The scientific contributions of this doctoral dissertation are:
• Development of a new active output filtering method, which consists of a passive LC circuit and a specific control of the circuit in order to produce voltage slopes of de-signed length to suppress the effects of fast transients in an electric drive.
• Formulation of the theoretical background for the application of the active du/dt filter-ing method in an electric drive.
• Development of guidelines for the filter component value selection and the basis for the corresponding control sequences for the application of the method in an electric drive.
• A method is introduced for correction of the error caused by the load current of the motor present in the drive.
• The method is proven to be a potential output du/dt filtering solution by a series of experimental measurements.
The author has published research results related to the subjects covered in the dissertation as a co-author in the following publications:
1) J.-P. Ström, J. Tyster, J. Korhonen, K. Rauma, H. Sarén and P. Silventoinen, "Active du/dtFiltering for Variable Speed AC drives," in13thEuropean Conference on Power Electronics and Applications, EPE 2009, 8–10 September, Barcelona, Spain, 2009, (Ström et al., 2009).
2) J. Korhonen, J.-P. Ström, J. Tyster, H. Sarén, K. Rauma and P. Silventoinen, "Control of an Inverter Output Active du/dt Filtering Method", inThe 35thAnnual Conference of the IEEE Industrial Electronics Society, IECON 2009, 3–5 November, Porto, Portugal, 2009, (Korhonen et al., 2009).
3) J. Tyster, M. Iskanius, J.-P. Ström, J. Korhonen, K. Rauma, H. Sarén and P. Silventoinen,
"High-speed gate drive scheme for three-phase inverter with twenty nanosecond mini-mum gate drive pulse," in13thEuropean Conference on Power Electronics and Appli-cations, EPE 2009, 8–10 September, Barcelona, Spain, 2009, (Tyster et al., 2009).
4) J.-P. Ström, H. Eskelinen and P. Silventoinen, "Manufacturability and Assembly Aspects of an Advanced Cable Gland Design for an Electrical Motor Drive,"Intl. Journal of Design Engineering, Vol. 1, Issue 4, 2009.
5) J.-P. Ström, M. Koski, H. Muittari and P. Silventoinen, "Analysis and filtering of common mode and shaft voltages in adjustable speed AC drives," in12thEuropean Conference on Power Electronics and Applications, EPE 2007, 2–5 September, Aalborg, Denmark, 2007.
6) J.-P. Ström, M. Koski, H. Muittari and P. Silventoinen, "Transient Over-Voltages in PWM Variable Speed AC Drives - Modeling and Analysis," inNordic Workshop on Power and Industrial Electronics, 12–14 June, Lund, Sweden, 2006.
1.4 Scientific contribution 21
J.-P. Ström has been the primary author in publications 1 and 4–6. The background research for publications 1–3 has been done together by J.-P. Ström, Mr. J. Korhonen, and Mr. J.
Tyster. The prototype used in the measurements of publications 1–2 was developed by Mr.
J. Tyster and Mr. J. Korhonen. The prototype used in publication 3 was developed by Mr. J.
Tyster and Mr. M. Iskanius. Measurements for publications 1–3 were carried out by the first authors of the corresponding publications.
Background research for publication 4 was carried out by the authors. The research on the manufacturability and assembly aspects in publication 4 was carried out by Dr. H. Eskelinen.
The cable gland prototypes were constructed by the Department of Mechanical Engineering at Lappeenranta University of Technology and the measurements were carried out by J.-P.
Ström.
For publication 5, background research was carried out by Ms. H. Muittari. Filter prototype construction and the measurements were carried out by J.-P. Ström and Ms. H. Muittari. For publication 6, J.-P. Ström was in the major role in the background research, measurements and writing, with the help of the co-authors.
The author is designated as a co-inventor in the following patents or patent applications con-sidering the subjects presented in this dissertation:
FI Patent 119669 B "Jännitepulssin rajoitus". Patent granted Jan 30 2009, (Sarén et al., 2009).
EU Patent application 08075493.0 - 1242 "Limitation of voltage pulse". Application filed May 19 2008, (Sarén et al., 2008a).
US Patent application 20080316780 "Limitation of voltage pulse". Application filed Dec 25 2008, (Sarén et al., 2008b).
23
Chapter 2
Cable-reflection-induced terminal overvoltages in variable-speed drives
Along with the development of new power semiconductor switching components and identi-fication of the side effects produced by the frequency converters applying these components, the topic of cable reflection has been under extensive research, and numerous scientific pub-lications can be found considering both the phenomenon itself and various means to mitigate its effects. Some key publications on cable reflections are for example (Persson, 1992) and (Saunders et al., 1996).
As presented in the introductory chapter, three-phase motors are controlled by means of variable voltage and frequency, and in a very typical case, this is implemented by using a switching-mode DC to a three-phase AC converter, typically a voltage source inverter (VSI) applying pulse width modulation (PWM). The energy from the utility source is rectified into a DC link capacitor by using a rectifying bridge, and the DC link capacitor acts as the low-impedance voltage source for the inverter bridge.
The AC voltage is formed from the DC link voltage by the inverter bridge as a series of pulses, which have a constant amplitude – neglecting the DC link fluctuations – and a varying width, the output of the phases being connected either to the positive or negative DC link rail; therefore, the phase-to-phase voltage between two phases can be either the positive or negative DC bus voltage. A schematic of a main circuit of a frequency converter is shown in Figure 2.1. Further, a possible output filter connection is shown along with a typical motor common-mode current path.
In order to keep the losses produced in the switching operation of a single power semiconduc-tor component in the inverter bridge to a minimum, the transition time between the on- and off-states (and vice versa) of the switching component should be made as short as possible.
This is because the voltage across the component is larger than the on-state saturation voltage
CL
M
Figure 2.1. Frequency converter main circuit. Power from the grid is rectified into the DC link. The motor AC voltage of variable frequency and voltage is generated from the DC link voltage using the three-phase inverter bridge shown. A possible output du/dtfilter, and a typical motor common-mode current path are also presented.
of the component and a possible current flowing through the component will generate power loss (heat) during the transition according to the following equation
P= 1 T Z
uidt. (2.1)
On this account, the transitions in the voltage pulses generated by the DC to AC converter in the adjustable speed drive are kept as short as possible, leading to the fact that the steepness of the edges of the voltage pulses is high. In an inverter power switch component generally applied, that is, the insulated gate bipolar transistor (IGBT), the transition time between the states is at fastest in the order of tens of nanoseconds, as can be seen for instance in the next section. In addition to the benefits presented above, the fast switching voltage transient and thereby the output voltage of the inverter contains a lot of high-frequency components as a byproduct of the switching mode operation. The frequency components beside the base frequency of the electric drive are by definition unnecessary and even harmful to the operation of the drive, but are not irrelevant for the operation of the drive. This is the key difference between the voltage waveforms in a traditional direct-on-line (DOL) and VSI-converter-fed drives.
The switching transients occuring in the inverter are – and have to be – fast, when compared with the fundamental and switching frequencies. Therefore, the output voltage waveform contains in addition to the fundamental base frequency, switching frequency, and their har-monics, high-frequency components resulting from the steep voltage pulse edges extending up to the megahertz range (Skibinski et al., 1999). If the speed of propagation in the mo-tor cable is for example in the order of 0.5c, see for example (Skibinski et al., 1997; Ahola, 2003), the wavelength of a 50 Hz signal is in the order of thousands of kilometers, whereas the wavelength of a signal of 1 MHz is only 300 meters.
2.1 Frequency spectrum of the output voltage of a typical three-phase switching mode
inverter 25
Hence, the lengths of a typical motor cable, which are in the order of tens to a few hundred meters, are substantial compared with the high-frequency components present in the inverter output voltage. Therefore, each switching in the inverter output stage induces a traveling wave into the motor cable, and the transmission line theory must be applied in the analysis of the behavior of the traveling waves in the motor cable (Persson, 1992); see Chapter 4 for mea-surements of the propagation speed for the MCMK power cables used in the meamea-surements of this dissertation.
This also sets special requirements for the motor cabling and the insulations in the electric motor, because the motor and the motor cable are typically designed for low operating fre-quencies, and also the effects caused by the high frequency content in the output voltage must be taken into account in a converter drive, for example the overvoltages caused by wave reflections, as will be discussed later in this chapter.
2.1 Frequency spectrum of the output voltage of a typical three-phase switching mode inverter
As presented in (Skibinski et al., 1999), the output voltage of a pulse-width-modulated (PWM) voltage source inverter can be approximated as a series of trapezoids of varying width, and the frequency spectrum of the signal can be approximated by means of Fourier analysis (Zhong et al., 1998). An example of an inverter output voltage and corresponding differential-mode voltage spectrum presented in (Skibinski et al., 1999) are shown in Fig-ures 2.2a and 2.2b.
tr
T = 1 / fc t [ s ] f [ H z ]
Uphase [V] fc fB W
U D C
Uphase [dB] 0 - 1 0 0 - 2 0 0
a ) b )
Figure 2.2. a) Inverter phase output voltage and b) corresponding voltage spectrum. In this example, from (Skibinski et al., 1999), the switching frequency fcis 500 Hz, the duty cycle 50 % andtr200 ns.
The frequency axis is logarithmic.
The main parameters that the spectral width of the signal depends on are the rise timetrand the switching frequency fc. According to (Zhong et al., 1998), the theoretical spectrum is flat until fc, and it begins to attenuate after this frequency by 20 dB/decade and after fBWby
40 dB/decade. Therefore, fBWcan be used as a rough approximate for the spectral width of the inverter output voltage waveform (Skibinski et al., 1999):
fBW≈ 1
πtr. (2.2)
When IGBT power switches with typical transition times between 50 and 400 ns (Saunders et al., 1996; IEC, 2007) are employed in the inverter output stage, the frequency spectrum of the output voltage extends up to the radio frequency region, from hundreds of kilohertz up to several megahertz. As an example, the rise and fall times and the calculated bandwidth estimate using (2.2) for some Semikron Semitrans packaged IGBT modules are presented in Table 2.1. These modules are selected as an example, because they fit in the Vacon NXP series frame size 6 industrial frequency converter, which is also used in the prototype equipment and tests. The total switching energy at the rated, continuous collector current is also presented.
Table 2.1. Rise and fall times, the total switching energies and the calculated bandwidth estimates of some Semikron Semitrans packaged IGBT modules, as stated by the manufacturer
Module Typical Typical Total switching Bandwidth
type rise time fall time energy estimate
tr tf @100 A Eq. (2.2)
Semikron SKM
100GB123D 70 ns 70 ns 27 mJ 4.5 MHz
1200 V Standard Semikron SKM
100GB125DN 40 ns 20 ns 22 mJ 16 MHz
1200 V Ultra fast Semikron SKM
100GB176D 40 ns 145 ns 100 mJ 10.6 MHz
1700 V Trench
Spectrum measurements of an inverter output voltage are presented for example in (Skibinski et al., 1999), in which the spectral width was found to reach up to the megahertz range. In the example, rise time was 200 ns and the spectral width was more than 1 MHz.