Full-duplex information and energy simultaneous transmission system for distributed power generation

Abstract

The invention discloses a full-duplex information and energy simultaneous transmission system for distributed power generation, and the system comprises the steps: modulating the numerical information of a to-be-transmitted digital signal into the phase transformation information of a phase-shift intermediate-frequency sinusoidal signal through phase-shift keying modulation; then the phase shift intermediate frequency sinusoidal signal is superposed on a voltage fundamental wave signal output by an inverter control loop to obtain a reference signal for pulse width modulation of a single-phase inverter, pulse width modulation is carried out on the reference signal, an intermediate frequency harmonic signal capable of being transmitted on a power line is generated, and transmission of the signal on the power line is achieved; meanwhile, output information of different inverters is distributed on different frequency bands, and the frequency division multiplexing type full duplex signal / energy synchronous transmission technology is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of renewable energy power generation, and more specifically, relates to a full-duplex signal-energy simultaneous transmission system for distributed power generation. Background Art

[0002] The communication system can transmit the status data, fault information and control instructions of power equipment, so that the control center can quickly respond to and handle various situations, ensure the stable supply and quality of electricity, and is an indispensable part of the power system. At present, communication in the power system is mainly achieved through wireless communication technology and wired communication technology. Wireless communication has good adaptability and rapidity, but due to signal attenuation, the transmission distance is limited, the signal may be interfered by various noises and network attacks, and the transmission reliability is low. Wired communication is mainly achieved by adding other communication lines, so it has strong anti-interference and small attenuation. However, the independent communication system of each component will bring additional costs for the construction and maintenance of the signal line. Therefore, a new and reliable communication technology is urgently needed for power system signal transmission.

[0003] With the advancement of communication technology and power electronics technology, a power system communication method called power line carrier communication has emerged and has received widespread attention. Power line carrier communication uses the existing power system (power lines, cables, wireless) as a channel for signal transmission and transmits signals in real time, thereby avoiding the use of independent communication systems or communication lines. It has the characteristics of small investment, strong flexibility, and wide coverage. In the context of the country's vigorous development of energy Internet and smart grid, using power lines as a medium to transmit information can not only fully supplement and improve my country's communication platform, but also enhance my country's global competitiveness and cutting-edge nature in the construction of smart grids. However, traditional PLC requires additional circuits to inject and amplify sinusoidal signals of specific frequencies, which will undoubtedly increase the cost of the power grid system. At the same time, studies have shown that power line carrier communication will affect the normal operation of power electronic converters and reduce the energy transmission efficiency of the system. How to reduce the burden of power line carrier communication on system cost and system energy density has become an urgent issue to be solved.

[0004] As a common energy conversion device used to connect new energy and electrical equipment, power electronic converters play an important role in renewable energy power generation systems. They play an important role in power conversion and control, and can achieve efficient and reliable energy conversion. Among them, pulse width modulation is an important means for power electronic converters to achieve energy conversion. By comparing the reference signal and the triangular carrier signal, the gate drive signal is obtained to control the on and off of the circuit switch devices on the bridge arm of the converter, so that a series of pulses with equal amplitude but inconsistent width are obtained at the output end, and these pulses are used to replace the sine wave or the required waveform. In the pulse width modulation process, the amplitude, frequency and phase of the sinusoidal reference signal and the triangular carrier signal provide additional degrees of freedom for signal encoding, so power electronics has the potential to load and transmit information. In recent years, the method of using the additional degrees of freedom of pulse width modulation of power electronic converters for signal transmission has been named signal / energy synchronous transmission technology, referred to as signal-energy synchronous transmission technology.

[0005] At present, signal / energy synchronous transmission technology is mainly used in DC power grids, and is less used in inverters and AC power grids. In addition, signal transmission of signal / energy synchronous transmission technology mainly adopts simplex communication or half-duplex communication, lacking the full-duplex communication function that can transmit information in both directions at the same time. For multi-inverter distributed power generation systems, non-full-duplex communication methods will lead to untimely information exchange and waste of frequency band resources. Therefore, modern power systems are in urgent need of a full-duplex signal / energy synchronous transmission solution between inverters for multi-inverter distributed power generation. Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a full-duplex signal and energy simultaneous transmission system for distributed power generation, so that multiple distributed power generation systems can simultaneously realize synchronous transmission of signals and energy.

[0007] To achieve the above-mentioned purpose, the present invention provides a full-duplex signal and energy transmission system for distributed power generation, characterized by comprising: N distributed signal and energy transmission power generation systems connected in parallel to an AC bus, an AC bus and an AC load;

[0008] Each distributed signal-energy simultaneous transmission power generation system generates an AC voltage consistent with the phase, amplitude and frequency of the AC bus voltage through droop control and is connected to the AC bus to supply power to the AC load; at the same time, each distributed signal-energy simultaneous transmission power generation system couples the digital signal to be transmitted into the harmonic signal of the inverter output voltage through signal modulation, and then transmits the harmonic signal to the remaining distributed signal-energy simultaneous transmission power generation systems through the AC bus. The remaining distributed signal-energy simultaneous transmission power generation systems can obtain the digital signals to be transmitted by the remaining distributed signal-energy simultaneous transmission power generation systems through the internal demodulation system.

[0009] The object of the invention of the present invention is achieved in this way:

[0010] The present invention discloses a full-duplex signal-energy simultaneous transmission system for distributed power generation. The system modulates the numerical information of a digital signal to be transmitted into the phase change information of a phase-shifted intermediate frequency sinusoidal signal through phase-shift keying modulation. The phase-shifted intermediate frequency sinusoidal signal is then superimposed on the voltage fundamental signal output by the inverter control loop to obtain a reference signal for pulse width modulation of a single-phase inverter and performs pulse width modulation on the reference signal to generate an intermediate frequency harmonic signal that can be transmitted on a power line, thereby realizing signal transmission on the power line. Meanwhile, different inverter output information is distributed on different frequency bands, thereby realizing frequency division multiplexing type full-duplex signal / energy synchronous transmission technology.

[0011] At the same time, the full-duplex signal-energy simultaneous transmission system for distributed power generation of the present invention also has the following beneficial effects:

[0012] (1) The present invention can realize the conversion of digital signals into intermediate frequency harmonics of output voltage by performing phase shift keying on the pulse width modulated reference signal through the controller, without the need to inject additional harmonic signals and signal coupling devices, thus reducing the cost and volume of the system, and making full use of the spectrum resources of the inverter output voltage to realize full-duplex communication of information between multiple inverters, thereby improving the utilization rate of the frequency band and realizing simultaneous bidirectional information transmission between inverters;

[0013] (2) Different inverters use different frequency bands to transmit information, making full use of the frequency spectrum resources of the inverter output voltage to achieve full-duplex communication of information between multiple inverters, improving the frequency band utilization rate and achieving simultaneous two-way information transmission between inverters;

[0014] (3) The present invention analyzes the types of harmonics that will be generated on the output voltage of a single-phase inverter when a phase-shifted intermediate frequency sinusoidal signal is injected into a pulse width modulation reference signal, and provides relevant precautions for injecting phase-shifted intermediate frequency sinusoidal signals into different inverters: the frequencies and frequency bands of the phase-shifted intermediate frequency sinusoidal signals used by different inverters should be different, and a frequency band buffer should be set between different harmonics to prevent the generated harmonic signals from interfering with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is an architecture diagram of a full-duplex signal-energy simultaneous transmission system for distributed power generation according to the present invention;

[0016] Figure 2 It is the structural diagram of the distributed signal-energy simultaneous transmission inverter power generation system;

[0017] Figure 3 is a schematic diagram of a signal modulation system 1 and a signal modulation system 2;

[0018] Figure 4 is a schematic diagram of a signal demodulation system 1 and a signal demodulation system 2;

[0019] Figure 5 It is the AC bus voltage and spectrum analysis result diagram under different working conditions;

[0020] Figure 6 is the signal demodulation result on the inverter 1 side, including the intermediate frequency harmonic signal from the inverter 2 extracted by the bandwidth filter 1, the coherent demodulation result of the signal demodulation system 1, and the input and output signal comparison result;

[0021] Figure 7 It is the signal demodulation result on the inverter 2 side, including the intermediate frequency harmonic signal from the inverter 1 extracted by the bandwidth filter 2, the coherent demodulation result of the signal demodulation system 2, and the input and output signal comparison result. DETAILED DESCRIPTION

[0022] The specific implementation of the present invention is described below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0023] Example

[0024] Figure 1 This is an architecture diagram of a full-duplex signal and energy simultaneous transmission system for distributed power generation according to the present invention.

[0025] In this embodiment, if Figure 1 As shown, the present invention is a full-duplex signal and energy simultaneous transmission system for distributed power generation, comprising: N distributed signal and energy simultaneous transmission power generation systems connected in parallel to an AC bus, an AC bus and an AC load;

[0026] Each distributed signal-to-energy simultaneous power generation system generates an AC voltage that is consistent with the phase, amplitude and frequency of the AC bus voltage through droop control and is connected to the AC bus to supply power to the AC load; at the same time, each distributed signal-to-energy simultaneous power generation system couples the digital signal to be transmitted into the harmonic signal of the inverter output voltage through signal modulation, and then transmits the harmonic signal to the remaining distributed signal-to-energy simultaneous power generation systems through the AC bus.

[0027] like Figure 2 As shown, taking the nth distributed signal-energy simultaneous transmission power generation system as an example, its structure specifically includes: control system, signal modulation system, adder, DC voltage source, pulse width modulation module, single-phase full-bridge inverter circuit, LC filter circuit, voltmeter, bandpass filter, phase-locked loop and signal demodulation system;

[0028] In this embodiment, the control system adopts droop control to realize the grid connection of the distributed signal-energy simultaneous transmission power generation system; first, the control system detects the output current and voltage of the distributed signal-energy simultaneous transmission power generation system, obtains the active and reactive power of the distributed signal-energy simultaneous transmission power generation system, and then decouples the active power and reactive power; then, the control system extracts the frequency and amplitude of the output voltage of the distributed signal-energy simultaneous transmission power generation system and uses them as reference values ​​for droop control, thereby reasonably allocating the active and reactive power of the distributed signal-energy simultaneous transmission power generation system and generating a stable low-frequency voltage fundamental wave signal;

[0029] The signal modulation system is used to receive the digital signal to be transmitted and the given intermediate frequency sinusoidal signal, and then perform phase shift keying modulation on the intermediate frequency sinusoidal signal according to the transmitted digital signal to generate a phase-shifted intermediate frequency sinusoidal signal carrying information;

[0030] In this embodiment, the digital signal to be transmitted is a square wave signal, and the phase-shifted intermediate frequency sinusoidal signal is an intermediate frequency sinusoidal signal with a continuously changing phase, and its phase is determined by the input square wave signal and represents different signals; wherein, when the value of the input square wave signal is 0, the phase of the intermediate frequency sinusoidal signal is 0°, and when the value of the square wave signal is 1, the phase of the intermediate frequency sinusoidal signal is 180°;

[0031] The adder adds the low-frequency voltage fundamental signal and the phase-shifted intermediate-frequency sinusoidal signal to obtain a reference signal;

[0032] The pulse width modulation module generates a gate drive signal by comparing the reference signal and the triangular carrier.

[0033] The single-phase full-bridge inverter circuit is used to receive a gate drive signal and control the switching action of each active switch in the single-phase full-bridge inverter circuit through the gate drive signal, thereby generating a pulse width modulation signal;

[0034] In this embodiment, the single-phase full-bridge inverter circuit is also called an inverter, and its topology is as follows: the single-phase full-bridge inverter circuit consists of an active switch S n1 ~S n4 and diode D n1 ~D n4 Composition, active switch S n1 ~S n4 Both are insulated bipolar transistors with diodes;

[0035] The four switch tubes are divided into two groups, each with two switch tubes; the two groups of switch tubes are connected to the positive and negative ends of the DC power supply respectively, and the conversion from DC to AC is realized by controlling the conduction and shutdown of the two groups of switch tubes; the collector of each active switch is connected to the cathode of the corresponding diode, the emitter of each active switch is connected to the anode of the corresponding diode, and the drain of each active switch is connected to the gate drive signal input by the controller; the active switch S n1 ,Sn2 The collector of the active switch S is connected to the positive terminal of the DC voltage source. n3 ,S n4 The emitter of is connected to the negative terminal of the DC voltage source; the active switch S n1 The emitter of the active switch S n3 The collector of the active switch S is connected as a group. n2 The emitter of the active switch S n4 The collectors of the inverter are connected as a group to form a bridge arm of a single-phase full-bridge inverter circuit; an LC filter circuit is connected at the midpoint of each group;

[0036] The LC filter circuit is composed of an inductor L and a capacitor C, and is connected to a single-phase full-bridge inverter circuit to filter the pulse width modulation signal to obtain a sinusoidal voltage signal with good power quality. Finally, the sinusoidal voltage signal is merged into the public AC bus to provide power for the load.

[0037] Finally, the outputs of multiple inverters are filtered by the LC filter circuit and then connected to the public AC bus together with other inverters to provide power to the load, thus completing the grid connection of the distributed power generation system;

[0038] In order to realize the signal transmission of the distributed power generation system, the voltage across the capacitor C in the LC filter circuit is read by a voltmeter in each distributed power generation system to obtain the voltage of the AC bus, and then the AC bus voltage is filtered by a bandpass filter to obtain the intermediate frequency harmonic signal H of the target frequency band. i ;Intermediate frequency harmonic signal H i satisfy:

[0039] H i =A H cos(2πf i t+θ i )

[0040] Among them, A H is the intermediate frequency harmonic H i The amplitude after passing through the LC filter circuit, f i is the frequency of the phase-shifted intermediate frequency carrier signal, θ i is the intermediate frequency harmonic H i Phase on the AC bus after passing through the LC filter circuit;

[0041] In this embodiment, the intermediate frequency harmonic signal H iThe phase-shifted intermediate frequency sinusoidal signal of the i-th distributed signal and energy simultaneous transmission and power generation system is generated through pulse width modulation, and its frequency is equal to the frequency of the injected phase-shifted intermediate frequency sinusoidal signal, and its phase changes with the phase of the phase-shifted intermediate frequency sinusoidal signal; in the entire full-duplex signal and energy simultaneous transmission system, the frequency and frequency band of the phase-shifted intermediate frequency sinusoidal signal used in different distributed signal and energy simultaneous transmission and power generation systems should be different.

[0042] Finally, the intermediate frequency harmonic signal H is obtained through the phase-locked loop i The phase θ i , and transmit it to the signal demodulation system. The signal demodulation system uses the intermediate frequency harmonic signal H i The frequency f i and phase θ i Generate synchronous demodulation signal and IF harmonic signal H i Coherent demodulation is performed to obtain the digital signals transmitted by the remaining distributed signal-energy simultaneous transmission and power generation systems, where i represents the remaining N-1 distributed signal-energy simultaneous transmission and power generation systems except the nth distributed signal-energy simultaneous transmission and power generation system.

[0043] Figure 3 A schematic diagram of a signal modulation system 1 and a signal modulation system 2 is shown;

[0044] In this embodiment, if Figure 3 As shown, the signal modulation system 1 is responsible for the signal modulation of the distributed signal-energy simultaneous transmission and power generation system 1, and the signal modulation system 2 is responsible for the signal modulation of the distributed signal-energy simultaneous transmission and power generation system 2. The distributed signal-energy simultaneous transmission and power generation systems 1 and 2 are any two submodules in a full-duplex signal-energy simultaneous transmission system for distributed power generation of the present invention; both signal modulation systems use phase shift keying modulation to couple the information of the input square wave signal into the sinusoidal signal to generate a phase-shifted intermediate frequency sinusoidal signal; the phase-shifted intermediate frequency sinusoidal signal is an intermediate frequency sinusoidal signal with continuous phase change and constant frequency, and its phase is determined by the input square wave signal, wherein when the value of the input square wave signal is 0, the phase of the intermediate frequency sinusoidal signal is 0°; when the value of the square wave signal is 1, the phase of the intermediate frequency sinusoidal signal is 180°, so the data information of the input square wave signal is modulated into the phase information of the phase-shifted intermediate frequency sinusoidal signal; it is worth noting that the frequencies of the sinusoidal signals used by the two signal modulation systems are inconsistent, which is intended to distinguish the intermediate frequency harmonics from different inverters in the signal transmission frequency domain.

[0045] Figure 4 The schematic diagrams of the signal demodulation system 1 and the signal demodulation system 2 are shown;

[0046] In this embodiment, if Figure 4As shown, the signal demodulation system 1 is responsible for the signal demodulation of the distributed signal energy simultaneous transmission and power generation system 1, and the signal demodulation system 2 is responsible for the signal demodulation of the distributed signal energy simultaneous transmission and power generation system 2; the structures of the two signal demodulation systems are the same, both of which include a synchronous signal generator, a coherent demodulator, and a threshold judgement device; the input harmonic phase of the two systems is obtained by a phase-locked loop, and the harmonic frequency is consistent with the center frequency of the bandpass filter; the synchronous signal generator generates the initial intermediate frequency harmonic H extracted by the bandpass filter according to the input harmonic phase and harmonic frequency i =A H cos(2πf i t+θ i ) frequency and phase consistent synchronous demodulation signal and used for coherent demodulation.

[0047] Figure 5 The AC bus voltage and spectrum analysis results under different working conditions are shown;

[0048] In this embodiment, Figure 5 (a) Figure 5 (b) and Figure 5 (c) are the AC bus voltage and spectrum analysis results without injection signal, with injection signal and transmission rate of 0 bps, and with signal and transmission rate of 300 bps; the frequency of the phase-shifted intermediate frequency sinusoidal signal injected by inverter 1 is 6 kHz, and the frequency of the phase-shifted intermediate frequency sinusoidal signal injected by inverter 2 is 4 kHz; by comparison Figure 5 (a) and Figure 5 (b) It can be found that after injecting 6kHz and 4kHz phase-shifted intermediate frequency sinusoidal signals, 6kHz and 4kHz intermediate frequency harmonic signals and harmonic frequency bands will be generated in the AC bus voltage accordingly; by comparison Figure 5 (b) and Figure 5 (c) It can be found that when the transmission rate increases from 0bps to 200bps, the frequency band of harmonic signal transmission becomes wider. Therefore, a buffer zone needs to be reserved when designing two frequency bands. In this embodiment, the 5kHz frequency band is between the 6kHz and 4kHz frequency bands to prevent data from interfering with each other between the two frequency bands.

[0049] Figure 6 The signal demodulation results on the inverter 1 side are shown;

[0050] In this embodiment, if Figure 6 As shown, the signal demodulation result on the inverter 1 side includes the intermediate frequency harmonic signal extracted by the bandwidth filter 1 from the inverter 2, the coherent demodulation result of the signal demodulation system 1, and the input and output signal comparison result; the information collected on the inverter 1 side is the information from the inverter 2, so the center frequency of the bandpass filter 1 is selected as the frequency of the phase-shifted intermediate frequency sinusoidal signal injected by the inverter 2, that is, 4kHz is selected as the center frequency of the bandpass filter 1; by comparing Figure 6 (a) and Figure 6 (b) It can be found that the signal demodulation system 1 in this embodiment can accurately demodulate to obtain an output square wave signal that is consistent with the input square wave signal.

[0051] Figure 7 The signal demodulation results on the inverter 2 side are shown;

[0052] In this embodiment, if Figure 7 As shown, the signal demodulation structure on the inverter 2 side includes the intermediate frequency harmonic signal extracted from the inverter 1 by the bandwidth filter 2, the coherent demodulation result of the signal demodulation system 2, and the input and output signal comparison result; the information collected on the inverter 2 side is the information from the inverter 1, so the center frequency of the bandpass filter 2 is selected as the frequency of the phase-shifted intermediate frequency sinusoidal signal injected by the inverter 1, that is, 6kHz is selected as the center frequency of the bandpass filter 2; by comparing Figure 7 (a) and Figure 7 (b) It can be found that the signal demodulation system 2 in this embodiment can accurately demodulate to obtain an output square wave signal that is consistent with the input square wave signal.

[0053] Although the above describes the illustrative specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A full-duplex signal-energy simultaneous transmission system for distributed power generation, characterized in that: include: N distributed signal-energy simultaneous generation systems connected in parallel to the AC bus, the AC bus and the AC load; Each distributed signal-to-energy simultaneous power generation system generates an AC voltage that is consistent with the phase, amplitude and frequency of the AC bus voltage through droop control and is connected to the AC bus to supply power to the AC load; at the same time, each distributed signal-to-energy simultaneous power generation system couples the digital signal to be transmitted into the harmonic signal of the inverter output voltage through signal modulation, and then transmits the harmonic signal to the remaining inverters through the AC bus.

2. According to claim 1, a full-duplex signal and energy simultaneous transmission system for distributed power generation, characterized in that: The distributed signal-energy simultaneous transmission power generation system comprises: a control system, a signal modulation system, an adder, a DC voltage source, a pulse width modulation module, a single-phase full-bridge inverter circuit, an LC filter circuit, a bandpass filter, a phase-locked loop and a signal demodulation system; The control system adopts droop control to realize the grid connection of the distributed signal-energy simultaneous transmission power generation system; first, the control system detects the output current and voltage of the distributed signal-energy simultaneous transmission power generation system, obtains the active power and reactive power of the distributed signal-energy simultaneous transmission power generation system, and then decouples the active power and reactive power; then, the control system extracts the frequency and amplitude of the output voltage of the distributed signal-energy simultaneous transmission power generation system and uses them as reference values ​​for droop control, thereby reasonably allocating the active and reactive power of the distributed signal-energy simultaneous transmission power generation system and generating a stable low-frequency voltage fundamental wave signal; The signal modulation system is used to receive a digital signal to be transmitted and a given intermediate frequency sinusoidal signal, and then perform phase shift keying modulation on the intermediate frequency sinusoidal signal according to the transmitted digital signal to generate a phase-shifted intermediate frequency sinusoidal signal carrying information; The adder adds the low-frequency voltage fundamental signal and the phase-shifted intermediate-frequency sinusoidal signal to obtain a reference signal; The pulse width modulation module generates a gate drive signal by comparing a reference signal with a high-frequency triangular carrier signal; The single-phase full-bridge inverter circuit is used to receive a gate drive signal and control the switching action of each active switch in the single-phase full-bridge inverter circuit through the gate drive signal, thereby generating a pulse width modulation signal; The LC filter circuit is composed of an inductor L and a capacitor C, and is connected to a single-phase full-bridge inverter circuit to filter the pulse width modulation signal to obtain a sinusoidal voltage signal with good power quality, and finally merge the sinusoidal voltage signal into a public AC bus to provide power for the load; The voltmeter reads the voltage across the capacitor C in the LC filter circuit, thereby obtaining the voltage of the AC bus; The bandpass filter is used to filter the AC bus voltage to obtain the intermediate frequency harmonic signal H of the target frequency band. i ; The intermediate frequency harmonic signal H obtained by the phase-locked loop i The phase θ i , i represents the number of the distributed signal-energy simultaneous transmission and power generation system; The signal demodulation system is based on the intermediate frequency harmonic signal H i The frequency f i and phase θ i Generate a synchronous demodulation signal and use the synchronous demodulation signal to modulate the intermediate frequency harmonic signal H i Coherent demodulation is performed to obtain the digital signals transmitted by the other distributed signal and energy simultaneous power generation systems.

3. According to claim 1, a full-duplex signal and energy simultaneous transmission system for distributed power generation, characterized in that: The digital signal to be transmitted is a square wave signal.

4. According to claim 1, a full-duplex signal and energy simultaneous transmission system for distributed power generation, characterized in that: The frequencies and frequency bands of the phase-shifted intermediate frequency sinusoidal signals used in the different distributed energy transmission inverter power generation systems should be different.

5. According to claim 1, a full-duplex signal and energy simultaneous transmission system for distributed power generation, characterized in that: The intermediate frequency harmonic signal H i satisfy: H i =A H cos(2πf i t+θ i ) Among them, A H is the intermediate frequency harmonic signal H i The amplitude after passing through the LC filter circuit, f i is the frequency of the phase-shifted intermediate frequency carrier signal, θ i It is the phase of the intermediate frequency harmonic on the AC bus after passing through the LC filtering circuit.

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