Download I consumer

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: [email protected]
1
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)
2
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.
3
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.
4
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
5
Experimental set-up
6A
System for fine tuning of electrode position
A
S
A
CT
25A/5A
220V
50Hz
AT
0-300V
V 250V
Consumer
Digital
oscilloscope
PC
B
LEM +- M
Rs
Supply
of the
LEM
+- M
Consumers:
resistor 220 A, 30 A
fluorescent lamps
mono-phase motor with
speed control
microwave furnace
compact fluorescent
lamp with electronic
ballast
PC
6
Results of experiments
Materials and arc characteristics
Electrodes
copper - copper
copper - messing
messing - messing
messing - alloy of aluminium
Minimal current of a stable arc (A)
10
 6.5
7
2
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
Voltage (V), Current (A)
30
Current
Voltage
20
10
0
-10
-20
-30
0
5
10
15
20
Time (ms)
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 %
10
2
1.5
1
Current
(A)
struja(A)
0.5
0
-0.5
-1
-1.5
-2
0
0.02
0.04
0.06
(s)
Timevreme
(s)
0.08
0.1
0.12
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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.
13
Realised prototype of the arc protective device
Voltage input
1
Current input
2
fo
3
LF
HF
4
f’o
5
P
LCD

6
7
11.0682MHz
14
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.
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