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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 i EMI in Substations December 2000 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. 1 EMI in Substations December 2000 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. 2 EMI in Substations December 2000 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 3 EMI in Substations December 2000 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 4 EMI in Substations December 2000 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. 5 EMI in Substations December 2000 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 6 EMI in Substations December 2000 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. 7 EMI in Substations December 2000 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. 8 EMI in Substations December 2000 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 9 EMI in Substations December 2000 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. 10 EMI in Substations December 2000 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” 11