Vacuum Interrupter / X-Ray Generation

[saladhawks]

The utility I work for decided years ago to transition from oil to SF6 for medium voltage equipment insulation due to environmental reasons.

Now, consideration is being given to transitioning from SF6 to Solid Dielectric w/ Vacuum Interrupters for medium voltage equipment insulation due to new environmental reasons.

I have read in manufacturer literature that the hi-pot testing of solid dielectric equipment with vacuum interrupters above 30 kV can result in X-Rays being generated. What exactly is the mechanism that causes such X-Ray generation and how is this being mitigated in 25 and 35 kV class rated solid dielectric equipment?

 

[dpc]

X-rays are produced in a vacuum by slamming electrons into an metal target. This is what happens during hi-pot testing of the vacuum bottles. It's only an issue during testing of the bottles and just requires keeping your distance from the bottle during the test. Should be well discussed in the manufacturer's test procedure. This has been a known issue with vacuum breakers for a long time.

For medium-voltage applications within vacuum interrupter's range, they have a lot of advantages over SF6, especially these days.

http://en.wikipedia.org/wiki/X-rays#History

 

[Gianoli]

**
What exactly is the mechanism that causes such X-Ray generation and how is this being mitigated in 25 and 35 kV class rated solid dielectric equipment?
**
A description of the mechanisms that can cause X-radiation to be produced during testing or during normal operation is complex and cannot be accurately described here. A very brief and incomplete explanation is as follows:

With the contacts of a vacuum interrupter open and an electrical potential (AC or DC) applied across the contacts one contact becomes a cathode and one an anode. As the electrical field strength increases it is possible to pull electrons of the cathode (field emissions) and accelerate them towards the anode. When the electrons reach the cathode they have achieved a high velocity and the collision with the anode material can produce some x-ray photons. The higher the electrical potential across the contacts the more electrons pulled off the cathode and the more collisions with the anode material and consequently more X-radiation produced.

It is important to follow the manufactures recommendations when testing your equipment. In general, if you do not exceed 40kV no special precautions are necessary when testing. In our company the testing of vacuum interrupters as all but ceased. The failure rate is so low, almost zero, that we don’t bother checking any more.

 

[itsmoked]

35kV Xrays are pretty punie. A lot of TVs run around that voltage. Those Xrays can be stopped by a little sheetmetal so I can't imagine much hazard outside any enclosure.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[unclebob]

Anyone ever smelled the ozone produced while hi-poting?
A somewhat pleasent odour...

 

[Zogzog]

I love the smell of Ozone in the morning!

Good topic, good way to explain it DPC. I have never had a good,simple answer for that.

I must say that I have seen several Vac bottles fail the integrity test and IMHO (And NETA's) it is still a valid and important test.

 

[dpc]

Vacuum bottles are extremely reliable, but they do fail on occasion. I'd still do the hi-pot test as recommended.

 

[Gianoli]

For those who feel they need test the integrity of vacuum interrupters on a regular basis I would advise caution. Adherence to the manufacuters instructions is critical. The flowing is taken from the service manual for a General Electric Power/Vac vacuum circuit breaker:

http://www.geindustrial.com/publibrary/checkout/Application and Technical|DET-456|generic

III. VACUUM INTERRUPTER INTEGRITY TEST
This test of the vacuum interrupter will determine its internal dielectric condition
and vacuum integrity. Prior to performing any vacuum interrupter integrity tests,
the outside (external surface) of the interrupter should be wiped clean of any
contaminates with a non-linting cloth. This is critical that the entire external
surface is to be completely free of all dirt, dust, oil, etc. With the breaker open,
individually check each interrupter by connecting the hi-pot machine across the
primary studs (bars) on the breaker side of the disconnect fingers. Ground the
other two interrupters line and load side of the primary studs, the frame, and the
secondary wiring. Increase the hi-pot machine voltage to 36kV (rms) 60 Hz. and
maintain for 60 seconds. If no breakdowns occur during any of the three hi-pot
tests, the interrupters have passed and the breaker can be put into service. If a
breakdown occurs in an interrupter, it must be replaced.
GE Burlington continues to strongly recommend the use of an AC high
potential machine for vacuum interrupter integrity tests. DC testing of vacuum
interrupters should only be utilized if an AC tester is not available, and should be
used for quick field checks only. Our experience with DC testers over many
years indicates they frequently yield false negative test results, due partially to
the capacitive component of the vacuum interrupter during DC testing, and to
the fact that most lightweight DC testers have a very low leakage current trip
setting. They will however, reliably indicate a truly failed bottle if the voltage
output is set at 50kV DC.
If using a DC tester, and a test indicates a bad interrupter, retest with the polarity
of the DC test voltage reversed. If this results again in a failure, we would
recommend a final AC test prior to contacting GE Post Sales Service or
discarding the interrupter.
No attempt should be made to try and compare the condition of one vacuum
interrupter with another, or to correlate the condition of any interrupter to low
values of DC leakage current. There is no significant correlation.
After the high potential voltage is removed, discharge any electrical charge
that may be retained.
CAUTION: MANY OLDER DC HIGH POTENTIAL MACHINES ARE HALFWAVE
RECTIFIERS. THIS TYPE OF HI-POT TESTER MUST NOT BE USED
TO TEST VACUUM INTERRUPTERS. THE CAPACITANCE OF THE
POWER/VAC BOTTLE IS VERY LOW AND THE LEAKAGE IN THE RECTIFIER
AND ITS DC VOLTAGE MEASURING EQUIPMENT IS SUCH THAT THE
PULSE FROM THE HALF-WAVE RECTIFIER MAY ACTUALLY BE IN THE
NEIGHBORHOOD OF 120KV, WHEN THE METER IS ONLY READING 40KV.
IN THIS CASE, SOME PERFECTLY GOOD BOTTLES CAN SHOW A
RELATIVELY HIGH LEAKAGE CURRENT SINCE IT IS THE PEAK VOLTAGE
OF 120KV THAT IS PRODUCING ERRONEOUS BOTTLE LEAKAGE
CURRENT. IN ADDITION, THE X - RADIATION WILL BE OF CONCERN.
23
An acceptable AC high potential test machine is available from the GE Electrical
Equipment Business, Burlington, Iowa, catalog number 282A2610P001.
Other acceptable manufacturers of portable AC and DC insulation testers
include Hipotronics models numbers 7BT60A, 60HVT, 860PL (DC) and
880PL (DC), or GE/Programma model VIDAR (DC).

 

[Zogzog]

Sure, that should be common knowledge to anyone that test these. But maybe not everyone knows about the DC hipot concerns.

Funny thing about this warning is the adder "An acceptable AC high potential test machine is available from the GE Electrical Equipment Business, Burlington, Iowa, catalog number 282A2610P001."

GE is always selling the extras.

 

[dpc]

GE - the one stop slop shop! - but not any more. They don't even make switchgear anymore - sold that to Powells, along with the Burlington, IA facility.

 

[Zogzog]

Yeah I know, we bought up the majority of the surplus GE switchgear and breakers.