CN108387768B - Hybrid MMC module capacitance and voltage measuring method based on master-slave structure - Google Patents
Hybrid MMC module capacitance and voltage measuring method based on master-slave structure Download PDFInfo
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Abstract
The invention discloses a hybrid MMC module capacitance and voltage measuring method based on a master-slave structure, and belongs to the technical field of modular multilevel converter submodule capacitance and voltage measurement. The method classifies sampling moments according to the level change rule of bridge arm output voltage and the running state of each submodule, provides a submodule capacitor voltage calculation method with higher accuracy for each type of sampling moments by utilizing measurement data and the variable quantity of the measurement data of a master voltage sensor and a slave voltage sensor, and the switch state of the submodule at the adjacent sampling moments and the capacitance voltage variable quantity of the submodule with the changed switch state, compensates voltage drop caused by energy loss of an additional circuit, and improves the capacitance voltage measurement accuracy of the submodule while reducing the hardware complexity of a system. The method not only realizes the voltage measurement function but also can detect the short-circuit fault of a single module in the sub-module pre-charging stage; the master-slave structure can reliably measure in the process of clearing the direct-current short-circuit fault, so that the safety of equipment is ensured.
Description
Technical Field
The invention discloses a hybrid MMC module capacitance and voltage measuring method based on a master-slave structure, and belongs to the technical field of modular multilevel converter submodule capacitance and voltage measurement.
Background
At present, in high-voltage and high-power occasions, the modularized multi-level technology attracts wide attention due to the characteristics of modularization, high expansibility and flexibility. The Modular Multilevel Converter (MMC) can realize flexible change of Voltage and power grade by adjusting the serial number of the submodules, can be expanded to any level output, and reduces electromagnetic interference and harmonic content of output Voltage, so that the Modular Multilevel Converter is widely applied to occasions such as High-Voltage Direct Current (HVDC) and the like.
How to handle direct current short-circuit fault is an important problem that needs to be solved in MMC application, direct current short-circuit fault refers to short-circuit fault that MMC direct current side generating line takes place, there is very big injection fault current and the afterflow current of bridge arm inductance in the alternating current side after the trouble takes place and arouses that the transverter locks, this is very unfavorable for the switching element (diode) inside the submodule, especially when ground resistance is less, inductance current decay is slower, can lead to the device to bear long-time overcurrent, and prolong the fault clearing time, it is unfavorable to MMC's steady operation. At present, the main scheme for handling the MMC direct-current short-circuit fault is to improve the topology structure of the sub-modules.
In a sub-module topological structure commonly used by the MMC, a half-bridge sub-module does not have direct-current fault ride-through capability, a full-bridge sub-module has direct-current fault ride-through capability, but the number of switching devices required by the full-bridge sub-module is twice that of the half-bridge sub-module, so that the full-bridge-half-bridge sub-module mixed type MMC is a commonly used topological structure with the direct-current fault ride-through capability for reducing the number of the switching devices. When a direct current side short-circuit fault occurs, because the time of a direct current side short-circuit fault clearing process is extremely short and the value of short-circuit current is very large, the capacitance voltage values of all full-bridge sub-modules are close to and are all larger than the capacitance voltage values of all half-bridge sub-modules, and in order to ensure the safety of equipment, the capacitance voltage values of the full-bridge sub-modules need to be reliably monitored in real time.
In the full-bridge-half-bridge submodule mixed MMC, in order to realize the safe and stable operation of a system, the capacitance voltage value of each submodule needs to be kept balanced, and in order to achieve the control purpose, the capacitance voltage value of each submodule needs to be measured. At present, a direct measurement method is mainly adopted, namely, each submodule alternating current side is connected with one voltage sensor, but the method needs a large number of voltage sensors, so that not only is the hardware complexity of the system increased, but also the risk of hardware failure is increased.
Indirect measurement methods using a reduced number of voltage sensors have received attention from a number of scholars, and research has focused on three main aspects: firstly, a mathematical model-based submodule capacitor voltage estimation algorithm is used, but the method lacks a voltage correction step, so that errors exist in submodule capacitor voltage calculation, and the design of a control system is not facilitated; secondly, the method for estimating the capacitance voltage of the submodule based on the mathematical model corrects the estimation method through the voltage value measured at the alternating current side of the submodule connected in series, but the method increases the calculation complexity of a control system and causes the accumulated error of the capacitance voltage calculation of other input submodules; and thirdly, the capacitance voltage of the submodule is estimated by measuring the output voltage of the bridge arm and utilizing the voltage variation between adjacent sampling moments, although the method reduces the hardware complexity and is simple and easy to calculate, the measurement error of the method can change along with the change of the output voltage level of the bridge arm, and the updating frequency of the capacitance voltage of the submodule in the method is not a fixed value and is not beneficial to the balance control of the capacitance voltage of the submodule. Therefore, it is very important to provide a reliable voltage measurement method which can reduce the complexity of system hardware and improve the calculation accuracy of the sub-module capacitor voltage.
Disclosure of Invention
The invention aims to provide a hybrid MMC module capacitance and voltage measuring method based on a master-slave structure, which aims to overcome the defects of the prior art, realize the sub-module fault detection in the pre-charging stage and the reliable monitoring of a full-bridge sub-module under the direct-current side short-circuit fault, improve the sub-module capacitance and voltage measuring precision, reduce the hardware complexity of the system, and solve the technical problems that the existing method for estimating the capacitance and voltage of the sub-module according to the voltage variation between adjacent sampling moments has the measuring error which changes along with the change of the bridge arm output voltage and the existing hybrid MMC module capacitance and voltage measuring method has poor reliability under the sub-module fault and the direct-current side short-circuit fault.
The invention adopts the following technical scheme for realizing the aim of the invention:
a method for measuring capacitance and voltage of a hybrid MMC module based on a master-slave structure is characterized in that a hybrid modular multilevel converter with at least one full-bridge submodule and at least one half-bridge submodule connected in series to each phase of bridge arm is used, a master voltage sensor is used for measuring output voltage of each bridge arm, a slave voltage sensor is used for measuring output voltage of one full-bridge submodule in each bridge arm, capacitance and voltage of each submodule are calculated and faults of the single submodule are diagnosed according to bridge arm current directions and measured values of the master voltage sensor and the slave voltage sensor in a module pre-charging stage, capacitance and voltage of each submodule are calculated when the hybrid MMC works normally, and capacitance and voltage of the submodule connected with the hybrid MMC is directly measured by the slave voltage sensor when the hybrid MMC has a direct-current side.
As a further optimization scheme of the hybrid MMC module capacitance and voltage measuring method based on the master-slave structure, the capacitance and voltage of each submodule are calculated and the fault of a single submodule is diagnosed according to the bridge arm current direction and the measured values of the master-slave voltage sensor in the module pre-charging stage according to the following method,
for the precharge uncontrollable phase:
when the current of the bridge arm is less than zero, the capacitance voltage of the full-bridge sub-modules directly connected with the slave voltage sensor is equal to the measured value of the slave voltage sensor, the capacitance voltage of other full-bridge sub-modules not directly measured by the voltage sensor is determined by the ratio of the measured value of the main voltage sensor to the number of the full-bridge sub-modules in the bridge arm, the capacitance voltage of the half-bridge sub-modules is kept unchanged, the number of the full-bridge sub-modules in the bridge arm which are kept in the switching-in state is determined by the ratio of the measured value of the main voltage sensor to the capacitance voltage of the full-bridge sub-modules in the bridge arm which are measured by the slave voltage sensor, when the number of the full-bridge sub-modules in the bridge arm,
when the bridge arm current is larger than zero, the capacitance voltage of the full-bridge submodule directly connected with the slave voltage sensor is equal to the measured value of the slave voltage sensor, the measured value of the slave voltage sensor is taken as the capacitance voltage of other full-bridge submodules which are not directly measured by the voltage sensor, the measured value of the slave voltage sensor can be taken as the capacitance voltage of other full-bridge submodules which are not directly measured by the voltage sensor, the capacitance voltage of the half-bridge submodule is obtained by the ratio of the difference between the measured value of the master voltage sensor and the total capacitance voltage value of the full-bridge submodule of the bridge arm to the total number of the half-bridge submodules connected in series in the bridge arm, the total capacitance voltage value of the full-bridge submodule of the bridge arm is obtained by the product of the measured value of the slave voltage sensor and the total number of the full-bridge, judging that one submodule in the bridge arm has a short-circuit fault;
and a precharge controllable stage for respectively carrying out grouping charging on the half-bridge submodule and the full-bridge submodule:
when full-bridge submodule groups are put into for charging, the capacitance voltage of each submodule in each full-bridge submodule group is obtained by the ratio of the measured value of the main voltage sensor to the number of submodules in each full-bridge submodule group, the capacitance voltage value of each full-bridge submodule in the full-bridge submodule group where the full-bridge submodule is positioned is measured by the slave voltage sensor, the number of the full-bridge submodules which are kept in the put-in state in the bridge arm is determined by the ratio of the measured value of the main voltage sensor to the measured value of the slave voltage sensor, and when the number of the full-bridge submodules which are kept in the put-in state in the bridge arm is less than the number of the submodules contained in the group, the fact that one,
when half-bridge submodule groups are put into charge and bridge arm current is larger than zero, capacitance voltage of each submodule in each half-bridge submodule group is obtained by the ratio of the measured value of the main voltage sensor to the number of submodules in each half-bridge submodule group, the calculated value of the capacitance voltage of each half-bridge submodule in each half-bridge submodule group is determined by the ratio of the measured value of the main voltage sensor to the number of the submodules contained in the group, and when the variation of the capacitance voltage of each half-bridge submodule in each half-bridge submodule group exceeds the variation of the capacitance voltage of the half-bridge submodule in the bridge arm at adjacent sampling time, it is judged that one half-bridge submodule in the.
As a further optimization scheme of the hybrid MMC module capacitance and voltage measurement method based on the master-slave structure, the method for calculating the capacitance and voltage of each submodule when the hybrid MMC works normally comprises the following steps: the method comprises the steps of classifying sampling moments according to a change rule of a bridge arm output voltage level and working states of submodules, calculating capacitance voltages of the submodules according to switch states of the submodules at adjacent sampling moments, capacitance voltage variation of the submodules with the switch states changed between the adjacent sampling moments, variation of measurement data of a main voltage sensor and measurement data of the main voltage sensor between the adjacent sampling moments, and variation of measurement data of the voltage sensor between the measurement data of the voltage sensor and the measurement data of the auxiliary voltage sensor between the adjacent sampling moments, and compensating capacitor voltage drop of the submodules caused by energy loss generated by an additional circuit comprising a balance resistor of the submodules and a switching power supply to correct the capacitance voltages of the submodules which are not directly measured by the voltage sensor and keep the switch states unchanged after a sampling period.
As a further optimization scheme of the hybrid MMC module capacitance and voltage measurement method based on the master-slave structure, the method for classifying the sampling time according to the change rule of the bridge arm output voltage level and the working state of each submodule comprises the following steps:
dividing the sampling time when only one submodule represented by the bridge arm when the output voltage level is 1 is in the switching-on state into a first type of sampling time,
dividing the sampling time when the bridge arm output voltage level rises but is not 2 and is kept in the on state between the adjacent sampling time by the full bridge submodule measured by the voltage sensor into a second type of sampling time,
the sampling time when the bridge arm output voltage level is reduced but the bridge arm output voltage level is not 2 and the full bridge submodule measured by the voltage sensor keeps the switching-on state between the adjacent sampling time is divided into a third type of sampling time,
dividing the sampling time of the full-bridge submodule which is measured by the voltage sensor after the output voltage level of the bridge arm rises and is converted from the cut-off state to the put-on state between the adjacent sampling time into a fourth type of sampling time,
dividing the sampling time of the full-bridge submodule which is measured by the voltage sensor after the output voltage level of the bridge arm is reduced from the on state to the off state between the adjacent sampling time into a fifth type of sampling time,
dividing the sampling time when the bridge arm output voltage level rises and the full-bridge submodule measured by the voltage sensor keeps the cutting-off state between the adjacent sampling time into a sixth type of sampling time,
dividing the sampling time at which the bridge arm output voltage level is reduced and the full-bridge submodule measured by the voltage sensor keeps the cutting-off state between the adjacent sampling time into a seventh type of sampling time,
the sampling time when the output voltage level of the bridge arm rises to 2 and the full-bridge submodule measured by the voltage sensor keeps the on state between the adjacent sampling time is divided into an eighth type of sampling time,
and dividing the sampling time when the output voltage level of the bridge arm is reduced to 2 and the full-bridge submodule measured by the voltage sensor keeps the switching-on state between the adjacent sampling time into a ninth type sampling time.
As a further optimization scheme of the hybrid MMC module capacitance and voltage measurement method based on the master-slave structure, the sampling frequency of the measurement method and the updating frequency of capacitance and voltage of each submodule are both twice of the equivalent switching frequency of bridge arm output voltage.
As a further optimization scheme of the master-slave structure-based hybrid MMC module capacitance and voltage measurement method, a method for calculating capacitance and voltage of each submodule by using a submodule switch state at adjacent sampling time, a submodule capacitance and voltage variation in which a switch state changes between adjacent sampling times, a variation in main voltage sensor measurement data and main voltage sensor measurement data between adjacent sampling times, and a variation in slave voltage sensor measurement data and slave voltage sensor measurement data between adjacent sampling times is as follows:
for a first type of sampling instant: u. ofcd(t)＝um_u(t)；
For a second type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠e,s，Δucj(t)＝Δucs(t)＝us(t)-us(t-1),j≠e,s，uce(t)＝Δum(t)-∑SjΔucj(t),j≠e,s，
For a third type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠f,s，Δucj(t)＝Δucs(t)＝us(t)-us(t-1),j≠f,s，ucf(t)＝ucf(t-1)+Δuecf(t)，
For a fourth type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠s，
j≠s，ucs(t)＝us(t)，
For a fifth type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠s，
j≠s，ucs(t)＝us(t)，
For a sixth type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠e,s，
Sj(t)＝1，uce(t)＝Δum(t)-∑SjΔucj(t),j≠e,s，
For a seventh type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠f,s，
Sj(t)＝1，ucf(t)＝ucf(t-1)+Δuecf(t)，
For a eighth class of sampling instants: u. ofce(t)＝Δum(t)-us(t)，e≠s，ucs(t)＝us(t)，
For a ninth type of sampling instant: u. ofcr(t)＝Δum(t)-us(t),r≠f,s，ucs(t)＝us(t)，ucf(t)＝ucf(t-1)+Δuecf(t),f≠r,s，
Wherein u iscd(t) is the capacitance voltage of the d-th sub-module in the on state at the sampling time t, d is 1,2, …, N, N is the number of sub-modules connected in series in one bridge arm, um_u(t) bridge arm output voltage, u, measured by the main voltage sensor at the time of t samplingcj(t-1) the capacitance voltage u of the jth submodule which is not directly measured by the voltage sensor and has a constant switch state between the t sampling moment and the previous sampling moment at the t-1 sampling momentcj(t) is the capacitance voltage of the jth submodule which is not directly measured by the voltage sensor and has a constant switch state between the sampling time t and the previous sampling time, delta ucj(t) is the variation of the sub-module capacitor voltage which is not directly measured by the voltage sensor and is the jth sub-module capacitor voltage of which the switch state is kept unchanged between the sampling time t and the previous sampling time, Sj(t) is the switching function of the jth submodule which is not directly measured by the voltage sensor and whose switching state remains unchanged between the sampling time t and the previous sampling time, SjWhen t is 1, the switching state of the jth submodule which is not directly measured by the voltage sensor and is not directly measured at the sampling time t is kept unchanged between the sampling time t and the previous sampling time S is in an on statejWhen the (t) is 0, the j-th submodule which is not directly measured by the voltage sensor and the switch state of which is kept unchanged between the t sampling moment and the previous sampling moment is in a cut-off state, s is a full-bridge submodule measured by the voltage sensor in the bridge arm, u is a full-bridge submodule measured by the voltage sensor in the bridge armcs(t) is the capacitance voltage of the full-bridge submodule measured by the slave voltage sensor in the bridge arm at the sampling time t, delta ucs(t) is the variation of the output voltage of the full-bridge submodule measured by the voltage sensor between the sampling time t and the previous sampling time，us(t-1)、us(t) measured values from the voltage sensor at the sampling time t-1 and at the sampling time t, uce(t) is the capacitance voltage at the sampling time t, Δ u, of the submodule e which has been switched from the off-state to the on-state and is not directly measured by the voltage sensor between the sampling time t and the previous sampling timem(t) is the amount of change in the measured value of the primary voltage sensor between the sampling instant t and the sampling instant immediately preceding it, ucf(t-1) is the capacitance voltage u at the sampling time t-1 of the submodule f which is not directly measured by the voltage sensor and is changed from the on state to the off state between the sampling time t and the previous sampling timecf(t) is the capacitance voltage at the sampling time t, Δ u, of the submodule f, which is switched from the on-state to the off-state and is not directly measured by the voltage sensor, between the sampling time t and the previous sampling timeecf(t) is the change in capacitance voltage of the submodule f, which is not directly measured by the voltage sensor, from the on-state to the off-state between the sampling time t and the previous sampling time, ucr(T) is the capacitance voltage at the sampling time T of the submodule r which is kept in the on state except the submodule measured from the voltage sensor between the sampling time T and the previous sampling time T, TCIs the system sampling period, CfThe capacitance value i of the submodule f, which is not directly measured by the voltage sensor, is changed from the switched-on state to the switched-off state between the sampling time t and the previous sampling timearm_u(t-1)、iarm_u(t) bridge arm currents at t-1 sampling time and t sampling time, respectively, CjAnd the capacitance value of the jth sub-module which is not directly measured by the voltage sensor and the switching state of which is kept unchanged between the sampling time t and the previous sampling time is obtained.
As a further optimization scheme of the method for measuring the capacitance and voltage of the hybrid MMC module based on the master-slave structure, the method for compensating the sub-module capacitance and voltage drop caused by the energy loss generated by the additional circuit including the sub-module balance resistor and the switching power supply to correct the capacitance and voltage of the sub-module which is not directly measured by the voltage sensor and has the same switching state at the adjacent sampling time after the sampling period comprises the following steps:
when the full-bridge submodule measured by the voltage sensor keeps the working state unchanged between adjacent sampling moments, the variation of the output voltage of the full-bridge submodule measured by the voltage sensor between the adjacent sampling moments is the submodule capacitor voltage drop caused by the energy loss generated by an additional circuit comprising the submodule balance resistor and the switching power supply,
when the full-bridge sub-module measured by the voltage sensor is switched to the on-state or the off-state at the adjacent sampling time, the variation of the measured data from the voltage sensor at the sampling time which is closest to the current sampling time and the full-bridge sub-module measured by the voltage sensor at the adjacent sampling time is in the off-state is taken as the voltage drop of the sub-module capacitor caused by the energy loss generated by an additional circuit comprising the sub-module balance resistor and a switching power supply,
by the expression: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t)+(Sj(t)-1)ΔucebAfter the sampling period is corrected, the switch state of the submodule is kept unchanged between adjacent sampling moments and the capacitor voltage delta u of the submodule is not directly measured by the voltage sensorcebIs the drop in voltage of the sub-module capacitor caused by the energy loss generated by the additional circuit comprising the sub-module balancing resistors and the switching power supply.
By adopting the technical scheme, the invention has the following beneficial effects:
(1) the invention provides a master-slave structure-based voltage measuring method for a hybrid modular multilevel converter module, which is characterized in that a measuring device of a master-slave structure is used for detecting a fault submodule at a pre-charging stage, and the measuring device of the master-slave structure is used for reliably monitoring data of a full-bridge submodule under a direct-current side fault so as to ensure the safety of equipment;
(2) the invention also provides a sub-module capacitance voltage measuring method aiming at the normal working mixed MMC, the sampling time is classified according to the change rule of the bridge arm output voltage level and the working state of the measured full-bridge sub-module, the sub-module capacitance voltage change quantity of the sub-module with the switch state changed between the adjacent sampling time, the change quantity of the main voltage sensor measuring data and the main voltage sensor measuring data between the adjacent sampling time, and the change quantity of the slave voltage sensor measuring data and the slave voltage sensor measuring data between the adjacent sampling time are utilized to calculate the capacitance voltage of each sub-module.
Drawings
Fig. 1 is a circuit diagram of a main circuit topology and sensor connection locations of a hybrid modular multilevel converter.
Fig. 2 is a flowchart of a method for measuring module capacitance and voltage of a hybrid modular multilevel converter based on a master-slave structure.
Fig. 3 is a schematic diagram of the operation of the hybrid modular multilevel converter during the precharge phase.
Fig. 4 is a schematic diagram of the operation of the hybrid modular multilevel converter in the event of a dc short-circuit fault.
Detailed Description
The technical scheme of the invention is explained in detail in the following with reference to the attached drawings.
The main circuit topology and the sensor connection position of the hybrid modular multilevel converter system related to the invention are shown in fig. 1. Each phase of the hybrid MMC system is composed of an upper Bridge arm and a lower Bridge arm, and each Bridge arm is composed of a plurality of groups of Full Bridge Sub-modules (FBSM) and a plurality of groups of half Bridge Sub-modules (HBSM) which are connected in series. The invention provides a Master-Slave Structure Based mixed type modular multilevel converter module Voltage Measuring method (MS-MT), which uses a Master Voltage sensor to measure the output Voltage of each bridge arm, uses a Slave Voltage sensor to measure the alternating current output Voltage of any full-bridge submodule in each bridge arm (hereinafter, the submodule is referred to as a Slave submodule), and uses a current sensor to measure the current of each bridge arm. In FIG. 1, UdcIs the system DC side bus voltage ua、ub、ucThree-phase alternating current output voltage respectively, N is the number of the submodules connected in series with each bridge arm,iarm_auand iarm_al、iarm_buAnd iarm_bl、iarm_cuAnd iarm_clThe currents of the upper and lower bridge arms of the A phase, the B phase and the C phase of the system are respectively LauAnd Lal、LbuAnd Lbl、LcuAnd LclBridge arm inductances u of A, B and C phases of the systemoIs the AC output voltage of the submodule, C is the capacitance value in the submodule connected in parallel with the switching device, ucIs the sub-module capacitance voltage.
The method for measuring the capacitance and voltage of the hybrid modular multilevel converter module based on the master-slave structure is shown in figure 2, the capacitance and voltage of each submodule are calculated and the fault of a single submodule is diagnosed according to the bridge arm current direction and the measured value of a master-slave voltage sensor in the module pre-charging stage, the capacitance and voltage of each submodule are calculated when a hybrid MMC works normally, and the capacitance and voltage of a full-bridge submodule connected with the hybrid MMC is directly measured by the slave voltage sensor when the hybrid MMC has a direct-current side short-circuit fault.
Firstly, calculating the capacitance voltage of each submodule when the mixed MMC works normally
①, taking the strategy of phase-shifting modulation of carrier as an example, each bridge arm output voltage is measured by a Master voltage sensor, the AC output voltage of any full-bridge submodule inside each bridge arm is measured by a Slave voltage sensor, each bridge arm current is measured by a current sensor, the voltage and current sampling time is the peak and trough time of all module carriers, the sampling frequency and the updating frequency of the capacitance and voltage of each submodule of the measuring method are both the equivalent switching frequency of the bridge arm output voltage, the voltage measuring range of the Master voltage sensor needs to be larger than the sum of the output voltages when all submodules in the measured bridge arm are switched in, and the voltage measuring range of the Slave voltage sensor needs to be larger than the AC output voltage of a single FBSM.
②, dividing the sampling time into nine classes according to the change rule of the bridge arm output voltage level and the working state of each submodule, the first class is the sampling time when the bridge arm output voltage level is 1 (only one submodule is in the on state at this time), the second class is the sampling time when the bridge arm output voltage level is increased but the bridge arm output voltage level is not 2 and the Slave submodule keeps the on state during the adjacent sampling time (a non-Slave submodule in the bridge arm is converted from the off state to the on state at this time), the third class is the sampling time when the bridge arm output voltage level is decreased but the output voltage level is not 2 and the Slave submodule keeps the on state during the adjacent sampling time (a non-Slave submodule in the bridge arm is converted from the on state to the off state) the fourth class is the sampling time when the Slave submodule is increased and the Slave submodule is converted from the off state to the on state during the adjacent sampling time (only the switch state of the Slave submodule is changed from the off state after the bridge arm output voltage level is reduced and the Slave submodule is converted from the sampling time when the Slave submodule is in the non-Slave submodule from the switch from the off state after the bridge arm output voltage level is converted into the sampling time (the switch from the first state when the bridge arm output voltage level is converted into the non-Slave submodule (the switch from the switch off state after the sampling time when the bridge arm output voltage level is converted into the sampling time) and the Slave submodule is converted into the non-Slave submodule (the switch from the switch off state when the sampling time when the bridge arm output sub-Slave submodule (the bridge arm output sub-Slave submodule) when the bridge arm output sub-Slave submodule is converted into the bridge arm output voltage level is converted into the sampling time when the bridge arm output sub module is converted into the bridge arm output sub module), the sampling time when the Slave submodule (the Slave submodule), the switch from the Slave submodule), the bridge arm output sub-Slave submodule is converted from the bridge arm output sub-Slave.
③, a method for calculating the capacitance voltage of each sub-module is provided for each type of sampling time:
a. for a first type of sampling instant, t is the measured value u of the main voltage sensor at the sampling instantm_u(t) is the accurate value u of the capacitance voltage of the d sub-module in the switching state at the sampling moment tcd(t) is represented by the formula (1):
ucd(t)＝um_u(t) (1)，
other sub-modules are in an off state, and the voltage is unchanged;
b. for the sampling instants of the second type of sampling instants,
firstly, the variation delta u of the measured value of the bridge arm voltage between two adjacent sampling moments is calculated according to the formula (2)m(t)：
Δum(t)＝um_u(t)-um_u(t-1) (2)，
Wherein, Δ um(t) is the bridge arm output voltage variation measured by the Master voltage sensor between the sampling time t and the sampling time t-1, um_u(t)、um_u(t-1) bridge arm output voltages measured by the Master voltage sensor at the t-sampling time and the t-1 sampling time respectively;
secondly, calculating the capacitance voltage variation delta u of the jth sub-module which is not directly measured by the voltage sensor and has the working state kept in the input state between the adjacent sampling moments in each sampling period according to the formula (3)cj(t)：
Δucj(t)＝Δucs(t)＝us(t)-us(t-1),j≠e,s (3)，
Where the subscript s denotes the Slave submodule,. DELTA.ucs(t) is the variation of the capacitor voltage of the Slave submodule between adjacent sampling moments, us(t-1)、us(t) are respectively the measured values of the Slave voltage sensor at the sampling time t-1 and the sampling time t,
the capacitor voltage u of the submodule e which is not directly measured by the voltage sensor and is switched from the cut-off state to the put-on state between the t sampling time and the t-1 sampling time at the t sampling timece(t) is calculated from the formula (4):
uce(t)＝Δum(t)-∑SjΔucj(t),j≠e,s (4)，
wherein, sigma Sj(t)Δucj(t) is the sum of the capacitance voltage variations of the submodules which are kept in the switching state between adjacent sampling moments and are not directly measured by the voltage sensor, Sj(t) is the switching function of this sub-module, when SjWhen (t) is 1, the submodule is a throwIn a state of Sj(t) is 0, the submodule is in an excision state,
at this time, the working state of the jth sub-module between the adjacent sampling moments is kept unchanged, and the capacitive voltage u of the jth sub-module is not directly measured by the voltage sensorcj(t) can be calculated from equation (5):
ucj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠e,s (5)，
wherein u iscj(t-1) is the capacitance voltage of the jth submodule which is not directly measured by the voltage sensor and has a constant switch state between the sampling time t and the previous sampling time at the sampling time t-1;
c. for a third type of sampling time, the capacitance voltage u of the submodule which remains unchanged between adjacent sampling times for the switch state and is not measured by the voltage sensorcj(t) the amount of change Δ u, between adjacent sampling times, in the capacitance voltage of the submodule f that is cut off between adjacent sampling times and is not measured by the voltage sensor can be calculated from the equations (3) and (5)ecf(t) can be obtained from formula (6):
wherein iarm_u(T-1) is the value of the bridge arm current at the sampling time T-1, TCIs a sampling period, CfIs the capacitance value of sub-module f;
the capacitance voltage of sub-module f is calculated as equation (7):
ucf(t)＝ucf(t-1)+Δuecf(t) (7)，
wherein u iscf(t-1) is the capacitance voltage value of the submodule f at the sampling moment of t-1;
d. for the fourth type of sampling time, the switch state keeps switched on at the adjacent sampling time and the capacitance voltage variation delta u of the jth sub-module which is not measured by the voltage sensorcj(t) can be calculated from equation (8):
capacitor voltage u of submodule jcj(t) can be calculated from the equations (5) and (8),
since the submodule with the switching state changed between the adjacent sampling time is the Slave submodule, the capacitance voltage u of the Slave submodule is changedcs(t) and us(t) equal;
e. for the fifth type of sampling time, the switch state keeps switched on at the adjacent sampling time and the capacitance voltage variation delta u of the jth sub-module which is not measured by the voltage sensorcj(t) can be calculated from equation (9):
capacitor voltage u of submodule jcj(t) can be calculated from the formulas (5) and (9),
similar to d, since the sub-module whose switching state changes between adjacent sampling time instants is the Slave sub-module, the capacitance voltage u thereof iscs(t) and us(t) equal;
f. for the sixth sampling time, the switch state keeps switched on at the adjacent sampling time and the capacitance voltage variation delta u of the jth sub-module which is not measured by the voltage sensorcj(t) can be calculated from equation (10):
wherein iarm_u(t) and iarm_u(t-1) is the value of the bridge arm current at sampling times t and t-1, CjThe capacitance value of the jth sub-module which is not directly measured by the voltage sensor and the switching state of which is kept unchanged between the sampling time t and the previous sampling time is obtained,
capacitor voltage u of submodule jcj(t) can be calculated from the formulae (5) and (10);
capacitance voltage u at sampling time t for submodule e which is switched into on state between adjacent sampling time and is not directly measured by voltage sensorce(t) can be represented by the formula (11)And calculating to obtain:
uce(t)＝Δum(t)-∑SjΔucj(t),j≠e,s (11)，
g. for the seventh kind of sampling time, the switch state keeps on being switched on at the adjacent sampling time and the capacitance voltage variation delta u of the jth sub-module which is not measured by the voltage sensorcj(t) can be calculated from equation (10) and the capacitor voltage u of sub-module jcj(t) can be calculated from the equations (5) and (10),
capacitive voltage u for submodule f which is cut off between adjacent sampling instants and is not directly measured by a voltage sensorcf(t) can be calculated from the formulae (6) and (7);
h. for the eighth type of sampling time, at this time, the Slave sub-module keeps the on state at the adjacent sampling time, and the sub-module capacitance voltage on the adjacent sampling time can be calculated by equation (12):
uce(t)＝Δum(t)-us(t) (12)，
at this time, since the other sub-module which is kept in the on state between the adjacent sampling time instants is the Slave sub-module, the capacitor voltage u thereof is equal to the Slave voltagecs(t) and us(t) equal;
i. for the ninth type of sampling time, the capacitance voltage of the other sub-module r, except the Slave sub-module, which is kept in the on state during the adjacent sampling time can be calculated by equation (13):
ucr(t)＝Δum(t)-us(t) (13)，
the capacitor voltage for the sub-module that is cut off between adjacent sampling instants can be calculated from equations (6) and (7),
at this time, since the other sub-module which is kept in the on state between the adjacent sampling time instants is the Slave sub-module, the capacitor voltage u thereof is equal to the Slave voltagecs(t) and us(t) equal;
④ the sub-module capacitor voltage generated by the additional circuit containing the sub-module balancing resistor and the switch power supply drops by delta U against the energy losscebThe following category classification calculation:
a.Slave submodule remaining in cut-off state, at this time Δ UcebCan be calculated from equation (14):
Δuceb＝Δucs(t)＝us(t)-us(t-1) (14)，
the slave submodule remains in the on state, at this time, Δ ucs(t) already contains Δ UcebWhile being Δ UcebThe temperature of the molten steel is kept unchanged,
when the slave submodule is newly put in or cut off, delta U does not occur due to sudden change of the capacitance voltage of the submodulecebIs invariant and can be computed at the sampling instant closest to this instant and at which the Slave sub-module is in the cut-out state,
the formula (5) for calculating the capacitance voltage of the cut-off submodule can be corrected to formula (15):
ucj(t)＝ucj(t-1)+Sj(t)Δucj(t)+(Sj(t)-1)Δuceb(15)，
the capacitor voltage of other sub-modules in the switching state does not need to be corrected, because the voltage drop is considered when the voltage variation of the adjacent sampling time is calculated;
(II) diagnosing the fault of the single submodule according to the measured value of the master-slave voltage sensor in the module pre-charging stage
The precharge phase hybrid modular multilevel converter operates as shown in fig. 3, where RlFor each phase of pre-charge resistance, sensors distributed in a master-slave configuration can detect a single sub-module short circuit fault during a sub-module pre-charge stage. The sub-module capacitor voltage measurement of the sub-module pre-charging link is divided into a controllable stage and an uncontrollable stage for measurement respectively.
In an uncontrollable link, all trigger pulses are blocked, and the sub-module capacitor voltage is calculated by the following equations (16) to (18):
uc_F＝ucs(16)，
wherein u isc_FIs the capacitor voltage, u, of one of the FBSMsc_HIs the capacitance voltage of a certain HBSM, nFNumber of FBSMs in a bridge arm, nHIs the number of HBSM in a bridge arm, um_uSingle bridge arm output voltage, u, measured for Master Voltage sensorcs_FThe single full-bridge sub-module capacitance voltage measured for the Slave voltage sensor. When the bridge arm current is less than 0, the capacitance voltage of the half-bridge sub-modules is kept unchanged, and when the bridge arm current is greater than 0, the measured value of the Slave voltage sensor can be regarded as the capacitance voltage value of each full-bridge sub-module.
In a controllable link, the FBSM and the HBSM are respectively grouped and put into each group in sequence for charging, and the formulas (17) and (18) are modified into formulas (19) to (20):
wherein n isgFnFor the number of sub-modules in each FBSM packet, ucj_FFor the capacitor voltage, n, of each submodule in each FBSM groupgHnFor the number of sub-modules in each HBSM packet, ucj_HCapacitance voltages of the sub-modules in each HBSM group;
in the uncontrollable stage, when the bridge arm current is less than 0, calculating the quantity of FBSMs by using the formula (17) and the measured values of the two sensors and comparing the quantity with nFComparing if the calculated value is less than nFIt indicates that one FBSM has a short-circuit fault; when the bridge arm current is greater than 0, the equations (18) and n are usedF、nHAnd the voltage u of HBSM is calculated from the measured values of two sensorsc_H(t) and uc_H(t-1)+(iarm_u(t)+iarm_u(t-1)). C/2 comparison, if uc_H(t) is greater than uc_H(t-1)+(iarm_u(t)+iarm_u(t-1)) C/2, indicating that one submodule has a short-circuit fault.
In the controlled phase, when the FBSM packet is charged, the FBSM number is calculated using equation (19) and the two sensor measurements and compared to ngFnComparing if the calculated value is less than ngFnIt indicates that there is a short-circuit fault in the FBSM in the packet; when the packet of HBSM is charged, equation (20), n is usedgHnCalculating the voltage u of HBSM according to the measured value of Master sensorcj_H(t) and ucj_H(t-1)+(iarm_u(t)+iarm_u(t-1)). C/2 comparison, if ucj_H(t) is greater than ucj_H(t-1)+(iarm_u(t)+iarm_u(t-1)) C/2, indicating that a short circuit fault has occurred with one of the HBSMs. When a voltage sensor fails, its measurement value is generally a constant, and when this value is held for more than three sampling periods, the sensor fails. If the Slave voltage sensor fails, the Master voltage sensor can complete the voltage measurement process of the rest precharge stage, but if the Master voltage sensor fails, the whole precharge link must be stopped;
(III) when the mixed MMC has a direct-current side short-circuit fault, the slave voltage sensor directly measures the capacitance voltage of the full-bridge submodule connected with the slave voltage sensor
Operation of the hybrid modular multilevel converter in the event of a dc short circuit fault is shown in fig. 4, where R is0、L0Respectively an equivalent resistance and an equivalent inductance L of a direct current side bus after a direct current short circuit faults、RsRespectively, a load inductance and a resistance usa、usb、uscRespectively, the three-phase grid side voltages. When a direct current side short-circuit fault occurs, the voltage measurement and fault measurement method is the same as the single submodule fault detection method, but the difference is that the time of the direct current side short-circuit fault clearing process is extremely short, the short-circuit current value is very large, the capacitance voltage values of all FBSMs are close to each other and are all larger than the capacitance voltage values of all HBSMs, and in order to ensure the safety of equipment, the capacitance voltage value of the full-bridge submodule is directly measured by a Slave voltage sensor.
In summary, compared with the existing measurement method, the method for measuring the capacitance and voltage of the hybrid modular multilevel converter module based on the master-slave structure is suitable for being applied to various occasions, not only improves the measurement precision of the capacitance and voltage of the sub-module, but also reduces the hardware complexity of the system. Meanwhile, in the sub-module pre-charging stage, the method not only realizes the voltage measurement function, but also can detect the short-circuit fault of the module; in the process of clearing the direct-current short-circuit fault, the master-slave structure and the measurement method can reliably measure, so that the safety of equipment is ensured.
The above description is only of the preferred embodiments of the present invention, and it should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and are intended to be included within the scope of the invention.
Claims (6)
1. A method for measuring capacitance and voltage of a hybrid MMC module based on a master-slave structure is characterized in that a hybrid modular multilevel converter with at least one full-bridge submodule and at least one half-bridge submodule connected in series to each phase bridge arm is adopted, a master voltage sensor is adopted to measure the output voltage of each bridge arm, a slave voltage sensor is adopted to measure the output voltage of one full-bridge submodule in each bridge arm, the capacitance and voltage of each submodule are calculated and the fault of each submodule is diagnosed according to the current direction of the bridge arm and the measured value of the master-slave voltage sensor in the module pre-charging stage, the capacitance and voltage of each submodule are calculated when the hybrid MMC works normally, the capacitance and voltage of the full-bridge submodule connected with the hybrid MMC is directly measured by the slave voltage sensor when the hybrid MMC has a direct-current,
the method for calculating the capacitance voltage of each submodule when the hybrid MMC works normally comprises the following steps: the method comprises the steps of classifying sampling moments according to a change rule of a bridge arm output voltage level and working states of submodules, calculating capacitance voltages of the submodules according to switch states of the submodules at adjacent sampling moments, capacitance voltage variation of the submodules with the switch states changed between the adjacent sampling moments, variation of measurement data of a main voltage sensor and measurement data of the main voltage sensor between the adjacent sampling moments, and variation of measurement data of the voltage sensor between the measurement data of the voltage sensor and the measurement data of the auxiliary voltage sensor between the adjacent sampling moments, and compensating capacitor voltage drop of the submodules caused by energy loss generated by an additional circuit comprising a balance resistor of the submodules and a switching power supply to correct the capacitance voltages of the submodules which are not directly measured by the voltage sensor and keep the switch states unchanged after a sampling period.
2. The method for measuring the capacitance and voltage of the hybrid MMC module based on the master-slave structure of claim 1, wherein the capacitance and voltage of each sub-module are calculated and the fault of the single sub-module is diagnosed according to the bridge arm current direction and the measured values of the master-slave voltage sensor during the module pre-charging stage according to the following method,
for the precharge uncontrollable phase:
when the current of the bridge arm is less than zero, the capacitance voltage of the full-bridge sub-modules directly connected with the slave voltage sensor is equal to the measured value of the slave voltage sensor, the capacitance voltage of other full-bridge sub-modules not directly measured by the voltage sensor is determined by the ratio of the measured value of the main voltage sensor to the number of the full-bridge sub-modules in the bridge arm, the capacitance voltage of the half-bridge sub-modules is kept unchanged, the number of the full-bridge sub-modules in the bridge arm which are kept in the switching-in state is determined by the ratio of the measured value of the main voltage sensor to the capacitance voltage of the full-bridge sub-modules in the bridge arm which are measured by the slave voltage sensor, when the number of the full-bridge sub-modules in the bridge arm,
when the current of a bridge arm is greater than zero, the capacitance voltage of a full-bridge submodule directly connected with a slave voltage sensor is equal to the measured value of the slave voltage sensor, the measured value of the slave voltage sensor is used as the capacitance voltage of other full-bridge submodules which are not directly measured by the voltage sensor, the capacitance voltage of the half-bridge submodule is obtained by the ratio of the difference between the measured value of a main voltage sensor and the total capacitance voltage value of the full-bridge submodule of the bridge arm to the total number of the half-bridge submodules connected in series in the bridge arm, the total capacitance voltage value of the full-bridge submodule of the bridge arm is obtained by the product of the measured value of the slave voltage sensor and the total number of the full-bridge submodules connected in series in the bridge arm, and when the variation of the capacitance voltage of the;
and a precharge controllable stage for respectively carrying out grouping charging on the half-bridge submodule and the full-bridge submodule:
when full-bridge submodule groups are put into for charging, the capacitance voltage of each submodule in each full-bridge submodule group is obtained by the ratio of the measured value of the main voltage sensor to the number of submodules in each full-bridge submodule group, the capacitance voltage value of each full-bridge submodule in the full-bridge submodule group where the full-bridge submodule is positioned is measured by the slave voltage sensor, the number of the full-bridge submodules which are kept in the put-in state in the bridge arm is determined by the ratio of the measured value of the main voltage sensor to the measured value of the slave voltage sensor, and when the number of the full-bridge submodules which are kept in the put-in state in the bridge arm is less than the number of the submodules contained in the group, the fact that one,
when half-bridge submodule groups are put into charge and bridge arm current is larger than zero, capacitance voltage of each submodule in each half-bridge submodule group is obtained by the ratio of the measured value of the main voltage sensor to the number of submodules in each half-bridge submodule group, the calculated value of the capacitance voltage of each half-bridge submodule in each half-bridge submodule group is determined by the ratio of the measured value of the main voltage sensor to the number of the submodules contained in the group, and when the variation of the capacitance voltage of each half-bridge submodule in each half-bridge submodule group exceeds the variation of the capacitance voltage of the half-bridge submodule in the bridge arm at adjacent sampling time, it is judged that one half-bridge submodule in the.
3. The method for measuring the capacitance and voltage of the hybrid MMC module based on a master-slave structure of claim 1, wherein the method for classifying the sampling time according to the variation rule of the output voltage level of the bridge arm and the working state of each submodule comprises the following steps:
dividing the sampling time when only one submodule represented by the bridge arm when the output voltage level is 1 is in the switching-on state into a first type of sampling time,
dividing the sampling time when the bridge arm output voltage level rises but is not 2 and is kept in the on state between the adjacent sampling time by the full bridge submodule measured by the voltage sensor into a second type of sampling time,
the sampling time when the bridge arm output voltage level is reduced but the bridge arm output voltage level is not 2 and the full bridge submodule measured by the voltage sensor keeps the switching-on state between the adjacent sampling time is divided into a third type of sampling time,
dividing the sampling time of the full-bridge submodule which is measured by the voltage sensor after the output voltage level of the bridge arm rises and is converted from the cut-off state to the put-on state between the adjacent sampling time into a fourth type of sampling time,
dividing the sampling time of the full-bridge submodule which is measured by the voltage sensor after the output voltage level of the bridge arm is reduced from the on state to the off state between the adjacent sampling time into a fifth type of sampling time,
dividing the sampling time when the bridge arm output voltage level rises and the full-bridge submodule measured by the voltage sensor keeps the cutting-off state between the adjacent sampling time into a sixth type of sampling time,
dividing the sampling time at which the bridge arm output voltage level is reduced and the full-bridge submodule measured by the voltage sensor keeps the cutting-off state between the adjacent sampling time into a seventh type of sampling time,
the sampling time when the output voltage level of the bridge arm rises to 2 and the full-bridge submodule measured by the voltage sensor keeps the on state between the adjacent sampling time is divided into an eighth type of sampling time,
and dividing the sampling time when the output voltage level of the bridge arm is reduced to 2 and the full-bridge submodule measured by the voltage sensor keeps the switching-on state between the adjacent sampling time into a ninth type sampling time.
4. The method for measuring the capacitance and voltage of the hybrid MMC module based on the master-slave structure of claim 1, wherein the sampling frequency of the method and the update frequency of the capacitance and voltage of each submodule are both the equivalent switching frequency of the bridge arm output voltage.
5. The method for measuring the capacitance and voltage of the hybrid MMC module based on the master-slave structure as claimed in claim 3, wherein the method for calculating the capacitance and voltage of each submodule using the switch status of the submodule at the adjacent sampling time, the variation of the capacitance and voltage of the submodule with the switch status changed between the adjacent sampling times, the variation of the measurement data of the master voltage sensor and the measurement data of the master voltage sensor at the adjacent sampling times, and the variation of the measurement data of the slave voltage sensor and the measurement data of the slave voltage sensor at the adjacent sampling times comprises:
for a first type of sampling instant: u. ofcd(t)＝um_u(t)；
For a second type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠e,s，Δucj(t)＝Δucs(t)＝us(t)-us(t-1),j≠e,s，uce(t)＝Δum(t)-∑SjΔucj(t),j≠e,s，
For a third type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠f,s，Δucj(t)＝Δucs(t)＝us(t)-us(t-1),j≠f,s，ucf(t)＝ucf(t-1)+Δuecf(t)，
For a fourth type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠s，
ucs(t)＝us(t)，
For a fifth type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠s，
ucs(t)＝us(t)，
For a sixth type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠e,s，
uce(t)＝Δum(t)-∑SjΔucj(t),j≠e,s，
For a seventh type of sampling instant: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t),j≠f,s，
ucf(t)＝ucf(t-1)+Δuecf(t)，
For a eighth class of sampling instants: u. ofce(t)＝Δum(t)-us(t)，e≠s，ucs(t)＝us(t)，
For a ninth type of sampling instant: u. ofcr(t)＝Δum(t)-us(t),r≠f,s，ucs(t)＝us(t)，ucf(t)＝ucf(t-1)+Δuecf(t),f≠r,s，
Wherein u iscd(t) is the capacitance voltage of the d-th sub-module in the on state at the sampling time t, d is 1,2, …, N, N is the number of sub-modules connected in series in one bridge arm, um_u(t) bridge arm output voltage, u, measured by the main voltage sensor at the time of t samplingcj(t-1) is sampling at tThe capacitor voltage u of the jth submodule which is not directly measured by the voltage sensor and has a constant switch state between the moment and the previous sampling moment at the t-1 sampling momentcj(t) is the capacitance voltage of the jth submodule which is not directly measured by the voltage sensor and has a constant switch state between the sampling time t and the previous sampling time, delta ucj(t) is the variation of the sub-module capacitor voltage which is not directly measured by the voltage sensor and is the jth sub-module capacitor voltage of which the switch state is kept unchanged between the sampling time t and the previous sampling time, Sj(t) is the switching function of the jth submodule which is not directly measured by the voltage sensor and whose switching state remains unchanged between the sampling time t and the previous sampling time, SjWhen t is 1, the switching state of the jth submodule which is not directly measured by the voltage sensor and is not directly measured at the sampling time t is kept unchanged between the sampling time t and the previous sampling time S is in an on statejWhen the (t) is 0, the j-th submodule which is not directly measured by the voltage sensor and the switch state of which is kept unchanged between the t sampling moment and the previous sampling moment is in a cut-off state, s is a full-bridge submodule measured by the voltage sensor in the bridge arm, u is a full-bridge submodule measured by the voltage sensor in the bridge armcs(t) is the capacitance voltage of the full-bridge submodule measured by the slave voltage sensor in the bridge arm at the sampling time t, delta ucs(t) is the variation of the output voltage of the full bridge submodule measured from the voltage sensor between the sampling time t and the previous sampling time, us(t-1)、us(t) measured values from the voltage sensor at the sampling time t-1 and at the sampling time t, uce(t) is the capacitance voltage at the sampling time t, Δ u, of the submodule e which has been switched from the off-state to the on-state and is not directly measured by the voltage sensor between the sampling time t and the previous sampling timem(t) is the amount of change in the measured value of the primary voltage sensor between the sampling instant t and the sampling instant immediately preceding it, ucf(t-1) is the capacitance voltage u at the sampling time t-1 of the submodule f which is not directly measured by the voltage sensor and is changed from the on state to the off state between the sampling time t and the previous sampling timecf(t) is the capacitance voltage, delta, at the sampling time t of the submodule f which is not directly measured by the voltage sensor and is switched from the on state to the off state between the sampling time t and the previous sampling timeuecf(t) is the change in capacitance voltage of the submodule f, which is not directly measured by the voltage sensor, from the on-state to the off-state between the sampling time t and the previous sampling time, ucr(T) is the capacitance voltage at the sampling time T of the submodule r which is kept in the on state except the submodule measured from the voltage sensor between the sampling time T and the previous sampling time T, TCIs the system sampling period, CfThe capacitance value i of the submodule f, which is not directly measured by the voltage sensor, is changed from the switched-on state to the switched-off state between the sampling time t and the previous sampling timearm_u(t-1)、iarm_u(t) bridge arm currents at t-1 sampling time and t sampling time, respectively, CjAnd the capacitance value of the jth sub-module which is not directly measured by the voltage sensor and the switching state of which is kept unchanged between the sampling time t and the previous sampling time is obtained.
6. The method for measuring the capacitive voltage of the hybrid MMC module based on a master-slave structure of claim 5, wherein the method for compensating the sub-module capacitive voltage drop caused by the energy loss generated by the additional circuit including the sub-module balancing resistor and the switching power supply to correct the capacitive voltage of the sub-module which remains unchanged in the switch state and is not directly measured by the voltage sensor at the adjacent sampling time after the sampling period comprises:
when the full-bridge submodule measured by the voltage sensor keeps the working state unchanged between adjacent sampling moments, the variation of the output voltage of the full-bridge submodule measured by the voltage sensor between the adjacent sampling moments is the submodule capacitor voltage drop caused by the energy loss generated by an additional circuit comprising the submodule balance resistor and the switching power supply,
when the full-bridge sub-module measured by the voltage sensor is switched to the on-state or the off-state at the adjacent sampling time, the variation of the measured data of the voltage sensor at the sampling time which is closest to the current sampling time and is kept in the off-state by the full-bridge sub-module measured by the voltage sensor at the adjacent sampling time is taken as the voltage drop of the sub-module capacitor caused by the energy loss generated by an additional circuit comprising the sub-module balance resistor and a switching power supply,
by the expression: u. ofcj(t)＝ucj(t-1)+Sj(t)Δucj(t)+(Sj(t)-1)ΔucebAfter the sampling period is corrected, the switch state of the submodule is kept unchanged between adjacent sampling moments and the capacitor voltage delta u of the submodule is not directly measured by the voltage sensorcebIs the drop in voltage of the sub-module capacitor caused by the energy loss generated by the additional circuit comprising the sub-module balancing resistors and the switching power supply.
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