IMPLEMENTATION OF A CONTROL SYSTEM FOR RECLOSERS USING A DIGITAL SIGNAL
PROCESSOR
WANIR J. MEDEIROS JR., JOSÉ W. L. NERYS, ENES G. MARRA
PEQ, Escola de Engenharia Elétrica e de Computação, Universidade Federal de Goiás
Praça Universitária S/N, 74605-220 Goiânia, GO, Brasil
wanirjr@ig.com.br, jwilson@eee.ufg.br, enes@eee.ufg.br
Abstract This paper presents the experimental results of a digital control for hydraulic reclosers using Digital Signal Processors (DSP). Some characteristic curves are implemented and some experimental results are shown to illustrate the recloser operation. Hydraulically controlled reclosers normally present good mechanic performance, however they present a limited operating
set, which makes it difficulty to implement a coordinated operation with modern reclosers and other protective devices.
Keywords automatic recloser, digital signal processor, overcurrent protection, electrical distribution protection.
Resumo Religadores hidráulicos possuem boa característica mecânica, porém têm apresentado problemas de coordenação com
outros religadores modernos devido a limitada capacidade de alteração de suas características de operação. A idéia básica deste
trabalho é apresentar os resultados experimentais da digitalização do controle de religador usando Processador Digital de Sinais
(DSP).
Palavras-chave religadores automáticos, processador digital de sinais, proteção de sobre-corrente.
1
Introduction
Reclosers have been in use for more than half a century and their operation principle consists in interrupting the main current of a system, when an over
current condition is detected, and in automatically
restoring the energy supply after a previously programmed time. In those cases when the fault is
cleared no other action is taken. On the other hand,
when an over current state persists, the recloser will
open and close the main circuit for a preset number
of times. After the fourth opening action, the main
switches are kept opened and locked-out, which requires the intervention of a technical operator.
Automatic Reclosers are protective equipments
that are used to reduce the interruption time of distribution systems and, consequently, to increase the
reliability of those systems. They allow the power to
be restored after a short time when temporary faults
occur, such as those caused by trees, strong winds
and animal contact. Previous studies show that 60%
to 75% of the electric faults in distribution systems
are due to transient causes.
The expansion of the electrical distribution system has led to a more and more difficult coordinating
task of the reclosers. This has required more versatile and precise protective equipments, so that the
older ones (hydraulic reclosers) cannot any more
fulfil the requirements of the modern distribution
systems, because they have limited operating characteristics.
The main purpose of this work is present experimental results of a digital control system for hy-
draulic reclosers belonging to an Energy Company in
Brazil, so as to allow them to use their good physical
characteristics with a more versatile and precise control system.
2 DSP Development
This work has been developed using the DSP kit
ADMC401 by Analog Devices. It has a high performance in applications such as industrial process
control and motor drive and its main characteristics
are:
• 26 MIPS fixed-point DSP core;
• single cycle instruction execution, 38.5 ns;
• 12 –Bit pipeline flash analog-to-digital converter;
• digital-to-analog converter, 8 channels and
12 bits;
• two double buffered synchronous serial
ports;
• three-phase PWM generation unit;
• 12 programmable digital I/O ports.
The ADMC401 has a flexible architecture and
single instructions that allows to the processor several parallel processing. It also contains a fast, high
accuracy, multiple-input analog-to-digital converter
unit with simultaneous sampling capabilities.
The described DSP kit was chosen due its high
capability for acquisition and processing data, which
may result in a more precise calculus of the RMS
values of the currents. The implemented program
was developed in assembly of the chosen DSP and
its processing sequence is: acquisition of the three
main currents; calculus of their RMS values cycle by
cycle; comparison between the RMS values and their
corresponding limits obtained from the time-current
curves, which will be described further on. The calculated rms values of the currents are stored for future analysis and visualization. The RMS value of
the current is given by the square root of the sum of
each sampled value, squared, divided by the number
of samples for a complete cycle (S). The expression
used for calculating the rms value of the current is:
I
rms
=
∑i
2
(1)
S
(
The sample time is 50µs, which results in 334
samples for a 16.667 ms period.
A serial communication channel is used to transfer the necessary configuration set from a microcomputer to the DSP platform so as to attend the operational requirements.
3 Interface Software
To support the DSP platform another program was
developed using Delphi programming language. This
tool is used to plot curves based on standard equations by IEEE, IEC and ABB, which allow a comparative analysis between different curves, for a protective study. The plotted curves are called inverse
time-current characteristics and they are used to calculate the delay time between an over current detection and the necessary action to protect the distribution system.
To a better understanding of those inverse-time
curves it is necessary to understand the operating
principle of an induction relay. Its characteristic
curve is the result of the integration of a current
function with respect to time. It has basically three
types of curves: moderately inverse, very inverse and
extremely inverse curves. These curves are chosen so
as to keep a coordinated operation with other protective equipments. Expression (2), by IEEE C37.901989, is used to calculate the pickup time of an inverse-time over current curve.
Where:


A

+ B n
 M P −1



(
)
Expression (2) is similar to the IEC 255-03
equation, except for the addition of constant B,
added to the IEEE equation. This constant defines
the definite time component resulting from the core
saturation of an induction type relay. Expression (3)
is a more generic form of inverse-time over current
curve, and it is used by ABB Power Company:


A
14n − 5

t(I ) =
+ B
P
 M −C
 9


Where:
i = sampled value of the current;
S = number of samples for a complete cycle.
t(I ) =
t(I) = trip time in seconds for M > 1;
M = multiple of pickup currents;
A, B and P = constants to provide selected curve
characteristics. Their values define whether the
curves will be moderately, very or extremely inverse;
n = time dial.
(2)
)
(3)
Where:
C = constant to provide selected curve characteristics.
The developed program allows plotting any of
these three types of curves, with and automatic adjust
of the constant values. Furthermore, the user may
prefer to choose any other values for constants of
expression (3), so as to create a personal inverse-time
over current curve for a particular project.
The resulting curves after applying the discussed
expressions may be saved as a bitmap file to be
printed and analysed on a later occasion. Fig. 1
shows the moderately, very, and extremely inverse
curves resulting from standard IEEE C37.90. Fig. 2
shows similar curves, resulting from standard IEC
255-4. It can be seen from Fig. 1 and Fig. 1 that the
IEEE curves present shorter time responses to a specific value of over current, when compared to the
IEC curves.
After choosing the most appropriate curve, the
user should transfer it to the DSP platform through
the microcomputer serial port.
The transmitted curve is used by the DSP algorithm to calculate the time-response equivalent to the
fault current. The calculated time is used to drive the
recloser.
The developed program also allows the user to
set the following parameters of the recloser: delayed
time; reclosing time, i.e., interval of time between
one opening action and the next closing action; reset
time, i.e., necessary time to clear the register of the
last faults; minimum trip current and the number of
open/close actions.
4 Experimental Results
Experimental tests were performed using the recloser model ESV3810 by Cooper Power Systems.
The main characteristics of the recloser are:
• nominal voltage – 34.5 kV;
• nominal pickup current – 800 A;
• maximum current – 10 kA;
• vacuum interruption;
• transformation ratio – 1000/500:1 A.
Before carrying out these experimental tests, the
the original recloser control (Kyle Form 6 control)
was replaced by the implemented DSP control system. The adopted transformation ratio was 500:1 A.
An AC current source was used for simulating a
short circuit current of 670 A through the recloser
terminals.
Figure 3 shows the system assembly in the laboratory.
Figure 1. IEEE inverse time-current characteristic.
Figure 3. System assembly.
Figure 2. IEC inverse time-current characteristic.
Fixed delayed times can be set for each one of
the trip operations, and these times may be added, or
not, to the delayed times defined through the inversetime curve. The minimum trip current may be set per
phase on a three-phase system and the number of
open/close action may be adjusted between one and
four.
Figures 4 to 7 show the results for the recloser
currents. The adopted scale is 1000 A/div. The programmed pickup current for these results is 200 A,
and the short circuit current is 670 A, and it was applied to one of the recloser three phases. The programmed time for the first reclosing action is 0.5
seconds.
In figure 4, the IEC extremmely inverse time
curve was used with dial time of 0.5. The calculated
time for the recloser action was of 3.91 s. The measured time was of 4.2 s, which corresponds to an error
of aproximately 7.5%.
The same test as before, however using the
IEEE extremmely inverse time curve, with dial time
of 0.5, resulted in an operating time of 1.57 s, against
a calculated time of 1.45 s. Therefore, for the test
conditions above, the IEEE curve resulted in a faster
response, when compared to the IEC response.
The same test as before was also performed using the ABB extremmely inverse time curve. Figure
6 shows the corresponding results. The calculated
time for the first pickup was 144 ms. The second
reclosing action was programmed for 5 s.
measured time was 1.36 s, which corresponds to na
error of 7.1%.
Figure 4. Experimental result using IEC extremely inverse curve,
with n = 0.5.
Figure 7. Experimental result using specific inversse curve.
5 Conclusion
This research work is intended to improve the distribution system coordination of the protective equipments belonging to CHESP Energy Company – Brazil, by applying a more versatile and modern control
system, based in Digital Signal Processors. It is seen,
from the experimental results that the error between
the calculated operating time and the effectively
measured time is less then 10% for all four implemented curves, which confirms the high response of
the system.
Figure 5. Experimental result using IEEE extremely inverse curve,
with n = 0.5.
Acknowledgment
The authors would like to express their gratitute to
CHESP Energy Company for the financial support to
this research work and FUNAPE for the presentation
support.
References
Figure 6. Experimental result using ABB extremely inverse curve,
with n = 0.5.
Figure 7 shows the result for a fourth time inverse curve, which was created by the user. The used
constants are: A = C = n = 1, B = 0.05 e P = 0.5. The
calculated time for the first pickup was 1.27 s. The
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