CN115912930A - Isolation converter, photovoltaic system and insulation impedance detection method - Google Patents
Isolation converter, photovoltaic system and insulation impedance detection method Download PDFInfo
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- CN115912930A CN115912930A CN202211450614.0A CN202211450614A CN115912930A CN 115912930 A CN115912930 A CN 115912930A CN 202211450614 A CN202211450614 A CN 202211450614A CN 115912930 A CN115912930 A CN 115912930A
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Abstract
The application discloses isolation converter, photovoltaic system and insulation impedance detection method, and the isolation converter includes: the device comprises a controller, a bridging resistor, a primary side conversion circuit, a transformer and a secondary side conversion circuit; the input end of the primary side conversion circuit is used for inputting direct current, the output end of the primary side conversion circuit is connected with the primary side of the transformer, the secondary side of the transformer is connected with the input end of the secondary side conversion circuit, and the secondary side conversion circuit is used for outputting alternating current; the first end of the bridging resistor is connected with the primary side conversion circuit, and the second end of the bridging resistor is connected with the secondary side conversion circuit; and the controller changes the voltage at the two ends of the bridging resistor and obtains the insulation impedance of the isolation converter according to the changed voltages at the two ends of the front and rear bridging resistors, the changed voltage of the first ends of the front and rear bridging resistors to the ground and the changed resistance value of the bridging resistor. When the insulation resistance is small, insulation fault occurs on the direct current side, and measures are taken in time. Because only one bridging resistor is added, hardware devices are fewer, the size of the circuit is reduced, and the cost is reduced.
Description
Technical Field
The application relates to the technical field of power electronics, in particular to an isolation converter, a photovoltaic system and an insulation impedance detection method.
Background
At present, due to the difference between the input voltage and the output voltage, or the requirement of safety isolation, the micro inverter and the energy storage converter usually adopt an isolation topology, that is, the micro inverter and the energy storage converter include a transformer, and hereinafter, the isolation inverter or the converter including the transformer is generally referred to as an isolation converter.
The input end of a general micro inverter is connected with a photovoltaic module, and the input end of an energy storage converter is connected with a battery. Therefore, the isolation converter has a requirement for detecting the insulation resistance to the ground for the power supply such as the photovoltaic module or the battery at the input end, that is, whether the insulation fault occurs is judged according to the size of the insulation resistance.
The traditional resistance bridge method requires adding a resistor and a switch circuit to the ground, which increases the volume of the isolation converter, reduces the reliability of equipment and has higher cost. Taking micro-inverter as an example, the advantage of small volume and long life is applied, and the traditional resistance bridge method for measuring the insulation resistance can increase the volume and reduce the reliability to influence the life.
Disclosure of Invention
In view of this, the present application provides an isolation converter, a photovoltaic system and an insulation resistance detection method, which can detect the insulation resistance of the isolation converter without significantly increasing the volume.
The present application provides an isolated converter comprising: the device comprises a controller, a bridging resistor, a primary side conversion circuit, a transformer and a secondary side conversion circuit;
the input end of the primary side conversion circuit is used for inputting direct current, the output end of the primary side conversion circuit is connected with the primary side of the transformer, the secondary side of the transformer is connected with the input end of the secondary side conversion circuit, and the secondary side conversion circuit is used for outputting alternating current;
the first end of the bridging resistor is connected with the primary side conversion circuit, and the second end of the bridging resistor is connected with the secondary side conversion circuit;
and the controller is used for changing the voltage at the two ends of the bridging resistor and obtaining the insulation impedance of the isolation converter according to the changed voltages at the two ends of the front and rear bridging resistors, the changed voltage of the first ends of the front and rear bridging resistors to the ground and the resistance value of the bridging resistor.
Preferably, the first end of the bridging resistor is connected with the negative electrode or the positive electrode of the primary side conversion circuit.
Preferably, the second end of the bridging resistor is connected with the positive electrode or the negative electrode of the secondary side conversion circuit.
Preferably, the secondary side conversion circuit includes: an inverter circuit;
and a controller, in particular for varying the secondary side voltage of the transformer to vary the voltage across the resistor.
Preferably, the secondary side conversion circuit includes: an inverter circuit;
and the controller is specifically used for changing the switching state of a switching tube in the inverter circuit to change the voltage across the resistor.
Preferably, the inverter circuit includes: the first switching tube, the second switching tube, the third switching tube and the fourth switching tube;
the first switching tube and the second switching tube are connected in series to form a first bridge arm, and the third switching tube and the fourth switching tube are connected in series to form a second bridge arm;
the middle point of the first bridge arm is connected with the live wire through a first switch, and the middle point of the second bridge arm is connected with the zero line through a second switch.
Preferably, the first end of the bridging resistor is connected to the negative electrode of the primary side conversion circuit, and the voltage across the first end of the bridging resistor to the ground is sampled at the same instant of the instantaneous value of the output voltage of the inverter circuit before and after the change.
Preferably, the first end of the cross-over resistor is connected to the negative electrode of the primary side conversion circuit, the voltage across the first end of the cross-over resistor to the ground is superposed with a preset alternating voltage at the output voltage of the inverter circuit before and after the change, and the amplitude of the preset alternating voltage is the same as the amplitude of the grid voltage but opposite in direction.
Preferably, the primary side conversion circuit is a forward circuit, a flyback circuit, a push-pull circuit or a half-bridge circuit.
Preferably, the primary side conversion circuit comprises an inverter H-bridge circuit, and the secondary side conversion circuit comprises a bidirectional switch bridge arm and a capacitor bridge arm which are connected in parallel.
Preferably, a first end of the bridging resistor is connected with a direct current negative electrode or a direct current positive electrode of the inverter H-bridge circuit, and a second end of the bridging resistor is connected with a midpoint, a positive electrode or a negative electrode of a capacitor bridge arm of the transformer.
Preferably, the capacitor bridge arm comprises two capacitors connected together in series;
the controller is specifically used for changing the voltage of at least one capacitor on the capacitor bridge arm to change the voltage at two ends of the bridging resistor, or changing the switching state of a switching tube of the bidirectional switch bridge arm to change the voltage at two ends of the bridging resistor.
Preferably, the ground of the control circuit where the controller is located is connected with the input cathode of the primary side conversion circuit, and the bridging resistor is a voltage sampling resistor of the secondary side conversion circuit;
or the like, or, alternatively,
the ground of the control circuit where the controller is located is connected with the input negative electrode of the secondary side conversion circuit, and the bridging resistor is a voltage sampling resistor of the primary side conversion circuit.
Preferably, the primary side conversion circuits comprise a plurality of primary side conversion circuits, and the cathodes of the plurality of primary side conversion circuits are connected together; the number of the transformers is multiple, and each transformer comprises a primary side and a secondary side;
the plurality of primary side conversion circuits are in one-to-one correspondence with the plurality of transformers, and each primary side conversion circuit is connected with a corresponding primary side;
all the secondary sides are coupled together directly or through power semiconductor devices;
or the like, or, alternatively,
the transformer comprises a plurality of primary sides and a plurality of secondary sides, and the plurality of primary sides share a magnetic core; each primary side conversion circuit is connected with a corresponding primary side, and all secondary sides are directly coupled together or coupled together through a power semiconductor device.
Preferably, the secondary side conversion circuit is grounded through a protection zero system.
The present application also provides a photovoltaic system, comprising the isolated converter described above, further comprising: a photovoltaic module;
at least one primary side conversion circuit is provided;
the input end of each primary side conversion circuit is connected with at least one photovoltaic module.
The present application also provides an insulation impedance detection method of an isolation converter, the isolation converter including: the transformer comprises a bridging resistor, a primary side conversion circuit, a transformer and a secondary side conversion circuit; the input end of the primary side conversion circuit is used for inputting direct current, the output end of the primary side conversion circuit is connected with the primary side of the transformer, the secondary side of the transformer is connected with the input end of the secondary side conversion circuit, and the secondary side conversion circuit is used for outputting alternating current; the first end and the second end of the bridging resistor are respectively connected with the primary side conversion circuit and the secondary side conversion circuit;
the method comprises the following steps:
changing the voltage across the resistor;
and obtaining the insulation impedance of the isolation converter according to the voltages of the two ends of the front and rear bridging resistors, the voltages of the first ends of the front and rear bridging resistors to the ground and the resistance value of the bridging resistor.
Therefore, the application has the following beneficial effects:
according to the isolation converter, the insulation resistance of the direct current side of the isolation converter can be detected only by adding one bridging resistor, and therefore when the insulation resistance is small, it is judged that insulation faults occur on the direct current side, and measures are taken in time. Because only one bridging resistor is added, compared with a resistor bridge method in the prior art, the number of hardware devices is reduced, the size of the circuit can be reduced, and the cost is reduced.
Drawings
Fig. 1 is a schematic diagram of an isolated converter according to an embodiment of the present application;
FIG. 2 is a schematic diagram of another isolated converter provided by an embodiment of the present application;
FIG. 3 is an equivalent circuit diagram of the insulation resistance corresponding to FIG. 2;
FIG. 4 is a schematic diagram of another isolated converter provided in an embodiment of the present application;
FIG. 5 is a schematic diagram of a voltage sampling circuit corresponding to FIG. 4;
FIG. 6 is a corresponding equivalent circuit diagram of FIG. 5;
FIG. 7 is a diagram of insulation resistance corresponding to FIG. 6;
fig. 8 is a schematic view of a photovoltaic system provided in an embodiment of the present application;
fig. 9 is a flowchart of an insulation resistance detection method of an isolation converter according to an embodiment of the present application.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present application more comprehensible, embodiments accompanying the drawings are described in detail below.
Referring to fig. 1, a schematic diagram of an isolated converter according to an embodiment of the present application is shown.
The isolated converter provided by the embodiment comprises: a controller (not shown in the figure), a cross-over resistor Rm, a primary side conversion circuit 100, a transformer T and a secondary side conversion circuit 200;
the input end of the primary side conversion circuit 100 is used for inputting direct current, the output end of the primary side conversion circuit 100 is connected with the primary side of the transformer, the secondary side of the transformer T is connected with the input end of the secondary side conversion circuit 200, and the secondary side conversion circuit 200 is used for outputting alternating current;
a first end of the bridging resistor Rm is connected with the primary side conversion circuit 100, and a second end of the bridging resistor Rm is respectively connected with the secondary side conversion circuit 200;
the specific location where the bridging resistor is connected to the primary side conversion circuit is not specifically limited in the embodiments of the present application, for example, the first end of the bridging resistor is connected to the negative electrode or the positive electrode of the primary side conversion circuit.
The embodiment of the application does not specifically limit the specific position of the bridging resistor for connecting the secondary side conversion circuit, and the second end of the bridging resistor is connected with the anode or the cathode of the secondary side conversion circuit.
And the controller is used for changing the voltage at two ends of the bridging resistor Rm and obtaining the insulation resistance of the isolation converter according to the changed voltages at two ends of the front and rear bridging resistors Rm, the changed voltage of the first end of the front and rear bridging resistors Rm to the ground and the resistance value of the bridging resistor Rm. Namely, the insulation resistance to the ground is obtained by five values of the voltage at the two ends of Rm before the voltage is changed, the voltage at the two ends of Rm after the voltage is changed, the voltage of the first end of Rm to the ground before the voltage is changed, the voltage of the first end of Rm to the ground after the voltage is changed, and the resistance value of Rm.
Since the first end of the bridge resistor is not specifically limited to be connected to the positive electrode or the negative electrode of the primary side conversion circuit in the embodiment of the application, the voltage of the first end of the bridge resistor to the ground may be the voltage of the positive electrode of the primary side conversion circuit to the ground or the voltage of the negative electrode of the primary side conversion circuit to the ground.
For example, when the first end of the bridge resistor is connected to the negative electrode of the primary-side conversion circuit, the voltage of the negative electrode of the primary-side conversion circuit may be a voltage of the ground, or the input voltage of the primary side may be subtracted from the voltage of the positive electrode of the primary-side conversion circuit.
It should be understood that the insulation impedance is an equivalent integrated impedance, and is not an impedance to ground of a certain end, for example, an equivalent value of a positive electrode to ground impedance and a negative electrode to ground impedance of the primary side conversion circuit, and when the input end of the primary side conversion circuit includes a plurality of input ends, an equivalent impedance to ground of a plurality of direct current input ends.
In order to prevent Rm from destroying the function of the isolation converter, rm may be designed in such a way that, for example, to avoid destroying the function of the transformer isolation, the resistance value of Rm should be large, for example, 1M Ω may be obtained, so that the leakage current between the primary side and the secondary side of the transformer is lower than a limited current value. In order to avoid damaging the insulation function of the transformer, the resistance value of Rm and the pipelining span should be large enough, for example, a plurality of resistors with larger resistance values can be selected to be connected in series to realize Rm, so as to maintain the original insulation level of the transformer. To avoid degrading the electromagnetic compatibility (EMC) design, rm can be made as small as possible with the transformer to reduce loop emissions, and inductive devices (e.g., beads) can be connected in series with Rm to increase the high frequency impedance. In order not to affect the normal operation of the isolation converter, a switch may be provided, which is connected in series with Rm, and closes the switch when detecting the insulation resistance, and opens the switch when detecting no insulation resistance, that is, cuts Rm out of the circuit.
According to the isolation converter provided by the embodiment of the application, the insulation resistance of the direct current side of the isolation converter can be detected only by adding one bridging resistor, so that when the insulation resistance is small, the insulation fault of the direct current side is judged to be generated, and measures are taken in time. Because only one bridging resistor is added, compared with the resistance bridge method in the prior art, the number of hardware devices is reduced, the volume of the circuit can be reduced, and the cost is reduced.
The embodiment of the application does not specifically limit the specific circuit structure of the primary side conversion circuit, and does not specifically limit the specific circuit structure of the secondary side conversion circuit, and the insulation impedance detection mode provided by the application can be adopted for the insulation impedance detection as long as the isolation converter comprises a transformer, and the isolation converter is in direct current input and alternating current output. The manner in which the isolation resistance is obtained is described below in connection with a specific circuit. First, a primary side conversion circuit is taken as a flyback circuit, and a secondary side conversion circuit is taken as an inverter circuit for example.
The embodiment of the present application does not specifically limit the dc source connected to the dc side of the isolated converter, and for example, the dc source may be a dc source such as a photovoltaic module or a storage battery, and the ac side may be connected to an ac grid or to an electric device such as a load.
Referring to fig. 2, a schematic diagram of another isolated converter provided in the embodiments of the present application is shown.
In the isolation converter provided in this embodiment, a primary side conversion circuit including a flyback circuit is taken as an example for description, and in addition, the primary side conversion circuit may also be a forward circuit, a push-pull circuit, or a half-bridge circuit.
The number of the transformers can be one or more.
When the number of the transformers is multiple, each transformer comprises a primary side and a secondary side; the primary side conversion circuits comprise a plurality of primary side conversion circuits, and the cathodes of the plurality of primary side conversion circuits are connected together; the transformer comprises a plurality of primary sides, a plurality of primary side conversion circuits are in one-to-one correspondence with the plurality of transformers, and each primary side conversion circuit is connected with one corresponding primary side of the transformer; all the secondary sides are coupled together directly or through power semiconductor devices.
For convenience of description, in the following embodiments, the primary side conversion circuit includes two paths, and an input end of each path is connected to a corresponding direct current.
DC1 and DC2 in fig. 2 respectively represent two direct current sources, the first primary side conversion circuit includes a first primary side winding and a power tube S1, the second primary side conversion circuit includes a second primary side winding and a power tube S2, the first primary side winding and S1 are connected in series and then connected between the positive electrode and the negative electrode of C1, and the second primary side winding and S2 are connected in series and then connected between the positive electrode and the negative electrode of DC 2. The cathodes of the two primary side conversion circuits are connected together, namely the DC1 and the cathode are connected with the cathode of the DC 2.
Each primary winding corresponds to a secondary winding, the first secondary winding is connected in parallel with the second secondary winding through a first diode D1 and a second diode D2, it should be understood that after passing through the diodes, the output is direct current, and the direct current is inverted into alternating current through an inverter circuit.
In this embodiment, the secondary side conversion circuit includes a full-bridge inverter circuit as an example, that is, the inverter circuit includes: a first switching tube S3, a second switching tube S4, a third switching tube S5 and a fourth switching tube S6;
the first switching tube S3 and the second switching tube S4 are connected in series to form a first bridge arm, and the third switching tube S5 and the fourth switching tube S6 are connected in series to form a second bridge arm;
the middle point of the first bridge arm is connected with a live wire through a first switch K1, and the middle point of the second bridge arm is connected with a zero line through a second switch K2.
It should be understood that the figures are only schematic, and in actual operation, the output terminal of the isolated converter is connected to the power grid through a filter, for example, the filter includes an inductor, a capacitor, etc., in addition to the power grid through K1 and K2.
In this embodiment, the inverter circuit outputs a single-phase alternating current as an example.
In this embodiment, an example will be described in which both ends of the bridge resistor Rm are connected to the negative electrode of the primary side conversion circuit and the negative electrode of the secondary side conversion circuit, respectively.
In order to derive the insulation resistance on the dc side of the isolating transformer, an equivalent circuit diagram is described below.
Referring to fig. 3, an equivalent circuit diagram of the insulation resistance corresponding to fig. 2 is shown.
Assuming that the positive pole-to-ground insulation resistance of DC1 is R1, the positive pole-to-ground insulation resistance of DC2 is R2, and the negative pole of DC1 and the negative pole of DC2 share the ground insulation resistance of R0, since the negative pole of DC1 and the negative pole of DC2 are connected together, the two negative poles share one ground insulation resistance.
The secondary side of the transformer is connected with the ground, for example, the TN system is grounded through the protection zero-connection. Assuming that the input voltage of DC1 is V1, the input voltage of DC2 is V2, the voltage of ground to DC negative electrode is V0, and the voltage of second end of Rm to DC negative electrode is Vm.
The following can be obtained:
the resistance R0 of the direct current negative pole to the ground can be obtained by the formula (2):
and obtaining a new voltage Vm 'of the second end of the Rm to the direct current negative electrode by changing the operation state of the isolation converter or the switching state of a switching tube in the isolation converter, and acquiring a new value of the voltage V0' of the earth to the direct current negative electrode, wherein the values of V1 and V2 are kept unchanged in the process, for example, the operation state change of the isolation converter is completed in a short time. A new equation can be listed:
(4) - (1), the influence of V1 and V2 can be eliminated, yielding:
the equivalent parallel values of the positive electrode-to-ground resistance R1 of DC1 and the positive electrode-to-ground resistance R2 of DC2 can be calculated ("//" represents the parallel calculation):
according to the formula (3) and the formula (6), the equivalent parallel values of the direct current negative electrode ground resistance R0, the positive electrode ground resistance R1 of the DC1, and the positive electrode ground resistance R2 of the DC2, that is, the final direct current side ground insulation resistance value, can be calculated:
the voltages V0 and V0 'may be obtained by direct voltage sampling, or by sampling other voltage values and performing conversion to obtain V0 and V0'.
In the embodiment, two-way dc input is taken as an example for introduction, and the above conclusion is also applicable to the case of single-way dc input, three-way dc input, or more-way dc input. Moreover, the two ends of Rm are connected to other positions, for example, the first end of the bridge resistor Rm may also be connected to the positive electrode of DC1, or the second end of the bridge resistor Rm may be connected to the positive electrode of the secondary side direct current, and the like, and the same conclusion may also be obtained, which is not described herein again.
From equation (7), the key to calculating the dc-side ground insulation resistance value is to change the terminal voltage across the resistance Rm (for example, change the voltage of the second terminal to the dc negative terminal) Vm and Vm 'and cause a change in the dc negative voltage to ground voltages V0 and V0'. A corresponding change occurs because the secondary side of the converter has another connection branch, in fig. 2 an ac network, with respect to ground, so that changing the voltage across the resistor causes a change in the dc negative voltage to ground.
Several specific implementations of varying the voltage across the resistor are described below in conjunction with fig. 2.
The first method comprises the following steps:
the controller is specifically configured to vary the voltage on the secondary side of the transformer to vary the voltage across the resistor, for example, to vary the voltage on the secondary side dc bus.
Continuing to refer to fig. 2, keeping S5 and K2 closed, and disconnecting the other switching tubes and switches, so that the positive electrode of the secondary side dc bus is connected with the N line (zero line) of the power grid through the closed S5 and K2, which is equivalent to grounding the positive electrode of the dc bus. And the controller of the isolation converter controls the output voltage on the secondary side direct current bus to be Vbus, and then Vm is:
V m ＝-V bus +V 0 (8)
by changing the value of Vbus to Vbus ', vm' can be obtained:
V’ m ＝-V’ bus +V’ 0 (9)
substituting equations (8) and (9) into equation (7) can obtain a DC-side insulation resistance value to ground as follows:
as can be seen from equation (10), the dc-to-ground insulation resistance can be calculated by changing the value of Vbus and causing a change in the dc negative voltage-to-ground voltage V0. The secondary side voltage may vary considerably, for example from 50V to 300V, thereby improving the accuracy of the detection of the dc side insulation resistance value to ground.
And the second method comprises the following steps:
and the controller is specifically used for changing the switching state of a switching tube in the inverter circuit to change the voltage across the resistor.
Continuing to refer to fig. 2, maintaining the voltage Vbus of the direct-current bus on the secondary side unchanged, maintaining S5 and K2 to be closed, and keeping the rest of the switch tubes and the switches to be open, wherein Vm is represented by formula (8); changing the states of the switch tube and the switch, for example, maintaining the switch tube S6 and the switch K2 to be closed, and the other switch tubes and the switches to be open, so as to obtain Vm':
V’ m ＝V’ 0 (11)
substituting equations (8) and (11) into equation (7) can obtain a DC-side insulation resistance value to ground as follows:
as can be seen from equation (12), when the Vbus value is maintained, the dc-side insulation resistance to ground can be calculated by changing the secondary-side switch state to change the dc negative voltage to ground V0.
In both of the above-described two ways of changing the voltage across the across resistor, the connection relationship between the secondary side conversion circuit and the N line of the power grid is changed, and in addition, the connection relationship between the secondary side conversion circuit and the L line (live line) can also be changed. This solution is particularly suitable for a scenario where the output of the isolated converter is connected to only the L-line of the power grid, for example, a three-phase three-line connection. For example, when the switch tube S3 and the switch K1 are kept closed and the other switch tubes and switches are all opened, the positive electrode of the secondary side dc bus is connected to the L line through the closed S3 and K1. Assuming that the AC voltage is Vac, vm is:
V m ＝-V bus +V ac +V 0 (13)
since the output of the secondary side inverter circuit is connected to the ac grid or the load, the ac voltage may interfere with the detection of the insulation resistance, and several ways of reducing the ac voltage interference are described below.
The first method comprises the following steps:
the ac voltage effect is eliminated by the period average.
Eliminating the influence of Vac on the calculation of the insulation resistance value, continuously collecting the voltage of at least 1 power frequency period, and then averaging, so that the influence of Vac can be eliminated, wherein the voltage has a transverse line to represent an average value in the following formula:
wherein:
the dc-to-ground insulation resistance value can be calculated as:
and the second method comprises the following steps:
the ac voltage effect is eliminated by sampling the same instantaneous value. Namely, the first end of the bridging resistor is connected with the cathode of the primary side conversion circuit, and the voltage of the first end of the bridging resistor to the ground is sampled at the same moment of the instantaneous value of the output voltage of the inverter circuit before and after the change.
In changing Vm, sampling of V0 is performed when the instantaneous voltages of Vac are the same. Assuming that the instantaneous voltage of Vac is Vac (T), for example, a forward peak 311V of a 220V ac grid, when the voltage value for changing Vm changes from Vm1 to Vm2, the values of V0 are respectively recorded as V0 and V0' when the instantaneous voltage of Vac is Vac (T). In the calculation process, vac (T) will be cancelled out, and the dc side insulation resistance to ground calculation result will not be affected.
And the third is that:
the influence of the alternating voltage is eliminated by the inverter outputting the reverse voltage. The first end of the bridging resistor is connected with the negative electrode of the primary side conversion circuit, the voltage of the first end of the bridging resistor to the ground is superposed with a preset alternating current voltage at the output voltage of the inverter circuit before and after the change, the amplitude of the preset alternating current voltage is the same as the amplitude of the power grid voltage but opposite in direction, and the amplitude is the same as the amplitude of the power grid voltage but opposite in direction, so that when the voltage of the first end of the bridging resistor to the ground is sampled, the preset alternating current voltage is offset with the power grid voltage, and the influence of the power grid voltage on the sampled voltage is eliminated.
By the operation of the isolated converter, a dc superimposed ac voltage is generated on the secondary capacitor, wherein the superimposed ac voltage has the same magnitude as the ac voltage Vac but exactly cancels Vac in the circuit, for example:
V bus ＝V bus_dc +V ac (16)
the compound of formula (13) can be:
V m ＝-V bus_dc +V 0 (17)
therefore, vac is exactly offset in the circuit analysis, so that the Vac does not influence the calculation result of the DC-to-ground insulation resistance.
The isolation converter described in the above embodiment is described by taking the primary side as a flyback circuit and the secondary side including an inverter circuit as an example, and the case where the primary side is a full-bridge inverter circuit, the secondary side is a secondary side bridge arm circuit, and the secondary side does not include a dc bus is described below. The primary side conversion circuits comprise a plurality of primary side conversion circuits, and the cathodes of the plurality of primary side conversion circuits are connected together; the transformer comprises a plurality of primary sides, and each primary side conversion circuit is connected with one primary side of the transformer.
Referring to fig. 4, a schematic diagram of another isolated converter provided in the embodiments of the present application is shown.
In this embodiment, the example that the isolated converter includes two dc inputs is continuously described.
DC1 and DC2 in fig. 2 respectively represent two DC sources, and the cathodes of the two DC sources are connected together.
Assuming that the positive pole-to-ground insulation resistance of DC1 is R1, the positive pole-to-ground insulation resistance of DC2 is R2, and the negative pole of DC1 and the negative pole of DC2 share the ground insulation resistance of R0, because the negative poles of DC1 and DC2 are connected together, the two negative poles share one ground insulation resistance.
For example, the primary side conversion circuit includes an inverted H-bridge circuit, and the secondary side conversion circuit includes a bidirectional switching leg and a capacitive leg connected in parallel.
The transformer comprises two primary windings and two secondary windings, wherein each primary winding is connected with the output end of a corresponding inversion H-bridge circuit, the output ends of the secondary windings are connected in parallel and are connected with the input end of a secondary bridge arm circuit, namely, the positive pole of each secondary winding is connected with the midpoint of a bidirectional switch bridge arm, and the negative pole of each secondary winding is connected with the midpoint of a capacitor bridge arm.
In this embodiment, a first end of the bridge resistor is connected to a dc negative electrode of the H-bridge inverter circuit, and a second end of the bridge resistor is connected to a midpoint of a capacitor bridge arm of the transformer.
The capacitor bridge arm comprises two capacitors connected in series; as shown, the first capacitor C1 and the second capacitor C2 are connected in series.
The controller is specifically used for changing the voltage of at least one capacitor on the capacitor bridge arm to change the voltage across the resistor, for example, changing the voltage across the second capacitor C2 to change the voltage across Rm;
or the voltage across the two ends of the bridging resistor is changed by changing the switching state of the switching tube of the bidirectional switching bridge arm. The bidirectional switch bridge arm comprises an upper bridge arm and a lower bridge arm, wherein the upper bridge arm comprises two switch tubes S1 and S2 which are connected in series to form a bidirectional switch; the lower bridge arm comprises two switching tubes S3 and S4 which are connected in series to form a bidirectional switch.
The positive output end of the isolation converter is connected with the L line through a switch K1, and the negative output end of the isolation converter is connected with the N line through a switch K2.
In the isolation converter provided by this embodiment, the second capacitor C2 may be used as a secondary dc bus capacitor, and the isolation converter is controlled to generate a dc voltage on the second capacitor C2, or an ac voltage with a dc bias, so as to change the voltage across the across resistor Rm by changing the voltage on the second capacitor C2. Meanwhile, the states of the switches S1 to S4 and K1 to K2 may be changed, i.e., the dc side-to-ground insulation resistance value may be calculated according to the principle described in the embodiment corresponding to fig. 2, which is not described herein again.
In practice, the bridge resistor Rm may also utilize the existing sampling resistor in the circuit. For example, the GND of the control circuit is connected to the negative input terminal of the primary conversion circuit, and the bridge resistor Rm may be configured by a voltage sampling resistor on the secondary side. It should be understood that the input negative of the primary side conversion circuit is a dc negative.
In addition, when the ground of the control circuit where the controller is located is connected with the input negative electrode of the secondary side conversion circuit, the bridging resistor is a voltage sampling resistor of the primary side conversion circuit.
In practice, the voltage sampling resistor present in the isolated converter may be used as a bridge resistor. The ground of a control circuit where the controller is located is connected with the direct current negative electrode, and the bridging resistor is a voltage sampling resistor of the secondary side conversion circuit.
Generally, voltage sampling is resistance differential sampling, and at least comprises two strings of resistors. Taking fig. 4 as an example, the voltage sampling circuit of the second capacitor C2 is shown in fig. 5, and includes four resistors Rs1 to Rs4 and an operational amplifier, wherein each resistor of Rs1 to Rs4 may be formed by one resistor or a plurality of resistors connected in series, and the signal ground SGND of the sampling circuit is connected to the dc negative electrode.
Since each pin of the operational amplifier is a low voltage circuit, which can be considered as an equal potential, and is equal in potential to the signal ground SGND, the voltage sampling circuit of the capacitor C2 can be equivalent to two resistors Rm1 and Rm2.
In the resistance differential sampling circuit, the resistance values of the sampling resistor at the positive end and the sampling resistor at the negative end are usually selected to be consistent, so that Rm1 is approximately equal to Rm2.
As shown in fig. 6, the bridge resistor Rm, which is equivalent to the voltage sampling circuit using the second capacitor C2, actually includes the following two resistors: rm1 and Rm2.
The resistance R0 of the direct current negative pole to the ground and the resistance R1/R2 of the positive pole to the ground are as follows:
from the equation (19), the DC insulation resistance to ground R0// R1// R2 can be obtained.
Rm1 ≈ Rm2, so equation (19) can also be simplified as:
for the circuit shown in fig. 6, in the first state, K2 is closed, and the dc voltage Vc1 is formed on C2, then
In the second state, K2 is closed, and the DC voltage Vc2 is formed on C2 by control, then
Where Vc1 and Vc2 are the output voltage of the isolation converter at C2, and can be sampled, and V0' are sampled, so after the equation (19) is carried out, the dc-side negative-to-ground insulation resistance R0 and the negative-to-ground insulation resistance R1// R2, and the dc-side negative-to-ground insulation resistance R0// R1// R2 can be calculated.
Describing the case where the isolation converter includes a plurality of bridge resistors, for a scenario including more bridge resistors, for example, there are other sampling resistors (e.g., ac port differential resistor sampling, other capacitance voltage differential resistor sampling, etc.), the formula may be derived according to rules.
For example, a bridge resistor string including N primary and secondary sides, the resistance R0 of the dc negative electrode to ground and the resistance R1// R2 of the positive electrode to ground are calculated as:
to sum up, when the isolation converter provided in the embodiment of the present application obtains the dc side insulation impedance to ground, the shunt resistor may be connected between the primary side conversion circuit and the secondary side conversion circuit, the number of the shunt resistors is not limited in the present application, and may be one or multiple, and the insulation impedance may be derived by changing the voltage at two ends of the shunt resistor and the voltage at one end of the shunt resistor to ground.
The isolation converters provided in the above embodiments are described by taking a plurality of transformers as an example, and may further include a transformer, where the transformer includes a plurality of primary sides and a plurality of secondary sides, the plurality of primary sides correspond to the plurality of primary side conversion circuits one to one, each primary side conversion circuit is connected to a corresponding primary side, and all the secondary sides are directly connected in parallel or coupled together through a power semiconductor device.
Based on the isolated converter provided by the above embodiments, the embodiments of the present application further provide a photovoltaic system, and the following description refers to the accompanying drawings for detailed description.
Referring to fig. 8, the figure is a schematic view of a photovoltaic system provided in an embodiment of the present application.
The photovoltaic system provided by this embodiment includes the isolated converter 1000 described in the above embodiment, and further includes: a photovoltaic module PV.
At least one primary side conversion circuit 100;
the input end of each primary side conversion circuit 100 is connected with at least one photovoltaic module PV.
According to the system provided by the embodiment of the application, the insulation resistance of the direct current side of the isolation converter can be detected only by adding one bridging resistor in the isolation converter, so that when the insulation resistance is small, the insulation fault of the direct current side is judged to be timely taken. Because only one bridging resistor is added, compared with a resistor bridge method in the prior art, the number of hardware devices is reduced, the size of the circuit can be reduced, and the cost is reduced.
Based on the isolation converter and the photovoltaic system provided by the above embodiments, the embodiments of the present application further provide an insulation impedance detection method for the isolation converter, and the following description is provided with the accompanying drawings for detailed description.
Referring to fig. 9, the figure is a flowchart of an insulation resistance detection method of an isolation converter according to an embodiment of the present application.
In the method for detecting an insulation resistance of an isolation transformer provided in this embodiment, the isolation transformer includes: the transformer comprises a bridging resistor, a primary side conversion circuit, a transformer and a secondary side conversion circuit; the input end of the primary side conversion circuit is used for inputting direct current, the output end of the primary side conversion circuit is connected with the primary side of the transformer, the secondary side of the transformer is connected with the input end of the secondary side conversion circuit, and the secondary side conversion circuit is used for outputting alternating current; the first end and the second end of the bridging resistor are respectively connected with the primary side conversion circuit and the secondary side conversion circuit;
the method comprises the following steps:
s901: changing the voltage across the cross-over resistor;
s902: and obtaining the insulation impedance of the isolation converter according to the voltage of the two ends of the front and rear bridging resistors, the voltage of the first ends of the front and rear bridging resistors to the ground and the resistance value of the bridging resistor.
Namely, the insulation resistance to the ground is obtained by five values of the voltage at the two ends of Rm before the voltage is changed, the voltage at the two ends of Rm after the voltage is changed, the voltage of the first end of Rm to the ground before the voltage is changed, the voltage of the first end of Rm to the ground after the voltage is changed, and the resistance value of Rm.
It should be understood that the insulation impedance is an equivalent integrated impedance, and is not an impedance to ground of a certain end, for example, an equivalent value of a positive electrode to ground impedance and a negative electrode to ground impedance of the primary side conversion circuit, and when the input end of the primary side conversion circuit includes a plurality of input ends, an equivalent impedance to ground of a plurality of direct current input ends.
The method provided by the embodiment of the application can accurately detect the insulation impedance of the direct current side, and the insulation impedance of the direct current side of the isolation converter can be detected only by adding one jumper resistor, so that when the insulation impedance is small, the insulation fault of the direct current side is judged and measures are taken in time. Because only one bridging resistor is added, compared with a resistor bridge method in the prior art, the number of hardware devices is reduced, the size of the circuit can be reduced, and the cost is reduced.
It should be noted that, in the present specification, each embodiment is described in a progressive manner, and each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments may be referred to each other. For the system or the device disclosed by the embodiment, the description is simple because the system or the device corresponds to the method disclosed by the embodiment, and the relevant points can be referred to the method part for description.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (17)
1. An isolated converter, comprising: the device comprises a controller, a bridging resistor, a primary side conversion circuit, a transformer and a secondary side conversion circuit;
the input end of the primary side conversion circuit is used for inputting direct current, the output end of the primary side conversion circuit is connected with the primary side of the transformer, the secondary side of the transformer is connected with the input end of the secondary side conversion circuit, and the secondary side conversion circuit is used for outputting alternating current;
the first end of the bridging resistor is connected with the primary side conversion circuit, and the second end of the bridging resistor is connected with the secondary side conversion circuit;
and the controller is used for changing the voltage at the two ends of the bridging resistor, and obtaining the insulation impedance of the isolation converter according to the voltages at the two ends of the bridging resistor before and after changing, the voltage of the first end of the bridging resistor to the ground before and after changing and the resistance value of the bridging resistor.
2. The isolated converter of claim 1, wherein the first end of the shunt resistor is connected to the negative or positive terminal of the primary side conversion circuit.
3. The isolated converter according to claim 1 or 2, wherein the second end of the bridge resistor is connected to the positive or negative pole of the secondary side conversion circuit.
4. An isolated converter according to any of claims 1-3, wherein the secondary side conversion circuit comprises: an inverter circuit;
the controller is specifically configured to change a secondary side voltage of the transformer to change a voltage across the cross-over resistor.
5. An isolated converter according to any of claims 1-3, wherein the secondary side conversion circuit comprises: an inverter circuit;
the controller is specifically configured to change a switching state of a switching tube in the inverter circuit to change a voltage across the bridge resistor.
6. The isolated converter of claim 4 or 5, wherein the inverter circuit comprises: the first switching tube, the second switching tube, the third switching tube and the fourth switching tube;
the first switching tube and the second switching tube are connected in series to form a first bridge arm, and the third switching tube and the fourth switching tube are connected in series to form a second bridge arm;
the middle point of the first bridge arm is connected with a live wire through a first switch, and the middle point of the second bridge arm is connected with a zero line through a second switch.
7. An isolated converter according to any of claims 4-6, wherein the first terminal of the across resistor is connected to the negative terminal of the primary side conversion circuit, and the voltage across the first terminal of the across resistor to ground is sampled before and after the change at the same instant as the instantaneous value of the output voltage of the inverter circuit.
8. The isolated converter according to any one of claims 4 to 6, wherein the first end of the bridging resistor is connected to the negative electrode of the primary side conversion circuit, and the voltage across the first end of the bridging resistor to the ground is superposed with a preset alternating voltage on the output voltage of the inverter circuit before and after the change, wherein the preset alternating voltage has the same amplitude as the grid voltage but is opposite in direction.
9. An isolated converter according to any of claims 1-8, wherein the primary side conversion circuit is a forward circuit, a flyback circuit, a push-pull circuit or a half-bridge circuit.
10. An isolated converter according to any of claims 1-3, characterized in that the primary conversion circuit comprises an inverted H-bridge circuit and the secondary conversion circuit comprises a bidirectional switching leg and a capacitive leg connected in parallel.
11. The isolated converter of claim 10, wherein a first end of the cross-over resistor is connected to a dc negative electrode or a dc positive electrode of the H-bridge inverter circuit, and a second end of the cross-over resistor is connected to a midpoint, a positive electrode or a negative electrode of a capacitor leg of the transformer.
12. The isolated converter of claim 11, wherein the capacitive leg comprises two capacitors connected together in series;
the controller is specifically configured to change a voltage of at least one capacitor of the capacitor bridge arm to change a voltage across the bridge resistor, or change a switching state of a switching tube of the bidirectional switch bridge arm to change a voltage across the bridge resistor.
13. The isolated converter according to any one of claims 10-12, wherein the ground of the control circuit in which the controller is located is connected with the negative input terminal of the primary side conversion circuit, and the across resistor is a voltage sampling resistor of the secondary side conversion circuit;
or the like, or, alternatively,
the ground of a control circuit where the controller is located is connected with the input negative electrode of the secondary side conversion circuit, and the bridging resistor is a voltage sampling resistor of the primary side conversion circuit.
14. An isolation converter as claimed in any one of claims 1 to 13, wherein said primary side conversion circuit comprises a plurality of said primary side conversion circuits, the cathodes of said plurality of said primary side conversion circuits being connected together; the number of the transformers is multiple, and each transformer comprises a primary side and a secondary side;
the plurality of primary side conversion circuits are in one-to-one correspondence with the plurality of transformers, and each primary side conversion circuit is connected with a corresponding primary side;
all the secondary sides are coupled together directly or via power semiconductor devices;
or the like, or, alternatively,
the transformer comprises a plurality of primary sides and a plurality of secondary sides, and the primary sides share a magnetic core; each primary side conversion circuit is connected with a corresponding primary side, and all secondary sides are directly coupled together or coupled together through a power semiconductor device.
15. An isolated converter according to any of claims 1-14, wherein the secondary side conversion circuit is connected to ground through a protection zero system.
16. A photovoltaic system comprising the isolated converter of any of claims 1-15, further comprising: a photovoltaic module;
at least one primary side conversion circuit is arranged;
the input end of each primary side conversion circuit is connected with at least one photovoltaic module.
17. A method of detecting an insulation resistance of an isolation transformer, the isolation transformer comprising: the transformer comprises a bridging resistor, a primary side conversion circuit, a transformer and a secondary side conversion circuit; the input end of the primary side conversion circuit is used for inputting direct current, the output end of the primary side conversion circuit is connected with the primary side of the transformer, the secondary side of the transformer is connected with the input end of the secondary side conversion circuit, and the secondary side conversion circuit is used for outputting alternating current; the first end and the second end of the bridging resistor are respectively connected with the primary side conversion circuit and the secondary side conversion circuit;
the method comprises the following steps:
varying the voltage across the cross-over resistance;
and obtaining the insulation impedance of the isolation converter according to the voltages at the two ends of the bridging resistor before and after changing, the voltages of the first ends of the bridging resistor to the ground before and after changing and the resistance value of the bridging resistor.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117872194A (en) * | 2024-03-11 | 2024-04-12 | 西安奇点能源股份有限公司 | Detection method for insulation resistance, fault battery pack and short-circuit battery pack in energy storage system based on H bridge |
| CN117872194B (en) * | 2024-03-11 | 2024-05-14 | 西安奇点能源股份有限公司 | Detection method for insulation resistance, fault battery pack and short-circuit battery pack in energy storage system based on H bridge |
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2022
- 2022-11-18 CN CN202211450614.0A patent/CN115912930A/en active Pending
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117872194A (en) * | 2024-03-11 | 2024-04-12 | 西安奇点能源股份有限公司 | Detection method for insulation resistance, fault battery pack and short-circuit battery pack in energy storage system based on H bridge |
| CN117872194B (en) * | 2024-03-11 | 2024-05-14 | 西安奇点能源股份有限公司 | Detection method for insulation resistance, fault battery pack and short-circuit battery pack in energy storage system based on H bridge |
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