Download Doble Test Procedures - Richmond Community College

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, dielectricloss, 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 offline 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.