CN117810950A - Direct current power supply system - Google Patents
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Abstract
The invention relates to the field of direct current power supply, in particular to a direct current power supply system. The system of the invention comprises: m medium-voltage alternating current power supply devices, m alternating current-direct current converters, a medium-voltage direct current bus, n direct current converters, a low-voltage direct current bus and a control device; the alternating current side of each alternating current-direct current converter is respectively connected with a medium-voltage alternating current power supply device; the direct current sides of the m alternating current-direct current converters are connected in parallel with a medium-voltage direct current bus; the AC-DC converter converts the AC power provided by the medium-voltage AC power supply device into DC power and transmits the DC power to the medium-voltage DC bus, and the voltage of the medium-voltage DC bus is maintained stable; the high-voltage sides of the n direct-current converters are connected in parallel with the medium-voltage direct-current bus, and the low-voltage sides of the n direct-current converters are connected in parallel with the low-voltage direct-current bus, so that the voltage stability of the low-voltage direct-current bus is maintained; the low-voltage direct-current bus provides direct-current voltage for the direct-current load; the control device distributes power and controls current sharing, so that each AC/DC converter and each DC converter share load electricity according to the rated power of the control device in equal proportion.
Description
Technical Field
The invention relates to the field of direct current power supply, in particular to a direct current power supply system.
Background
The development of power electronics technology and the wide application of direct current load lead to the wider and wider application of direct current power supply systems. In the urban direct current power supply system, firstly, medium-voltage alternating current is converted into medium-voltage direct current through an alternating current-direct current conversion device; and then connecting the medium-voltage direct current with a direct current load through a direct current conversion device. The direct current load comprises a data center, an office building, a charging pile, a residential building and the like, and multiple paths of loads are connected in parallel to a direct current power supply system through load switches to form a direct current power utilization network.
The direct current power utilization system has various accessed load types and quantity, and higher requirements on power supply reliability are provided; the failure of the medium voltage alternating current system or the medium voltage direct current system is light, so that the power supply is interrupted, and the direct current load is damaged and bad influence is caused.
In the prior art, a set of AC/DC conversion device is generally adopted to maintain the DC bus voltage, and when the AC/DC conversion device fails or the AC distribution system fails, the power supply of the whole DC power supply system is interrupted, thereby greatly reducing the reliability and continuity of the power supply of users, and particularly, the power supply system cannot be applied to the demand scenes of continuous power supply of sensitive loads and the like.
Prior art related to the invention:
1. technical proposal of the prior art
The Chinese patent application with publication number of CN112751330A discloses a low-voltage direct-current continuous power supply system and a control method thereof; according to the scheme, the direct current limiter and the energy storage system are arranged on the low-voltage side, so that the low-voltage direct current distribution system can quickly and clearly break down and continuously supply power when the medium-voltage side and the low-voltage side break down, the possibility of breaking down the low-voltage direct current load when the fault occurs is minimized, and the power supply reliability is effectively improved.
Drawbacks of the first prior art: according to the technical scheme, the energy storage system needs to be configured at the low-voltage side, the power of the energy storage system is equivalent to all loads, the energy storage system cannot be separated from the medium-voltage system for long time to work independently, and the applicability is limited.
2. Technical proposal of the second prior art
The Chinese patent application with publication number of CN113013931A discloses a power self-adaptive balance control method and system for an AC/DC power distribution system; according to the scheme, an alternating current sagging control strategy is applied to power control, direct current end voltage and current signals of a three-phase full-control bridge DC/AC converter are introduced to outer loop control of the alternating current sagging control strategy, and power of a DC bus and power of an AC bus are automatically balanced and distributed under the condition of no communication.
Drawbacks of the second prior art: the technical scheme focuses on AC/DC power distribution, mainly researches the control strategy of the AC/DC, and does not research the whole control of the DC distribution network; in the scheme, the AC/DC is a three-phase full-control bridge, and the direct-current voltage range is limited; the scheme has limited applicability in medium voltage direct current distribution networks.
3. Prior art III related to the invention
The Chinese patent application with publication number of CN111313527A discloses a method for controlling power balance of a DC micro-grid system; according to the scheme, master-slave and battery SOC control is added on the basis of direct-current sagging control, the stability of direct-current bus side voltage can be effectively guaranteed, the dynamic response capability of the system is improved, and the cycle service life of the storage battery of the energy storage unit is prolonged.
Drawbacks of the third prior art: the scheme adopts a master-slave control mode, and a host computer is required to be selected for power monitoring and distribution; once the host is abnormal, the technical scheme shown will fail, so the technical scheme has limited applicability.
Disclosure of Invention
The invention aims to solve the problems in the prior art and provides a multi-port direct current power supply system operation control method which can effectively improve the power supply reliability and continuity of a direct current power supply system.
In order to achieve the above purpose, the present invention is realized by the following technical scheme.
The invention provides a direct current power supply system which is used for supplying power to a direct current load; the direct current power supply system includes: m medium voltage ac power supply units, m ac-dc converters and medium voltage dc bus, the system further comprising: n DC converters, a low-voltage DC bus and a control device; wherein,
the alternating current side of each alternating current-direct current converter is respectively connected with a medium-voltage alternating current power supply device; the direct current sides of the m alternating current-direct current converters are connected in parallel with a medium-voltage direct current bus;
the alternating current-direct current converter is used for converting alternating current provided by the medium-voltage alternating current power supply device into direct current and transmitting the direct current to the medium-voltage direct current bus to maintain the voltage stability of the medium-voltage direct current bus;
the high-voltage sides of the n direct-current converters are connected in parallel with the medium-voltage direct-current bus, and the low-voltage sides of the n direct-current converters are connected in parallel with the low-voltage direct-current bus, so that the voltage stability of the low-voltage direct-current bus is maintained;
the low-voltage direct current bus is used for providing direct current voltage for a direct current load;
the control device is used for enabling the AC/DC converters and the DC converters to share load electricity according to the rated power of the DC/DC converters in equal proportion through power distribution and current sharing control.
As one of the improvements of the above technical solutions, in the control device, through power distribution and current sharing control, each ac/dc converter and each dc converter share load power according to their rated power in equal proportion, specifically including:
step 1, adjusting the output power of each AC-DC converter in a droop control mode according to the output power and the rated power of each AC-DC converter, so that the difference value between the output power of each AC-DC converter and the rated power of each AC-DC converter is within a set threshold deviation range;
step 2, adjusting the output power of the direct current converters in a droop control mode according to the output power and rated power of each direct current converter, so that the difference value between the output power of each direct current converter and the rated power of each direct current converter is within a set threshold deviation range;
step 3, calculating the voltage of the medium-voltage direct-current bus, and judging whether the set range is exceeded or not: if the preset range is not exceeded, the step 4 is entered; if the lower limit or the upper limit of the set range is exceeded, the voltage of the medium-voltage direct-current bus is adjusted by translating the direct-current sagging curve of the alternating-current-direct-current converter;
step 4, calculating the voltage of the low-voltage direct-current bus, and judging whether the set range is exceeded or not: if the set range is not exceeded, the direct current power supply system normally supplies power; and if the lower limit or the upper limit of the set range is exceeded, the low-voltage direct-current bus voltage is regulated by a direct-current sagging curve of the translational direct-current converter, and finally, the alternating-current and direct-current converters and the direct-current converters share load electricity according to the rated power of the direct-current and direct-current converters in equal proportion.
As one of the improvements of the above technical solution, the step 1 includes:
step 1-1. Calculating the total power P of the AC/DC converter under droop control Total (S) ：
Wherein P is i Representing the rated power of the ith AC-DC converter; i=1, 2,. -%, m;
step 1-2, calculating the active power P required to be output by the ith AC-DC converter according to the sagging coefficient and the total power of each AC-DC converter iexp ：
Wherein k is dci Representing the droop coefficient of the ith AC-DC converter;
step 1-3. Calculating the active power, i.e. the power increment DeltaP, which the ith AC/DC converter needs to adjust i ：
ΔP i ＝P iexp -P i
Step 1-4, sequencing the power increment required to be adjusted by each AC/DC converter, and preferentially adjusting the AC/DC converter with the largest power increment;
and step 1-5, repeating the steps 1-1 to 1-4 until the power deviation of the AC-DC converter is smaller than the set deviation value.
As one of the improvements of the above technical solution, the step 2 includes:
step 2-1. Calculating the total power P of the DC converter under droop control dc total ：
Wherein P is dcj Indicating the rated power of the j-th direct current converter; j=1, 2,. -%, n;
step 2-2, calculating the active power P required to be output by the jth direct current converter according to the droop coefficient and the total power of each direct current converter dcjexp ：
Wherein k is dcj Representing a droop coefficient of the jth dc converter;
step 2-3. Calculating the active power, i.e. the power increment DeltaP, to be adjusted by the jth DC converter dcj ：
ΔP dcj ＝P dcjexp -P dcj
Step 2-4, sequencing the power increment required to be adjusted by each direct current converter, and preferentially adjusting the direct current converter with the largest power increment;
and step 2-5, repeating the steps 2-1 to 2-4 until the power deviation of the direct current converter is smaller than the set deviation value.
As one of the improvements of the above technical solution, in the step 3, if the lower limit or the upper limit of the set range is exceeded, the dc sag curve of the ac/dc converter is translated to adjust the voltage of the medium-voltage dc bus, which specifically includes:
if the voltage of the medium-voltage direct-current bus exceeds the lower limit, adopting a return difference control mode to move the direct-current sagging curve of the alternating-current/direct-current converter to the right until the voltage of the medium-voltage direct-current bus is restored to be within a set range;
and if the voltage of the medium-voltage direct-current bus exceeds the upper limit, the direct-current sagging curve of the alternating-current and direct-current converter is shifted left by adopting a return difference control mode until the medium-voltage direct-current voltage bus is restored to be within a set range.
As one of the improvements of the above technical scheme, the offset of the dc-dc converter, which is shifted right or left in the dc-sag curve, is calculated by PI control.
As an improvement of the above-mentioned technical solution, in the step 4, if the lower limit or the upper limit of the set range is exceeded, the low-voltage dc bus voltage is adjusted by a dc sag curve of the translational dc converter, which specifically includes:
if the voltage of the low-voltage direct-current bus exceeds the lower limit, a return difference control mode is adopted to move the direct-current sagging curve of the direct-current converter to the right until the voltage of the low-voltage direct-current bus is restored to be within a set range;
and if the voltage of the low-voltage direct-current bus exceeds the upper limit, the direct-current sagging curve of the direct-current converter is shifted leftwards by adopting a return difference control mode until the voltage of the low-voltage direct-current bus is restored to be within a set range.
As one of the improvements of the above technical scheme, the offset of the dc-converter dc-sag curve shifted right or left is calculated by PI control.
As an improvement of the foregoing technical solution, the control device is further configured to start the dc power supply system, and the starting process includes:
step (1) detecting three-phase voltages of alternating current sides of the alternating current-direct current converters: if the three-phase alternating voltage is greater than M times rated voltage, entering the step (2); if the three-phase alternating voltage is not more than M times of rated voltage, entering the step (3);
step (2) adopting a sagging control mode to control the voltage of the medium-voltage direct-current bus, ensuring that the multi-AC-DC converter jointly maintains the voltage of the medium-voltage direct-current bus stable, and entering step (4);
step (3) adopts a zero-voltage soft start mode on the alternating-current side of the alternating-current/direct-current converter, adopts independent constant-frequency constant-voltage control on the three-phase voltage, and enters step (4);
step (4) detecting the direct current voltage at the high-voltage side of the direct current converter, if the direct current voltage is greater than N times of rated voltage, entering step (5), otherwise stopping starting the direct current power supply system;
and (5) controlling the low-voltage direct-current bus voltage by adopting a sagging control mode, so as to ensure that the multiple direct-current converters jointly maintain the low-voltage direct-current bus voltage stable, and the starting of the direct-current power supply system is completed.
As one of the improvements of the technical scheme, the values of M and N are respectively 0.8-0.9.
Compared with the prior art, the invention has the advantages that:
1) The invention realizes the parallel operation of the AC/DC converter (AC/DC) and the DC/DC converter (DC/DC) of the multi-port DC power supply system, and improves the power supply reliability of the medium-voltage DC system and the low-voltage DC system;
2) The invention enables each converter to share the load according to the rated power of the converter in equal proportion through the power distribution and the current sharing control, thereby improving the effective utilization rate of each converter and the utilization rate of the whole system.
Drawings
FIG. 1 is a block diagram of a system of the present invention;
FIG. 2 is an AC/DC status identification flow chart;
FIG. 3 is a direct current droop control block diagram;
FIG. 4 is a schematic view of direct current droop;
FIG. 5 is a schematic diagram of the change of the reference value of the AC voltage in the zero voltage soft start mode;
FIG. 6 is an AC voltage control block diagram;
FIG. 7 is a DC/DC status identification flow chart;
FIG. 8 is a low side DC bus voltage control block diagram;
FIG. 9 is an AC/DC power adjustment flow;
FIG. 10 is a DC/DC power adjustment flow;
FIG. 11 is a block diagram of an AC/DC droop curve offset calculation;
fig. 12 is a block diagram of DC/DC sag curve offset calculation.
Detailed Description
The technical scheme of the invention is described in detail below with reference to the accompanying drawings and examples.
The direct current power supply system of the invention comprises: an AC/DC converter (AC/DC), a DC converter (DC/DC), and a DC load, as shown in fig. 1. The alternating current side of the alternating current-direct current conversion device is connected into different medium voltage alternating current systems, and the direct current side is connected in parallel with a medium voltage direct current bus for maintaining the voltage stability of the medium voltage direct current bus; the high-voltage side of the DC converter is connected to the medium-voltage DC bus and the low-voltage side of the DC converter is connected to the low-voltage DC bus in parallel, so that the low-voltage DC bus is maintained stable, and a stable DC power supply is provided for a DC load. The medium voltage alternating current voltage in the embodiment is about 10 kV; the medium-voltage direct current bus is not more than 20kV; the low voltage DC bus does not exceed 750V.
The operation of the multi-port direct current power supply system mainly comprises the following steps:
1 System Start
(1) AC/DC status identification, the process is as shown in fig. 2: the AC/DC detects the three-phase voltage of the self alternating-current side, if the three-phase alternating-current voltage is more than 0.85 times of rated voltage, the method enters (2), otherwise, the method enters (3);
(2) Controlling the voltage of an AC/DC constant direct current bus; the voltage control of the direct current bus adopts a sagging control mode, so that the voltage stability of the medium voltage direct current bus is ensured to be commonly maintained by multiple AC/DC, and the power supply redundancy and reliability of the medium voltage direct current bus are improved; the direct current sagging control block diagram is shown in fig. 3, and the direct current sagging schematic diagram is shown in fig. 4;
(3) Constant frequency and constant voltage control of an AC/DC side; adopts a zero-voltage soft start mode, and adopts independent control for three-phase voltage. The ac voltage reference value changes are shown in fig. 5. Starting from 0, the voltage is gradually increased according to the set time until the set voltage is expected. The ac voltage control block diagram is shown in fig. 6.
(4) DC/DC status identification, the process is as shown in fig. 7: the DC/DC detects the direct current voltage at the high-voltage side of the self-body, if the direct current voltage is larger than 0.85 times of rated voltage, the self-body enters (5), otherwise, the self-body is not started;
(5) DC/DC constant low voltage direct current bus voltage control: the low-voltage direct-current bus voltage control adopts a sagging control mode, so that the voltage stability of the low-voltage direct-current bus is ensured to be commonly maintained by multiple DCs/DCs, and the power supply redundancy and reliability of the low-voltage direct-current bus are improved; a low-side direct current bus voltage control block diagram is shown in fig. 8;
2AC/DC power distribution and current sharing control
According to the actual power and rated power of each AC/DC, the output power is finely adjusted on the basis of droop control, so that the output power of each AC/DC can be adapted to the rated power of the AC/DC, and the stability of a medium-voltage direct-current system is improved;
the AC/DC power adjustment flow is shown in fig. 9, and the specific process is as follows:
(1) Calculating the total power P of the AC/DC under droop control according to Total (S) ；
(2) Calculating the active power required to be output by each AC/DC according to the sagging coefficient and the total power of each AC/DC;
(3) Calculating the active power to be adjusted of each AC/DC;
ΔP i ＝P iexp -P i
(4) Sequencing the power increment, and preferentially adjusting the AC/DC with the maximum power increment;
(5) Repeating the steps (1) - (4) until the AC/DC power deviation is smaller than the set deviation value;
3DC/DC power distribution and current sharing control
According to the actual power and rated power of each DC/DC, the output power is finely adjusted on the basis of droop control, so that the output power of each DC/DC can be adapted to the rated power of the DC/DC, and the stability of a low-voltage direct current system is improved;
the DC/DC power adjustment flow is shown in fig. 10, and the specific process is as follows:
(1) Calculating the total power P of the DC/DC under droop control according to dc total ；
(2) Calculating the active power required to be output by each DC/DC according to the sagging coefficient and the total power of each DC/DC;
(3) Calculating the active power to be adjusted of each DC/DC;
ΔP dci ＝P dciexp -P dci
(4) Sequencing the power increment, and preferentially adjusting the DC/DC with the maximum power increment;
(5) Repeating the steps (1) - (4) until the DC/DC power deviation is smaller than the set deviation value;
4 medium voltage DC bus voltage regulation
When the voltage of the medium-voltage direct-current bus exceeds a set deviation range, the voltage of the direct-current bus is adjusted by translating an AC/DC direct-current sagging curve; the specific process is as follows:
(1) Calculating the voltage of the medium-voltage direct-current bus, if the voltage exceeds the set range, adjusting the voltage, otherwise, not adjusting the voltage;
(2) If the medium voltage DC voltage is lower than the lower limit, the AC/DC direct current droop curve is shifted to the right by adopting a return difference control mode (i.e. U is increased dc0 ) Until the medium-voltage direct-current voltage is restored to be within a set range;
(3) If the medium voltage DC voltage exceeds the upper limit, the AC/DC direct current sagging curve is shifted left by adopting a return difference control mode (i.e. U is reduced dc0 ) Until the medium-voltage direct-current voltage is restored to be within a set range;
the calculation of the AC/DC direct current droop curve offset is realized by a PI control mode, and a control block diagram is shown in figure 11.
5 low voltage DC bus voltage regulation
When the voltage of the low-voltage direct-current bus exceeds a set deviation range, the direct-current bus voltage is adjusted by translating the DC/DC direct-current sagging curve; the specific process is as follows:
(1) Calculating the voltage of the low-voltage direct-current bus, if the voltage exceeds the set range, adjusting the voltage, otherwise, not adjusting the voltage;
(2) If the low-voltage DC voltage is lower than the lower limit, the DC/DC direct-current droop curve is shifted to the right (i.e. U is increased) by adopting a return difference control mode dc0 ) Until the low-voltage direct-current voltage is restored to be within a set range;
(3) If the low-voltage direct-current voltage is higher than the upper limit, the DC/DC direct-current sagging curve is shifted leftwards by adopting a return difference control mode (namely U is reduced dc0 ) Until the low-voltage direct-current voltage is restored to be within a set range;
the calculation of the DC/DC direct current droop curve offset is realized by a PI control mode, and a control block diagram is shown in figure 12.
Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present invention and are not limiting. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made thereto without departing from the spirit and scope of the present invention, which is intended to be covered by the appended claims.
Claims (10)
1. A DC power supply system for supplying power to a DC load; the direct current power supply system includes: m medium voltage ac power supply units, m ac-dc converter and medium voltage dc bus, its characterized in that, the system still includes: n DC converters, a low-voltage DC bus and a control device; wherein,
the alternating current side of each alternating current-direct current converter is respectively connected with a medium-voltage alternating current power supply device; the direct current sides of the m alternating current-direct current converters are connected in parallel with a medium-voltage direct current bus;
the alternating current-direct current converter is used for converting alternating current provided by the medium-voltage alternating current power supply device into direct current and transmitting the direct current to the medium-voltage direct current bus to maintain the voltage stability of the medium-voltage direct current bus;
the high-voltage sides of the n direct-current converters are connected in parallel with the medium-voltage direct-current bus, and the low-voltage sides of the n direct-current converters are connected in parallel with the low-voltage direct-current bus, so that the voltage stability of the low-voltage direct-current bus is maintained;
the low-voltage direct current bus is used for providing direct current voltage for a direct current load;
the control device is used for enabling the AC/DC converters and the DC converters to share load electricity according to the rated power of the DC/DC converters in equal proportion through power distribution and current sharing control.
2. The dc power supply system according to claim 1, wherein the control device is configured to cause each ac/dc converter and each dc converter to share load power according to its rated power in equal proportion by power distribution and current sharing control, and the control device specifically comprises:
step 1, adjusting the output power of each AC-DC converter in a droop control mode according to the output power and the rated power of each AC-DC converter, so that the difference value between the output power of each AC-DC converter and the rated power of each AC-DC converter is within a set threshold deviation range;
step 2, adjusting the output power of the direct current converters in a droop control mode according to the output power and rated power of each direct current converter, so that the difference value between the output power of each direct current converter and the rated power of each direct current converter is within a set threshold deviation range;
step 3, calculating the voltage of the medium-voltage direct-current bus, and judging whether the set range is exceeded or not: if the preset range is not exceeded, the step 4 is entered; if the lower limit or the upper limit of the set range is exceeded, the voltage of the medium-voltage direct-current bus is adjusted by translating the direct-current sagging curve of the alternating-current-direct-current converter;
step 4, calculating the voltage of the low-voltage direct-current bus, and judging whether the set range is exceeded or not: if the set range is not exceeded, the direct current power supply system normally supplies power; and if the lower limit or the upper limit of the set range is exceeded, the low-voltage direct-current bus voltage is regulated by a direct-current sagging curve of the translational direct-current converter, and finally, the alternating-current and direct-current converters and the direct-current converters share load electricity according to the rated power of the direct-current and direct-current converters in equal proportion.
3. The direct current power supply system according to claim 2, wherein the step 1 comprises:
step 1-1. Calculating the total power P of the AC/DC converter under droop control Total (S) ：
Wherein P is i Representing the rated power of the ith AC-DC converter; i=1, 2,. -%, m;
step 1-2, calculating the active power P required to be output by the ith AC-DC converter according to the sagging coefficient and the total power of each AC-DC converter iexp ：
Wherein k is dci Representing the droop coefficient of the ith AC-DC converter;
step 1-3. Calculating the active power, i.e. the power increment DeltaP, which the ith AC/DC converter needs to adjust i ：
ΔP i ＝P iexp -P i
Step 1-4, sequencing the power increment required to be adjusted by each AC/DC converter, and preferentially adjusting the AC/DC converter with the largest power increment;
and step 1-5, repeating the steps 1-1 to 1-4 until the power deviation of the AC-DC converter is smaller than the set deviation value.
4. The direct current power supply system according to claim 2, wherein the step 2 comprises:
step 2-1. Calculating the total power P of the DC converter under droop control dc total ：
Wherein P is dcj Indicating the rated power of the j-th direct current converter; j=1, 2,. -%, n;
step 2-2, calculating the j-th direct current transformer according to the droop coefficient and the total power of each direct current converterActive power P to be output by converter dcjexp ：
Wherein k is dcj Representing a droop coefficient of the jth dc converter;
step 2-3. Calculating the active power, i.e. the power increment DeltaP, to be adjusted by the jth DC converter dcj ：
ΔP dcj ＝P dcjexp -P dcj
Step 2-4, sequencing the power increment required to be adjusted by each direct current converter, and preferentially adjusting the direct current converter with the largest power increment;
and step 2-5, repeating the steps 2-1 to 2-4 until the power deviation of the direct current converter is smaller than the set deviation value.
5. The dc power supply system according to claim 2, wherein in the step 3, if the lower limit or the upper limit of the set range is exceeded, the dc sag curve of the ac/dc converter is translated to adjust the dc bus voltage, and the method specifically comprises:
if the voltage of the medium-voltage direct-current bus exceeds the lower limit, adopting a return difference control mode to move the direct-current sagging curve of the alternating-current/direct-current converter to the right until the voltage of the medium-voltage direct-current bus is restored to be within a set range;
and if the voltage of the medium-voltage direct-current bus exceeds the upper limit, the direct-current sagging curve of the alternating-current and direct-current converter is shifted left by adopting a return difference control mode until the medium-voltage direct-current voltage bus is restored to be within a set range.
6. The dc power supply system of claim 5, wherein the offset of the dc-dc converter dc sag curve shifted right or left is calculated by PI control.
7. The dc power supply system according to claim 2, wherein in the step 4, if the lower limit or the upper limit of the set range is exceeded, the dc link voltage of the low-voltage dc bus is adjusted by shifting the dc sag curve of the dc converter, and the method specifically comprises:
if the voltage of the low-voltage direct-current bus exceeds the lower limit, a return difference control mode is adopted to move the direct-current sagging curve of the direct-current converter to the right until the voltage of the low-voltage direct-current bus is restored to be within a set range;
and if the voltage of the low-voltage direct-current bus exceeds the upper limit, the direct-current sagging curve of the direct-current converter is shifted leftwards by adopting a return difference control mode until the voltage of the low-voltage direct-current bus is restored to be within a set range.
8. The dc power supply system of claim 7, wherein the offset of the dc converter dc sag curve to the right or left is calculated by PI control.
9. The direct current power supply system according to one of claims 1 to 8, wherein the control means is further adapted to start the direct current power supply system, the start-up procedure comprising:
step (1) detecting three-phase voltages of alternating current sides of the alternating current-direct current converters: if the three-phase alternating voltage is greater than M times rated voltage, entering the step (2); if the three-phase alternating voltage is not more than M times of rated voltage, entering the step (3);
step (2) adopting a sagging control mode to control the voltage of the medium-voltage direct-current bus, ensuring that the multi-AC-DC converter jointly maintains the voltage of the medium-voltage direct-current bus stable, and entering step (4);
step (3) adopts a zero-voltage soft start mode on the alternating-current side of the alternating-current/direct-current converter, adopts independent constant-frequency constant-voltage control on the three-phase voltage, and enters step (4);
step (4) detecting the direct current voltage at the high-voltage side of the direct current converter, if the direct current voltage is greater than N times of rated voltage, entering step (5), otherwise stopping starting the direct current power supply system;
and (5) controlling the low-voltage direct-current bus voltage by adopting a sagging control mode, so as to ensure that the multiple direct-current converters jointly maintain the low-voltage direct-current bus voltage stable, and the starting of the direct-current power supply system is completed.
10. The direct current power supply system according to claim 9, wherein the values of M and N are each 0.8 to 0.9.
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