Digital Current Control of a Voltage Source Converter With Active ...

WU AND LEHN: DIGITAL CURRENT CONTROL OF A VOLTAGE SOURCE CONVERTER 1371
Fig. 15. d- and q-axis currents during a 10-A step in I , damping and integrator
loops enabled.
Fig. 17. d- and q-axis currents, and three-phase bus voltages during three-phase
fault.
Fig. 16. Oscilloscope phase A signals during a 10-A step in I , damping and
integrator loops enabled. Time: 5 ms/div. Top to bottom: i (20 A/div), v
(100 V/div), i (20 A/div), v (100 V/div).
Fig. 18. Oscilloscope screen capture of phase A signals during three-phase
fault. Time: 5 ms/div. Top to bottom: i (10 A/div), v (100 V/div), i
(10 A/div), v (100 V/div).
shows that the step response is fast, the system is well-damped,
and there is no ringing.
B. Disturbance Rejection
In order to demonstrate the ability of the controller to reject
disturbances and its ability to damp the resonance from excitation
by external sources, a balanced three-phase fault was introduced
which instantaneously reduced the magnitude of
by approximately 2/3 for 15 ms before being cleared. The response
of the system to this fault is shown in Figs. 17 and 18.
There is a brief transient in due to the speed of the fault exceeding
the controller bandwidth, and because of the approximation
in the calculation of the feedforward gain.
Otherwise, the system is well-damped despite the high speed
and severity of the fault, as shown in the oscilloscope screen capture
of Fig. 18. Despite imbalances caused by different clearing
times for each phase, the controller maintains good regulation,
demonstrating its ability to operate in an unbalanced system.
C. Overcurrent Protection
The current controller regulated the current during the fault
so effectively that overcurrent limits were rarely reached. As
a result, this controller could not be used to test the overcurrent
protection. However, use of a less aggressive controller or
a larger filter capacitor will make overcurrent limiting imperative.
Fig. 19 shows the sampled and calculated - and -axis currents
for a less aggressive damping controller during the fault,
with the current limits set to 30 A in each axis. It can be seen
that this controller allows high current levels to occur for a significant
period of time. Despite the change in overall gain, the
deadbeat controller maintained the same performance, tracking
its reference with a fixed delay. Fig. 20 shows the corresponding
phase A currents and voltages.
The overcurrent limit was then set to 10 A in each axis and the
balanced, three-phase fault was reintroduced to demonstrate the
transient overcurrent protection capabilities of the controller. As
shown in Fig. 21 the -axis current reaches the new overcurrent
limit immediately after the fault but is clipped to 10 A. Ringing
can be seen in the oscilloscope traces of and of Fig. 22
following the fault. This occurs when the current is clipped because
the sum of the damping and integral feedback is saturated,
thus only the deadbeat controller is in operation with the current
limit as its reference. With the damping feedback disabled
by saturation, the fault excites ringing in the LC tank formed