Measuring Signals in the Time Domain to Understand and Design EMI Filters

Günther Herrmann at thyssenkrupp Elevator Innovation

The design of EMC filters for drives are often based on experience. One of the main condition to have a well working interference suppression are related to the switching frequency of the inverter. For understanding this it is very helpful to measure voltages and currents also in the time domain, what is quite unusual during EMC measurements.

Frequency inverters have high conductive related interference levels due to the switching mode of operation. In order to reduce this, an EMV filter is provided at the line input. The EMC filter typically consists of one or more filter stages with capacitors and current-compensated chokes.

The measurement setup for conductive emission measurement is quite troublesome, since a motor generator set and the EMC measuring devices with line impedance stabilization network (LISM) must be setup in a standardized manner. If the required limits for the emissions are exceeded, the filter is modified and measured again.

Often only one of many possible frequency inverter-motor combinations can be measured. It remains unclear whether the limit values can also be maintained in combination with other motors.

The following describes a method by which the EMC filters can be designed and optimized in a systematical manner. The main focus in this article is the behavior of the filter in the frequency range from the switching frequency of the frequency inverter up to about 1 MHz. It is assumed that typical high-frequency requirements of the filter design are taken into account.

Operation-mode of the drive

To assess the EMC situation, the knowledge of the pulse pattern generated by the frequency inverter is important. According to the state of the art, the so-called SpaceVectorModulation (SVM) is often used. This type of modulation generates, for example, an average output voltage of 0V by switching the zero vector "7" (all upper IGBT's "on") with 50% duty cycle and the zero vector "0" (all lower IGBTs "on”) also with 50% duty cycle. Therefore, the common mode voltage corresponds the DC-link voltage (respective + / - DC-link-voltage / 2). This operating point represents the one with the highest common mode voltage. Thus a distinctive operating point is found. At this operating point, the capacitors CP and CY (see Figure 1) form a simple voltage divider for alternating voltages in the range of the switching frequency of the frequency inverter.

UCY = 3*CP/CY*Ubus (1)

Modulation methods with low common mode voltages are also known [1], [2]. However, the ripple currents in the motor windings are then increased.

Figure 1: Capacitors CP and CY form a simple voltage divider

Design of the EMC filter

The necessary insertion attenuations can be determined by interference level measurements without an EMC filter in order to fall below the desired interference level. In addition, the differential mode (DM) and common mode (CM) interference level can be measured with the aid of an RF current measuring clamp [3] or a DM / CM separator [4], [5] and the ratio of the differential mode component to the common mode component can be determined. Thus the values for CX1 * LDM, CX2 * LDM and CY * LCM can be calculated.

Despite knowledge of these values, it occurs often that the EMC filter does not have the desired insertion damping. The data sheets of many EMC filters contain attenuation values, which were measured with network analyzers. These analyzers are far from reflecting the operating mode of the filter in use with the frequency inverter drive.

The saturation current (see also [5]) is still missing for the current compensated choke.

The magnetization of the core due to the differential mode current can be estimated as follows ([5], [6]):

BDM = Lsigma*IDM/(N*AFe),

Where N is the number of turns and AFe is the effective magnetic cross-section of the core, and Lsigma is the leakage inductance of the current compensated choke, and IDM is the current at the network input.

The magnetization due to the common mode current can be determined approximately by means of the alternating voltage at UCY. From:

Results with UL = UCY:

ICM,L = (TPeriod/2)*UCY/LCM.                   (2)

With (1) follows:

ICM,L = 3*CP*(Tperiod/2)*Ubus/(CY*LCM) (3)

The values Tperiod and Ubus are known from the operating conditions of the frequency inverter, CY and LCM or the product CY * LCM is determined by the necessary interference suppression. The value for CP is still missing. It is possible to measure this value directly or measure the voltage at CY. If the measurement setup corresponds to the final arrangement (shielded motor cable, ...), this measurement includes all parasitc capacities to PE. With the help of the voltage profile at CY, the magnetization current ICM for the current-compensated choke can be calculated:

BCM = UCY*TPeriod/2 /(N*AFe) bzw.       (4a)

BCM = LCM * ICM / (N*AFe)                                   (4b)

The demand for the saturation induction results overall Bsat > BDM + BCM.

For frequency inverters with space vector modulation, the highest common mode control occurs in the range around the output voltage zero (zero speed), the largest period duration TPeriod, and the highest DC-link voltage. As already mentioned, this is selected as the first operating point for a disturbance level measurement.
The highest differential mode is to be expected at the highest input current of the frequency inverter. Therefore, this is selected as a further operating point for emission level measurement.
If the operating point with the shortest period duration (that corresponding to the highest switching frequency and thus the highest power density spectrum) as well as a braking operation for an emission level measurement are also measured, it can be assumed that the EMV filter performs its function at all operating points.
All these operating points occur, for example, in the case of elevator drives.

Figure 2: Measurement of UCY (50V / div) and ICM,L (0,5A / div) (top) and noise level (bottom)

Figure 3: Measurement UCY (50V / div) and ICM,L (2A / div) (top) and noise level (bottom) in the case of saturation of the current-compensated choke.

Optimization of the EMC filter

In order to check the basic functionality of an EMC filter in frequency inverter operation, simple measurements in the time domain can be carried out as shown in Figure 2 and 3 (left pictures). Knowing the saturation limit of the current-compensated choke (measurement of LCM as a function of the current LCM = LCM (I)), it is also possible to estimate the limit of the parasitic Y - capacities. Consequently, the size of the Y-capacitor can be chosen, where as low as possible values are preferred.

EMC and residual current circuit breaker

Today, for the protection of people and equipment the installation of a residual current circuit breaker is required. In order to ensure a high availability of the system, it must be ensured that this residual current circuit breaker does not trigger due to excessive capacitive leakage currents of the frequency inverter drive. If, for example, the EMC filter is not designed for the lowest switching frequency, a residual current circuit breaker may trigger when the frequency inverter reduces the switching frequency due to external conditions (e.g. increased temperature in summer in a glass shaft of an elevator). In the case of leakage currents ICM as shown in Figure 3, for example, a residual current circuit breaker with 30 mA will trip. This is followed by the obvious rule: "If a residual current circuit breaker trips (leakage current too high), the emission level of the system must be checked.".


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More information: thyssenkrupp    Source: Bodo's Power Systems, January 2018