over voltage
DESCRIPTION
gffTRANSCRIPT
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-0
Part II: Over-voltages and Insulation Coordination in Power Systems
Chapter 4: Traveling waves Chapter 5: Lightning over-voltage protection Chapter 6: Switching over-voltage protection Chapter 7: Insulation coordination in power systems
Instructor: Dr. Jian Li
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-1
Chapter 5: Lightning over-voltage protection
z Main contents Electrical parameters of lightning Lightning protection devices Lightning protection of power transmission lines Lightning protection of power plants and substations Lightning protection of transformers Lightning protection of generators
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-2
5.1 Electrical parameters of lightning
z Lightning phenomenon
Inter-cloud lightning
Cloud-to-ground stroke
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-3
5.1 Electrical parameters of lightning1. Basic characteristics of lightning
Lightning discharges are sparks in extra long air gaps. An average bolt of negative lightning carries an electric current
of 30 kA, and transfers a charge of five coulombs and 500 MJ. Large bolts of lightning can carry up to 120 kA and 350 coulombs.
The voltage is proportional to the length of the bolt. Lightning rapidly heats the air in its immediate vicinity to
around 20,000 C.
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-4
5.1 Electrical parameters of lightning2. Formation of lightning discharges
Because of the temperature difference, humid air is lifted to higher altitudes and raindrops are formed.
Raindrops with size of a few millimeters are polarized by the electrical field between the lower part of ionosphere and the earth surface.
The majority of thunderclouds is negatively charged.
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-5
5.1 Electrical parameters of lightning2. Formation of lightning discharges
Negative ions are accumulated in thunder clouds. The strong field initiates discharges inside clouds. Negative stream of electrons emerges as a dim spark called a
stepped leader that jumps in steps of approximately 30 meters and reaches the earth in about 10 millimeters.
The stepped leader reaches close to the earths surface, reaching an upward positive leader, and forms the main channel.
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-6
5.1 Electrical parameters of lightning3. Terms and definitionsz Annual number of thunderstorm days Td
The annual number of days where thunder can be heard.z Annual number of thunderstorm hours Thz Keraunic level
To describe the lightning and thunder activity in a given area, defined as the annual number of thunderstorm days.
z Ground flash density The average number of lightning strokes to ground per unit area (1 km2)
per unit time (1 thunderstorm day) at a particular location.z Flash collection rate of transmission lines N
Annual number of lightning strokes to a transmission line per 100 km.
km/yr)100/(flashes100100010
dThN =
Keraunic level Low Medium High Very highTd 15 >15 and
40 >40 and
90 >90
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-7
5.1 Electrical parameters of lightningz Wave impedance Z0 of lightning channels
Recommended by national standards: Z0= 300 ohmsz Polarity of lightning flashes
75 - 90% of lightning flashes are negative.z Magnitude of stroke current I
Stroke current is defined as the current flowing through an object with grounding resistance smaller than 30 ohms, when lighting strikes the object.
where I0 is the magnitude of incident stroke current. The probability of stroke current, p, is calculated as:
02II
88lg Ip =
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-8
5.1 Electrical parameters of lightningz Front duration, front increasing rate, and width of stroke current
Front duration: 1-4 s; average value 2.6 s. Width: 20-100 s; most values 40 s. Front increasing rate: I/2.6 kA/s
z Wave shapes of stroke currents Double exponential wave:
Oblique Wave :
Oblique-front and flat-top Wave :
)(0 tt eeIi ati
>
11
1
TtaTTtat
i
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-9
jLZ ZZ
Zvi += 00
a) Leader b) main discharge c) calculation model d) equivalent circuit
5.1 Electrical parameters of lightningz Equivalent circuit for calculating stroke current
ZZZ
iiZiu z +== 00
0000 2
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-10
5.2 Lightning protection devicesz Lightning protection
approaches Lightning rods Ground wires Protection gaps Arresters Reactors Capacitor banks Arc suppression coils Auto reclose
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-11
5.2.1 Lightning rods and earth wiresz Single rod
=
otherwise)25.1(5.0)(
phhhhphh
rx
xx
h30 m: p1; 30 m
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-12
5.2.1 Lightning rods and ground wiresz Single ground wire
=
otherwise)53.1(5.0)(47.0
phhhhphh
rx
xx
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-13
5.2.1 Lightning rods and ground wiresz ground wires
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-14
5.2.2 Arrestors1. Main parameters of arresters
Residual voltage Ur The voltage that appears between the terminals of an arrester
during the passage of the discharge current.
Power frequency follow current The current from the connected power source that flows
through an arrester during and following the passage of discharge current.
Current capability The ability of an arrester to withstand current flowing through
itself. Rated voltage
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-15
5.2.2 Arrestors2. Basic requirements for arresters
z Proper voltage-time characteristics of discharge voltages and protection margins.
z Well reinsulated property to shut off power frequency follow current.
SCI protective level (PL): the rise time of discharge voltage equal to 1 s.
LIPL: the rise time of discharge voltage is 8-10 s.
SIPL: the rise time of discharge voltage is 45-60 s.
The insulation curve should be more than 115% of the arrester discharge voltagecurve at a minimum.
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-16
5.2.2 Arrestors2. Basic requirements for arresters
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-17
5.2.2 Arrestors3. Spark gaps and tube type arresters
Demerits:z Voltage-time characteristics are
steep and scattered.z Chopped voltages are generated
after discharges, which is harmful to power equipment with windings.
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-18
5.2.2 Arrestors4. Valve type arresters
1- series-connected spark gaps;2- valve plates; 3- copper electrode;4- mica gasket; 5 single spark gap;6- copper cover; 7- by-pass resistors8- porcelain sleeve
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-19
5.2.2 Arrestorsz Merits of multiple spark gaps
Small arcs in multiple gaps are more easily to be extinct than a arc in one gap with equal length as the total length of multi-gaps, because the cooling effect in multi-gaps can generate stronger deionization by recombination than in one gap.
Reignition of small arcs are more difficult than that of long arcs. z Non-linear resistance of valve plates
Low resistance at the discharge period. High resistance after arcs in spark gaps are extinct. Power frequency follow current, chopped voltage, and oscillation of
impulse over-voltages, are limited by the non-linear resistor.
ciu = - non-linearity coefficient
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-20
5.2.2 Arrestorsz Parameters of valve type arrestors
Impulse wave discharge voltage Extinction voltage Protection ratio: the ratio of residual voltage to extinction voltage. DC conductive current
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-21
5.2.2 Arrestors5. Metal oxide surge arrestors (MOA)
V-I characteristic of MOA
Comparison of V-I characteristics of MOA, SiC arrestor, et al.
=0.02-0.05
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-22
5.2.2 Arrestorsz Merits of MOA in comparison to SiC arresters
Simple structure without multiple sparking gaps. Flat V-t characteristics for protection. Low leakage current under operation voltage. No power frequency follow current. Low impulse overvoltages. Large current capability. Suitable for overvoltage protection in DC systems in particular.
z Parameters of MOA Initial discharge voltage (U1mA) Residual voltage ratio: the ratio of the residual voltage to U1mA. Applied voltage ratio: the ratio of peak value of the maximum
operational voltage (Um) to U1mA. Protection ratio k:
ratio voltage Appliedratio voltage Residual
2==
m
r
UUk
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-23
5.2.3 Ground devices for lightning protectionz In electricity supply systems, ground or earth is a direct physical
connection to the Earth.z Classification of earth in power systems
Operational earthing: earthing resistance is 0.5-10 ohms. Protective earthing: earthing resistance is 1-10 ohms. Lightning protection earthing: earthing resistance is 1-30 ohms.
z Earthing resistance Re Equal to the ratio of the voltage Ue between a grounded conductor
and a neutral surface underground to the current Ie flowing through the grounded conductor.
Impulse coefficient The ratio of earthing resistance Re under power frequency
current to the earthing resistance Ri under impulse current.
eee IUR =
ei RR=
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-24
5.2.3 Ground devices for lightning protectionz In the majority, impulse coefficient < 1.z The spark effect and the inductance effectz Contact voltage and pace voltage
Contact voltage is the voltage between the grounded equipment and the standing point. (height 1.8 m; horizontal distance 0.8 m)
Pace voltage is the voltage between the two standing points.
Dangerous threshold of current flowing through human being is 10 mA.
-
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
E
P
T
O
F
H
I
G
H
V
O
L
T
A
G
E
A
N
D
I
N
S
U
L
A
T
I
O
N
E
N
G
C
H
O
N
G
Q
I
N
G
U
N
I
V
E
R
S
I
T
Y
Lecture 10-25
5.2.3 Ground devices for lightning protection1. Ground conductorsz Vertical ground conductors
z Horizontal ground conductors
z Ground grids
)18(ln2
=dl
lRe
nR
R e=8.065.0 =
)(ln2
2A
dhl
lRe +=
)1(nlLS
BRe ++=
Part II: Over-voltages and Insulation Coordination in Power SystemsChapter 4: Traveling wavesChapter 5: Lightning over-voltage protection Chapter 6: Switching over-voltage protectionChapter 7: Insulation coordination in power systemsChapter 5: Lightning over-voltage protection5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.1 Electrical parameters of lightning5.2 Lightning protection devices5.2.1 Lightning rods and earth wires5.2.1 Lightning rods and ground wires5.2.1 Lightning rods and ground wires5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.2 Arrestors5.2.3 Ground devices for lightning protection5.2.3 Ground devices for lightning protection5.2.3 Ground devices for lightning protection