Mit s1 Nse an en v01

8/10/2019 Mit s1 Nse an en v01

1/4

5kV and 10 kV Insulation Testing Testing in Noisy EnvironmentsApplication Note

1

KEEP THE NOISE DOWNApplication note

What is electrical noise? What effects will noise have on insulation testing?

How does a Megger insulation tester deal withnoise?

What does the noise immunity specification mean?

What else can reduce the effects of noise?

How and when to use screened test leads?

What is electrical noise?

The term electrical noise is used to describe a whole range of electricalphenomena, signals, RF, electromagnetic, electronic, etc. Generally itis unwanted electrical energy, in other words electrical energy that willproduce an unwanted effect of some kind. When listening to musicelectrical noise can produce an unwanted hiss in the background.

Electrical noise we are interested in is produced at a variety offrequencies, and can appear in the sample being tested. It is usuallyinduced currents or voltages from adjacent equipment and is verycommon in substations, particularly high voltage substations wherepower frequencies predominate.

What effects will noise have on insulation testing?

This electrical noise superimposes an AC signal onthe DC test current and can cause considerablevariations in readings and may prevent theoperator getting a reading at all if it is beyond thecapabilities of your instrument. Many operators seethis as something they have to live with, a sort ofoccupational hazard. One solution is to implementa complete and expensive shutdown to remove thesource of noise. In the worst case it can lead toincomplete test schedules and as a key objective

of diagnostic insulation testing is to prevent expensive and dangerous failures this is not good news.

How does a Megger insulation tester deal with noise?

For fear of stating the obvious Megger builds in immunity to noise. Megger has over 100 years of experience inthe innovation and design of insulation testers. Each generation of instruments improves on the last, combiningour experience with cutting edge technology. The result is instruments that are designed to operate in anyenvironment the operator is ever likely to encounter, including electrical noise.


2/4


2

In Europe we have to meet stringent EMC standards, to meet these standards Megger has invested in its veryown EMC test laboratory at the Dover manufacturing site. The latest generation of MIT and S1 5kV and 10kVinsulation testers where all tested there and pass the latest edition of IEC61326-1 that came into force inFebruary 2009. Not only that, they passed the standard to the heavy industrial limits.

However, it doesnt stop there, in extreme testing environments experience has shown that the electrical noiseinstruments are subjected to can be far in excess of the limits laid down by IEC61326. One example is HVsubstations, especially in locations where earthing or grounding is difficult.

What does the noise immunity specification mean?

So when choosing an 5kV or 10kV insulation tester how can you be confident that it will be able to provideaccurate insulation measurements in noisy environments. Any manufacturer can say that an instrument hashigh noise immunity, but are they willing to add a specification to their literature?

Megger have for many years specified the noise immunity of insulation testers, the latest MIT and S1

instruments are no exception. The level of immunity specified for these instruments is 2mA at 50/60Hz, but whatdoes that mean?

Basically nearby powered up equipment, for example corona on HV bushings, radiates electrical noise that getspicked up either by the test leads or the item being tested.

Nearby cables can also induce current due to their electromagnetic field. This effectively results in unwantednoise current flowing in the test. The amount of current induced depends on the amount of electrical noise in thelocality of course.

Adjacent live cablesInducing noise current

Adjacent equipmentradiating noise

Test piece


3/4


3

So, back to the specification of 2mA at 50/60Hz. This is actually a worst case, the MIT and S1 instrumentsemploy capacitive filtering which means that as the noise frequency increases then so does the level ofimmunity. The noise generated from the effects of corona on HV bushes is typically in the kHz region, hencewhy we can hear it crackling.

Noise immunity of 2mA is sufficient for the majority of applications, but what happens in extreme environments

such as >150kV substations. In these locations it is not uncommon to encounter noise currents up to 4mA. Theanswer is the Megger S1-554/2 and its 10kV counterpart the S1-1054/2. These instruments represent theultimate in noise immunity available on an insulation tester. Specially developed input filtration efficiently removeany high frequency effects on the reading, and then firmware filtering techniques remove the low frequencyvariation on the reading. The S1-554/2 and S1-1054/2 have been successfully tested and used in the noisiestswitchyards in the world, including locations in India.

Can the noise current exceed 4mA, and what happens if it does, I hear you ask? Connecting to the tips ofbushing on the top of transformers can involve the use of very long test leads making very effective aerials, sothe answer is occasionally it will. In these circumstances the answer is prevent the noise inducing a current inthe first place.

What else can reduce the effects of noise?

The first thing that can be done is take care with test lead layout. Keep them as short as possible and where youcan keep them near to earth / ground to minimize noise pick up.

XX


4/4


4

How and when to use screened test leads?

By far the most effective precaution is to use screened / shielded test leads. Megger have a comprehensiverange of screened test leads to suit your application. The short test lead connected to earth or ground will notpick up enough noise to cause a problem. The long test lead connected to the equipment under test is the one

to screen / shield and Megger can supply 3m, 10m or 15m leads to suit.The screen / shield are connected to the Guard terminal, which takes the noise current away from themeasurement circuit, and therefore ignored. The Guard terminal is then employed to remove unwanted leakagecurrent (see the Be on Guard for effective testing application note) in the usual way.

However, remember the screened test leads will only remove the effects of noise picked up on the test leads. Ifnoise is picked up on the test piece (e.g. long overhead power cable) there is no substitute for an instrumentwith excellent noise immunity.

The combination of the excellent noise immunity of the MIT and S1 range of 5kV and 10kV instruments and agood quality set of screened / shielded test leads will provide excellent noise immunity.

Moving to the S1-554/2 and S1-1054/2 with their 4mA of noise immunity with screened / shielded leads providesthe ultimate in high noise immunity able to take accurate measurements in the most arduous locations in theworld.

The Use of

Screened TestLeads

Screened test lead.Available from Megger3m, 10m or 15m long, 5

or 10kV rated

Screen

connected toGuard terminal

Guard used to ignoreleakage currents in

the usual manor

The Use of

Screened TestLeads

Screened test lead.Available from Megger3m, 10m or 15m long, 5

or 10kV rated

Screen

connected toGuard terminal

Guard used to ignoreleakage currents in

the usual manor

UK

Archcliffe Road Dover

CT17 9EN England

T +44 (0) 1304 502101

F +44 (0) 1304 207342

[email protected]

UNITED STATES

4271 Bronze Way

Dallas TX 75237-1019 USA

T 800 723 2861 (USA only)

T +1 214 333 3201

F +1 214 331 7399

[email protected]

OTHER TECHNICAL SALES OFFICES

Valley Forge USA, College Station USA, Sydney

AUSTRALIA, Tby SWEDEN, Ontario CANADA,

Trappes FRANCE, Oberursel GERMANY, Aargau

SWITZERLAND, Kingdom of BAHRAIN, Mumbai

INDIA, Johannesburg SOUTH AFRICA, Chonburi

THAILAND

CERTIFICATION ISO

Registered to ISO 9001:2008 Cert. no. Q 09250

Registered to ISO 14001-2004 Cert. no. EMS 61597

MIT_S1_NSE_AN_en_V01www.megger.com

Megger is a registered trademark

Mit s1 Nse an en v01

Documents

Transcript of Mit s1 Nse an en v01