CN115776225B - Redundant fault-tolerant control method for power electronic transformer - Google Patents
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Abstract
A redundant fault-tolerant control method for a power electronic transformer comprises the following steps: calculating the number of fault power modules allowed by redundancy; calculating N paths of carrier phase-shifting pulse width modulation signals and distributing the N paths of carrier phase-shifting pulse width modulation signals to N power modules in input states; monitoring the working state of each power module in real time, and identifying the fault power module; judging whether the cascade power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition or not; after the blocking fault power module is calculated, the low-voltage stage voltage is calculated when the intermediate frequency transformer with the intermediate tap is not switched, the low-voltage stage voltage is compared with the rated voltage of the low-voltage stage, the change proportion of the low-voltage stage voltage is calculated, the change proportion of the low-voltage stage voltage is compared with the corresponding proportion of each intermediate tap of the intermediate frequency transformer with the intermediate tap, and the intermediate tap with the smallest difference is selected as a switching target. The invention has low cost, simple control and high reliability.
Description
Technical Field
The invention relates to the technical field of power electronic transformers, in particular to a redundant fault-tolerant control method of a power electronic transformer.
Background
In the field of power systems, conventional transformers convert high-voltage delivered electrical energy into low-voltage power for use by consumers. However, the power frequency transformer has large volume and weight, low efficiency and low power density, increases operation energy consumption and maintenance cost, has uncontrollable power factor and output voltage, and is easy to bring about harmonic pollution and reactive power pollution of a power grid. In order to change various problems brought by the power frequency transformer, a power electronic transformer has been developed. Besides the voltage class conversion and electric isolation functions of the traditional transformer, the power electronic transformer has multiple functions of flow bidirectional control, electric energy quality control, self-protection and self-diagnosis of devices, communication and information exchange, and the like, has alternating current and direct current ports with different voltage classes, and is suitable for flexible access of various types of distributed energy sources, energy storage and loads and interconnection of alternating current/direct current power grids. The power electronic transformer plays an irreplaceable important role in a smart grid, an energy internet and a future alternating current-direct current interconnection grid, and is a core device for realizing electric energy conversion and processing.
The power electronic transformer converts high-voltage alternating current into multi-unit direct current through an AC/DC (alternating current/direct current) converter, then realizes electric isolation and energy transfer through a plurality of unit/high-frequency isolation type DC/DC (direct current/direct current) converters, obtains direct current at parallel output sides thereof, and then generates low-voltage alternating current through inversion of the DC/AC (direct current/alternating current) converter. The reliability of the power electronic transformer device itself is particularly important for safe and reliable operation of the distribution network. Because the power electronic transformer is a multi-unit cascade topology, the stable operation of the whole power electronic transformer can be influenced by the failure of any one unit. When designing the power electronic transformer, the selected power electronic device will set a certain voltage and current margin, generally the low voltage element is 50%, the high voltage element can reach 100%, so the voltage and current margin can be regarded as a redundant part of the power unit in the power electronic transformer, and the reliability of the power electronic transformer can be effectively improved by utilizing the redundant part and the corresponding fault tolerance technology.
At present, two redundancy schemes aiming at a multi-unit structure exist, one redundancy scheme is that one or more units are used as standby redundancy units, the redundancy units do not participate in working when the power electronic transformer normally operates, when one unit fails, the redundancy units replace the failure units to work, the failure units are out of operation, and the transmission power of each unit does not change. The other scheme is that all units are put into normal operation, the fault unit is cut off when the fault occurs, and the power and the voltage of the fault unit are shared by the rest working units, namely the redundant voltage and current margin of the power electronic element is utilized. The redundant units in the first scheme not only increase the hardware cost of the system, but also increase the complexity of the control circuit, and cause impact to the system when the fault unit is removed and the standby unit is put into operation. Compared with the first scheme, the second scheme is simpler in structure, has no new hardware cost, and cannot impact the system due to the input of a fault unit; however, due to the changed number of operating units, the control parameters of the DC/AC (direct current/alternating current) converter are changed when the power and voltage are re-leveled, so that the DC/AC (direct current/alternating current) converter works under the working conditions of over-high voltage and over-high power, and very high requirements are imposed on the voltage and current stress of the power electronic devices and the control system.
Disclosure of Invention
The invention aims to overcome the defects and problems of high hardware cost, complex control and low system reliability in the prior art and provides a redundant fault-tolerant control method for a power electronic transformer, which has the advantages of low hardware cost, simple control and high system reliability.
In order to achieve the above object, the technical solution of the present invention is: the power electronic transformer comprises N power modules, wherein the input ends of the N power modules are connected in series, the output ends of the N power modules are connected in parallel, the power modules comprise a high-voltage side H-bridge converter, an isolated DC/DC converter and a low-voltage side H-bridge converter, the input ends of the N high-voltage side H-bridge converters are connected in series, the output ends of the N high-voltage side H-bridge converters are not connected with each other, the output ends of the N low-voltage side H-bridge converters are connected in parallel, the input ends of the N low-voltage side H-bridge converters are not connected with each other, the isolated DC/DC converter comprises an intermediate-frequency transformer with a middle tap, the input ends of the isolated DC/DC converter are connected with the output ends of the high-voltage side H-bridge converter, and the output ends of the isolated DC/DC converter are connected with the input ends of the low-voltage side H-bridge converter; the control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy;
s2, calculating N paths of carrier phase-shifting pulse width modulation signals, and distributing the N paths of carrier phase-shifting pulse width modulation signals to N power modules in input states;
s3, monitoring the working state of each power module in real time, and identifying a fault power module;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition; if yes, jumping to the step S5; if not, a tripping command is sent, and the power electronic transformer cascade power circuit is stopped and is withdrawn from operation;
s5, a bypass instruction is issued to the fault power module to lock the fault power module, after the fault power module is locked, the low-voltage level voltage is calculated when the intermediate frequency transformer with the intermediate tap is not switched, then the low-voltage level voltage is compared with the rated voltage of the low-voltage level, the change proportion of the low-voltage level voltage is calculated, then the change proportion of the low-voltage level voltage is compared with the corresponding proportion of each intermediate tap of the intermediate frequency transformer with the intermediate tap, the intermediate tap with the minimum difference is selected as a switching target, and then the isolated DC/DC converter is required to select the intermediate taps of the intermediate frequency transformer with the intermediate tap, corresponding to the number of the fault power module.
In step S1, the method for calculating the number of fault power modules allowed by redundancy includes:
first calculating the base of each device of the power electronic transformerThe margin is selected, the minimum basic margin is selected as the reference margin, and the number of fault power modules allowed by redundancy is calculated by adopting the following formula
；
Wherein,,
for the number of power modules +.>
Is the smallest basic margin among the basic margins of all devices of the power electronic transformer.
In step S4, the power electronic transformer cascade power circuit meeting the redundant fault-tolerant operation condition means that: the number of the fault power modules is smaller than or equal to the number of the fault power modules allowed by redundancy.
In step S5, a bypass switch is connected to the ac terminal of the high-voltage side H-bridge converter
The low-voltage side H-bridge converter and the breaking switch>
The power electronic transformer is connected with the output end of the power electronic transformer after being connected in series;
the step of issuing a bypass instruction to the fault power module means that: closed bypass switch
Breaking switch->
。
In step S5, the low-voltage stage voltage change ratio
The calculation formula of (2) is as follows:
wherein,,
for the number of power modules +.>
Is the number of failed power modules.
The isolated DC/DC converter comprises a high-voltage stage chopper converter and a low-voltage stage inverter converter, and the high-voltage stage chopper converter is connected with the low-voltage stage inverter converter through an intermediate frequency transformer with a middle tap.
Compared with the prior art, the invention has the beneficial effects that:
in the redundant fault-tolerant control method of the power electronic transformer, firstly, only the fault power module is cut off, the redundant module and the investment thereof are not involved, and the hardware cost is saved; secondly, after the power module fails, a bypass is closed without changing a phase shift angle, so that a control algorithm is simplified; and finally, after the fault power module is cut off, the low-voltage direct-current bus voltage of each path of isolation type DC/DC converter can rise due to the fact that the direct-current bus voltage of the fault power module is split, the original direct-current bus voltage can be maintained unchanged by changing the transformer transformation ratio through a middle tap, the power electronic devices of the isolation type DC/DC converter and the low-voltage side H-bridge converter are prevented from working under higher voltage stress, and the reliability of the system is improved. Therefore, the invention has low cost, simple control and high reliability.
Drawings
Fig. 1 is a schematic circuit diagram of a single-phase power electronic transformer in accordance with the present invention.
FIG. 2 is a flow chart of a redundant fault tolerant control method for a power electronic transformer in accordance with the present invention.
In the figure: power module 1, high-side H-bridge converter 101, isolated DC/DC converter 102, low-side H-bridge converter 103, intermediate frequency transformer with intermediate tap 104, bypass switch
105. Breaking switch->
106. A high-voltage chopper converter 107, and a low-voltage inverter converter 108. Concrete embodiments
The invention is described in further detail below with reference to the accompanying drawings and detailed description.
Referring to fig. 1, the power electronic transformer includes N power modules 1, N input ends of the power modules 1 are connected in series, output ends of the power modules 1 are connected in parallel, the power module 1 includes a high-voltage side H-bridge converter 101, an isolated DC/DC converter 102 is connected in series with an input end of a low-voltage side H-bridge converter 103, output ends of the N high-voltage side H-bridge converters 101 are not connected to each other, output ends of the N low-voltage side H-bridge converters 103 are connected in parallel, input ends of the N low-voltage side H-bridge converters are not connected to each other, the isolated DC/DC converter 102 includes an intermediate-frequency transformer 104 with an intermediate tap, an input end of the isolated DC/DC converter 102 is connected to an output end of the high-voltage side H-bridge converter 101, and an output end of the isolated DC/DC converter 102 is connected to an input end of the low-voltage side H-bridge converter 103; the ac terminal of the high-voltage side H-bridge converter 101 is connected with a bypass switch
105, the low-voltage side H-bridge converter 103 and the breaking switch +.>
106 are connected in series and then are connected with the output end of the power electronic transformer; the isolated DC/DC converter 102 includes a high-voltage stage chopper converter 107 and a low-voltage stage inverter converter 108, and the high-voltage stage chopper converter 107 is connected to the low-voltage stage inverter converter 108 through a mid-tap intermediate-frequency transformer 104.
Referring to fig. 2, a redundant fault-tolerant control method for a power electronic transformer includes the following steps:
s1, calculating the number of fault power modules allowed by redundancy;
the method for calculating the number of fault power modules allowed by redundancy comprises the following steps:
the basic margin of each device of the power electronic transformer is calculated firstly, then the minimum basic margin is selected as the reference margin, and then the number of fault power modules allowed by redundancy is calculated by adopting the following formula
；
Wherein,,
for the number of power modules 1 +.>
The minimum basic margin in the basic margins of all devices of the power electronic transformer is provided;
s2, calculating N paths of carrier phase-shifting pulse width modulation signals, and distributing the N paths of carrier phase-shifting pulse width modulation signals to N power modules 1 in input states;
s3, monitoring the working state of each power module 1 in real time, and identifying a fault power module;
s4, judging whether the power electronic transformer cascade power circuit meets the redundant fault-tolerant operation condition, namely whether the number of fault power modules is smaller than or equal to the number of fault power modules allowed by redundancy; if yes, jumping to the step S5; if not, a tripping command is sent, and the power electronic transformer cascade power circuit is stopped and is withdrawn from operation;
s5, firstly, a bypass instruction is issued to the fault power module, namely a bypass switch is closed
105, breaking the breaking switch->
106, after the fault power module is locked, the low-voltage stage voltage is calculated by the intermediate frequency transformer 104 with the intermediate tap when the tap is not switched, and then the low-voltage stage voltage is in phase with the rated voltage of the low-voltage stageCalculating the low-voltage level voltage change proportion, comparing the low-voltage level voltage change proportion with the corresponding proportion of each intermediate tap of the intermediate-tap intermediate-frequency transformer 104, selecting the intermediate tap with the smallest difference as a switching target, and then requiring the isolated DC/DC converter 102 to select the intermediate taps of the intermediate-tap intermediate-frequency transformer 104 corresponding to the number of fault power modules;
the low-voltage stage voltage change ratio
The calculation formula of (2) is as follows:
wherein,,
for the number of power modules 1 +.>
Is the number of failed power modules.
Example 1:
in the embodiment, a 10 kilovolt power electronic transformer prototype of 500 kilovolt ampere applied to the site of a Chinese Baowu Wu Steel group Limited company is taken as an example for illustration, and a single-phase circuit of the power electronic transformer is formed by cascading 6 power modules and can independently operate, and the input phase voltage is 5774VAC; the high-voltage direct current bus of the high-voltage side H-bridge converter has a rated voltage of 1500V, an IGBT with a rated voltage of 3300V is adopted, the low-voltage direct current bus of the isolated DC/DC converter has a rated voltage of 400V, and the low-voltage part of the converter and the low-voltage side H-bridge converter both adopt IGBTs with rated voltage of 650V; it can be seen that the withstand voltage design margins of the high-side and low-side power electronic switching devices are 120% and 62.5%, respectively, and thus, the reference margin may be set to 62.5%.
The transformation ratio of the normal tap of the intermediate frequency transformer with the intermediate tap is 3.75:1, the taps of the intermediate frequency transformer with the intermediate tap are arranged one every 1 percent, namely 41 taps are arranged from 0 percent to-40 percent, and the power electronic transformer is required to be connected in seriesNumber of power modules
。
As can be seen from the above, when
When the tap is not switched, the low-voltage level (secondary) voltage +.>
The voltage change amplitude is 100 percent, which is more than->
The number of faulty power modules that the system can withstand at maximum +.>
。
The cascade H-bridge circuit of the high-voltage alternating-current side of the power electronic transformer adopts a carrier phase-shifting pulse width modulation method, and corresponding proper middle taps of the intermediate-frequency transformer with middle taps are respectively arranged according to 1-path faults, 2-path faults and m-path faults; once 1, 2 and m power modules have faults, a locking bypass instruction is required to be executed to bypass the fault power module, and meanwhile, a middle tap corresponding to the faults on the middle-tap medium-frequency transformer is put into the middle tap, so that the voltage of a low-voltage direct-current bus of the middle/high-frequency isolation type DC/DC converter is basically kept unchanged.
In the invention, as long as the fault-tolerant operation condition is satisfied, the number of fault power modules is less than or equal to the number of redundant fault-tolerant power modules
The fault power module can be bypassed and put into the middle tap with the corresponding fault on the middle tap intermediate frequency transformerSmooth fault-tolerant operation of the power electronic transformer is realized; the specific control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy
；
S2, calculating 6 paths of carrier phase-shifting pulse width modulation signals, and distributing the 6 paths of carrier phase-shifting pulse width modulation signals to the power modules in 6 input states;
s3, monitoring the working state of each power module in real time, and identifying that 1 power module fails;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition, namely the number of fault power modules
Whether or not the number of fault power modules allowed by redundancy is less than or equal to +.>
The method comprises the steps of carrying out a first treatment on the surface of the If the redundant fault-tolerant operation condition is met, jumping to the step S5;
s5, firstly, a bypass instruction is issued to the fault power module to lock the fault power module, at the moment, the system is changed from 6-path cascade operation to 5-path cascade operation, the input current ripple of the system can be increased to some extent, the basic operation is not influenced, and the total DC bus voltage is kept unchanged
) The rest 5 high-voltage side H-bridge converters increase the bus voltage drop of each high-voltage side H-bridge converter to +.>
In the IGBT withstand voltage range still lower than the rated 3300V, fault-tolerant operation can be realized; after the blocking fault power module is calculated, the low-voltage stage voltage is calculated when the intermediate-frequency transformer with the intermediate tap does not switch taps, the low-voltage stage voltage is compared with the rated voltage of the low-voltage stage, the change proportion of the low-voltage stage voltage is calculated, and then the low-voltage stage voltage is poweredThe voltage change proportion is compared with the corresponding proportion of each intermediate tap of the intermediate-tap intermediate-frequency transformer, the intermediate tap with the smallest difference is selected as a switching target, and then the isolated DC/DC converter is required to select the intermediate taps of the intermediate-tap intermediate-frequency transformer corresponding to the number of fault power modules.
Thus, after comparing all of the taps 0% to-40%, it is determined that the tap 17% is closest to
Then a tap switch instruction of-17% of the intermediate frequency transformer with intermediate taps is sent and switched to-17% taps.
At this time, since the high-voltage bus voltage rises to 1800V, the low-voltage DC bus voltage of the isolated DC/DC converter is equal to that before-17% tap is switched
The rated operating voltage of the H-bridge converter exceeds 400V by 20 percent, the voltage stress of corresponding devices is increased, and the controller is greatly influenced. After switching-17% taps, the low-voltage DC bus voltage of the isolated DC/DC converter is +.>
Compared with 400V, the voltage is only 1.6V lower, namely 0.4%, so that the smoothness of fault switching can be effectively ensured, and the voltage stress of power electronic devices of the isolated DC/DC converter and the low-voltage side H-bridge converter is basically maintained unchanged.
Example 2:
if a serious 2-path power module fails at the same time, the specific control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy
；
S2, calculating 6 paths of carrier phase-shifting pulse width modulation signals, and distributing the 6 paths of carrier phase-shifting pulse width modulation signals to the power modules in 6 input states;
s3, monitoring the working state of each power module in real time, and identifying that 2 power modules have faults;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition, namely the number of fault power modules
Whether or not the number of fault power modules allowed by redundancy is less than or equal to +.>
The method comprises the steps of carrying out a first treatment on the surface of the If the redundant fault-tolerant operation condition is met, jumping to the step S5; />
S5, firstly, a bypass instruction is issued to the fault power module to lock the fault power module, at the moment, the system is changed from 6-path cascade operation to 4-path cascade operation, the input current ripple of the system can be increased to some extent, the basic operation is not influenced, and the total DC bus voltage is kept unchanged
) The voltage drop of the bus of each high-voltage side H-bridge converter is increased to +.A residual 4-path high-voltage side H-bridge converter is added due to the DC bus voltage of the high-voltage side H-bridge converter of a shared fault path>
In the IGBT withstand voltage range still lower than the rated 3300V, fault-tolerant operation can be realized; and after the blocking fault power module is calculated, the low-voltage stage voltage is calculated when the intermediate frequency transformer with the intermediate tap is not switched, the low-voltage stage voltage is compared with the rated voltage of the low-voltage stage, the voltage change proportion of the low-voltage stage is calculated, the voltage change proportion of the low-voltage stage is compared with the corresponding proportion of each intermediate tap of the intermediate frequency transformer with the intermediate tap, the intermediate tap with the smallest difference is selected as a switching target, and then the isolated DC/DC converter is required to select the intermediate taps of the intermediate frequency transformer with the intermediate tap, corresponding to the number of the fault power module.
Thus, after comparing all of the taps from 0% to-40%, it is determined that the tap of-33% is closest to
Then a tap switch instruction of-33% with the intermediate tap intermediate frequency transformer is sent and switched to the tap of-33%.
At this time, the high-voltage bus voltage rises to 2250V, so that the low-voltage DC bus voltage of the isolated DC/DC converter is before the-33% tap is switched
The rated operating voltage of the H-bridge converter at the low voltage side exceeds 400V by 50%, the rated operating voltage limit of the IGBT is approximated to 650V, the voltage stress of corresponding devices is increased, and the controller is greatly influenced. After switching-33% taps, the isolated DC/DC converter has a low voltage DC bus voltage of
Compared with 400V which exceeds 2V, namely 0.5%, the method can effectively ensure the smoothness of fault switching and basically maintain the voltage stress of power electronic devices of the isolated DC/DC converter and the low-voltage side H-bridge converter unchanged.
The above method can be applied to three-phase power electronic transformers in a generalized manner, and the taps may not be limited to 0% to-40%, and one tap may be provided every 1%.
After the system has a power module fault, the fault power module can be bypassed as long as fault-tolerant operation conditions are met, namely, the number of the fault power modules is smaller than or equal to the voltage and current margin of each device, and a proper middle tap of the middle-tap middle-frequency transformer of the middle/high-frequency isolation type DC/DC converter is selected according to the specific fault condition, so that the transformation ratio of the middle-frequency transformer is changed, the voltage change of a direct-current bus of the low-voltage side H-bridge converter is greatly reduced, the power electronic transformer is smoothly and redundantly switched, and the risk of voltage stress rise of the power electronic devices of the low-voltage side H-bridge converter is reduced.
Claims (2)
1. A redundant fault-tolerant control method for a power electronic transformer is characterized by comprising the following steps of:
the power electronic transformer comprises N power modules (1), wherein the input ends of the N power modules (1) are connected in series, the output ends of the N power modules are connected in parallel, the power modules (1) comprise a high-voltage side H-bridge converter (101) and an isolated DC/DC converter (102) which are connected in series with a low-voltage side H-bridge converter (103), the input ends of the N high-voltage side H-bridge converters (101) are connected in series, the output ends of the N high-voltage side H-bridge converters (101) are not connected with each other, the output ends of the N low-voltage side H-bridge converters (103) are connected in parallel, the input ends of the N isolated DC/DC converters (102) are not connected with each other, the isolated DC/DC converter (102) comprises an intermediate-frequency transformer (104) with an intermediate tap, and the input end of the isolated DC/DC converter (102) is connected with the output end of the high-voltage side H-bridge converter (101), and the output end of the isolated DC/DC converter is connected with the input end of the low-voltage side H-bridge converter (103).
The control method comprises the following steps:
s1, calculating the number of fault power modules allowed by redundancy;
firstly, calculating basic margin of each device of the power electronic transformer, then selecting the minimum basic margin as a reference margin, and then adopting the following method to calculate the number m of fault power modules allowed by redundancy;
wherein N is the number of the power modules (1), M min The minimum basic margin in the basic margins of all devices of the power electronic transformer is provided;
s2, calculating N paths of carrier phase-shifting pulse width modulation signals, and distributing the N paths of carrier phase-shifting pulse width modulation signals to N power modules (1) in input states;
s3, monitoring the working state of each power module (1) in real time, and identifying a fault power module;
s4, judging whether the cascade power circuit of the power electronic transformer meets the redundant fault-tolerant operation condition; if yes, jumping to the step S5; if not, a tripping command is sent, and the power electronic transformer cascade power circuit is stopped and is withdrawn from operation;
the power electronic transformer cascade power circuit meeting the redundant fault-tolerant operation condition means that: the number of the fault power modules is smaller than or equal to the number of the fault power modules allowed by redundancy;
s5, a bypass instruction is issued to the fault power module to lock the fault power module, after the fault power module is locked, the low-voltage level voltage is calculated when the intermediate frequency transformer (104) with the intermediate tap is not switched, then the low-voltage level voltage is compared with the rated voltage of the low-voltage level, the change proportion of the low-voltage level voltage is calculated, then the change proportion of the low-voltage level voltage is compared with the corresponding proportion of each intermediate tap of the intermediate frequency transformer (104) with the intermediate tap, the intermediate tap with the smallest difference is selected as a switching target, and then the isolated DC/DC converter (102) is required to select the intermediate taps of the intermediate frequency transformers (104) with the intermediate taps corresponding to the number of the fault power module;
the alternating current terminal of the high-voltage side H-bridge converter (101) is connected with a bypass switch K p (105) The low-voltage side H-bridge converter (103) and the breaking switch K f (106) The power electronic transformer is connected with the output end of the power electronic transformer after being connected in series;
the step of issuing a bypass instruction to the fault power module means that: closing bypass switch K p (105) Breaking switch K f (106)；
The low-voltage stage voltage change ratio R T The calculation formula of (2) is as follows:
wherein N is the number of the power modules (1), N EROR Is the number of failed power modules.
2. The power electronic transformer redundancy fault-tolerant control method according to claim 1, wherein: the isolated DC/DC converter (102) comprises a high-voltage stage chopper converter (107) and a low-voltage stage inverter converter (108), wherein the high-voltage stage chopper converter (107) is connected with the low-voltage stage inverter converter (108) through an intermediate-frequency transformer (104) with a middle tap.
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