FIELD OF THE INVENTION

The invention relates to monitoring and indentifying appliances connected to an electrical installation. BACKGROUND OF THE INVENTION

There is an increasing need for metering the power consumption of appliances, e.g. in a home or a small-medium enterprise. This may be done by power meters which are connected to each appliance. Clearly, this requires numerous power meters or if only a single power meter is available, it has to be moved between appliances to monitor the power load. The power meter may also not have the capability of monitoring the variation of power consumption during a day, e.g. for detecting undesired high power consumption of an appliance.

Accordingly there is a need for monitoring appliances in a more advanced way, for example to enable monitoring of all appliances in a house and to enable monitoring of the power consumption as a function of time.

WO 2009103998 discloses a method of inference of appliance usage from a point measurement on a supply line, said supply line being common to multiple appliances and/or components of appliances and comprising the steps of: obtaining data from said measurement point; sampling power and reactive power at intervals substantially throughout periods of operation of said appliances or components of appliances corresponding to appliances or components of appliances being in ON and/or OFF modes of use; identifying characteristics of events by assessing power and reactive power change during an event; and by assessing one or more additional characteristics derivable from said power and reactive power to characterize an appliance; grouping events and/or cycles of events into clusters of similar characteristics; and inferring appliance usage based on said grouping.

Whereas WO 2009103998 discloses a method for monitoring appliances from a point measurement by identifying characteristics of events there is a need to improve the reliability of such identified characteristics and monitoring of appliances. SUMMARY OF THE INVENTION

It would be advantageous to achieve improved reliability of determining operational states of appliances. It would also be desirable to enable an apparatus connected to an electrical installation to determine the operational status of a plurality of appliances connected to the electrical installation. In general, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination. In particular, it may be seen as an object of the present invention to provide a method that solves the above mentioned problems, or other problems, of the prior art.

To better address one or more of these concerns, in a first aspect of the invention a training apparatus for a load monitoring apparatus according to a second aspect is presented, the load monitoring apparatus being configured for determining an operational state of each of a plurality of electrical appliances connected to an electrical installation powered by a power source, where the training apparatus comprises,

a voltage controller connectable with the electrical installation for controlling a voltage on the electrical installation independently of the voltage supplied by the electrical installation,

a training sensor connectable to an appliance and the electrical installation for recording appliance signatures of the appliance obtained from electrical measurements of the appliance,

a transmitter for transmitting one or more appliance signatures of the appliance to the load monitoring apparatus.

Normally, the voltage supplied to an appliance may vary, e.g. the root-mean square (RMS) or peak voltage value may vary. The current drawn by the appliance or the power usage of the appliance may depend linearly or nonlinearly on the supplied voltage. Accordingly, appliance signatures obtained from measurements of the appliance may depend on the voltage and, therefore, it may be difficult to obtain reliable appliance signatures.

However, by controlling the voltage using a voltage controller the voltage will not fluctuate unpredictably and, therefore, it is possible to obtain reliable appliance signatures. The voltage controller may be permanently fixed to the electrical installation so that the voltage is controlled both during training and during normal operation. Alternatively, the voltage controller may be removably connected so that the voltage is only controlled during use of the training apparatus. The transmitter may be a plug or a socket connectable with a corresponding plug or socket of the load monitoring apparatus. The transmitter may also be configured as a wired or wireless transmitter. A second aspect of the invention relates to a load monitoring apparatus for determining an operational state of each of a plurality of electrical appliances connected to an electrical installation powered by power source, the apparatus comprises,

a receiver for receiving one or more appliance signatures transmitted by the training apparatus according to the first aspect,

a state detector connectable with the electrical installation for determining an operational characteristic of one of the appliances determined from one or more

measurements on the electrical installation,

a processor for comparing the determined operational characteristic with the appliance signatures,

a processor for attributing an operational state determined from the operational characteristic to one of the appliances selected depending on the result of the comparison of the operational characteristic with the appliance signatures.

Thus, the second aspect relates to a load monitoring apparatus or equivalently a state monitoring apparatus. By comparing operational characteristics, i.e. electrical characteristic, determined from measurements of an appliance with appliance signatures it possible to infer the actual operational status of the appliance, e.g. the actual power usage. Accordingly, the actual operational state may then be attributed to an applicance which was selected as a result of the comparison of the determined operational characteristic with the applicance signatures. An appliance may have a single appliance signature being

characteristic for the ON status of an appliance, or an appliance may have a plurality of appliance signatures being characteristic for different states of the appliance, e.g. washing, tumbling and heating states of a washing machine. The load monitoring apparatus advantageously has a receiver for receiving the appliance signatures obtained by the training apparatus. The load monitoring apparatus may be fixedly or removably connected to the electrical installation.

A third aspect of the invention relates to a load monitoring system for determining an operational state of each of a plurality of electrical appliances connected to an electrical installation powered by power source, the system comprises,

- a voltage controller connectable with the electrical installation for controlling a voltage on the electrical installation independently of the voltage supplied by the electrical installation, a training sensor connectable to an appliance and the electrical installation for recording appliance signatures of the appliance obtained from electrical measurements of the appliance,

a state detector connectable with the electrical installation for determining an operational characteristic of one of the appliances determined from one or more

measurements on the electrical installation,

a processor for comparing the determined operational characteristic with the appliance signatures,

a processor for attributing an operational state determined from the operational characteristic to one of the appliances selected depending on the result of the comparison of the operational characteristic with the appliance signatures.

The training apparatus according to the first aspect and the load monitoring apparatus according to the second aspect may be combined as a system in which case the transmitter/receiver pair may be omitted since the system may be an integrated system.

However, this does not exclude that the system may be modular and provided with a transmitter/receiver pair.

In an embodiment according to the first or third aspect the voltage controller is capable of maintaining a substantially constant average voltage on the electrical installation independently of the voltage from the power source. The voltage may be constant or substantially constant, e.g. within some tolerances of a few percent, in the sense that the RMS voltage or peak voltage is maintained at a constant level. When the voltage is kept at a constant level, a more reliable and correct attribution of the operational state to an appliance may be obtained since the appliance signature and the operational characteristic may become independent of voltage fluctuations of the power source. Accordingly, uncertainties of a match between appliance signatures and operational characteristics during the comparison may be reduced significantly.

In an embodiment according to the first or third aspect the voltage controller is capable of varying the voltage on the electrical installation independently of the voltage from the power source. By varying the voltage it is possible to obtain appliance signatures for a range of voltages. This may be an advantage since operational characteristics may include the voltage at which the operational characteristic was obtained and, therefore, the operational characteristic can be compared with an appliance signature which was obtained at a similar voltage during the training. In this case, a voltage controller does not have to be used by the load monitoring apparatus or the load monitoring system. Since the operational characteristics may be compared with appliance signatures at a range of different voltages a more reliable and correct attribution of the operational state to an appliance may be obtained since the appliance signature and the operational characteristic may become independent of voltage fluctuations of the power source. Accordingly, uncertainties of a match between appliance signatures and operational characteristics during the comparison may be reduced significantly.

In an embodiment the load training apparatus or the load monitoring apparatus according to the first and third aspects further comprises a training controller for controlling the variation of voltage output of the voltage controller, during a training period, and where the training sensor is further configured for recording the voltage output of the voltage controller as appliance signatures. Accordingly, by recording the voltage output of the voltage controller it is possible to obtain appliance signatures for a range of voltages where each signatures pertains to a particular voltage.

In an embodiment according to the first, second and third aspects, the appliance signatures comprises one or more parameters selected from the list: current, power, electrical phase, admittance, transients characteristics and frequency spectrum. Any of the electrical values may be obtained as real electrical values, imaginary values or complex values.

In an embodiment according to the second or third aspects the determined operational characteristic is stored as a new appliance signature if the operational

characteristic is not comprised by the set of appliance signatures. In this way new appliances or previously unrecognized operational characteristics may be recognized during monitoring as operational characteristics which do not have matching appliance signatures.

An embodiment according to the second or third aspects further comprises a user interface configured to enable a user to attribute an operational state to a new appliance signature. Accordingly, a user may attribute a particular state, e.g. an ON state, to a new appliance signature which has been obtained from an operational characteristic which did not have a matching appliance signature. The user may further attribute an appliance name to the new appliance signature for identification of the appliance.

In an embodiment according to the second or third aspects, an ON or OFF transition is attributed to the selected appliance depending on the monitored operational characteristic. In an embodiment the number of ON or OFF transitions attributed to a particular appliance is counted. By counting the number of ON/OFF transitions the number of identical appliances which are currently ON can be determined.

In an embodiment according to the first, second or third aspect the operational characteristic comprises a plurality of samples for characterizing the time evolution of the operational characteristic. In this way an operational characteristic may advantageously characterize high frequency components and spikes of electrical measurements on the electrical installation. It is understood that corresponding appliance signatures are obtained during the training phase.

In an embodiment according to the second or third aspects, the state detector is configured to obtain at least first and second operational characteristics where each of the first and second characteristics may be selected from the list of: voltage, current, power, phase and phase shift between current and output voltage during an ON/OFF transition. Accordingly, two or more operational characteristics may advantageously be obtained and compared with a same number of appliance signatures in a multidimensional signature space. Use of two or more operational characteristics and appliance signatures may improve the reliability in the attribution of operational states and identification of operational

characteristics.

A fourth aspect relates to a method for determining an operational state of each of a plurality of electrical appliances connected to an electrical installation powered by power source, the method comprises,

controlling a voltage on the electrical installation independently of the voltage supplied by the electrical installation,

determining an operational characteristic of one of the appliances determined from one or more measurements on the electrical installation,

comparing the determined operational characteristic with stored appliance signatures,

attributing an operational state determined from the operational characteristic to one of the appliances depending on the result of the comparison of the operational characteristic with the appliance signatures.

A fifth aspect relates to a computer program enabling a processor to carry out the method of the fourth aspect.

In summary the invention relates to a method for identifying the status of appliances connected to an electrical installation, for example a status indicating whether the appliance is ON or OFF. The identification is obtained by measuring one or more electrical values from the electrical installation such as voltages, power and current in various forms. The measured values are compared with previously obtained appliance signatures in order to obtain a match with one of the signatures. A positive match is used to set a status of the corresponding appliances. A status may be the current power consumption or the power consumption may be determined subsequently from the status. Since appliance signatures may depend on the supply voltage, a signature may contain a plurality of supply voltages and an electrical parameter, e.g. power consumption, for each of the voltages. The signatures may be obtained during a training process where the voltage supplied to the appliance is varied in a controlled way.

In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 shows a load monitoring system for determining an operational state of appliances,

Fig. 2 shows an embodiment of the load monitoring system where the training apparatus is detachable,

Fig. 3 shows a signature space of different appliance signatures for identification of operational characteristics,

Fig. 4A shows measurements of real power and complex power for an incandescent lamp,

Fig. 4B shows measurements of real power and complex power for a compact fluorescent lamp,

Fig. 5A shows current variations due to the load of a compact fluorescent lamp,

Fig. 5B shows current variations due to the load of an incandescent lamp, Fig. 6 illustrates a method of an embodiment. DESCRIPTION OF EMBODIMENTS

In an embodiment the invention relates to a method for determining the operational state, for example the power consumption, of electrical appliances, such as lamps, a television and a washing machine. According to an embodiment a monitor connected to an electrical installation is capable of determining operational characteristics of appliances connected to the electrical installation. An operational characteristic could be a change in current, a voltage harmonic in the electrical installation caused by turning ON a television, or a parameter determined from an electrical measurement. The operational characteristic gives an electrical fingerprint or signature of each appliance, or of different loads comprised by an appliance, such as pump and heater loads of a washing machine. By comparing a monitored operational characteristic with stored appliance or load signatures, it is possible to attribute the operational characteristic, e.g. the actual power consumption, to a particular appliance if the operational characteristic matches a stored signature. The stored appliance or load signatures may have been obtained during a separate training process from operational characteristics obtained by operating, e.g. turning ON, individual appliances in turn.

Accordingly, the appliance or load signature may be a direct measurement, or a processed measurement.

Whereas a single appliance may have several associated load signatures, e.g. signatures of the drive and the heater of a washing machine, in the following reference to an appliance signature may also be understood as a load signature.

Fig. 1 shows an embodiment of a load monitoring system 100 for determining an operational state, e.g. power or energy consumption, of each of a plurality of electrical appliances 120 connected to an electrical installation 110 powered by a power source 111. The electrical installation 110 is comprised by electrical wireing 110a located between the power source 111 and the monitoring system 100, and electrical wireing 110b after the monitoring system. The power source 111 may be a utility grid, a local power generator, a solar panel, the battery of an electrical car or other power sources. The appliances 120 are connected to the electrical installation 110 via electrical cables 121 for example using sockets (not shown) of the electrical installation.

The load monitoring system 100 may be connected in series or in parallel with the electrical installation. When the load monitoring system 100 is connected in parallel, the system may simply be connected via a plug to a socket of the electrical installation. When the load monitoring system is connected in series, the system is merely inserted in series with the power source 111 located on one side of the system and the electrical installation 110 located on the other side of the system.

The load monitoring system 100 comprises a voltage controller 102 connected to the electrical installation 110 for controlling the voltage on the electrical installation independently of the voltage supplied by the electrical wireing 110a of the electrical installation 110 being powered by the power source 111. Accordingly, even when the voltage of the power source 111 fluctuates, the DC or AC voltage on the electrical installation may be controlled independently of these fluctuations. Depending on the voltage controller 102, the voltage on the electrical installation 110 may be controlled to be constant or substantially constant, or the voltage on the electrical installation 110 may be controlled to vary as a predetermined function of time.

The load monitoring system 100 further comprises a separate training monitor or training sensor 104 connected to an appliance 120 and the electrical installation 110. Thus, the training sensor 104 may be connected to a socket of the electrical installation 110 and the appliance 120 may be connected to the training sensor 104, i.e. in series with the appliance.

Alternatively, the training sensor 104 may connected to any other socket of the electrical installation 110, i.e. in parallel with the appliance. During operation of the appliance the training sensor obtains appliance signatures from the appliance. The appliance signatures may be obtained from measurements of electrical values on the electrical supply cables 121, e.g. current or voltage values. Operation of the appliance 120 may be invoked by manually switching the appliance ON and OFF and monitoring electrical changes on the supply cables 121. Alternatively or additionally, operation of the appliance may involve the operation cycle, e.g. the washing cycle of a washing machine. Thus, during the operation of the appliance, electrical values, for example any changes on the supply cables 121 are recorded.

Accordingly, the training sensor 104 may be seen as a portable device which may have to be relocated to different sockets of the electrical installation 110 for recording of appliance signatures of different appliances. However, in case the training sensor is connected in parallel with different appliances 104, the monitor may not have to be relocated.

Since the voltage on the electrical installation 110 is controlled, the appliance signatures obtained from measurements by the training sensor are not inappropriately affected by voltage fluctuations on the electrical installation. The voltage may deliberately be controlled to vary, e.g. stepwise as a function of time, during a training phase. This variation of the supply voltage may be recorded by the training sensor 104 as a first appliance signature together with second appliance signatures. Thereby, the dependence of the second appliance signatures on the supply voltage can be obtained, e.g. as a range of first appliance signatures (supply voltages) linked with different second appliance signatures. This relationship between supply voltages and appliance signatures is useful during the load monitoring process as described below. The voltage controller may be controlled by a training controller 115. The training controller may be a separate device or may be comprised by a training apparatus 201 (see Fig. 2), the load monitoring system 100, the voltage controller 102, the training sensor 104 or any other component of the load monitoring system 100 or the training apparatus 201.

The load monitoring system 100 further comprises a state detector 101 which is connected with the electrical installation for determining operational characteristics of the appliances 120. The state detector 101 is arranged to measure electrical values on the supply connectors of the electrical installation 110. The state detector 101 may obtain the operational characteristics directly from the measurements or the operational characteristics may be a function of measured values. Examples of operational characteristics are described below.

The load monitoring system 100 further comprises a processor 105 for comparing the determined operational characteristic with the appliance signatures. The processor 105 may be part of the state detector 101 or the processor may be a separate device, e.g. a computer, located elsewhere. The appliance signatures may have been stored previously in a storage comprised by the load monitoring system, for example by the state detector 101 or the processor 105.

If an operational characteristic matches an appliance signature during the comparison, an operational state, e.g. power or energy consumption, may be assigned to a particular appliance. Also, a plurality of possibly different operational states may be assigned to a plurality of different appliances.

The assignment of the operational state may be performed by a processor comprised by the load monitoring system 100, for example the processor 105 or a different processor. Thus, the actual assignment is performed depending on the result of the

comparison of the operational characteristic with the appliance signatures.

The load monitoring system 100 may further comprise a user interface 103 enabling a user to see results of assignments of operational states. The user interface 103 may also enable the user to enter names for appliances which are monitored during the training phase, and to manually enter or modify such operational states. The voltage controller 102 and the training sensor 104 may be seen as a training apparatus where the voltage controller 102 is connectable to a connector of the load monitoring system 100. Accordingly, the voltage controller and the training sensor 104 may be connected only during the training phase and then removed.

The state detector 101, the processor 105 and possibly the user interface 103 may be seen as a load monitoring apparatus which may be fixedly or detachably connected to the electrical installation 110.

Fig. 2 shows an embodiment of a load monitoring system 100 which comprises the same devices as in Fig. 1, but where the training apparatus 201 is a detachable apparatus which comprises both the voltage controller 102 and the training sensor 104.

Thus, whereas in Fig. 1 the voltage controller may be used both during training and normal operation, in Fig. 2 the voltage controller is only used during training. In Fig. 2 the load monitoring apparatus 202, which does not contain the voltage controller, may be fixedly or detachably connected to the electrical installation 110.

The training sensor 104 or the training apparatus 201 may comprise a transmitter 108 for transmitting monitored appliance signatures, obtained during the training phase, to a receiver 109 comprised by the monitoring apparatus 202 or the state detector 101. The data transmission may be wired or wireless. Alternatively, during the training phase the training sensor 104 or the training apparatus 201 may store monitored appliance signatures, which are then subsequently communicated via the transmitter 108 and the receiver 109.

In an embodiment, the transmitter 108 of the training sensor 104 or the training apparatus 201 comprises a transmitter connector being connectable with a matching connector of the receiver 109 comprised by the monitoring apparatus 202, a storage device, the processor 105 or, for example, a computer. According to this embodiment, during training measurements are obtained by the training sensor 104 or the training apparatus 201 and stored in a storage component. When the training process has been completed the training sensor 104 or the training apparatus 201 is connected to a receiver 109 so as to transfer the monitored appliance signatures so that they are available for the monitoring apparatus 202.

The appliance signatures may comprise any electrical value which may determined from measurements of the current in the supply wires 121 and/or the voltage over the supply wires. Such measurements enable determination of changes in current, power, phase difference between voltage and current, appliance impedance or appliance admittance in response to a change of the operational state of an appliance. Any of these appliance signatures may be determined as real, imaginary or complex values. Alternatively appliance signatures may comprise electrical transients, e.g. voltage transients. As another alternative, appliance signatures may comprise harmonics, e.g. amplitudes of a number of harmonics or the number of harmonics which exceed a threshold amplitude. The harmonics may be obtained by Fast Fourier Analysis of voltage or current measurements.

One or a plurality of appliance signatures may be used to characterize an appliance 120.

The operational characteristics may comprise any of the same measurements or processed measurements as may be used for appliance signatures. Thus, operational characteristics may be seen as equivalents to appliance signatures, except that operational characteristics are only obtained during normal operation of appliances. Appliance signatures may be obtained both during the training phase and normal operation of appliances. For example, an appliance signature obtained during the training phase may be corrected or modified by measurements obtained by the state detector 101 during the normal operation.

The operational state of an appliance may be the current ON or OFF state, the current power or energy consumption, or other operational states or electrical values.

Transitions between ON and OFF states may be determined from measurements of real current by the state detector 101. For example, an increase of the real current would indicate an ON transition of an appliance or a load of an appliance. Similarly, the decrease of the real current would indicate an OFF transition of an appliance or a load. The appliance to which the operational state transition can be assigned is given by the comparison of an operational characteristic with appliance signatures, where the operational characteristic and the operational state transition occur within the same time interval.

Transitions between ON and OFF states may be determined from other measurements by the state detector 101, for example measurements of harmonics in the voltage or current. Measurements of harmonics can be used to determine a change in the power consumption which again can be used to determine an ON/OFF transition. Details of use of harmonics for determining power are described in published patent US 4,858,141 which is hereby incorporated by reference.

Once an operational state has been attributed to an appliance the energy usage per appliance can be determined. The determination of the energy usage may be determined by the state detector 101, a processor 105 or other processing means. For example, when ON and OFF switching states have been attributed to different appliances together with time information of the transitions, then the energy usage can be determined from knowledge of the real power consumption between ON/OFF transitions. These power consumptions may have been determined from measurements of real current or current harmonics. Alternatively, the power consumption may have been manually entered by a user via the user interface. For example, the power consumption of lamps may be entered manually as an alternative to measure the consumption.

The appliance signatures of different appliances 120 may be represented as a signature space 300 as illustrated in Fig. 3. The signature space may have one or more dimensions 301, 302, e.g. a first dimension 301 representing a first appliance signature relating to or equal to real power consumption and a second dimension 302 representing a second appliance signature relating to or equal to voltage which may be a voltage change commanded by the controlled voltage controller 102. The appliance signatures of first, second and third appliances 120 are represented by confined spaces 303-305, respectively. Accordingly, when the state detector 101 obtains first and second operational characteristics which match for example confined space 303, then an operational state, e.g. a switch to an ON state or a power consumption can be attributed to the appliance having the confined space 303.

The dimension of the signature space 300 need not be constant for each signature. For example, if only one appliance has the operational characteristic of a real power consumption of 2000 Watt, and no other appliances have a similar operational characteristic, it may be sufficient to describe this appliance with a single dimension in the signature space. However, if two or more appliances have one or more characteristics which are identical or similar, the signature space for these appliances may be expanded with more signature characteristics. Accordingly, the signature space can be considered as a dynamic space whose dimensionality changes depending on the appliance.

The state detector 101 may obtain operational characteristics which does not match any of the confined signature spaces 303-305 but is located between such spaces at a point 306 as illustrated in Fig. 3. The state detector 101, a processor 105 or generally, the load monitor 202 or system 100, may be configured to store such operational characteristics which do not match any previously stored signature spaces 303-305 as a new appliance signature. When such new appliance signature has been stored, the user interface 103 may prompt the user to indicate if the storage of the new appliance signature can be accepted or should be canceled and to enter which appliance should be associated with this new appliance signature. Often a plurality of identical appliances 120, e.g. a plurality of identical lamps, are connected to the electrical installation 120. In order to keep track on the operational status of identical appliances the monitoring apparatus 202 or the load monitoring system 100 may have an appliance counter associated with a particular appliance signature. If the state detector 101 determines an ON transition of an appliance signature the appliance counter is incremented by one. If the state detector 101 determines an OFF transition of an appliance signature the appliance counter is decremented by one. In this way the maximum number of identical appliances can be determined by the state detector 101 and the maximum number can be confirmed by the user via the user interface 103 or the user may edit the maximum number of a particular appliance.

The number of appliances given by the appliance counter may be used as an operational characteristic which can be compared with an appliance signature indicating the stored maximum number of appliances. Thus, if the appliance counter is equal to or below the maximum number of appliances this may be used to confirm a comparison between an operational characteristic and an appliance signature.

The actual total power consumption or an accumulated change of power consumption as determined by the load monitoring apparatus 202 may be used as an operational characteristic and equivalently as an appliance signature. For example, a number of appliances may have a unique power consumption which cannot be obtained by any other combination of appliances. Therefore, this unique power consumption can be used as a signature and an operational characteristic which indicates that a certain selection of appliances is ON.

Fig. 4A shows measurements of real power P and complex power Q of the complex power S for an incandescent lamp for supply voltages of 210 V, 230 V and 250 V as illustrated by references 401, 402, 403, respectively. I Fig 4A the real and imaginary power components have been normalized with respect to the supply voltage V. If the load of the appliance 120 has a constant admittance Y, the measured power is given by S=Y V ² and, accordingly, the normalized complex power Sn=S (230/V) ² would be independent of the voltage. However, normally the admittance Y is not constant and may depend on the supply voltage V. The dependence of the power consumption on the voltage is clear from FIG 4A, where particularly the real power P is shown to be dependent on the voltage.

Fig 4B is equivalent to Fig 4A and shows the real power P and complex power Q of the complex power S for a compact fluorescent lamp for supply voltages of 210 V, 230 V and 250 V as illustrated by references 411, 412, 413, respectively. Fig 4B shows some dependency on voltage of the real power P as in Fig 4A.

As described previously, this dependency on voltage may be compensated for by determining both first appliance signatures, e.g. voltage, together with second appliance signatures, e.g. power, so that the dependency of the second appliance signatures on the supply voltage can be obtained. When operational characteristics are obtained in a similar way, i.e. where both voltage and power are measured, the operational characteristic can be compared with signatures having the same voltage.

Using Fig 4B as an example, the complex power S is recorded together with the actual voltage. Thus, at the voltage of 250 V (ref. 413), the power is approximately

15.5+j21.6 VA , at a voltage of 230V (ref. 412) the power is 16.2+j22 VA, and at a voltage of 210V (ref. 411) the power is 17.5+j24.2. During the training, the appliance can be exposed to different voltages by controlling the voltage controller 102 to generate different voltages.

Fig 5 A shows how the current A due to the load of a compact fluorescent lamp varies as a function of time (msec) during one period of a 50 Hz supply voltage. Three curves indicated by references 501, 502 and 503 are obtained for supply voltages with RMS values of 210, 230 and 250 V, respectively. The current curves 501-503 shows that in addition to e.g. power or RMS values of current, also the shape of e.g. the current may depend on the RMS voltage value. Fig 5B shows a current measurements 511-513 equivalent to those in Fig 5A, for an incandescent lamp. In Fig. 5B the shape is substantially independent of the voltage RMS value as expected due to the linear behavior of this lamp.

Since the shape of curves, for example the current curves 501-503 and 511- 513 may be used as appliance signatures the shape dependency on voltage again suggests that appliance signatures may be obtained as a function of supply voltage to sharpen the recognition of appliances under varying supply voltages. The appliance signatures may be obtained as a function of RMS voltage values, peak voltage values or voltage shape that could include introducing high frequency components in the voltage waveform.

Fig 6 shows a method according to an embodiment comprising the steps: 601 : controlling a voltage on the electrical installation 110 independently of the voltage supplied by the electrical installation 110, 110a,

602: determining an operational characteristic of one of the appliances 120 determined from one or more measurements on the electrical installation 111,

603 : comparing the determined operational characteristic with stored appliance signatures, 604: attributing an operational state determined from the operational characteristic to one of the appliances 120 depending on the result of the comparison of the operational characteristic with the appliance signatures.

In step 601, the voltage may be controlled by a voltage controller 102 connectable to the electrical installation or fixedly connected to the electrical installation. In step 602 the operational characteristic may be determined by a state detector 101, e.g. a processor in a computer. In step 603 the stored appliance signatures may be stored in a memory, e.g. in a computer. Any of the steps 602, 603, 604 may be performed by an electronic circuit or a computer program enabling a processor to perform the steps. Also the voltage controller in step 601 may be controlled by an electronic circuit or a computer program.

The method may comprise an additional training step 605 of recording appliance signatures of appliances obtained from electrical measurements of the appliances by training sensor connectable to an appliance and the electrical installation. The training sensor may be controlled by a training controller, e.g. a computer or a computer program enabling a processor control the training sensor.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.