Rjeas Research Journal in Engineering and Applied Sciences 2(4) 343-348 Rjeas Emerging Academy Resources (2013) (ISSN: 2276-8467)

www.emergingresource.org

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INVESTIGATING HARMONIC RESONANCE AND CAPACITOR BANK

SWITCHING AT A POWER DISTRIBUTION SUBSTATION USING A FIXED CAPACITOR BANK

J. C. Attachie and C. K. Amuzuvi

University of Mines and Technology, Department of Electrical and Electronic Engineering, Tarkwa, Ghana.

Correspondence Author: J. C. Attachie __________________________________________________________________________________________

ABSTRACT Power systems are designed to operate at nominal frequencies of 50 Hz or 60 Hz. However, certain types of loads produce currents and voltages with frequencies that are higher than the nominal, resulting in the following: voltage and current distortion, low voltage notching, communication systems interference and high voltages and currents in case of resonance. Harmonics can also cause relay malfunction, programmable logic controller (PLC) interference, capacitor fuse interruptions and high overall system losses. Moreover, the interruption of a capacitive current causes dielectric problems for the switching device. The major concern arising from the use of capacitors is the possibility of system resonance which is one of the main consequences of harmonics in power systems. This paper investigates the frequent capacitor bank tripping and damages in a distribution substation of the Electricity Company of Ghana (ECG) with the aid of a fixed capacitor bank. The study was conducted using the Electromagnetic Transient Program (EMTP) software for the simulation. The results showed that, the damages and failures had a direct bearing with harmonic resonance. Carefully selected series-connected inductors were recommended to move the resonant frequencies of the network below characteristics harmonic frequencies. The significance of the study is to investigate the effect of harmonic resonance on capacitor banks in distribution systems. Emerging Academy Resources KEYWORDS: Harmonic Resonance, Natural Frequency, Harmonic Distortion, Capacitance Switching,

Resonant Frequency, Capacitor Banks. __________________________________________________________________________________________INTRODUCTION The objective of the electric utility is to deliver sinusoidal voltage at fairly constant magnitude throughout their system. This objective is complicated by the fact that there are loads on the system that produce harmonic currents. These currents result in distorted voltages and currents that adversely impact the system performance in different ways including overheating and malfunction of equipment, communications interference, fuse and breaker mis-operation and overheating of conductor Since the number of harmonic producing loads has increased over the years, it has become increasingly necessary to address their influences when making any additions or changes to an installation. The application of shunt capacitors for voltage support and power factor correction is a common practice in the power industry. With the proliferation of harmonic-producing loads (such as adjustable speed drives and uninterruptible power supply) and the increasing awareness of harmonic effects, the possibility of system-capacitor resonance has become a routine concern for shunt capacitor applications. Whenever a shunt capacitor is to be added to a

network or resized, system planners are interested to know if the proposed capacitor installation would resonate with the system and if there is a resonance, then the problem is much severe. Furthermore, one cannot determine the severity of the resonance as not all resonance conditions will cause setbacks. However, engineering judgment must be used when applying capacitors in power systems with high harmonic currents. Capacitors might not survive long enough in such environments if they are inappropriately applied (Ronald, 2005). The objective of this paper is to investigate the frequent tripping and occasional failure of fixed and automatically switched capacitor banks in the ECGs distribution network. The network has a number of 33/11 kV power distribution substations with shunt capacitor banks installed. The main purpose of installing the capacitors was to minimize system losses, improve the system power factor and release the capacity to enable transfer of maximum power. However, upon commissioning, some of the capacitor banks were reported to have tripped persistently on over voltages and in some cases capacitor failure had occurred.

Research Journal in Engineering and Applied Sciences (ISSN: 2276-8467) 2(4):343-348 Investigating Harmonic Resonance And Capacitor Bank Switching At A Power Distribution Substation Using A Fixed Capacitor Bank

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In this paper, a distribution substation was used as a case study to aid in the investigation of the problem. Power disturbance analyzer was installed at the stations to monitor and measure power quality parameters. Harmonic currents measured were observed to be significant. Consequently, the network was modeled and examined using the EMTP software to help identify the exact disturbing electrical phenomenon. It was found that the failures were strongly related to harmonic resonance. MATERIALS AND METHODS Concept of Harmonic Resonance Phenomena The reactance of an electrical network is dependent on the frequency. At certain frequencies, the inductive and capacitive components of the network begin to resonate with each other at the resonance frequency. That frequency is the natural resonance frequency that is determined by the combination of the inductances and capacitances of the components. The resonant frequency for a particular inductance/capacitance combination can be computed from a variety of different formulae depending on what data are available. The basic resonant frequency equation is:

where L is the inductance and C is the capacitance of the network (Lukasz et al., 2009). At high voltages, the resistance of a network is usually small compared to the capacitance and inductance and therefore, the impedance can change considerably. The situation becomes severe when the resonance frequency coincides with a frequency of any harmonic current or voltage. If this occurs, the harmonic current or voltage will be amplified, which can lead to damage of some network components. Most often, resonance frequencies occurs between harmonic frequencies (inter-harmonic resonance) (Gunther, 2001). One system can have several resonance frequencies depending on the grid configuration (Patel, 2010). A relatively small distortion at resonance frequency can lead to serious consequences, which emphasizes the importance of the advance analysis of harmonics (Arana et al., 2009). Two different types of resonance can be identified; parallel resonance and series resonance (Wakileh, 2001). Parallel Resonance In parallel resonance, the impedance of the circuit is high. In an ideal resonance (circuit without resistance), impedance becomes infinitely high, which leads to extremely high overvoltage. At parallel resonance frequency, the voltage obtains its highest possible value at a given current. (Young and Freedman, 2011). Parallel resonance can occur when a source of a harmonic current is connected to the electrical circuit that can be simplified as a parallel connection of

inductive and capacitive components (Zheng, 2010). In an extreme case, a relatively small harmonic current can cause destructively high voltage peaks at resonance frequency (Aro et al., 2003) Parallel resonance is common when there are capacitor banks or long AC lines connected with large transformers. In this case, large capacitances and inductances start to resonate with each other (Lukasz et al., 2009). Fig. 1 shows an example of a parallel harmonic resonance circuit.

Fig. 1: Example of parallel resonant circuit Fig. 2 shows a plot of the equivalent system impedance and frequency scan as seen from BUS1 (Fig. 1). In Fig. 1, the circuit has a high impedance at its resonant frequency: impedance of the circuit as seen from BUS1 is 147 at 630 Hz using EMTP. Under normal condition, this should not pose any threat to the system at all and therefore a clean sine-wave voltage was recorded, using the measuring device M, at BUS1. However, it becomes a problem only if a harmonic source exists at or near this frequency.

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Fig. 2: Frequency scan and impedance as seen from BUS1 Parallel Resonant Equation The parallel resonant frequency of a system involving a transformer can be estimated. At a secondary side of a transformer, resonant frequency of a network can be determined using the following relation (Thomas and Carnovale, 2013).

Research Journal in Engineering and Applied Sciences (ISSN: 2276-8467) 2(4):343-348 Investigating Harmonic Resonance And Capacitor Bank Switching At A Power Distribution Substation Using A Fixed Capacitor Bank

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where, h is the harmonic frequency in per unit, Z is impedance of the transformer in percent, kVA the power rating and kVAR the reactive power rating of the capacitor bank. Series Resonance Series resonance differs from the parallel resonance in its low impedance at a resonance frequency. At the resonance frequency, the inductive and the capacitive reactance at a certain point becomes equal (Sankaran, 2002). In this case, the capacitive reactance annuls the inductive reactance and the network impedance only consists of the resistance of the network. As the cable resistances are normally very low, the reduction of impedance can be seen as noticeably high currents (Aro et al., 2003). The case is analogous to the parallel resonance but instead of high voltages, high currents flow through a low impedance circuit. To prevent the resonance from becoming dangerous, the system natural frequency point is forced below any of the frequencies where significant current harmonic distortion occurs. To do this, the series resonant circuit is used. In a series resonance circuit, the inductive reactance components are in series to a source of harmonic current. Essentially, series resonant is a harmonic filter tuned at a fixed frequency to attract harmonic currents and consequently reduce harmonic distortion. An example of the series resonant circuit is shown in Fig. 3.

Fig. 3: Example of series resonant circuit A plot of the impedance as a function of frequency in the series resonance circuit is illustrated in Fig. 4 (for the circuit of Fig. 3). The effect of the series inductance can be seen as: while the parallel resonant

circuit produces a high impedance of 147-ohms at the 13th harmonic current, the series circuit, tuned to 13th harmonic order, produces a low impedance path to the 13th harmonic current and changes the resonant frequency point. The resonant frequency is now moved to the 8th harmonic order which is considered not detrimental.

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Fig. 4: Frequency scan and impedance as seen from BUS1 (reference Fig. 3). Fig. 5 depicts the effect of the series inductor on the bus voltage and current amplification in the capacitive circuit. At the instant the harmonic source was energised, a voltage spike was observed. This can be destructive but can be managed with a surge arrestor. Compared with the case of the parallel resonance, the series inductor has significantly reduced the current to a tolerable range. It is important to note that, the primary purpose of the series resonant is not essentially to reduce the harmonic distortion but to ensure that the capacitor does not resonate with the impedance of the circuit (IEEE 519-1992, 1994).

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Fig. 5: Bus voltage and current amplification Series Resonant Equation The value of the inductance required to tune or move a circuits natural frequency away from a characteristic harmonic frequency, can be determined using the following relation:

where, XC is the capacitive impedance of all the capacitors connected to the secondary bus of the transformer and XL is the inductive impedance of the transformer.

Research Journal in Engineering and Applied Sciences (ISSN: 2276-8467) 2(4):343-348 Investigating Harmonic Resonance And Capacitor Bank Switching At A Power Distribution Substation Using A Fixed Capacitor Bank

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RESULTS AND DISCUSSIONS At the power distribution substation, the cause of frequent tripping and occasional failure of a 2400 kVAR, fixed capacitor bank is investigated. An equivalent circuit representation of the network of the substation is shown in Fig. 6. The transformers were independently operated as the 11 kV bus sectionalizer was kept normally open. Although the study considered two sections of the 11 kV bus (BUS1 and BUS2), to avoid repetition of procedures and ensure clarity, report of the analysis is limited to BUS1.

Fig. 6 A single line diagram of the power distribution substation The Circuit Data The source was represented as Thevenin equivalent with an X/R value of 5.3. The short circuit power is 1800 MVA at a voltage of 33 kV as base. The 10 MVA, 33/11 kV transformer at the station was represented with 9.9 % impedance. The true and reactive (PQ) load of the station are 7200 kW and 4081 kVAR with a lagging power factor of 0.87. The calculated resonance frequency of the network using equation (1) yielded 8.5. This means the system exhibits a resonant frequency that is close to a problematic frequency of the 9th harmonic order. The calculated resonant frequency was confirmed using the EMTP RV software. Fig. 7 depicts the frequency scan and impedance as seen from BUS1. The network present a high impedance at a resonant

frequency of 450 Hz (9th harmonic order) which is in agreement with the calculated natural frequency.

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Fig. 7: Frequency scan and impedance as seen from BUS1 SIMULATION RESULTS AND DISCUSSIONS A harmonic current source of 34 A of the 9th harmonic order (value taken from the measured harmonic content of the station) was injected into BUS1 so as to determine the harmonic resonant effect on the system voltage. The resulting overvoltage which is about 30% of the nominal value is illustrated in Fig. 8.

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Fig. 8: Harmonic effect on BUS1 voltage The limit on steady-state voltage is generally taken to be 110% of the rated voltage. If the voltage is allowed to rise above this point, transformers will saturate and overheat. According to IEEE standard 18-1992 and IEEE standard 1036-1992, capacitor bank short time overvoltage of about 130% should be limited to one minute. From this standpoint, we conclude that the existence of the 9th harmonic of such magnitude in the system explains why persistent tripping of the capacitors was experienced.

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Research Journal in Engineering and Applied Sciences (ISSN: 2276-8467) 2(4):343-348 Investigating Harmonic Resonance And Capacitor Bank Switching At A Power Distribution Substation Using A Fixed Capacitor Bank

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harmonic resonant condition The current drawn by the capacitors under the harmonic resonant condition was also examined. The result showed that, peak current of about 270% of the normal current was drawn by the capacitors. This is shown in Fig. 9. Capacitor standards by IEC 60871 and AS 2897 indicate that capacitors should be able to withstand inrush currents up to 100 times the nominal current. Large and high frequency steady-state current of about 270% is enough to damage the capacitors. After investigating the possible cause of the frequent tripping and the damage of the capacitor bank, our secondary aim was to find the solution to the problem. The network at this stage exhibited the 9th harmonic resonant frequency. The problem was then solved by the introduction of an inductance of 0.0059 H connected in series with the capacitor bank to drain all the 9th harmonic current. The network now gives a very low impedance path to the 9th harmonic current as shown in Fig. 10. The resonant frequency of the network is now shifted to the 8th harmonic order which is a harmless frequency.

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Fig. 10: Frequency scan and impedance as seen from BUS1 The effect of the series inductor on BUS1 voltage is shown in Fig. 11. It is evident that the 30% overvoltage effect occasioned by the parallel harmonic resonance condition in Fig. 8 has been removed by the series inductor.

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Fig. 12: Current in the capacitor as a result of the series inductor The effect of the series inductor in relation to the current drawn by the capacitors is illustrated in Fig. 12. The harmonic induced peak current amplification is now reduced significantly from 270% to about 20%. According to the IEEE standard 18-2002, shunt capacitors should be able to tolerate 135% continuous current overload. Hence, the 20% current amplification is considered safe for the capacitors. CONCLUSION Harmonic current can have a significant impact on electrical distribution systems and the facilities that they feed. It is important to consider their impact when contemplating additions or changes to a system. In addition, identifying the size and location of non-linear loads should be an important part of any maintenance and troubleshooting. Accordingly, mitigation of these harmonics is important especially for industrial applications where any small downtime period may lead to great economic losses. We have investigated the frequent capacitor bank tripping and damages in two distribution substations of the ECG using a fixed capacitor bank. The study was conducted using the EMTP software for the simulations. The results showed that, the damages and failures were related to harmonic resonance. Harmonic content of the substation was measured and the resonant frequency point of the network forced below the characteristic harmonic frequencies. The limitation of this study is that the capacitor bank sizes and locations where not considered in the analysis. Further research will be conducted to consider this effect in future work. ACKNOWLEDGEMENT The authors wish to acknowledge Geroge Eduful of the Electricity Company of Ghana for his assistance and efforts during the data collection and analysis. REFERENCE Aro, M., J. Elovaara, M. Karttunen, K. Nousiainen, V. Palva, 2003. Suurjnnitetekniikka, ISBN: 9789516723207, Helsinki: Otatieto.

Research Journal in Engineering and Applied Sciences (ISSN: 2276-8467) 2(4):343-348 Investigating Harmonic Resonance And Capacitor Bank Switching At A Power Distribution Substation Using A Fixed Capacitor Bank

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Arana, I., L. Kocewiak, J. Holboll, L. Bak and A. H. Nielsen, 2009. How to Improve the Design of the Electrical System in Future Wind Power Plant, Nordic Wind Power Conference (NWPC2009), Bornholm, Denmark. Gunther, E. W., 2001. Inter-harmonics in Power Systems, IEEE Power Engineering Society Summer Meeting 2001, pp. 813817 vol. 2. IEEE Standard 519-1992, (1994). Power System Harmonics Causes and Effects of Variable Frequency Drives Relative to the IEEE 519-1992 Standard. Square D Product Data Bulletin, Bulletin No. 8803PD9402. Lukasz, H. K., J. Hjerrild and C. L. Bak, 2009. Harmonic Models of a Back-to-Back Converter in Large Offshore Wind Farms Compared with Measurement Data, Nordic Wind Power Conference 2009, Bornholm, Denmark. Patel, D., 2010. Impact of Wind Turbine Generators on Network Resonance and Harmonic Distortion, Electrical and Computer Engineering (CCECE), 2010 23rd Canadian Conference. Ronald, H. S., 2005. Misapplication of Power Capacitors in Distribution Systems with Nonlinear Loads Three Case Histories. IEEE Transactions on Industry Applications. Vol. 41. No. 1. Sankaran, C., 2002. Power Quality, ISBN: 0849310407, Boca Raton, FL: CRC Press. Thomas, M. B, and D. J. Carnovale, 2013. Capacitor Application Issues. Online source, date accessed: 18th January, Available at http://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/capacitor-application-issues Wakileh, G. J., 2001. Power System Harmonics - Fundamentals, Analysis and Filter Design, ISBN: 3540422382, Springer. Young, H. D. and R. A. Freedman, 2011. Sears and Zemanskys University Physics with Modern Physics, ISBN: 0201603365, San Francisco: Addison-Wesley. Zheng, R., 2010. Harmonic Resonances Due to a Grid-Connected Wind Farm, Harmonics and Quality of Power (ICHQP), 14th International Conference on, Bergamo, Italy, pp. 1.