Battery power supply system

Abstract: A battery power supply system permits inexpensive introduction of a transfer break system to a solar cell generation system and quick release of reverse charge phenomenon. The battery power supply system comprises a solar cell array 1, a DC-AC inverter 2, a brake device 3 for a commercial power system, and wireless communication means 4 as control means for controlling the conduction and break of the break device based on the information derived through wireless communication.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a battery power supply system, and more particularly to an inexpensive battery power supply system which may quickly release a reverse charge phenomenon. Related Background Art Interest to the global environment has recently been very much enhanced and a great expectation has been paid to a solar cell which is representative of battery power supplies which are inexhaustible and clean energy sources. Today, the cost of a solar cell is less than 1000 Yen/W and it is 1/10 when compared with that of a few years ago. The electricity generated by the solar cell is more expensive than the existing commercial electric power as of 1994 but it is said that the cost of the solar sell will go down to 200 Yen/W in 2000, when the electricity can be generated by the solar cell with a comparable cost to that of the commercial power supply. Since the solar cell is operative only during sun shine, some backup means for the night and rainy day is required when it is to be used in a home. To this end, a system for using the power supply and the solar cell output in linkage through an inverter has been proposed and it has been put into practice. One example of such linkage system is shown in Fig. 5. A DC power generated by a solar cell is converted to an AC power by a DC-AC inverter 2 and it is supplied to a load 5. In the absence of sun shine, the commercial power is supplied to the load 5 through a break device 3, which disconnects the solar generation system from the system line in response to a short circuiting accident in the system or an accident of the commercial system. In most cases, it stops the DC-AC inverter 2. However, in many cases, the power generated by the solar cell is in excess in daytime. The flow of the excess power to the power system is called a "reverse current". By the reverse current, the electricity generated by the solar cell can be used without waste. Such a reverse current type solar cell generation system would be an ideal manner of use of the solar cell. However, a problem of "reverse charge phenomenon" which is inherent to the reverse current system has not yet been solved. The "reverse charge phenomenon" may occur when the quantity of power generated by the solar cell generation system and the power consumption of a load connected thereto are substantially equal. In such a case, even if the electricity of the commercial power supply system is stopped or it stops by some cause, the solar cell generation system cannot detect the stop of the commercial power system and the solar cell generation system continues its independent operation. As a result, lines which are not to be active are charged and a risk occurs in a maintenance operation of the lines. Further, when the commercial power system is reclosed, the commercial power system and the solar cell generation system are not in synchronism and an excess current is generated and the reclosure may fail. When many solar cell generation systems are used in future, the "reverse charge phenomenon" may occur over a wide area. In order to prevent the "reverse charge phenomenon", various method (see e.g. DE-A-4 021 552 and Patent Abstracts of Japan vol. 011, no. 375 (P-644) 8 december 1987 & JP-A-62 145 314) have been proposed as shown in Table 1 and tests of those methods have been conducted but no definite solution therefor has been found yet. Method Content Remarks 1. Countermeasure on solar cell generation system (1) Methodic countermeasure a. Adoption of current controlled inverter · Use constant current source and no function to maintain voltage. · Can localize condition of occurence of reverse charge operation. (2) Passive countermeasure a. Phase monitor system · Detect transitional variation of line voltage phase. · Detect abrupt change in phase difference between inverter output voltage and current. · Not operable in a complete balanced condition. Very effective for other cases. b. Harmonics monitor system · Monitor harmonic voltage generated by line load. · Efect is influenced by line load status and is unstable. · Combine with current controlled inverter to primarily detect third harmonic component generated by pole transformer. · Not influenced by load status and is stable, but inverter output voltage should be sine wave. (3) Active countermeasure a. Frequency variation system · Diverse inverter output frequency when system power supply losses. · System for modulating phase of feedback waveform of system voltage and system for biasing frequency of oscillation circuit. Both are normally synchronized with frequency of line. b. Output voltage or current variation system · Continuously and finely vary inverter output voltage or current. · Sine wave variation system and variation system by quasi-random pattern. Effect may be lowered when a number of solar cell generation systems are parallelly operated. c. Output power variation system · System for initially imparting large AC amplitude variation and system for imparting small variation and forming positive feedback loop in reverse charge operation mode to amplify amplitude to detect resulting increase of voltage or frequency variation. In both systems, effect may be lowered when a number of solar cell generation systems are parallelly operated. (1) Active power variation · Continuously and finely vary inverter output active power. (2) Reactive finely vary inverter · Continuously and finely vary inverter output reactive power. 2. Countermeasure on line (1) Passive countermeasure a. Line zero-voltage check device · Check line zero-voltage after sub-station breaker has been opened. Need to parallelly use transfer break. (2) Active countermeasure a. Reactive power injection system (Capacitor throw-in system) · Check line zero-voltage after substation breaker has been opened. If voltage is present, balance of reactive power is destroyed by connecting capacitor. Very effective as post protection. Note) There is shown an equilibrium range which can be detected by a conventional frequency and voltage monitor function at the time of equilibrium between power generation amount and load amount. On the other hand, in a spacial high potential power transmission line, a "transfer break system" is used to avoid the reverse charge status. In the "transfer break", a break device is actuated by a break signal from a host substation to stop the system. However, in order to apply it to a low potential solar cell generation system, it is necessary to install a communication line to each of the solar cell generation systems, and it is not feasible because of a huge amount of costs. A telephone line may be used as the communication line for the transfer break. In this case, a cost to install the communication line for each system may be significantly reduced. However, as described above, when a number of solar cell generation systems are widely used, the reverse charge phenomenon may possibly occur over a wide area. In such a case, all or most of the solar cell generation systems in the area of the reverse charge phenomenon must be broken. In order to simultaneously break several tens to several thousands solar cell generation systems, a telephone line system which assigns the number to each of the users is not appropriate. To break one hundred systems, for example, telephone calls must be made at least one hundred times. Assuming that one user can be accessed in five seconds, it takes 500 seconds or more than eight minutes. SUMMARY OF THE INVENTION It is an object of the present invention to provide a device for preventing the reverse current and the reverse charge without modifying an inverter by adding it to a solar cell generation system which may produce a reverse current. The present invention provides a battery power supply system comprising a DC-AC inverter for converting a DC output of a battery power supply to an AC output to supply it to a load, first power measurement means for measuring the output of the DC-AC inverter, second power measurement means for measuring an input power from a power system for powering to the load or a power consumption of the load, a dummy load device electrically connected between the power system and the load for controlling the amount of load, and control means for controlling the amount of load of the dummy load device based on the measurements of the first and second power measurement means. The dummy load includes a secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a battery power supply system having a reverse current and reverse charge prevention device of the present invention. Fig. 2 shows an operation chart of the reverse current and reverse charge prevention device used in the battery power supply system of the present invention. Fig. 3 shows another embodiment of the battery power supply system having the reverse current and reverse charge prevention device of the present invention. Fig. 4 shows a further embodiment of the battery power supply system having the reverse current and reverse charge prevention device of the present invention. Fig. 5 shows a configuration of a prior art battery power supply system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is now described in detail with reference to the drawings. (Embodiment 1) An embodiment of the solar cell generation system which uses the reverse current and reverse charge prevention device of the present invention is shown in Fig. 1. In Fig. 1, a portion enclosed by broken lines is the reverse current and reverse charge prevention device of the present invention. A solar cell array 1 comprising a plurality of solar cell modules converts sun shine to a DC power. A DC-AC inverter 2 converts the DC power from the solar cell array to an AC power. The inverter 2 may be self-excited type or separately excited type, and a PWM self-excited inverter which uses IGBT, power MOSFET and power transistor as switching devices is preferable. The inverter includes gate control means for the switching devices to start and stop the inverter. A maximum power tracking function for the solar cell is also provided so that a maximum available power can always be taken out or the solar cell array. A break device 3 mechanically separates the commercial power system from the solar cell system. Numeral 4 denote a load such as a user, and specifically it is a home electric equipment. Numeral 101 denotes power measurement means for measuring the output of the inverter 2. It may be a power meter or a combination of an ammeter and a volt meter. In the latter case, the measurements are multiplied to each other to calculate an integration in a period of the commercial power system frequency to determine the power. The measurement means may be a current transformer, a voltage transformer or a digital power meter. Numeral 102 denotes power measurement means for measuring an input power from the commercial power system. It may be a power meter or a combination of an ammeter and a volt meter, as the above power measurement means is. Numeral 103 denote a control unit (controller) which receives the measurement outputs from the power measurement means 101 and 102 and processes them to produce a control output. The control means may be a programmable controller. Numeral 104 denotes a dummy load for controlling the amount of load. It may be an electronic load device, a combination of a resistive load and a slidac transformer, a combination of a resistive load and a triac device, a plurality of resistors selectable by relays or a secondary battery. The control unit 103 continuously monitors the output of the inverter 2 and the power consumption of the load 4 by the two power meters 101 and 102 and controls the dummy load 104 such that the dummy load 104 consumes the output of the inverter 2 to maintain the minimum receiving power when a difference between the power consumption of the load 4 and the output of the inverter 2 becomes smaller than the minimum receiving power. In the reverse current and reverse charge prevention device of the present invention, the inverter output and the power consumption of the load are continuously monitored, and when the difference between the power consumption of the load and the inverter output is larger than the minimum receiving power, the control command is issued to the load to consume more power than the minimum receiving power. As a result, the output from the inverter is totally consumed in the premises of the user and the reverse current and reverse charge to the system is prevented. (Embodiment 2) A solar cell generation system which uses the device of the present invention is constructed as shown in Fig. 1. The solar cell array 1 comprises 20 amorphous silicon solar cell modules (rated voltage 12 V, rated power 22 W) to form an array of 240 V, 440 W. The inverter 2 is a self-excited transistor inverter (rated output 500 W, 100 V, operable in a reverse correct mode). The break device 3 is a combination of a sequencer and a transducer which breaks the commercial system when an excess current or excess voltage occurs. The load 4 comprises three 100 W bulbs. A minimum receiving power is 10 W. The power measurement means 101 is a power meter which measures the output power of the inverter and produces a digital output, and the power measurement means 102 is a power meter which measures the power consumption of the load and produces a digital output. The measurement outputs are supplied to a measurement control computer which serves as the control unit 103, through a GPIB interface. The control output of the control unit 103 is supplied to an electronic load device which serves as the dummy load 104, through the GPIB interface. An operation chart of the reverse current and reverse charge prevention device in the solar cell generation system of the present invention on a fine weather day is shown in Fig. 2. An abscissa represents a time and an ordinate represents a power. From around 8:00 am when the sun has risen and the sun shine becomes strong, the inverter output starts to increase as shown by a solid line A. Around 10:33 am, the inverter output exceeds 290 W and the electronic load device starts to operate. Until 13:30 thereafter, the receiving power is kept at a minimum receiving power 10 W as shown by a solid line B so that the reverse current and the reverse charge are prevented. The power shown by a broken line C in Fig. 2 shows the reverse current power created when the present device is not used. (Embodiment 3) The solar cell array 1, the inverter 2, the break device 3 and the load 4 are of the same construction as those of the embodiment 5 and the reverse current and reverse charge prevention device is constructed as shown in Fig. 3. Current transformers (CT) 201 and 202 and potentio-transformers (PT) 203 and 204 are connected to power transducers 205 and 206 to form two sets of power meters. They are arranged to measure the inverter output and the input power from the commercial power system. Those outputs are produced as 8-bit digital signals, which are supplied to general purpose parallel port of the control unit 103. In the present embodiment, the dummy load 104 comprises eight 30 W bulbs and eight relays serially connected thereto. Drive inputs of the relays are connected to other general purpose parallel port of the control unit 103. As a result, the dummy load 104 has a power consumption which is an integer multiple of 30 W. In the present embodiment, since the dummy load 104 assumes only the integer multiple of 30 W, the receiving power is little bit larger than that of the embodiment 2. However, an expensive electronic load device is not required and the control unit is compact. (Embodiment 4) The solar cell array, the inverter, the brake device and the load are of the same construction as those of the embodiment 2 and the reverse current and reverse charge prevention device is constructed as shown in Fig. 4. The power measurement means 101 and 102 are power meters which produce analog measurement outputs and they are arranged to measure the inverter output and the input power from the commercial power system, respectively. Those outputs are supplied to the control unit 103 which comprises an operational amplifier. The operational amplifier 11 is an adder which calculates a difference between the output of the converter 2 and the power consumption of the load 4 based on the measurements of the power meters 101 and 102. The output of the operational amplifier 11 is supplied to a comparator 12 which compares it with a voltage 13 corresponding to the minimum receiving power, and produces an output when the difference between the power consumption of the load 4 and the output of the inverter 2 is smaller than the minimum receiving power. The output of the control unit 103 is either digital value "1" or "0", which drives a relay 104R in the dummy load device 104. A 400 W heater is connected to the relay 104R as a load. In the present embodiment, the receiving power is larger than that of the embodiment 3 but the configuration of the present embodiment is very simple. As described above, the embodiments 2-4 relate to the reverse current and reverse charge prevention device to be added to the solar cell generation system comprising the solar cell array, the DC-AC inverter which may operate in the reverse current mode, and the load. It comprises the power measurement means for measuring the output of the inverter, the power measurement means for measuring the input power from the commercial system or the power consumption of the load, the dummy load device capable of controlling the amount of load, and the control means for supplying the load control signal to the dummy load device in accordance with the measurements by the power measurement means. The solar cell generation system free from the reverse current and the reverse charge can be constructed by merely adding the present device as the reverse current and reverse charge prevention device, without modifying the inverter. In accordance with the present invention, the reverse charge is quickly realesed and the reverse current is prevented.

Claims: A battery power supply system comprising: a battery power supply (1); a DC-AC inverter (2) for converting a DC output of said battery power supply (1) to an AC output to supply the converted AC output to a load (4); first power measurement means (101, 201, 203, 205) for measuring the output of said DC-AC inverter; second power measurement means (102) for measuring a power consumption of said load; a dummy load (104) electrically connected between a further power system and said load for controlling the amount of load, and control means (103) for controlling the amount of load of said dummy load based on the measurements of said first and second power measurement means. A battery power supply system comprising: a battery power supply (1); a DC-AC inverter (2) for converting a DC output of said battery power supply (1) to an AC output to supply the converted AC output to a load (4); first power measurement means (101, 201, 203, 205) for measuring the output of said DC-AC inverter; second power measurement means (102, 202, 204, 206) for measuring an input power from a further power system for powering to said load; a dummy load (104) electrically connected between said further power system and said load for controlling the amount of load, and control means (103) for controlling the amount of load of said dummy load based on the measurements of said first and second power measurement means. A battery power supply system according to claim 1 or 2, wherein said dummy load (104) includes a secondary battery.

Inventors: TAKEHARA NOBUYOSHI, TAKEHARA, NOBUYOSHI, TAKEHARA, NOBUYOSHI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H02J9
IPC codes: H02J7, H02J13, H02J3