Doble Test Procedures

Richmond Community College

Insulation Test Specimens

• Insulation systems associated with many types of high-voltage power apparatus and devices often consist of a mix of non-homogeneous component parts.

• Representation of an apparatus insulation system may consisting of a number of resistor and capacitor elements arranged in many varied ways

• A convenient way to represent an insulation specimen by a single capacitor combined with a single resistor

Insulation Test Specimens

• The capacitor element represents the specimen's fundamental capacitance (i.e., its ability to store electrically separated charges), while the resistor element represents the dissipated loss in the insulation when voltage is applied

• The M4000 test circuit commonly views the specimen as a parallel capacitor/resistor network

Insulation Test Specimens

• Resistor element of equivalent dielectric circuit represents the watts-loss dissipated in the insulation when a voltage is applied. Rp considered undesirable in a dielectric.

• There is always a certain amount of measurable loss is normal for most dielectrics and, therefore, the existence of loss, per se, does not necessarily suggest an operating hazard.

• In a perfect or no-loss capacitor, the current leads the test voltage by exactly 90°. In a perfect resistor, the current and voltage are exactly in-phase

Test Modes Of The M4000 Analyzer

Three basic test circuit configurations of M4000

• GST-GROUND

• GST-GUARD

• UST-MEASURE

Grounded-Specimen Test Mode (GST)

• With the M4000 Analyzer in GST-GROUND (R,B), the LV Leads are brought to ground potential. Thus, the LV Leads may be used to connect ground to a specimen terminal.

GST-Guard Test Modes

With the M4000 Analyzer in GST-GUARD (R,B) Figure 1.5, the LV Leads are both connected to the test circuit Guard.

GST-Guard Test Modes

• Blue will be measured and red not.

• In both cases, the grounded lead is measured and the guarded lead is not

IN ALL THREE CASES, THE TEST SET GROUND LEAD IS MEASURED

GST-Guard Test Modes

red will be measured and blue not

Ungrounded-Specimen Test Mode (UST) • With the M4000 Analyzer in UST-MEASURE (R,B) , the only input to the

measuring circuit is through the LV Leads

• Test set Guard and Ground are common in the UST mode; thus, current and losses to ground are not measured

•red and blue leads are both measured

Ungrounded-Specimen Test Mode (UST)

• Only the red lead is measured

Ungrounded-Specimen Test Mode (UST)

•Only the blue lead is measured

Doble High-Voltage Test Cable

Placement Of The High Voltage “Hook”

• The high voltage cable’s outboard pothead, commonly called the “hook” includes a Guard and a Ground ring, or terminal, which

must be KEPT at a distance from any ENERGIZED SURFACES

• Never hold onto the High Voltage Cable during a test!

• If draping the high voltage cable over an adjacent structure to keep the Guard and Ground rings away from the energized surface

• Do not drape it over a terminal or surface that will be energized during the test

Test Voltages • The basic principle of the Doble Test is to measure the fundamental AC

electrical parameters of insulation (i.e., power factor, capacitance, dielectric-loss, etc.), applying test voltages which are moderate compared with the design rating of the insulation

• With due regard for properly selected test voltages, the Doble Test is NONDESTRUCTIVE, in that it should cause no measurable harm to serviceable insulation

• Should determine beforehand the tests to be performed and the test potentials to be applied for the various measurements. In particular, the test engineer should note those tests which must be made at reduced voltages (i.e., less than 10 kV)

Suspect Apparatus

In following situation make an initial measurement at low voltage (2 kV or less), gradually increasing, in steps, up to the normal test level only if the readings at the previous lower voltage indicate no problem

1. Equipment which has become badly contaminated with moisture in transit from the factory, repair shop, or between substations

2. Power transformers and related equipment that have tripped off-line due to the operation of protective relays

3. Equipment suspected of having become badly contaminated with moisture after being stored outdoors for prolonged periods

Surface Leakage • Two environmental variables arise which cannot be controlled easily are

temperature and humidity (also humidity in combination with air pollutants) • It should also be recognized that there will be times when it will be best to

postpone tests until another day • There are two basic approaches to minimizing the effects of surface leakage:

1. Clean and dry exposed external surfaces (usually porcelain) to reduce the losses.

2. Employ Guard Collars to divert the undesirable surface leakage currents from the measuring circuit.

It is apparent that a combination of both approaches will be most effective.

Surface Cleaning

• Wiping

a moist/dirty porcelain surface can be cleaned with a clean and dry cloth, provided that the amount of contamination is not too great.

• Caution

Sometimes wiping the surface simply spreads the contamination, and may actually increase surface losses.

In such cases it may be necessary to apply a silicone-base or hard-base wax or grease (after surface dirt has been removed)in order to BREAK UP conducting paths.

Surface Cleaning Ctd… • Solvents may be used to help clean surfaces which are stained or laden with

particulate matter such as dirt, soot, etc. Upon evaporation, solvents tend to cool a porcelain surface and this, in turn, may increase moisture condensation.

• Caution Solvents should not be used where the problem is one of moisture

condensation alone.

• Heat

An excellent method for reducing surface leakage due to moisture. Infrared lamps and hot-air blowers have been used successfully.