CN106992535B - Constant current pre-charging method for high-voltage direct-current bus capacitor of electric energy router - Google Patents

Constant current pre-charging method for high-voltage direct-current bus capacitor of electric energy router Download PDF

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CN106992535B
CN106992535B CN201710248565.5A CN201710248565A CN106992535B CN 106992535 B CN106992535 B CN 106992535B CN 201710248565 A CN201710248565 A CN 201710248565A CN 106992535 B CN106992535 B CN 106992535B
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converter
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CN106992535A (en
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谷庆
袁立强
孙建宁
赵争鸣
李婧
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Tsinghua University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses a constant current pre-charging method for a high-voltage direct current bus capacitor of an electric energy router. According to the invention, the use of a charging resistor and a circuit breaker with high voltage grade can be avoided, the cost of the high-voltage direct-current bus capacitor pre-charging circuit is reduced, and the safety and reliability of the pre-charging circuit are improved; the problem of uncontrollable precharge current among the prior art is solved for the speed of precharging.

Description

Constant current pre-charging method for high-voltage direct-current bus capacitor of electric energy router
Technical Field
The invention belongs to the field of power electronic technology application, and particularly relates to a constant current pre-charging method for a high-voltage direct-current bus capacitor of an electric energy router.
Background
In order to deal with energy crisis and environmental problems and restrain global climate change, governments around the world are actively exploring new energy power generation and distributed power supply systems. Except for a few distributed power systems that can be directly grid-connected or supplied to a user, most others require access to a traditional power grid through power electronic converters. The electric energy router has functions of electrical isolation, voltage conversion, power bidirectional transmission and the like, and can well meet the requirements, so that the electric energy router is researched more and more widely in recent years. The power electronic transformer based on the cascade H-bridge structure is a key component of an electric energy router, and generally adopts a three-stage structure, namely an input rectifier stage, an intermediate isolation stage and an output inverter stage. With the intermediate isolation stage as a boundary, the power router is generally divided into two parts, a high-voltage side and a low-voltage side, with the part of the three-phase high-voltage ac port having electrical connections being the high-voltage side and the part of the three-phase low-voltage ac port having electrical connections being the low-voltage side. Typically, the intermediate isolation stage is a high frequency isolation DC/DC converter, including a high side high frequency DC/AC converter, a high frequency isolation transformer, and a low side high frequency AC/DC converter. The capacitor connected to the DC port of the high-side high-frequency DC/AC converter is generally referred to as the high-voltage bus capacitor, and the capacitor connected to the DC port of the low-side high-frequency AC/DC converter is generally referred to as the low-voltage bus capacitor. Before the electric energy router normally operates, if the high-voltage bus capacitor and the low-voltage bus capacitor of the electric energy router are not pre-charged, but are directly connected to a power grid, overlarge dv/dt can be generated in a very short time, so that the impact current flowing through the capacitor is overlarge, and the capacitor and the whole circuit system are damaged. Therefore, the research on the pre-charging of the linear bus capacitor of the electric energy router is of great significance.
In the existing pre-charging scheme of the direct-current bus capacitor of the electric energy router, the high-voltage direct-current bus capacitor is charged by adding a pre-charging circuit consisting of a charging resistor with a high voltage grade and a breaker outside the high-voltage side of the electric energy router. The charging resistor and the circuit breaker with high voltage level are large in size and expensive, so that the improvement of the power density and the reduction of the cost of the whole electric energy router system are not facilitated, and meanwhile, the danger in the charging process is increased due to the pre-charging operation of the high-voltage side. In addition, in addition to the need of pre-charging the high-voltage bus capacitor, the low-voltage bus capacitor also needs to be pre-charged in order to prevent the low-voltage bus capacitor from generating excessive current surge and voltage oscillation, so that a set of pre-charging circuit needs to be additionally added on the low-voltage side, and the volume and cost of the system are further increased. In order to ensure that the current value of the high-voltage direct-current bus capacitor in the charging process is not too large, the low-voltage side inward shift ratio of the high-frequency isolation DC/DC converter is controlled to be slowly reduced from 1, so that the charging voltage applied to the high-voltage direct-current bus capacitor is slowly increased from 0. The method has the defects that the charging current is not controllable, the setting of the inward shift ratio is blindness, and the problems that the charging current exceeds a limit value, the charging current is too small, the charging time is too long and the like are caused. In order to ensure constant charging current, the current is often sampled and closed-loop controlled. Accurate sampling and operation of high frequency current puts high demands on the current sensor and the processor, increasing the complexity of the pre-charging scheme.
Disclosure of Invention
The invention aims to provide a constant current pre-charging method for a high-voltage direct-current bus capacitor of an electric energy router, which avoids using a charging resistor and a circuit breaker with high voltage grade, reduces the cost of a pre-charging circuit and improves the safety and reliability of the pre-charging circuit; the problem of uncontrollable precharge current among the prior art is solved for the speed of precharging.
Therefore, the invention provides a constant current pre-charging method for a high-voltage direct-current bus capacitor of an electric energy router, which comprises the following steps:
1) charging a low-voltage direct-current bus capacitor by using a low-voltage side auxiliary power supply;
2) setting a current value in the charging process of the high-voltage direct-current bus capacitor;
3) calculating the internal shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter in different charging periods according to the set charging current value;
4) and applying a driving signal to a low-voltage side switching tube of the high-frequency isolation DC/DC converter according to the calculated internal shift ratio change rule, keeping the high-voltage side switching tube in a locked state in the process, and charging a high-voltage direct current bus capacitor.
Further, the electric power router has a three-stage structure, i.e., an input AC/DC rectifier, an intermediate high-frequency isolation DC/DC converter, and an output DC/AC inverter; the electric energy router is divided into a high-voltage side and a low-voltage side by taking the high-frequency isolation DC/DC converter as a boundary, wherein the part electrically connected with the three-phase high-voltage alternating current port is the high-voltage side, and the part electrically connected with the three-phase low-voltage alternating current port is the low-voltage side; the high-frequency isolation DC/DC converter consists of a high-frequency DC/AC converter, a high-frequency isolation transformer and a high-frequency AC/DC converter; the high-voltage direct current bus capacitor is a high-voltage side bus capacitor of the high-frequency isolation DC/DC converter; and the low-voltage direct-current bus capacitor is a low-voltage side bus capacitor of the high-frequency isolation DC/DC converter.
Further, the current value in the charging process of the high-voltage direct current bus capacitor in the step 2) refers to a current peak value flowing through a primary winding of the high-frequency isolation transformer; the inward shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter in the step 3) refers to the ratio of the phase difference between two bridge arm driving signals of the low-voltage side high-frequency AC/DC converter to a half period of the driving signal.
Furthermore, the charging process of the high-voltage direct-current bus capacitor is divided into two stages, wherein the first stage is the pre-charging of the low-voltage direct-current bus capacitor, and the second stage is the pre-charging of the high-voltage direct-current bus capacitor; the first phase is a necessary condition of the second phase, namely, the pre-charging of the high-voltage direct-current bus capacitor is carried out after the pre-charging of the low-voltage direct-current bus capacitor is completed. The pre-charging does not need a charging resistor and a circuit breaker with high voltage level, and only needs a charging resistor, a circuit breaker and an auxiliary power supply with low voltage level.
Further, after the first pre-charging stage is completed, a driving signal with a certain inward shift ratio change rule is applied to a switching tube of a high-frequency AC/DC converter at the low-voltage side of the high-frequency isolation DC/DC converter, and meanwhile, the switching tube of the high-frequency DC/AC converter at the high-voltage side of the high-frequency isolation DC/DC converter is kept in a locked state, so that the energy of the low-voltage side auxiliary power supply is transmitted to the high-voltage side through the high-frequency AC/DC converter, the high-frequency isolation transformer and the high-frequency DC/AC converter, and the purpose of charging the high-voltage direct current bus capacitor.
Further, the change rule of the internal shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter in the step 4) is calculated according to the following method: and calculating the inward shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter required in different charging periods according to the constraint condition that the charging current is equal to the preset current value by deducing the mathematical relationship between the peak value which can be reached by the current of the primary winding of the transformer in each charging period of the second pre-charging stage and the inward shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter.
The invention provides a constant current pre-charging method for a high-voltage direct current bus capacitor of an electric energy router, which has the characteristics and advantages that:
1. the constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router comprises the two stages of pre-charging the low-voltage direct current bus capacitor and pre-charging the high-voltage direct current bus capacitor, the low-voltage direct current bus capacitor is already charged before the high-voltage direct current bus capacitor is pre-charged, and the low-voltage direct current bus capacitor does not need to be additionally pre-charged.
2. The constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router, provided by the invention, has the advantages that the charging resistor and the circuit breaker with high voltage levels are not needed in circuit realization, and only the charging resistor, the circuit breaker and the auxiliary power supply with low voltage levels are needed, so that the cost of the high-voltage direct current bus capacitor pre-charging circuit is reduced, and the safety and the reliability of the pre-charging circuit are improved.
3. According to the constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router, in the charging process of the high-voltage direct current bus capacitor, the current passing through the primary winding of the high-frequency transformer can be controlled to be constant through reasonably setting the inward shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter, compared with a traditional current uncontrollable pre-charging scheme, the constant current pre-charging method simultaneously meets two requirements of charging current and charging speed, and on the premise that the charging current cannot be too large, the charging speed is accelerated as much as possible.
4. The constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router is realized based on the mathematical relationship between the peak value which can be reached by the primary winding current of the transformer and the internal shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter in the charging process of the high-voltage direct current bus capacitor, and is simple and reliable and independent of a high-bandwidth sensor and a high-bandwidth processor.
Drawings
Fig. 1 is a schematic diagram of an electric energy router topology and a constant current pre-charging method of a high voltage dc bus capacitor according to an embodiment of the present invention;
FIG. 2 is a flow chart of the constant current pre-charging of the high voltage DC bus capacitor of the electric energy router according to the embodiment of the present invention;
FIG. 3 is a topology diagram of another power conversion sub-module that may be used in embodiments of the present invention;
FIG. 4 is an equivalent circuit diagram of the first stage of precharging in accordance with the embodiment of the present invention;
FIG. 5 is a schematic diagram illustrating a second stage of pre-charging according to an embodiment of the present invention;
FIG. 6 is a waveform diagram illustrating the second stage of pre-charging operation according to an embodiment of the present invention;
FIG. 7 is a block diagram illustrating the calculation of the low-voltage side step-in ratio of the high-frequency isolated DC/DC converter according to the embodiment of the present invention;
FIG. 8 is a simulation waveform diagram of the method of the present invention applied to the constant current pre-charging of the high voltage DC bus capacitor of the 10kV electric energy router.
Detailed Description
The technical scheme of the invention is clearly and completely described below with reference to the accompanying drawings.
The invention provides a constant current pre-charging method for a high-voltage direct current bus capacitor of an electric energy router, which is combined with a figure 1 and a figure 2 and comprises the following steps:
s1, charging of a low-voltage direct-current bus capacitor. Specifically, according to the limiting conditions of factors such as charging current flowing through all capacitors, power loss on a current limiting resistor, charging time and the like in the charging process of the low-voltage direct-current bus capacitor, the current limiting resistor with a proper resistance value is selected and a low-voltage side pre-charging circuit is constructed. The low-voltage side pre-charging circuit consists of an auxiliary power supply, a circuit breaker, a current-limiting resistor and bypass switches connected in parallel at two ends of the current-limiting resistor; the auxiliary power supply is a 220V alternating current power supply or a direct current power supply in the forms of photovoltaic power, storage batteries and the like, and the output of the auxiliary power supply is adjusted to the voltage set value of the low-voltage direct current bus through a three-phase PWM rectification system or a DC/DC converter. When the pre-charging is started, the low-voltage side circuit breaker is switched on, and the auxiliary power supply starts to charge all capacitors connected to the low-voltage direct-current bus through the current-limiting resistor; when the voltage on the low-voltage direct-current bus capacitor reaches a set value, the bypass switches connected in parallel at two ends of the current-limiting resistor are closed, and the current-limiting resistor is cut off from the circuit. And then, charging the low-voltage direct-current bus capacitor.
And S2, setting a current value in the charging process of the high-voltage direct-current bus capacitor.
And S3, calculating the internal shift ratio between two bridge arms of the low-voltage side AC/DC converter of the high-frequency isolation DC/DC converter. Specifically, after the low-voltage direct-current bus capacitor is charged, a certain driving signal is applied to the low-voltage side AC/DC converter of the high-frequency isolation DC/DC converter, so that the low-voltage direct-current bus voltage is induced to the primary side through the high-frequency isolation transformer, and the high-voltage direct-current bus capacitor is charged through the leakage inductance or the external inductance of the transformer. According to the relation between the maximum value of the current flowing through the high-voltage direct-current bus capacitor or the primary winding of the transformer in each period and the internal shift ratio of the low-voltage side AC/DC converter in the charging process, the internal shift ratio of the low-voltage side AC/DC converter in each charging period is calculated according to the charging current value set in S2.
And S4, charging the high-voltage direct-current bus capacitor. Specifically, when the first stage of pre-charging, namely the charging of the low-voltage direct-current bus capacitor is completed, the charging of the second stage, namely the charging of the high-voltage direct-current bus capacitor, is started. And according to the internal shift ratio change rule obtained by the calculation of S3, applying a driving signal to a switching tube of a low-voltage side AC/DC converter of the high-frequency isolation DC/DC converter, and keeping the switching tube of the rectification stage and the switching tube of the high-voltage side DC/AC converter of the high-frequency isolation DC/DC converter in a locked state in the process, so that the energy of the auxiliary power supply is transmitted to the high-voltage direct-current bus capacitor through the low-voltage side AC/DC converter, the high-frequency isolation transformer and the high-voltage side DC/AC converter to charge the high-voltage direct-current bus. And when the voltage on the high-voltage direct-current bus capacitor reaches a set value, the low-voltage side circuit breaker is disconnected, and the auxiliary power supply is cut off from the circuit. And then, charging the high-voltage direct-current bus capacitor.
In the constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router, the electric energy router has a three-stage structure, namely an input AC/DC rectifier, a middle high-frequency isolation DC/DC converter and an output DC/AC inverter. As shown in fig. 1, the output DC/AC inverter adopts a three-phase four-leg structure; the input AC/DC rectifier and the intermediate high-frequency isolation DC/DC converter form a power conversion submodule, the input end of the power conversion submodule is connected with a three-phase high-voltage alternating-current power grid, and the output end of the power conversion submodule is connected with an output DC/AC inverter through a low-voltage direct-current bus. The constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router provided by the embodiment of the invention can be used for carrying out constant current charging on the high-voltage side bus capacitor of the high-frequency isolation DC/DC converter. The high-frequency isolation DC/DC converter may adopt, but is not limited to, a structure shown in fig. 1 in which a high-voltage side is a diode-clamped three-level structure and a low-voltage side is a two-level full bridge, and the input AC/DC rectifier may adopt, but is not limited to, a diode-clamped three-level structure shown in fig. 1.
Fig. 3 shows another power conversion submodule structure to which the constant current pre-charging method for the high voltage dc bus capacitor of the electric energy router according to the embodiment of the present invention can be applied. The input AC/DC rectifier is a two-level full-bridge structure, the high-voltage side of the middle high-frequency isolation DC/DC converter is a two-level full-bridge structure, and the low-voltage side of the middle high-frequency isolation DC/DC converter is a diode clamping three-level structure. In fact, the constant current pre-charging method for the high-voltage direct current bus capacitor of the electric energy router can be applied to, but is not limited to, the electric energy router with the power conversion sub-modules shown in fig. 1 or fig. 3. For an input AC/DC rectifier of a power conversion submodule, a high-voltage side DC/AC converter and a low-voltage side AC/DC converter of a middle high-frequency isolation DC/DC converter, the input AC/DC rectifier, the high-voltage side DC/AC converter and the low-voltage side AC/DC converter adopt any one combined structure consisting of a two-level full-bridge structure and a diode clamping three-level structure, and the high-voltage DC bus capacitor can be subjected to constant-current pre-charging by adopting the pre-charging method of the embodiment of the invention.
Embodiments according to the present invention are further described in detail below using an actual power router circuit as an example.
The power router has a structure as shown in fig. 1. The power conversion submodule adopts the structure shown in FIG. 1, the rectification stage is a diode clamping three-level structure, the high-voltage side of the high-frequency isolation DC/DC converter is a diode clamping three-level structure, and the low-voltage side of the high-frequency isolation DC/DC converter is a two-level full-bridge structure; each phase is connected to a three-phase alternating current power grid in a mode of inputting, connecting in series and outputting in parallel by N (N is 6) power conversion sub-modules with the same structure and parameters, a star connection method is adopted among three phases, and the effective line voltage value of the three-phase power grid is 10 kV; the high-voltage direct-current bus voltage of the high-frequency isolation DC/DC converter is 1600V, and the low-voltage direct-current bus voltage is 700V; the primary and secondary side transformation ratio of the high-frequency isolation transformer is 8: 7; the high-voltage direct-current bus capacitor is formed by connecting two equivalent capacitors in series, the middle point of the capacitor is connected with one winding of the transformer, and the two capacitors have the same capacitance value; the low-voltage direct-current bus capacitor consists of an equivalent capacitor; the output inverter stage is of a three-phase four-leg structure, a direct current bus of the output inverter stage is directly connected with a low-voltage direct current bus of the power conversion submodule, the voltage level is 700V, a bus capacitor consists of an equivalent capacitor, and the output end of the output inverter stage is a 380V three-phase alternating current port.
First, precharge first stage-charging of low-voltage DC bus capacitor
The first stage of the pre-charging process is to charge a capacitor connected to the low voltage dc bus from the auxiliary power supply. All the switching tubes of the input rectifier, the intermediate high-frequency isolation DC/DC converter and the output three-phase four-leg inverter are in a locked state, so that an equivalent circuit at the stage can be regarded as that 700V direct-current voltage obtained by modulation of an auxiliary power supply directly charges a capacitor on a low-voltage direct-current bus, and the equivalent circuit is shown in fig. 4. Wherein, the low-voltage DC bus capacitor CDCLThe sum of the low-voltage bus capacitors of all the power conversion sub-modules and the bus capacitors of the output inverter is obtained.
The current limiting resistor of fig. 4 is selected to take into account the following factors, i.e., the charging current limit through the capacitor, the power loss limit across the current limiting resistor, and the charging time limit, as will be described separately below:
first consider the charging current limit through the capacitor. Recording the sum of the low-voltage bus capacitors of all the power conversion sub-modules as CL_totalThe sum of the bus capacitors of the inverter is Cinv_total(ii) a The maximum current allowed to flow through the low-voltage direct-current bus capacitor of the power conversion submodule and the direct-current bus capacitor of the three-phase four-bridge-arm inverter is Imax_SM，Imax_inv. Considering that the three-phase four-leg inverter is not necessarily connected to the main circuit when pre-charging is carried out, the three-phase four-leg inverter has
Figure BDA0001271287790000071
Figure BDA0001271287790000072
In the formula of UDCLObtained by rectifying or chopping the low-voltage side auxiliary power supplyThe low voltage bus voltage value of (2); rchargeIs a low-side current limiting resistor.
Second, the power loss limit on the current limiting resistor is considered. As shown in fig. 4, let the charging time constant be τ ═ RchargeCDCLThe capacitor voltage during charging can be expressed as
Uc(t)＝UDCL(1-e-t/τ) (3)
During the precharge process, any time period [ t ]1,t2]The average power dissipated by the current limiting resistor is
Figure BDA0001271287790000073
Since the current during the pre-charging process is continuously reduced along with the progress of the charging process, the power loss on the current-limiting resistor is the largest in the initial stage of the pre-charging. The maximum average power allowed to be consumed on the current limiting resistor within 1 second of pre-charging is recorded as Pmax_resThen there is
Figure BDA0001271287790000081
Finally, the charging time limit is considered. Considering that the time of the pre-charging process cannot be too long, the charging time constant is also limited during circuit design. The maximum allowable time length of the first precharge stage is tmaxConsidering that the capacitor voltage can reach a stable value within a time of 3-5 tau, the voltage is determined by
5RchargeCDCL≤tmax(6)
In summary, the value range of the current limiting resistor in fig. 4 can be obtained by comprehensively considering the expressions (1), (2), (5), and (6).
Second and pre-charging second stage-charging of high-voltage direct-current bus capacitor
After the first-stage charging of the pre-charging is finished, the low-voltage direct-current bus capacitor voltage of all the power conversion sub-modules reaches the rated operation value, namely UDCL. At this time, the bypass switches at both ends of the low-voltage side current limiting resistor are closed to start prechargingAnd in the second stage, charging the high-voltage direct-current bus capacitor.
Fig. 5 is a diagram illustrating the operation of the second stage of precharging. Because each power conversion sub-module has the same structure and parameters, only one sub-module needs to be analyzed. In the second stage of pre-charging, the fluctuation of the low-voltage DC bus capacitor voltage in the charging process is ignored, the low-voltage DC bus capacitor can be approximately regarded as a voltage source, and the voltage value is UDCL. Submodule rectifier stage switching tube S11～S14、S21～S24High-voltage side switch tube S of high-frequency isolation DC/DC converter31～S34All are in a locked state, and only the anti-parallel diode is used as a current path; low-voltage side switch tube S of high-frequency isolation DC/DC converter41～S44The driving signals shown in fig. 6 are applied. The charging principle of the high voltage dc bus capacitor will be described in detail with reference to fig. 6.
As shown in fig. 6, a switch tube S is defined44The drive signal lags behind the switch tube S41The ratio of the phase of the driving signal to the half period is the internal shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter, which is recorded as D2。
At [ t ]2,t3]In time period, switch tube S41And S44And at the same time, conducting, the current path is as shown by line 1 in fig. 5. At this time, the terminal voltage u of the low-voltage side winding of the high-frequency isolation transformerLIs + UDCLAnd further, the terminal voltage u of the high-voltage side winding of the transformerHIs + nUDCLThen add to the leakage inductance LsVoltage u onLsIs nUDCL-UCH1. If at t2Before the moment, the current flowing through the inductor is reduced to 0, and then the inductor current linearly rises from 0 and becomes a high-voltage direct-current bus capacitor CH1And (6) charging.
At [ t ]3,t5]In time period, switch tube S42And S44And is simultaneously turned on. Low-voltage side end voltage u of transformer L0 and hence the high side end voltage u H0, adding to the leakage inductance LsVoltage u onLsis-UCH1. At this time, the inductor is electrically connectedThe flow will drop linearly from the maximum and continue to CH1Charging until t4At that moment, the energy stored in the inductor is totally converted into the capacitor CH1In the middle, the inductive current is reduced to 0, and then the inductive current is always maintained at 0 until the next time period because the current cannot reversely pass through the anti-parallel diode of the switching tube.
At [ t ]5,t6]In time period, switch tube S42And S43And at the same time, conducting, the current path is shown as line 2 in fig. 5. Similar to [ t ]2,t3]Analysis in time period, the system is a high-voltage direct-current bus capacitor CH2And (6) charging.
At [ t ]0,t2]In time period, switch tube S41And S43And is simultaneously turned on. Similar to [ t ]3,t5]Analysis over a period of time, at which the capacitance CH2The voltage on the leakage inductor is reversely applied to the leakage inductor, and the current flowing through the leakage inductor linearly decreases to 0.
When the second pre-charging stage is started, the low-voltage side inward shift ratio of the high-frequency isolation DC/DC converter is gradually reduced from 1, so that the effective value of the low-voltage side full-bridge output alternating voltage is gradually increased, and the purpose of smoothly charging the high-voltage direct-current bus capacitor is achieved. Because of two capacitors C of the high-voltage direct-current bus in the pre-charging processH1And CH2And the alternating charging is continuously carried out, so that the voltage balance of the two capacitors can be ensured in the charging process. And when the voltages of the two capacitors respectively reach half of the voltage of the high-voltage direct-current bus, the whole charging process is finished.
Third, calculation of low-voltage side internal shift phase ratio of high-frequency isolation DC/DC converter
The calculation of the internal shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter is carried out on the basis of ensuring that the current in the charging process of the high-voltage direct-current bus capacitor is kept unchanged. Because the switching frequency in the pre-charging process is very high, usually 20 to 50kHz, it can be considered that the voltage on the high-voltage direct-current bus capacitor is kept unchanged in one charging period and is marked as Uc. The capacitor C of the high-voltage direct-current bus is not used belowH1Half period of charging [ t ]2,t5]For example, pairThe calculation of the low-pressure side in-shift ratio is illustrated.
As shown in fig. 6, at t2,t3]During the time period, the inductive current rises from 0 to the maximum value
Figure BDA0001271287790000091
In the formula, n is the turn ratio of primary and secondary windings of the high-frequency transformer; l issThe leakage inductance is equivalent to the sum of the leakage inductance of the primary side and the external inductance attached to the primary side of the high-frequency transformer; f. ofsThe switching frequency of the DC/DC converter is isolated for high frequencies.
During this time period, the total energy transmitted from the secondary side to the primary side is
Figure BDA0001271287790000101
In the formula, C11Is CH1And CH2The capacity value of (c); u shapec0Is t2The capacitance voltage value at a moment; delta Uc0Is [ t ]2,t3]The amount of change in the capacitor voltage over the time period. Delta Uc0Can be obtained from the average current
Figure BDA0001271287790000102
Substituting (9) into (8) to obtain
Figure BDA0001271287790000103
At [ t ]3,t5]In the time period, the inductive current is reduced to 0, and all energy is charged into the capacitor CH1Above, then there are
Figure BDA0001271287790000104
In the formula, Δ UcIs [ t ]2,t5]The amount of change in the capacitor voltage over the time period.
Therefore, in each charging period, the voltage of each high-voltage direct-current bus capacitor changes
Figure BDA0001271287790000105
As shown in fig. 7, if the current limit value during the charging process of the high-voltage direct current bus capacitor is given, the inward shift ratio D between the two arms on the low-voltage side of the high-frequency isolation DC/DC converter in each charging cycle can be calculated according to equation (7)2And further obtaining a voltage change curve of the high-voltage direct-current bus capacitor according to the formula (12).
To illustrate the effectiveness of the present invention, FIG. 8 shows the low side shift-in ratio D during the pre-charging process of the high voltage DC bus capacitor2The voltage of the two high-voltage direct current bus capacitors and the simulation result of the current waveform flowing through the leakage inductance in the pre-charging process, wherein the current set value in the charging process of the high-voltage direct current bus capacitors is 10A. Simulation results show that in the charging process, the voltages of the two high-voltage direct-current bus capacitors can be kept balanced, and the current flowing through the leakage inductance can be basically kept at a set value, so that the effectiveness of the invention is proved.
Although the embodiments of the present invention have been described in connection with the accompanying drawings, it is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the present invention, such modifications and variations fall within the scope defined by the appended claims.

Claims (6)

1. A constant current pre-charging method for a high-voltage direct-current bus capacitor of an electric energy router is characterized by comprising the following steps:
1) charging a low-voltage direct-current bus capacitor by using a low-voltage side auxiliary power supply;
2) setting a current value in the charging process of the high-voltage direct-current bus capacitor;
3) calculating the internal shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter in different charging periods according to the set charging current value;
4) applying a certain driving signal to a low-voltage side switching tube of the high-frequency isolation DC/DC converter according to the calculated internal shift ratio change rule, keeping the high-voltage side switching tube in a locked state in the process, and charging a high-voltage direct current bus capacitor;
wherein the calculation of the step 3) with respect to the phase shift on the low pressure side is as follows:
in the high voltage DC bus capacitor CH1Half cycle of charging [ t2,t3]During the time period, the inductive current rises from 0 to the maximum value
In the formula, n is the turn ratio of primary and secondary windings of the high-frequency transformer; u shapeDCLThe low-voltage bus voltage value is obtained after rectification or chopping conversion is carried out on the low-voltage side auxiliary power supply; l issThe leakage inductance is equivalent to the sum of the leakage inductance of the primary side and the external inductance attached to the primary side of the high-frequency transformer; f. ofsThe switching frequency of the high-frequency isolation DC/DC converter;
during this time period, the total energy transmitted from the secondary side to the primary side is
Figure FDA0002279070900000012
In the formula, C11Is CH1And CH2The capacity value of (c); u shapec0Is t2The capacitance voltage value at a moment; delta Uc0Is [ t ]2,t3]The amount of change in the capacitor voltage over a period of time; delta Uc0Can be obtained from the average current
Figure FDA0002279070900000013
Substituting (3) into (2) to obtain
Figure FDA0002279070900000014
At [ t ] of said half period3,t5]In the time period, the inductive current is reduced to 0, and all energy is charged into the capacitor CH1Above, then there are
Figure FDA0002279070900000021
In the formula, Δ UcIs [ t ]2,t5]The amount of change in the capacitor voltage over a period of time;
therefore, in each charging period, the voltage of each high-voltage direct-current bus capacitor changes
Figure FDA0002279070900000022
Wherein D is2The method is characterized in that the internal shift phase ratio between two bridge arms on the low-voltage side of a high-frequency isolation DC/DC converter is obtained.
2. The constant current pre-charging method for the capacitor of the high-voltage direct current bus of the electric energy router according to claim 1, characterized in that: the electric energy router has a three-stage structure, namely, an input AC/DC rectifier, an intermediate high-frequency isolation DC/DC converter and an output DC/AC inverter; the electric energy router is divided into a high-voltage side and a low-voltage side by taking the high-frequency isolation DC/DC converter as a boundary, wherein the part electrically connected with the three-phase high-voltage alternating current port is the high-voltage side, and the part electrically connected with the three-phase low-voltage alternating current port is the low-voltage side; the high-frequency isolation DC/DC converter consists of a high-frequency DC/AC converter, a high-frequency isolation transformer and a high-frequency AC/DC converter; the high-voltage direct current bus capacitor is a high-voltage side bus capacitor of the high-frequency isolation DC/DC converter; and the low-voltage direct-current bus capacitor is a low-voltage side bus capacitor of the high-frequency isolation DC/DC converter.
3. The constant current pre-charging method for the capacitor of the high-voltage direct current bus of the electric energy router according to claim 1, characterized in that: the current value in the charging process of the high-voltage direct-current bus capacitor in the step 2) refers to the peak value of the current flowing through the primary winding of the high-frequency isolation transformer; the inward shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter in the step 3) refers to the ratio of the phase difference between two bridge arm driving signals of the low-voltage side high-frequency AC/DC converter to a half period of the driving signal.
4. The constant current pre-charging method for the capacitor of the high-voltage direct current bus of the electric energy router according to claim 1, characterized in that: the charging process of the high-voltage direct-current bus capacitor in the step 2) is divided into two stages, wherein the first stage is the pre-charging of the low-voltage direct-current bus capacitor, and the second stage is the pre-charging of the high-voltage direct-current bus capacitor; the first stage is a necessary condition of the second stage, namely, the pre-charging of the high-voltage direct-current bus capacitor is carried out only after the pre-charging of the low-voltage direct-current bus capacitor is completed.
5. The constant current pre-charging method for the capacitor of the high-voltage direct current bus of the electric energy router according to claim 4, characterized in that: after the first pre-charging stage is completed, a driving signal with a certain inward shift ratio change rule is applied to a switching tube of a high-frequency AC/DC converter at the low-voltage side of the high-frequency isolation DC/DC converter, and meanwhile, the switching tube of the high-frequency DC/AC converter at the high-voltage side of the high-frequency isolation DC/DC converter is kept in a locked state, so that the energy of the low-voltage side auxiliary power supply is transmitted to the high-voltage side through the high-frequency AC/DC converter, the high-frequency isolation transformer and the high-frequency DC/AC converter, and the high-voltage direct current bus capacitor is.
6. The constant-current pre-charging method for the capacitor of the high-voltage direct-current bus of the electric energy router according to claim 4, wherein the change rule of the inward shift ratio of the low-voltage side of the high-frequency isolation DC/DC converter in the step 4) is calculated according to the following method:
and calculating the internal shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter required in different charging periods according to the constraint condition that the charging current is equal to the preset current value by deducing the mathematical relationship between the peak value which can be reached by the current of the primary winding of the transformer in each charging period of the second pre-charging stage and the internal shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter.
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