2-ci sual

Effect of exposure time and electrode geometry on breakdown

voltage in liquid dielectrics.

When exposed to voltage pulses with a duration of τ < 10–4 s (region I), the influence of impurities is
significantly weakened, i.e., they do not have time to move over noticeable distances. An increase in the
time of exposure to voltage τ > 10–3 s leads to a rapid decrease in Ubr due to the influence of moisture
and fibers, as well as the formation of gas bubbles. With a further increase in the time of exposure to
stress, thermal processes begin to have a decisive influence on the reduction of Ubr.

Insulation of high voltage capacitors and transformers.

High Voltage Capacitor Insulation
Purpose of capacitors:
• 1) improvement of cos ;
• 2) HF communication;
• 3) compensation of the phase shift between current and voltage;
• 4) rectifier units - filters, etc.;
• 5) high-voltage pulse installations.

Insulation: gas, liquids, solid inorganic and organic

materials.
Solid insulation in high-voltage capacitors is often
organic - paper, films impregnated with oil. The
capacitor is characterized by the specific stored
energy, J / dm^3:
2
  0  E
p
Wуд  .
2

• High-voltage capacitors are designed to compensate reactive power and

filter harmonics in high-voltage networks.
High voltage transformer insulation
 • The main insulation in transformers includes transformer oil, electrical
cardboard, laminated pressboard (textolite), and delta wood.
In gas-filled transformers, high-strength insulating gases are used.
• For interlayer and turn-to-turn insulation, materials such as oil-impregnated
cable paper, various2types of varnished fabric, 1enamel insulation, as well as
glass fiber impregnated with varnishes or epoxy resin are used.

Wave processes in lines.

When a direct lightning strike (DLM) occurs on or near a line (into the
ground), electromagnetic waves are generated that propagate along the power line
wire. Atmospheric overvoltages on lines and substations are determined by the
movement and refraction of these waves. Therefore, the analysis of wave processes
when calculating lightning protection devices is of fundamental importance.
 The wave propagates along a line in air at a speed of m/μs – the speed of light
(μ is the relative magnetic permeability of the medium; ε is the dielectric constant
of the medium).

For air μ0 = 1; ε = 1.

For cable lines μ = 1; ε ≈ 4.

Therefore, in cables υ ≈ 0.5 C.

Voltage and current waves are related:

ZCLIU==00 – characteristic impedance.

Characteristic impedance of a single overhead line wire Z = 400…450 Ohm.

Cable lines have Z = 50…100 Ohm.

In the general case, the wave process in lines is determined by four main
parameters: capacitance C, inductance L, active resistance of the wire r and active
conductivity of the dielectric g.

Insulation moisture control and high voltage testing.

The insulation capacitance at a constant temperature and frequency of
applied voltage is a constant value. Therefore, an abrupt change in the
capacitance value indicates the presence of defects in the insulation.
Moistening of the insulation has a particularly strong effect on the change in
capacitance, therefore, to control the moistening of the insulation, a method
was found for measuring the capacitance of the insulation at different
frequencies, which was called the “capacitance-frequency” method.
As the frequency increases, the insulation capacity of any device
(transformer, cable, insulator, etc.) decreases. This phenomenon forms the
basis of the capacitance-frequency method. The capacitance-frequency
method consists of comparing capacitance values measured at two different
frequencies f = 2 Hz and f = 50 Hz (C2 and C50) at t = 10–20 °C. The quality
of insulation is judged by the ratio C2/C50. The smaller this ratio, the better
(drier) the insulation.

High voltage test equipment and measurements.

1. Generating High Voltage
  AC Voltage: Test transformers up to 1200 kV; for higher voltages, cascade connection is
used (up to 2200 kV+).
 DC Voltage: Obtained using rectifier circuits—half-wave, full-wave, bridge, and voltage
multipliers (doublers, triplers, cascades).
 Cascade DC Generator: Uses multiple voltage doubling stages; output voltage is
2⋅n⋅Um2 \cdot n \cdot U_m2⋅n⋅Um.

2. Impulse Testing

 Used for: Testing insulation against lightning and switching surges.

 Impulse generator (GIN): Charges capacitors in parallel and discharges them in series
through spark gaps.
 Standard pulses: Full and chopped lightning waveforms (e.g., 1.2/50 µs).

3. Pulse Current Generator

 Simulates high current pulses during breakdowns.

 Uses capacitors, spark gaps, and inductors.
 Current depends on charging voltage, capacitance, and inductance.

4. High Voltage Measurement

 Ball Gaps: Simple and universal for DC, AC, and impulse voltages. Voltage found from
tables based on breakdown distance.
 Electrostatic Voltmeters: Measure effective voltage using electrostatic force.
 Voltage Dividers:
o Ohmic: Uses resistors; good for long pulses.
o Capacitive: Uses capacitors; better for short pulses.
o Mixed: Combines both; accurate but complex.

Pulse testing facilities.

To test the insulation of high-voltage electrical equipment with lightning and
switching pulses, pulse voltage generators (GVG) are used.

Lightning effects are reproduced by standard voltage pulses: full and cut
waves. Standard pulses (1.2/50 or 2.0) can be obtained using the installation, the
diagram of which is shown in Fig. 3.7.

V Т RЗАЩ R1 R3 R19 RФ
+U0 +U0

F11
C C C C СФ
RН
F1 F2 F3 F10 RР

R2 R4 R6 R20
–
 Fig. 3.7. Schematic diagram of a GIN with one-way charging: T – high-voltage transformer; V –
rectifier; RZASCH – resistance to limit the charging current; R1–R20 – charging resistances; F1–F11 –
spark gaps; C – capacity of the GIN stage; C ' – “parasitic” containers;
RF, SF – front resistance and capacitance; RP – discharge resistance; RN – load resistance

Capacitors C are charged in parallel, and they are discharged sequentially,

which leads to the addition of the charging voltages of the stages.
To ensure almost identical charging of all capacitors up to U0, it is necessary
to comply with the following condition: R1…R20 << Rprotect. At voltage U0,
only F1 breaks through. The capacity is discharged in circuit C – R2 – F1, but R2
is large (tens of kilo-ohms). At the first moment, the discharge occurs along C – C '
– F (Х = 1/С,  is the circular frequency of the order of megahertz, and
consequently, XC is small). C' quickly charges to U. Then twice the charging
voltage U0 is applied to F, so F2 can have a distance 2 times greater than F1, etc.
To regulate the parameters of the voltage pulse and obtain a standard wave,
the following elements are used: RF - front resistance, SF - front capacitance, RP -
discharge resistance.
The front length is formed by SF and RF, the pulse length is RФ, i.e. RP
together with Rн:

tф=3,24RФ СФ;
tв  0,7  Сгин  R .
The change in pulse amplitude is regulated by changing the distance between
the ball electrodes F1, F2, ..., F10. The gap F11 serves to separate the charging
capacity of the GIN from the load when charging capacitors with constant voltage,
to eliminate the effect of constant charging voltage on the load.
GIN is used to test the insulation of high-voltage equipment. Internal
insulation is tested by applying three full pulses and three chopped pulses of
positive and negative polarity.

Pulse current generator.

Current pulse generators (CPGs) are used to simulate the effects of large
amplitude current pulses. The electrical circuit of the GIT is shown in Fig. 3.8.

Т V RЗАЩ L Р

управление

C1 C2 RН
 Cn
~U

Fig. 3.8. Electrical circuit of the GIT: V – high-voltage rectifier;

RZASCH – resistance to limit the charging current;
C1–Cn – capacitor bank; R – controlled arrester;
RН – load; L – inductance of the discharge circuit

After the spark gap P is triggered, the capacitor bank is discharged into the load
resistance, for example, into the discharge channel after a breakdown. The magnitude of
the current is determined primarily by the inductance and capacitance of the discharge
circuit:
U0
Im  ,
L
C
here U0 is the charging voltage; L – circuit inductance; С = n·С1
(if C1 = C2 = ... = Cn) – capacity of the discharge circuit.

Ball arresters and electrostatic voltmeters.

Ball arresters are commonly used to measure high voltages. They are universal devices that can
measure DC, AC, high-frequency, and pulse voltages. The breakdown voltage (the voltage at
which a spark jumps between the balls) depends on:

 The distance between the balls

 The size of the balls
 The connection type (both balls live, or one grounded)
 The air density (affected by pressure and temperature)

To ensure accurate measurements, these rules should be followed:

1. Distance between the balls should be less than or equal to half the ball's diameter (S ≤
0.5D). Different ball sizes are used for different voltages.
2. The balls must be clean and smooth. Dust can lower the breakdown voltage.
3. Keep other live or grounded objects far away—at least 5 times the ball diameter.
4. Use UV light or radiation to help create consistent sparks, especially at small distances.
5. Do the test 4–5 times and use the average value, since results can vary.

How measurements are done:

 For DC and AC: Start with a large distance to prevent breakdown. Then slowly reduce
the distance until a spark occurs. Repeat and average the distance. Use a table to find the
corresponding voltage.
  For pulse voltage: Adjust the gap so sparks occur about half the time. This is called the
50% breakdown voltage. Use the average distance to find the voltage from a table.

Calibration tables from the International Electrotechnical Commission (IEC) are used. They
give voltage values based on ball size and distance under standard conditions (760 mm Hg,
20 °C). If conditions differ, a correction is applied using the formula:

Uи = Uт·,
here Ut is the table value of breakdown voltage; δ = 0.386Р/(273 + T),
here P and T are pressure in mmHg, respectively. Art. and temperature in
degrees Celsius of the environment during measurements.

Electrostatic voltmeters
Electrostatic voltmeters measure the effective value of voltage. The operating
principle is based on the mechanical movement of one of the voltmeter electrodes
under the influence of electrostatic forces. The measurement is made by balancing
this mechanical force with a weight or spring:

0  S
F U 2 KU 2 ,
l 2 2
here S is the area of the movable electrode; l – distance between electrodes;

The design of the electrostatic voltmeter by A. A. Chernyshev is shown in

Fig. 3.9.

N
k1

С А С k2

l
В
ВН
F
Г

Fig. 3.9. Design of an electrostatic voltmeter by A. A. Chernyshev:

A – movable grounded disk, B – fixed high-voltage disk,
C – protective grounded ring, N – metal grounded rocker arm,
k1, k2 – contacts of the galvanometer circuit, G – galvanometer

Ball voltmeters are available, such as the Sorensen, Hobson and Rameau
voltmeter.
In technical electrostatic kilovoltmeters, for example S100 for voltages up to
75 kV, the movable electrode is balanced by an elastic stretcher on which a mirror
 is mounted. The readings are taken using a light beam.

Lightning rod protection zone.

Lightning rods protect buildings and equipment from lightning damage. Depending on what is
being protected, either rod-type (used at substations) or cable-type (used for overhead lines)
lightning rods are used. For lightning protection to work well, the lightning rod must have good
grounding.

The highest lightning voltages happen when lightning directly hits a power line or substation.
This creates a very high voltage (millions of volts) at the strike point, which is higher than the
insulation strength of power lines and electrical devices. To keep the electrical system working
safely and reliably, strong and cost-effective lightning protection is needed.

For full protection, the object must be completely inside the lightning rod's protection zone.
This zone is the area around the lightning rod where lightning is unlikely to strike.

Protection zone of a lightning rod

The surface delimiting the protection zone of a lightning rod can be
represented by a broken line (Fig. 4.3).

a
0,2h
в
h

hx
с

0,75h rх

1,5h
сечение зоны защиты
на высоте hx

rх

Protection zone of two lightning rods

Open distribution substations are located over a large area. They have to be
protected with several lightning rods. The protection zone is determined in the
same way as the protection zone of two lightning rods.
Protection zone of cable lightning rod

Rope lightning rods (or cable-type rods) are mainly used to protect overhead power lines.
Instead of using a protection zone like rod-type systems, they use protection angles — the angle
between a vertical line under the cable and the line connecting the cable to the wire it protects.

For long power lines (up to 1000 km), like 500 kV lines, lightning can strike over 200 times in
one storm season. That’s why cable-type lightning protection is very important. Experience
shows that the best protection angle is 20–25°.

The grounding resistance must be:

 Less than 5 Ohms for systems with a grounded neutral,

 Less than 10 Ohms for systems with an isolated neutral.

For substations, lightning protection should include not only protection from direct strikes but
also:

1. Protection from flashovers (when lightning hits grounded parts and causes sparks to
jump to live equipment).
2. Protection from voltage waves that come from the power line.

Fig. 4.5. Protection zone of cable lightning rod

To fulfill the first requirement, it is necessary to make the grounding

resistance of the substation small. For voltages above 1000 V, the substation
grounding resistance is Rз ≈ 0.5 Ohm. Reducing Rз is the most effective way to
protect against backflow.
To fulfill the second requirement, valve-type arresters (VR) and surge
suppressors (OSL) are used. The valve-type arrester has a flat volt-second
characteristic (VSC). This allows it to protect equipment over a wide range of
changes in wavelengths coming from the line (Fig. 4.6).
For effective protection it is necessary that:
1) the remaining voltage at the operating resistance of the radio did not
exceed the permissible limit;
2) the steepness of the wave approaching the substation was limited.
To fulfill these conditions, all lines approaching and departing from the
substation are equipped with cable protection 2–3 km long - protective approaches.

Lightning resistance of objects.

Due to their large length, overhead power lines (OHLs) are most often
affected. Therefore, disruption of the operation of power systems is mainly caused
by a violation of the insulation of overhead lines.
 When calculating the lightning resistance of overhead lines, the concept of
lightning resistance level is introduced. The level of lightning resistance is assessed
by the maximum amplitude of the lightning current I0 and its slope a, at which the

line insulation does not break (slope is the duration of the current wave
front).
The indicator of lightning resistance is the probable number of years of
operation of the installation without lightning outages:

where M is the number of years of operation without lightning

outages; Noff – the expected number of cases of dangerous lightning overvoltages
per year.
For example, for VL

where h is the average height of the

cable or wire suspension;

L – line length;
nd – number of thunderstorm days per year;
υper – probability of overlapping of overhead line insulation during a
lightning strike;
η – probability of transition of a pulse overlap into a power arc.
The lightning resistance of other objects (substations) is calculated similarly.