I would like to understand if it is possible to use a circuit to regulate the countertorque provided by a permanent magnet synchronous generator (PMSG) all the way from zero torque to a maximum torque. I am thinking mostly about PMSGs like those used in wind turbines.
There are several papers online that discuss "direct torque control" (DTC) of a PMSG. E.g.:
http://www.icrepq.com/icrepq%2712/448-abdolghani.pdf
http://pe.org.pl/articles/2016/10/56.pdf
I am trying to understand how this works in detail and what range of torque values is available (as a % of the generator's maximum countertorque).
Specific questions:
How does DTC work, in layman's terms, when applied to a generator? Are these methods effectively decreasing the load felt by the generator? Or, are they putting a current into the generator to drive it like a motor and compensate for the inherent electromagnetic torque? Or some other explanation?
What is the range of torque values that such methods enable? How rapidly and reliably can the torque be controlled? In particular, can these methods enable the PMSG to have zero countertorque at one moment and a maximum countertorque a few moments later?
I have read about both "field-oriented control" (FOC) and "direct torque control" (DTC). Can both of these methods be used to adjust the countertorque provided by a PMSG?
Finally, in an electrically excited synchronous generator (EESG), it seems intuitive that you could reduce the countertorque provided the generator all the way to zero by simply decreasing the DC current in the field coils, without a complicated control circuit. Is this correct? How does this torque-control approach compare to using DTC?
Thank you!
Best Answer
Torque control of the generator works by changing the electrical load felt by the generator. The grid-side converter is adjusted to supply whatever power to the grid will result in the desired generator driving torque. The generator driving torque is determined by measuring current, voltage, frequency and phase angle of the power transferred from the generator to the machine-side converter. Using a mathematical model of the PMSG, the generator torque and speed can be calculated from the measured operating data.
The desired torque, the torque required for the maximum power that can be produced at a given wind speed, is calculated from the wind turbine characteristics and the wind speed. In one of the papers cited, the turbine pitch angle is also set by the machine control system.
The system could not control the counter torque down to zero, because the turbine must produce enough power to supply the losses in the system before there is any power available to transfer to the grid. However, the system can probably control the system quite well over the torque range that is useful for turbine operation.
Torque Control for Machines other than Wind Turbines
Similar control systems could be used for torque control of generators driven by machines other than wind turbines. An absorbing dynamometer might require torque control to torque and speed operating points approaching zero. The operating range of both torque and speed might be very wide. For best performance, it would be desirable to implement field oriented control with shaft torque meter and shaft speed / position encoder feedback. Such a system could hold torque against a driving force at zero speed. In that case, the losses of the absorbing machine would be supplied by the grid-side converter. Direct torque control without shaft transducers has been used to approach the performance of a systems with such transducers, but the performance has not been adequate for the most demanding applications. The PMSG may or may not be the best machine for the most demanding torque control applications.