Download Table 3.1 Table 3.2

HIGH VOLTAGE ENGINEERING
Dr. Alexey V. Mytnikov
UDK 621.3.027.3(075.8)
BBK 31.24я73
М99
Author: A.V. Mytnikov
М99 High Voltage Engineering: – Tomsk: TPU, 2012. – 36 с.
ISBN 0-00000-000-0
In the practical workshop course basic fundamentals of practise part of
high voltage engineering are given. Every laboratory workshop is devoted to
practise studying of some high voltage phenomenon. Every workshop is experimental research of electrical discharge and high voltage circuit.
The textbook is appropriated for foreign students who study bachelor
electric engineering course.
UDK 621.3.027.3(075.8)
BBK 31.24я73
© Tomsk polytechnical university, 2012
© Mytnikov A.V., 2012
© Design. Publishing of Tomsk polytechnical university, 2012
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Preface
This laboratory workshop course has been written for the foreign students who come to study Bachelor’s course in Electrical Engineering of
Tomsk Polytechnical University. The author has an experience of course
“High Voltage Engineering” teaching for students who study on English language only, both from non-English (France, China, Czech Republic, Vietnam) and English-speaking countries (India, Jamaica).
It is very important for high qualify specialist in electrical engineering
to be familiar with real high voltages which are used in electrical energy systems. Practical experience of high voltage measurements, different types of
electrical discharge generation, real high voltage circuit engineering is very
useful for future practical engineering activity.
All workshops are united in one course. All workshops are fulfilled on
real high voltage equipment with the same voltage and current levels as at
power engineering industrial systems.
Every workshop is devoted to separate subject of high voltage engineering. It is necessary to note that successful studying of the course will be much
more effective in combination with textbook with the same title “High voltage engineering”.
Tomsk, 2012
Dr. Alexey V. Mytnikov
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Introduction
High voltage engineering has very wide range of application in many
areas of modern industry. High voltage engineering occupies an important
place in power engineering development all over the World. Problem of stable work supply of any electric energy system is solved by means of high
voltage technology using: reliable external and internal insulation performance, timely and high quality diagnostic of all high voltage equipment
range, overvoltages restriction and reliable protection, high voltage equipment testing, advanced high voltage equipment development and many others.
High voltage is used in electro–physical installations for pulsed power
purposes: charge particle accelerators, power lasers, plasma sources and
thermonuclear reactors. High voltage is widely used in technological processes such as: electroseparation, electrofiltering, pulsed magnetic field
treatment, plasmachemistry, nanostructure material synthesis with new properties.
In this connection, a studying of High Voltage Engineering course is
necessary for every high qualified specialist in electrical engineering.
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1. DISCHARGES IN ATMOSPHERE AIR
AT ALTERNATING VOLTAGE
MAIN GOAL OF THIS WORK: experimental research of discharge voltage dependence from different electric field forms at atmosphere air at AC
voltage of main frequency as source of electromagnetic interferences.
Short theoretical description
Atmosphere air is main kind of dielectric material for most of external
insulation of high voltage equipment and facilities. At normal atmosphere
conditions electric strength of air gaps in uniform field is 30 kV/cm. As rule,
electric field of real insulation constructions is non-uniform. Electrical
strength in inhomogeneous fields is lower compare uniform ones. This fact is
explained by discharge process law in inhomogeneous fields.
Three configurations of electric fields are existed:
uniform, weakly inhomogeneous, strongly inhomogeneous fields.
Parameter, which allows to determine and classify a type of electric
field is non-uniformity coefficient – KNON. Non-uniformity coefficient is ratio
of maximum electrical intensity to mean electrical intensity. Classification of
electric field types are following:
KNON=1 – uniform field
1>KNON>3 – weakly inhomogeneous field
KNON>3 – strongly inhomogeneous field
On practice, type of electric field is determined by configuration of
electrode system. Uniform field is correspond “ball-ball” electrode system,
when diameter of balls is bigger than distance between them. Weakly inhomogeneous field could be formed by “ring-ring” system of electrodes. Exam-
5
ple of strongly inhomogeneous field is “sharp rod-plate” electrode system. In
this laboratory workshop it is necessary to measure breakdown voltages in
three electrode systems for all electric field configurations.
Value of discharge voltage depends on anode-cathode distance, electrode configuration, kind of gas, pressure of gas and other factors.
Calculation of discharge voltage is fulfilled according to following formulas:
“Ball-ball” system:
U
DIS


0,54 

27,2 S 1


R  , kV



2
S

S




0,25 1  1 8
R

R



“Ring-ring” system:
U DIS  93,39S  , kV
“Sharp rod-plate” system:
U DIS  73,36S  , kV
where:
 - relatively density of air,
S - anode-cathode distance, cm
R - radius of ball, cm
ORDER OF WORK.
1. Install a “ball-ball” electrode system in high voltage testing facility.
2. Measure the discharge voltage for 1-5 cm range of anode-cathode
distance, at least three times and put results to Table (below).
3. Repeat the same measurements for “ring-ring”, “sharp rod-plate”,
“sharp – sharp”, “ball-sharp”, “ring-sharp” electrode systems.
6
4. Determine mean discharge voltage and electrical strength intensity
for all experimental points.
5. Calculate discharge voltage for all electrode systems, taking into
account that

is 1 for experimental conditions of high voltage
hall.
6. Make graphs for experimental and calculated dependences.
7. Make comparison and conclusions.
8. Prepare report containing experimental results, graphs, calculations
and conclusions.
9. Be ready to explain the experimental results.
Table 1.1 Experimental results
Mean
Distance
Breakdown Breakdown Electric
Electrode con-
between
voltage,
figuration
electrodes,
first run
S, cm
kV
“Ball-ball”
1
3
4
5
1
2
3
7
Electric
breakdown breakdown
second run field
kV
2
“Ring-ring”
voltage,
Mean
field
voltage,
intensity,
kV
kV/cm
4
5
“Sharp rodplate”
1
2
3
4
5
The same table structure
for other electrode systems
2. CHARACTERISTICS OF CORONA DISCHARGE
AT THE AC VOLTAGE
Main goal of this work: be familiar with main characteristics of corona discharge at the AC voltage as electromagnetic interfaces source. To study
experimental methods of corona parameters determination.
1. Short theoretical description
Corona discharge or just corona is self-sustain discharge in gas when
pressure is high enough. Main condition of corona existence is strong nonhomogeneity of electric field in electrode gap. Ionization processes are accompanied by light emission. Name of this kind of discharge is connected
with agleam aureole like “corona” form. Charge carriers are generated in ionization area under influence of electric field and move in the same field. So,
electric current of corona discharge is arisen.
Initial gradients of electric field E for AC corona voltage ignition
could be designated by empirical formulas of Pick. There are following:
for coaxial cylinders:
8
0,308
)
r0  
for parallel conductors of equal radii r0 :
0,301
E0  29,8   (1 
)
r0  
where r0 - radius of conductor,
E0  31  (1 

(1)
(2)
- special coefficient of wire surface evenness
а
b
c
g
f
Fig.1. Structure of charge distribution for different situation
of corona at wire area
Corona could be defined by three main electro-physical methods.
One of them is volt-ampere characteristic (VAC) determination. It could be
done with help of electron oscilloscope. Splash of current during VAC measurement is evidence of corona ignition. Typical picture of corona VAC is
shown on the Fig.2,a.
Fig.2. Characteristics of corona:
а) volt-ampere characteristic;
b) volt-coulomb characteristic
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Corona discharge is source of energy losses at the electric power stations and electric energy transmission lines. Small sized micro-sharps and
nano-sharps are presented on the transmission conductors always. So, necessary condition of corona ignition takes place practically on every transmission line and power station.
Energy losses due to corona presence are strongly depended on atmosphere conditions. The more bad weather the more serious losses take
place. This factor is taken into account in the coefficient of wire surface
evenness (it is so called weather factor). At the sunshine and dry weather the
coefficient is about 0,2-0,3. At the nasty, raining or snowing weather this factor is increased and reached value about 0,9.
Volt-coulomb characteristic (VCC) are shown on the Fig.2, b.
Current curve is typical sinusoid so as we use Ac voltage of 50 Hz.
Fig.3. Dependence of weather coefficient versus rain intensity.
Corona can cause serious problems in stable work and regimes of electrical
engineering processes and facilities. There are following:
- Corona is source of electric field. It is intensity is determined by
corona fields and currents;
- Corona activates electromagnetic interferences (EMI). EMI level
is determined by discharge processes. Corona can cause serious
problems in radio and TV broadcast in surrounding areas. Some-
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-
-
times it is impossible at all to use TV and radio devices. Services
like airports or emergency medicine must place far enough from
possible corona sources.
Corona is source of ozone in atmosphere air. Ozone could be generated very effective because all necessary condition of ozone synthesis are in corona area.
Ozone in turn can oxidize all matter around and spoil equipment
very seriously.
Corona can activate further discharge and breakdown phenomena up to arc ignition and emergency short-circuit regime.
Corona makes fire threat, because a lot of flammable materials
and properties are used in electrical engineering systems.
- There is just one positive feature of corona from EMI. It is property of corona to reduce of lightning discharge pulse. Due to wave
processes pulse level of lightning discharge could be serious diminished.
Electrical scheme of experimental set-up for corona studying is
shown on Fig.4.
Fig.4. Electrical scheme of experimental set-up:
АТ – adjusting transformer; Т – high voltage transformer, Rз – protective resistance; R1R2 – high voltage divider for measurement; C1-C2 – high voltage capacitive divider for
measurement; N – oscilloscope; С – divider for volt-coulomb characteristic determination; R – divider for current curve and volt-ampere characteristic determination;
Ц – metal cylinder.
11
Order of work
1. Be familiar with experimental set-up.
2. Measure the corona voltage ignition for 4 conductors of different diameters (r1, r2, r3, r4) by means of three methods: а) current curve; b) using
volt-ampere characteristics; c) using volt-coulomb characteristics. Put
these data to Table #1. Measurement procedure is fulfilled by voltage determination on primary winding of high voltage transformer. Using
converter curve to find voltage on secondary winding.
Table 2.1. Experimental results of corona parameters measurement.
Technique of corona voltage ignition
Corona voltage ignition, kV
Radius of wire, mm
r1 =
r2 =
r3 =
r4 =
Current curve
Volt-Ampere characteristic
Volt-Coulomb characteristic
Report content
1. Fill the Table #1, according to experimental measurements.
2. Plot the dependence of ignition voltage of corona versus wire radius.
3. Answer control questions.
Control questions
1.
2.
3.
4.
5.
Which is main special feature of corona discharge?
Why corona has this name?
Which is main and necessary condition of corona ignition and existence?
Which adverse effects could be caused by corona?
Describe shortly main experimental techniques of corona voltage ignition?
What can you say about advantage and drawback of any experimental
method?
6. Which problems could be caused by corona from electromagnetic compatibility point of view?
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3. POLARITY AND BARRIER EFFECTS RESEARCH
Goal of this laboratory workshop: research of influence both factors polarity
of electrodes and barrier presence to the breakdown voltage of electrode system “rod-plate” type at the air atmosphere pressure.
Short description
Atmosphere air is main kind of electrical insulation in the industrial
electric power systems. That’s why it is very important to estimate breakdown voltages of air gaps. Discharge voltages at the constant electrode gap
depend on degree of electric field uniformity, time of voltage application, polarity of electrodes and others. At uniform and weak non-homogeneous fields
an influence of polarity is not serious. At strongly non-homogeneous electric
fields the polarity of electrodes influences on the physical processes in the
discharge gap very much.
Polarity effect could be reason of activation of electromagnetic interferences of different range and intensities. Polarity effect should be taking
into account as source of electromagnetic interferences.
At the positive polarity of rod (sharp electrode) electrons moving to the
sharp, produce impact ionization and form an electron avalanche. When avalanche comes to sharp electrons are neutralized at the anode. At the same
time positive ions due to small velocities, move in the sharp region providing
positive volume charge. Electric field of positive volume charge diminishes
field at the sharp region and increases of it in the space of electrode gap. As
result of the processes streamer is formed to the cathode direction and increases external electric field. In this case breakdown voltage is decreased. At
the negative polarity of sharp electric field at the sharp region leads to intensive electron cathode emission, impact ionization and big amount of avalanches. Avalanche’s electrons coming to anode lost a velocity, are captured
by neutral molecules transforming to negative ions. Positive ions form volume charge at the sharp which increases field at the sharp diminishing electrical strength in the gap. As result – breakdown voltage is higher at the negative polarity of rod.
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Fig.1. Anode streamer formation.
Fig.2. Cathode streamer formation.
Е – electrical strength of external field; Еоб – electrical strength of positive
volume charge; Ерез – resulting strength in the gap after ionization.
Positive volume charge influence is used to increase discharge voltages of
insulating gaps. It could be reached by barrier installation. Presence of barrier
made from dielectric (electrical paper) allow to increase breakdown voltage
significantly (Fig.3).
14
Рис. 3. Electrical field intensity distribution with barrier in the gap.
Order of work
1. Study and collect electrical scheme as it is shown on the Fig.4.
2. To measure the breakdown voltage for both polarity for 4-6 points of electrode gap. Repeat experiment 3 times. Data should be marked in the Table
#1.
15
Fig.4. Electrical scheme of facility:
AT – adjusting transformer;
Т – high voltage transformer; RЗАЩ – protective resistor;
V – rectifier; С – capacitor; R – resistor for current limitation;
А – device for current measurement
3. To measure the breakdown voltage for 4-6 points at the sharp-barrier distance - S1 Repeat 3 times and put data in Table #2. Electrode gap length is
constant.
S,
Cm
UP, kV
1 2
3
UР.CP
S1,
cm
UP, kV
1
16
2
UР.CP
3
Polarity of
sharp
Table 3.2
Polarity of
sharp
Table 3.1
Report content
1. Prepare plotted dependences breakdown voltage versus gap length at both
positive and negative polarities.
2. Prepare plotted dependences breakdown voltage versus S1 at the positive
and negative sharps.
3. Explain an influence of electrode polarity and barrier presence to the discharge voltage.
Control questions
1. Call factors, which is responsible for discharge voltages?
2. Explain, please, influence of polarity to the discharge voltage breakdown
process?
3. How is it possible to increase the discharge voltages of insulating gaps?
4. Explain, please, influence the barrier to the discharge voltages
5. Which materials are used as barrier? What about optimum distance between rod and barrier for maximum efficiency providing?
6. How polarity effect should be taking into account from electromagnetic
compatibility point of view?
4. DISCHARGES ON THE SURFACE
OF SOLID DIELECTRIC
Goal of this work: research of surface discharge characteristics which are
depended on electrical field configuration, electrode distance and dielectric
thickness.
Short description
Surface discharge is type of electrical discharge which is burning alone
surface of solid dielectric. Discharge voltage of surface discharge in the air
is much low than discharge voltage of the same gap and electric field configuration without dielectric between electrodes. Discharge voltage is determined by discharge channel length, electric field configuration, conditions on the solid dielectric surface, temperature, pressure and humidity of
air.
17
Three main types of electric fields on surface of solid dielectric
could take place:
1. Uniform field (Fig.1).
2. Non-homogeneous field with prevailing tangential part of electric
field (Fig.2).
3. Non-homogeneous field with prevailing normal part of electric
field (Fig.3).
Fig.1. Electrode system with uniform field
а
b
Fig.2. Electrode system with prevailing tangential (a) and prevailing normal (b)
part of electric field
Any dielectric material, which is placed into electrode gap influences to
discharge processes a lot. As result discharge is formed on the surface of
dielectric. At this case discharge voltages are sufficient low compare air
gap.
Surface discharge passes a couple stages of development during applying
voltage increasing:
1. First stage is corona. Corona foreruns to all next processes. At the
low voltages corona discharge is firstly formed independently of dielectric material, electrode form, surface conditions.
2. Under grow of applying voltage streamer channels are arisen. Character of discharge processes is determined by current level in
streamer channels. Current in the streamer channels increase as re-
18
sult of thermal ionization process. At his stage discharge is characterized by intensive luminescence of channel, diminution of channel
resistance. This form of streamer discharge is called “sliding discharge”.
3. Length of sliding discharges grows fast enough. This process is finished by anode-cathode gap crossing.
Intensity of sliding discharges is determined by surface capacity. Surface capacity depends on dielectric width.
Humidity presence plays important role at discharge process. Dry surface
has much higher electrical strength. Humidity diminishes a electrical
strength of any surface. Dry surface is one of main way to increase of electrical strength and compatibility.
Contamination of surface diminishes the electrical strength also.
Electrical strength of anode-cathode gap (atmosphere air only) in uniform
field (“ball-ball” system) is about 30 kV per cm. With dielectric material
surface between balls, discharge will spread on surface always. In this case
electrical strength of the same gap is about 5-10 kV per cm. If pollutions
are on dielectric surface – electrical strength can drop up to
1 kV
per cm. In case moisture presence with contaminations on the dielectric
surface – electrical strength can come down to several hundred volts per
cm. These factors are very important and have to take into account in design and engineering.
Dielectric surfaces are everywhere in energetic systems and power facilities. So, probability of surface discharges is high enough. Pollutions and
moisture are presence constantly in power systems, especially which are in
atmosphere air. Dielectric surfaces and corona (as first stage) cause all
electromagnetic interferences and have to be taking into account from EMI
point of view.
Typical measures are:
- to remove all dielectric surface contaminations;
- to avoid of moisture presence on the dielectric surface;
- to use special ribbed form of insulating constructions (this increases a
way of sliding discharge and diminishes a probability of discharge ignition).
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Experimental set-up
Electrical scheme of experimental set-up is shown on the Fig.3.
Fig.3. Electrical scheme of experimental set-up
S1, S2 – switches,
AT – adjusting transformer,
V – voltage measuring device,
T – high-voltage transformer,
RЗ – protective resistor,
SDS – surface discharge system.
Basic principles of scheme is the same like in lab #1.
Order of work
1. Be familiar with main features of experimental set-up, surface discharge system and safety rules.
2. Install on dielectric surface (glass) grounded and potential electrodes. It should be possibility of length reading between electrodes.
3. Provide reliable supply of potential electrode by bushing of high
voltage transformer.
20
4. Measure the corona discharge and discharge voltages of surface
discharge with prevailing normal part of electric field for 6-8 points of anodecathode gap. Put the results in the Table #1. Corona ignition is fixed by specific “corona-hissing” sound.
5. Change the surface capacity by means of changing of dielectric
width. Repeat paragraph 4 for other width of dielectric.
6. Measure the corona discharge and discharge voltages of surface
discharge with prevailing tangential part of electric field for 6-8 points of anode-cathode gap. Put the results in the Table #1.
7. Change the surface capacity by means of changing of dielectric
width. Repeat paragraph 6 for other width of dielectric.
8. Plot the dependences of discharge voltage versus anode-cathode
gap length for both types of surface discharges.
Тable 4.1 Sample of table
l,
d,
UC,
Usd.exp,
cm
cm
kV
kV
Here are:
Notes
l – anode-cathode length (cm),
d - thickness of dielectric layer (mm),
UC– voltage of corona ignition (kV),
Usd. exp. – experimental slide discharge voltage (kV)
9. Make a report about experimental work - an analysis the obtained
results and formulate the conclusions about specific features of discharge
processes.
21
Тable4.2. Experimental results
l,
d,
UC,
Usd.exp,
cm
mm
kV
kV
22
Notes
5. RESEARCH OF VOLTAGE DISTRIBUTION
OF SUSPENDED INSULATOR STRING
Main goal of this work: experimental research of voltage distribution of
typical suspended insulator string.
General information
One of the main insulation system in modern energy industry is suspended insulation string (string). Many thousands miles of energy transmission lines use insulation strings. Typical insulation string consists of six dielectric plates and connected armature. Most spreading dielectric material for
string’s plates is electrical glass and electrical porcelain. Suspended insulation string could serve as long as necessary, because their electric strength is
high enough to provide many years of stable work. But main reason of string
damage is non-homogeneity of voltage distribution along string. One of the
measure to make electric field more homogeneous along string is special leveling (smoothing) armature using. Typical smoothing armature is metal ring,
which is suspended under string. It is very effective way to make electric
field more homogeneous and prong life time of insulation string.
In this laboratory workshop two insulation strings are researched. One
of them is produced from electric glass. Other one is from porcelain.
Three experimental runs should be fulfilled: a) voltage distribution research along “normal” string; b) the same measurements at the damaged plate
of string; c) voltage distribution research along string with leveling armature.
Device which is used to measure of voltage distribution alon string is special
“ball discharger”.
Table 5.1 Experimental results
Glass string
“Normal”
Number
string
of plate Voltage, kV
String with
damaged
plate
Voltage, kV
1
2
3
4
5
23
String with
leveling armature
Voltage, kV
Comments and notes
6
Porcelain string
“Normal”
Number
string
of plate Voltage, kV
String with
damaged
plate
Voltage, kV
String with
leveling armature
Voltage, kV
Comments and notes
1
2
3
4
5
6
Report about this laboratory workshop should contain graphs of voltage versus plate number for all measured cases.
Be ready explain experimental results.
6. MARX GENERATOR
FOR PULSED VOLTAGE GENERATION
Main goal for this work: be familiar with engineering, electrical scheme and
working regimes of Arkad’ev-Marx pulsed voltage generator (PVG).
Short theoretical description
High voltage pulses have wide using range for atmosphere electricity
imitation and electrical insulation testing. Scientific applications like high
voltage source for both fundamental and technological research are significant enough. Main principle of high voltage formation is Arkad’ev-Marx one.
Main features of Arkad’ev-Marx scheme
Working regime of PVG has two stages. First stage is charge of pulsed
capacitors in parallel circuit; second one is their discharge in consequence
circuit. So, voltage on exit of PVG is charging voltage of every capacitor
multiplied to number of capacitors - nU0, where n – number of capacitors, U0
– charging voltage. Scheme of PVG is shown on the Fig.1. Capacitors C is
charged by means of resistors R1 (protective resistor) and R0 (charge resistors). Necessary condition is R1>>R0. This condition provides charge of ca-
24
pacitors at the same time. At the finish of charge capacitors, potentials of
points 0, 2, 4, 6 equal 0; potentials of points 1, 3, 5, 7 equal maximum voltage +U0 of transformer T at the same time. Length of spark gaps F1…. Fn
are established so that voltage U0 is enough for breakdown of first gap F1
only.
Figure.1. Electrical scheme of Pulsed Voltage Generator
All capacitors are in consequence circuit as result of gaps F1…Fn
breakdown. Their voltages are summarized. So, potential of point 21 will be
equal nU0, where n – number of PVG stages.
3. Form of testing waves
Value of discharge voltage of PVG depends on voltage wave form.
Wave form should have special configuration in order to it could be possible to compare waves of atmosphere over voltages. Applying wave form
for pulsed tests is characterized by fast grow of voltage up to maximum
value (wave front) and slow decreasing of one (wave tail). As sample wave
is admitted the one with following parameters:
ф = 1,2  0,4 microseconds
and
в = 50  10 microsec-
onds
Serious danger at the energetic power systems have so called cut
waves. This kind of waves could take place after lightning discharge process. So, it is necessary to imitate this process to predict insulation work
after lightning strikes around electric power stations.
25
а
b
Figure.2. Forms of full and cut waves of pulsed voltages
4. Adjusting of wave form
Necessary wave form could be reached by changing of forming elements which are included in charge circuit of PVG. Different variants of
wave forms and corresponding working regime of PVG are shown on the
Fig.3.
Fig.3,a corresponds to regime of “empty work”. Here Rк – active resistance of charge circuit; Сп – “harmful” capacity of discharge circuit; Rд
– resistance of voltage divider. Period of hesitation is determined as:
Тхх  2 L п  С п ,
26
Т кз  2 L п  С г .
Figure.3. Substitution schemes and corresponding forms of PVG
27
To avoid hesitations on the wave front at the discharge circuit “quiet” resistance - rУ - is placed (Fig.1.), which is chosen from the expression:
rу  2
L/п
С п/
To obtain non-periodic wave only “front” resistance is placed at the discharge circuit of PVG:
Rф  2
Lп
Сп
Discharge resistance Rр is placed in discharge scheme to adjust wave duration.
Order of work
1. Be familiar with scheme and basic idea of PVG.
2. Research “empty work” regime of PVG.
3. Placed front resistance and than front capacity in the discharge circuit of
PVG. Research working regime in this case.
4. Switch on ball gap and obtain cut wave.
5. To make “short circuit” regime. Research work of PVG in this case.
Report content
1. Illustrate electric scheme of PVG.
2. Explain basic idea of PVG work.
3. Plot forms of sample wave forms.
Control questions
1. Explain, please, main idea of pulsed voltage generator work?
2. Where PVG could be applied?
3. Which is role of protective resistor?
28
7. MEASUREMENT OF ELECTRIC FIELD STRENGTH OF MAIN
FREQUENCY, GENERATED BY HIGH VOLTAGE FACILITIES
Goal of this laboratory workshop: to study requirements and standards of electrical field strength of mains frequency; to be familiar with special
device to measure electric field intensity (P3-50); to measure an electric field
intensity in high voltage laboratory and make an analysis of experimental results.
INTRODUCTION
High voltage engineering sources can cause adverse effects to technoand bio-sphere. Voltages and currents in transmission facilities generate an
electromagnetic field in space. Electric and magnetic fields of mains frequency (50 or 60 Hz) can produce dangerous influence to service staff and population. Prediction and control of electric field influence important task of
safety measure protection.
Electric field of mains frequency. General information.
Sources of electric field of mains frequency (EFMF) are charges on
conductor lines of different circuit elements such as generators, transformers
and so on. EFMF can cause adverse influence to human organism. There are
following kinds of adverse influence:
Direct action of EFMF, which person is in electric field area.
Adverse effect is increased with electric field intensity growth
and time of presence in field area. Long EFMF action can
cause nervous and heart diseases.
Action of electrical discharges to man due to touch to circuit
elements or to surrounding equipment.
EFMF can cause fire or explosion due to heat action of discharges, which, in turn, are generated by electric field action.
(for example flammable liquids are present in many power facilities).
Limits for acceptable value of strength of EFMF (E, kV/m) are following:
E > 25 kV/m - presence in EFMF area is forbidden without special defensive measures.
20 < E < 25 kV/m – acceptable time of presence in EFMF no more
than 10 minutes.
5 < E < 20 kV/m - acceptable time of presence in EFMF no more than
1 hour.
29
E < 5 kV/m – no limits for presence in EFMF area.
EFMF of conductor lines influence to stable work of different devices.
First sign of the electromagnetic interference (EMI) availability is – often
faults in work of TV, monitors, computers, consumer electronics and so on.
Protection measures from EFMF exposure.
One of the effective and simple protection measure form EFMF is
screen using. Metal shield is good enough to protect from negative EFMF influence. Solid plate of metal, set of metal rods, metal net are high usage protective equipment in power station and many other power energetic objects.
General requirement to shields is – they should have anticorrosive coating
and must be grounded.
Important task is EFMF intensity restriction under conductor lines.
Most simple of this way is to use a grounded wire rope under conductor line.
Physical mechanism of grounded wire rope is based on opposite charge compensation. Charges of opposite signs (compare with conductor line signs) are
generated on the grounded wire rope. Common field of these signs diminishes EFMF intensity. But cost of grounded wire rope is high enough. So, they
are used in populated area and facilities and plants where EFMF has dangerous level.
Good method of EFMF intensity restriction is using of natural green
and vegetation. Trunks, steams of trees and bushes have high surface conductivity during whole year and could be effective way of EFMF protection.
30
Device for Electric and Magnetic field control
Figure.1.1. Scheme for EFMF generation
Table 7.1 Results of measurement of electric field strength of mains frequency, generated by high voltage facilities
Primary Secondary
EX,
EY,
EZ,
ETOTAL
winding
winding
Comments
kV/m
kV/m
kV/m
kV/m
voltage,
voltage,
Volts
Volts
0
0
Facility
is
switch off
0
0
Facility
is
turn on
10
3500
Distance
from facility
31
to measurement point =
0
20
7000
30
10500
Voltage converter factor - USECONDARY = UPRIMARY * 350
Table 7.2 Results of measurement of electric field strength of mains frequency, generated by high voltage facilities
Primary Secondary
EX,
EY,
EZ,
ETOTAL
winding
winding
Comments
kV/m
kV/m
kV/m
kV/m
voltage,
voltage,
Volts
Volts
0
0
Facility
is
switch off
0
0
Facility
is
turn on
Distance
from facility
to measurement point =
0
32
Voltage converter factor - USECONDARY = UPRIMARY * 350
Table 7.3 Results of measurement of magnetic field strength of mains frequency, generated by high voltage facilities
Current
in circuit,
A
0
BX,
A/m
BY,
A /m
BZ,
A /m
BTOTAL
A /m
Comments
Facility
is
switch off
Facility
is
turn on
Distance
from facility
to measurement point =
0
0
33
Table 7.4 Results of measurement of magnetic field strength of mains frequency, generated by high voltage facilities
Current
in circuit,
A
0
BX,
A/m
BY,
A /m
BZ,
A /m
BTOTAL
A /m
Comments
Facility
is
switch off
Facility
is
turn on
Distance
from facility
to measurement point =
0
0
34
Contents
Preface………………………………………………………………………..3
Introduction ………………………………………………………………… 4
1. Discharges in atmosphere air at alternating voltage ……………………... 5
2. Characteristics of corona discharge at the AC voltage ……………………8
3. Polarity and barrier effect research ………………………………………13
4. Discharge on the surface of solid dielectric ………………………...……17
5. Research of voltage distribution of suspended insulator string ……….... 23
6. Marx generator for pulsed voltage generation ………………………….. 24
7. Measurement of electric field strength of mains frequency, generated by
high voltage facilities ……………………………………………………... 29
Contents …………………………………………………………………… 35
35
Alexey Mytnikov Author was born 2 June 1966. He received Ph. D. degree in 2001. He is Associate Professor of Electrical Power System Department, Power Engineering Institute of Tomsk Polytechnical University. Area of scientific interests: power transformer diagnostics, high voltage technologies.
Textbook
MYTNIKOV Alexey Vladimirovich
HIGH VOLTAGE ENGINEERING
Textbook
Computer design A.V. Mytnikov
Cover design A.V. Mytnikov
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