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Cover page for contributions to the PC62.36 revision
PC62.36 Clause
Number and Title
Comment
Submitted By
New clause: GDT oscillation
Mick Maytum, Mick Maytum, and Wolfgang Oertel, Bourns.
Proposal Summary
Define a test arrangement for identifying GDTs that oscillate in
digital subscriber line, DSL, applications.
Technical Rational
for Proposal
Damaging oscillations occur when certain GDTs are used in
digital subscriber line, DSL, applications. The proposed test
arrangement enables such oscillation problems to be tested for in
the laboratory and permit the selection of GDT types that don’t
cause oscillation problems in given systems.
Proposed Change
or New Material
WG Resolution
See following pages
Accept/ Reject / Accept with Modifications
NOTICE
This correspondence is a draft document and subject to change. It does not reflect the
corporate policies or the views of the Institute of Electrical and Electronics Engineers, Inc
or of any company. The Institute, the companies and individuals involved, take no
responsibility in the applications of this document. It is possible that this material will at
some future date be included in a copyrighted work by Institute of Electrical and
Electronics Engineers, Inc.
By submitting this correspondence, the contributor and his company, if any, has agreed
to comply with the IEEE patent policy that can be found in clause 6 of the IEEE -SA
Standards Board Bylaws.
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1.1
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GDT-based SPD Oscillation Test
1.1.1
Background
Figure 1 shows the VI characteristic of a Gas Discharge Tube, GDT, in one quadrant.
When the voltage across a GDT reaches its sparkover voltage it conducts current. Low
currents are conducted in the glow mode and h igh currents are conducted in the arc
mode. A GDT has transitional conditions between the various regions. Figure 1 shows
transitions of sparkover to glow, glow to arc and arc to glow. The left characteristic of
Figure 1 shows the inherent GDT characteristic. The right characteristic shows the VI
locus that occurs when the voltage source has a high resistance value. The glow region
operation is in two sections; one for increasing current and the other for decreasing
current. The transitions are constrained to what the source resistance allows. Low
resistance source values can remove any glow region operation. There will be two
transitions; sparkover to arc and arc to non-conduction.
400
400
Sparkover
Voltage — V
300
250
350
200
150
100
0 to Sparkover
Sparkover-to-Glow Transition
Glow Region - Current Increasing
Glow-to-Arc Transition
Arc Region
Arc-to-Glow Transition
Glow Region - Current Decreasing
Glow to 0
Sparkover
300
Voltage — V
350
0 to Sparkover
Sparkover-to-Glow Transition
Glow Region
Glow-to-Arc Transition
Arc Region
Arc-to-Glow Transition
Glow to 0
250
200
GDT VI Locus with an AC Voltage Source of 2 kV peak
(1.4 kV rms) and a resistance of 2 k
150
100
Glow Region
Glow Region
50
50
Arc Region
Arc Region
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Current — A
Current — A
Figure 1 — Inherent GDT VI characteristic (left) and
GDT VI locus with a 2 k AC source (right)
GDT formulations can be classified by the value of arc voltage. Table 1 shows the typical
parameter ranges for arc voltages of 10-volts or less and those greater 10-volts.
Formulations with high arc voltage will typically have less voltage overshoot on fast rising
impulses than those with low arc voltage.
Table 1 — GDT Parameter value ranges
Parameter
Low Arc
Voltage GDT
High Arc
Voltage GDT
DC Sparkover Voltage
V
65 — 600
200 — 600
Glow Voltage
V
70 — 80
130 — 200
Arc Voltage
V
8 — 10
20 — 35
Glow-to-Arc Transition
Current A
0.2 — 0.5
1—2
Arc-to-Glow Transition
Current A
0.05 — 0.1
1 typical
During laboratory power-fault testing with digital subscriber line equipment, it has been
discovered that certain GDTs can powerfully oscill ate generating hundreds of volts at
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many megahertz for long periods. This oscillation results from the interaction of the
characteristic of that GDT and the series C and L input impedance of the DSL equipment.
This oscillation generates interference to other lines and can cause failure of the DSL
equipment transceiver IC, both in the laboratory and in field service. Figure 2 shows such
an oscillation. The oscillation may occur in one or both AC voltage polarities.
Figure 2 — GDT oscillation in the negative AC polarity
yellow trace is voltage and the cyan trace is current
1.1.2
Purpose
The purpose of this test is to determine the likelihood of a SPD, using a GDT, generating
damaging oscillations when used with DSL equipment.
1.1.3
Equipment
a) 600 V rms AC power fault equipment capable of currents up to 3 A peak.
b) DSL equipment input emulation, capacitor C1 and transformer inductance L P1 and
L P2 . Typically C = 33 nF and 250 µH < L P1 + L P2 < 1000 µH.
c) Quad channel digital oscilloscope or waveform recorder capable of capturing
megahertz events
d) Voltage and current probes
1.1.4
Equipment States Subject To Test
All power fault test configurations that cause GDT conduction
1.1.5
Procedures
Figure 3 shows a longitudinal power fault test configuration. The SPD may contain two
GDTs, GDT1 and GDT2, or one single chamber device, GDTA. In some cases there may
be a GDT connected directly across the twisted pair. Currents, I 1 and I 2 , can be
monitored at the SPD input terminals and voltages, V 1 and V 2 , are recorded on the SPD
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output terminals. Alternatively just the oscillation voltage, V 3 , can be monitored on the
transformer secondary.
I1
SPD
GDT1
GDTA
V1
LP1
Longitudinal
Power fault
source
C1
LP2
I2
GDT2
LS1
V3
V3
LS2
V2
Figure 3 — Longitudinal Power fault test configuration
1) Connect up the chosen power fault configuration.
2) Set the power fault voltage at 600 V rms and the prospective current to the generator
lowest setting.
3) Apply the power fault condition to the SPD for 1 second (or as shorter time as
practical) and record the circuit waveforms during a AC cycle.
4) Increase the prospective current by a factor of two or the next current leve l setting
and repeat item 3.
5) Continue to increase the prospective current by repeating item 4 until a value in
excess to 3 A peak is reached.
NOTE: The test sequence can be stopped when an unacceptable level of oscillation is found, see 1.1.7
1.1.6
Alternative Methods
1.1.6.1
Measuring oscillation level
A visual determination of unacceptable self -oscillation levels will not be exact for
borderline cases. Crudely the stress levels are going to be dependent on the number of
impulses in a whole AC cycle and the amplitude of the pulses. A charge pump added to
the Figure 3 test circuit can be used as an amplitude sensitive counter . This would enable
a qualitative rather than quantitive a method of evaluating the level of oscillation, see
Figure 4. The charge pump circuit without the load, R1, becomes a (peak-to-peak)
voltage doubler and so the output is limited by a clamp diode D1. Suggested component
values are:
R1 = 1 k
C2 = 22 pF (for 2.5 primary-secondary ratio transformer)
C3 = 500 nF
D1 = D2 = 100 V signal diodes
D3 = 18 V
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SPD
GDT1
GDTA
C2
LP1
Longitudinal
Power fault
source
LS1
C1
LP2
D2
D1
C3
R1 D3
Average
voltage V O
LS2
GDT2
Figure 4 — Figure 3 with a charge pump circuit to give an average oscillation
voltage value
1.1.6.2
Data sheet selection
It has been found that high arc voltage GDTs tend to be the ones that oscillate. Although
not infallible, selecting GDTs with low data sheet arc voltage values is a simplistic
method of reducing the possibility of oscillation occurring.
1.1.7
Requirements
Any oscillations shall be of less than a few cycles and oscillation during most of the AC
conduction time constitutes a reject.
1.1.8
Comments