DETECTION OF SERIAL ELECTRIC ARC IN LOW VOLTAGE ELECTRICAL INSTALLATIONS

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• DETECTION OF SERIAL ELECTRIC ARC IN LOW VOLTAGE
  ELECTRICAL INSTALLATIONS
• Zoran Radakovic, PhD EE
• Department of power converters and drives
• Faculty of electrical engineering
• University of Belgrade
• Bulevar Kralja Aleksandra 73, 11120 Beograd,
  Yugoslavia
• mailto radakovic_at_galeb.etf.bg.ac.yu

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Fire hazard

• According to data published by Consumer Products
  Safety Commission,
• there are more than 450 thousand fire events per
  year in USA,
• one third of them being caused by failure in
  electrical installations.
• The most critical electrical faults are those
  containing electric arc
• of longer duration. The arc produces a high
  amount of heat and
• temperature increase, causing appearing of fire.
• Types of electrical arc
• parallel (between parts of installation on
  different potentials)
• serial (on clamps in switch boards and inside the
  consumers,
• contacts in receptacles, plugs and switchers)

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Characteristics of parallel and serial arcs

• by a parallel arc
• a current of the circuit increases
• in the case of phase to protective conductor
  fault,
• the differential current appears
• the parallel arc usually grows to a standard
  direct short circuit
• by a serial arc
• a current of the circuit decreases !!!
• There is the need to search some special kind
• of protection against serial electrical arc.

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State of the art

• 1. GREGORY, G.D., and SCOTT, G.W., The Arc
  Fault Circuit Interrupter An
• Emerging Product, IEEE Trans. on industry
  applications, September/October 1998
• 2. http//www.cutlerhammer.eaton.com
• From their description it is hardly possible to
  realise the physical base of protective
• algorithm. It seems that it is necessary to
  protect any idea in this subject by a patent.
• By the product 1. A complex adjustment is
  required at the specific place installation.
• The quoted devices are based on analysis of
  electrical signal.
• There is also possibility of arc detection based
  on
• acoustic signals or
• light
• These detection techniques are practically
  limited to the places with high number
• of contacts, for example at switchboards.

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Our idea and research done

• Electric arc is non-linear element (impedance).
• It means that distort current will flow through
  the linear consumer - electric arc serial
  connection.
• The idea is to detect the distortion caused by
  the electric arc.
• The questions to be investigated
• spectral characteristics of the electric arc
• the minimal current of stable arc maintaining
• influence of material to arc characteristics
• influence of the consumers to the arc
  characteristics
• investigation of the harmonics caused by
  non-linear consumers
• influence of the voltage distortion
• structure of the consumers in household and
  industry
• setting-up the limitations of the algorithm
  application

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Experimental set-up

• Consumers
• resistor 220 A, 30 A
• fluorescent lamps
• mono-phase motor with speed control
• microwave furnace
• compact fluorescent lamp with electronic ballast
• PC

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Results of experiments
Materials and arc characteristics
At 1.5 kW consumer the existence of electric arc
is already possible.
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Spectral characteristics of the electric arc
resistive load
By effective current of 6.5 A the higher current
components are I3g  1.95 A I5g is our great
sicret I7g and I9g are about 3 times lower than
I5g
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Harmonics caused by non-linear consumers
Fluorescent lamps. By the fundamental harmonic
current of 3.35 A, the third harmonics was
0.60 A and the fifth 0.015 A. Mono-phase motor
with speed control. Simular existance of a high
third harmonic was detected by the fundamental
current of 3 A, the third harmonic was
1.6 A. Conclusions 1. Detection algorithm will
be based on the fifth harmonic. Criteria
detected fifth harmonic greater than I5g. 2. The
idea was to have simple algorithm, easy and chip
in hardware realisation and without adjustment
by the installation. 3. That is why the objective
application is in household. All circuits, first
of all in industry, having consumers with highly
distort currents can not be protected against
the arc using this simple algorithm. 4. There is
the need to test other consumers appearing in
households which could jeopardise the algorithm
application.
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Specific problems with the non-linear consumers

• by the usage of welding device, containing
  controlled electric arc, protection does not
  function, i. e. have to be blocked
• microwave has the fifth harmonic of 45.5  I5g
  the microwave decrease the total fifth harmonic
  appearing in parallel connection microwave -
  circuit of resistive load with electric arc.
  Solution fortunately, microwave operates
  intermittent, i. e. there are long periods with
  no load
• By the motor start, there is an appearance of
  high fifth harmonics. Solution short time delay
  of the protection response. We hope nothing will
  be burned ?.
• Usage of some consumers in the circuit has to be
  limited
• compact fluorescent lamps 20 W, up to 20.
  Measured THD  78.4
• PC up to 4. Measured THD  76.7

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Influence of the voltage distortion

• The fifth current harmonic can appear in the case
  of pure resistive load in the case when the
  network voltage is distorted.
• Quantification of the phenomena (the worst case
  study)
• U5max IEC 0.06 x 230 13.8 V
• Ifuse  max 16 A
• Iconsumer  ? Ifuse max
• Rconsumer  ? 230 V / 16 A 14.38 ?
• Iconsumer 5max   13.8 V / (230 V / 16 A) 0.96
  A ? I5g

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Structure of the consumers in household

• Having in mind noticed minimal currant of stable
  arc and the power of real consumers in household,
  the critical ones are thermal (boilers, heaters,
  furnaces).
• Central protection, in the supply of the whole
  flat is not suitable due to the quoted non-linear
  consumers.
• It is explained that harmful effect of non-linear
  consumers to the efficacy and reliability can be
  avoided.

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Realised prototype of the arc protective device
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Concept of the commercial device
Separate circuits should be monitored using
analogue multiplexer. Analogue multiplexed
signals would be secondary currents from current
transformers or voltage signals from the
shunts. Uniform scanning cycle would be
interrupted in the case of detecting the signal
of electric arc on one of the channels. Than, the
monitoring of this channel only would be
undertaken the following 10 seconds. For every
circuit two diodes (the green, indicating normal
operation and the red would be installed) in
order to distinguish the protection due to the
electric arc. Further research Extension of the
algorithm and its elaboration to a more general,
industry applicable case. The idea is to train
the detector with the harmonic characteristic of
the consumer every time at the specific place of
its instalment.