development of a model of the alpha / beta auto …
TRANSCRIPT
D E V E L O P M E N T O F A MODEL OF THE ALPHA / BETA 765kV LINE FOR THE EVALUATION O F
AUTO-RECLOSING
by
PETER ROBERT VAN HEERDEN
Submitted in partial fulfilment of the requirements for the
Degree of
MSc (POWER AND ENERGY SYSTEMS)
FACULTY OF ENGINEERING University of Kwazulu-Natal
Supervisors: Prof. R Zivanovic Prof. N M Ijumba
February 2009
DECLARATION
I, the undersigned declare that the material presented in this dissertation is
my own work, except where specific acknowledgement is made in the form
of a reference
P R van Heerden
Student No. 204001288
18/03/2009
ABSTRACT
The intention of this dissertation is to determine, on the Eskom system, if the pre-insertion
closing resistors installed on the Alpha-Beta 765 kV line breakers are preventing overvoltages
from being caused during auto-reclosing. Other possible solutions of reclosing protection are
investigated. It has been shown on two occasions, from actual field data that overvoltages have
occurred on the lines after reclosing. High overvoltages on this network could be the cause of
the many reactor failures that have occurred.
A mathematical model of the Alpha-Beta 765kV system was produced on Matlab/Simulink to
simulate the resonance of the line during opening and then the effect on the voltage when
reclosing takes place. The effects of installing pre-insertion resistors to reduce overvoltages on
reclosing were analysed, as well as looking at controlled reclosing at the optimal voltage across
the line breakers.
It was shown from the studies, that pre-insertion resistors do limit the overvoltages to within the
surge capabilities of the line (10% overvoltage) and that the cause of the previous overvoltages
were actually due to insertion resistor operations failure. It was also shown that the method of
controlled closing at optimal voltage across the breakers is also a successful method of
preventing overvoltages. This dissertation also evaluates a design specification for a switching
relay for controlled re-closing of the line.
11
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude and appreciation to:
- My mentor, Prof Rastko Zivanovic.
- Armien Edwards for assistance in the design of the switching relay.
- Brian Berry for help in formatting the document.
- Eskom, for financial support.
TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION & BACKGROUND 1
1.1 Introduction 1
1.2 Background 1 1.2.1 Problem Overview 1 1.2.2 Methods for controlling overvoltages whilst switching shunt compensated, EHV lines 3
CHAPTER 2: INVESTIGATION INTO BC HYDRO'S USE OF CONTROLLED CLOSING ON SHUNT REACTOR COMPENSATED LINES 6
2.1 BC Hydro 500 kV line configuration 6
2.2 Resonant voltage signal produced on the open shunt reactor compensated line 7
2.3 Feasibility study of controlled reactor switching on the B. C. Hydro 500 kV line 8
2.3.1 Electromagnetic Transient Program (EMTP) Studies 8
2.4 Closing Control Device Strategy 11
2.5 Line Switching Tests 12
2.6 Interphase Coupling Phenomena 15
2.7 Conclusions 16
CHAPTER 3: SIMULATION MODEL OF THE ALPHA-BETA 765KV SYSTEM 18
3.1 Simulation Model Design 18
3.2 Testing of the Simulation Model 19 3.2.1 Comparison between study and case study results 21
CHAPTER 4: ALPHA-BETA 765KV SYSTEM SIMULATION 23
4.1 Simulation Aim 23
4.2 Simulation Methodology 23
4.3 Simulation Results 23 4.3.1 Simulation 1 - Reclosing at minimum differential voltage with pre-insertion resistors
included 23 4.3.2 Simulation 2 - Reclosing at minimum differential voltage without pre-insertion resistors
included 25 4.3.3 Simulation 3 - Reclosing at maximum differential voltage with pre-insertion resistors
included 26 4.3.4 Simulation 4 - Reclosing at maximum differential voltage without pre-insertion resistors
included 28
4.4 Interpretation of Simulation Results 30
IV
CHAPTER 5: DESIGN SPECIFICATIONS FOR A CONTROLLED SWITCHING RELAY 31
5.1 Switching Constraints 31
5.2 Relay Functional Overview 32 5.2.1 Analogue Input Quantities 32 5.2.2 Frequency Tracking and Phasor Calculation 32 5.2.3 Relay Algorithm and Output Decision Logic 32
5.3 Algorithm Functional Design and Implementation 33 5.3.1 Analogue Input Design 33 5.3.2 Frequency Tracking 34 5.3.3 Decision Variables and Logic 36
CHAPTER 6: CONCLUSION 44
V
LIST OF FIGURES PAGE
FIGURE 1: ONE LINE DIAGRAM OF ALPHA-BETA 765 KV SYSTEM 2 FIGURE 2: ACTUAL RING DOWN BEFORE AND AFTER BREAKER CLOSED 26/05/2006 3 FIGURE 3: THE EXTRACTED RING DOWN VOLTAGE WAVEFORM 4 FIGURE 4: (A) 50 HZ SINE WAVE, (B) 40 HZ SINE WAVE, (C) DIFFERENCE BETWEEN
WAVE (A) AND (B) 5 FIGURE 5: BC HYDRO 500KV LINE ARRANGEMENT 6 FIGURE 6: RESONATING RING-DOWN VOLTAGE SIGNAL FOR VARIOUS DEGREES OF
COMPENSATION 7 FIGURE 7: CUMULATIVE FREQUENCY DISTRIBUTIONS FOR OVERVOLTAGES AT
REMOTE LINE END 9 FIGURE 8: OVERVOLTAGE PROFILES ALONG TRANSMISSION LINES 10 FIGURE 9: CLOSING STROKE FOR SYSTEM BREAKER SUPERIMPOSED ON MEASURED
VOLTAGE ACROSS CIRCUIT BREAKERS 12 FIGURE 10: CONTROLLED AUTO-RECLOSING WITH TWO SHUNT REACTORS
CONNECTED 13 FIGURE 1 1: CONTROLLED AUTO-RECLOSING WITH ONE SHUNT REACTOR CONNECTED
AT LEAD END 14 FIGURE 12: EXAMPLE OF COUPLING BETWEEN PHASES 15 FIGURE 13: INTERPHASE COUPLING BETWEEN PHASES DURING CONTACT CLOSURE.. 16 FIGURE 14: ALPHA-BETA 765KV SYSTEM MODEL IN SIMULINK 18 FIGURE 15: MODEL BREAKER WITH A 450 OHM PRE-INSERTION RESISTOR CONNECTED
IN PARALLEL 19 FIGURE 16: RESONANT RING DOWN VOLTAGE AFTER OPENING 20 FIGURE 17: ACTUAL RING DOWN BEFORE AND AFTER BREAKER CLOSED 26/05/06 20 FIGURE 18: THE EXTRACTED RING DOWN VOLTAGE WAVEFORM 21 FIGURE 19: VOLTAGE ACROSS OPEN CIRCUIT BREAKER - DEGREE OF COMPENSATION
80% CASE STUDY 21 FIGURE 20: VOLTAGE ACROSS OPEN CIRCUIT BREAKER - DEGREE OF COMPENSATION
60% 22 FIGURE 21: THE ALPHA - BETA LINE BREAKER RECLOSING AT MINIMUM VOLTAGE
WITH THE INCLUSION OF PRE-INSERTION RESISTORS AT ALPHA 24 FIGURE 22: THE ALPHA-BETA LINE BREAKER RECLOSING AT MINIMUM VOLTAGE
WITH THE INCLUSION OF PRE-INSERTION RESISTORS AT BETA 24 FIGURE 23: DIFFERENCE IN VOLTAGE BETWEEN LINE AND BUSBAR AT ALPHA 25 FIGURE 24: THE ALPHA - BETA LINE BREAKER RECLOSING AT MINIMUM VOLTAGE
WITHOUT THE INCLUSION OF PRE-INSERTION RESISTORS AT ALPHA 25 FIGURE 25: THE ALPHA - BETA LINE BREAKER RECLOSING AT MINIMUM VOLTAGE
WITHOUT THE INCLUSION OF PRE-INSERTION RESISTORS AT BETA 26 FIGURE 26: DIFFERENCE IN VOLTAGE BETWEEN LINE AND BUSBAR AT ALPHA BEFORE
RECLOSING 26 FIGURE 27: THE ALPHA - BETA LINE BREAKER RECLOSING AT MAXIMUM VOLTAGE
WITH THE INCLUSION OF PRE-INSERTION RESISTORS AT ALPHA 27 FIGURE 28: THE ALPHA - BETA LINE BREAKER RECLOSING AT MAXIMUM VOLTAGE
WITH THE INCLUSION OF PRE-INSERTION RESISTORS AT BETA 27 FIGURE 29: DIFFERENCE IN VOLTAGE BETWEEN LINE AND BUSBAR AT ALPHA BEFORE
RECLOSING 28 FIGURE 30: THE ALPHA - BETA LINE BREAKER RECLOSING AT MAXIMUM VOLTAGE
WITHOUT THE INCLUSION OF PRE-INSERTION RESISTORS AT ALPHA 28 FIGURE 31: THE ALPHA - BETA LINE BREAKER RECLOSING AT MAXIMUM VOLTAGE
WITHOUT THE INCLUSION OF PRE-INSERTION RESISTORS AT BETA 29 FIGURE 32: DIFFERENCE IN VOLTAGE BETWEEN LINE AND BUSBAR AT ALPHA BEFORE
RECLOSING 29 FIGURE 33: DIFFERENTIAL VOLTAGE ACROSS THE BREAKER AT ALPHA WITH THE LINE
COMPENSATED BY BOTH LINE RE ACTORS 31 FIGURE 34: RELAY MODEL IN SIMULINK 33 FIGURE 35: FILTER DESIGN PARAMETERS 34
vi
FIGURE 36: FILTER PERFORMANCE 34 FIGURE 37: FREQUENCY TRACKING PERFORMANCE WITH AN INITIAL ESTIMATE OF
50HZ ON BREAKER OPENING 35 FIGURE 38: FREQUENCY TRACKING PERFORMANCE WITH SMOOTHING FUNCTION AND
GOOD INITIAL ESTIMATE OF FREQUENCY 35 FIGURE 39: MODIFIED SIMULINK DISCRETE 1 -PHASE PHASE LOCKED LOOP FUNCTION..
36 FIGURE 40: INSTANTANEOUS AND RMS DIFFERENTIAL VOLTAGE COMPARISON 37 FIGURE 41: MINIMUM RMS DIFFERENTIAL THRESHOLD DETECTOR PERFORMANCE... 38 FIGURE 42: DELTA DIFFERENTIAL VOLTAGE MEASUREMENT LOGIC 38 FIGURE 43: DELTA DIFFERENTIAL VOLTAGE MEASUREMENT COMPARISON 39 FIGURE 44: INTEGRATED DELTA DIFFERENTIAL VOLTAGE MEASUREMENT
COMPARISON 39 FIGURE 45: INTEGRATED DELTA DIFFERENTIAL VOLTAGE MINIMUM DETECTION 40 FIGURE 46: MINIMUM NUMBER OF COUNTS BEFORE THE OUTPUT LOGIC 40 FIGURE 47: LOGIC FOR CALCULATING THE BEAT FREQUENCY 41 FIGURE 48: CALCULATED BEAT FREQUENCY 41 FIGURE 49: INSTANTANEOUS AND SAMPLED RESULTS FOR THE BEAT FREQUENCY
PERIOD 41 FIGURE 50: LOGIC TO CALCULATE CLOSING TIME BASED ON THE PREDICTED TIME... 42 FIGURE 51: COUNTER LOGIC AND FINAL CLOSING LOGIC 43 FIGURE 52: CLOSING LOGIC PERFORMANCE WITH A COUNT OF TWO SETTING 43
vii
CHAPTER 1: INTRODUCTION & BACKGROUND
1.1 Introduction
The aim of this project is to determine whether the use of pre-insertion resistors on Eskom's 765
kV, Alpha-Beta transmission line is an effective method of protection against switching transients.
This study is brought about due to the excessive damage caused to the line reactors. Other methods
of protection are investigated and a model of the Alpha-Beta system is constructed. The model is
then used to determine the effect of the different types of protection.
1.2 Background
As mentioned before, this research is focused around Eskom's Alpha-Beta Transmission lines. It is
therefore imperative that some background to the lines' configuration, existing protection scheme
and problems which have occurred on these lines are discussed. Alternative protection schemes are
then discussed.
1.2.1 Problem Overview
Presently in Eskom, the 765 kV, Alpha-Beta transmission line is an important corridor within the
South African national grid. Should one of these parallel lines be disconnected, the stability of the
entire South African network would be affected. The Alpha-Beta corridor is important because it
connects the North-East and Cape Regions. The North-East region contains the majority of the
country's generation capacity (a total of approximately 25000 MW) [1 ]. The Cape region, however,
lacks sufficient generation and thus to meet its demand, power is imported from the North-East
region via the Alpha-Beta corridor. Should this supply be interrupted, it would result in over 3000
MW of load being shed in the Cape region [1]. It is therefore imperative that these lines are well
protected to avoid unnecessary interruptions.
There have, however, been recorded incidents where these lines have tripped and failed to
reconnect due to the failure of the existing protection scheme. In order to understand the protection
scheme, the configuration of the Alpha-Beta line is first explained.
1.2.1.1 Alpha-Beta Transmission Line Configuration
The 765 kV Alpha-Beta lines are approximately 435 km long and have a maximum apparent power
rating of 6495 MVA [ 1 ], [2]. Their long length and Extra High Voltage (EHV) cause the lines to be
capacitive under light-load conditions [3]. To compensate for this effect, line reactors are installed
on both sides of the line [l].This can be seen from Figure 1 which shows a one line representation
of the Alpha-Beta line. These reactors are then used to prevent overvoltages when the line is lightly
loaded or if load is suddenly lost as well as to decrease the Ferranti effect [3].
1
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Tutuka 3
Majuba 1
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Figure 1: One line Diagram of Alpha-Beta 765 kV System
1.2.1.2 Alpha-Beta Protection Scheme Overview
There are breakers at each side of the line which will open, should a fault occur anywhere along the
line. These breakers will attempt to reconnect via the Automatic Reclose (ARC) scheme after one
second for a single-phase fault, or three seconds for a three-phase fault [1]. However, when the
lines are opened, an exponentially decaying transient voltage waveform will remain on the lines.
This is due to the capacitive and reactive components of the line causing resonance [3J. The
resonant frequency is between 30 Hz and 45 Hz, and is dependant on the line parameters and the
level of compensation [4].
This effect can cause an overvoltage surge when the line attempts to automatically reclose. This is
because the transient voltage will be superimposed on the source voltage creating dangerous
overvoltages along the line and on the shunt reactors. Resistors are therefore switched in parallel
with the breakers and will connect 2 to 8 ms before the line breakers close, in order to dampen the
overvoltage. These resistors are typically accompanied by surge arrestors which also help to reduce
the overvoltage. These resistors are known as Pre-insertion Resistors and are valued at 450 ohms at
both ends of the Alpha-Beta line [1].
1.2.1.3 Alpha-Beta Pre-insertion Resistor Failures
Although the use of pre-insertion resistors is a world-wide practice, there are many recorded
incidents of them failing to decrease the overvoltage. The steep overvoltages, caused during
switching, are unevenly distributed along the reactor winding and thus cause a high risk of
insulation failure. There were nine single pole reactor failures on the Alpha-Beta lines between
1999 and 2002. These failures were all due to insulation failure. The costs associated with these
failures were in excess of R 30 million.
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Two occasions (26/05/1997 and 26/01/1999) where the pre-insertion resistors failed to close before
the breaker, on the Alpha-Beta line, are examined. The 26/01/1999 incident occurred due to a three
phase line fault, which resulted in all the breakers opening at both the Alpha and Beta end. The line
was able to reclose after 3 seconds but an overvoltage, in the order of 1.7 p.u, was experienced. In
the 26/05/1997 incident, the breakers at Alpha opened for a fault on the line and reclosed after 3
seconds. However, the breakers on the Beta end failed to close due to an auto-reclose (ARC) relay
failure [4].
The transient voltages from an incident which occurred on 26/05/2006 can be seen in Figure 2. It is
evident from the figure that a resonant frequency does occur after the line is opened. Figure 3
shows an extract of the ring down voltage from Figure 2. It is then observed that the ring down
voltage has an amplitude of 186.4 kV and a resonant frequency of 43.23 Hz. This data was
obtained from the field and then manipulated using the methods detailed in Appendix A.
1.2.2 Methods for controlling overvoltages whilst switching shunt reactor compensated, EHV lines
Presently, there are seven countries using or intending to use 765 kV transmission systems. They
are Russia, India, China, Korea, Canada, Brazil and South Africa. For these systems, there are two
methods which are used to control the surges caused by the resonant frequency on the shunt
compensated lines. These can be either the more commonly used method of pre-insertion resistors
or the uncommon method of controlled shunt reactor compensated line switching [5]. These
methods are discussed and compared below.
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Figure 3: The extracted ring down voltage waveform [4]
1.2.2.1 Pre-insertion resistors
Information was obtained by contacting the above-mentioned power utilities and enquiring as to
the method presently in use for the auto-reclosing of their 765 kV shunt compensated lines. Further
investigation indicated that all the above utilities make use of pre-insertion resistors whilst closing
the breakers.
The most commonly used protection measure for reclosing on EHV shunt compensated lines, is the
use of metal oxide (Mol) surge arrestors combined with pre-insertion closing resistors. The
problem with this solution is that it is both expensive and mechanically complex. The mechanical
complexity results in the solution requiring extensive maintenance and thus it is prone to failure
and unplanned outages [5].
1.2.2.2 Controlled shunt reactor compensated line switching
As discussed previously, the compensated line will hold a resonant ring-down voltage when
opened. This resonant signal has a specific frequency which is typically between 30 and 45 Hz [6].
Since the frequencies of the open line and the busbar (which remains at the grid frequency) differ, a
waveform containing beats will occur in the voltage across the open breaker. This can be seen from
Figure 4, (a) represents the 50 Hz busbar system frequency, (b) represents the open line's resonant
frequency (an example of 40 Hz is chosen) and (c) represents the difference between the two
waves. It can be seen in (c) that beats form.
The basic idea behind controlled shunt reactor switching is to close the line, at the point when the
voltage difference between the line and busbar is within the minimum of the beat cycle. This
should therefore minimise the overvoltage surge which occurs when closing these lines.
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5
CHAPTER 2: INVESTIGATION INTO BC HYDRO'S USE OF CONTROLLED CLOSING ON SHUNT REACTOR COMPENSATED
LINES
Presently, the Canadian power utility, BC Hydro, is the only utility which makes use of controlled
shunt reactor switching. This utility made use of the controlled closing methodology on its 500kV
network. BC Hydro has implemented this system over several lines in their 500 kV network,
however, only one such transmission line is studied. BC Hydro's latest 500kV transmission line,
which was constructed during 1993 and 1994, is discussed [7J.
2.1 BC Hydro 500 kV line configuration
The 500 kV BC Hydro line is 330km long and the configuration of this line can be seen in Figure 5
[7]. It is evident from the figure that the line is compensated using a mid-line series capacitor bank
as well as reactors on either side of the line. The mid-line capacitor bank is rated at 605 Mvar, but
is bypassed during auto-reclosing. The shunt reactors are rated as 550kV, 135 Mvar at each end.
The lead end is fixed and ungrounded through a metal oxide varistor and the remote end is
switchable and grounded through a neutral reactor [5]. The circuit breakers are 500kV single
pressure SF6 type with spring-hydraulic operating mechanisms with required mechanical operating
time consistency over the ambient temperature range of -50°C to +40°|7]. The time consistency is
relevant because controlled switching is highly dependent on time.
Figure 5: BC Hydro 500kV line arrangement [7]
6
2.2 Resonant voltage signal produced on the open shunt reactor compensated line
As the line is opened, an exponentially decreasing resonant voltage signal is formed. The resonant
frequency of the voltage signal is usually lower than the power frequency and dependent on the
inductance of the shunt reactors and the capacitance of the line [7].
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Figure 6: Resonating ring-down voltage signal for various degrees of compensation [7]
The tripping of an unloaded uncompensated transmission line leaves a d.c. voltage on the line and a
voltage minimum across the circuit breaker will occur every half-cycle assuming no decay of the
line voltage (Figure 6 (a)) [7].
7
With both shunt reactors connected on the BC Hydro line, the signal resonates at approximately 45
Hz. In this case, the voltage appearing across the open circuit breaker exhibits a very distinct
beating pattern (see Figure 6 (b)) [7].
With only one of the shunt reactors connected, the resonant frequency drops to approximately 33
Hz and the voltage pattern is less distinct than the previous case [7]. This voltage signal is not
obviously recognizable as a periodic beating pattem (see Figure 6 (c)). The reason for this is the
reduced damping of both the aerial and ground modes due to larger phase impedance and near
infinite ground impedance with the ungrounded shunt reactor neutral.
2.3 Feasibility study of controlled reactor switching on the B. C. Hydro 500 kV line
Due to the protection constraints of the line, a decision was made to limit the switching surge level
of the line to 1.7 p.u. This limit is defined as a 2 % value, i.e. there is a probability of 2 % or less
that the 1.7 p.u limit will be exceeded. Traditionally, pre-insertion resistors would be installed to
achieve this constraint [5]. However, due to the mechanical complexity and maintenance associated
with pre-insertion resistors, an investigation into a controlled reactor switching scheme was
conducted.
2.3.1 Electromagnetic Transient Program (EMTP) Studies
A large number of Electromagnetic Transient Program (EMTP) studies were run to define and
specify the requirements for the overall controlled switching scheme [5]. All the protection
schemes applicable to the scenario were studied in order to compare results. The following
protection scheme cases were studied:
- Closing with no switching surge control
Closing with the use of pre-insertion resistors
- Control with staggered closing (half-cycle time delay between close signal application to
successive poles/phases)
- Control with staggered closing using surge arresters with a 1.5 p.u. protective level, two (at line
terminations) or three (at line terminations and mid-line)
- Controlled point-of-wave closing using staggered closing and three surge arresters connected
as noted above.
Figure 7 compares the results of the EMTP studies by way of the cumulative frequency
distributions of the switching surge overvoltage at the remote end for the different cases (a better
description would probably be 'Cumulative Frequency (%)' on the y-axis). In all the cases, a shunt
8
reactor is connected at the lead end only. It is evident from the figure that the value of adding
surge arresters is quite significant and with the addition of controlled closing, the 1.7 p.u surge
limit is achieved. Note that for purposes of the study, controlled closing means closing at a 60Hz
source side voltage zero crossing, rather than at a minimum beat voltage point across the circuit
breaker. The implication of this is that the overvoltage distribution curve for actual controlled
closing at the minimum beat voltage across the circuit breaker will show an increase in
performance (i.e. it will be to the left of the shown curves). Viewed in terms of mitigation, the
curves show that in the extreme case with the control device out of service, the risk of exceeding
1.7 p.u. increases only to 10% which is considered acceptable [5].
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a) no switching surge control b) with 400 Q closing resistors c) with 1.5 p.u. surge arresters at line terminations and staggered closing d) as for (c) and additional surge arrester at mid-line e) with controlled closing and surge arrester per (d) above
Figure 7: Cumulative frequency distributions for overvoltages at remote line end [51
The voltage profile along the transmission line was also examined in the studies. Figure 8 shows
this voltage profile for a number of the studied overvoltage control options. By consideration of
both Figure 7 and Figure 8, it is evident that only the controlled closing with the three surge
arrester option will meet the 1.7 p.u. limit at every point along the transmission line.
9
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a) with 400 Q, closing resistors b) with 1.5 p.u. surge arresters at line terminals and staggered closing c) as for (b) with additional surge arrester at mid-line
Figure 8: Overvoltage profiles along transmission lines [5]
Specialised surge arresters are used compared to standard surge arrestors normally applied on a
500 KV system. A comparison of ratings is shown in Table 1 based on IEC 99-4[8].
These surge arrestors are considered specialised for a number of reasons:
1. The ratio of system voltage to surge arrestor rated voltage is 1.48 versus a typical value of
1.39 or less.
2. The ratio of Continuous Operating Voltage (COV) to rated voltage is 0.8 versus a typical
value of about 0.85
3. The switching surge protective level is 1.575 p.u. versus a typical value of approximately
1.8 p.u.
4. The energy absorption capability is higher than that normally required on EHV systems.
However, the rating is conservative since two consecutive surges of the noted magnitude
are highly improbable.
10
Requirement
Continuous Operating Voltage (kV rms)
Rated Voltage (kV rms)
Switching Surge Protective Level at 2 kA, kV peak
Line Class
Energy Absorption Duty Cycle
Temporary Overvoltage Capability
Standard
Surge Arrester
318
396
792
5
1.98 M J - 60 s -
1.98 MJ
Per IEC 99-4 or
better at rated
voltage
Special
Surge Arrester
318
372
709
6.7 (equivalent at
protective level)
2.5 MJ - 60s - 2.5
MJ
Per IEC 99-4 or
better at rated
voltage
Table 1: Comparison of Requirements for Standard and Specialised Surge Arresters on B.C. Hydro 500kV System [51
2.4 Closing Control Device Strategy
A control device (relay) was designed to measure the transient voltage signal and trigger the
breakers to close within the beat minimum on all the phases. The beat minimum is approximately
20 ms in duration and can be seen in Figure 9 as the "window of opportunity". Closing within the
window of opportunity is mandatory. If the control device aims to close the breaker within the
window of opportunity but misses, the overvoltage will exceed the surge capabilities of the line [6J.
It is therefore important that the control device is accurate and robust.
The strategy for the closing device is to delay the closing command appropriately such that closing
or prestrike of the contacts occur in the desired time window [6]. The control device utilises an
algorithm that recognises and analyses the voltage signal across the circuit breaker and then
predicts the occurrence of future optimal instants for closing (i.e. the beat minima) [6]. After de-
energization of the line, up to 300 ms are available for the control device to analyse the
characteristics of the voltage signal across the circuit breaker [61. Possible closing windows can
then be predicted. The scheme must incorporate mitigation measures to cover all possible
contingencies within the allocated computation time (approximately 300 ms) [6J. These
contingencies range from control device unavailability to failure to determine the optimal closing
instant [6]. Every circuit breaker has a certain closing time in the range of 50 ms. This means that
after the control device issues the closing command, it takes another 50 ms until the contacts
actually close [6]. Hence, the control device has to predict the shape of the voltage signal at least
50 ms into the future. The signal across the circuit breaker is periodically repeated at least two to
11
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three times during the acquisition time of 300 ms after the line is de-energised; therefore, a fairly
accurate prediction of the future waveform can be made [7].
In terms of functionality and of being fail-safe with respect to auto-reclosing of the circuit breaker,
the control device is required to meet the following requirements:
- Work when either one and two shunt reactors are connected
- Bypass in the event of control device failure (e.g. loss of power, malfunction of the process,
etc.)
- Reclose at 60 Hz source side zero crossing, if the optimum closing instant is not selected on
application of the close signal
- Successfully complete Transient Network Analyser (TNA) and EMTP based testing before
installation.
2.5 Line Switching Tests
A series of line switching tests were carried out to verify the performance of the design concepts
and associated hardware [5]. Two line condition cases were considered, in the first case, the shunt
reactors were connected at both line ends; in the second case, only the fixed shunt reactor at the
lead end of the line was connected.
12
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- B O O . O B O O O
V R A o (kV) - S O O B O O . O
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o . o 1 L
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VTA, B, C source side phase voltages VRA, B, C line side phase voltages VBA, B, C voltages across circuit breaker poles ILA, B, C line currents
Figure 10: Controlled auto-reclosing with two shunt reactors connected [5]
With two shunt reactors connected, the line oscillates at about 45 Hz on being switched out. The
voltage across the circuit breaker has a very pronounced beat with distinctive and repetitive
maximum and minimum amplitude points. The control device closed each pole of the circuit
breaker independently at a zero crossing within the beat minimum. As seen in Figure 10, this
resulted in no overvoltages and thus no energy was absorbed by the line surge arresters [5].
With one shunt reactor connected (low compensation as discussed earlier) the line oscillates at
approximately 33 Hz on being switched out. However, the voltage across the circuit breaker has a
less pronounced beat making it difficult to determine the periodic maximum and minimum
amplitude points. The control device therefore experienced difficulty in achieving optimal closing
on two poles (see Figure 11).
13
VTA tkV) U - B O O B O O . O
V R A „ _ C K V ] 0 °
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s o o . o ..
— o O O , C J 8 0 0 O
: ^?o ' o W^AW\AAAA^^vv^^^V\AA,^^AAy^/^^A/^^ ,V
VvYVWW\rf/T\WWW^ — s o o _ o _.
— B O O . O 2 . O _,
i K A ) . J - W V V / V * / ^ .. _ y W W > -E>
O L-wv/
1 0 0 ? o o 3 0 0 -aoo s o o B O O 7 0 0 B O O s o o • "I I ME Cms J
VTA, B , C source side phase voltages VRA, B, C line side phase voltages VBA, B, C voltages across circuit breaker poles ILA, B, C line currents
Figure 11: Controlled auto-reclosing with one shunt reactor connected at lead end [51
The reclosing in all such instances was successful, however, overvoltages did occur. As expected,
the measured overvoltages did not exceed 1.7 p.u. in any instance and the worst case single-shot
energy absorption by a line surge arrester was approximately 1 MJ [7]. This is well below the rated
energy absorption capability of the surge arrester. This was because of the one feature of the
control device. That is, if the device has not chosen an optimal closing instant by the time the close
signal is applied, then a special default mode is activated to mitigate this circumstance. In this
mode, the closing target is a zero crossing of the source side voltage.
The control device and overall controlled switching scheme has been in-service since March 1995
and has functioned correctly without incident at the time that the paper was published (April 1997)
[7]. To improve performance for the one shunt reactor case, the control device software was
upgraded and successfully tested in November 1997 [7J.
14
tkV)
tkA)
tkV)
(kA)
(kV)
(kA)
9 0 O
VBA, B,C - source side phase voltage ILA, B,C - line currents
Figure 12: Example of coupling between phases [51
2.6 Interphase Coupling Phenomena
Interphase coupling phenomena are caused by capacitive coupling and by coupling through the
neutral of the reactors, for the case where the reactor is grounded through a neutral reactor. This
means that as the first phase is reenergized during the reclosing operation, a phase shift due to
coupling occurs in the second and third phases. Figure 12 shows an example of the circuit breaker
closing on one phase and influencing the oscillation on another phase [6]. The same effect occurs
in the third phase again when the second phase is reenergized. The second and third phase voltages
show a dynamic phase shift. This causes a significant deviation from the original signal used by the
control device to predict the optimum closing instant which means that a signal measurement error
could possibly occur [6]. This deviation is shown in Figure 13. It is evident from the figure that the
coupling effect is significantly less in the second phase to close than this in the third (healthy)
phase to close [6].
15
Compensation of this error can be done only by adding a correction factor programmed into the
control device provided that the deviation from the optimal closing moment is consistent.
However, numerous EMTP simulations showed that the considered effect scatters statistically.
This means that neither a simple nor more complex systematic correlation between line
characteristics, switching angles and deviation from the optimal closing is derivable [6]. The value
of increase in the second phase is negligible and that in the third phase to close is not more than
20%. Both values are not too high and thus the maximum possible error will still allow the breaker
to close within the surge limits of the line [6]. The effect of coupling becomes relevant only if all
three phases are unfaulted during auto-reclosing, which is rarely the case. Therefore, for the
development of the control device such coupling effects were neglected [6].
Figure 13: Interphase coupling between phases during contact closure [6]
2.7 Conclusions
The following conclusions are drawn about the use of pre-insertion resistors or controlled breaker
closing using surge arresters on the BC Hydro 500 kV line.
Switching surge limitation on EHV systems can be achieved by making use of pre-insertion
resistors with line surge arresters and traditionally, pre-insertion resistors are used. Switching surge
limitation on systems can, however, be achieved through the combined use of line surge arresters
and controlled circuit breaker closing. This scheme displaces the use of circuit breaker closing
16
resistors. Specific requirements are placed on the components of the scheme and the control device
provides fail-safe or default modes to guarantee that the circuit breaker will close. In the event of
such mode closing, a higher than design risk for exceeding the switching surge limitation level
must be accepted. The controlled closing scheme allows for the use of lower line insulation levels
which induces cost savings. Extensive field testing provided evidence of the scheme's robust
design and operation.
17
CHAPTER 3: SIMULATION MODEL OF THE ALPHA-BETA 765KV SYSTEM
3.1 Simulation Model Design
In order to predict the effects of faults and the reclosing of the Alpha-beta line with or without pre-
insertion resistors, a simulation model was designed. The distributed parameter line model was
used to represent the transmission line, as this is the preferred model for greater accuracy over
longer length lines (>250km) [10]. The distributed line parameters are determined from the
physical line parameters, i.e. resistivity, length, Geometric Mean Radius (GMR), etc. These
parameters are obtained from the manufacturer, and the model parameters are calculated by the
program detailed in Appendix B.
A Matlab-Simulink model is then constructed using the line parameters. The Simulink model is
shown in Figure 14. The two parallel transmission lines are clearly visible from the figure and both
lines use the distributed parameter model. To emulate the voltage transformer (VT) measurement, a
3 phase V-l measurement model was used to measure the voltage across the breakers.
StiluralJe 1 rsnrfonner E
Voltage Meaairernonn
Figure 14: Alpha-Beta 765kV system model in Simulink
The top line is compensated using two line reactors, whereas the bottom line is compensated with
the primary windings of saturable transformers. This is done to represent the effects of the reactors
saturating because there is no saturable reactor model in Simulink. The transformer is open
circuited on the secondary side and star connected to earth on the primary side. The parameters
used for the saturable transformers are obtained from the saturation curve provided by ABB.
A load of 2500 MW and 500 Mvar is attached to the Beta substation, and a generating input of
2600 MW is supplied to the Alpha substation. The source block models at the Alpha and Beta
substations implement 3 voltage sources connected in star with the neutral internally grounded. The
inductance value is specified by the source inductive shunt circuit X/R level ratio. These values
were obtained via the PSS/E system model presently representing the Eskom transmission network
and are based on a typical operating condition for the line [1 ]. The breaker models on either side of
the line were adapted by putting two breakers in parallel, one with an internal resistance of 450
ohms. This parallel branch represents the pre-insertion resistor which closes 8 msec, before the
source breaker. The breaker block is shown in Figure 15. A further description of the block
parameters and model configuration is detailed in Appendix C.
O CDB CD'
If? 3-Phase
Breaker Ftea stance
TH Breater6
< m o
< fi o
4 CD O
Breater l
7W A2 B1 B2 C1 C2
•CD - ^ • C D
Cb
Figure 15:
Subsyaem
Model breaker with a 450 ohm pre-insertion resistor connected in parallel
3.2 Testing of the Simulation Model
In order to ensure that the model's output is accurate, a past incident must be recreated. The
measured and simulated values can then be compared to determine the simulation error. The
incident which occurred on 26/01/1999 was chosen [4]. During this incident, a three phase line
fault caused all the phase breakers to open at both Alpha and Beta substations. The ring down
voltage waveform was extracted and is displayed in Figure 16.
19
.'IfIf . ,
'•% : o r • i -
tt) • 1 D
cp !' _ J C '•' : . I . . I . •
4 ) .' ' '
- ' • f • ,' > . ' '
' ' f * H i ', • * * ' J ' ' ' ' ' i 1 1 . . ! . _ . . ._ I_. _J _.! . . . J . . i ' c? o.-i o.Ci :\e 0 7 o.t o.e
Time (s)
Figure 16: Resonant ring down voltage after opening [4]
Data from another incident (26/05/2006) was recorded [4]. Figure 17 shows the actual reclosing of
the Alpha breaker onto the ring down voltage after 3 seconds. The breaker opened for a fault on the
line and reclosed after 3 seconds. However, the Beta breaker failed to reciose and thus the Alpha
breaker reopened. Figure 18 shows an extract from the ring down voltage shown in Figure 17.
900.0
-900.0:
-nnun-
Figure 17: Actual ring down before and after breaker closed 26/05/06 f4]
The amplitudes and frequencies from each incident are calculated using Matlab and the program
code appears in Appendix A. The actual ring down frequency from the two incidents is calculated
to be 43.23 Hz. Running the simulation under the same conditions yields a ring down frequency of
42.5 Hz as seen in Figure 20. Therefore, the simulation results differ from the measured results by
0.73 Hz (+1.7 % error). This provides confidence in the accuracy of both the simulation model and
simulation results.
O - / I." '
••40!
White Phase Volt White 435.3KV/ cm
20
32C
I =g -i;Ki' >
2C0 ' v;
y / \ '
0.08 o.io o.i;
l ime (rns)
10 0.18
Figure 18: The extracted ring down voltage waveform [4]
3.2.1 Comparison between study and case study results
The figures (Figure 19 and Figure 20) below show the comparison between the voltage measured
across the breakers of the BC Hydro 500 kV system in the case study and the Eskom 765kV system
in this study. The BC Hydro 500 kV system is 80% compensated and the Eskom 765kV system is
60% compensated, explaining the slight difference in the resonant frequency of 45Hz on the BC
Hydro system and 43Hz on the Eskom system.
The overvoltages resulting from the compensation are not compared, as the purpose of this study is
to find where the minimum voltage occurs to enable the closing of the breakers without the use of
pre-insertion resistors. The case study presented was performed to investigate how compensation
affects the closing voltage for controlled switching.
% \ | !' r 1
I • I
| ! j i I I j j i h , , / i ! i i j i i j
1 : i - i I ! : I I I ' ' ' ' !
S
* i i !
Figure 19: Voltage across open circuit breaker - Degree of Compensation 80% Case Study (BC Hydro) [6]
21
x 10
> 0 5
Voltage across open ,breaker is minimium
I i, \i\
Ir '.]•
Il l Ml
\} I
M 1
11
m Kl'l
I * i
l l . i
'id
iii
ik i l I 1 i
I . I
II i I1
I;
ill 1 i
0.2 0.6 0.8 Time in seconds
Figure 20: Voltage across open circuit breaker - Degree of Compensation 60% (ESKOM)
22
CHAPTER 4: ALPHA-BETA 765KV SYSTEM SIMULATION
4.1 Simulation Aim
The aim of the simulations is to determine what effects the various protection schemes will have
on the transient voltages. Both the pre-insertion resistor and controlled shunt reactor switching
schemes will be simulated to determine which scheme yields the best results.
4.2 Simulation Methodology
Several simulations are undertaken to emulate the various protection schemes and system
conditions. In each case a fault is simulated by simply opening the breakers at both the Alpha
and Beta sides of the line. For the purpose of the line reclosing, only two frequencies are
relevant: the power system frequency on the breaker source-side and the fundamental ring down
frequency on the line-side. The voltages at the source and at the line breaker sides are of similar
magnitude, and the voltage measured across the breaker has a beating form. The following
conditions are simulated.
• Reclosing at minimum differential voltage between busbar and line with pre-insertion
resistors included.
• Reclosing at minimum differential voltage between busbar and line without pre-
insertion resistors included.
• Reclosing at maximum differential voltage between busbar and line with pre-insertion
resistors included.
• Reclosing at maximum differential voltage between busbar and line without pre-
insertion resistors included.
Note that when the pre-insertion resistors are used, they are connected 8 ms before the line
breakers are closed.
4.3 Simulation Results
4.3.1 Simulation 1 - Reclosing at minimum differential voltage with pre-insertion resistors included
This simulation is used to determine the voltage transient effects caused when the pre-insertion
resistors are used. The line breakers are closed at 0.8 sec. when the beating differential voltage
across the line breaker (Seen in Figure 23) is at a minimum. It is evident from both Figure 21
23
and Figure 22 that the overvoltages are + 1 0 % of the system voltage at Alpha and Beta. This
overvoltage is within the surge capabilities of the equipment on the line [2].
x 10v
0.6 0.8 Time h seconds
Figure 21: The Alpha breaker reclosing at minimum voltage with the inclusion of pre-insertion resistors
> 2<~-
'.D 0
l'i!!!M!!!'i!: II l l l j i ipjM
It I I I
ii
I I*. "i.'1 i1 I
I l l l ^ t ^ i J i m * ijii«i.;.f;
overvol tage 10%
!l HI
!!l I
'! ' M i l l 1 ' 1 ' I J l i l l '
0.2 0.4 0.6 0.8 T ime in seconds
1.2
Figure 22: The Beta breaker reclosing at minimum voltage with the inclusion of pre-insertion resistors
24
i i : 1 1.2 1.4
Figure 23: Difference in voltage between line and busbar at Alpha
4.3.2 Simulation 2 - Reclosing at minimum differential voltage without pre-insertion resistors included
This simulation is used to determine the voltage transient effects caused when the pre-insertion
resistors are not used but the line is reclosed when the differential voltage across the breaker is
at a minimum. The line breakers are closed at 0.8 sec. when the beating differential voltage
across the line breaker (Seen in Figure 26) is at a minimum. It is evident from both Figure 24
and Figure 25 that the overvoltages are +10 % of the system voltage at Alpha and Beta. This
overvoltage is within the surge capabilities of the equipment on the line[2].
;-^- -overvol tage 1 0%
Figure 24: The Alpha breaker reclosing at minimum voltage without the inclusion of pre-insertion resistors.
x i c r
0.5 -
Voltage across open [breaker is miri imium
"!/ I, ' • ^ i ; '
i
tf
if J !
li
Ijl'ljli I fil
J I
m '<M
HI II
i ' i
v w
0.6 0.8 Time in seconds
25
< 1 0
111"! 1 ;v| n l
| O
2
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6 ;
I |
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I1
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i1
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i
i. i II
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i
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I 1 1 1
1 '
r
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i i| f ji|i I I l ) ! |
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l i l i ' ii ii ' P
il h i t 'J \i f '1 Ml 1 i
! | M 1 1 i i i
i i ' •
i • J ,i i II
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+ai- -overvoltage 10%
! i ,i
i '
11 i
i
HUH I
||
If 11,
!'l Ii
,li I I
1
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i 1 ] •
1:
if > Ii
;-
i
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ii
1 iPli I hi 1
1
|) l
i i;i 1
is'i in
i
i
i
s 1
'i
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Il i i
ill'1
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III I I I !
(•
1 1
j j i • i •i
SPl'f ti in
i l l II I:, i
ii 1!
: 1 III
" I3' i i i i
Mi
|l
i1
i
i
0.6 0.8 T ime in seconds
Figure 25: The Beta breaker reclosing at minimum voltage without the inclusion of pre-insertion resistors
C.5
0
-C.5
- l .S
x 10 '
Voltage across cpen ,breakei * minim um
- ! i
i
ii
i i
- jji-
1 i1
- 1 ) 1
il 1 1 - i
II
1/ i • i i i ' i i :
U
II m:--[\ h ,f • I f ! . !
ill IS 1 pi Ii! Y
i , ii l.'i"l 1 ' I 1
I' lit) 1 | II'
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ii'l l i l f tP lwi\
1
vv! - I l - - ' f- : • -ipJ:iii M
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IP If If If •j i- l i i:l
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_._!. _
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Hill,., |;
' 1 i
i . . . . . . .
iiffl; pi li I'1- :
• ' t - - i i -
| l|'; :
I I ' ; ; .
D.6 0.8 T ime in seconds
Figure 26: Difference in voltage between line and busbar at Alpha before reclosing
4.3.3 Simulation 3 - Reclosing at maximum differential voltage with pre-insertion resistors included
This simulation is used to determine the voltage transient effects caused when the pre-insertion
resistors are used but the line is reclosed when the differential voltage across the breaker is at a
maximum. The line breakers are closed at 0.9 sec. when the beating differential voltage across
26
the line breaker (Seen in Figure 29) is at a maximum, ll is evident from both Figure 27 and
Figure 28 that the overvoltages are +10 % of the system voltage at Alpha and Beta. This
overvoltage is within the surge capabilities of the equipment on the line [2],
x 1CT
jv . 2
-6
.Point of.sv.'ttcbung:
ill ! -!i i l ,! ! 11 j
I I I ) illl'H't'lfiVRii i
I I H l U P I I'I T U ' - ' • -I -rl- l:l- il H1 rfll Hiii • \M
l!
:, i j; - t - i
1 0 % ove rvo l tage
111
1 - ' h i ' M '• i i
Resonan t v/3hage
i i 0 6 0 8 ~ir-iR n s^r.oncl5=
ili'.illll!
Figure 27: The Alpha breaker reclosing at maximum voltage with the inclusion of pre-insertion resistors
I I t 1 hi n
1, , '!.• 1 M l i I1
i 1 Ml - 1 - i
''! in, •
:;
1 il 'h 'i
':? J i i l l 1 1 >\ 1
Mil Hi i i l , 1
is 'ip Hi1'1
H s;i i ! 1,1 i ' M " . ' ;
; , , ] h ; i ( '
j
0 2
Point of sw i t ch ing :
i :
MM | J!
i i • •
li m | Mil \ i -
ii1 i :i ' 4 ! '
1
0-4
. i l i ' : . \ I M ' l l r H . l . l .
I ' : , i:,-'i . :; '"i !-4l i
I I I I 1 I 1 , 1 1 , j ' . ! '• 1 1
' I ' l 'Hi '•, I ! j , ,
1
1 1 il ;ii 1
1 M Mi it!"'MM ii !
. ' ' IP 1 : 11 1 :r 1
•' i i
• / h i 1 -v ' 1' 1 , 1. !
1 11 . ' ! ' 1 ' 1 ' i '
i J , 1 „ ; i i i ' ihv; [ ; ! i i ! 1 8 ' : ' j i • 1 i 1 ' t
Resonan t vo l tage
1 1
0.6 0.8 T ime ir i secon ds
- 1 0 % ovecvo
I I i I \ A 1 I I 1 1 1 i ; l l
! I IM ' 1 '1 'H ' I ' l l jl j i | : !| 1 [1
ll 1
ii III ' 'iiiill' :i
(i . | l | l ( l i . . i i
mil' I ' 1 1
n I In
I • i
|||!|
• I
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I tage
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11'l
i l l 1
1. 1
|i| iii
1 1 .
i 1.2
1
"
i
Figure 28: The Beta line breaker reclosing at maximum voltage with the inclusion of pre-insertion resistors
27
3.&
Figure 29: Difference in voltage between line and busbar at Alpha before reclosing
4.3.4 Simulation 4 - Reclosing at maximum differential voltage without pre-insertion resistors included
This simulation is used to determine the voltage transient effects caused when the pre-insertion
resistors are not used (i.e. the pre-insertion resistors fail to close) but the line is reclosed when
the differential voltage across the breaker is at a maximum. The line breakers are closed at 0.9
sec. when the beating differential voltage across the line breaker (Seen in Figure 32) is at a
maximum. It is evident from both Figure 30 and Figure 31 that the overvoltages are
approximately +50 % of the system voltage at Alpha and Beta. This overvoltage is not within
the surge capabilities of the equipment on the line and therefore there is a chance that the
components will be damaged [2]
o o
point of swi tch ing
fll'i.l Hi . . ' , '.",!. " I'"'"'-'- '
i|
i (i
hi; t II I Ii ;
III •
iil (SfCltiijIltliUli 11 §M WW
1 ii lililll 1, illill - I 1 I1 tillIIij i
i !i i 11 ! I si 1 i l " 'MM
J i
h ' ! l U ' ' ' It:' 1 i " "
"• '" l " : •". : r e s o n a n t v o l t a g
r i 1
fiirt
lii",! S I H
< - . ."". :oyervo ! ta j
I • •
1
Hi II p i t if 1 : i: : ' '< 1 I
llii'l In i 111,11,1 1 1 1
i 1 ' 1 ! : i i i j111
I
e 1
i
If I ' l l .,, J1!!
11 Illill.
e
Ii" l f -i i ! | l ' 11
I1" ni l
....
0.6 0.8 Time in seconds
Figure 30: The Alpha line breaker reclosing at maximum voltage without the inclusion of pre-insertion resistors
28
" 2
point of swi tching
'Hi
i1
iiiiii i hi
r .'.'-ii in
i
i:' ill niil'PW
!|! | | ! f | i | l !! ih ' I I I ! ! h i « «>'' mm if
H'i!! i i i i i i
i
resonant vofltage
0.6 OS T ime in seconds
Figure 31: The Beta breaker reclosing at maximum voltage without the inclusion of pre-insertion resistors
— 0 . 6 - 1 - •
Closed at voltage maximum
m HI
I'r
C.S 0 8 Tirve :~i seconds
Figure 32: Difference in voltage between line and busbar at Alpha before reclosing
29
4.4 Interpretation of Simulation Results
Several conclusions can be drawn from the four simulations presented above. It is concluded
that pre-insertion resistors are able to reduce the transient overvoltage to within the surge
capabilities of the line. However, should the resistors fail to close, the overvoltage exceeds the
line surge capabilities and damage can be done to its components. Controlled switching within
the beat minimum without the use of pre-insertion resistors, also yields an overvoltage which is
within the surge capabilities of the line. It is therefore concluded, from a theoretical perspective,
that controlled switching is a viable means of transient protection on the Alpha-Beta lines.
30
CHAPTER 5: DESIGN SPECIFICATIONS FOR A CONTROLLED SWITCHING RELAY
The convenience of controlled switching devices for use on the Alpha-Beta line is evident from
the discussion in the previous chapters. Such a device is not yet readily available and must
therefore be designed. A design specification for this device is given in order to initiate the
design process of such a device. These devices will possibly be used for the future 765 kV
transmission network in South Africa.
5.1 Switching Constraints
There are several criteria which must be met in order for the proposed protection scheme to
function correctly. The relay must measure the differential voltage across the line breaker and
attempt to reclose within the beat minimum for each phase. This objective is highly dependent
on time and thus all possible time errors and constraints must be considered.
It is evident from Figure 33 that the beat minimum or reclosing period exists for approximately
50 ms on the Alpha-Beta line when it is compensated by both reactors on each end of the line.
This means that if the breaker is set to close in the middle of the beat minimum, a closing error
of up to ±25 ms can occur without stressing the line equipment. The errors due to the sampling
and measurement of the voltage signal must therefore not produce a closing error of more than
±25 ms.
-6 x 10
o >
-1.5
Figure 33: Differential voltage across the breaker at Alpha with the line compensated by both line reactors.
31
Other errors could occur due to the varying time taken to close a breaker. The relevant breakers
have a closing time of approximately 50 ms [7]. This means that there is a 50 ms gap between
the issuing of the close command by the relay, and the closing of the breaker. However this time
can vary by as much as ±1 ms with a standard deviation (a) of 0.6 ms for a compensated
line [7].
5.2 Relay Functional Overview
A relay consists of three main functions as well as a group of auxiliary functions where the main
functions are the input sampling of the voltages on the busbar and line side, the hardware
filtering of these quantities to cutoff any unwanted frequencies, frequency tracking of the
waveforms, computation of the phasor quantities, computation of decisional variables and lastly
the computation of the close logic output. The auxiliary functions include the control logic,
auto-verification, event recording, synchronization and communication.
A description of the primary functions is presented hereunder.
5.2.1 Analogue Input Quantities
These consists of three busbar voltage transformer input quantities and a single channel line side
voltage transformer input quantity connected as a phase-to-phase measurement. The analogue
input circuitry is responsible for converting these power system voltage waveforms into digital
form for use by the relay processors. These consist of hardware filters and analogue to digital
converters with appropriate sampling circuitry.
5.2.2 Frequency Tracking and Phasor Calculation
Frequency tracking is an essential component of any relay to ensure that the sampling period for
digitally calculating the phasor quantities is correct as the power system frequency varies. The
phasor quantities are are typically calculated using techniques such as the full cycle Discrete
Fourier Transform.
5.2.3 Relay Algorithm and Output Decision Logic
This is the main software function of the relay and consists of various signal processing, logic
operations, level detection techniques that allow the algorithm to be fine tuned for a wide range
of applications.
32
5.3 Algorithm Functional Design and Implementation
A Matlab Simulink model has been developed for the proposed algorithm based on the design
criteria discussed earlier. The Simulink model is shown in Figure 34.
Figure 34: Relay model in Simulink
The busbar and voltage input waveforms are generated from the Matlab-Simulink power system
model discussed in Chapter 3. These quantities are then passed to a low-pass filter and then
frequency tracked using a phase locked loop Simulink block for the A phase only. This provides
the frequency for correct calculation of the phasor voltage quantities which represents the
system voltages in terms of rms magnitude and angle quantities. These quantities are used to
calculate the beat frequency of the difference voltage across the circuit breaker. The function
also calculates a rms voltage from the discrete sample values for the beat voltage. These filtered
analogue quantities are used to calculate decision variables such as the delta voltage (rate of
change of voltage over a set period), closing time predictor and threshold detector which feeds
the control close output logic decision. These components are discussed below:
5.3.1 Analogue Input Design
The voltage inputs are sampled at 48 samples/cycle, which is typical for a modern numerical
relay, passed through a gain function which represents the voltage transformer ratio for the
application and a low pass filter with a cut-off of 200Hz which is up to the 4th harmonic. The
performance of the filter is shown below showing a small phase error at nominal frequencies.
33
Discrete 2nd-Order
Filter 3
Figure 35: Filter design parameters
Bode Diagram
Figure 36: Filter Performance
5.3.2 Frequency Tracking
The frequency on the line side on breaker opening is at nominal 50Hz and immediately after
breaker opening is at frequencies of between 30Hz and 45Hz depending on the level of
compensation. Simulation results for the Eskom example shows frequencies in the order of
33Hz and 43Hz for the condition where one or two reactors are in service. The frequency
tracking algorithm starting from an initial frequency of 50Hz would take time to track to the
resonant frequency as shown below for the 43.23Hz example. Here the breaker is opened for 0.8
seconds and the initial frequency was at 50Hz and as can be seen the function struggles to track
the frequency very quickly. In fact the requirement is that the tracking algorithm should lock
onto the frequency by the arrival of the first minima which could be within 150msec. It is
34
proposed that the breaker status trip and reactor status bits be mapped to the function to change
the initial frequency estimate on breaker opening. The performance is shown below where the
breaker is opened for 0.8 seconds and the initial frequency was estimated at 43Hz so therefore
the function quickly locks onto this frequency.
frequency Hz
Figure 37: Frequency tracking performance with an initial estimate of 50Hz on breaker opening
frequency,
Hz
Figure 38: Frequency tracking performance with smoothing function and good initial estimate of frequency
35
As can be seen from Figure 38, the resonant frequency is not constant and causes oscillations in
the estimate. To smooth the frequency over a few cycles, the standard PLL function was
modified with a weighted moving average function which block samples and holds the most
recent samples for averaging.
Discrete 1-phase PLL
Discrete
Variable Frequency
Mean ralje
PI w w
Discrete
PI Con'roller
Discrete
Rate
Discrete
Rale Limher
\^}-*
. . i l l l l l l l l l l l l l
Weiglitec
Mow rig A*rage
-*& Discrete
2 n-3-Order
Filter
-+CD
S;n_Cos
LK±) Freq
Note: sin (a )* cos( b )= 1/2" sin(a+b) +1 /2 ' sin ia-bi
Figure 39: Modified Simulink Discrete 1-phase phase locked loop function
5.3.3 Decision Variables and Logic
The decision variables are the primary quantities used to describe and detect the phenomena and
used to feed the actual decision logic. Three quantities have been derived from the measured
data namely, the rms value of the differential voltage across the line breaker, the beat frequency
and period of the differential voltage and the rate of change of frequency of the differential
voltage measured over a number of cycles.
5.3.3.1 Differential RMS Voltage Threshold
The rms voltage is calculated from the measured instantaneous voltage inputs and calculated
over a running window of one cycle of the fundamental frequency.
36
200
Voltage
kV o
200 0 0.1
Instantaneous Differential Voltage
RMS Voltage
200
100
0.9
Figure 40: Instantaneous and RMS Differential Voltage comparison
The voltage minimum of the rms value corresponds very well to the instantaneous differential
voltage and is used to track and provide a robust signal for when the voltage minimum occurs. It
is also used as minimum rms voltage threshold detector below which the voltage must be to
allow the closing logic to proceed. This is a settable threshold which can be modified depending
on the application requirements. The performance of the minimum threshold detection logic is
shown below.
37
200 ( r
Voltage kV o
Instantaneous Differential Voltage
i i f
0.1 0.2 0.3 0.4 0.5 0.6 time (sec)
0.9 1
Figure 41: Minimum rms differential threshold detector performance
5.3.3.2 Delta Differential RMS Voltage
This function calculates an average rms differential voltage over a user settable buffer size (in
cycles), stores it and then calculates a delta measurement as the difference between two
consecutive average values as follows:
AV/ AT = Vave, blllTer(T) - Vave blI1Ter(T-l)
This tracks the changing measurement and provides a means to check for a gradient change or
zero crossing. The function with its performance is shown in the following figures:
Q >
&
PEP
III!! Weijhtad
Figure 42: Delta differential voltage measurement logic
38
Voltage
kV 0
5111)
1 Instantaneous differential voltage
1
i
4 i
i i
•) I !j
1. 1 * ,
/
ill' V
II "'
! j
Nl
;'
f i l
1
1 ' 'i
IP
i i I
1 1 1
V
i
^
il ( Delta diffential voltage
y / ft
0.2 0.3 0.4 0.5
Time (sec)
0.6 0.7 0.8
Figure 43: Delta differential voltage measurement comparison
The delta measurement quantity is further integrated to provide a robust signal for detecting the
minimum differential voltage point and provides a trigger input for the resonant frequency
function discussed next. The integrated signal is delayed by a cycle but this is compensated for
in the output logic decision.
voltage
kV
Figure 44: Integrated delta differential voltage measurement comparison
39
The trigger is determined from the voltage minimum of the integrated waveform and provides a
consistent signal for detecting the voltage minimum. This in turn is used to trigger a counter
which is used to determine a minimum number of counts before the output logic is allowed to
issue a close command.
Voltage (kV) ?"
/ \ 1 1 .' 1
1
1 i
/' 1 ;
.11,. _±
Integrated delta differential voltage
/ ; •
'''• ' 1 / '
: • \ ; 1
1 . . * •
n
• \ ! 1 I I
imum detects
i . . . „. ...J..
•
1
trigger
i i
11 II ^ 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
time (sec)
Figure 45: Integrated delta differential voltage minimum detection
Trigger
i. threshold detector
0 0 1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Figure 46: Minimum number of counts before the output logic
5.3.3.3 Beat frequency and period function
Trigger
Minimum Counter
This is calculated as the difference in frequency of the busbar frequency and line frequency,
which is the beat frequency and represents the resonant frequency of the ringdown voltage (i.e.
the beat frequency). This is shown as follows:
40
Bus_Frequoncy
From Workspace
• leatfrequency
To Workspaces
J Divid
beatperiod
To Workspace2
Line Frequency
From Workspacel
From_Delta_Measurement _Trigger
Discrete Sample & Hold
From Workspace2
Figure 47: Logic for calculating the beat frequency
Hz
h e a l f r e q u e n c y
Figure 48: Calculated Beat frequency
Busxoltages I 1
0 2 0 . 3 0 . 4 0 . 5 0 .6 0 . 7 0 . 8 0.1
0 2
0 IB
0 16
0 14
0 12 beat period(sec)
0 02
I)
~i r-
Instantaneous Output
Sample-Hold Output
0 2 0.3 0.4 0.5 0 6 0.7 time (sec)
0.8 0.9 1
Figure 49: Instantaneous and sampled results for the beat frequency period
The actual beat frequency varies with time as seen in Figure 48 thus the instantaneous value for
its period follows the same trend. The actual beat period value used in the decision logic is
41
sampled at the instant the minimum differential voltage occurs using a trigger from the delta
differential rms voltage function and then using a sample and hold logic to provide an average
measurement over the beat period.
5.3.3.4 Closing Logic Function
The final closing logic is shown below and combines the output from the counter function and
the predicted closing time which is the beat period minus the compensation time with a running
timer which starts on a voltage minimum detection trigger and resets on the next trigger. This
allows the running time to be compared to the predicted closing time and a closing decision is
allowed if the running timer has exceeded the predicted closing time.
The closing condition is therefore only valid if the following conditions are met:
1. The number of minimum counts has exceeded a set value of two
2. The rms voltage is below the set threshold
3. The running timer has exceeded the predicted closing time and therefore allows a close
command
beat period
Subtract 5
lata Store
Read
Data Store Write
n
Relational Operator
Data Store Write 1
From Delta trigger
To closing logic
Data Store Memory
Data Store Memory 1
Figure 50: Logic to calculate closing time based on the predicted time
42
Prom integrated delta function
• ->
->
U> U7z
Detect Increase
U< U/z
Detect Decrease
Trigger tor beat period function Da, f
-*r
— • NOT -
Logical Operator 1
iniB (SI)
Type Convers
A N D
Logical • •
Jpera 01 *
Discrete
I
°n W*1 1_
! '
n Memory
^ Constant 2
From timer logic
Trigger for timer function
+
1 i B Itaae* -3
k .
£ • h
jbtract 4 ' •
2
nstart!
>-—' Relational
Operator 1 L^. 1 ->
A N D
Logical Operator
— 2
Figure 51: Counter logic and final closing logic
The overall performance of the closing logic is shown in the Figure 52 taking note that this is
based on a counter setting of two which means the closing pulse is only given after two
minimum thresholds are detected.
200
i Instantaneous differential voltage ;l
i
| l
" '! V
{
i l l I1
ll
/
1 1 i f
f
1 ..
I ' l l
i
1 • 1
II i ll
i •
1 \\
t 1 J i'l'll
1 1
•
\'\ 1
i M ji i w
11 . '
• . 1
1
«
, 1
1
1
i 1 )
III nimi • i j 1
|l 1 | 1
' M l
' t
II k r 1 '
l|
1 \ I1 !' x\
| i
;, | : '
lift :l \\\y
>K 1
1 1 _ _ L 1 J 1
> !
i
Closing pulse
/ '
|
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
time (sec)
i
U..'i
u
Figure 52: Closing logic performance with a count of two setting
43
CHAPTER 6: CONCLUSION
Automatic reclosing on the Alpha-Beta, 765kV, shunt-reactor compensated line can cause very
high transient overvoltages. These overvoltages are caused due to a resonance voltage
remaining on the open line. The resonant signal is caused due to the capacitive and reactive
components of the line and its frequency is therefore dependent on the degree of compensation
used. Traditionally, pre-insertion resistors are used to reduce the transient overvoltages to within
the surge capabilities of the line. A Simulink model of the Alpha-Beta 765kV line was
constructed and used to study the performance of the existing pre-insertion resistor protection
scheme. It has been shown that the resistors are able to reduce the overvoltages to within the
surge capabilities of the line. However, if the pre-insertion resistors fail to operate, the line
components are exposed to dangerous overvoltages. Several such incidents on the Alpha-Beta
line were recorded and analysed.
An alternative protection scheme, which has been used by the BC Hydro utility, was
investigated. The protection scheme involves closing the line breakers at the moment when the
differential voltage between the busbar and line is at a minimum. Simulations of the controlled
closing scheme, on the Alpha-Beta line, proved that it is able to reduce the overvoltages to
within the surge capabilities of the line without the use of pre-insertion resistors. A relay must
be designed to facilitate the controlled closing. This relay will be able to predict the optimal
reclosing moment and then issue the close command to the breaker. The relay must be robust
because the resonant voltage signal is dependent on the specific conditions of the line such as
the degree of compensation and faulted phase.
Since pre-insertion resistors are currently installed on the Alpha-Beta 765kV system, the
controlled closing relays could be installed in conjunction with the resistors. This can be done to
facilitate field testing without jeopardising the stability of the network. Should the solution
prove to be feasible after field testing, the controlled closing methodology could be
implemented throughout Eskom's future 765 kV network. This would directly save costs
because the use of expensive pre-insertion resistors would be unnecessary.
44