ABNORMALITY DETECTION DEVICE FOR GRID INTERCONNECTION RELAY AND POWER CONDITIONER

Abstract

An abnormality detection device for a grid interconnection relay to detect an abnormality of the grid interconnection relay upon switching to grid independent operation, and includes an abnormality detector to execute commercial power system voltage determination of determining whether or not there is a commercial power system voltage, and first voltage determination to be executed if it is determined that there is no commercial power system voltage through the commercial power system voltage determination, of causing the power conditioner to chronologically alternately output monitor voltages having different values in a state where a contact of the grid interconnection relay is controlled to open and executing abnormality determination as to the grid interconnection relay according to whether or not each of the monitor voltages is followed by difference between a voltage of the power conditioner and voltage of the commercial power system with respect to corresponding one of the monitor voltages.

Claims

1.-13. (canceled)

14. An abnormality detection device for a grid interconnection relay, the device incorporated in a power conditioner configured to be switched between grid connected operation by interconnection with a commercial power system via the grid interconnection relay and grid independent operation by power supply to a stand-alone power system via a stand-alone power system relay, the power conditioner including an inverter configured to convert DC power to AC power and an LC filter configured to remove a high frequency component from an output voltage of the inverter, the abnormality detection device configured to detect an abnormality of the grid interconnection relay upon switching to grid independent operation and comprising an abnormality detector configured to execute: commercial power system voltage determination of determining whether or not there is a commercial power system voltage; and first voltage determination to be executed if it is determined that there is no commercial power system voltage through the commercial power system voltage determination, of causing the power conditioner to chronologically alternately output monitor voltages having different values in a state where a contact of the grid interconnection relay is controlled to open and executing abnormality determination as to the grid interconnection relay in accordance with whether or not each of the monitor voltages is followed by a difference between a voltage of the power conditioner and a voltage of the commercial power system with respect to corresponding one of the monitor voltages.

15. The abnormality detection device for the grid interconnection relay according to claim 14, wherein the first voltage determination includes abnormality determination as to the grid interconnection relay according to whether or not a product of the differences each between the voltage of the power conditioner and the voltage of the commercial power system with respect to the corresponding one of the monitor voltages is less than a predetermined reference value.

16. The abnormality detection device for the grid interconnection relay according to claim 15, wherein, assuming that a command value of a variable output voltage of the power conditioner is ΔE*, a confidence coefficient for adjustment of the command value ΔE* of the variable output voltage and a reference value E.sub.chk is a, and a confidence coefficient for a commercial power system voltage E.sub.Grid is b, when chronologically alternately outputting at least monitor voltages having command values E.sub.min and E.sub.max (E.sub.min<E.sub.max) of an output voltage effective value indicated in mathematical expressions [Expression 1], the first voltage determination includes calculating a product ΔE.sub.CST of differences ΔE each between the voltage of the power conditioner and the voltage of the commercial power system in accordance with mathematical expressions [Expression 2] assuming that a sampling time point is k, and executing abnormality determination as to the grid interconnection relay in accordance with whether or not the product ΔE.sub.CST of the differences ΔE is less than the predetermined reference value E.sub.chk; $\begin{array}{cc}\{\begin{array}{c}{E}_{\max }=E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}+a.Math.\Delta .Math..Math.E^{*}\\ E_{\min }=E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}-a.Math.\Delta .Math..Math.E^{*}\\ b.Math.E_{\mathrm{Grid}}\le E_{\max }-E_{\min }\le 2.Math.a.Math.\Delta .Math..Math.E^{*}\\ 0<a<1\\ 0<b<1\end{array}& \left[\mathrm{Expression}.Math..Math.1\right]\\ \{\begin{array}{c}\Delta .Math..Math.E\left(k\right)=.Math.E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}\left(k\right)-E_{\mathrm{uw}..Math.\mathrm{rms}}\left(k\right).Math.\\ \Delta .Math..Math.E_{\mathrm{CST}}=\Delta .Math..Math.E\left(k-2\right).Math.\Delta .Math..Math.E\left(k-1\right).Math.\Delta .Math..Math.E\left(k\right)\\ \Delta .Math..Math.E_{\mathrm{CST}}<E_{\mathrm{chk}}\\ E_{\mathrm{chk}}<{\left(a.Math.\Delta .Math..Math.E^{*}\right)}^3\end{array}.& \left[\mathrm{Expression}.Math..Math.2\right]\end{array}$

17. The abnormality detection device for the grid interconnection relay according to claim 16, wherein the command values E.sub.min and E.sub.max of the output voltage effective value are set in a range from b×E.sub.Grid to 2×a×ΔE* with respect to the rated voltage E.sub.Grid of the commercial power system.

18. The abnormality detection device for the grid interconnection relay according to claim 14, wherein, if it is determined that there is a commercial power system voltage through the commercial power system voltage determination, the abnormality detector is configured to set the output voltage of the power conditioner to zero and execute second voltage determination of abnormality determination as to the grid interconnection relay according to a magnitude relation between a difference between the voltage of the power conditioner and the voltage of the commercial power system, and a value obtained by multiplying the predetermined reference value by a predetermined confidence coefficient in a state where the contact of the grid interconnection relay is controlled to open.

19. The abnormality detection device for the grid interconnection relay according to claim 14, wherein the abnormality detector is configured to execute if it is determined that there is a commercial power system voltage through the commercial power system voltage determination, first current determination of abnormality determination as to the grid interconnection relay according to whether or not there is an input current to the power conditioner in the state where the contact of the grid interconnection relay is controlled to open, and if it is determined that there is no commercial power system voltage through the commercial power system voltage determination, second current determination of abnormality determination as to the grid interconnection relay according to whether or not there is an output current from the power conditioner in the state where the contact of the grid interconnection relay is controlled to open.

20. The abnormality detection device for the grid interconnection relay according to claim 19, wherein, assuming that the LC filter has capacitor capacity C.sub.inv, internal resistance R.sub.c, and a capacitor current i.sub.c, and a stand-alone power system voltage e.sub.sd is a measurement value, the first current determination includes calculation, as the input current, of the capacitor current i.sub.c in accordance with a mathematical expression [Expression 3]: $i_c=\frac{{\mathrm{sC}}_{\mathrm{inv}}}{{\mathrm{sR}}_c.Math.C_{\mathrm{inv}}+1}.Math.e_{\mathrm{sd}}.$

21. The abnormality detection device for the grid interconnection relay according to claim 20, wherein the first current determination includes determining that the grid interconnection relay has an abnormality if the input current measured in a predetermined sampling cycle has an absolute value not less than a predetermined threshold a plurality of consecutive times and the absolute value of the input current increases every time the output current is measured.

22. The abnormality detection device for the grid interconnection relay according to claim 19, wherein, assuming that the LC filter has capacitor capacity C.sub.inv, internal resistance R,, an output voltage e.sub.sd at grid independent operation, and a capacitor current i.sub.c, and an inverter current i.sub.inv is a measurement value, the second current determination includes calculating, as the output current, an output current i.sub.sp of the power conditioner in accordance with a mathematical expression [Expression 4]:
i.sub.sp=i.sub.inv−i.sub.c.

23. The abnormality detection device for the grid interconnection relay according to claim 22, wherein the second current determination includes determining that the grid interconnection relay has an abnormality if a difference in absolute value of peak values of the output current of the power conditioner is not less than a predetermined threshold a plurality of consecutive times and the absolute value of the peak value decreases every time the output current is measured.

24. The abnormality detection device for the grid interconnection relay according to claim 14, wherein the abnormality detector is configured to execute abnormality detection processes after all contacts of the grid interconnection relay are controlled to open and execute the abnormality detection processes every time one of the contacts is controlled to close independently.

25. The abnormality detection device for the grid interconnection relay according to claim 14, wherein the commercial power system voltage determination includes determining whether or not there is a commercial power system voltage in accordance with a magnitude relation between a value obtained by multiplying a preliminarily set output voltage set value of the power conditioner by a predetermined confidence coefficient and the commercial power system voltage, and a magnitude relation between a value obtained by multiplying a stand-alone power system frequency by a predetermined confidence coefficient and a commercial power system frequency.

26. A power conditioner of a single-phase or three-phase type, provided with an inverter configured to convert DC power to AC power and an LC filter configured to remove a high frequency component from an output voltage of the inverter, the power conditioner comprising: a control unit configured to switch between grid connected operation by interconnection with a commercial power system via a grid interconnection relay and grid independent operation by power supply to a stand-alone power system via a stand-alone power system relay; and an abnormality detection device incorporated in the control unit and functioning as an abnormality detector configured to execute: commercial power system voltage determination of determining whether or not there is a commercial power system voltage; and first voltage determination to be executed if it is determined that there is no commercial power system voltage through the commercial power system voltage determination, of causing the power conditioner to chronologically alternately output monitor voltages having different values in a state where a contact of the grid interconnection relay is controlled to open and executing abnormality determination as to the grid interconnection relay according to whether or not each of the monitor voltages is followed by a difference between a voltage of the power conditioner and a voltage of the commercial power system with respect to corresponding one of the monitor voltages.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] FIG. 1 is a circuit block configuration diagram of a distributed power supply including a power conditioner.

[0065] FIG. 2 is an explanatory chart of ON/OFF operation of a grid interconnection relay upon abnormality detection.

[0066] FIG. 3 is a flowchart of commercial power system voltage determination.

[0067] FIG. 4 is a flowchart of a method of detecting an abnormality of the grid interconnection relay.

[0068] FIG. 5 is a flowchart of first voltage determination.

[0069] FIG. 6 is an explanatory graph of first current determination.

[0070] FIG. 7 is an explanatory graph of second current determination executed when a current is not less than a determination threshold.

[0071] FIG. 8 is an explanatory graph of the second current determination executed when a current is not more than the determination threshold.

[0072] FIG. 9A is an explanatory chart of an output voltage switching sequence of the power conditioner executing the first voltage determination, and FIG. 9B is an explanatory chart of a voltage detection sequence of a commercial power system executing the first voltage determination.

[0073] FIG. 10A is an explanatory chart of a contact control sequence for detection of welding at a contact S.sub.w upon the first voltage determination, FIG. 10B is an explanatory chart of an output voltage waveform of the power conditioner during the contact control sequence indicated in FIG. 10A, and FIG. 10C is an explanatory chart of a voltage waveform of the commercial power system corresponding to FIG. 10B in a case where welding at the contact S.sub.w is detected.

[0074] FIG. 11A is an explanatory chart of the contact control sequence for detection of welding at the contact S.sub.w upon the first voltage determination, FIG. 11B is an explanatory chart of an output voltage waveform of the power conditioner during the contact control sequence indicated in FIG. 11A, and FIG. 11C is an explanatory chart of a voltage waveform of the commercial power system corresponding to FIG. 11B in a case where welding at the contact S.sub.w is not detected.

DESCRIPTION OF EMBODIMENTS

[0075] An abnormality detection device for a grid interconnection relay and a power conditioner according to the present invention will now be described below with reference to the drawings.

[0076] FIG. 1 depicts a solar power generator 1 exemplifying a distributed power supply. The solar power generator 1 includes a solar panel SP and a power conditioner PCS connected to the solar panel SP.

[0077] DC power generated by the solar panel SP is supplied to the power conditioner PCS via a DC circuit breaker and a surge suppressor (not depicted).

[0078] The power conditioner PCS includes a DC/DC converter 2 configured to raise a DC voltage generated by the solar panel SP to a predetermined DC link voltage V.sub.dc, a DC/AC inverter 3 configured to convert the DC link voltage V.sub.dc raised by the DC/DC converter 2 to a predetermined AC voltage, an LC filter 4 configured to remove a higher harmonic wave from the AC voltage outputted from the DC/AC inverter 3, a control unit 5 configured to control the DC/DC converter 2 and the DC/AC inverter 3, and the like.

[0079] AC power converted by the power conditioner PCS is supplied to an AC load via a grid interconnection relay Ry1 in interconnection with a commercial power system 100. When the commercial power system 100 is separated due to power cut or the like, AC power is supplied to a load R.sub.sd connected to a stand-alone power system via a stand-alone power system relay Ry2.

[0080] FIG. 1 depicts the grid interconnection relay Ry1 having contacts S.sub.u and S.sub.w and the stand-alone power system relay Ry2 having two contacts S.sub.sd.

[0081] The control unit 5 of the power conditioner PCS includes a microcomputer, a memory, a peripheral circuit having an input/output circuit provided with an AD conversion unit and the like. The microcomputer includes a CPU configured to achieve expected functions by causing control programs stored in the memory to be executed.

[0082] Specifically, embodied by the control unit 5 are functional blocks as a converter controller 5a configured to control a boosting switch of the DC/DC converter 2, an inverter controller 5b configured to control a switch included in a bridge of the DC/AC inverter 3, and an abnormality detector 5c configured to detect an abnormality of the grid interconnection relay Ry1.

[0083] The converter controller 5a is configured to monitor an input voltage, an input current, and an output voltage of the DC/DC converter 2 and control maximum power point tracking (MPPT) of operating the solar panel SP at a maximum power point, as well as control to boost the DC/DC converter 2 and output the predetermined DC link voltage V.sub.dc to the DC/AC inverter 3.

[0084] The inverter controller 5b is configured to control the inverter 3 so as to achieve grid connected operation via the grid interconnection relay Ry1, or to control the inverter 3 so as to achieve grid independent operation via the stand-alone power system relay Ry2.

[0085] The inverter controller 5b includes functional blocks such as a current control block configured to control an output current of the inverter 3 so as to synchronize with a phase of a commercial power system voltage upon grid connected operation, a voltage control block configured to supply the stand-alone power system with AC power having a predetermined voltage upon power system separation, and an islanding operation detection block configured to detect whether or not grid connected operation is in an islanding operation state.

[0086] The abnormality detector 5c is configured to detect whether or not the grid interconnection relay Ry1 has an abnormality upon transition from grid connected operation to grid independent operation. If detecting that the grid interconnection relay Ry1 has an abnormality of contact welding, the abnormality detector 5c is configured to turn ON an alarm indicative of trouble and stop grid independent operation control by the inverter controller 5b. In other words, the abnormality detector 5c functions as the abnormality detection device according to the present invention.

[0087] The AD conversion unit in the control unit 5 receives a monitor signal of an output current detected by a current transformer provided downstream of an inductor L configuring the LC filter 4.

[0088] Furthermore, the AD conversion unit in the control unit 5 receives a monitor signal of a stand-alone power system voltage e.sub.sd of the power conditioner PCS detected by a resistance voltage divider circuit provided upstream of the stand-alone power system relay Ry2, as well as a monitor signal of a commercial power system voltage e.sub.uw detected by a resistance voltage divider circuit provided downstream of the grid interconnection relay Ry1.

[0089] The stand-alone power system voltage e.sub.sd and a stand-alone power system frequency f.sub.sd of the power conditioner PCS as well as the commercial power system voltage e.sub.uw and a commercial power system frequency f.sub.Grid are obtained in accordance with the monitor signals received by the AD conversion unit.

[0090] The inverter controller 5b closes the grid interconnection relay Ry1 to achieve grid connected operation if power generated by the solar panel SP has a value enabling interconnection with the commercial power system, and opens the grid interconnection relay Ry1 to achieve separation from the commercial power system if power generated by the solar panel SP decreases or the islanding operation detection block detects the islanding operation state.

[0091] If power generated by the solar panel SP has an adequate value for grid independent operation during separation from the commercial power system due to the islanding operation state, the inverter controller 5b starts the abnormality detector 5c for detection of an abnormality of the Grid interconnection relay Ry1.

[0092] If the abnormality detector 5c determines that the grid interconnection relay Ry1 is normal, the inverter controller 5b is configured to start the inverter 3 and open the stand-alone power system relay Ry2 to achieve grid independent operation. In contrast, if the abnormality detector 5c determines that the grid interconnection relay Ry1 is abnormal, the inverter controller 5b is configured to stop the DC/AC inverter 3 without closing the stand-alone power system relay Ry2.

[0093] Described below is a method of detecting an abnormality of the grid interconnection relay Ry1, which is executed by the abnormality detector 5c.

[0094] Abnormality detection executed by the abnormality detector 5c includes contact control of controlling to open or close the contacts of the grid interconnection relay Ry1, commercial power system voltage determination, setting an output voltage of the inverter 3, voltage determination, and current determination.

[0095] The abnormality detector 5c is configured to execute the current determination and the voltage determination to be described later, of determining whether or not the contacts are welded after all the contacts S.sub.u and S.sub.w of the grid interconnection relay Ry1 are controlled to open, and execute the current determination and the voltage determination every time the contact S.sub.u or S.sub.w is controlled to close.

[0096] If the abnormality detector 5c executes the current determination and the voltage determination after all the contacts S.sub.u and S.sub.w of the grid interconnection relay Ry1 are controlled to open and determines that the contacts are welded, it is clarified that all the contacts S.sub.u and S.sub.w are welded.

[0097] If the abnormality detector 5c determines that the grid interconnection relay Ry1 is normal, the abnormality detector 5c executes the current determination and the voltage determination every time the contact S.sub.u or S.sub.w is controlled to close independently. If the abnormality detector 5c determines that the contact is welded in either one of the cases, it is clarified that the contact being controlled to open upon the determination is welded.

[0098] FIG. 2 indicates timing of ON/OFF control of the contacts S.sub.u and S.sub.w of the grid interconnection relay Ry1. When the abnormality detector 5c starts, the abnormality detector 5c executes the current determination and the voltage determination to check the contacts S.sub.u and S.sub.w that are controlled to open, controls to close the contact Su after a predetermined delay period T.sub.dly and executes the current determination and the voltage determination to check the contact S.sub.w, and controls to open the contact S.sub.u then controls to close the contact S.sub.w after another predetermined delay period T.sub.dly and executes the current determination and the voltage determination to check the contact S.sub.u.

[0099] Each check period is set to 1 sec. and the delay period T.sub.dly is set to 300 msec. in the present embodiment. At least a check period T.sub.on has only to be set to satisfy T.sub.on≧3 T.sub.dly. Such an open/close control sequence for the grid interconnection relay Ry1 corresponds to the step of the contact control described above. The delay period T.sub.dly is variable appropriately in accordance with the type of the grid interconnection relay.

[0100] FIG. 3 depicts a welding determination preparation flow of the grid interconnection relay Ry1 executing the steps of the commercial power system voltage determination and the voltage setting.

[0101] In the commercial power system voltage determination step, in a case where grid independent operation is required (S1), all the contacts of the grid interconnection relay Ry1 are controlled to open (S2), the commercial power system voltage is checked by the resistance voltage divider circuit provided downstream of the grid interconnection relay Ry1 (S3), and whether or not there is a commercial power system voltage is checked in accordance with the following mathematical expressions [Expression 5] (S4).

[00003]$\begin{array}{cc}\{\begin{array}{c}.Math.e_{\mathrm{uw}}.Math.\ge x.Math.E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}\\ f_{\mathrm{Grid}}\ge x.Math.f_{\mathrm{sd}}\end{array}& \left[\mathrm{Expression}.Math..Math.5\right]\end{array}$

[0102] In the mathematical expressions, E*.sub.sd.rms is a command value of an output voltage effective value at grid independent operation, and x is a confidence coefficient set in the range 0<x<1 for securing determination accuracy and set to 0.5 in the present embodiment. The stand-alone power system frequency f.sub.sd is set to be equal to the commercial power system frequency f.sub.Grid. The command value E*.sub.sd.rms according to the present embodiment is set to 40 V that is lower than 100 V of a rated output voltage effective value at grid independent operation.

[0103] The commercial power system voltage is measured for at least one cycle (20 msec. in a case where the commercial power system frequency is 50 Hz) and an absolute value |e.sub.uw| of a maximum instantaneous value is obtained. The absolute value |e.sub.uw| is compared with a product of the command value E*.sub.sd.rms of the output voltage effective value at grid independent operation and the confidence coefficient x. The commercial power system voltage is alternatively measured for a plurality of cycles to obtain an average of the absolute values |e.sub.uw| of the maximum instantaneous values of the respective cycles.

[0104] Furthermore, the commercial power system frequency f.sub.Grid is compared with a product of the stand-alone power system frequency f.sub.sd and the confidence coefficient x. The confidence coefficient x has a value for securement of determination reliability. As the value is more approximate to 1, determination is stricter with more influence of noise. In contrast, as the value is more approximate to 0, determination is less strict with less influence of noise. An intermediate value of 0.5 is preferred to be adopted typically.

[0105] In a case where the commercial power system voltage |e.sub.uw| is 0 V, the command value E*.sub.sd.rms of the effective value of the stand-alone power system voltage is 40 V, and x is 0.5, the mathematical expressions [Expression 5] are as follows.

|e.sub.uw|=0<0.5×40=20

f.sub.Grid=0<0.5×50=25

[0106] In a case where the commercial power system voltage |e.sub.uw| is 283 V, the command value E*.sub.sd.rms of the effective value of the stand-alone power system voltage is 40 V, and x is 0.5, the mathematical expressions [Expression 5] are as follows.

|e.sub.uw|=283<0.5×40=20

f.sub.Grid=50<0.5×50=25

[0107] Specifically, in step S4, it is determined that there is a commercial power system voltage if the two mathematical expressions [Expression 5] are both satisfied, and it is determined that there is no commercial power system voltage if none of the mathematical expressions is satisfied.

[0108] Steps S3 and S4 described above correspond to the commercial power system voltage determination step of determining whether or not there is a commercial power system voltage in accordance with a magnitude relation between a value obtained by multiplying a preliminarily set output voltage set value of the power conditioner PCS by a predetermined confidence coefficient and the commercial power system voltage, and a magnitude relation between a value obtained by multiplying a stand-alone power system frequency by a predetermined confidence coefficient and a commercial power system frequency.

[0109] The preliminarily set output voltage set value of the power conditioner PCS can have a voltage value necessary for grid independent operation, or can have an exclusive voltage value for abnormality detection, which is less than the voltage value. Even in a case where the commercial power system voltage is different from the grid independent operation voltage, whether or not there is a commercial power system voltage is determined accurately by appropriately setting the output voltage set value and the confidence coefficient.

[0110] As in the mathematical expressions [Expression 5], checking the commercial power system voltage as well as the commercial power system frequency enables accurate determination as to whether or not there is a commercial power system voltage with no error due to noise or the like.

[0111] If it is determined that there is a commercial power system voltage in the commercial power system voltage determination step, a reference value E.sub.chk for contact welding determination in the voltage determination is set to an effective value E.sub.uw.rms of the commercial power system voltage, and a delay period T.sub.chk for determination of a difference between the stand-alone power system voltage and the commercial power system voltage is set to a reciprocal of the commercial power system frequency (S5).

[0112] If it is determined that there is no commercial power system voltage, a command value of the output voltage of the power conditioner PCS at abnormality detection is set (S6), the reference value E.sub.chk for contact welding determination is set to be less than (.Math.ΔE*).sup.3 included in mathematical expressions [Expression 14] to be mentioned later, and the delay period T.sub.chk in this case is set to a reciprocal of the stand-alone power system frequency (S7).

[0113] In other words, if it is determined that there is a commercial power system voltage, the power conditioner PCS is stopped to have the output voltage of 0 V. Steps S5 to S7 described above correspond to the voltage setting step.

[0114] The output voltage mentioned above has a command value e satisfying the following mathematical expression [Expression 6].

e*.sub.sd=√2E*.sub.sd.rmssin(θ.sub.sd)   [Expression 6]

[0115] In the mathematical expression, E*.sub.sd.rms is the command value of the effective value of the stand-alone power system voltage and θ.sub.sd is a phase angle of the stand-alone power system voltage. In the present embodiment, E*.sub.sd.rms=40 V is the command value at detection of an abnormality of the grid interconnection relay Ry1, and the command value E*.sub.sd.rms after normality determination is 100 V. The command value for the abnormality detection and the command value after normality determination are merely exemplary and can be set appropriately.

[0116] The commercial power system voltage determination step of determining whether or not there is a commercial power system voltage is executed before the voltage setting step, and the voltage setting step includes setting the stand-alone power system voltage of the power conditioner PCS and the reference value E.sub.chk for abnormality determination as to the grid interconnection relay Ry1 to different values in accordance with the result of the commercial power system voltage determination step.

[0117] The power conditioner PCS is controlled by the control unit 5 so as to transition to grid independent operation upon power cut of the commercial power system, but the commercial power system voltage temporarily decreases and recovers shortly in some cases. If the grid interconnection relay Ry1 has welding in such cases, an inconvenient situation with asynchronous input or the like may cause damage to the power conditioner PCS.

[0118] By executing the commercial power system voltage determination before the voltage setting so as to set the stand-alone power system voltage of the power conditioner PCS and the reference value for abnormality determination as to the grid interconnection relay Ry1 to different values in accordance with the result of the commercial power system voltage determination, abnormality determination as to the grid interconnection relay is executed accurately with secured safety to avoid asynchronous input and reverse charge.

[0119] In a case where a commercial power system voltage is detected, contact welding determination is executed accurately with no damage to the power conditioner PCS by setting the stand-alone power system voltage of the power conditioner PCS to 0 V and the commercial power system voltage to the reference value.

[0120] FIG. 4 depicts a contact welding determination flow of the grid interconnection relay Ry1 according to the present invention.

[0121] If grid independent operation starts (S21), the contact control step is executed (S22). If it is determined that there is a commercial power system voltage through the commercial power system voltage determination (S23, Y), first current determination (S27) of abnormality determination as to the grid interconnection relay Ry1 is executed in accordance with whether or not the power conditioner PCS has an input current in the state where the contacts of the grid interconnection relay Ry1 are controlled to open.

[0122] If it is determined that there is no commercial power system voltage through the commercial power system voltage determination (S23, N), second current determination (S24) of abnormality determination as to the grid interconnection relay Ry1 is executed in accordance with whether or not the power conditioner PCS has an output current in the state where the contacts of the grid interconnection relay Ry1 are controlled to open.

[0123] If the grid interconnection relay Ry1 is determined to have no abnormality through the first current determination (S27, N), the output voltage e.sub.sd of the power conditioner PCS and the effective value of the commercial power system voltage e W are measured during a predetermined delay period n˜T.sub.chk (n is a positive integer) (S28) and second voltage determination is executed (S29).

[0124] If the grid interconnection relay Ry1 is determined to have no abnormality through the second current determination (S24, N), the output voltage e.sub.sd of the power conditioner PCS and the effective value of the commercial power system voltage e.sub.uw are measured during a predetermined delay period n.Math.T.sub.chk (n is a positive integer) (S25) and first voltage determination is executed (S26).

[0125] Determined in step S32 is an error flag state. If an error flag is set (S32, Y), a corresponding relay contact is determined to be welded and accordingly executed is abnormality handling of turning ON abnormality indication on the display panel of the power conditioner PCS or the like (S33).

[0126] As long as no set error flag is found in step S32 (S32, N), the process from steps S22 to S34 is repeated until abnormality determination is completed in each of the three states, namely, the state where all the contacts S.sub.u and S.sub.w of the grid interconnection relay Ry1 are controlled to open, and the states where either one of the contacts S.sub.u and S.sub.w is controlled to close. Described in detail below are processes of the current determination and the voltage determination.

[0127] In the first current determination (S27), assuming that the LC filter 4 has capacitor capacity C.sub.inv, internal resistance R.sub.c, and a capacitor current i.sub.c, and the stand-alone power system voltage e.sub.sd is a measurement value, calculated as an input current is the capacitor current i.sub.c in accordance with the following mathematical expression [Expression 7]. In the mathematical expression, s is a Laplacian operator (Laplace transform).

[00004]$\begin{array}{cc}{i}_c=\frac{{\mathrm{sC}}_{\mathrm{inv}}}{{\mathrm{sR}}_c.Math.C_{\mathrm{inv}}+1}.Math.e_{\mathrm{sd}}& \left[\mathrm{Expression}.Math..Math.7\right]\end{array}$

[0128] The stand-alone power system voltage e.sub.sd is measured with use of the resistance voltage divider circuit configured to detect the output voltage e.sub.sd of the inverter, and the measurement value is substituted into the mathematical expression [Expression 7] to calculate a value of a current flowing into the capacitor of the LC filter 4.

[0129] If the grid interconnection relay Ry1 has a welded contact, the commercial power system voltage e.sub.uw and the stand-alone power system voltage e.sub.sd are regarded as having equal detection values.

[0130] As indicated in FIG. 6, in the first current determination, in order to determine a variation state of an absolute value |i.sub.c| of the instantaneous capacitor current i.sub.c measured and calculated in a predetermined sampling cycle T.sub.s, if satisfying conditions that the absolute value |i.sub.c| is not less than a threshold I.sub.c.chk at least three consecutive times and its value tends to increase (S27, Y), the grid interconnection relay Ry1 is determined to have a welded contact and a flag is set in an error flag memory area set in the memory (S31). Determination is optionally repeated a plurality of times in consideration of erroneous detection due to noise and in order for reliable detection of the variation state.

[0131] The threshold I.sub.c.chk can be obtained in accordance with the following mathematical expression [Expression 8]. In the mathematical expression, P.sub.sd.rated is rated output power at grid independent operation, E*.sub.sd.rms is the command value of the effective value of the stand-alone power system voltage, and y is a confidence coefficient having a positive number satisfying y<1.

[00005]$\begin{array}{cc}{I}_{c.\mathrm{chk}}=y.Math.\frac{P_{\mathrm{sd}..Math.\mathrm{rated}}}{\sqrt{2}.Math.E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}}& \left[\mathrm{Expression}.Math..Math.8\right]\end{array}$

[0132] The display panel of the power conditioner PCS is configured to turn ON abnormality indication when an error flag is set. In a case where the present embodiment is designed such that the command value E*.sub.sd.rms of the effective value of the stand-alone power system voltage is 100 V and the threshold I.sub.c.chk is 10% of a rated current (y=0.1) with the rated power P.sub.sd.rated of 1.5 kW at grid independent operation as a reference value, the threshold I.sub.c.chk is set to 1 A and the sampling cycle T.sub.s is set to 50 μsec. (corresponding to a switching cycle of the DC/AC inverter). The threshold is set to 10% of the rated current ((0.1×1500)/(100×1.414)=1). The predetermined sampling cycle T.sub.s has only to satisfy a condition of a reciprocal of a maximum switching frequency of a switching element configuring the inverter.

[0133] In order to determine the variation state of the absolute value |i.sub.c| of the instantaneous capacitor current i.sub.c in step S27, if a state not satisfying the conditions that the absolute value |i.sub.c| is not less than the threshold I.sub.c.chk at least three consecutive times and its value tends to increase lasts for a predetermined period (e.g. several cycles), it is determined that there is no current flowing from the commercial power system to the capacitor of the power conditioner PCS (S27, N) and the process flow proceeds to the voltage determination in step S28. Determination is optionally repeated a plurality of times in consideration of erroneous detection due to noise and in order for reliable detection of the variation state. If there is a commercial power system voltage and the contacts of the grid interconnection relay Ry1 are normal with no welding, the absolute value |i.sub.c| of the instantaneous capacitor current i.sub.c is constantly zero.

[0134] In the second current determination (S24), calculated from the capacitor capacity C.sub.inv, the internal resistance R.sub.c, the stand-alone power system voltage e.sub.sd, and the capacitor current i.sub.c of the LC filter 4 and the inverter output current i.sub.inv as a measurement value in accordance with the following mathematical expression [Expression 9] is an output current i.sub.sp of the power conditioner PCS as an output current to the load connected with the commercial power system.

i.sub.sp=i.sub.inv−i.sub.c   [Expression 9 ]

[0135] If there is no commercial power system voltage, the inverter 3 is driven and the power conditioner PCS outputs the predetermined stand-alone power system voltage e.sub.sd, the capacitor current i.sub.c obtained from the inverter current i.sub.inv thus measured and the stand-alone power system voltage e.sub.sd in accordance with the mathematical expression [Expression 7] is substituted into the mathematical expression [Expression 9] to calculate the output current i.sub.sp of the power conditioner PCS. The stand-alone power system voltage e.sub.sd is detected by the resistance voltage divider circuit configured to detect the output voltage e.sub.sd of the inverter.

[0136] If the grid interconnection relay Ry1 has a welded contact, there is detected a current flowing out of the power conditioner PCS to the load R.sub.uw connected to the commercial power system.

[0137] As indicated in FIG. 7, in the second current determination, in order to determine a variation state of a difference in maximum value I.sub.sp.max of the output current i.sub.sp of the power conditioner PCS, if the difference is not less than a predetermined threshold I.sub.sp.chk three consecutive times and the instantaneous output current i.sub.sp has an absolute value |i.sub.sp| tending to decrease (S24, Y), the grid interconnection relay Ry1 is determined to have a welded contact and a flag is set in the error flag memory area set in the memory (S31). Determination is optionally repeated a plurality of times in consideration of erroneous detection due to noise and in order for reliable detection of the variation state.

[0138] The maximum value I.sub.sp.max of the output current i.sub.sp is obtained in accordance with a mathematical expression [Expression 10], and the threshold I.sub.sp.chk is obtained in accordance with a mathematical expression [Expression 11]. In the mathematical expression [Expression 11], E*.sub.sd.rms is the command value of the effective value of the output voltage at grid independent operation.

[00006]$\begin{array}{cc}{I}_{\mathrm{sp}..Math.\max }=\sqrt{\frac{4}{T_{\mathrm{sd}}}.Math.{\int }_0^{\frac{T_{\mathrm{sd}}}{2}}.Math.i_{\mathrm{sp}}^{.Math.2}\left(t\right).Math.\mathrm{dt}}& \left[\mathrm{Expression}.Math..Math.10\right]\\ I_{\mathrm{sp}..Math.\mathrm{chk}}=y.Math.\frac{P_{\mathrm{sd}..Math.\mathrm{rated}}}{\sqrt{2}.Math.E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}}& \left[\mathrm{Expression}.Math..Math.11\right]\end{array}$

[0139] If a current flows out of the power conditioner PCS to the load R.sub.uw connected with the commercial power system, the voltage decreases and the current value gradually decreases. In the second current determination, if the calculated difference in maximum value of the output current of the power conditioner PCS is not less than the predetermined threshold a plurality of consecutive times and the absolute value |i.sub.sp| of the instantaneous output current i.sub.sp tends to decrease, it is determined that a current flows out of the power conditioner PCS to the load R.sub.uw connected with the commercial power system.

[0140] If the contacts of the grid interconnection relay Ry1 are normal with no welding and the commercial power system is connected with no load or a light load, the maximum value I.sub.sp.max of the output current i.sub.sp of the power conditioner PCS is regarded as constantly being not more than the predetermined threshold I.sub.sp.chk, as indicated in FIG. 8.

[0141] The display panel of the power conditioner PCS turns ON abnormality indication when an error flag is set in step S31. In a case where the present embodiment is designed such that the command value E*.sub.sd.rms of the effective value of the stand-alone power system voltage is 100 V and the threshold I.sub.sp.chk is 10% of the rated current (y=0.1) with the rated power P.sub.sd.rated of 1.5 kW at grid independent operation as a reference value, the threshold I.sub.sp.chk is set to 1 A and the sampling cycle is set to 0.5 T.sub.sd (see FIG. 6). The sampling cycle is 10 msec. if the stand-alone power system frequency is 50 Hz.

[0142] In step S27, as to a variation state of the maximum value I.sub.sp.max of the output current i.sub.sp, if a state not satisfying conditions that the maximum value I.sub.sp.max is less than the threshold I.sub.sp.chk at least three consecutive times and its value tends to increase lasts for a predetermined period (e.g. several cycles) as indicated in FIG. 8, it is determined that the commercial power system is connected with no load or a light load (S27, N). The process flow then proceeds to the second voltage determination starting in step S28. Determination is optionally repeated a plurality of times in consideration of erroneous detection due to noise and in order for reliable detection of the variation state.

[0143] In the second voltage determination, the stand-alone power system voltage e.sub.sd of the power conditioner PCS and the effective value of the commercial power system voltage e.sub.uw are measured during the predetermined delay period n.Math.T.sub.chk (n is a positive integer), and a magnitude relation is determined between an absolute value |E.sub.sd.rms−E.sub.uw.rms| of the difference between these values and a value obtained by multiplying the reference value E.sub.chk set in the voltage setting step by a predetermined confidence coefficient z (S29).

[0144] In a case where the grid interconnection relay Ry1 has no welded contact, it is detected that the commercial power system voltage is 200 V and the output voltage of the power conditioner PCS is 0 V. Assuming the confidence coefficient z=0.5 (z is a positive number satisfying z<1), a comparison value 100 V (=200×0.5) is less than the absolute value |E.sub.sd.rms−E.sub.uw.rms|=200 V of the difference.

[0145] In another case where the grid interconnection relay Ry1 has a welded contact, it is detected that the commercial power system voltage is 200 V and the output voltage of the power conditioner PCS is 200 V. In this case, the comparison value 100 V (=200×0.5) is more than the absolute value |E.sub.sd.rms−E.sub.uw.rms|=0 V of the difference.

[0146] If the value obtained by multiplying the reference value E.sub.chk by the confidence coefficient z is determined to be more than the absolute value |E.sub.sd.rms−E.sub.uw.rms| of the difference in step S29, the grid interconnection relay Ry1 is determined to have a welded contact and a flag is set in the error flag memory area set in the memory (S31).

[0147] If the value obtained by multiplying the reference value E.sub.chk by the confidence coefficient z is determined to be less than the absolute value |E.sub.sd.rms−E.sub.uw.rms| of the difference, the grid interconnection relay Ry1 is determined to be normal and the error flag in the error flag memory area set in the memory is reset (S30).

[0148] The delay period T.sub.chk is set to three cycles (n=3) of the commercial power system frequency or the stand-alone power system frequency so as to enable calculation of the effective value according to sampling values for at least the three cycles. The delay period T.sub.chk has only to be set to a plurality of cycles and is not limited to the three cycles.

[0149] If no welding of the grid interconnection relay Ry1 is detected in the second current determination in step S24 (S24, OK), the first voltage determination starting in step S25 is executed.

[0150] FIG. 5 depicts a specific procedure of the first voltage determination.

[0151] In the first voltage determination, a mathematical expression [Expression 12] sets a processing period T.sub.1 for determination as to whether or not the grid interconnection relay Ry1 has an abnormality (S41).

T.sub.1=(T.sub.ON−T.sub.dly)/  [Expression 12]

[0152] As indicated in FIG. 2, TON in the mathematical expression [Expression 11] is a period for measurement of mainly a voltage of the power conditioner PCS and a voltage of the commercial power system in a state where each of the contacts of the grid interconnection relay Ry1 is controlled to open or close in the contact control of the grid interconnection relay Ry1, and T.sub.dly is a period necessary for calculation with the measurement values. In the present embodiment, T.sub.ON is set to 1.0 sec. and the delay period T.sub.dly is set to 300 msec.

[0153] Upon completion of the setting, an initial value of a measurement time point k is set to “−2”, and the power conditioner PCS outputs a monitor voltage E.sub.min as a command value E.sub.sd.rms of the output voltage effective value included in mathematical expressions [Expression 13] in the state where all the contacts S.sub.u and S.sub.w of the grid interconnection relay Ry1 are controlled to open (S42).

[0154] The commercial power system voltage e.sub.uw is then measured and the effective value E.sub.uw.rms is calculated, an absolute value ΔE(k) of a difference between the command value E.sub.min of the output voltage effective value of the power conditioner PCS and the voltage effective value E.sub.uw.rms of the commercial power system is calculated and stored in the memory (S43).

[0155] The process in step S43 is continuously executed until the processing period T.sub.1 elapses. If the processing period T.sub.1 elapses (S44, Y), “1” is added to the measurement time point k and the output voltage of the power conditioner PCS is updated to a command value E.sub.max of the output voltage effective value included in the mathematical expressions [Expression 13] and is outputted (S45).

[0156] The process of measurement in steps S43 to S45 is repeated three times until the measurement time point k set to the initial value k=“−2” reaches “1” (S46), and the monitor voltage outputted from the power conditioner PCS during this period is switched from E.sub.min to E.sub.max, and then to E.sub.min chronologically. In the mathematical expressions, ΔE* is a command value of a variable output voltage of the power conditioner, whereas a and b are confidence coefficients set to the ranges 0<a<1 and 0<b<1, respectively.

[00007]$\begin{array}{cc}\{\begin{array}{c}{E}_{\max }=E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}+a.Math.\Delta .Math..Math.E^{*}\\ E_{\min }=E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}-a.Math.\Delta .Math..Math.E^{*}\\ b.Math.E_{\mathrm{Grid}}\le E_{\max }-E_{\min }\le 2.Math.a.Math.\Delta .Math..Math.E^{*}\\ 0<a<1\\ 0<b<1\end{array}& \left[\mathrm{Expression}.Math..Math.13\right]\end{array}$

[0157] If the measurement time point k is determined to be “1” in step S46, three differences ΔE(k) chronologically calculated at the time points k (k=−2, −1, and 0) are read out of the memory and a product ΔE.sub.CST of the differences is calculated and stored in the memory (S47).

[0158] The product ΔE.sub.CST calculated in step S47 is compared with the predetermined reference value E.sub.chk. If the product ΔE.sub.CST is less than the predetermined reference value E.sub.chk, the grid interconnection relay Ry1 is determined to have a welded contact (S48, N) and an error flag is set (step S31 in FIG. 4).

[0159] If the product ΔE.sub.CST is not less than the predetermined reference value E.sub.chk, the grid interconnection relay Ry1 is determined to have no welded contact (S48, Y) and the error flag is reset (step S30 in FIG. 4).

[0160] FIGS. 9A and 9B indicate timing of the first voltage determination depicted in the flowcharts in FIGS. 4 and 5.

[0161] As indicated in FIG. 9A, the monitor voltage E.sub.min is outputted as a command value E*.sub.sd.rms(k−2) of the output voltage effective value of the power conditioner PCS during the first processing period T.sub.1.

[0162] As indicated in FIG. 9B, a voltage effective value E.sub.uw.rms(k−2) of the commercial power system is measured during the first processing period Ti, and a difference ΔE(k−2) between E*.sub.sd.rms(k−2) and E.sub.uw.rms(k−2) is calculated and stored in the memory.

[0163] As indicated in FIG. 9A, the monitor voltage E.sub.max is outputted as a command value E*.sub.sd.rms(k−1) of the output voltage effective value of the power conditioner PCS during the subsequent processing period T.sub.1.

[0164] As indicated in FIG. 9B, a voltage effective value E.sub.uw.rms(k−1) of the commercial power system is measured during the subsequent processing period Ti, and a difference ΔE(k−1) between the E*.sub.sd.rms(k−1) and E.sub.uw.rms(k−1) is calculated and stored in the memory.

[0165] As indicated in FIG. 9A, the monitor voltage E.sub.min is outputted as a command value E*.sub.sd.rms(k) of the output voltage effective value of the power conditioner PCS during the last processing period T.sub.1.

[0166] As indicated in FIG. 9B, a voltage effective value E.sub.uw.rms(k) of the commercial power system is measured during the last processing period T.sub.1, and the difference ΔE(k) between E*.sub.sd.rms(k) and E.sub.uw.rms(k) is calculated and stored in the memory.

[0167] The product ΔE.sub.CST of the data ΔE(k−2), ΔE(k−1), and ΔE(k) stored in the memory during the period T.sub.chk is subsequently calculated in accordance with the mathematical expressions [Expression 14] and is compared with the predetermined reference value E.sub.chk. Assume that the variable voltage of the command value of the output voltage of the power conditioner is ΔE*, and the confidence coefficient for adjustment of the variable voltage ΔE* and the reference value E.sub.chk is a. The confidence coefficient a is set to 0.5, the command value ΔE* of the variable output voltage is set to 20 V, the command value E*.sub.sd.rms is set to 40 V, and the confidence coefficient b is set to 0.1 in the present embodiment. These values are set appropriately in accordance with actual design conditions and the like.

[00008]$\begin{array}{cc}\{\begin{array}{c}\Delta .Math..Math.E\left(k\right)=.Math.E_{\mathrm{sd}..Math.\mathrm{rms}}^{*}\left(k\right)-E_{\mathrm{uw}..Math.\mathrm{rms}}\left(k\right).Math.\\ \Delta .Math..Math.E_{\mathrm{CST}}=\Delta .Math..Math.E\left(k-2\right).Math.\Delta .Math..Math.E\left(k-1\right).Math.\Delta .Math..Math.E\left(k\right)\\ \Delta .Math..Math.E_{\mathrm{CST}}<E_{\mathrm{chk}}\\ E_{\mathrm{chk}}<{\left(a.Math.\Delta .Math..Math.E^{*}\right)}^3\end{array}& \left[\mathrm{Expression}.Math..Math.14\right]\end{array}$

[0168] The command values E.sub.min and E.sub.max of the output voltage effective value are preferably set in the range from b×E.sub.Grid to 2×a×ΔE* with respect to a rated voltage E.sub.Grid of the commercial power system. If there is a small difference between the command values E.sub.min and E.sub.max of the output voltage effective value, there may not be recognized a very large difference in product ΔE.sub.CST between the case where the grid interconnection relay Ry1 has a welded contact and the case where the grid interconnection relay Ry1 has no welded contact, due to hum noise. In consideration of such cases, the command values E.sub.min and E.sub.max of the output voltage effective value are set to fall within the range from b×E.sub.Grid to 2×a×ΔE* so as to cause a significant difference in product ΔE.sub.CST between the case where the grid interconnection relay Ry1 has a welded contact and the case where the grid interconnection relay Ry1 has no welded contact.

[0169] As described above, in the abnormality detection device for the grid interconnection relay Ry1 according to the present invention, a circuit element configured to detect an output voltage and an output current of the power conditioner PCS, as well as a commercial power system voltage is originally required for control of the power conditioner PCS and there is thus no need to separately provide any sensor or any circuit element for determination of welding of the grid interconnection relay Ry1.

[0170] Whether or not the contacts of the grid interconnection relay Ry1 have a welding abnormality is reliably detected regardless of whether or not the AC load R.sub.uw is connected, and regardless of whether or not there is a commercial power system voltage.

[0171] FIGS. 10A to 10C and FIGS. 11A to 11C each exemplify a test result of the first voltage determination according to the present invention. Specifically, in the state where the contact S.sub.u is closed and the contact S.sub.w is opened out of the contacts S.sub.u and S.sub.w of the grid interconnection relay Ry1 as indicated in FIGS. 10A and 11A in the contact control in step S22 in FIG. 4, the first voltage determination is executed. FIGS. 10B, 10C, 11B, and 11C indicate results thereof, i.e. results of welding determination as to the contact S.

[0172] Such tests were conducted under conditions of the check period T.sub.ON=1 sec. and the delay period T.sub.dly=0.3 sec. indicated in FIG. 2, as well as the monitor voltage E.sub.min=30 V, the monitor voltage E.sub.max=50 V, the check period T1=0.2 sec., and the reference value E.sub.chk=125 V.

[0173] As indicated in FIG. 10B, using the grid interconnection relay Ry1 having the welded contact S.sub.w, in the state where the contact Su is controlled to close and the contact S.sub.w is controlled to open in the contact control step, the power conditioner PCS outputted the voltage e.sub.sd chronologically alternately in the order of E.sub.min (=30 V), E.sub.max (=50 V), and E.sub.min (=30 V) in the cycles T.sub.1 (=0.2 sec.≈(T.sub.ON−T.sub.dly)/3).

[0174] As indicated in FIG. 10C, the voltage e.sub.uw substantially equal to the voltage e.sub.sd is also detected at the commercial power system because the contact S.sub.w of the grid interconnection relay Ry1 is welded. The values ΔE calculated in the respective cycles T.sub.1 are thus substantially zero. The product ΔE.sub.CST of ΔE(k−2), ΔE(k−1), and ΔE(k) was subsequently calculated during the period T.sub.chk and the product was determined to be less than E.sub.chk (=125 V), so that the grid interconnection relay Ry1 was determined to have a welded contact. In FIG. 10C, there is recognized slight hum noise in a waveform of the commercial power system voltage e.sub.uw while the power conditioner PCS outputs no voltage.

[0175] As indicated in FIG. 11C, using the grid interconnection relay Ry1 having the normal contact S.sub.w with no welding, in the state where the contact S.sub.u is controlled to close and the contact S.sub.w is controlled to open in the contact control step, the power conditioner PCS outputted the voltage chronologically alternately in the order of E.sub.min (=30 V), E.sub.max (=50 V), and E.sub.min (=30 V) in the cycles T.sub.1 (=0.2 sec.≈(T.sub.ON−T.sub.dly)/3).

[0176] As indicated in FIG. 11C, the voltage e.sub.sd is not detected at the commercial power system because the contact S.sub.w of the grid interconnection relay Ry1 is not welded. The hum noise, however, has a larger value due to the voltage e.sub.sd outputted from the power conditioner PCS. Determination only according to the value ΔE(k) will lead to erroneous determination that the grid interconnection relay Ry1 has a welded contact. It is because the differences ΔE(k−2), ΔE(k−1), and ΔE(k) in FIG. 10C are not significantly different from the differences ΔE(k−2), ΔE(k−1), and ΔE(k) in FIG. 11C.

[0177] However, as indicated in FIGS. 11B and 11C, the hum noise is not largely varied in level in a case where the voltage e.sub.sd outputted from the power conditioner PCS is varied stepwise from 30 V to 50 V. The product ΔE.sub.CST of ΔE(k−2), ΔE(k−1), and ΔE(k) was thus determined to be more than E.sub.chk (=125 V), so that the grid interconnection relay Ry1 was accurately determined to have no welded contact.

[0178] If the grid interconnection relay Ry1 in FIGS. 10B and 10C has welding, the differences ΔE are designed to a half of the command value of total variable voltages (0.5×a×ΔE*). The differences ΔE are about 5 V, and the reference value E.sub.chk set in the range from 5.sup.3 (=125 V) to 6.sup.3 (=216 V) is evaluated as being appropriate. The value Ethic is appropriately determined in consideration of the command values E.sub.min and E.sub.max of the output voltage effective value. This exponent 3 corresponds to the number of the sampled differences.

[0179] Other embodiments will be described below.

[0180] The embodiment described above exemplifies the case where the command value of the output voltage effective value of the power conditioner PCS is outputted in the three cycles by alternately adopting the two values E.sub.min and E.sub.max in the first voltage determination. The number of the command values is not necessarily two and at least voltages having different values are applicable. Furthermore, the number of cycles is not necessarily three and has only to be at least two.

[0181] The power conditioner PCS has only to chronologically alternately output the different monitor voltages E.sub.min and E.sub.max for abnormality determination as to the grid interconnection relay Ry1 according to whether or not the difference between the voltage of the power conditioner PCS and the voltage of the commercial power system with respect to each of the monitor voltages E.sub.min and E.sub.max follows the corresponding monitor voltage.

[0182] The monitor voltages E.sub.min and E.sub.max can obviously be chronologically alternately outputted in the order of E.sub.min, E.sub.max, and E.sub.min, as well as in the order of E.sub.max, and E.sub.min, and E.sub.max. In other words, the monitor voltages having the command values E.sub.min and E.sub.max of the output voltage effective value have only to be outputted chronologically alternately.

[0183] The above embodiment exemplifies the aspect in which the product ΔE.sub.CST of the differences ΔE(k−2), ΔE(k−1), and ΔE(k) is compared with the reference value E.sub.chk for abnormality or normality determination. Alternatively, the differences are each compared with a preliminarily set reference value so as to be determined to be abnormal or normal, and final abnormality or normality determination is executed in a case where all the differences are determined to be normal.

[0184] Still alternatively, abnormality or normality determination is executed in accordance with whether or not the differences are similarly varied in accordance with the variation between the monitor voltages E.sub.min and E.sub.max. If the differences are varied similarly with the varied monitor voltages E.sub.min and E.sub.max, the grid interconnection relay Ry1 is determined to be normal. In contrast, if the differences are not varied in accordance with the varied monitor voltages E.sub.min and E.sub.max, the grid interconnection relay Ry1 is determined to be abnormal.

[0185] The abnormality detection device for the grid interconnection relay according to the present invention has only to include the abnormality detector configured to execute the commercial power system voltage determination of determining whether or not there is a commercial power system voltage, and, if it is determined that there is no commercial power system voltage through the commercial power system voltage determination, execute the first voltage determination of causing the power conditioner to chronologically alternately output the monitor voltages having different values in the state where the grid interconnection relay has a contact controlled to open, and determining whether or not the grid interconnection relay has an abnormality in accordance with whether or not the difference between the power conditioner voltage and the commercial power system voltage with respect to each of the monitor voltages follows the corresponding monitor voltage.

[0186] The above embodiments of the present invention exemplify the case where the power conditioner PCS is configured for single-phase output. The present invention is also applicable to a case where the power conditioner PCS is configured for three-phase output and the grid interconnection relay Ry1 includes three contacts S.sub.u, S.sub.v, and S.sub.w.

[0187] The abnormality detection device for the grid interconnection relay according to each of the above embodiments exemplifies the distributed power supply including the solar panel SP and the power conditioner PCS connected to the solar panel SP. The power generator incorporated in the distributed power supply is not limited to the solar panel SP but is appropriately selected from a wind power generator, a fuel cell, and the like.

[0188] The embodiments described above merely exemplify the method of detecting an abnormality of the grid interconnection relay and the power conditioner according to the present invention. The description is not intended to limit the technical scope of the present invention. It is obvious that the specific circuit configuration and abnormality detection algorithm can appropriately be modified in design as long as the functional effects of the present invention are exerted.

REFERENCE SIGNS LIST

[0189] 1: Distributed power supply [0190] 2: DC/DC converter [0191] 3: DC/AC inverter [0192] 4: LC filter [0193] 5: Control unit [0194] 5a: Converter controller [0195] 5b: Inverter controller [0196] 5c: Abnormality detector [0197] PCS: Power conditioner [0198] Ry1: Grid interconnection relay [0199] Ry2: Stand-alone power system relay [0200] S.sub.u, S.sub.w: Contact