WO2009133494A1

WO2009133494A1 - Switching off an ac electrical appliance when local power supply generator is used

Info

Publication number: WO2009133494A1
Application number: PCT/IB2009/051645
Authority: WO
Inventors: Balakrishnan Srinivasan; Randeep Singh; Nagaraju Bussa; Rajendra S. Sisodia
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-04-29
Filing date: 2009-04-22
Publication date: 2009-11-05

Abstract

A switching system (SY) switches on and off an electrical appliance (EA) receiving power from an AC power signal (APS). The switching system (SY) comprises a detection device (DE) and a switching device (SW). The detection device (DE) detects whether the AC power signal (APS) is supplied by a mains grid (MG) or by a local power supply generator (INV). The switching device (SW) switches a supply of the power to the electrical appliance (EA) in dependence on a control signal (OS) from the detection device (DE).

Description

SWITCHING OFF AN AC ELECTRICAL APPLIANCE WHEN LOCAL POWER SUPPLY GENERATOR IS USED

FIELD OF THE INVENTION

The invention relates to a switching system for an electrical appliance receiving power from an AC power signal supplied by either a mains grid or by a local power supply generator. The invention further relates to an electrical appliance comprising such a switching system, and to a method of switching an electrical appliance receiving power from the AC power signal.

BACKGROUND OF THE INVENTION

Power cuts and outages in the mains grid commonly occur in many developing countries or on the country side of developed countries. These power outages are often unscheduled and thus really a nuisance. With the mains grid is meant the power supply grid interconnecting a power plant with a plurality of electricity consuming electrical appliances in households and/or plants or any other place. Usually, such a power plant is a public plant operated by or under the supervision of the government. Alternatively, such a power plant may be owned by a private firm.

To cater for the power outages, diesel driven electricity generators may be used as local power supply generators to supply AC power to the appliances in the household or elsewhere. However, these diesel generators create prohibitive levels of noise and smoke. Therefore, often battery-based inverters are installed which generate the AC power by converting the DC voltage of a battery (or a plurality of batteries) into an AC voltage resembling the AC voltage supplied by the mains grid.

However, these battery-based inverters have a limited power capacity and usually cannot supply power to all household appliances. Such a battery-based inverter may have a safety system which switches off the inverter if the power drawn is above its maximum capacity. Consequently, the user first has to switch off a sufficient number of appliances before the inverter is able to start operating.

A common practice is to partition the appliances in the household in two groups, one which will be backed up by the inverter and one which will not be backed up. The electrical circuit in the home has to be specially wired in accordance with the groups such that the circuit associated with the inverter is powered by the inverter when the mains grid has an outage. However, this requires an a priori planning of the wiring or a separate wiring of the circuit.

SUMMARY OF THE INVENTION It is an object of the invention to provide a switching system which does not require a special wiring for a subset of the electrical appliances.

A first aspect of the invention provides a switching system as claimed in claim 1. A second aspect of the invention provides an electrical appliance as claimed in claim 10. A third aspect of the invention provides a method of switching as claimed in claim 11. Advantageous embodiments are defined in the dependent claims.

A switching system for an electrical appliance in accordance with the first aspect of the invention receives power from an AC power signal. The switching system forwards this AC power signal to the electrical appliance if it is detected that the AC power signal is supplied by the mains grid. The switching system blocks the AC power signal to the electrical appliance if it is detected that the AC power signal is supplied by the local power supply generator. Although the local power generator may be of any type, for example a diesel generator or a battery operated inverter, in the following the local power supply generator is referred to as the inverter. The inverter supplies electrical power independent of the mains grid. Usually, the mains grid supplies an AC power signal with a voltage between 90 and 240 Volts with a frequency of 50 or 60 Hz, dependent on the country. Usually, the inverter supplies a comparable voltage.

The switching system comprises a detection device and a switching device. The detection device may detect whether the AC power signal is supplied by the mains grid or by the inverter on the basis of (differences between) characteristics of the mains power signal supplied by the mains grid on the one hand (and/)or characteristics of the local power signal supplied by the local power supply generator on the other hand. For example, such characteristics may be a shape of the voltage or current supplied by the power supply.

The switching device switches off the supply of the power to the electrical appliance if the detection device has detected that the AC power signal is supplied by the local power supply generator, and connects the AC power signal to the electrical appliance if the detection device has detected that the AC power signal is supplied by the mains grid. The detection device may do so by just detecting whether the AC power signal has the characteristics of a power signal supplied by the local power generator, and by switching the switching device accordingly. Alternatively, the detection device may do so by just detecting whether the AC power signal has the characteristics of a mains power signal, and by switching the switching device accordingly. Both alternatives are within the scope of the independent claims. The switching device may comprise a single switch in one of the power leads of the electrical appliance, or may be a double switch which is able to disconnect both leads of the electrical appliance from the AC power source.

Such a switching system may be a separate entity which is arranged in series with the power cord of the electrical appliance or may be incorporated in the electrical appliance. The switching system in accordance with the invention has the advantage that the electrical appliances which should be switched off when the mains grid has an outage are automatically switched off as soon as is detected that the inverter is (becoming) active. It is not required that the user has to switch off these appliances by hand, and no special electrical circuit is required in the home. A single switching system may control several electric appliances.

In an embodiment, the detection device comprises a band-pass filter, a comparator and a driver. The band-pass filter filters the AC power signal to obtain a filtered signal. The comparator compares the filtered signal with a threshold. The driver switches off the switching device if the filtered signal has a level indicating that a level of frequency components within the band-pass of the band-pass filter surpasses a predetermined level. Thus, if in the frequency band of the filter a higher level of the frequency components is detected than usual for a mains grid voltage, it is concluded that the inverter is active and thus the appliance should be disconnected from the inverter.

Usually, the inverter comprises electronic switches which generate an AC voltage from a DC battery voltage. Such inverters are well known and may comprise a full bridge of electronic switches and an inductor. The output of the inverter is connected between the junctions of the two half bridges which form the full bridge. In one phase of the full bridge, the top switch of one of the half bridges and the bottom switch of the other one of the half bridges is closed to supply the DC voltage in a first polarity to the output. In the other phase the bottom switch of the one of the half bridges and the top switch of the other one of the half bridges is closed to supply the DC voltage in the opposite polarity to the output. The inductor is used together with the switches mentioned to create an up-converter which converts the relatively low battery voltage to the required amplitude of the AC mains voltage. In such a half bridge, always only one of the series arranged switches must be closed to prevent a very large short circuit current through the series arrangement. In practical implementations this will lead to a dead-time between two consecutive phases during which dead-time both switches are open. During the dead-time a zero level may occur in the AC voltage generated which causes an increase of higher harmonics with respect to the sinusoidal AC voltage supplied by the mains grid. It has to be noted that it is expensive, both in terms of money and also in terms of efficiency of operation, to generate a sinusoidal signal comparable to that supplied by the mains power supply. Such inverters have an efficiency of about 80%-90% and hence are not very common. Pulse/signal shapers can also be used to get as close as possible to a sinusoidal signal but the fact remains that this is less efficient. Thus, instead of the dead-time related characteristics, other characteristics may be checked to determine whether the power is supplied by a local power supply or not. The increased level of the higher harmonics can be detected by comparing the band-pass filtered voltage with a threshold level. The threshold level is selected such that the band-pass filtered AC voltage of the mains grid is below the threshold while the band-pass filtered AC voltage of the inverter is above the threshold.

The selection of the center frequency and the bandwidth of the band-pass filter, and the threshold of the comparator can be determined experimentally by first filtering the AC voltage of the mains grid and then filtering the AC voltage of the inverter. Preferably the center frequency and the bandwidth are selected such that the difference between the filtered signals is maximal. It has to be noted that the actual shape of the AC voltage and thus the amount of harmonics usually depends on the input impedance of the electrical appliance. Especially electrical appliances with a capacitive input impedance (for example, air conditioners and display apparatuses) may cause higher high harmonics on the mains grid. It may be sorted out, dependent on the actual electrical appliance connected, how to select the band-pass filter and the threshold to optimally distinguish between the AC voltage from the mains grid and the AC voltage from the inverter. In a practical implementation, for example, the band-pass filter has a center frequency at or near to the 9^th harmonic of the mains power signal, thus in 50 Hz countries at 450 Hz, and the bandwidth may be 50 Hz.

The switching system may have a selection switch for switching the center frequency and/or bandwidth of the band-pass filter and/or the threshold of the comparator. The center frequency, bandwidth and threshold may be predefined or may be adjusted per home. In an embodiment, the selection switch may be implemented to switch between electric appliances which form a predominantly resistive load and electric appliances which form a predominantly capacitive load.

In an embodiment the switching system is digitally implemented and comprises an analog low-pass filter and an analog to digital converter. The low-pass filter low-pass filters the AC power signal to obtain a low-pass filtered signal to decrease or minimize aliasing when sampling the signal with the digital to analog converter. The analog to digital converter converts the low-pass filtered signal into a digital signal. The band-pass filter, which for example is a FIR band-pass filter, filters the digital signal. The comparator compares the value supplied by the digital band-pass filter with a threshold value. The digital implementation may use dedicated circuits or may use a processor which performs the actions described. An advantage of a digital implementation with a processor is that it is easy to build in the flexibility for coping with different situations wherein different characteristics occur of the AC voltage supplied by the mains grid or the inverter and of the electrical appliance.

In an embodiment, the switching system is digitally implemented and comprises an analog low-pass filter, an analog to digital converter and a Fourier analyzer. Again, the low-pass filter is selected to decrease the aliasing caused by the analog to digital converter. The Fourier analyzer performs a fast Fourier analysis on the digital signal supplied by the analog to digital converter to obtain an actual representation of the AC power signal in the frequency domain. The comparator compares the actual representation with a reference footprint in the frequency domain of the AC power signal. The reference footprint may either be the representation of the AC power signal in the frequency domain when supplied by the mains grid, or when supplied by the inverter. A decision on whether the AC power signal is supplied by the inverter can be taken by comparing the actual representation with the stored footprint. Alternatively, both footprints may be stored and the comparison of the actual representation with the footprint which has the smallest difference is decisive for which AC power supply signal is actually received.

In an embodiment, the switching system comprises a low-pass filter, an analog to digital converter and a cross-correlation analyzer. Again, the low-pass filter low-pass filters the AC power signal to obtain a low-pass filtered signal which is converted into a digital signal by the analog to digital converter. The cross-correlation analyzer determines a cross-correlation between the footprint of the actual digital signal and a reference footprint in the time domain of the AC power signal. The reference footprint may either be the representation of the AC power signal in the time domain when supplied by the mains grid, or when supplied by the inverter. A decision on whether the AC power signal is supplied by the inverter can be taken by comparing the actual footprint with the stored footprint. Alternatively, both footprints may be stored and the comparison of the actual footprint which has the smallest difference is decisive for which AC power supply signal is actually received. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically shows a block diagram of an embodiment of a switching system and an electric appliance,

Fig. 2 schematically shows a block diagram of an embodiment of a detection device of the switching system, Fig. 3 schematically shows a block diagram of another embodiment of the detection device,

Fig. 4 schematically shows a block diagram of yet another embodiment of the detection device,

Fig. 5 schematically shows a block diagram of yet another embodiment of the detection device, and

Fig. 6 shows a detailed electric diagram of an implementation of the switching system.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

DETAILED DESCRIPTION

Fig. 1 schematically shows a block diagram of an embodiment of a switching system and an electric appliance. The electrical appliance EA receives its power via the switch SW from either the mains grid MG which supplies the mains power signal MPS, or from the local power supply generator INV which supplies the local power signal LPS. The local power supply generator INV is further referred to as the inverter INV. The detection device DE receives the AC power signal APS which is either the mains power signal MPS or the local power signal LPS. The detection device DE checks at least one characteristic of the AC power signal APS and accordingly controls the switch SW with the control signal OS. Although a single switch SW is shown, alternatively, a double switch may be implemented which completely disconnects the electric appliance EA from the mains grid MG and the inverter INV. The switching system SY comprises the switch SW and the detection device DE. The switching system SY may be an add-on to the electric appliance EA or may be integrated into the electric appliance EA.

The characteristic of the AC power signal APS may be any characteristic which enables to distinguish between the mains power signal MPS of the mains grid MG and the local power signal LPS supplied by the inverter INV. For example, the characteristic may be the shape of, or the level of frequency components present in, the AC voltage received from the mains grid MG or the inverter INV.

Fig. 2 schematically shows a block diagram of an embodiment of a detection device of the switching system. A band-pass filter BPF filters the AC power signal APS to obtain a filtered signal FS. The comparator COM compares the filtered signal FS with a threshold value TH to obtain a comparator output signal CS. The optional flip flops FF may be included to monitor the output of the inverter over a longer term before switching the electrical appliance EA off. In this manner it is sure that the characteristic detected (for example, the 9th harmonic) is not due to a power surge in the mains grid. If required, the driver DR converts the improved comparator signal CS' into a switch signal OS suitable for switching the electronic switch SW.

The selection of the center frequency and the bandwidth of the band-pass filter BPF and the threshold TH supplied to the comparator COM can be determined experimentally by first filtering the AC voltage of the mains grid MG and then filtering the AC voltage of the inverter INV. The center frequency and the bandwidth may be selected such that the difference between the filtered signals FS in response to the mains power signal MPS or the local power signal LPS is maximal. It has to be noted that the actual shape of the AC voltage and thus the amount of harmonics usually also depends on the input impedance of the electrical appliance EA. Especially electrical appliances EA with a capacitive input impedance (for example, air conditioners and display apparatuses) may cause an increase of high harmonics on the mains power signal MPS. Consequently, the mains power signal MPS gets characteristics which more resemble the characteristics of the local power signal LPS. It may be sorted out dependent on the actual electrical appliance EA how to select the characteristics of the band-pass filter BPF and the threshold TH to optimally distinguish between the AC voltage MPS from the mains grid MG and the AC voltage LPS from the inverter INV.

For example, in a particular practical embodiment, the band-pass filter BPF has a center frequency at or near to the 9^th harmonic of the mains power signal, thus in 50 Hz countries at 450 Hz, and the -3dB bandwidth may be in the order of 50 Hz. Fig. 3 schematically shows a block diagram of another embodiment of the detection device. In this digital implementation, the detection device DE comprises a low- pass filter LPF, an analog to digital converter ADC, a digital band-pass filter BPF, a digital comparator COM, a driver DR and a clock generator CLK. The clock generator CLK generates the clock signal(s) CL for the digital circuits. The digital circuits may be realized by dedicated hardware or their activities may be partly or completely performed by a suitable programmed processor. The low-pass filter LPF filters the AC power signal APS to at least suppress the frequencies above half the sample frequency supplied by the clock generator CLK to the analog to digital converter ADC to prevent too much aliasing. The analog to digital converter ADC converts the analog filtered signal LFS from the low-pass filter LPF into the digital signal DS. The digital band-pass filter BPF is a digital equivalent of the band-pass filter discussed with respect to Fig. 2 and supplies the band-pass filtered signal BFS to the comparator COM. The band-pass filter BPF may be implemented as a well known FIR filter. The comparator COM compares the band- pass filtered signal BFS with the threshold TH to decide in the same manner as elucidated with respect to Fig. 2 whether the AC power signal APS originates from the mains grid MG or from the inverter INV. If the digital comparator output signal DCO is not suitable to drive the electronic switch (or switches) SW directly, a driver DR is added to convert the digital comparator output signal DCO into the drive signal OS suitable for driving the switch SW. The clock generator CLK may generate different clock signals for the analog to digital converter than for the digital circuits.

Fig. 4 schematically shows a block diagram of yet another embodiment of the detection device. In this digital implementation, the low-pass filter LPF, the analog to digital converter ADC and the driver DR are identical to the ones elucidated with respect to Fig. 3. The Fourier analyzer FFT applies a Fast Fourier Transform on the digital signal DS to obtain a representation RP in the frequency domain of the AC power signal APS received. The comparator COM compares this representation RP with either a representation RFl in the frequency domain of the mains power signal MPS or a representation RF2 of the local power signal LPS, or with both. The representations RFl and RF2 may be predefined. Alternatively, the representation RFl of the mains power signal MPS and the representation RF2 of the local power signal LPS may be obtained, or the predefined representations may be altered, using the detection device DE during periods in time that is know that the mains grid MG or the inverter INV is active, respectively. The respective representations RP obtained may be stored in a memory to be used later as the representations RFl and RF2, respectively.

The comparator COM may compare the representation RP of the actual AC power signal APS with either the representation RFl or RF2. For example, if the representation RP is compared with only the representation RFl, a substantial fit indicates that the actual AC power signal APS originates from the mains grid MG and the comparator output signal DCO' indicates that the switch SW should be closed. If there is no fit, the AC power signal APS originates from the inverter INV and the comparator output signal DCO' indicates that the switch SW should be opened. Alternatively, the representation RP of the actual AC power signal APS may be compared with both stored representations RFl and

RF2. The comparator COM outputs the comparator output signal DCO' indicating which one of the stored representations RFl, RF2 most resembles the actual representation RP.

Fig. 5 schematically shows a block diagram of yet another embodiment of the detection device. In this digital implementation, the low-pass filter LPF, the analog to digital converter ADC and the driver DR are identical to the ones elucidated with respect to Fig. 4. Instead of the Fast Fourier analysis, now a cross-correlation analysis is performed.

The cross-correlation analyzer CCA determines the cross-correlation between the footprint FP of the actual AC power signal APS and the footprint FPl of either the mains power signal MPS, or the footprint FP2 of the local power signal LPS, or both. With footprints is meant that the AC power signal is processed in a particular manner to obtain a representation defining the type of AC power signal received. The footprints FPl, FP2 may be predetermined or may be obtained by the reference footprint determiner RFD during periods in time that the AC power signal APS is the mains power signal MPS or the local power signal LPS, respectively. The footprint determiner RFD receives the AC power signal APS and stores the footprints FPl of the mains power signal MPS and the footprint FP2 of the local power signal LPS in the memory MEM. The actual footprint FP is generated from the digital signal DS by the footprint generator FPC. The footprints FP, FPl, FP2, should correspond with a same period in time during which the samples are accumulated. Usually, the samples are accumulated or averaged over multiple single half or full periods of the AC power signal APS. A zero crossing detector may be used to detect the start and end of the period in time the samples are accumulated. Alternatively, it is possible to use a single half or full period of the AC power signal APS. However, this may impair the accuracy of the cross- correlation. The cross-correlation analyzer CCA may determine the cross-correlation with only one of the stored footprints FPl or FP2. If the cross-correlation is above a particular threshold it is concluded that the actual AC power signal APS originates from the source (mains grid MG or inverter INV) associated with the stored footprint FPl, FP2 used in the cross-correlation. However, a more reliable detection of which source is actually supplying the AC power signal APS is possible by performing the cross-correlation determination between the footprint FP of the actual AC power signal APS and both stored footprints FPl and FP2. The one of the stored footprints FPl, FP2 which has the highest correlation with the actual footprint FP indicates which source is supplying the actual AC power signal APS. Fig. 6 shows a detailed electric diagram of an implementation of the switching system. This analog embodiment is based on the block diagram shown in Fig. 2. The operational amplifier (opamp) Ol is an input buffer with unity gain. The opamps 02, 03 and 04 form the analog band-pass filter BPF which in this embodiment is a three-pole Butterworth filter with a quality factor often. The band-pass filter has a centre frequency of 450 Hz and a -3 dB bandwidth of 50 Hz. The gain of each stage is 1.5. Opamp 05 operates as the comparator COM which compares the filtered signal FS supplied by opamp 04 with the threshold TH.

The square wave pulse CS generated by the comparator COM is supplied to a flip-flop chain IC3 to prevent that the signal CS' which is supplied by the output of the flip- flop chain IC3 to the driver DR is influenced by a power glitch of the mains grid MG. In the embodiment shown, the flip-flop chain IC3 comprises 8 series arranged flip-flops. The square wave pulse generated by the comparator COM is also supplied to charge the capacitor C14. The resistor R23 is arranged in parallel with the capacitor C 14 to slowly discharge the capacitor C 14. A buffer comprising the transistors T2 and T3 is connected between the capacitor C14 and a common reset input of the flip-flops IC3. If the square wave pulse CS has a low level, indicating that the AC power signal APS is the mains power signal MPS, the capacitor C 14 will be discharged and the flip-flops ICs will be reset. Now, the flip-flops IC3 are in the starting mode for a next detection of whether the mains grid MG has an outage with a sufficient long duration that the inverter INV will be activated and the switch SW should be opened.

The output signal CS' of the flip-flop chain IC3 is supplied via the transistors T4, T5 and the opto-coupler U6 to the control electrode G of the switch SW. In the embodiment shown, the switch SW is a triac. Alternatively, other controllable electronic switches may be implemented in series with the load LO. The series arrangement of the switch SW and the load LO receives the AC power signal APS. The configuration comprising the transistors T4, T5 and the opto-coupler is designed to turn-off the triac to decouple the load LO from the AC power signal APS when the signal CS' has a high level. The signal CS' has a high level if the output CS is a pulse train and thus indicates that the mains grid MG has an outage of a sufficient long duration.

A digital implementation, especially if a microcontroller is used, has a high degree of flexibility to cope with the actual characteristics of the AC voltage or AC current supplied by the mains grid MG or the inverter INV. Both the characteristics of the band-pass filter BPF, the threshold TH used by the comparator COM, and the duration during which an mains grid MG outage is considered to be a glitch can be adapted automatically in use, when installing the switching system SY, or by the user. In a digital implementation wherein the band-pass filter BPF has a centre frequency of 450 Hz, a relatively low sample frequency of 1 to 2 kHz suffices as the sample signal for the analog to digital converter ADC. If aliasing is a problem, for example if a Fast Fourier Transform has to be performed, a low-pass filter LPF should be added in front of the analog to digital converter ADC. In an embodiment, the low-pass filter LPF is a one-opamp third order filter at 500 Hz.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, in another embodiment within the scope of the claims a particular signal (message) is added to the AC power supply voltage if the AC power originates from the inverter INV. For example, the absence of mains grid power can be detected and the particular signal or information (e.g. a digital bit stream) is supplied to the mains wiring. This can be detected by the detection circuit DE, which then turns off the electrical appliance EA. If the message is absent it indicates that the supply is from the mains grid.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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.

Claims

CLAIMS:

1. A switching system (SY) for an electrical appliance (EA) receiving power from an AC power signal (APS), the switching system (SY) comprising: a detection device (DE) for detecting whether the AC power signal (APS) is supplied by a mains grid (MG) or by a local power supply generator (INV), and a switching device (SW) for switching a supply of the power to the electrical appliance (EA) in dependence on a control signal (OS) from the detection device (DE).

2. A switching system (SY) as claimed in claim 1, wherein the detection device (DE) comprises a band-pass filter (BPF) for filtering the AC power signal (APS) to obtain a filtered signal (FS), a comparator (COM) for comparing the filtered signal (FS) with a threshold (TH), and a driver (DR) for switching off the switching device (SW) if the filtered signal (FS) has a level indicating that a level of frequency components within a band-pass of the band-pass filter (FS) surpasses a predetermined level.

3. A switching system (SY) as claimed in claim 2, wherein the band-pass filter (BPF) has a 9^th harmonic of the mains power signal (MPS) as its center frequency.

4. A switching system (SY) as claimed in claim 2, wherein the band-pass filter (BPF) has a bandwidth of 50 Hz.

5. A switching system (SY) as claimed in claim 2, further comprising a low-pass filter (LPF) for low-pass filtering the AC power signal (APS) to obtain a low-pass filtered signal (LFS), and an analog to digital converter (ADC) for converting the low-pass filtered signal (LFS) to a digital signal (DS), wherein the band-pass filter (BPF) is arranged for filtering the digital signal (DS).

6. A switching system (SY) as claimed in claim 2, further comprising a low-pass filter (LPF) for low-pass filtering the AC power signal (APS) to obtain a low-pass filtered signal (LFS), an analog to digital converter (ADC) for converting the low-pass filtered signal (LFS) into a digital signal (DS), and a Fourier analyzer (FFT) for fast Fourier analysis of the digital signal (DS) to obtain a representation (RP) in the frequency domain of the AC power signal (APS), wherein the comparator (COM) is constructed for comparing the representation (RP) with a reference footprint (RFl, RF2)) in the frequency domain of the AC power signal (APS).

7. A switching system (SY) as claimed in claim 6, wherein the comparator (COM) is constructed for comparing the representation (RP) both with a reference footprint (RFl)of the mains power signal (MPS) and with a reference footprint (RF2) of the local power signal (LPS) to switch the switching device (SW) off if a difference between the representation (RP) and the reference footprint (RF2) of the local power signal (LPS) is smaller than a difference between the representation (RP) and the reference footprint (RFl) of the mains power signal (MPS).

8. A switching system (SY) as claimed in claim 2, further comprising a low-pass filter (LPF) for low-pass filtering the AC power signal (APS) to obtain a low-pass filtered signal (LFS), an analog to digital converter (ADC) for converting the low-pass filtered signal (LFS) into a digital signal (DS), and a cross-correlation analyzer (CCA) for determining a cross-correlation between an actual footprint of the digital signal (DS) and a reference footprint (FPl, FP2) in the time domain of the AC power signal (APS).

9. A switching system (SY) as claimed in claim 8, wherein the comparator (COM) is constructed for comparing the cross-correlation between the digital signal (DS) and the reference footprint (FPl) of the mains power signal (MPS) and between the digital signal (DS) and the reference footprint (FP2) of the local power signal (LPS) to switch the switching device (SW) off if a cross-correlation between the digital signal (DS) and the reference footprint (FP2) of the local power signal (LPS) is higher than a cross-correlation between the digital signal (DS) and the reference footprint (FPl) of the mains power signal (MPS).

10. An electrical appliance (EA) comprising the switching system (SY) as claimed in any of the preceding claims.

11. A method of switching an electrical appliance (EA) receiving power from an

AC power signal (APS), the method comprises: detecting (DE) whether the AC power signal (APS) is supplied by a mains grid (MG) or by a local power supply generator (INV) to provide a control signal, and switching (SW) a supply of the power to the electrical appliance (EA) in dependence on the control signal (OS).