Download Electromagnetic Interference Modeling Research on the Electrical

Progress In Electromagnetics Research Symposium, Beijing, China, March 23–27, 2009
235
Electromagnetic Interference Modeling Research on the Electrical
Machine and Converter Systems
Lingyun Wang, Ruifang Liu, and Hui Huang
School of Electrical Engineering, Beijing Jiaotong University, Beijing, China
Abstract— Nowadays, electrical machine and converter systems are widely used in renewable
energy power generation system and speed regulation of motors. With the applications of power
electronic devices in a variety of equipments and systems, the electromagnetic interference (EMI)
is attracting more and more attentions. Modeling to EMI is the fundamental of forecasting and
removing EMI. Compared with the radiated EMI, conducted EMI is the main problem in the
electrical machine and converter system. EMI modeling technologies to the converter systems
and electrical machines are discussed in this paper, and the modeling methods to the parasitic
parameters of the long cable and the grounding loops are also analyzed here.
1. INTRODUCTION
With the rapid development of power electronics technology, the switching frequency in the converter system is raising. On the one hand it improves the performance of converters; on the other
hand it also brings bad effects due to EMI. For example, it can produce high dv/dt and di/dt by
fast switching in the PWM inverter-fed motor system, leading to motor bearing material erosion
and early mechanical failure; the grounding current produced by high frequency (HF) commonmode voltage through the parasitic capacitance brings EMI issues in the grounding loop; voltage
reflection will be occurred at machine terminals because of the effects of distribution parameters
existed in cable, and this will cause HF oscillation [1]. In order to acquiring good performances,
we need to forecast and remove EMI in the systems. Modeling to EMI is an effective and essential
method.
According to different transmission routes of EMI, EMI can be divided into radiated EMI and
conducted EMI. In electrical machine and converter system, conducted interference is usually the
main EMI. In detail, this paper discusses and analyzes the conducted EMI modeling methods and
technology widely used today in the system, including converter EMI modeling methods, electrical
machine EMI modeling technology under HF, the cable modeling method after considering the
effects of the distributed parameters existed in cable, and the grounding loop modeling technology
of the system after taking account of the grounding currents among different circuit loops. Finally,
the current status of conducted EMI modeling research of the system is presented.
2. TIME DOMAIN AND FREQUENCY DOMAIN MODELING METHODS ON
CONDUCTED EMI
The method of modeling EMI can be divided into two categories, time domain method and frequency domain modeling method. Under time domain modeling, resistances, inductances and
capacitors are used to establish the model, which is used to simulate EMI of the system. Time
domain model is easy to understand the mechanism of system EMI. Frequency domain modeling is
based on time domain modeling. Waveforms under time domain are converted into wave pattern
under frequency range through the Fourier transformation. After the transformation, the parameters under frequency range are used as impedance parameters in the modeling circuit. Compared
with time domain modeling method, frequency domain modeling method is more complex, but it
is easier to analyze EMI spectrum distribution, and faster to forecast EMI.
3. EMI MODELING METHOD OF ELECTRICAL MACHINE AND CONVERTER
SYSTEMS
3.1. EMI Modeling of Converters
In the electrical machine and converter systems, EMI emission of converter has significant influence
on electrical machine. No matter under time domain or frequency domain, most of the modeling
technologies combine the analysis of the EMI mechanism with the experiment and simulation results
to get parameters of the equivalent circuit.
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PIERS Proceedings, Beijing, China, March 23–27, 2009
Under time domain modeling, paper [2] described conducted EMI coupling mechanism in power
converter based on the lumped circuit modeling method. The paper proposed a simple and practical method which estimates the EMI noise source and impedance using LISN (Line Impedance
Stabilization Networks). In this paper, the information of EMI noise sources and impedances were
extracted at the same time. Due to the lumped circuit modeling method for describing EMI mechanism is suitable for other power electronic devices. Because coupling paths were not considered
there, the method of extracting EMI source parameters was too simple to use. Paper [3] combined
time domain measured waveforms with analysis of EMI coupling mechanism and coupling paths to
establish the EMI model. Common mode (CM) EMI, difference mode (DM) EMI and mix-mode
(MM) EMI in the system were analyzed and investigated. The model, which included the measured
and calculated component parameters, was given to evaluate the EMI for each mode of noise. This
method can effectively grasp the EMI mechanism and the interference coupling paths. Filter design
can be proposed based on the simulation of the model.
As frequency domain modeling is based on time domain modeling, the changes of high voltage
and current during the switching instant was considered in paper [4]. An improved and simplified
EMI modeling method was proposed under considering the IGBT switching behavior model. In
the model, the real device switching voltage and current were approximated by piece-wise linear
lines and expressed using multiple dv/dt and di/dt superposition. Since the double Fourier integral
is an effective method to analyze switching devices in PWM inverter, a model included CM source
and DM source of PWM inverter was proposed in paper [5]. The paper also analyzed the effects
of different modulation schemes on EMI spectrum. The equivalent modeling method, which didn’t
rely on the simulation, improved the forecast accuracy of conducted EMI in PWM inverter in this
paper.
In the converter modeling, combined the analysis of the EMI emission mechanism (switching
of the devices and coupling paths) with experimental data, most modeling methods effectively
establish the equivalent circuit or model through calculation of the parameters. At the same time,
the defects of “black box” prediction method are also changed. Based on these modeling methods,
we can propose better solutions to suppress EMI in the systems.
3.2. EMI Modeling of Electrical Machines
Under HF band of conducted EMI, the frequency characteristic of the electrical machines becomes
extremely complex. The HF parasitic parameters can’t be ignored any more. Hence, in the analysis of conducted EMI, the HF model about electrical machines is needed. Since the electrical
machine is non-linear equipment, it involves too many parameters to measure precisely. When
electrical machine is regarded as a capacitive network, mutual inductances are often neglected in
EMI analysis.
Based on the method that combines EMI mechanism analysis and experiment results, paper [6]
established time domain model that includes electrical machine parameters, converter parameters,
and long cable distributed parameters. Especially, the model of electrical machine not only includes
HF parameters (phase or neutral to ground capacitance Cg , phase to phase capacitance Cp , phase to
neutral capacitance Ci and eddy loss resistor Rf ), but also includes medium frequency parameters
(phase or neutral to ground Rg , Ls , Cs ). The Figure 1 shows the equivalent motor circuit for wide
frequency range. On top of that, based on the analysis, four over-voltage mitigation technologies
are presented: RLC inverter output filter, a modified RLC inverter output filter, RC filter at motor
terminals and dv/dt control.
Under frequency domain modeling methods, the sensitivity analysis technique of the parasitic
capacitance is applied on the investigation of CM EMI in paper [7]. The research proved that the
inverter-input-side parasitic parameters of the DC cable have petty effects on CM EMI; parasitic
capacitors between the inverter and ground, parasitic capacitors between motor windings and motor
shell have significant effects on CM EMI. The sensitivity analysis is helpful to solve the problem
of determining the CM EMI spreading path. The results also provided a useful guidance for EMI
analysis.
3.3. EMI Modeling of Long Cable
In electrical machine and converter systems, sometimes long cable is needed to connect electrical
machine and converter in a system. In EMI problems, various devices in the system are connected
by transmission line. When the length of transmission line and the on-line transmission signal
wave λ are at the same geometric magnitude, on-line voltage function and on-line current function
are not only a function of time, but also a function of location. Under this condition, as lumped
Progress In Electromagnetics Research Symposium, Beijing, China, March 23–27, 2009
237
parameter model is not suitable for EMI modeling, distributed parameter model is required.
In inverter drive systems, transmission speed of PWM pulse produced by switching devices
relies on different cable structure and media. The longer the cable, the longer the on-line pulses’
transmission time is. When the transmission time is close to the rise time of the HF PWM pulses,
and the characteristic impedance of the cable and motor doesn’t match, the reflection is more
obvious. Sometimes, there will be a second reflection of the on-line pulses transmission, so the
over-voltage at motor terminal will be doubled. Figure 2 shows transmission and reflection steps
of PWM pulses (Zc , Zm , Us stands for characteristic impedance of cable, characteristic impedance
of motor and similar DC voltage of PWM pulse voltage).
Figure 1: Induction motor model under wide frequency range.
Figure 2: Transmission and reflection steps of PWM
pulses.
Based on the theory, because the not-matching characteristic impedance directly influences
amplitude of the over-voltage, paper [8, 9] proposed a method of the characteristic impedance
matching to suppress the EMI of the system. Compared with the motor terminal over-voltage
without long cable between motor and converter, paper [10] proved that the motor terminal CM
over-voltage doubled due to voltage reflection on the long cable.
3.4. EMI Modeling of the Grounding Loops
In the power electronics system, conducted EMI caused by the grounding net has already got
people’s attention in recent years. In the electrical machine and converter systems, the current
distribution in the grounding net not only affects electrical machine and converter, but also affects
other sub-systems or devices through coupling distributed parameters among different circuits.
Paper [11] analyzed the effects of input and output decoupling capacitances on the grounding
current EMI in DC motor devices. Through the grounding current mechanism analysis and experimental results in a specific system, the paper established equivalent circuit model, and proposed a
scheme to reduce the grounding current interference. The effects of different decoupling capacitance
on the grounding current EMI was also studied using the calculation method.
Based on the theory of Partial Element Equivalent Circuit (PEEC), paper [12, 13] established
equivalent model of the grounding loop in the system. Considering the magnetic field coupling
between different grounding loops and the magnetic field coupling between grounding loops and
external circuits, the impedance characteristic was proposed and verified with experimental results
in this paper.
In present research of the impedance characteristic across two grounding points in the grounding
net, following factors have been considered based on PEEC theory, impedance frequency characteristic between two grounding points, the interference current distribution in the grounding net in
a certain frequency range, and the affects of different interference currents on the impedance.
In some occasions, if the geometry size of the grounding net is longer than λ/2π (λ is the length
of the researched signal), the loss and delay of the EMI transmission must be taken into account.
Based on the theory, even the geometry size of the researched system is longer than λ/2π, or the
length between EMI sources and tampering devices needs to be considered when comparing with
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λ/2π, PEEC is still valid and feasible. Paper [14] proposed a HF model of grounding loop in a
large scale system. The model is verified through simulation and experimental results.
4. CONCLUSIONS
The paper analyzes and concludes the EMI modeling methods of converter, electrical machine, long
cable, and grounding loop in the electrical machine and converter systems. The models under time
domain are intuitive to see the mechanism of conducted EMI, but in the high frequency, when
analyzes the time domain EMI waveforms, the models based on frequency domain is required.
When establishing model of the system, no matter under time domain or frequency domain, the
models are based on the simulation and experimental results. Through analyzing the mechanism
of the equivalent modeling, better solutions of EMI elimination can be proposed.
In recent years, PEEC method has showed its advantage in the EMI analysis. It can be applied
perfect in small scale systems. But in large scale systems, due to too many parameters need to be
calculated, it limits the application of PEEC method. At present, although the PEEC modeling
method has applied to the grounding loop in large scale systems, research on electromagnetic wave’s
loss and delay is only put forward on theory. There is no modeling analysis based on this problem
when applying PEEC to specific systems. In the future research on large scale systems when the
delay and loss of electromagnetic wave needs to be considered, the PEEC modeling method should
be improved.
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