Method for optimizing output voltage waveform quality of cascade type electric energy quality treatment device
Technical Field
The invention relates to the technical field of control of power electronic devices, in particular to a method for optimizing the output voltage waveform quality of a cascade type power quality control device.
Background
In recent years, the construction of intelligent power grids and ubiquitous power Internet of things is greatly promoted by the nation, and high requirements are provided for the aspects of safety stability, intelligent interactivity and the like of the power grids. The power distribution network is used as the last gateway of a company directly facing customers, and the reliability and intelligent development level directly determine the obtaining feeling of the customer energy consumption. However, at present, a power grid distribution network is weak in architecture and low in equipment configuration level, and a large amount of potential value data acquired by devices such as a distribution transformer intelligent terminal, a fault indicator and a monitoring sensor are difficult to effectively apply. Therefore, the problems of user side fault and power loss, low voltage, harmonic pollution, three-phase imbalance and the like are difficult to eliminate all the time. Particularly, in the urban power supply area and the sensitive power utilization fields of medical treatment, scientific research, education, government offices and the like, the influence caused by the problems is more and more serious. The demand of users for realizing the comprehensive management of the quality of various types of electric energy is strong day by day, so that the installation of the electric energy quality management device is an important means for improving the stable operation capacity of the electric power system.
Generally, a controllable voltage source is connected in series between a system power supply and a sensitive load, so that an ideal sinusoidal voltage is provided for the sensitive load under the conditions of power supply voltage interruption and electric energy quality not meeting the national standard, and finally comprehensive treatment on the load power supply voltage is realized. The H-bridge-based cascaded multilevel converter is a cascaded circuit topological structure formed by directly connecting basic power unit modules in series and overlapping, and has the advantages of high efficiency, quick response, modularization, easiness in expansion, redundancy control and the like, so that the H-bridge-based cascaded multilevel converter is widely applied to the aspect of electric energy quality control, such as a medium-voltage high-capacity electric energy quality control device provided by the Chinese utility model patent CN 209016707U.
However, in an actual cascaded power quality control device system, under the influence of input diode rectification, output fundamental wave and harmonic current, 2-fold frequency, 6-fold frequency and a small amount of 6 n-order (n is a positive integer) pulsation occur in the direct-current voltage of the power unit. For example, for a three-phase diode rectification circuit with capacitance filtering, when the three-phase diode rectification circuit is in no load, the direct current voltage is 1.414Uin of the input line voltage peak value; when the load is heavy, the direct current voltage is about 1.35 Uin; and the actual voltage is pulsed at a frequency of 6 times between 1.35Uin and 1.414 Uin. According to the principle of H-bridge PWM (Pulse width modulation), the output voltage expression is provided
Wherein, UdIs a direct-current voltage, m is a modulation degree, and sin (ω t + α) is a modulation wave. When U is turneddWhen the value is not a constant value any more, but contains 6-fold-frequency pulsating components, the expression is assumed to be Ud=Ud0+Ud6sin (6 ω t + β), the expression of the output voltage under this condition can be derived
It can be seen that 5 th and 7 th harmonic voltages are added to the output voltage. More generally, the n-fold frequency ripple component on the direct current voltage is mapped into the output waveform through the PWM inversion output, so as to generate n +/-1 (n is a positive integer) subharmonic voltage, further deteriorate the quality of the output waveform of the device, and form the second-order superposition of the output voltage harmonic.
Therefore, how to optimize and improve the output voltage waveform quality of the cascade electric energy quality control device and reduce the capacitance value of the direct current capacitor under the condition of ensuring the stability of the direct current voltage of the device unit, thereby reducing the volume and the cost of the capacitor, and is very important for the safe and stable operation of the cascade electric energy quality control device.
Disclosure of Invention
In order to solve the problems, the invention provides a method for optimizing the output voltage waveform quality of a cascade type electric energy quality treatment device, which optimizes the output voltage waveform quality of the electric energy quality treatment device by compensating the DC side voltage ripple of a cascade unit in the electric energy quality treatment device.
In order to achieve the purpose, the invention adopts the following technical scheme:
the method for optimizing the output voltage waveform quality of the cascade type electric energy quality treatment device is characterized by comprising the following steps of:
s1: obtaining the current operation information of the cascade electric energy quality control device, including the zone bit information C of the control signal of the energy storage systemflagEffective value of output current IkMaximum harmonic content U of output voltagekharDc bus voltage u of cascaded cellskdjAnd the filtered DC bus voltage U of the cascade unitkdjDC/DC converter current i of a cascade unitkdjTerminal voltage U of energy storage system after filtering of cascade unitkbj(ii) a Wherein, k is a, B and C, which represent A, B, C three phases of a cascaded electric energy quality control device, and j is 1, 2.
S2: comparing the current operation information obtained in the step S1 with a preset output voltage waveform quality optimization threshold of the cascade type power quality control device, and calculating and generating a DC/DC converter control signal of each cascade unit according to a comparison result, wherein the method specifically comprises the following steps:
s2.1: if Ik≥αIrThen go to stepS2.2, where α is the output current compensation factor, IrTo output rated current; otherwise, the energy storage system control signal flag bit information C is orderedflagAnd returns to step S1;
s2.2: if U iskhar≥UhThen, the energy storage system control signal flag bit information C is orderedflagIf 1, go to step S2.3, where UhCompensating a threshold value for the harmonic content of the preset output voltage meeting the national standard; otherwise, returning to the step S1;
s2.3: if U iskbj≥UbcAnd U iskbj≤UkdjThen step S2.4 is entered, where UbcIs the lowest discharge voltage of the energy storage system; otherwise, returning to the step S1;
s2.4: according to the DC bus voltage u of the cascade unit
kdjAnd a DC bus reference voltage
Calculating voltage error amount, and generating first stage feedback control signal via regulator
S2.5: a first-stage feedback control signal obtained according to S2.4
DC/DC converter current i with said cascade unit
kdjCalculating current error amount, and generating DC/DC converter control signal U of cascade unit through regulator
pwm-kDCj(k=A,B,C,j=1,2,...,n);
S3: DC/DC converter control signal U of cascade unit obtained according to S2.5pwm-kDCjAnd adjusting the working state of the energy storage system corresponding to the cascade unit to reduce the output voltage ripple of the cascade unit, and continuously repeating the steps S1-S3 to realize the continuous optimization of the output voltage waveform quality of the cascade electric energy quality control device.
Further, the specific step of obtaining the current operation information of the cascaded electric energy quality control device in step S1 includes:
s1.1: obtaining energy storage system control signal zone bit information C through controller in DC/DC converterflagThe values are '0' and '1', which respectively represent standby and starting;
s1.2: the three-phase output current of the cascading type power quality control device A, B, C is sampled through a current sensor and is recorded as iA、iB、iCThen calculating the effective value I of the output currentk(k=A,B,C);
S1.3: the three-phase output voltage of the cascading type power quality control device A, B, C is sampled through a voltage sensor and is recorded as uA、uB、uCAnd performing fast Fourier transform processing on the output voltage, and obtaining the maximum harmonic content U of the output voltage according to the result of the fast Fourier transform processingkhar(k=A,B,C);
S1.4: sampling the direct-current bus voltage of the cascade unit of the cascade electric energy quality control device through a voltage sensor, and recording the direct-current bus voltage as:
wherein u isAdj、uBdj、uCdjRespectively represents the direct current bus voltage of the jth cascade unit in the three phases of the cascade type electric energy quality control device A, B, C, and the direct current bus voltage of the cascade unit is abbreviated as ukdj(k ═ a, B, C, j ═ 1,2,. n); then, the filter unit is used for filtering ripples in the DC bus voltage to obtain the filtered DC bus voltage U of the cascade unitkdj(k=A,B,C,j=1,2,...,n);
S1.5: sampling the current of a DC/DC converter of a cascade unit of the cascade electric energy quality control device through a current sensor, and recording the current as:
wherein iAdj、iBdj、iCdjRespectively representing cascade electricityThe energy quality control device A, B, C is characterized in that the DC/DC converter current of the jth cascade unit in the three phases is abbreviated as the DC/DC converter current i of the cascade unitkdj(k=A,B,C,j=1,2,...,n);
S1.6: sampling the end voltage of an energy storage system of a cascade unit of the cascade electric energy quality control device through a voltage sensor, and recording as follows:
wherein u isAbj、uBbj、uCbjRespectively representing the voltage of the energy storage system terminal of the jth cascade unit in the three phases of the cascade type electric energy quality control device A, B, C; then, the filtering unit is used for filtering ripples in the terminal voltage to obtain the filtered terminal voltage U of the energy storage system of the cascade unitkbj(k=A,B,C,j=1,2,...,n)。
Further, the regulator in steps S2.4 and S2.5 is a PI regulator.
The invention has the beneficial effects that:
compared with the prior art, the method for optimizing the output voltage waveform quality of the cascade type electric energy quality treatment device has the following beneficial effects:
(1) the invention relates to an output current effective value I of a cascade electric energy quality control devicekHarmonic wave U of output voltagekharThe harmonic content of the device is higher than a preset output voltage harmonic content compensation threshold U meeting the national standardhWhen the device is used, the cascade unit DC/DC converter controls the energy storage system to charge and discharge, so that voltage ripples at the direct current side of the cascade unit of the device are reduced, otherwise, the energy storage system does not charge and discharge, the working time of the energy storage system is greatly optimized, the service life of the energy storage system is prolonged, the output voltage waveform quality of the device is improved, and the total harmonic distortion rate of the output voltage is reduced;
(2) when the output current of the device exceeds a certain threshold value, the direct-current bus voltage ripple of the power unit can be effectively reduced through the working mode of the direct-current side energy storage system of the cascade unit of the dynamic adjusting device, so that the capacitance value and the volume of the direct-current side capacitor of the cascade unit of the device can be reduced when system hardware is designed, and the compact layout and the cost optimization of the whole structure of the device are realized.
(3) The method of the invention has simple algorithm and is easy to realize in engineering.
Drawings
Fig. 1 is a topological diagram of a cascaded electric energy quality control device provided in embodiment 1 of the present invention;
fig. 2 is a flowchart of a method for optimizing the output voltage waveform quality of a cascaded electric energy quality control device according to embodiment 1 of the present invention;
fig. 3 is a waveform comparison diagram before and after the output voltage waveform quality of the cascade type power quality management device provided in embodiment 1 of the present invention is optimized; the voltage is the superposition of direct current voltages of the A-phase n cascade units, and the output voltage of the A1 cascade unit;
fig. 4 is a comparison graph of harmonic content before and after the output voltage waveform quality of the cascade power quality control device provided in embodiment 1 of the present invention is optimized; wherein (a) is before optimization and (b) is after optimization.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described with reference to the following embodiments and the accompanying drawings.
Example 1
The embodiment provides a method for optimizing the output voltage waveform quality of a cascading type electric energy quality treatment device, wherein a topological graph of the cascading type electric energy quality treatment device is shown in fig. 1 and comprises 3 × n cascading units; the cascade unit comprises a three-phase uncontrolled rectifier bridge, a cascade H-bridge converter, a DC/DC converter, an energy storage system, a power diode and a direct current bus capacitor, wherein the DC/DC converter comprises a controller;
the cascade unit has 4 working modes:
mode 1: in the normal working mode, namely, the alternating current power grid transmits power to the load, the three-phase uncontrolled rectifier and the cascaded H-bridge converter work normally, at the moment, the cascaded unit can solve the problems of harmonic distortion, flicker, three-phase imbalance and other electric energy quality at the grid side, stable 10kV voltage is output in a superposed mode, and high-quality power supply of the load is realized;
mode 2: the energy storage system is in a passive discharge mode, namely, an alternating current power grid and the energy storage system transmit power to a load, the direct current bus voltage of the cascade unit is lower than the voltage of the energy storage system, namely, the power distribution grid generates voltage sag or short-time interruption, and the cascade unit carries out zero-delay passive discharge on the energy storage system through a power diode, so that the output voltage of the cascade unit is stable, and the uninterrupted power supply of the load is realized;
mode 3: the energy storage system is in an active charging and discharging mode, namely, an alternating current power grid transmits power to a load and the energy storage system, and when the voltage input of a power distribution network is normal, the device control system conducts active charging and discharging management on the energy storage system through the bidirectional DC/DC converter according to the SOC state of the energy storage system and the range of the direct current bus voltage of the cascade unit, so that the service life of the energy storage system is prolonged;
mode 4: optimizing the output voltage waveform quality mode of the cascade type electric energy quality management device, wherein the device complete machine control system outputs a voltage waveform quality optimization threshold value through a preset device at the moment, then comparing the obtained current operation information of the device with the preset threshold value, calculating and generating a device cascade unit DC/DC converter control signal according to a comparison result, and controlling the DC/DC converter to charge and discharge the energy storage system, so that the voltage ripple of the direct current side of the device cascade unit is reduced, and the waveform quality of the output voltage of the device is improved.
The method for optimizing the output voltage waveform quality of the cascaded electric energy quality control device provided by the invention is described in detail below with reference to fig. 2, and specifically comprises the following steps:
s1: the method comprises the following steps of obtaining current operation information of a cascading type electric energy quality control device, and specifically comprises the following steps:
s1.1: obtaining energy storage system control signal zone bit information C through controller in DC/DC converterflagThe values are '0' and '1', which respectively represent standby and starting;
s1.2: tong (Chinese character of 'tong')The overcurrent sensor samples the three-phase output current of the cascade type power quality control device A, B, C, which is recorded as iA、iB、iCThen calculating the effective value I of the output currentk(k=A,B,C);
S1.3: the three-phase output voltage of the cascading type power quality control device A, B, C is sampled through a voltage sensor and is recorded as uA、uB、uCAnd performing fast Fourier transform processing on the output voltage, and obtaining the maximum harmonic content U of the output voltage according to the result of the fast Fourier transform processingkhar(k=A,B,C);
S1.4: sampling the direct-current bus voltage of the cascade unit of the cascade electric energy quality control device through a voltage sensor, and recording the direct-current bus voltage as:
wherein u isAdj、uBdj、uCdjRespectively represents the direct current bus voltage of the jth cascade unit in the three phases of the cascade type electric energy quality control device A, B, C, and the direct current bus voltage of the cascade unit is abbreviated as ukdj(k ═ a, B, C, j ═ 1,2,. n); then, the filter unit is used for filtering ripples in the DC bus voltage to obtain the filtered DC bus voltage U of the cascade unitkdj(k=A,B,C,j=1,2,...,n);
S1.5: sampling the current of a DC/DC converter of a cascade unit of the cascade electric energy quality control device through a current sensor, and recording the current as:
wherein iAdj、iBdj、iCdjRespectively represents the current of the DC/DC converter of the jth cascade unit in the three phases of the cascade electric energy quality control device A, B, C, which is abbreviated as the current i of the DC/DC converter of the cascade unitkdj(k=A,B,C,j=1,2,...,n);
S1.6: sampling the end voltage of an energy storage system of a cascade unit of the cascade electric energy quality control device through a voltage sensor, and recording as follows:
wherein u isAbj、uBbj、uCbjRespectively representing the voltage of the energy storage system terminal of the jth cascade unit in the three phases of the cascade type electric energy quality control device A, B, C; then, the filtering unit is used for filtering ripples in the terminal voltage to obtain the filtered terminal voltage U of the energy storage system of the cascade unitkbj(k=A,B,C,j=1,2,...,n);
S2.1: if Ik≥αIrThen step S2.2 is entered, where α is the output current compensation factor, IrTo output rated current; otherwise, the energy storage system control signal flag bit information C is orderedflagAnd returns to step S1;
s2.2: if U iskhar≥UhThen, the energy storage system control signal flag bit information C is orderedflagIf 1, go to step S2.3, where UhCompensating a threshold value for the harmonic content of the preset output voltage meeting the national standard; otherwise, returning to the step S1;
s2.3: if U iskbj≥UbcAnd U iskbj≤UkdjThen step S2.4 is entered, where UbcIs the lowest discharge voltage of the energy storage system; otherwise, returning to the step S1;
s2.4: according to the DC bus voltage u of the cascade unit
kdjAnd a DC bus reference voltage
Calculating voltage error amount, and generating first stage feedback control signal via regulator
The regulator is a PI regulator;
s2.5: a first-stage feedback control signal obtained according to S2.4
DC/DC converter current i with said cascade unit
kdjCalculating current error amount, and generating DC/DC converter control signal U of cascade unit through regulator
pwm-kDCj(k ═ a, B, C, j ═ 1,2,. n); the regulator is a PI regulator;
s3: DC/DC converter control signal U of cascade unit obtained according to S2.5pwm-kDCjAnd adjusting the working state of the energy storage system corresponding to the cascade unit to reduce the output voltage ripple of the cascade unit, and continuously repeating the steps S1-S3 to realize the continuous optimization of the output voltage waveform quality of the cascade electric energy quality control device.
Fig. 3 is a waveform comparison diagram before and after the output voltage waveform quality of the cascaded electric energy quality control device provided by the embodiment is optimized; the voltage is the superposition of direct current voltages of the A-phase n cascade units, and the output voltage of the A1 cascade unit; fig. 4 is a comparison graph of harmonic content before and after the output voltage waveform quality of the cascade power quality control device provided in this embodiment is optimized; wherein (a) is before optimization and (b) is after optimization. As can be seen from fig. 3 and 4, when the cascade electric energy quality control device operates in no-load, the ripple of the output voltage of the device is small; then, a rated inductance resistance load is added at 0.06 second, the superposition of the direct current voltages of the n cascade units of the A phase and the waveform of the output voltage of the A1 cascade unit both have fluctuation phenomena, namely, the direct current power supply double-frequency pulsation caused by the cascade H-bridge converter is superposed into the output waveform, and the harmonic content (THD) of the output voltage of the device is 2.71 percent; then, at the time of 0.14 second, the method for optimizing the output voltage waveform quality of the cascade type electric energy quality control device is adopted, at this time, the superposition of the direct-current voltages of the n cascade units of the A phase and the ripple of the output voltage of the A1 cascade unit are rapidly reduced, the voltage amplitude tends to be stable, the head deviation phenomenon of the output waveform also disappears, and at this time, the harmonic content (THD) of the output voltage of the device is reduced to 0.30%.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.