Partial Discharge
Meter (DTM-D)
| Intro |
Measuring Principle
| Description
| DTM-Test
Circuits | Pulse
Generator |
General
The measurement of partial discharges is a non destructive text on electrical power
apparatus. The measured quantities are available data to determine the quality of an insulation. In high voltage test techniques, special importance is therefore attached to
measurement of partial discharges.
The reading of partial discharge intensity is in pico-coulombs (pC) or, alternatively, Radio Interface Voltage (RIV) in
micro volts (mV). The Voltage AC voltage display is also available to measure peak, peak and RMS using suitable Voltage Divider.
The partial discharge/RIV measuring system consist of :
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Partial discharge meter (type DTM) which is the basic measuring unit with built-in Digital Storage Oscilloscope.
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Filter insert option for either Narrow band (Type DTF-1) or Broad band (type Eb1)
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Measuring impedance and coaxial connecting cable.
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Pulse generator (type
PDG)
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Laptop Computer with suitable GPIB Card.
Functions :
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Measurement
of partial discharges in pC and RIV in mV.
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Measurement on number of pulses above
threshold and details of its phase position.
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Narrow band selective filter (type DTF1) with variable center frequency from 600 kHz to 2400 kHz, band width 9kHz for PD and RIV
measurements. Alternatively, Broad band filter insert with frequency range from 40 kHz to 220 kHz having fixed bandwidth of 180 kHz for PD
measurement.
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Built-in oscilloscope to display PD pulse.
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Facility to measure high voltage AC as Peak, Peak/2 or RMS Values.
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Software for recording and further processing of PD pulses in Computer.
Measuring Principle
Partial discharges in the test object cause charge transfers in the high-voltage circuit, giving rise to voltage pulse variations on the measuring impedance
(Quadripole). These pulses are evaluated by the partial discharge meter. The filter with adjustable centre frequency allows to select parts of the high-voltage spectrum for
measurement purpose.
Description
Partial discharge meter (DTM D)
The measured partial discharge intensity is displayed either in pC or in mV in accordance with IEC60270/IS6209 or NEMA 107. According to IEC 60270, the measured quantity shall be multiplied by a correction factor, which takes care of the circuit
characteristics of the complete test arrangement. With built-in correction circuit, it is possible to incorporate the correction factor into the display. The actual partial discharge intensity can then be read directly without the usual calculations. At switch position "CORR", the correction factor is displayed directly. The PD pulses are tapped from the analogue output terminal and displayed of the built-in Digital Storage Oscilloscope.
AC Test voltage Display
The power frequency test voltage is displayed as Peak, Peak /2 or RMS
values in conjunction with suitable voltage divider.
Software for PD Analysis
The built-in Digital Storage Oscilloscope is used to capture/transfer the Analog PD pulses measured by the basic PD Meter (clause 3.1)
The Digital PD data is transferred to a computer through GPIB and can be manipulated using Microsoft Windows compatiable software loaded in the Laptop.
With use of this built-in software the following features can be obtained :
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The main PD display window shows the peak value, average value and the number of pulses above threshold.
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Phase position of PD activity.
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Provides noise immunity used to cancel-out unwanted noise.
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Provides automatic statistical data analysis for post inspection review.
Filter Inserts :
A selective narrow band filter with variable center frequency and a broadband filter can be supplied.
Narrow Band Filter :
Variable center frequencies from 600 kHz to 2400 kHz, Bandwidth 9kHz
Broad-band Filter :
Frequency range 40 kHz to 220 kHz, Bandwidth 180kHz. The basic noise level of the instrument is less than 0.6 pC in the most sensitive measuring range. the measuring accuracy and linearity correspond to IEC 60270 and IS 6209
Measuring ranges :
2.5 pC to 50,000 pC (IEC 60270) / 1mV to 25,000 mV (VDE, CISPR)
Measuring Impedance :
The measuring impedance (quadripole) filters out the power frequency currents from the high-voltage test circuit and allows only HFPD pulses. It can be continuously loaded with 4A. Built-in surge arresters protect the instrument against over voltages.
Computer :
Laptop PC with suitable GPIB card.
Cable :
Co-axial cables to connect quadripoles to meter.
DTM-Test Circuits
Fig. 1 :
Connection of partial discharge meter DTM between test object and ground (Test circuit according to IEC60270 and IS6209).
1a. A coupling capacitor may be used to improve the transfer characteristics of the test circuit.
1b. PD measurement during applied potential test of inductive test objects.
1c. PD measurement during induced potential test of inductive test objects.
1d. PD measurement on capacitive test objects.
Fig. 2 :
Connection of partial discharge meter DTM between coupling capacitor and ground (Test Circuit according to NEMA Publ. 107)
2a. PD measurement during applied potential test of inductive test objects.
2b. PD measurement during induced potential test of inductive test objects.
2c. PD Measurement on capacitive test objects
2d. Coupling capacitors (C=1000pF) according to NEMA publ. 107
