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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@galeb.etf.bg.ac.yu

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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)

Fire hazard

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by a parallel arc: a current of the circuit increases in the case of phase to protective conductor fault, the differential current appearsthe 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.

Characteristics of parallel and serial arcs

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1. GREGORY, G.D., and SCOTT, G.W.,: ’The Arc – Fault Circuit Interrupter: An Emerging Product’, IEEE Trans. on industry applications, September/October 19982. 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 onacoustic signals orlight

These detection techniques are practically limited to the places with high number of contacts, for example at switchboards.

State of the art

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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 arcthe minimal current of stable arc maintaininginfluence of material to arc characteristicsinfluence of the consumers to the arc characteristicsinvestigation of the harmonics caused by non-linear consumersinfluence of the voltage distortionstructure of the consumers in household and industrysetting-up the limitations of the algorithm application

Our idea and research done

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Experimental set-up

Consumers:resistor 220 A, 30 Afluorescent lampsmono-phase motor with speed controlmicrowave furnacecompact fluorescent lamp with electronic ballastPC

6A

220V50Hz

A

+- M

Supply of the LEM

Digitaloscilloscope

PC

Consumer

LEM

S

AT0-300V

CT25A/5A

A

B

V 250V

System for fine tuning of electrode position

Rs

+- M

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Materials and arc characteristics

At 1.5 kW consumer the existence of electric arc is already possible.

Results of experiments

Electrodes Minimal current of a stable arc (A)copper - copper 10copper - messing 6.5messing - messing 7

messing - alloy of aluminium 2

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By effective current of 6.5 A the higher current components areI3g = 1.95 AI5g is our great sicretI7g and I9g are about 3 times lower than I5g

Spectral characteristics of the electric arc: resistive load

0 5 10 15 20-30

-20

-10

0

10

20

30Current Voltage

Vo

ltag

e (V

), C

urr

ent

(A)

Time (ms)

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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 beprotected 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.

Harmonics caused by non-linear consumers

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by the usage of welding device, containing controlled electric arc, protection does not function, i. e. have to be blockedmicrowave 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 loadBy 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 %

Specific problems with the non-linear consumers

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0 0.02 0.04 0.06 0.08 0.1 0.12-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

str

uja

(A)

vreme (s)Time (s)

Cur

rent

(A

)

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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 AIconsumer   Ifuse max

Rconsumer   230 V / 16 A =14.38 

Iconsumer 5max  = 13.8 V / (230 V / 16 A) =0.96 A  I5g

Influence of the voltage distortion

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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.

Structure of the consumers in household

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Realised prototype of the arc protective device

Voltage input

HF

fo

LF

f’o

PLCD

Current input

11.0682MHz

1 2 3

4 5 6

7

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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.

Concept of the commercial device