CN106992535B - A constant current precharging method for high voltage DC bus capacitors of power routers - Google Patents

Abstract

The invention discloses a constant-current precharging method for the capacitor of a high-voltage DC bus of an energy router. The method firstly uses the auxiliary power supply on the low-voltage side to charge the capacitor of the low-voltage DC bus. The current value is calculated to compare the internal shift between the two bridge arms of the low-voltage side AC/DC converter corresponding to different charging times, so as to realize the constant current charging of the high-voltage DC bus capacitor. The invention can avoid the use of high-voltage level charging resistors and circuit breakers, reduce the cost of the high-voltage DC bus capacitor pre-charging circuit, improve the safety and reliability of the pre-charging circuit, and solve the problem of uncontrollable pre-charging current in the prior art. problem, speed up the precharge.

Description

A constant current precharging method for high voltage DC bus capacitors of power routers

technical field

The invention belongs to the application field of power electronics technology, and in particular relates to a constant current precharging method for a high voltage direct current bus capacitor of an electric energy router.

Background technique

In order to cope with the energy crisis and environmental problems and curb global climate change, governments around the world are actively exploring new energy power generation and distributed power systems. Except for a few distributed power systems that can be directly connected to the grid or supplied to users, most of them need to be connected to the traditional grid through power electronic converters. Power routers have functions such as electrical isolation, voltage conversion and bidirectional power transmission, which can well meet the above requirements, so they have been more and more widely studied in recent years. The power electronic transformer based on the cascaded H-bridge structure is a key component of the power router, and it generally adopts a three-stage structure, that is, an input rectification stage, an intermediate isolation stage and an output inverter stage. Taking the intermediate isolation level as the boundary, the power router is generally divided into two parts: the high-voltage side and the low-voltage side. The part that is electrically connected to the three-phase high-voltage AC port is the high-voltage side, and the part that is electrically connected to the three-phase low-voltage AC port is the low-voltage side. Usually, the intermediate isolation stage is a high-frequency isolation DC/DC converter, including a high-frequency side high-frequency DC/AC converter, a high-frequency isolation transformer, and a low-voltage side high-frequency AC/DC converter. The capacitor connected to the DC port of the high-frequency DC/AC converter on the high-voltage side is generally called a high-voltage bus capacitor, and the capacitor connected to the DC port of the high-frequency AC/DC converter on the low-voltage side is generally called a low-voltage bus capacitor. Before the power router operates normally, if it does not pre-charge its high-voltage busbar capacitors and low-voltage busbar capacitors, but directly connects them to the power grid, it may generate excessive dv/dt in a very short period of time, causing the capacitors to flow through. The inrush current is too large, which will cause harm to the capacitor and the entire circuit system. Therefore, it is of great significance to study the precharge of the linear bus capacitor of the power router.

In the existing power router DC bus capacitor precharging scheme, the charging of the high voltage DC bus capacitor is generally realized by adding a precharging circuit composed of a high voltage charging resistor and a circuit breaker on the high voltage side of the power router. High-voltage charging resistors and circuit breakers are bulky and expensive, which is not conducive to the improvement of power density and cost reduction of the entire power router system. At the same time, the pre-charging operation on the high-voltage side also increases the danger in the charging process. In addition, in addition to the pre-charging of the high-voltage bus capacitors, in order to prevent the low-voltage bus capacitors from generating excessive current surge and voltage oscillation, it is often necessary to pre-charge the low-voltage bus capacitors. , which further increases the size and cost of the system. In order to ensure that the current value during the charging process of the high-voltage DC bus capacitor will not be too large, generally, the inward shift of the low-voltage side of the high-frequency isolated DC/DC converter can be controlled to slowly decrease from 1, so that the capacitor added to the high-voltage DC bus is reduced. The charging voltage on the battery increases slowly from 0. The disadvantage of this method is that the charging current is uncontrollable, and the setting of the internal shift comparison is blind, which may cause the charging current to exceed the limit value, or cause the charging current to be too small and the charging time to be too long. In order to ensure the constant charging current, it is often necessary to sample the current and perform closed-loop control. Accurate sampling and calculation of high-frequency currents place higher requirements on current sensors and processors, increasing the complexity of the pre-charging scheme.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a constant current precharging method for the high voltage DC bus capacitor of the power router, so as to avoid the use of high voltage charging resistors and circuit breakers, reduce the cost of the precharging circuit, and improve the safety and reliability of the precharging circuit. It solves the problem of uncontrollable pre-charging current in the prior art, and speeds up the pre-charging speed.

To this end, the present invention proposes a constant current precharging method for the capacitor of the high-voltage DC bus of the power router, which includes the following steps:

1) Use the low-voltage side auxiliary power supply to charge the low-voltage DC bus capacitor;

2) Set the current value during the charging process of the high-voltage DC bus capacitor;

3) Calculate the internal shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter in different charging cycles according to the set charging current value;

4) According to the change rule of the internal shift phase obtained by calculation, apply a drive signal to the low-voltage side switch tube of the high-frequency isolated DC/DC converter, and keep the high-voltage side switch tube in a latched state during this process to charge the capacitor of the high-voltage DC bus. .

Further, the power router has a three-stage structure, that is, an input AC/DC rectifier, an intermediate high-frequency isolated DC/DC converter, and an output DC/AC inverter; the power router uses a high-frequency isolated DC/DC conversion The device is divided into two parts, the high-voltage side and the low-voltage side, the part electrically connected to the three-phase high-voltage AC port is the high-voltage side, and the part electrically connected to the three-phase low-voltage AC port is the low-voltage side; the high-frequency isolation DC/DC The converter is composed of a high-frequency DC/AC converter, a high-frequency isolation transformer and a high-frequency AC/DC converter; the high-voltage DC bus capacitor is a high-frequency isolation DC/DC converter high-voltage side bus capacitor; the low-voltage DC bus The capacitor is the low-voltage side busbar capacitor of the high-frequency isolated DC/DC converter.

Further, the current value in the charging process of the high-voltage DC bus capacitor described in step 2) refers to the current peak value flowing through the primary winding of the high-frequency isolation transformer; the low-voltage of the high-frequency isolation DC/DC converter described in step 3) The internal shift comparison of the side refers to the ratio of the phase difference between the driving signals of the two bridge arms of the low-voltage side high-frequency AC/DC converter to the half period of the driving signal.

Further, the charging process of the high-voltage DC bus capacitor is divided into two stages. The first stage is the pre-charging of the low-voltage DC bus capacitor, and the second stage is the pre-charging of the high-voltage DC bus capacitor. The first stage is necessary for the second stage. condition, that is, the pre-charging of the high-voltage DC bus capacitors must be performed after the pre-charging of the low-voltage DC bus capacitors is completed. The pre-charging does not require high-voltage charging resistors and circuit breakers in circuit implementation, but only requires low-voltage charging resistors, circuit breakers and auxiliary power supplies.

Further, when the first stage of pre-charging is completed, a driving signal with a certain internal shift phase change rule is applied to the switching tube of the high-frequency AC/DC converter on the low-voltage side of the high-frequency isolated DC/DC converter, while maintaining The switching tube of the high-frequency DC/AC converter on the high-voltage side of the high-frequency isolation DC/DC converter is in a blocking state, so that the energy of the auxiliary power supply on the low-voltage side passes through the high-frequency AC/DC converter, the high-frequency isolation transformer and the high-frequency DC/DC converter. The AC converter is transmitted to the high-voltage side to achieve the purpose of charging the capacitor of the high-voltage DC bus.

Further, in step 4), the variation law of the low-voltage side inward shift of the high-frequency isolation DC/DC converter is calculated according to the following method: by deriving the energy of the transformer primary winding current in each charging cycle in the second stage of precharging. The mathematical relationship between the achieved peak value and the inward shift of the low-voltage side of the high-frequency isolation DC/DC converter, according to the constraint that the charging current is equal to the preset current value, calculate the high-frequency isolation required for different charging cycles The inward shift comparison on the low-voltage side of the DC/DC converter.

The present invention proposes a constant current precharging method for a high voltage DC bus capacitor of an electric energy router, which has the following characteristics and advantages:

1. A constant current precharging method for the capacitor of the high-voltage DC bus of a power router proposed by the present invention, the pre-charging process is divided into two stages: The charging of the low-voltage DC bus capacitor has been completed before charging, and there is no need to additionally precharge the low-voltage DC bus capacitor.

2. The constant current precharging method of the high-voltage DC bus capacitor of the power router proposed by the present invention does not require high-voltage charging resistors and circuit breakers, but only requires low-voltage charging resistors, circuit breakers and auxiliary circuits. The power supply reduces the cost of the high-voltage DC bus capacitor pre-charging circuit and improves the safety and reliability of the pre-charging circuit.

3. The constant current precharging method of the high-voltage DC bus capacitor of the power router proposed by the present invention, in the charging process of the high-voltage DC bus capacitor, by reasonably setting the internal shift of the low-voltage side of the high-frequency isolation DC/DC converter to compare , it can control the current flowing through the primary winding of the high-frequency transformer to remain constant. Compared with the traditional pre-charging scheme with uncontrollable current, it also takes into account the two requirements of charging current and charging speed, while ensuring that the charging current cannot be too large. Under the premise, the charging speed is accelerated as much as possible.

4. A constant current pre-charging method for the capacitor of the high-voltage DC bus of an electric energy router proposed by the present invention, the control of the charging current is based on the isolation of the peak value of the primary winding current of the transformer during the charging process of the capacitor of the high-voltage DC bus from the high frequency isolation. Realized by the mathematical relationship between the low-voltage side inward shift of the DC/DC converter, the pre-charging method is simple and reliable, and does not depend on high-bandwidth sensors and processors.

Description of drawings

1 is a schematic diagram of a power router topology and a method for constant current precharging of high-voltage DC bus capacitors according to an embodiment of the present invention;

Fig. 2 is the constant current precharging flow chart of the high voltage DC bus capacitor of the power router according to the embodiment of the present invention;

3 is a topology diagram of another power conversion sub-module that can be used in the embodiment of the present invention;

4 is an equivalent circuit diagram of the first stage of precharging in an embodiment of the present invention;

5 is a schematic diagram of the charging principle of the second stage of pre-charging in an embodiment of the present invention;

FIG. 6 is a waveform diagram of the working principle of the second stage of precharging in an embodiment of the present invention;

Fig. 7 is the calculation block diagram of the inward shift comparison of the low-voltage side of the high-frequency isolation DC/DC converter in the embodiment of the present invention;

FIG. 8 is a simulation waveform diagram when the method of the present invention is applied to the constant current precharging of the capacitor of the high voltage DC bus of the 10kV power router.

Detailed ways

The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings.

The present invention provides a constant current precharging method for the high voltage DC bus capacitor of the power router. With reference to FIG. 1 and FIG. 2 , the method includes:

S1. Charging of low voltage DC bus capacitors. Specifically, first, according to the limiting conditions of the charging current flowing through all capacitors, the power loss on the current limiting resistor, and the charging time during the charging process of the low-voltage DC bus capacitor, select the current-limiting resistor with an appropriate resistance value and construct the low-voltage side precharge. circuit. The pre-charging circuit on the low-voltage side consists of an auxiliary power supply, a circuit breaker, a current-limiting resistor, and a bypass switch connected in parallel at both ends of the current-limiting resistor; among them, the auxiliary power supply is a 220V AC power supply or a DC power supply in the form of photovoltaics, batteries, etc., and the auxiliary power supply The output is adjusted to the voltage setting value of the low-voltage DC bus by a three-phase PWM rectification system or a DC/DC converter. When the pre-charging starts, the low-voltage side circuit breaker is closed, and the auxiliary power starts to charge all the capacitors connected to the low-voltage DC bus through the current limiting resistor; when the voltage on the low-voltage DC bus capacitor reaches the set value, Close the bypass switch connected in parallel across the current-limiting resistor to remove the current-limiting resistor from the circuit. So far, the charging of the low-voltage DC bus capacitor is completed.

S2. Set the current value during the charging process of the high-voltage DC bus capacitor.

S3. Calculate the internal shift comparison between the two bridge arms of the low-voltage side AC/DC converter of the high-frequency isolated DC/DC converter. Specifically, after the low-voltage DC bus capacitor is charged, a certain drive signal is applied to the low-voltage side AC/DC converter of the high-frequency isolation DC/DC converter, so that the low-voltage DC bus voltage is induced to the primary side through the high-frequency isolation transformer, The high-voltage DC bus capacitors are charged through the leakage inductance of the transformer or external inductance. According to the relationship between the maximum value of the current flowing through the capacitor of the high-voltage DC bus or the primary winding of the transformer in each cycle and the inward shift of the AC/DC converter on the low-voltage side during the charging process, according to the charging current value set by S2, Calculate the in-shift comparison of the low-side AC/DC converter during each charging cycle.

S4. Charging of high voltage DC bus capacitors. Specifically, after the first stage of pre-charging, that is, the charging of the low-voltage DC bus capacitor is completed, the second stage of charging, that is, the charging of the high-voltage DC bus capacitor is started. According to the change rule of the internal shift phase calculated by S3, a driving signal is applied to the switching tube of the low-voltage side AC/DC converter of the high-frequency isolated DC/DC converter, and the rectifier stage and the high-frequency isolated DC/DC conversion are maintained during this process. The switch tube of the high-voltage side DC/AC converter is in a locked state, so that the energy of the auxiliary power supply is transmitted to the high-voltage DC bus capacitor through the low-voltage side AC/DC converter, high-frequency isolation transformer and high-voltage side DC/AC converter, which is to charge. When the voltage on the high-voltage DC bus capacitor reaches the set value, disconnect the low-voltage side circuit breaker to cut off the auxiliary power supply from the circuit. So far, the charging of the high-voltage DC bus capacitor is completed.

In the constant current precharging method of the high-voltage DC bus capacitor of the power router according to the embodiment of the present invention, the power router has a three-stage structure, that is, an input AC/DC rectifier, an intermediate high-frequency isolation DC/DC converter, and an output DC/AC inverter. As shown in Figure 1, the output DC/AC inverter adopts a three-phase four-arm structure; the input AC/DC rectifier and the intermediate high-frequency isolated DC/DC converter form a power conversion sub-module, and its input terminal is connected to the three-phase high-voltage AC The power grid is connected, and the output terminal is connected with the output DC/AC inverter through the low-voltage DC bus. The constant current precharging method for the high voltage DC bus capacitor of the power router proposed in the embodiment of the present invention can perform constant current charging on the high voltage side bus capacitor of the high frequency isolation DC/DC converter. Among them, the high-frequency isolated DC/DC converter can adopt but is not limited to the structure shown in Figure 1 where the high-voltage side is a diode-clamped three-level, and the low-voltage side is a two-level full-bridge structure. The input AC/DC rectifier can adopt but Not limited to the diode-clamped three-level structure shown in FIG. 1 .

As shown in FIG. 3 , another power conversion sub-module structure to which the constant current precharging method of the high voltage DC bus capacitor of the power 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 intermediate high-frequency isolated DC/DC converter is a two-level full-bridge structure, and the low-voltage side is a diode-clamped three-level structure. In fact, the constant current precharging method for the high voltage DC bus capacitor of the power router in the embodiment of the present invention can be applied to, but not limited to, the power router with the power conversion sub-module shown in FIG. 1 or FIG. 3 . For the input AC/DC rectifier of the power conversion sub-module, the high-voltage side DC/AC converter and the low-voltage side AC/DC converter of the intermediate high-frequency isolated DC/DC converter, the three use a two-level full-bridge structure and Any combination structure composed of the diode-clamped three-level structure can be precharged with a constant current of the capacitor of the high-voltage DC bus by using the precharging method of the embodiment of the present invention.

The following takes an actual power router circuit as an example to further describe the embodiments according to the present invention.

The power router has a structure as shown in FIG. 1 . The power conversion sub-module adopts the structure shown in Figure 1. The rectifier stage is a diode-clamped three-level structure, the high-voltage side of the high-frequency isolated DC/DC converter is a diode-clamped three-level structure, and the low-voltage side is a two-level structure. Full-bridge structure; each phase is connected to the three-phase AC power grid by N (N=6) power conversion sub-modules with the same structure and parameters through the input series and the output parallel mode. The effective value of the line voltage of the power grid is 10kV; the high-voltage DC bus voltage of the high-frequency isolation DC/DC converter is 1600V, and the low-voltage DC busbar voltage is 700V; the transformation ratio of the primary and secondary sides of the high-frequency isolation transformer is 8:7; The bus capacitor is composed of two equivalent capacitors in series, and the midpoint of the capacitor is connected to a winding of the transformer, and the two capacitors have the same capacitance; the low-voltage DC bus capacitor is composed of an equivalent capacitor; the output inverter stage is three-phase four. The bridge arm structure, its DC bus is directly connected to the low-voltage DC bus of the power conversion sub-module, the voltage level is 700V, the bus capacitor is composed of an equivalent capacitor, and the output terminal is a 380V three-phase AC port.

1. The first stage of pre-charging - charging of low-voltage DC bus capacitors

The first stage of the pre-charging process is to charge the capacitors connected to the low-voltage DC bus from the auxiliary power supply. All switches of the input rectifier, the intermediate high-frequency isolated DC/DC converter and the output three-phase four-bridge inverter are in the blocking state. Therefore, the equivalent circuit at this stage can be regarded as the 700V modulated by the auxiliary power supply. The DC voltage directly charges the capacitor on the low-voltage DC bus, and its equivalent circuit is shown in Figure 4. Wherein, the low-voltage DC bus capacitance C _DCL is the sum of the low-voltage bus capacitance of all power conversion sub-modules and the output inverter bus capacitance.

The selection of the current limiting resistor in Figure 4 should consider the following factors, that is, the charging current limit flowing through the capacitor, the power loss limit on the current limiting resistor, and the charging time limit, which will be explained separately below:

Consider first the charge current limit flowing through the capacitor. Note that the sum of the low-voltage bus capacitances of all power conversion sub-modules is C _{L_total} , and the sum of the inverter bus capacitances is C _{inv_total} ; the low-voltage DC bus capacitances of the power conversion sub-modules and the three-phase four-bridge inverter DC bus are allowed to flow. The maximum currents of the capacitors are I _{max_SM} , I _{max_inv} , respectively. Considering that the three-phase four-leg inverter is not necessarily connected to the main circuit during pre-charging, there are

In the formula, U _DCL is the low-voltage bus voltage value obtained after the low-voltage side auxiliary power supply is rectified or chopped; R _charge is the low-voltage side current limiting resistor.

Next, consider the power dissipation limit on the current limiting resistor. As shown in Figure 4, denoting the charging time constant as τ=R _charge C _DCL , the capacitor voltage during the charging process can be expressed as

U _c (t)=U _DCL (1-e ^-t/τ ) (3)

During the precharging process, in any time period [t ₁ , t ₂ ], the average power consumed by the current limiting resistor is

Since the current during pre-charging is decreasing as the charging process progresses, the power loss on the current-limiting resistor is the largest at the initial stage of pre-charging. Note that the maximum average power allowed to dissipate on the current-limiting resistor in the first second of pre-charging is P _{max_res} , then there is

Finally, consider the charging time limit. Considering that the time of the pre-charging process should not be too long, the charging time constant also needs to be limited in the circuit design. Denote the maximum allowable duration of the first stage of precharging as t _max . Considering that it is generally believed that the capacitor voltage can reach a stable value within a period of 3 to 5τ, there are

5R _charge C _DCL ≤t _max (6)

To sum up, the value range of the current limiting resistor in Figure 4 can be obtained by comprehensively considering equations (1), (2), (5), and (6).

2. The second stage of pre-charging - charging of high-voltage DC bus capacitors

After the first stage of pre-charging is completed, the capacitor voltages of the low-voltage DC busbars of all power conversion sub-modules have reached the rated operating value, that is, U _DCL . At this time, the bypass switch at both ends of the low-voltage side current limiting resistor is closed, and the second stage of pre-charging begins to charge the high-voltage DC bus capacitor.

Figure 5 shows the working principle diagram of the second stage of precharge. Since 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, ignoring the fluctuation of the voltage of the low-voltage DC bus capacitor during the charging process, the low-voltage DC bus capacitor can be approximately regarded as a voltage source, and the voltage value is U _DCL . Sub-module rectifier stage switch tubes S ₁₁ ～S ₁₄ , S ₂₁ ～ S ₂₄ , high-frequency isolation DC/DC converter high-side switch tubes S ₃₁ ～S ₃₄ are all in the blocking state, and only use anti-parallel diodes as current paths; high The low-side switching transistors S ₄₁ to S ₄₄ of the frequency-isolated DC/DC converter are applied with the driving signals shown in FIG. 6 . The charging principle of the high-voltage DC bus capacitor will be described in detail below with reference to FIG. 6 .

As shown in Fig. 6, the ratio of the phase of the drive signal of the switch _S44 to the half cycle of the drive signal of the switch _S41 is defined as the internal shift of the low-voltage side of the high-frequency isolated DC/DC converter, which is denoted as _D2 .

During the period of [t ₂ , t ₃ ], the switches S ₄₁ and S ₄₄ are turned on at the same time, and the current path is shown as line 1 in FIG. 5 . At this time, the terminal voltage u _L of the low-voltage side winding of the high-frequency isolation transformer is +U _DCL , and further, the terminal voltage u _H of the high-voltage side winding of the transformer is +nU _DCL , then the voltage u _Ls added to the leakage inductance L _s is nU _DCL -U _CH1 . If the current flowing through the inductor has dropped to 0 before time t ₂ , the inductor current will increase linearly from 0 at this time, and will charge the high-voltage DC bus capacitor _CH1 .

During the time period of [t ₃ , t ₅ ], the switches S ₄₂ and S ₄₄ are turned on at the same time. When the voltage u _L of the low-voltage side of the transformer is 0, and then the voltage u _H of the high-voltage side is 0, the voltage u _Ls added to the leakage inductance L _s is -U _CH1 . At this time, the inductor current will decrease linearly from the maximum value, and continue to charge _CH1 until time t4, when all the energy stored in the inductor is converted into the capacitor _CH1 , and the inductor current _drops to 0. After that, the current cannot pass through in the reverse direction. The anti-parallel diode of the switch tube keeps the inductor current at 0 until the next time period.

In the time period of [t ₅ , t ₆ ], the switches S ₄₂ and S ₄₃ are turned on at the same time, and the current path is shown as line 2 in FIG. 5 . Similar to the analysis in the time period [t ₂ , t ₃ ], the system charges the HVDC bus capacitor _CH2 at this time.

During the period of [t ₀ , t ₂ ], the switches S ₄₁ and S ₄₃ are turned on at the same time. Similar to the analysis in the time period of [t ₃ , t ₅ ], the voltage on the capacitor _CH2 is reversely applied to the leakage inductance, and the current flowing through the leakage inductance decreases linearly until it becomes zero.

At the beginning of the second stage of pre-charging, the inward shift ratio of the low-voltage side of the high-frequency isolated DC/DC converter gradually decreases from 1, so that the effective value of the output AC voltage of the low-voltage side full-bridge gradually increases, so as to achieve a high-voltage DC voltage. The purpose of smooth charging of bus capacitors. During the precharging process, the two capacitors _CH1 and _CH2 of the high-voltage DC bus are continuously charged alternately, so the voltage balance of the two capacitors can be ensured during the charging process. The entire charging process ends when the voltages of the two capacitors reach half of the high-voltage DC bus voltage respectively.

3. Calculation of the internal shift of the low-voltage side of the high-frequency isolated DC/DC converter

The calculation of the high-frequency isolation DC/DC converter low-voltage side inward shift comparison is based on ensuring that the current during the charging process of the high-voltage DC bus capacitor remains unchanged. Since the switching frequency of the pre-charging process is very high, usually 20-50 kHz, it can be considered that in a charging cycle, the voltage on the high-voltage DC bus capacitor remains unchanged, denoted as U _c . The following may take the half cycle [t ₂ , t ₅ ] of the charging of the high-voltage DC bus capacitor _CH1 as an example to illustrate the calculation of the inward shift ratio of the low-voltage side.

As shown in Figure 6, in the [t ₂ ,t ₃ ] time period, the inductor current starts to rise from 0, and the maximum value it rises to is

In the formula, n is the turns ratio of the primary and secondary windings of the high-frequency transformer; L _s is the sum of the leakage inductance equivalent to the primary side of the high-frequency transformer and the external inductance attached to the primary side; f _s is the high-frequency isolation DC/ The switching frequency of the DC converter.

During this time period, the total energy transmitted from the secondary side to the primary side is

In the formula, C ₁₁ is the capacitance of _CH1 and _CH2 ; U _c0 is the capacitor voltage value at time t ₂ ; ΔU _c0 is the variation of the capacitor voltage in the [t ₂ , t ₃ ] time period. ΔU _c0 can be obtained from the average current

Substituting (9) into (8), we get

In the [t ₃ , t ₅ ] time period, the inductor current drops to 0, then all the energy is charged to the capacitor _CH1 , then there is

In the formula, ΔU _c is the variation of the capacitor voltage in the time period of [t ₂ , t ₅ ].

Therefore, in each charging cycle, the voltage change of each high-voltage DC bus capacitor is

As shown in Figure 7, if the current limit during the charging process of the high-voltage DC bus capacitor is given, then the two bridges on the low-voltage side of the high-frequency isolated DC/DC converter can be calculated according to formula (7) in each charging cycle. The internal shift between the arms is compared with D ₂ , and the voltage change curve of the high-voltage DC bus capacitor can be obtained according to formula (12).

In order to illustrate the effectiveness of the present invention, Fig. 8 shows the inward shift ratio of the low-voltage side D ₂ during the pre-charging process of the high-voltage DC bus capacitors, the voltages of the two high-voltage DC bus capacitors, and the current waveforms flowing through the leakage inductance during the pre-charging process. The simulation results of , in which the current setting value during the charging process of the high-voltage DC bus capacitor is 10A. The simulation results show that during the charging process, the voltages of the two high-voltage DC bus capacitors can be kept in balance, and the current flowing through the leakage inductance can be basically kept at the set value, which proves the effectiveness of the present invention.

Although the foregoing describes the embodiments of the present invention in conjunction with the accompanying drawings, it is only a preferred embodiment of the present invention, and is not intended to limit the present invention. Those skilled in the art can do so without departing from the spirit and scope of the present invention. Various modifications and variations can be made below, such modifications and variations falling 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 C_H1Half 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 shape_DCLThe low-voltage bus voltage value is obtained after rectification or chopping conversion is carried out on the low-voltage side auxiliary power supply; l is_sThe 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. of_sThe 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

In the formula, C₁₁Is C_H1And C_H2The capacity value of (c); u shape_c0Is t₂The capacitance voltage value at a moment; delta U_c0Is [ t ]₂,t₃]The amount of change in the capacitor voltage over a period of time; delta U_c0Can be obtained from the average current

Substituting (3) into (2) to obtain

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 C_H1Above, then there are

In the formula, Δ U_cIs [ 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

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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