01_current for sag-dip testing - part 1

7/28/2019 01_Current for Sag-dip Testing - Part 1

1/4

____________________________________________________

Page 1 of 4

Voltage Dip Testing: How Much Current Must the Dip Generator Deliver ?

Part 1: Experimental Standard Test Load

Alex McEachern, Senior Member, IEEE

[email protected]

Power Standards Lab, Emeryville, California, USATEL ++1-510-658-9600 FAX ++1-510-658-9688

ABSTRACT

Current required by electronic loads during and aftervoltage dip testing is investigated. A standard test load is

constructed. This load requires only 45 amps nominal,

but actually requires 700 amps, or more, at the

conclusion of a voltage dip. If this peak current is notavailable, the load falsely appears to meet the testing

requirements. For this reason, test equipment for

performing voltage sag tests must be capable of providingvery high surge currents. Electronic sources, such asamplifiers, are generally not adequate. Transformer-

based sag generators with fixed taps (not variable

transformers) are generally capable of sufficient surgecurrent.

Keywords: sag, dip, immunity, testing, SEMI F47, IEC

61000-4-11, 61000-4-34, mains, power line

I. VOLTAGE SAG TESTING AND AVAILABLE CURRENT

Several standards, including SEMI F47 [1], IEC 61000-4-11 [2], and IEC 61000-4-34 [3], require testing electronic

equipment with voltage dips. Voltage dips are brief

reductions in AC voltage, typically lasting a second orless.

It is not widely understood that electronic loads, at the

conclusion of a voltage dip, may require far more currentthan their nominal current draw indeed, they may need

more current by a factor of 15 or more. In other words, a

load that is rated at 40 amps during normal operation may

actually require 600 amps or more for proper dip testing.

If the load is tested with a source that cannot provide this

increased peak current, failure modes such as blown fusesand open circuit breakers may be missed.

For this reason, electronic sources that provide limited

current such as amplifiers should be used for dip

testing with caution and skepticism about the results.

Transformer-based dip generators can generally deliver

sufficient current.

II. TEST LOAD FOR DEMONSTRATING DIP CURRENT

Fig. 1 Single-phase test load for determining voltage dip waveforms,typical of the power input circuit of single-phase electronic loads such

as adjustable speed drives. The bridge rectifiers BR1 are rated for

5 000 amps peak, and the capacitor C1 has an ESR of less than 8 m .

Fig. 2 Test load schematic shown in Fig 1. R1 is a 10-ohm, 10kW

air-radiant resistor. Hand-held meter (at left) shows scale. Nominal AC

current is 45 Amps RMS, but rises to 750 Amps peak after a voltage dip.


2/4

____________________________________________________

Page 2 of 4

After many dip tests of real-world equipment, a test load

was constructed (Fig 1 and Fig 2). To simplify analysis,

the test load was limited to single-phase. To make the

results useful throughout the world, a test voltage of 240V

was selected.

The nominal AC current drawn by the test load is

approximately 45 amps RMS. To be conservative, a 150-amp fuse (not shown in photos) was selected. However,

as will be seen below, even this fuse melted several times

during the dip testing. Testing was performed with a

commercially-available sag/dip generator [4], modified todeliver up to 1750 amps peak. Typical results from a

voltage sag are shown below.

Cursor: -15.7ms 177.82 Volts www.PowerStandards.com

-500

-300

-100

100

300

500

Volts

L2-L3 load - m illiseconds - 12/22/03 8:15:40 AM

-40 4 48 92 136 180 224 268 312 356 400

Fig. 3A Typical voltage sag on AC mains 50% for 18 cycles.

Cursor: -15.7ms 317.54 Volts www.PowerStandards.com

-125

0

125

250

375

500

Volts

Ch 21 (1000V) - m illiseconds - 12/22/03 8:15:40 AM

-40 4 48 92 136 180 224 268 312 356 400

Fig 3B Voltage on capacitor C1 during sag

Cursor: -15.7ms . Am ps www.PowerStandards.com

-1000

-600

-200

200

600

1000

Amps

L2 Ld (1000A) - m illiseconds - 12/22/03 8:15:40 AM

-40 4 48 92 136 180 224 268 312 356 400

Fig 3C Current on AC mains reaches 700 amps peak

Cursor: 0m s 44.58 Am ps(rms) www.PowerStandards.com

-125

0

125

250

375

500

Amps(rms)

L2 Ld (1000A) (rm s) - m illiseconds - 12/22/03 8:15:40 AM

-40 4 48 92 136 180 224 268 312 356 400

Fig 3D Same as Fig 3C, but expressed as RMS current.

As expected, capacitor C1 partially discharged through

resistor R1 during the dip. At the conclusion of the dip,

capacitor C1 recharged rapidly. The current into the

capacitor was limited only by the source impedance of the

AC mains, the Equivalent Series Resistance (ESR) of thecapacitors, and the forward impedance of the rectifiers.

The key observations for Fig 3 are as follows:

Figure 3B shows that useful voltage remains oncapacitor C1, so the equipment may continue to

operate during the dip

Figure 3D shows that the pre-dip nominal

current is 45 amps RMS

Figure 3C shows the peak current after the dip is

700 amps.

A careful review of the waveforms indicates that the

source impedance of the AC mains is the most significant

limit on the current. The source impedance, in this case,had a magnitude of approximately 100 m, which

includes the impedance of the cabling and the dip

generator, in contrast to the ESR of the capacitors

(approximately 8 m) and the forward impedance of thebridge rectifiers (approximately 1 m each, or 2 m

total).

At the conclusion of the sag, the voltage on the capacitorC1 is approximately 175 volts, and the peak AC mains

voltage is approximately 241 volts. The difference of 66

volts, with a source impedance magnitude ofapproximately 100 m, readily explains the 700 amp

peak current.

Of course, if the AC mains source impedance had beenlower, the peak current would have been higher. Also, if

the capacitor had been more completely discharged, the

peak current would have been higher.

In other tests, not shown here, with the same load andlonger duration sags, at the end of the sag the peak current

reached 1650 amps. The load, of course, is rated for 45

amps RMS, so the contrast is somewhat startling.

III. FUSE OPERATION AT THE END OF A SAG

For test purposes, the test load of Fig 1 was equipped witha 150-amp fuse. This rating seemed more than adequate,

as the test load actually draws approximately 45 amps

nominal.

However, the fuse operated (opened) after a few voltage

sags. Fig 4 shows the waveforms.


3/4

____________________________________________________

Page 3 of 4

Cursor: 20m s 178.8 Volts www.PowerStandards.com

-500

-300

-100

100

300

500

Volts

L2-L3 load - m illiseconds - 12/22/03 8:16:45 AM

-40 12 64 116 168 220 272 324 376 428 480

Fig 4A Voltage sag, with fuse opening at conclusion of sag.

Cursor: 0m s .49 Am ps www.PowerStandards.com

-1000

-600

-200

200

600

1000

Amps

L2 Ld (1000A) - m illiseconds - 12/22/03 8:16:45 AM

-40 12 64 116 168 220 272 324 376 428 480

Fig 4B The 800-amp current surge at the end of the dip was sufficient

to open the 150-amp fuse.

Cursor: 0m s 45.21 Am ps(rms) www.PowerStandards.com

-125

0

125

250

375

500

Amps(rms)

L2 Ld (1000A) (rm s) - m illiseconds - 12/22/03 8:16:45 AM

-40 12 64 116 168 220 272 324 376 428 480

Fig 4C Same as Fig 4B, but expressed as RMS amps. Note pre-dipcurrent is 45 amps RMS, a reasonable level for a 150-amp fuse.

Cursor: 0m s 318.52 Volts www.PowerStandards.com

-125

0

125

250

375

500

Volts

Ch 21 (1000V) - m illiseconds - 12/22/03 8:16:45 AM

-40 12 64 116 168 220 272 324 376 428 480

Fig 4D Voltage on capacitor C1. Note brief increase in voltage at endof dip, followed by continued decrease after fuse opens.

The key observations for Fig 4 are as follows:

Figure 4C shows that the pre-sag nominal

current is 45 amps RMS

Figure 4A shows that the 150-amp fuse operatedat the end of the sag.

This load, as designed, clearly cannot handle this voltagedip even with a 150-amp fuse, the fuse opens.

However, if this load had been incorrectly tested with a

dip generator capable of providing only a few hundred

amps peak which seems reasonable, but is in fact

wrong, for a load that requires only 45 amps nominal, the

load would not have blown its fuse. It would have been a

false passing result.

CONCLUSION

Voltage dip immunity testing is a useful, practical

engineering tool. It produces products that are strongerand more reliable. Like any engineering discipline,

experience leads to the discovery of pitfalls and incorrect

shortcuts.

One pitfall is testing using a dip generator that cannot

produce sufficient peak current. Sufficient is difficult to

define, but in the real-world examples shown in thispaper the peak current required was 15 times the rated

nominal current.

The commercially-available transformer-type dip/swellgenerator [4] used for the testing in this paper was capable

of delivering sufficient peak current.

Electronic sources are rarely rated for delivering

sufficient peak current for dip testing.

ACKNOWLEDGEMENTS

The author gratefully acknowledges the contribution of

Edward WINTERBERGER, who constructed the test loaddescribed in this paper. MIKE QUINN ELECTRONICS, of

Oakland, California, provided as always excellentsurplus parts at very low cost for the construction of the

test load. I am grateful for their help. Any errors, of

course, are solely my own responsibility.


4/4

____________________________________________________

Page 4 of 4

REFERENCES

[1] SEMI F47-0200, Specification for Semiconductor

Processing Equipment, Voltage Sag Immunity.SEMI, Santa Clara, California, USA. 02-2000

[2] IEC 61000-4-11 Ed. 2.0, Testing and MeasuringTechniques voltage dips and short interruptions

immunity tests.

[3] IEC 61000-4-34 DRAFT, Testing and MeasuringTechniques voltage dips and short interruptions

immunity tests equipment greater than 16 amps.

IEC Document 77A/444/CD

[4] Power Standards Lab Industrial Power Corruptor

Model IPC-480V-200A,

http://www.PowerStandards.com/SagGen.htm

Alex McEachern (M 1984, SM 1996) isthe President of Power Standards Lab inEmeryville,California.

Over the last 20 years he has taught

graduate-level power quality coursesand/or has supervised the installation ofelectric equipment in the United States,Canada, Croatia, Japan, Hong Kong,

China, South Africa, Germany, France,Singapore, Switzerland, England, Scotland, Mexico, NewZealand, and Australia.

01_current for sag-dip testing - part 1

Documents