5 . 4 Resolution

•The impulse can be separated is given by the resolution.

•The number of impulse can be distinguished in one quadrant of a 50c/s sine wave.

•The impulse are separated by equal time intervals.

•The resolution depends on

1. The resolution of the oscilloscope screen

2. The resolution of the detection circuit

3. The resolution of the amplifier

5 . 4 . 1 Resolution of the oscilloscope screen

•The impulses usually are so short that they appear as straight lines perpendicular to the zero line.

•The minimum thickness = about 0.2 mm

•Two quadrants of the 50c/s = maximum of 250 impulse

•A magnification of the time base is not used.

•An upper limit of the resolution of 250 impulses per quadrant is set by the optical resolution of the oscillation screen.

5 . 4 . 2 Resolution of the detection circuit

•The voltage impulses across the detection impedance consequently appear as the superposition of impulse.

•In both cases there appear impulse on the screen of the oscilloscope which are not of the right magnitude.

• = 1.5 to 3 times(a time interval)

•Resolution r = 1/300 to 1/600

5 . 4 . 3 Resolution of the amplifier

•The bandwidth of the amplifier is smaller than that which corresponds to the signal,the amplifier determines the resolution.

•The relation between bandwidth and time constant ,the resolution becomes r = B/100 impulse per quadrant, where B is the bandwidth of the amplifier

5 . 4 . 4 Total resolution

• The resolution of a discharge detector is equal to the smallest value of r which is determined by the smallest of the following three values:

1. The optical resolution of the oscilloscope screen:this is usually about 250 impulses per quadrant.(If a transient recorder is used.)

2. The time constant of the signal , r 1/500

3. The bandwidth B of the amplifier , r B/100.

5 . 4 . 5 Narrow vs wide band

•Narrow band

- a detector with a turned detection impedance

- bandwidth : order of the 3kHz

- the ensuring amplifier about 10kHz

- superposition errors in complex : long cable

lengths, transformers and machines

discharge response caused

•Narrow and broad-band

- bandpass filter or resistive unit as input circuit

- bandwidth : 80 or up to 150kHz

- the ensuring amplifier about 150 to 400kHz

•Ultra-wide band

- bandpass : up to 1GHz(nanosecond region)

- discharge location in very short cables and gas-insulated

switchgear

5 . 5 Observation

5 . 5 . 1 Oscilloscope

•5.16(a) : Recurrent discharge in the successive cycles cover each other and a fairly stationary picture is obtained

•5.16(b) : The impulse are often superimposed on to the sine wave of the test voltage

•5.16(c) : The picture on an elliptical time base is even clear

•5.16(d) : The time base has been made more characteristic by giving it sharp edges

5 . 5 . 2 Discharge magnitude

- The height of the calibrating pulse is expressed in pC

(compared of the discharge impulse)

5 . 5 . 3 Impulse crest voltmeter

•An oscilloscope may be too complicated for use in industrial tests.

good use : crest voltmeter

•X-Y diagram (Fig 5.17)

observed discharges vs test voltage

• order of magnitude than to the precise magnitude of the discharge

•Noise, unstable discharge ,etc

5 . 5 . 4 X-Y recorder

•X : voltage divider

•Y : logarithmic signal from the crest voltage measuring device

•sample given in Fig 5.17

•All the discharge patterns on the oscilloscope are checked

nonexisting discharge , disturbances

noise signal ,etc

5 . 5 . 5 Impulse counter

•Observation is obtain by the use of impulse counters.

•The impulse are electronically classified according to their amplitude and counted

•Disadvantage : separated , superimposed

5 . 5 . 6 Data processing

•Some examples of possible yardsticks are shown

•The digital analysis of discharge

many different ratings can be introduced in this way

• The difference between cavities within the dielectric and those situated at an electrode can be accentuated in this-way.