CN110336462B - Direct-current power electronic transformer and control method and device thereof - Google Patents
Direct-current power electronic transformer and control method and device thereof Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
- H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
- H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
- H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
- H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
- H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
- H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
Abstract
The invention provides a direct-current power electronic transformer and a control method and device thereof, and belongs to the technical field of direct-current power electronic transformers. The control method adopts a control strategy of a Boost voltage loop, a Boost current loop and a voltage balancing loop aiming at a Boost module; a control strategy of a DC/DC voltage loop and a DC/DC current loop is adopted for the DC/DC conversion module; calculating the ratio of a voltage measurement value of a high-voltage direct-current bus to the sum of voltage values of Boost buses of all power conversion units in the input state to obtain an initial control quantity, and subtracting the output value of a Boost current loop from the initial control quantity to be used as a control signal of a Boost module; the output value of the DC/DC current loop and the voltage gain of the DC/DC conversion module are used as control signals of the DC/DC conversion module. The invention can improve the operation stability of the control system.
Description
Technical Field
The invention relates to a direct-current power electronic transformer and a control method and device thereof, belonging to the technical field of direct-current power electronic transformers.
Background
The Input Series Output Parallel (ISOP) type power electronic transformer plays an important role in realizing the voltage conversion, energy exchange and electric appliance isolation functions of a medium-high voltage direct current distribution network (10kV voltage level or higher) and a low-voltage direct current micro-grid (750V, 380V voltage level or lower).
As shown in fig. 1, an existing iso p-type power electronic transformer includes k power conversion units and m redundant standby units, wherein input ends of each power conversion unit and each redundant standby unit are connected in series and then used for connecting a high-voltage direct-current bus, output ends of each power conversion unit and each redundant standby unit are connected in parallel and then used for connecting a low-voltage direct-current bus, k is greater than or equal to 1, and m is greater than or equal to 1; each unit comprises a switching switch, a Boost module and a DC/DC conversion module; one specific structure of each unit can be as shown in fig. 2, and the switching switch comprises a mechanical controllable switch K1 and a bidirectional thyristor T3 which are arranged in parallel; the Boost module consists of an input inductor L and a half-bridge module, wherein the half-bridge module consists of an upper IGBT switching tube Sb1 and a lower IGBT switching tube Sb2 which comprise anti-parallel diodes and are connected in series; the DC/DC conversion module comprises a primary side full bridge, a capacitor C1, an intermediate frequency transformer T, a secondary side full bridge and a capacitor C2, wherein the primary side full bridge and the secondary side full bridge are formed by connecting two bridge arms in parallel, each bridge arm is formed by connecting an upper IGBT switching tube and a lower IGBT switching tube which comprise anti-parallel diodes in series, one bridge arm of the primary side full bridge is formed by connecting IGBT switching tubes S1 and S2 in series, the other bridge arm is formed by connecting IGBT switching tubes S3 and S4 in series, one bridge arm of the secondary side full bridge is formed by connecting IGBT switching tubes S5 and S6 in series, and the other bridge arm is formed by connecting IGBT switching tubes S7 and S8 in series.
After the pulses of the upper IGBT switching tube and the lower IGBT switching tube of the Boost module are blocked, the capacitor cannot be directly discharged through a short circuit of a bypass loop due to the existence of the anti-parallel diode. Corresponding units can be bypassed by a lower tube Sb2 of the normally-open Boost module, a turn-on bidirectional thyristor T3 or a close mechanical controllable switch K1. The high-voltage side of each unit is controlled to supply power in an automatic power supply mode, and the power supplies of a Boost module, a primary side full bridge of a DC/DC conversion module, a closing control signal of a mechanical controllable switch and a bidirectional thyristor control signal are all obtained from the high-voltage side bus capacitor.
The general operation mode of the ISOP type power electronic transformer is as follows: in the initial stage of operation, k power conversion units are in an on state, m redundant standby units are in a bypass state, and after a fault of one power conversion unit is found, the power conversion unit with the fault is bypassed, and one redundant standby unit is put in, namely in the operation mode, the number of the units in the on state is always k. However, since the activation and precharging of the redundant standby unit requires a long process, the time from the time of the failure to the time when the control system reenters the steady-state operation is prolonged by using this operation. In order to solve this problem, the following operation method can be adopted: in the initial stage of operation, k power conversion units and m redundant standby units are all put into operation, after a certain power conversion unit fails, the failed power conversion unit is directly bypassed, the other units continue to work, and when the number of bypassed failed conversion units reaches m, only k units are put into operation. In the operation mode, the redundant standby unit operates in the control system in a hot standby mode, so that when a certain power conversion unit fails, the rapid isolation of the failure can be realized, and the control system can rapidly enter the steady-state operation.
As shown in fig. 3-1 and 3-2, when a redundant standby unit operates in a control system in a hot standby mode, according to the control strategy shown in fig. 3-1, when a faulty power conversion unit is bypassed, the number of units in an input state is reduced (the change trend is from k + m to k), and since a set value of a voltage of a Boost bus is not changed, when the number of units in the input state is reduced, a ratio of an input terminal voltage of a unit Boost to a bus terminal voltage is greatly changed, a corresponding duty ratio of the Boost bus is obviously changed, and a situation that the control system is unstable or has a poor regulation effect is easily caused.
In addition, the feedback value V of the voltage loop in the Boost module is calculated according to the control strategy shown in FIGS. 3-1 and 3-2b_averAnd calculating the feedback value I of the current loop in the DC/DC conversion moduleLV_averIn time, the voltage and current of the fault unit are calculated, and thus the control effect of the whole control system is necessarily influenced.
Disclosure of Invention
The invention aims to provide a control method of a direct-current power electronic transformer, which is used for solving the problem that a control strategy in the prior art is easy to cause the instability of a control system or the poor regulation effect; the invention also provides a control device of the direct current power electronic transformer, which is used for solving the problem that the control strategy in the existing control device is easy to cause the control system to be unstable or the regulation effect is poor; the invention also provides a direct current power electronic transformer, which is used for solving the problem that a control system is easy to be unstable or the regulation effect is poor due to a control strategy in the conventional power electronic transformer control device.
In order to achieve the purpose, the invention provides a control method of a direct current power electronic transformer, the direct current power electronic transformer comprises X power conversion units, the input ends of the power conversion units are connected in series and then are used for connecting a high-voltage direct current bus, the output ends of the power conversion units are connected in parallel and then are used for connecting a low-voltage direct current bus, X is more than or equal to 2, and each power conversion unit comprises a switching switch, a Boost module and a DC/DC conversion module;
the Boost module adopts a control strategy of a Boost voltage ring, a Boost current ring and a voltage balancing ring; the given value of the Boost voltage ring is a set value of the Boost bus voltage, and the feedback value of the Boost voltage ring is an average value of the Boost bus voltage of each power conversion unit; the given value of the voltage balance ring is the feedback value of the Boost voltage ring, and the feedback value of the voltage balance ring is the voltage value of the Boost bus of the controlled unit; adding the output value of the Boost voltage loop and the output value of the voltage balancing loop to obtain a value serving as a given value of a Boost current loop, wherein the feedback value of the Boost current loop is the Boost input current value of the controlled unit; calculating the ratio of the voltage measurement value of the high-voltage direct-current bus to the sum of the voltage values of Boost buses of all power conversion units in the input state to obtain an initial control quantity, and subtracting the output value of a Boost current loop from the initial control quantity to be used as a control signal of a Boost module;
the DC/DC conversion module adopts a control strategy of a DC/DC voltage loop and a DC/DC current loop; the given value of the DC/DC voltage loop is a low-voltage bus voltage set value, the feedback value of the DC/DC voltage loop is a low-voltage bus voltage measured value, the output value of the DC/DC voltage loop is used as the given value of the DC/DC current loop, the feedback value of the DC/DC current loop is the average value of the output current of each power conversion unit, and the output value of the DC/DC current loop and the voltage gain of the DC/DC conversion module are used as control signals of the DC/DC conversion module.
The invention also provides a control device of the direct current power electronic transformer, which comprises a memory and a processor, wherein the processor is used for operating the program instructions stored in the memory so as to realize the control method of the direct current power electronic transformer.
The invention also provides a direct current power electronic transformer which comprises X power conversion units, wherein the input ends of the power conversion units are connected in series and then are used for connecting a high-voltage direct current bus, the output ends of the power conversion units are connected in parallel and then are used for connecting a low-voltage direct current bus, X is more than or equal to 2, and each power conversion unit comprises a switching switch, a Boost module and a DC/DC conversion module; the power electronic transformer also comprises a power electronic transformer control device which comprises a memory and a processor, wherein the processor is used for operating the program instructions stored in the memory so as to realize the direct current power electronic transformer control method.
The direct current power electronic transformer and the control method and the device thereof have the advantages that: on the basis of not influencing the output value (namely the correction value) of the Boost current loop, a feedforward link is added in the control strategy of the Boost module, namely, the initial control quantity is added, the difference value of the initial control quantity and the correction value is used as a control signal (namely, the Boost duty ratio) of the Boost module, and the initial control quantity is correspondingly changed along with the change of the number of actually input units (namely, the number of all power conversion units in the input state), so that even if the number of actually input units is changed, the Boost duty ratio can not obviously change, the stability of the control system is improved, and the condition that the control system is unstable or poor in regulation effect due to the change of the number of actually input units is effectively avoided.
Further, in the above dc power electronic transformer and the control method and apparatus thereof, the feedback value of the Boost voltage loop is an average value of the Boost bus voltages of all the power conversion units in the input state.
The average value of the Boost bus voltages of all power conversion units in the input state is used as the feedback value of the Boost voltage ring, namely the feedback value of the Boost voltage ring is only related to the Boost bus voltage of the unit which is actually input, so that the influence of the voltage of a fault unit on the calculation of the feedback value of the Boost voltage ring can be eliminated, and the stability and the control effect of the control system are further improved.
Further, in the above-described DC power electronic transformer and the control method and apparatus thereof, the feedback value of the DC/DC current loop is an average value of the output currents of all the power conversion units in the on state.
The average value of the output currents of all the power conversion units in the switching state is used as the feedback value of the DC/DC current loop, namely the feedback value of the DC/DC current loop is only related to the output current of the unit which is actually switched in, so that the influence of the current of the fault unit on the calculation of the feedback value of the DC/DC current loop can be eliminated, and the stability and the control effect of the control system are further improved.
Further, in the above described direct current power electronic transformer and the control method and apparatus thereof, a ratio of a measured value of a low voltage bus voltage to a voltage value of a Boost bus of a controlled unit is obtained, and the obtained ratio is multiplied by a transformation ratio of a transformer in a DC/DC conversion module, and the obtained product is a voltage gain of the DC/DC conversion module.
The voltage gain of the DC/DC conversion module is calculated by the actual voltage values (namely the measured value of the low-voltage bus voltage and the voltage value of the Boost bus of the controlled unit) at two sides of the DC/DC conversion module and the transformation ratio of a transformer in the DC/DC conversion module, when the voltages at two sides of the DC/DC conversion module fluctuate, the voltage gain of the DC/DC conversion module can change automatically, so that the influence on the output value of a DC/DC current loop is reduced, the aim of improving the stability of a control system is further achieved, and the condition that the regulation effect of the control system is poor when the voltage of the Boost bus or the voltage of the low-voltage side bus fluctuates is effectively avoided.
Further, in the above dc power electronic transformer and the control method and apparatus thereof, the switching switch includes a mechanically controllable switch and a bidirectional thyristor, which are arranged in parallel.
The switching switch comprises a mechanical controllable switch and a bidirectional thyristor which are arranged in parallel, so that when the bypass or permanent cutting-off operation is performed on a fault unit, the rapidity of the fault bypass is ensured by using the bidirectional thyristor, the reliability of the permanent cutting-off of the fault is ensured by using the mechanical controllable switch, and the loss is reduced.
Further, in the above-mentioned dc power electronic transformer and the control method and apparatus thereof, when determining that any power conversion unit is in a fault state, controlling to close the triac corresponding to the power conversion unit in question, continuously detecting the power conversion unit in question, if it is still determined that the power conversion unit in question is in a fault state within a set time, controlling to close the mechanical controllable switch corresponding thereto, and controlling to open the triac corresponding thereto; and if the power conversion unit with the fault changes from the fault state to the non-fault state within the set time, controlling to disconnect the corresponding bidirectional thyristor.
When a certain power conversion unit is detected to be in a fault state, a rapid bypass operation is carried out on the power conversion unit by using a bidirectional thyristor, then the state of the power conversion unit is continuously detected, and if the power conversion unit is determined to be in a permanent fault state, a permanent cutting operation is carried out on the power conversion unit by using a mechanical controllable switch; if the fault is judged wrongly or the fault is cleared before the fault is found, the execution unit is put into operation again, and a certain time is reserved for the system to clear the fault by using the fault judgment method, so that the system is ensured to have certain fault clearing capacity and self-recovery capacity.
Drawings
FIG. 1 is a prior art topology structure diagram of an ISOP power electronic transformer;
FIG. 2 is a prior art topology diagram of a power conversion unit and a redundant standby unit of an ISOP type power electronic transformer;
FIG. 3-1 is a block diagram of a Boost module control strategy for an ISOP type power electronic transformer in the prior art;
FIG. 3-2 is a block diagram of a control strategy for a DC/DC converter module of an ISOP type power electronic transformer in the prior art;
fig. 4 is a block diagram of a Boost module control strategy of a power electronic transformer in an embodiment of the control method of the invention;
FIG. 5 is a block diagram of a control strategy of a DC/DC conversion module of a power electronic transformer in an embodiment of the control method of the invention;
fig. 6 is a flowchart of a method for determining a fault of a power electronic transformer according to an embodiment of the control method of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
Power electronic transformer embodiment:
the direct-current power electronic transformer (hereinafter referred to as power electronic transformer) of the embodiment comprises X power conversion units, wherein the input ends of the power conversion units are connected in series and then are used for connecting a high-voltage direct-current bus, the output ends of the power conversion units are connected in parallel and then are used for connecting a low-voltage direct-current bus, X is more than or equal to 2, and each power conversion unit comprises a switching switch, a Boost module and a DC/DC conversion module; the power electronic transformer further includes a control device (i.e., a dc power electronic transformer control device), where the control device includes a memory and a processor, and the processor is configured to run a program instruction stored in the memory to implement the control method (i.e., the dc power electronic transformer control method) of the present invention.
In this embodiment, a specific topology structure of each power conversion unit is shown in fig. 2 in detail, where the transformer T in fig. 2 may be an intermediate frequency transformer or a high frequency transformer; as other embodiments, the topology of the power conversion unit may also adopt other structures in the prior art, for example, a filtering module is added at an input signal of a certain part of a certain module (for example, a Boost module or a DC/DC conversion module) of the power conversion unit, for example, an inductance L' is added at the input side of the transformer T in fig. 2.
Control device embodiment:
the control device of this embodiment includes a memory and a processor, where the processor is configured to run a program instruction stored in the memory to implement the control method of the present invention, and details of the control method are described in the control method embodiment and are not described herein again.
The embodiment of the control method comprises the following steps:
the control method of this embodiment is applicable to a power electronic transformer in an embodiment of the power electronic transformer, assuming that the power conversion units of the power electronic transformer in the input state under the full power operation condition are k (k is greater than or equal to 1), and m (m is greater than or equal to 1) power conversion units are operated in the control system as redundant standby units in a hot standby mode, (X is k + m), N is the number of actually input units (i.e., the number of all power conversion units in the input state), and as a faulty unit is bypassed, the value of N changes, and the change range is as follows: k is less than or equal to N and less than or equal to k + m.
When the redundant standby unit runs in the control system in a hot standby mode, the control strategies shown in fig. 3-1 and fig. 3-2 are directly applied, so that the control system is easy to be unstable or have poor regulation effect. To solve this problem, the present embodiment provides a control method, which includes a Boost module control strategy and a DC/DC conversion module control strategy.
As shown in fig. 4, the Boost module adopts a control strategy of a Boost voltage loop, a Boost current loop and a voltage balancing loop, and the control process is as follows (the control strategy is applicable to all the Boost modules in the dc power electronic transformer of the present invention):
a given value V of a Boost voltage loop is obtainedb_erf(namely, the set value of the voltage of the Boost bus) and the feedback value V of the Boost voltage loopb_averMaking a difference, and adjusting the obtained difference value by PIAfter the processing, an output value I of the Boost voltage ring is obtainedbo_r0(ii) a The feedback value V of the Boost voltage loop is converted into a voltage valueb_aver(namely the given value of the voltage equalizing ring) and the Boost bus voltage value V of the controlled unitbi(namely the feedback value of the voltage balancing loop) is subjected to PI regulation processing to obtain a difference value, and the Boost input current value I of the controlled unitboiThrough in FIG. 4
Frame its current positive or negative, if Iboi>0, then
Frame output 1, if Iboi<0, then
The frame outputs-1, and then the output value after PI regulation is multiplied by 1 or-1 to obtain the output value delta I of the voltage balance loopboi(ii) a The output value I of the Boost voltage loop is measuredbo_r0Output value delta I of voltage equalization loopboiSumming to obtain the given value I of the Boost current loopbo_ri，Ibo_ri＝Ibo_r0+ΔIboi(ii) a Setting the given value I of the Boost current loopbo_riBoost input current value I of controlled unitboi(namely the feedback value of the Boost current loop) is subjected to PI regulation processing to obtain the output value of the Boost current loop, and the value is used as a correction value delta Di(ii) a Calculating the voltage measurement value V of the high-voltage direct-current busHVAnd the sum V of the Boost bus voltage values of all the power conversion units in the switching state (namely the actual switching units)bObtaining the initial control quantity D, D ═ VHV/Vb(ii) a Calculating an initial control quantity D and a correction quantity Delta DiThe obtained difference is used as a control signal D of a Boost modulei，Di＝D-ΔDi。
In this embodiment, the feedback value V of the Boost voltage loopb_averThe average value of the Boost bus voltages of all power conversion units in the switching state, namely the sum V of the Boost bus voltages of the actually-switched unitsbRatio to actual number of units charged N, Vb_aver＝Vb/N。
The Boost module control strategy in the embodiment does not influence the correction quantity delta DiOn the basis, a feedforward link is added in a control strategy of a Boost module, namely, an initial control quantity D is added, wherein D is equal to VHV/VbThe initial control quantity D and the correction quantity Delta DiThe difference value is used as a control signal (namely, the Boost duty ratio) of the Boost module, and because the initial control quantity D can be correspondingly changed along with the change of the number N of the actually input units, the Boost duty ratio can not be obviously changed even if the number N of the actually input units is changed, so that the stability of the control system is improved, and the condition that the control system is unstable or the regulation effect is poor due to the change of the number N of the actually input units is effectively avoided.
In this embodiment, the feedback value V of the Boost voltage loopb_averThe method is an average value of the Boost bus voltages of all power conversion units in the input state, namely the feedback value of the Boost voltage ring is only related to the Boost bus voltage of the power conversion units actually input, so that the influence of the voltage of a fault unit on the calculation of the feedback value of the Boost voltage ring is eliminated, and the stability and the control effect of the control system are further improved.
As shown in fig. 5, the DC/DC conversion module adopts a control strategy of a DC/DC voltage loop and a DC/DC current loop, and the control process is as follows (the control strategy is applicable to all DC/DC conversion modules in the DC power electronic transformer of the present invention):
setting the given value V of the DC/DC voltage loopLV_ref(i.e., low voltage bus voltage set point) and feedback value V of DC/DC voltage loopLVThe difference value is subjected to PI adjustment processing to obtain the output value of the DC/DC voltage loop, and the value is used as the given value I of the DC/DC current loopLVr(ii) a Setting the given value I of the DC/DC current loopLVrFeedback value I with DC/DC current loopLV_averMaking a difference, and obtaining an output value P of the DC/DC current loop after the obtained difference is subjected to PI regulation treatmentDAB_ri(ii) a Calculating the voltage measurement value V of the low-voltage busLVBoost bus voltage value V of controlled unitbiMultiplying the ratio n of the transformer T in the DC/DC conversion module by the ratio of (d) to obtain the voltage gain d of the DC/DC conversion modulei，di＝nVLV/Vbi；diAnd PDAB_riModulated signal obtained by modulation strategy0i、1i、2iAnd the control signal is used as a control signal of the DC/DC conversion module.
In this embodiment, the feedback value I of the DC/DC current loopLV_averIs the average value of the output currents of all the power conversion units in the switching state, namely the sum of the output currents of the actually switched units ∑ ILViRatio to the number of units actually charged N, ILV_aver＝∑ILVi/N。
In this embodiment, the feedback value I of the DC/DC current loopLV_averThe average value of the output currents of all the power conversion units in the switching state, namely the feedback value of the DC/DC current loop, is only related to the output current of the actually-switched unit, so that the influence of the current of the fault unit on the calculation of the feedback value of the DC/DC current loop is eliminated, and the stability and the control effect of the control system are further improved.
In this embodiment, the voltage gain of the DC/DC conversion module is calculated from the actual voltage values (i.e., the measured voltage value of the low-voltage bus and the voltage value of the Boost bus of the controlled unit) at the two sides of the DC/DC conversion module, and when the voltages at the two sides of the DC/DC conversion module fluctuate, the voltage gain of the DC/DC conversion module changes automatically, so as to reduce the influence on the output value of the DC/DC current loop, thereby achieving the purpose of improving the stability of the control system, and effectively avoiding the situation that the regulation effect is not good when the voltage of the Boost bus or the voltage of the low-voltage side bus fluctuates.
In this embodiment, a fault determination process of a certain unit in an operation process of a power electronic transformer is shown in fig. 6:
collecting a voltage signal (u), a current signal (i) and a hardware fault signal of a certain unit;
firstly, judging whether a hardware fault signal is effective or not, and if the hardware fault signal is effective, directly executing permanent removal operation on the unit;
if the hardware fault signal is invalid, continuously judging the acquired voltage signal and current signal;
if the voltage signal and the current signal are abnormal, comparing the abnormal existence time t with the set longest fault clearing time Tset, if the abnormal existence time t does not exceed the Tset, executing fault bypass operation on the unit, timing the abnormal existence time t, setting a fault Flag bit Flag to be 1, continuously acquiring the voltage signal, the current signal and the hardware fault signal of the unit for repeated judgment, and once the abnormal existence time t exceeds the Tset, executing permanent cutting-off operation on the unit;
if the voltage signal and the current signal are normal and the front fault Flag is not equal to 1, the unit has no fault and is always in a normal operation state; if the voltage signal and the current signal are normal and the previous fault Flag is 1, it indicates that the unit has an abnormality before, but if the fault is cleared, the unit is put into operation again.
According to the failure determination flow shown in fig. 6, if it is determined that the failure is cleared, the unit is thrown again, and if the failure is not cleared for the set time Tset, the unit is considered to be permanently abnormal, and the unit is permanently removed; and a certain time is reserved for the system to realize fault clearing in a fault judgment link, so that the system has certain fault clearing capacity and self-recovery capacity.
The following describes in detail the process of performing a fault bypass operation, a fault permanent removal operation and a reset operation on a certain unit by using a switching switch with reference to fig. 2.
(1) The process of performing a fault bypass operation on a cell is as follows:
according to the fault determination process shown in fig. 6, after it is determined that the unit is to be subjected to the fault bypass operation, the driving signals of all IGBT switching tubes in the unit are blocked, the triac T3 is triggered to conduct (i.e., the triac T3 is closed), the actual number N of input units in the control system is updated in time, and the stability of the control system is ensured according to the control strategies shown in fig. 4 and 5. In addition, no matter the power flows in the forward and reverse directions at the moment, a reliable bypass effect can be realized under the assistance of a half-bridge arm of the Boost module. The specific principle is as follows:
for the unit, in the time (mu s level) when the IGBT switch tube is driven to be blocked but the bidirectional thyristor T3 is not completely conducted, if power flows in the forward direction, the IGBT switch tubes on two sides in the DC/DC module are closed and have no power transmission, input current can momentarily charge an output bus capacitor of a half-bridge arm through an anti-parallel diode D1 of an upper tube Sb1 of the half-bridge arm of the Boost module, and the voltage of the bus capacitor at the moment rises momentarily in small amplitude, however, the difference of the response time is small enough, the voltage of the bus capacitor rises extremely limitedly, and the device or the bus capacitor cannot be influenced; if the power flows in the reverse direction, at this time, because the IGBT switching tubes on the two sides in the DC/DC module are turned off and no power is transferred, the input current flows through the anti-parallel diode D2 of the lower tube Sb2 of the Boost half-bridge arm, and the unit is directly bypassed by the anti-parallel diode D2, thereby achieving the effect of reliable bypass.
The bypass of the fault unit is realized by adopting the bidirectional thyristor T3, and the bidirectional thyristor T3 has the advantage of high response speed.
(2) The process of performing a permanent fault removal operation on a unit is as follows:
according to the failure determination flow shown in fig. 6, it is determined that the permanent cut-off operation is to be performed on the unit, and there are two cases:
case 1: if the unit is in a bypass state before abnormality and the unit abnormality existing time t exceeds Tset, the unit can be judged to be in a permanent fault state. At this time, a mechanical switch closing signal is sent to the mechanical controllable switch K1, and after the mechanical controllable switch K1 is reliably closed (for example, after the mechanical switch closing signal is sent for a set time), the triac T3 is turned off, that is, the unit is permanently cut off. The input current now flows entirely through the mechanically controllable switch K1 (corresponding to the transfer of input current from the triac T3 to the mechanically controllable switch K1).
The permanent cutting off of the fault unit is realized by adopting the mechanical controllable switch K1, and the advantage of less loss is realized.
Case 2: the unit hardware fault signal is active. At this time, the driving signals of all IGBT switching tubes in the unit are blocked, and the bidirectional thyristor T3 is triggered to be conducted; after the triac T3 is turned on, a mechanical switch closing signal is sent to the mechanical controllable switch K1, and after the mechanical controllable switch K1 is reliably closed, the triac T3 is turned off, so that the unit is permanently cut off.
The quick bypass of the fault unit is realized by using the bidirectional thyristor T3, and the permanent removal of the fault unit is realized by using the mechanical controllable switch K1, so that on one hand, when the fault occurs, the fault unit can be removed from the system quickly, and on the other hand, the reliability of fault removal is also ensured.
(3) The process of performing a reentering operation on a unit is as follows:
according to the fault determination flow shown in fig. 6, after the unit is determined to be in the bypass state after the fault is cleared, that is, the unit is in the bypass state before the fault is cleared, and now the unit is to be put into operation again after the fault is cleared, the triac T3 is turned off first, and after the triac T3 is turned off reliably (for example, after a set time is given by a command for turning off the triac), the driving signals of all IGBT switching tubes are turned on, and the actual number N of put-in units and the carrier phase information in the control system are updated in time, so as to ensure the stability of the control system according to the control strategy shown in fig. 4 and 5.
According to the control strategy shown in fig. 4 and 5, when a unit is bypassed or permanently cut off or re-put, the put-in and cut-off states S of the units in the control system are updated in timeiAnd the actual number of units N put into the control system. After N is updated, the feedback value V of the Boost voltage ringb_averInitial control quantity D, DC/feedback value I of DC current loopLV_averWill also be updated at the same time; at this time, the carrier phase of each unit Boost module
Adjustment is also needed to realize control and regulation, and the specific principle is as follows:
in order to reduce the inductance value of the direct current inductance of the Boost modules, the Boost modules of all the units adopt a staggered parallel connection mode,so as to improve the equivalent switching frequency of the inductor and reduce the input current fluctuation. After the unit is bypassed or permanently cut off or put into use again, the carrier phase of each unit Boost module also needs to be adjusted to keep the consistency and balance of the input current, and the carrier phase of the jth unit Boost module
The calculation formula of (a) is as follows:
wherein S isiFor the input and cut-off states of each unit, S is used when the unit is inputiIs 1, otherwise SiIs 0; and X is the sum of all the units which are put into and cut off (namely the total number of the power conversion units of the power electronic transformer).
The fault judgment method solves the problems of short circuit of the direct current bus capacitor at the serial side and possible system instability in the bypass process of the power electronic transformer fault unit, has good bypass control effect on various systems for controlling the power taking mode and various possible faults in each subunit in the system, and is high in reliability and wide in engineering applicability.
Claims (8)
1. A control method of a direct current power electronic transformer comprises X power conversion units, wherein the input ends of the power conversion units are connected in series and then are used for being connected with a high-voltage direct current bus, the output ends of the power conversion units are connected in parallel and then are used for being connected with a low-voltage direct current bus, X is more than or equal to 2, and each power conversion unit comprises a switching switch, a Boost module and a DC/DC conversion module; it is characterized in that the preparation method is characterized in that,
the Boost module comprises a full-control device valve bank branch, the full-control device valve bank branch comprises two full-control device valve banks which are connected in series, the fling-cut switch is connected with one of the full-control device valve banks in parallel, and the full-control device valve bank branch is connected with the DC/DC conversion module in parallel;
the Boost module adopts a control strategy of a Boost voltage ring, a Boost current ring and a voltage balancing ring; the given value of the Boost voltage ring is a set value of the Boost bus voltage, and the feedback value of the Boost voltage ring is an average value of the Boost bus voltage of each power conversion unit; the given value of the voltage balance ring is the feedback value of the Boost voltage ring, and the feedback value of the voltage balance ring is the voltage value of the Boost bus of the controlled unit; adding the output value of the Boost voltage loop and the output value of the voltage balancing loop to obtain a value serving as a given value of a Boost current loop, wherein the feedback value of the Boost current loop is the Boost input current value of the controlled unit; calculating the ratio of the voltage measurement value of the high-voltage direct-current bus to the sum of the voltage values of Boost buses of all power conversion units in the input state to obtain an initial control quantity, and subtracting the output value of a Boost current loop from the initial control quantity to be used as a control signal of a Boost module;
the DC/DC conversion module adopts a control strategy of a DC/DC voltage loop and a DC/DC current loop; the given value of the DC/DC voltage loop is a low-voltage bus voltage set value, the feedback value of the DC/DC voltage loop is a low-voltage bus voltage measured value, the output value of the DC/DC voltage loop is used as the given value of the DC/DC current loop, the feedback value of the DC/DC current loop is the average value of the output current of each power conversion unit, and the output value of the DC/DC current loop and the voltage gain of the DC/DC conversion module are used as control signals of the DC/DC conversion module.
2. The direct-current power electronic transformer control method according to claim 1, wherein the feedback value of the Boost voltage loop is an average value of Boost bus voltages of all power conversion units in the on state.
3. A DC power electronic transformer control method according to claim 1, characterized in that the feedback value of the DC/DC current loop is the average of the output currents of all power conversion units in the on-state.
4. The direct-current power electronic transformer control method according to claim 1, characterized by obtaining a ratio of a low-voltage bus voltage measurement value to a Boost bus voltage value of a controlled unit, multiplying the obtained ratio by a transformation ratio of a transformer in a DC/DC conversion module, and obtaining a product as a voltage gain of the DC/DC conversion module.
5. A method as claimed in claim 1, wherein the switching switches comprise a mechanically controllable switch and a triac arranged in parallel.
6. The method according to claim 5, wherein when any power conversion unit is determined to be in a fault state, the triac corresponding to the power conversion unit with the fault is controlled to be closed, the power conversion unit with the fault is continuously detected, and if the power conversion unit with the fault is still determined to be in the fault state within a set time, the mechanical controllable switch corresponding to the power conversion unit is controlled to be closed, and the triac corresponding to the power conversion unit is controlled to be opened; and if the power conversion unit with the fault changes from the fault state to the non-fault state within the set time, controlling to disconnect the corresponding bidirectional thyristor.
7. A dc power electronic transformer control apparatus comprising a memory and a processor for executing program instructions stored in the memory to implement the dc power electronic transformer control method of any one of claims 1 to 6.
8. A direct current power electronic transformer comprises X power conversion units, wherein the input ends of the power conversion units are connected in series and then are used for being connected with a high-voltage direct current bus, the output ends of the power conversion units are connected in parallel and then are used for being connected with a low-voltage direct current bus, X is more than or equal to 2, and each power conversion unit comprises a switching switch, a Boost module and a DC/DC conversion module; characterized in that said power electronic transformer further comprises a power electronic transformer control means comprising a memory and a processor for executing program instructions stored in said memory for implementing the direct current power electronic transformer control method of any one of claims 1 to 6.
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