The Seven Types of Power Problems

Overview

The paper groups disturbances by wave shape into seven categories: transients, interruptions, sags or undervoltage, swells or overvoltage, waveform distortion, voltage fluctuations and frequency variations. For each it describes causes, effects on IT and industrial loads and typical mitigations such as surge protective devices, UPSs, power conditioners, K-rated transformers and filters.

The numeric classes are precise. Interruptions fall into four duration classes: instantaneous (0.5 to 30 cycles), momentary (30 cycles to 2 seconds), temporary (2 seconds to 2 minutes) and sustained (longer than 2 minutes). Sag and swell are a voltage reduction or increase lasting 0.5 cycles to 1 minute; a starting motor can draw six times its normal running current or more. Voltage fluctuations are random changes between 95 and 105% of nominal, generally below 25 Hz.

For protection, cascading SPDs (service entrance, sub-panel, equipment) together with a UPS is described as the most effective approach; SPD voltage and energy ratings must be sized and coordinated, and the SPD should open the circuit when its MOV fails. On a shared neutral, pulsed SMPS return currents add up to a theoretical maximum of 1.73 times the maximum phase current; the mitigations are oversized neutrals, K-rated transformers and harmonic filters. Humidity between 40 and 55% reduces ESD potential (the summary Table 1 instead lists 35 to 50%).

The paper also gives a quick screening rule for voltage imbalance: the difference between the highest and lowest phase voltage should not exceed 4% of the lowest, so 220/225/230 V gives 10 V against an 8.8 V limit and is too great. An appendix describes IT power-supply tolerance on the ITIC (CBEMA) curve: 20 ms at zero volts, at least 10 s at 80% of nominal, at least 1 ms at 200% and 0.01 cycle at 500%.

Key points

1. Seven IEEE 1159 categories: transients, interruptions, sags, swells, waveform distortion, voltage fluctuations, frequency variations.
2. Interruption classes: instantaneous 0.5-30 cycles; momentary 30 cycles-2 s; temporary 2 s-2 min; sustained over 2 min.
3. Sags and swells last 0.5 cycles to 1 minute; a starting motor can draw six times its running current or more.
4. Cascaded SPDs (service entrance, sub-panel, equipment) with a UPS are the most effective protection; the SPD should open the circuit if its MOV fails.
5. Humidity of 40-55% reduces ESD potential (summary Table 1 lists 35-50%).
6. Pulsed SMPS currents on a shared neutral reach a theoretical 1.73 times the phase current.
7. Voltage fluctuations are random changes of 95 to 105% of nominal, generally below 25 Hz.
8. Imbalance screening: highest minus lowest phase voltage must not exceed 4% of the lowest.
9. ITIC: 20 ms at zero volts, 10 s at 80%, 1 ms at 200%, 0.01 cycle at 500%.

Using it in design

• Encode the seven IEEE 1159 categories as a list used in risk registers and mitigation pickers.
• Check shared neutrals on single-phase SMPS circuits: the neutral must be sized for up to 1.73 times phase current, or a K-rated transformer specified.
• Calculate the phase-voltage imbalance check (max minus min <= 4% of min) whenever phase voltages are entered.
• Recommend a cascaded SPD scheme in the one-line template (Type 1 at the service entrance, Type 2 at sub-boards, point-of-use), and validate standby or line-interactive UPS and STS transfer times against the 20 ms ITIC ride-through.

Who should read it

Electrical designers and maintenance engineers assessing power-quality risk.

Related standards

• IEEE 1159
• IEEE 1100
• IEC 61000-4-11

Publication details

Authors

Joseph Seymour

Publisher

Schneider Electric

Year

2011

Document number

WP 18

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation