Download An EMTP – RV Based Analysis of the Line Surge Arrester

An EMTP – RV Based Analysis of the Line Surge
Arrester Energy Duty Due to Lightning Discharges
S. Grebovic, S. Pack, S. Sadovic
Abstract—This paper describes modelling procedure for
calculating energy stresses of transmission line surge arresters.
A 110 kV shielded transmission line is modelled using
ElectroMagnetic Transient Program – Restructured Version
(EMTP – RV). Some parts of transmission line such as
transmission line towers and stations on both ends of line are
modelled thanks to EMTP – RV possibility of grouping
electrical elements into subcircuits. Also it was possible to create
subcircuit consists of number of other subcircuit. This
possibility was very important for dividing transmission line
into number of sections that consists of number of towers.
Arrester current and voltage, energy duty, insulator voltage and
voltage on tower top due to bipolar lightning are calculated and
presented.
Keywords: Bipolar Lightning Stroke, Line Surge Arrester,
Energy duty, EMTP – RV, Transmission line, Modelling,
Simulation.
I. INTRODUCTION
T
HERE are several methods used for the improvement of
lightning performance of the existing transmission lines,
such as: tower footing resistance reduction, increase of line
insulation level, installation of additional ground wires, etc.
Line surge arresters (LSA) are mainly used for line lightning
performance improvement. LSA may fail in service if it is not
correctly selected. In the selection of the line surge arrester
for the line lightning performance improvement it is very
important to determine arrester energy duty [1]. Energy
stresses of transmission line surge arresters (LSA) are
commonly analyzed due to unipolar flashes that transport to
ground charge of one polarity. Also, it is very important to
analyze energy stresses on transmission line surge arresters
due to bipolar flashes that can cause serious problems in
electrical power systems. Modelling is of great importance in
power system transient analysis. Modelling can be used to
describe system, to analyze the behavior of a system under
conditions that can occur in nature. The main goal is to
achieve precise simulation with all required modelling data
and within minimal computer timings. So, this paper presents
S. Grebovic is PhD student at Graz University of Technology and with Faculty
of Electrical Engineering University of Sarajevo (e-mail: [email protected]).
S. Pack is with Institute of High Voltage Engineering and System Management,
Graz University of Technology (e-mail: [email protected]).
S. Sadovic is with the Department of Electrical Engineering, Faculty of
Electrical Engineering University of Sarajevo (e-mail: [email protected]).
Paper submitted to the International Conference on Power Systems
Transients (IPST2015) in Cavtat, Croatia June 15-18, 2015
modelling of complete transmission line with large number
input and output data.
II. MODELLING PROCEDURE
Transmission line is modelled into several parts: towers,
insulators, phase conductors, shield wire, line surge arresters
and tower footing resistance. Lightning stroke is modelled as
a current source (so called 'I – point by point source'). The
studied 110 kV transmission line is 42 km long, span or line
length between two towers is considered as a mean value of
300 meters. It is taken that line has a different footing
resistance along the line route, so tower footing resistance
value is varied. Soil resistivity is 1200 Ωm. In the case when
tower is equipped with one LSA it is installed on bottom
phase. For installation configuration with two LSA, LSA are
installed on bottom and middle phase [2].
A. Tower model
Transmission line tower model, used in simulation is
presented in Figure 1.
Fig. 1. Tower representation.
Each tower is divided in four parts. First part is section of the
tower from bottom crossarm to the ground and it is
represented as the propagation element, which is defined by
the surge impedance and the propagation length. In EMTP –
RV software this part is modelled as constant parameter (CP)
and line model (1 – phase version). Second part is sections
between tower top and top crossarm. Third and fourth part
T1
are sections between crossarms. Sections on the tower top
(between tower top and top crossarm and between crossarms)
are modelled as inductance branches. On this way it was
possible to calculate transient on tower top. Tower surge
impedance is calculated according tower shape theory.
Branch inductance is determined according to the section
length, tower surge impedance and propagation velocity.
Wave propagation speed on the tower was taken to be equal to
the velocity of light [1].
Figure 2 shows described tower model in EMTP – RV with
insulators and pins that enable electrical connection with
other elements. On this way it is possible to connect line
surge arresters in parallel to the phase conductor insulators
and model tower footing resistance.
c
C1
0
L1
2.8uH
+
Pin for connection
with tower top
504kV
A2
A1
?vi>S +
Air1
+
L2
1.37uH
Air2
B2
C3
c
0
L3
1.37uH
+
0
C2
c
504kV
504kV
C2
C1
CP
20
TLM1
+
?vi>S +
Air3
T2
Pin for connection
with tower footing
resistance
Fig. 2. Tower model in EMTP – RV.
Pin for connection
with tower top
DEV1
Tower top
Phase A
B2
Pins for
connection with
phase B
Phase B
B1
Pin for connection
with tower footing
resistance


where:
vgap – voltage across insulator string
V0 – reference voltage
K=1
D=0,2045d
d – length of gap (m)
t – time to flashover (µs).
CFO – critical flashover voltage can be represented by
equation (2) [4].
710 

V0  0, 9CFO  0, 9  400  0,75  d
t


(2)
C. Line surge arrester
+ ?vi>S
B1
B. Line insulation flashover model
Insulators are modeled as Air Gap elements. It operates
based on equal area flashover model. Flashover occurs when
the following integral becomes greater or equal to D [3,4]:
K
t
dt  D
(1)
 v gap  t   V0
t0
Phase C
A2
Pins for
connection with
phase A
A1
C2
Pins for
connection with
phase C
C1
Grounding
Fig. 3. EMTP – RV symbol for tower subcircuit.
All elements shown on Figure 2 are grouped in subcircuit.
This subcircuit is illustrated on Figure 3 and can be use many
times for modelling complete transmission line.
Used arrester has the following characteristics [2]:
Rated voltage:
108 (kVrms)
MCOV:
86 (kV)
IEC class:
II
Nominal discharge current
10 (kA)
The non – linear behavior of line surge arrester is
represented by the U – I characteristic [2,3].
D. Lightning current model
Bipolar lightning flashes transport to ground both negative
and positive charges [5, 6]. A possible explanation of
observed bipolar lightning currents is given by Narita et al.
They suggested that, in a bipolar discharge, currents of both
polarities follow the same channel to ground [7]. There are
basically three types of bipolar lightning discharges, although
some events may belong to more than one category [6, 8].
Lightning current (bipolar) shown on Figure 4 was
measure on Corsica. The negative charge transfer is
determined by integrating the absolute value of the current
over the negative current component duration. The positive
charge transfer is determined by integrating the current over
the positive current component duration. The trapezoidal rule
is used for integration. The total transfer is summary of the
negative and positive charge transfers. The negative part has
peak current of -52,86 kA and positive part has peak current
of +26,3 kA, with total charge transfers being -3,3025 As and
+1,2414 As, respectively.
Lightning stroke is modelled as a current source (so called
'I – point by point source') because it is possible to import
measured data point by point. The current shapes and values
can be changed in order to study different lightning stroke
currents.
Bipolar Lightning Current
40
t =55,2 (s)
1
30
t2 =78,5 (s)
SADOVIC measurements
t =81,2 (s)
3
20
Current (kA)
10
0
-10
-20
-30
-40
-50
-60
0
100
200
300
Time (s)
400
500
600
Fig. 4. Bipolar Lightning Current.
E. Transmission line sections
Shield wires and phase conductors are subdivided into
number of segments. Each segment is presented with constant
parameter (CP) line model (multiphase).
Middle of
the span for
phase A
Middle of
the span for
shield wire
Middle of
the span for
phase B
Connection
with tower
top
Middle of
the span for
phase C
T1
T_1_2_s
T_1_2_a
T_1_2_b
T_1_2_c
T2
T_2_3_s
T_2_3_a
T_2_3_b
T_2_3_c
T3
T_3_4_s
T_3_4_a
T_3_4_b
T_3_4_c
T4
T_4_5_s
T_4_5_a
T_4_5_b
T_4_5_c
T5
T_5_6_s
T_5_6_a
T_5_6_b
T_5_6_c
T6
T_6_7_s
T_6_7_a
T_6_7_b
T_6_7_c
T7
T_7_8_s
T_7_8_a
T_7_8_b
T_7_8_c
T8
T_8_9_s
T_8_9_a
T_8_9_b
T_8_9_c
T9
T_9_10_s
T_9_10_a
T_9_10_b
T_9_10_c
T10
DEV1
S_1
A_1
B_1
C_1
S_10
A_10
B_10
C_10
Section_from_1_to_10
Fig. 5. EMTP – RV section of 110 kV transmission line.
Each span of 300 m is divided in two segments to enable
connection between current source and middle of the span for
shield wire and phase conductors.
Certain number of tower subcircuiet elements, spans, line
surge arresters, grounding footing resistances and
corresponding pins (for connection with other elements) are
grouped into subcircuit element that presents transmission
line section. Example for section that consists of 10 towers is
given on Figure 5. It is possible to model sections with 50 or
more towers. In this paper some sections are consists of 50
towers and some of them are consists of 10 towers. Stations
are modelled on both ends of 110 kV transmission line.
On Figure 5 is illustrated the main idea for EMTP – RV
modelling in this work. The idea was grouping subcircuits
inside other subcircuits with possible connections between all
elements in model. On both ends of section shown on Figure
5 are pin connections for shield wire and phase conductors.
This means that is possible to make connection between two
or more section subcircuit elements to get complete
transmission line. On the top of section subcircuit element
shown on Figure 5 are pins for lightning current source
connection with tower top and middle of the span. The
position of the lightning stroke is varied from hitting the
shield wire or the phase conductor in the middle of the span
or directly at tower top. Waveform shapes that can be
observed in this simulation are: currents for all line surge
arresters, energy duties for all line surge arresters, flashover
currents and voltages for all insulators and tower footing
resistance voltage and current.
III. CPU TIMERS
To the author’s best knowledge, it is first time that
complete transmission line is created and solved in EMTP –
RV for purposes of transient analyses. CPU time is the
amount of time for which a central processing unit (CPU) was
used for processing instructions of a computer program.
Amount of input and output data is large, so it is important to
show CPU timers. Simulation time was 600 (µs). Time – step
conductor induces a voltage on the phase conductor. Voltages
on the tower top and at the bottom phase are shown on figure
10.
Energy duty
9
Energy duty for arrester installed on bottom phase
8
7
6
Energy (kJ)
was 50 (ns). Simulation statistics are: total number of
network nodes 1742, size of the main system equations 2175,
total number of solution points in time-domain 15441, total
number of iterations 47972 and number of iterations per timepoint 3,11. The CPU timings are presented in Table 1. CPU
timings are based on a Quad – core i7 processor (logical
processors: 8 and 2,2 GHz) with 8 GB of RAM. There are no
parallel computation in this simulation.
TABLE I
CPU TIMINGS (S)
CPU timers (s)
5
4
3
Prepare data
Read data
Time-domain A matrix formulation
Time-domain B vector updating
Time-domain Ax=B solution
Solution of control systems
Time-domain history updating
Time-domain solution
Total
0,23438
0,01562
0,0
1,23438
22,29688
0,15625
2,20312
26,68750
27,18750
2
1
0
0
100
200
300
Time (s)
400
500
600
Fig. 8. Energy duty for arrester – case 1.
Voltage across insulator strings
800
Voltage across insulator strings - top phase
Voltage across insulator strings - middle phase
Voltage across insulator strings - bottom phase
600
In this section are given simulation results in the case
when lightning source is bipolar lightning current from
Figure 4.
Three cases are considered. The first case is that lightning
hitting tower top and on tower is installed one line surge
arrester and tower footing resistance is 35 Ω.
On Figures 6 and 7 are presented respectively current and
voltage for arrester installed on bottom phase.
Voltage(kV)
400
IV. SIMULATION RESULTS AND DISCUSSION
200
0
-200
-400
0
100
200
300
Time (s)
400
500
600
Fig. 9. Insulator voltages – case 1.
Voltage on tower top and at bottom phase
600
Voltage on tower top
Voltage at bottom phase
400
Arrester current
3
200
Arrester current - bottom phase
0
Voltage(kV)
2.5
Current (kA)
2
-200
-400
-600
1.5
-800
1
-1000
-1200
0.5
-1400
0
100
200
0
300
Time ( s)
400
500
600
Fig. 10. Voltages on the tower top and at the bottom phase – case 1.
-0.5
0
100
200
300
Time (s)
400
500
600
Fig. 6. Current through arrester – case 1.
Arrester voltage
300
Arrester voltage - bottom phase
200
Second case is that lightning hitting tower top and on
tower are installed two line surge arrester and tower footing
resistance is 50 Ω. On Figures 11 and 12 are presented
respectively currents and voltages for arresters installed on
bottom and middle phase.
Arrester current
4
Voltage (kV)
100
Arrester current - bottom phase
Arrester current - middle phase
3.5
0
3
2.5
Current (kA)
-100
-200
-300
0
100
200
300
Time (s)
400
500
600
Fig. 7. Arrester voltage – case 1.
On Figure 8 is shown energy duty for arrester installed on
bottom phase. Voltage waves across insulators are presented
on Figure 9. Coupling between the shield wire and the phase
2
1.5
1
0.5
0
-0.5
-1
0
100
200
300
Time (s)
400
Fig. 11. Currents through arresters – case 2.
500
600
Arrester voltage
300
Arrester voltage - middle phase
Arrester voltage - bottom phase
respectively currents and voltages for arresters installed on
bottom, middle and top phase.
200
Arrester current
5
Arrester current - top phase
Arrester current - middle phase
Arrester current - bottom phase
100
Voltage (kV)
4
0
Current (kA)
3
-100
2
-200
1
-300
0
100
200
300
Time (s)
400
500
600
0
Fig. 12. Voltages on arresters – case 2.
On Figure 13 is shown energy duty for arresters installed
on bottom and middle phase. Voltage waves across insulators
are presented on Figure 14. Voltages on the tower top and at
the bottom phase are shown on figure 15.
-1
0
200
300
Time (s)
400
500
600
Fig. 16. Currents through arresters – case 3.
Arrester voltage
300
Arrester voltage - top phase
Arrester voltage - middle phase
Arrester voltage - bottom phase
Energy duty
200
12
100
Voltage (kV)
10
8
Energy (kJ)
100
0
-100
6
-200
4
-300
0
2
100
Energy duty for arrester installed on middle phase
Energy duty for arrester installed on bottom phase
0
0
100
200
300
Time (s)
400
500
600
Fig. 13. Energy duties for arresters – case 2.
Voltage across insulator strings
800
Voltage across insulator strings - top phase
Voltage across insulator strings - middle phase
Voltage across insulator strings - bottom phase
200
300
Time (s)
400
500
600
Fig. 17. Voltages on arresters – case 3.
On Figure 18 is shown energy duty for arresters installed
on bottom, middle and top phase. Voltage waves across
insulators are presented on Figure 19. Voltages on the tower
top and at the bottom phase are shown on figure 20.
600
Energy duty
14
400
200
10
0
8
Energy (kJ)
Voltage(kV)
12
-200
6
4
-400
0
100
200
300
Time (s)
400
500
600
2
Energy duty for arrester installed on top phase
Energy duty for arrester installed on middle phase
Energy duty for arrester installed on bottom phase
Fig. 14. Insulator voltages – case 2.
Voltage on tower top and at bottom phase
0
0
1000
100
200
Voltage on tower top
Voltage at bottom phase
300
Time (s)
400
500
600
Fig. 18. Energy duties for arresters – case 3.
500
Voltage across insulator strings
300
Voltage across insulator strings - top phase
Voltage across insulator strings - middle phase
Voltage across insulator strings - bottom phase
200
-500
100
Voltage(kV)
Voltage(kV)
0
-1000
-1500
0
-100
-2000
0
100
200
300
Time (s)
400
500
600
-200
Fig. 15. Voltages on the tower top and at the bottom phase – case 2.
Third case is that lightning hitting tower top and on tower
are installed three line surge arrester and tower footing
resistance is 60 Ω. On Figures 16 and 17 are presented
-300
0
100
200
300
Time (s)
400
Fig. 19. Insulator voltages – case 3.
500
600
Voltage on tower top and at bottom phase
1000
Voltage on tower top
Voltage at bottom phase
500
Voltage(kV)
0
-500
-1000
-1500
-2000
0
100
200
300
Time (s)
400
500
subcircuits inside other subcircuits is very useful for
modelling large systems. It has been shown that is possible to
model complete transmission line and provide plenty of
output data. Despite of fact that there is a plenty of input and
output data CPU time is not long. Line surge arrester energy
duty depends of arrester current shape. So, a knowledge of the
occurrence and characteristics of bipolar lightning is needed
for the selection of the arrester class (arrester block size) as
well as for designing adequate lightning protection schemes
for various objects and systems.
600
Fig. 20. Voltages on the tower top and at the bottom phase – case 3.
In Table 2 are presented LSA current and voltage peaks
and energies for all three cases.
VI. REFERENCES
[1]
TABLE II
LSA DATA
CASE
Phase
Bottom
Middle
Top
Bottom
Middle
Top
Bottom
Middle
Top
1 LSA
2 LSA
3 LSA
U (kV)
256,27
263,15
263,27
269,28
261,18
250,67
I (kA)
2,86
3,52
3,54
4,23
3,32
2,28
W (kJ)
8,24
11,93
9,73
12,38
9,68
6,15
In table 3 are shown peak values of insulator voltages.
TABLE III
INSULATOR VOLTAGES
CASE
1 LSA
2 LSA
3 LSA
Phase
Bottom
Middle (no LSA)
Top (no LSA)
Bottom
Middle
Top (no LSA)
Bottom
Middle
Top
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Ui (kV)
256,35
771,38
786,59
263,15
263,53
793,29
269,49
261,63
251,37
From presented arrester current shapes it is possible to see
that the arrester current peaks are not so high as peak of
injected lightning stroke. The arrester voltage shape for all
cases is similar but with different peak values. It depends of
LSA position (is it installed on bottom, middle or top phase)
and tower footing resistance. From energy duties
consideration we can see that arrester energy rise up as
bipolar lightning current arriving at arrester. The
corresponding energy duties are not so high. Reasons for that
are short durations of arrester currents and fact that
considering transmission line is shielded. Insulator voltages
are high, especially for phases without LSA installed (for
example 771,38 kV). Line critical flashover voltage for 110
kV is 550 kV.
V. CONCLUSIONS
This paper presents powerful and effective EMTP – RV
modelling. Grouping of elements into subcircuits and creating
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