Abstract: Developing new renewable energy sources, reducing environmental pollution, and improving power quality have become a hot topic in China's energy industry. Wind power generation technology, with its advantages of safety and reliability, zero pollution, no fuel consumption, short construction period, flexible scale, and grid connection, has emerged as a leading force in the energy and power industry.
The extensive use of power electronic devices in wind power systems inevitably introduces electromagnetic compatibility (EMC) issues, which severely impact the system, and this impact intensifies with increasing system power. This paper takes a 2MW doubly-fed induction generator (DFIG) wind power converter system as the research object, analyzes the main interference sources, interference coupling paths, and sensitive devices of the converter system, and performs EMC design on the converter. These designs include the design of EMI filters for the power line at the grid-side converter input and the EMI filters for the rotor-side converter output. Experiments on the 2MW DFIG wind power converter system show that after complete EMC design, the converter system operates stably and has strong anti-interference capabilities.
Keywords: Doubly fed wind power converter, EMI filter, electromagnetic compatibility
Electro-Magnetic Compatibility Design of MW Grade Doubly-Fed Wind Power Converter
Abstract: It is a focus of attention of the world energy industry for utilization of renewable energy, reducing environmental pollution and improving power quality. Wind energy is a new force suddenly rises in energy sources and power industry for its characteristics of safety, credibility, no pollution, no expending fuel, short construction period, small scale, combines the net to run and so on.
Due to a mass of power electronic equipments are used in wind power system, it is inescapable introduce electromagnetic compatibility issues, and make serious influence to the system, furthermore, it becomes graveness as the system power increase.The paper is aimed to the study of 2MW grade doubly-fed wind-energy generation system. common mode noise EMI filter design of rotor-side converter.Via experimentation on 2.0MW doubly-fed wind power converter system, it indicates that: the converter work stably, it has the great capacity of with standing interference after integrated Electro-Magnetic Compatibility Design for converter system.
Key words : Doubly-Fed Wind Power Converter; EMI filter; Electro-Magnetic Compatibility
introduction
In today's rapidly developing and fiercely competitive global economy, new energy power generation, especially wind power technology, is receiving increasing attention from Western countries. Due to the extensive use of power electronic devices in wind power systems, electromagnetic compatibility (EMC) issues are inevitably introduced. Especially with the development of MW-level high-power wind power technology and the increasing reliability requirements for wind turbine units, the EMC problems of wind power converters have become increasingly prominent, becoming one of the important factors affecting the safe and stable operation of the converter itself and the entire wind turbine unit. This paper studies the EMC problem based on a 2.0MW doubly-fed wind power converter, aiming to achieve stable operation and enhanced anti-interference capability of the converter system.
1. Main interference sources and coupling paths of the converter system
1.1 Main sources of interference
The commutation process of power electronic devices generates pulses with very steep leading and trailing edges, resulting in severe electromagnetic interference. This interference, through conduction and radiation coupling, seriously pollutes the surrounding electromagnetic environment and the power electronic devices in the power supply system. The electromagnetic interference of this doubly-fed converter system mainly comes from the following aspects:
(1) du/dt: At the moment of switching on and off of power electronic devices, the voltage jump will generate a large charging or discharging current on the capacitor. Both the drive circuit and the main circuit will have stray distributed capacitance. A capacitor of a few nanofarads can generate a transient current pulse of several amperes, which will cause serious electromagnetic interference to the power electronic system.
(2) di/dt: The current change at the moment of switching on and off will induce a spike voltage on the stray inductance; in addition, a current loop with a large di/dt is also a radiation source, which will generate a radiated electromagnetic field in space.
(3) High frequency of power electronic devices: The switching frequency of the converter is 2.5 kHz, which will cause strong conducted and radiated electromagnetic interference.
(4) Control circuit power supply: Since the control system power supply of the converter system is a switching power supply, the operating frequency of the switching power supply is generally in the tens of kilohertz or even hundreds of kilohertz; electromagnetic leakage of high frequency transformers, etc. will interfere with the nearby circuits.
1.2 Main Interference Coupling Paths
Electromagnetic interference (EMI) in power electronic devices can be classified into two categories based on its propagation path: conducted interference and radiated interference. Conducted interference refers to the coupling of a signal from one electrical network to another through a conductive medium; radiated interference refers to the coupling (interference) of its signal from an interference source to another electrical network through space. Because radiated interference in power electronic devices is more complex, most research on EMI in power electronic devices focuses on the analysis, modeling, and solution of conducted interference problems.
1.2.1 Conductive Coupling
Interference signals are coupled into the circuit through power lines, signal lines, and control lines. This is the main coupling method in power electronic systems. In this paper, the switching frequency of the doubly-fed induction generator (DFIG) for wind power generation is 2.5 kHz, so conducted interference is our main consideration. In 1989, MJNave proposed equivalent circuit models for common-mode interference and differential-mode interference using a switching power supply as an example, and predicted conducted interference from the time and frequency domains. Figures 1.1(a) and (b) show the voltage spike caused when the switch is turned off and the current spike caused when the switch is turned on, respectively.
(l) Common-mode conducted interference
Common-mode interference is interference between the power line and the ground, so it cannot propagate through the electromagnetic mutual generation principle of the transformer, but must be transmitted through coupling capacitors. In converters, there is distributed capacitance between the switch tube casing and the heat sink, which provides a transmission path for common-mode interference; common-mode current interference sometimes transforms into radiated interference during transmission, making the system interference more complex.
(2) Differential-mode conducted interference
Differential mode interference is interference generated at the output of the rotor-side converter. Its impact on the system is mainly reflected in the following two aspects: First, the harmonic components in the voltage will be coupled from the rotor to the stator under the action of the rotating magnetic field of the motor, and then propagate to the power grid, forming power grid harmonics; Second, the impact of differential mode di/dt on the system will cause motor overvoltage through long-distance transmission.
1.2.2 Radiative Coupling
The intensity of radiated interference is related to the current intensity of the interference source, the emission frequency of the interference source, and the equivalent radiation impedance of the device. On the one hand, radiated interference can affect the control circuit and communication of the system itself; on the other hand, since the system is not in a fully enclosed metal casing, it can couple to other devices through holes and gaps. A representative example is the radiated interference source of the SCR rectifier circuit, which focuses on analyzing the relationship between common-mode current (time domain and frequency domain) and the radiated field. It is believed that the common-mode current is related to the drive pulse from the control section and stray parameters. The voltage gradient between stray capacitors promotes the propagation of the common-mode current, and the voltage gradient at the rising edge of the pulse generates a common-mode current in the stray capacitor. Moreover, fast current pulses induce unwanted voltages on the metal parts of the SCR (casing and heat sink), becoming radiation sources.
2 Electromagnetic Interference Analysis of Converter System
2.1 Analysis of Conducted Electromagnetic Interference at the Input Terminal of the Converter
As shown in Figure 2.1, the input terminal of the AC-DC-AC converter (i.e., the AC side of the grid-side PWM converter) is connected to the power grid. The grid-side converter has two main functions: first, to maintain the stability of the DC bus voltage by controlling the input current, which is a prerequisite for the normal operation of the two PWM converters; and second, to ensure good input characteristics, that is, the input current waveform is close to sine, the harmonic content is low, and the input power factor meets the requirements.
In this 2.0MW doubly-fed wind power generation system, the grid-side converter is typically connected to the grid via an LC or LCL filter. The AC side voltage is primarily a sinusoidal fundamental frequency, but some higher harmonics are also present. Due to the filtering effect of the inductor, the harmonic current generated by the higher harmonic voltage is very small, thus the AC side input current of the grid-side converter is approximately sinusoidal. One of the control objectives of the grid-side converter is to achieve unity power factor rectification or inversion. The main EMI problem at the input end is the mutual influence between the interference signals generated by the converter and the interference signals on the grid through the power lines. On the one hand, high-frequency interference signals from the grid may interfere with the normal operation of the converter through the power lines; on the other hand, high-frequency interference signals generated by the converter can also propagate to the grid through the power lines, thereby interfering with other equipment on the same power line. Moreover, power electronic products must meet electromagnetic compatibility standards.
2.2 Analysis of Conducted Electromagnetic Interference at Converter Output
The output voltage of the rotor-side converter contains positive and negative sequence components (differential-mode voltage) and a zero-sequence component (common-mode voltage). The differential-mode voltage is the system's operating voltage, comprising two components: the three-phase symmetrical voltage corresponding to the fundamental frequency (generating the magnetizing current) and harmonic voltages (voltages appearing at multiples of the fundamental and switching frequencies). Harmonic voltages generate additional losses, torque fluctuations, and noise, and also pollute the power grid. When the differential-mode voltage is transmitted over long distances, due to the distributed characteristics of long cables (i.e., leakage inductance and coupling capacitance), voltage reflection occurs, causing overvoltage and high-frequency damped oscillations at the motor terminals, further exacerbating the insulation stress on the motor windings. The common-mode voltage, a zero-sequence component of the three-phase voltage, does not generate magnetizing current, but due to its high-frequency characteristics and rapid voltage changes, it generates harmful EMI, leakage current, and bearing current. Furthermore, both differential-mode and common-mode signals generate radiated interference within the system.
Figure 2.2 shows the output pulse voltage waveform on the rotor side. When these signals are applied to the motor rotor, the resulting rotor current is: (1) the "fundamental" component related to the rotor operating frequency, i.e., the working current of the rotor winding generating magnetic flux; (2) the spike current generated by the switching frequency harmonics and their multiples; (3) the charging current generated by the interaction between du/dt and the parasitic capacitance C between the lines; (4) the transient current generated by the interaction between du/dt and the parasitic capacitance C between the line and ground. Figure 2.3 shows the rotor output current waveform when the AC-DC-AC converter drives a 2.0MW motor.
2.2.1 Differential-mode conducted interference
The differential-mode conducted interference generated at the output of the rotor-side converter is mainly manifested in two aspects. First, the harmonic components in the differential-mode voltage will be coupled from the rotor to the stator and then propagate to the power grid under the action of the rotating magnetic field of the doubly-fed motor. Second, the impact of differential-mode du/dt on the system is mainly manifested in the overvoltage problem generated at the motor end during long-distance transmission.
2.2.1.1 Harmonic Analysis
The causes of harmonic generation in doubly-fed wind power systems are mainly of two types: one is the inherent tooth harmonics determined by the structure of the generator itself. Generators generally adopt a wound-rotor asynchronous motor structure, with three-phase groups in both the stator and rotor. The inherent harmonics mainly manifest as air-gap space harmonic magnetomotive force and tooth harmonic magnetomotive force. The other type is generated by the AC excitation system. The generator stator and rotor are tightly coupled through the air gap. The harmonic current on the rotor side induces a corresponding harmonic potential response on the stator side. After amplification on the stator side, it is injected into the grid. The harmonic components are very complex and are the main harmonic source during grid-connected operation. In particular, the low-order harmonic potentials will seriously affect the output power quality. Figure 2.4 shows the voltage waveform distortion caused by harmonics.
When a variable-speed constant-frequency wind power generation system adopts AC excitation, the harmonics exhibit the following characteristics:
1. It belongs to the voltage source type of harmonics, and is characterized by a harmonic group distribution centered on the carrier frequency;
2. Due to the small slip of AC excitation, the output harmonics of the excitation converter are low-frequency or even ultra-low-frequency;
3. When the wind turbine's rotational speed changes, the wind power generation system itself becomes unstable. The fundamental and harmonic spectrum distributions are not fixed, and the harmonic distribution is very wide, resulting in uncertainty in the system's harmonics.
4. The generation of a large amount of no-load harmonic voltage on the generator stator side makes grid connection difficult. After grid connection, the generator will inject a large amount of harmonic current into the grid, causing harmonic pollution of the grid and affecting the power quality of the grid.
If the harmonics generated by the system cannot be effectively suppressed and filtered out, they will have many adverse effects on the wind power generation system, mainly in the following aspects:
1. Increases motor losses and heat generation, affecting the motor's insulation life;
2. It causes noise and vibration in the generator, and sometimes even causes the entire system to oscillate:
3. The power generation quality does not meet national standards, making this technology unusable for practical application.
2.2.1.2 Overvoltage Analysis During Long-Distance Transmission
In a doubly-fed induction generator (DFIG) wind power system, the output of the rotor-side converter needs to be transmitted to the rotor side of the DFIG motor via a long cable. Due to the impedance mismatch between the motor and the cable, voltage reflection occurs, resulting in overvoltage and high-frequency damped oscillations at the motor end. This not only increases the insulation stress of the motor windings but also reduces the lifespan of the windings and rotor slip rings. This reflection phenomenon is related to the rise time of the inverter output pulse and the length of the cable. Figure 2.5(a) shows the output voltage waveform of the rotor-side converter, and Figure 2.5(b) shows the voltage waveform at the motor side after transmission via the long cable. It can be seen that the maximum voltage can reach twice the output voltage.
2.2.2 Common-mode conducted interference
The output of a three-phase voltage-source inverter contains a common-mode voltage, also known as a zero-sequence component. This common-mode voltage interacts with parasitic capacitances in the system, generating a common-mode current. This current, through the parasitic capacitances inside the motor, causes leakage current to flow into the ground wire. Excessive leakage current will cause electromagnetic interference to the system and power supply; the high-amplitude shaft voltage and bearing current induced on the motor shaft can also cause premature failure of the motor bearings.
2.2.2.1 Common-mode voltage
Common-mode voltage is defined as the potential difference between the inverter output neutral point and the reference ground. Figure 2.6 shows a three-phase voltage-source PWM inverter. Lm and Rm can be regarded as a three-phase symmetrical resistive-inductive load or a motor load. From Figure 2.6, the following set of voltage-current relationships can be obtained:
2.2.2.2 Common-mode current and its path
As analyzed above, the magnitude of the common-mode current is related to the values of du/dt and parasitic capacitance, C. A longer output line results in a larger C, higher motor power leads to a higher system voltage, and a faster voltage rise time results in a larger du/dt. All of these factors contribute to an increase in current. Furthermore, this current is also related to the carrier frequency; a higher carrier frequency results in a larger current. Figure 2.7 shows the measured common-mode current waveform.
3. Converter Conducted Interference Suppression Based on EMI Filter
We analyzed the performance indicators of the power line EMI filter and the main factors affecting insertion loss, and selected a power line EMI filter for the wind power converter. Regarding the common-mode interference problem caused by the common-mode voltage at the converter output, we conducted a detailed analysis of three methods: passive filtering technology, active filtering technology, and modulation-based technology, comparing their respective advantages and disadvantages. The analysis concluded that the method using common-mode inductors is not only simple in structure and control, but also has lower cost, making it more suitable for this wind power converter.
3.1 Design of EMI Filter for Grid-Side Converter Power Line
3.1.1 Performance Indicators of Power Supply EMI Filters
The main performance indicators of a power supply EMI filter include insertion loss, frequency characteristics, impedance matching, rated current, insulation resistance, leakage current, physical dimensions and weight, operating environment, and reliability. When using such filters, the rated voltage and current, insertion loss, and leakage current are the three most frequently considered factors.
3.1.2 Circuit Structure Selection for Power Supply EMI Filter
Figures 3.1(a) and (b) show the basic structures of common single-phase and three-phase power line EMI filters, respectively. These are passive low-pass networks composed of L and C connectors. The insertion loss of an EMI filter is affected by the noise source impedance and load impedance. To achieve maximum noise attenuation, when both the noise source impedance and load impedance are estimable, the filter structure is generally selected according to the following principles:
1. Low source impedance and low load impedance: Select a T-type filter structure;
2. Low source impedance and high load impedance: LC filter structure is selected;
3. High source impedance and low load impedance: Select CL type filter structure;
4. High source impedance and high load impedance: Select a type II filter structure.
3.2 The power supply line EMI of the machine-side converter has an original filter design.
When the motor is under load, the common-mode voltage can still generate destructive current through the load bearing. Therefore, filters composed of passive components are being used, which are very effective in eliminating the effects of common-mode voltage. Active filtering technology and modulation-based methods are also being employed to reduce the inverter output common-mode voltage.
Designed to eliminate common-mode voltage at the motor terminals, passive filters consisting of common-mode chokes and common-mode chokes plus capacitors can effectively eliminate high-frequency leakage current and bearing current. However, their drawback is the need to adjust the parameters of passive components to ensure that the common-mode voltage at the motor terminals, which varies with the carrier frequency, can be effectively eliminated, making implementation difficult. Furthermore, their effect on reducing harmonic components in the inverter output is very limited as the carrier frequency changes. Proposed active filtering technologies include three-phase four-arm inverters, dual-bridge inverters (DBI), active common-mode noise cancellers (AcC), and active common-mode voltage compensators (AcCom), among other circuit structures. Their basic principle is to use a compensation voltage with the same amplitude but opposite polarity as the common-mode voltage output from the filter, superimposed on the inverter output to achieve the purpose of eliminating common-mode voltage.
3.2.1 Three-phase four-arm technology
The structure of a three-phase four-arm inverter is shown in Figure 3.2. It adds one arm to the traditional three-phase inverter. If the load is balanced, then:
The biggest drawback of using a three-phase four-arm bridge structure is that the addition of an arm increases the system cost. Furthermore, when space vector modulation is used, it introduces severe switching losses and harmonic distortion.
3.2.2 Active Common-Mode Noise Canceller (Acc)
Figure 3.3 shows the circuit structure of an active common-mode noise canceller (Acc). It consists of a common-mode voltage detection unit, a push-pull amplifier circuit, and a four-winding common-mode transformer. The current output from the voltage detection network, which is composed of capacitors, is amplified by the push-pull circuit to drive the common-mode transformer. The output of the push-pull circuit, through the four-winding common-mode transformer, superimposes a voltage with the same amplitude but opposite phase as the common-mode voltage output by the inverter onto the inverter output system, thereby eliminating the common-mode voltage generated by the PWM inverter.
In the formula, UcM is the common-mode voltage, and Ibas is the base current flowing through the transistor. When the common-mode voltage applied to the motor is approximately zero, it represents the difference between the ideal common-mode voltage and the voltage of the fourth winding of the common-mode transformer, i.e., the common-mode voltage elimination error. This structure uses a star-connected capacitor directly connected to the inverter output as a common-mode voltage detector, which will generate spike pulse currents on the power semiconductor devices, potentially causing device damage. Moreover, since there is no resistance on the capacitor, the damping is very small or even zero. This may cause system oscillations, affecting system stability. Simultaneously, this method requires an emitter follower connected to the DC bus voltage, and the rated voltage of the complementary transistor must be greater than the DC bus voltage. Due to the limitation of the transistor's operating voltage, it is difficult to apply in high-voltage applications.
Methods for eliminating common-mode voltage using active filtering techniques all rely on adding active components to the system. This not only increases the size and weight of the converter, making system control more complex, but also significantly increases costs. Due to these limitations, passive filtering techniques using common-mode inductors offer a simpler circuit structure and control method, and are less expensive, making them more suitable for MW-level wind power converter systems.
3.3 Design of Passive Common-Mode Inductor Filter for Machine-Side Converter
A common-mode inductor consists of three windings wound in the same direction around a common-mode core. It has very high common-mode impedance and extremely low differential-mode impedance, providing excellent common-mode signal attenuation while offering almost no attenuation for differential-mode signals. Therefore, a common-mode inductor can be inserted between the inverter output and the motor to reduce the high-frequency leakage current generated by the common-mode voltage.
Common-mode interference can be represented by an equivalent resonant circuit composed of RLC circuits, with leakage current and related variables, where L and C are parasitic inductance and capacitance, respectively, which can be obtained through measurement. When a common-mode inductor is inserted between the inverter and the motor, the inductance L increases, and considering the losses of the common-mode inductor core, the resistance R also increases. The suppression effect of the common-mode inductor on common-mode interference mainly depends on the relationship between the parameters of the common-mode inductor and the equivalent parasitic parameters of the common-mode interference. These parasitic parameters include the parasitic parameters of the converter system and the parasitic parameters of the motor. Appropriate selection of common-mode inductor parameters can effectively suppress common-mode interference at the converter output. In addition to selecting suitable parameters, the design also needs to consider the influence of factors such as the core saturation of the common-mode inductor and the high-frequency characteristics of the common-mode inductor on the filtering performance. Figure 3.5 shows a comparison of the oscillating current waveforms before and after inserting the common-mode inductor.
Figure 3.5 Comparison of oscillating current waveforms before and after inserting the common-mode inductor
4. Conclusion
EMI filters are one of the most effective and commonly used methods for suppressing conducted interference. In MW-level doubly-fed wind turbine converters, conducted interference mainly includes power line interference at the grid-side converter input and common-mode interference at the rotor-side converter output. This paper first analyzes the types and paths of interference sources, then analyzes the power EMI of the grid-side filter, and finally focuses on the common-mode interference problem at the rotor converter output. A detailed theoretical analysis and comparison of active and passive filtering techniques are presented. The analysis concludes that the common-mode inductor method is more suitable for this 2MW doubly-fed wind turbine converter due to its simple structure and control method, and lower cost.
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About the author:
Cheng Peng (1983-) is currently employed at Harbin Jiuzhou Electric Co., Ltd., where he is mainly engaged in the research and development of wind power converters and photovoltaic grid-connected inverters.
