Download Electromagnetic Interference in Substations

Electromagnetic Interference in
Substations
written by
Milos Todorovic
Texas A&M University
College Station, TX
submitted to
Dr. Mladen Kezunovic
Texas A&M University
College Station, TX
Reliant Energy HL&P
Houston, TX
December 2000
TABLE OF CONTENTS
Introduction .......................................................................................................................... 1
Measurement and Prediction of EMI ................................................................................. 2
Measured EMI Characteristics ............................................................................................ 2
Disconnect Switch Operations ......................................................................................... 3
Circuit Breaker Operations ............................................................................................. 5
Prediction of EMI ................................................................................................................ 6
Comparison of peak EMI values to standards ..................................................................... 7
Conclusions ........................................................................................................................... 9
References............................................................................................................................ 11
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INTRODUCTION
Increasing use of electronic equipment in switchyards and control houses has brought an
issue of protecting such equipment from potentially damaging levels of electromagnetic
interference (EMI). Substation switching operations (breakers and disconnects), lightning
strikes or spontaneous faults can cause high frequency - high levels EMI. This EMI can
couple into low voltage control circuits and electronic equipment unless it is protected. The
highest levels of interference are encountered in the immediate proximity of the buses and
switching gear, and the radiated EMI has transient nature.
This transient EMI environment needs to be characterized by waveforms and spectra for
the highest expected levels both in the switchyard and inside the control house. These
levels can be then compared with equipment susceptibility levels and with surge withstand
capability (SWC) levels.
Relatively high number of papers dealing with the EMI measurements or prediction
have been published, but no standards for either of these two activities (measuring and
prediction) have been produced.
The aim of this paper is to give an overview of the activities related to EMI conducted
and papers published so far. The particular interest was focused on papers dealing with the
problem of high frequency EMI in the radio spectrum (up to 1 GHz). The motivation
behind this was to investigate the possible interference that the wireless radio devices
would have to deal with. Unfortunately, not a single paper identified so far has been dealing
with such high frequencies. The bandwidth covered is usually up to 150 MHz. Even this
data is very useful because the modern microprocessors work on the clock frequencies in
that range.
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MEASUREMENT AND PREDICTION OF EMI
In order to characterize electromagnetic environment in substations two things have to
be done. It is necessary to develop a generalized computer model that can be used to model
electromagnetic fields in an arbitrary substation. This model should be used to predict the
EM fields in the places where it is not feasible to conduct measuring. Also, computer model
prediction approach is much cheaper than field-measuring approach. Another important
issue related to the characterization of the EMI is collecting the representative set of
measurements that should be used to test the computer model.
According to the literature [1-4], both issues were investigated and initial activities on
generated computer model were conducted.
Measured EMI Characteristics
A number of different types of transient measurements are required to completely
describe how typical substation EMI arises. Switching a disconnect or circuit breaker, for
example, produces a complex sequence of high frequency current and voltage transients on
each phase of the high voltage bus. Current transients excite the 3-D bus structure which
acts as a complex antenna, radiating energy into the substation as transient electric and
magnetic fields. Cumulative electric and magnetic fields at a given point in the substation
are the result of the superposition of direct and reflected EM fields. The reflection occurs
on all metal surfaces, as well as on the ground.
Different sets of equipment have to be used in order to measure EM fields. Various
types of sensors are used to measure the data. In addition to the electric and magnetic field
measurement, bus current measurements have to be collected too. This is important for
concluding the relationship between the current transients as the EMI source, and resulting
EM fields.
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Several different types of sensors should be used – bus current probes, electric field
sensors, and magnetic field sensors. These devices should have the bandwidths to cover the
entire frequency range of interest (0.010-1000 MHz) and peak detection measuring
characteristics as shown in [7].
Current probes produce an output voltage proportional to the current transient. Electric
and magnetic field sensors produce output voltage proportional to the time derivative of the
measured fields. Therefore, it is necessary to integrate the outputs of these sensors. All data
should be transmitted over fiber optic data links to prevent the field coupling that could
corrupt the measurements.
It is known from theory that the dominant magnetic field component is the lateral
component, and the dominant electric field component if radial component. This is the case
when the field source is a long, thin structure such as a wire. In order to measure the peak
values of the dominant components, it is necessary to position sensors in to sense those
components.
Current transients should be measured using clamp-on probes.
Another important part of the measuring set is the data acquisition equipment. Recording
devices used for this purpose should have sufficient number of simultaneously sampled
input channels in order to provide for capturing the correlation between incoming data. The
minimum number of channels is obviously 3 (current, electric field, and magnetic field
measurements). It is beneficial if the device can use more than one sampling rate since it
provides for acquiring longer but less accurate, or shorter but more accurate records, with
the fixed storage memory size. This is necessary to analyze both the short, high frequency
transients and longer, lower frequency transients.
Following paragraphs give the overview of the transients typical for the operations of
circuit breakers and disconnects.
Disconnect Switch Operations
This section of the paper characterizes the transients produced during the disconnect
operations. Disconnect switches are hand-operated devices. They are actually three separate
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switches, one per phase, mechanically linked to a single hand crank. When the switch is
operated, arcs occur between the movable and fixed contacts. These arcs produce current
transient waves on the bus, which travel away from the switch in both directions. As the
result, transient electromagnetic interference occurs. Due to the mechanical nature of the
mechanism, not all three-phase switches operate at the same time. This means that the
phase containing the switch that operates first will launch the initial transient. Resulting
fields can, and usually do, couple energy to the remaining two phases and they start to be
the sources of the secondary EMI. It has been shown [1, 4] that the secondary EMI is about
twice strong compared with the primary (initial) EMI. However, those components can
contribute to the overall EMI. In general, the total current or field transient measured at a
particular location over any small interval reflects the characteristic of the primary and
secondary transients including any scattered and reflected components of both.
Measurements have shown that a single transient occurs about once each half cycle
when the disconnects are closing. The peak amplitude value of the current transient can be
seen at the beginning of the closing sequence. The reason for this is that the distance
between the moving and fixed part is the biggest at that time and the strongest electric field
is needed to produce arcing. Magnetic field follows the transient current and both transients
have almost the same waveform. Slight differences occur because of the influence of the
secondary magnetic field resulting from the currents induced in the other two phases of the
3-phase bus. Electric field transient have different waveform due to the fact that it is
proportional to the charge transient on the bus. However, it also has a peak value at the
beginning of the close operation.
When disconnects are opening, the current transient has totally different behavior. The
amplitudes are small at the beginning and they increase as the time passes. On the other
hand, first few milliseconds are characterized by the very high frequency harmonics (well
above 50 MHz), while the transient frequency decreases as the time elapses. By the end of
the opening operation, transients occur about every half cycle, just as at the beginning of
the close sequence.
Comparison between the transient behavior of the system when disconnects are closing
and opening shows that the transients die out faster during the open operation. On the other
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hand, both cases produce transients of the very similar shapes. It also has to be stressed that
the magnetic field closely follows the current transient, while the electric field has
significantly different profile.
Table 1 gives a summary of the peak values for current, electric field and magnetic field
transients as presented in [1].
Table 1. Peak amplitudes during disconnects operations
System voltage (kV)
Bus current (A)
Electric field (kV/m)
Magnetic field (A/m)
115
466
7
56.1
230
1040
5.5
82.2
500
3560
13.5
157
It can be seen that the peak values are higher in the higher voltage substations, which
was expected.
Circuit Breaker Operations
In the case of the circuit breaker operations resulting transients are different. Since the
breakers are operating much faster transients are diminishing much faster. Typically they
exist for only few cycles.
The circuit breaker closing operation produces one transient per half cycle. This is
similar to what happens during disconnect close operation. Another similarity is that the
peak amplitude is observed at the beginning of the operations. The same explanation as in
the case of disconnect close operation holds true. The waveform exhibits a burst of very
high frequencies (up to 50 MHz) on the initial rise of the main peak. Again, the magnetic
field has the same waveform as the current transient, while the electric field has the
waveform influenced by the change in the bus to ground charge.
The opening of the breaker produces transients that last just for 5-6 half cycles. The peak
amplitude is reached with the last one or two transients. The frequency content is somewhat
the same as in the case of the closing operation.
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Table 2 gives the summary of the peak amplitude values of the measurements as
reported in [1].
Table 2. Peak amplitudes during circuit breaker operations
System voltage (kV)
Bus current (A)
Electric field (kV/m)
Magnetic field (A/m)
115
11.8
3.7
0.62
230
76.8
0.9
5.3
500
132
5.6
18.9
Comparing the measurements presented in tables 1 and 2, it can be concluded that the
disconnect switch operations produce transients of significantly higher amplitudes. On the
other hand, transients resulting from the circuit breaker operations have broader frequency
bandwidth.
Readers are referred to [1-4] for more detailed descriptions of the resulting waveforms.
Prediction of EMI
There are several issues that have to be taken into consideration in order to develop a
good software package that will be able to predict EM field values at any given point in the
substation and its surrounding. Such package should be flexible enough to allow users to
specify the placement of the equipment in the substation so the program can compute
accurate prediction. A 3-D model should be implemented. In addition to this, user should
be able to specify the mutual coupling between the lines and busses, as well as the
characteristics of the switching gear.
When the selection of the model for EM fields’ computation is concerned, there are
generally two approaches to modeling physical EM sources. The most significant sources
are buses. They can be seen as long, thin antennas radiating electromagnetic power in the
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space around them. It is know that such antennas can be modeled either as the sum of little
dipole antennas, or they can be seen as one big radiating source.
The selection of one of the two models should be motivated by the trade-off between
the computational complexity and accuracy. Generally speaking, dipole antenna modeling
tends to increase the complexity of the model but gives more accurate predictions,
especially in the cases when the influence of the parasitic and reflected currents and
corresponding fields is considered.
The issue of current transients’ generation can be addressed by incorporating one of the
standard programs for transient analysis such as EMTP. These programs can generate very
accurate waveforms reflecting the actual substation configuration.
Another important issue in prediction model development is the model validation. It
should be done with as much field data as possible. A program should be forced to predict
the EM fields for the same configurations as the ones existing when the measurements were
taken and the results should be compared with the actual measurements. Any differences
should be used to point out the potential problems.
Looking at the results obtained by the TRAFIC model introduced in [3], it can be seen
that the comparison between the predicted and measured EM fields has shown very small
differences. This illustrates that it is possible to give very good prediction, which is
beneficial since the field measuring is very expensive and cumbersome.
Comparison of peak EMI values to standards
Currently, there are no standard values for the maximum levels of electromagnetic
interference that should be still acceptable for the regular operation of the substation control
and protection devices.
Standards [5, 6] list the procedures for testing the surge withstand capability of different
devices. Following these procedures and applying the maximum predicted and/or measured
EMI levels, the behavior of different devices can be determined.
However, no reports on such tests were identified.
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At this point, it is interesting to mention one problem with the PC computer that was
encountered while it was used as part of the wireless subsystem testing. The tests were
performed as a part of the ongoing project at Texas A&M University. The wireless
connection between two PC computers was disrupted during disconnects operations and
couldn’t be automatically restored at the end of the operation. One of the computers was
placed in the immediate proximity of the operating disconnect and that computer
experienced the problems. It was not clear what has caused the malfunction. One of the
possibilities was that the processor was temporarily disabled by the high EMI. If we take a
look at the Table 1 we will see that the highest EMI levels are encountered during the
disconnect switch operations. Analysis performed by different authors indicates that this
EMI has high frequency components reaching into frequencies close to 100 MHz, which is
the processor operating frequency. This supports the explanation that the PC computer was
severely influenced by the EMI.
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CONCLUSIONS
The aim of this paper was to give an overview of the topics relevant to the
electromagnetic interference that can be encountered in power system substations.
A description of the characteristic transient waveforms was given. The differences
between opening and closing sequences for disconnect switches and circuit breakers
operations were presented. It has been shown that the opening sequences produce transients
whose peak values occur by the end of the operation. On the other hand, closing sequences
produce peak amplitudes at the beginning of the operation.
It has been also shown that the transients resulting from disconnect operations last
longer and maximum frequencies in the transients’ spectrum are lower than those specific
to circuit breaker operations.
Maximum amplitude values for current transients, electric field transients, and magnetic
field transients were presented in tabular form as functions of the substation voltage level.
The next issue that was covered was the possibility of predicting EMI using computer
simulations. This approach is much more affordable. Specific requirements for the program
structure were mentioned, such as the flexibility in defining the substation configuration (3D model).
The issue of selecting the right approach for modeling radiation sources was covered.
Two models were presented – one that represents the bus as the sum of many small dipole
antennas, and the other one that models the bus as one long and thin radiator structure.
Former approach is more accurate but more computationally involved.
At the end, a topic of comparing the predicted and/or measured peak amplitude values
with the standards for SWT was mentioned. It has been emphasized that currently there are
no general standards specifying the maximum values of EMI that devices should be able to
withstand. Two standards [5 and 6] give guidelines how to conduct such comparisons but
do not give any specific values.
To conclude, although several studies aimed at characterizing EMI in substations were
conducted, still there are no models that could give the prediction of the levels at any given
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point in the substation. Since the use of microprocessor-based devices in substations is
increasing, it is likely that more projects will deal with this subject in the future. It should
be expected that the results of this research will produce a standard which will define the
maximum transient amplitude values that the equipment will be able to withstand.
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REFERENCES
1. C.M. Wiggins et al. “Transient Electromagnetic Interference in Substations”, IEEE
Trans. on Power Delivery, Vol. 9, No. 4, October 1994
2. C.M. Wiggins and S.E. Wright, “Switching Transient Fields in Substations”, IEEE
Trans. on Power Delivery, Vol. 6, No. 2, April 1991
3. D.E. Thomas et al., “Prediction of Electromagnetic Field and Current Transients in
Power Transmission and Distribution Systems”, IEEE Trans. on Power Delivery,
Vol. 4, No. 1, January 1991
4. C.M. Wiggins et al. “Measurements of Switching Transients in a 115 KV
Substation”, IEEE Trans. on Power Delivery, Vol. 4, No. 1, January 1989
5. ANSI/IEEE C37.90.2-1987, “Withstand Capability of Relay Systems to Radiated
Electromagnetic Interference from Transceivers”
6. ANSI/IEEE C37.90.1-1989, “IEEE Standard Surge Withstand Capability Tests for
Protective Relays and Relay Systems”
7. ANSI/IEEE 430-1986, “IEEE Standard Procedures for the Measurement of Radio
Noise from Overhead Power Lines and Substations”
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