Whereas Total Harmonic Distortion (THD) looks at current distortion at the present load, Total Demand Distortion (TDD) considers it in terms of peak current a motor requires. Since the passage of IEEE 519-2014, some utilities are beginning to charge customers who have high TDD at the point of common coupling (PCC).
Small currents can have a high THD, but may not significantly affect your facility’s power quality. For example, most variable speed drives (VSDs) produce high THD values whenever the motor drives a low load; while the overall waveform distortion may be high, the magnitude — and thus, system impact and TDD — is low.
However, on “weak” power sources with large demand currents relative to the rated current of the circuit — i.e., where the ratio of the bolted short-circuit current divided by the load’s peak current less than 50 — even a small distortion can greatly impact the entire system because of its high magnitude. Thus, TDD thresholds vary by the ratio of the bolted short-circuit current divided by the load’s peak current, known as the short-circuit ratio and often abbreviated as Isc/Ipeak.
| Short Circuit Ratio | TDD Limit |
| < 20 | 5% |
| >= 20 & < 50 | 8% |
| >= 50 & < 100 | 12% |
| >= 100 & < 1000 | 15% |
| >= 1000 | 20% |
There are even tighter thresholds for particular orders of harmonics, such as motor-damaging triplen harmonics; refer to IEEE 519 for these thresholds.
Suggested actions
Investigate TDD on the low-voltage side of the transformer serving this motor, and at the meter where you receive utility service, to get a better sense of how the distortion has perpetuated throughout your system. If distortion is high at the meter, investigate installing harmonic filtration equipment.