IEC 60034-18-41-2014 pdf download.Rotating electrical machines – Part 18-41: Partial discharge free electrical insulation systems (Type I) used in rotating electrical machines fed from voltage converters – Qualification and quality control tests.
4 Machine terminal voltages arising from converter operation Modern converter output voltage rise times may be in the 0,05 µs – 2,0 µs range due to power semiconductor switching characteristics. The voltage appearing at the terminals of a converter driven machine may be calculated using IEC/TS 61 800-8 and depends upon several characteristics of the power drive system, such as,
a) operating line voltage of the converter;
b) architecture and control regime of the converter;
c) filters between the converter and machine;
d) length and type of cable between them;
e) design of the machine winding;
f) design and configuration of the installation.
In order to apply this Standard to the qualification and testing of the insulation system of a winding, it is necessary to specify the required parameters of the voltage appearing at the machine terminals (Clause 7). The amplitude and rise time of the voltage at the machine terminals depend upon the grounding system, various design aspects of the cable, the machine surge impedance and the presence of any filters that increase the impulse rise time. Common ranges of characteristics of converter impulses at the machine terminals are given in Table 1 .
In the case of 2-level or other U converters, depending on the rise time of the voltage impulse at the converter output and on the cable length and machine impedance, the impulses generate voltage overshoots at the machine terminals (typically U p up to 2U dc between phases). The voltage overshoot is created by reflected waves at the interface between cable and machine or converter terminals due to surge impedance mismatch.
It is fully explained by transmission line and travelling wave theory. Figure 2 shows the voltage that appears (during one period at the fundamental frequency) at the machine terminals when fed from a 3-level converter.
The maximum change in voltage, U j , at the impulse frequency is shown in Figure 3. This parameter is important in defining the voltage enhancement that can occur across the first or last coil in the winding. A double jump transition is possible but it is the duty of the drive system integrator to ensure that the software controlling the converter drive prevents this from happening.
For an “n” level converter, the phase/phase voltage can be estimated as follows:
Peak/peak fundamental frequency voltage = 2(U dc + U b ) (1 )
Peak/peak impulse frequency voltage = U dc /(n–1 ) + 2U b
The phase/ground values are estimated as follows:
Peak/peak fundamental frequency voltage = 0,7 × 2(U dc + U b ) (2)
Peak/peak impulse frequency voltage = 0,7(U dc /(n–1 ) + 2U b )
The jump voltage is given by 0,7(U dc /(n–1 ) + U b ) (3)
The proportion of jump voltage appearing across the first turn is obtained from Figure 7.
The value of U b in these formulae is the value shown in Figure 1 for the phase/phase voltage on the machine terminals. The values of the phase/ground voltages estimated from these formulae may be higher or lower in practice, depending upon the grounding system, converter control regime and other factors. It is known that a sudden rise can occur in the machine ground voltage level with respect to the d.c. zero point in the converter. The theoretical rise is determined capacitively to be 1 /3 which gives a residual effect of about 0,7. This would apply to simple systems where only travelling wave theory determines the factor, i.e. stress categories A, B and C (see Clause 7).IEC 60034-18-41 pdf download.