during a thunderstorm, it is possible that the controller is unable to reach a stable state for some time. This would be an unstable system. In these circumstances, the controller must cease diverting power to the load until stability returns.

Figure 6 summarises the operation of the surplus power detection system, which comprises three components, the sensor, the load controller and the system manager. In the preferred embodiment, the load controller and the system manager are combined into a single main unit, which is located in the mains supply to a local energy load that is using the surplus power. The io sensor is located at the connection to the general power grid and in the preferred embodiment is monitoring the current that is flowing through this connection.

The system manager, as discussed, is connected to the sensor and the load controller and changes the power diverted to the local energy load by a defined amount and then detects whether the change in the energy flow in the connection to the general power grid is equal or opposite to the change in the power diverted to the local energy load. This is processed by the system manager and the system manager will increase the power diverted to the local energy load if the change in the energy flow in the connection to the general power grid is opposite to the change in the power diverted to the local energy load and vice versa.

Claims (11)

1. CLAIMS1. A surplus power detection system for use in a power arrangement comprising a local power generator, a connection to a general power grid, a local energy load and a network connecting together the local power generator, the connection to the general power grid and the local energy load, wherein the surplus power detection system comprises: o a sensor connected to the connection to the general power grid and arranged to measure the magnitude of the energy flow in the io connection to the general power grid, o a load controller connected to the local energy load and arranged to control the flow of power to the local energy load, and o a system manager connected to the sensor and the load controller and arranged to: § change the power diverted to the local energy load by a defined amount, § detect whether the change in the energy flow in the connection to the general power grid is equal or opposite to the change in the power diverted to the local energy load, and § increase the power diverted to local energy usage if the change in the energy flow in the connection to the general power grid is opposite to the change in the power diverted to the local energy load.
2. 2. A system according to claim 1, wherein the load controller and system manager are contained in a single main unit.
3. 3. A system according to claim 1 or 2, wherein the sensor is connected to the system manager via a wireless connection.
4. 4. A system according to claim 1, 2 or 3, wherein the sensor comprises a current sensor arranged to measure the magnitude of the current flowing in the connection to the general power grid.s
5. 5. A system according to any preceding claim, wherein the sensor is self-powered and is arranged to take the power it requires from the connection to the general power grid.
6. 6. A system according to any preceding claim, wherein the sensor io is arranged, during normal operation, to periodically transmit the measured magnitude of the energy flow in the connection to the general power grid to the system manager with a predefined time interval between each transmission.
7. 7. A system according to any preceding claim, wherein the sensor is arranged to detect a change in the magnitude of the energy flow in the connection to the general power grid to the system manager above a predefined threshold and to immediately transmit the detected change in the magnitude of the energy flow in the connection to the general power grid to the system manager.
8. 8. A system according to any preceding claim, wherein the system manager is arranged to use phase control to change the power diverted to the local energy load by a defined amount.
9. 9. A system according to any preceding claim, wherein the system manager comprises a user interface arranged to provide information concerning current energy usage within the surplus power storage system.
10. 10. A system according to claim 9, wherein the system manager further comprises a user interface component to switch the system manager to a boost mode where the system manager increases the power diverted to the local energy store to the maximum possible.
11. 11. A system according to any preceding claim, wherein the system manager is arranged, when increasing the power diverted to local energy usage to increase the power diverted to the local energy load.S12. A system according to any preceding claim, wherein the local energy load comprises a local energy store.13. A system according to any preceding claim, wherein the system io manager is further arranged to track local power consumption, periodically changing the power diverted to the local energy load by a defined amount, detecting whether the change in the energy flow in the connection to the general power grid is equal or opposite to the change in the power diverted to the local energy load, and decreasing or increasing the power diverted to local is energy usage if the change in the energy flow in the connection to the general power grid is respectively equal or opposite to the change in the power diverted to the local energy load.