Mircea Chindris and Antoni Sudrià-Andreu
Source: Handbook of Power Quality Edited by Angelo Baggin, John Wiley & Sons, Ltd
As has already been mentioned in Chapter 14, adjustable-speed drives (ASDs) are probably the type of equipment most sensitive to voltage dips (sags). In order to appropriately assess the level of susceptibility for particular equipment, meticulous tests and measurements have to be performed. In this case study, experimental measurements on an ASD with rated power of 2.2 kW are presented.
The characteristics of the ASD are as follows:
Sn = 2.75 kVA (2.2 kW);
Un = 3 ×380/450 V,50−60 Hz;
fout = 0–400 Hz;
In =11A;
In (60s)=15A;
J = 0.0069 kg/m2
Testing of the ASD described below was conducted with the following two types of voltage sags:
- short-term power interruption (100 % voltage sag);
- single-phase voltage sag.
Figure C14.1 presents the input values of the three-phase supply voltage (blue, red and green), the line current of one phase (magenta) and the voltage of the d.c. bus (orange) during a short-term power interruption (120 ms).
Figure 1 Recorded waves during a short-term power interruption (120 ms)
Figure 2 Impact of a single-phase sag (40 % magnitude, 100 ms duration)
The two vertical dashed lines show the limits of the total disconnection of the ASD, in this case 109 ms; if the supply voltage is restored before this period, the ASD is able to reconnect itself automatically.
Figure C14.2 illustrates the ASD behavior during a 100 ms single-phase sag. In this case, the ASD does not disconnect, but it can be seen that the torque of the controlled motor (black line in the figure) oscillates for the duration of the voltage sag; this fact can lead to malfunction of the mechanical drive train in industrial applications.
Once the sensitivity of the ASD to voltage sags is determined, an assessment of possible solutions to mitigate the effects of voltage sags or short-term interruptions should be done. Both the frequency occurrence of disturbances and their possible damage should be evaluated in order to select the best mitigating techniques. If the mechanical load is able to withstand the pulsating torque, the most cost-efficient solution is the ride-through option. During voltage sag, the ASD detects the disturbance and stops supplying the drive; when the voltage is restored, the ASD control system detects that normal operating conditions are re-established and starts to supply the motor on the fly. If the extent of the sags is in the range of some hundreds of milliseconds, the speed change of the motor can be acceptable to the mechanical load under ordinary conditions.