PWM converter effects in a generator
PWM converter effects in a generator
(OP)
All,
I've been reading IEC standard 60034-18-41 on PD testing type I insulation machines fed by voltage converters. It talks about all the hazards (for the electrical machine) coming with the utilization of these drives (PWM converters), and how there may be voltage overshoots at the terminals of the machine and this will stress the insulation.
The thing is: for everything (not just this standard, also NEMA std and lots of papers) I've read, it seems like they are only speaking about MOTORS as the "machines", for the PWM converter will be connected to the motor side. Instead, for a generator, as I understand this is the configuration:
(wind) generator - rectifier - inverter (PWM) - line filter - grid.
So my question is: in this case will there be voltage overshoots and all the trouble that's specified for motors, mainly variable speed induction motors? Is it right to apply the same test procedures as described on IEC?
To enter in detail, the generator I'm interested in is a variable speed CDSG (Converter-Driven Synchronous Gen) for wind power application. Forgive my ignorance, I'm new on the subject and the only thing I can find is either about the hazard for motors or the effects on the grid (in the case of generators) due to converter operation.
I've been reading IEC standard 60034-18-41 on PD testing type I insulation machines fed by voltage converters. It talks about all the hazards (for the electrical machine) coming with the utilization of these drives (PWM converters), and how there may be voltage overshoots at the terminals of the machine and this will stress the insulation.
The thing is: for everything (not just this standard, also NEMA std and lots of papers) I've read, it seems like they are only speaking about MOTORS as the "machines", for the PWM converter will be connected to the motor side. Instead, for a generator, as I understand this is the configuration:
(wind) generator - rectifier - inverter (PWM) - line filter - grid.
So my question is: in this case will there be voltage overshoots and all the trouble that's specified for motors, mainly variable speed induction motors? Is it right to apply the same test procedures as described on IEC?
To enter in detail, the generator I'm interested in is a variable speed CDSG (Converter-Driven Synchronous Gen) for wind power application. Forgive my ignorance, I'm new on the subject and the only thing I can find is either about the hazard for motors or the effects on the grid (in the case of generators) due to converter operation.





RE: PWM converter effects in a generator
RE: PWM converter effects in a generator
So should I believe this dv/dt value may occur at the generator terminals? Even if so, IEC 60034-18-41 only defines the test procedures according to the "stress category"* of the insulation system, so I guess I should contact the Converter Drive manufacturer in order to discover it, is that right?
*Notice that the stress category is defined based on the overshoot factor (peak voltage by the DC bus voltage) and the impulse rise time.
If someone experienced in this matter could explain me why standards like NEMA only speaks about voltage overshoots (due to the converter) and dv/dt for inverter-fed MOTORS, I would be really grateful. Also, I must say I understand the definition of dv/dt according to NEMA (MG 1), but I realized this is not mentioned by IEC (they only define 'tr', the rise time). I mean, isn't it all a mess?!
RE: PWM converter effects in a generator
Now the reason you're seeing motors mentioned everywhere is simply because they're established and common. Generators and power electronics are coming on the back of those developments. The concepts are very similar, you just need to draw out the components.
For example, if you really do have a rectifier on the generator side, then there are no switching elements on the generator side - all the switching will be effectively isolated by the DC bus between the rectifier and inverter. There'll be harmonics and power quality issues on the generator side, but not the dv/dt or voltage doubling issues you're considering. If you do have an inverter on the generator side, then all the motor issues apply. There's be operational differences because the predominate power flow in the other direction, but I think you probably don't need to concern yourself with those at this stage.
dv/dt and rise time are two ways of describing the same thing. dv/dt means the rate of voltage increase per unit time, and rise time is the time taken per unit voltage increase. They're measuring the same thing. It's only a mess because it's new to you - it always seems this way at first!
RE: PWM converter effects in a generator
If it is not too much to ask, do you know where can I find material and literature concerning the operational differences you mentioned in the case there's an inverter on the generator side?
Still, about the rise time, of course you are right about new things that usually only seems messy at first, but I believe it is not just that in this case. The way I've read the definition of rise time, it is not the time taken per unit voltage increase, but the "time for the voltage impulse to go from 0% to 90% of its final value". It is closely related to the dv/dt definition, but they don't describe the same thing, in a way that when I have one, I don't necessarily have the other (so I could have, for example, dv/dt = 1000 volts/microssecond and a t_r = 2.5 microsseconds and a peak voltage of about 3125 V, considering dv/dt ~ 0.8*Vp/t_r). Also, the reference value of the voltage for the t_r definition is different in NEMA and IEC, the former being the steady-state value (= DC bus voltage) and for IEC it is the actual peak voltage, so it will be bigger.
RE: PWM converter effects in a generator
The committie working on NEMA MG1 decided to write the section for motors. I have had no involvement so I can't tell you why.
You do know that a motor can become a generator by injecting power into the shaft?
RE: PWM converter effects in a generator
Thanks for the explanation about using rectifier or back to back inverters.
RE: PWM converter effects in a generator
The converter is being used for a very specific application. I would believe the generator and wiring and converter would be standardized for a specific wind turbine so it's possible the system was designed to limit the overshoots and/or rise times.
RE: PWM converter effects in a generator
Unfortunately the "operational differences" have a lot to do with the particular strategy employed by the manufacturer. The reasons for active switching instead of diode rectification are complex and subject to optimisation strategies that differ in each application. As a conservative guess however, I think it would be safe to assume the same worst case scenario as the drive/motor standard specifies.