Electrical Energy Saving and Economic Benefits from Power System Harmonics Mitigation in the Petrochemical Plants

Transcription

1 Electrical Energy Saving and Economic Benefits from Power System Harmonics Mitigation in the Petrochemical Plants Sherif M. Ismael Electrical Engineering Division, Engineering for the Petroleum and Process Industries (ENPPI) Cairo, Egypt S.F. Mekhamer, A.Y. Abdelaziz Electrical Power and Machines Department Faculty of Engineering Ain Shams University, Cairo, Egypt Abstract- Due to the dramatic increase in the applications of nonlinear loads in the petrochemical plants, mainly the variable frequency drives (VFD's), power system harmonics problems arise and represent a big obstacle against the wide application of the VFD's although they enhance system efficiency and provide great energy saving. Power system harmonics cause many problems like equipment failures, malfunctions and plant shutdowns. Accordingly, mitigation of these harmonics is considered an important target especially for the petrochemical applications where any short downtime period may lead to great economic losses. Nowadays, there are various harmonic mitigation techniques in the market, each with specific technical advantages and disadvantages but the selection of the optimum technique is not only based on the technical aspects but the economic aspects as well. This paper focuses on the harmonics harmful effects on the plant economics and the electrical energy savings achieved through harmonics mitigation. In addition, this paper introduces comprehensive economic studies for most of the harmonic mitigation techniques supported by market surveys. Hence, this paper is considered as a helpful guide for the design engineers, consultants and customers of the petrochemical sector. Index Terms- Harmonics, Variable Frequency Drives (VFD), Economic, Energy Saving, Petrochemical Plants. I. INTRODUCTION Due to the dramatic increase in the applications and usage of the nonlinear loads in the petrochemical facilities such as the Polystyrene production plants and the Ethylene production plants, the power system harmonics problems arise and cause many harmful effects to the power systems such as: - Overheating of generators, motors, transformers, and power cables that lead to early equipment failures - Failure of capacitor banks - Nuisance tripping to protection relays and circuit breakers - Interference to communication systems and sensitive electronic devices From the economic point of view, power system harmonics lead to great economic losses for the industrial applications. Accordingly, mitigating these harmonics will lead to considerable energy saving, avoid unwanted plant shutdowns and enhance plant economics [1, 2]. The main feature of the power system harmonics is that they are silent problems, which means that they are continuously happening without being realized by the plant owner until a complete equipment failure happens. Some economic concerns of harmonic mitigation techniques are presented in [3, 4] but authors did not focus on the petrochemical sector, during an extensive literature review over the past twenty years, it was found that comprehensive economic comparisons between the various harmonic mitigation techniques was not clearly found even though it is strongly needed in the petrochemical sector. This paper goals are: 1- To provide a comprehensive understanding of power system losses occurring due to harmonic distortions. 2- To highlight the economic impacts of power system harmonics on various power system components. 3- To highlight the electrical energy savings achieved from the power system harmonics mitigation. 4- To perform a market survey on the available harmonic mitigation techniques and provide a comparative economic studies for these mitigation techniques. 1

2 II. UNDERSTANDING THE HARMONICS-RELATED LOSSES In an electrical network that contains no harmonic loads, the RMS current is equal to the fundamental current. But in a harmonics polluted network, the RMS current can be calculated as follow: I RMS (I 1 ) 2 n h 2 I h 2 Where, I RMS : The root mean square current I 1 : Fundamental current I h : Harmonic current at harmonic order (h) h: Harmonic order From equation (1), it is clearly found that the RMS current in a harmonic polluted environment is higher than its value in a pure sinusoidal electrical network. This increase in the RMS current leads to increased heat dissipation in the electrical equipment and deterioration to the equipment's insulation that may cause premature equipment failures. The heat dissipation losses in the electrical networks are calculated from the product of the square of the current by the system impedance (I 2. Z). In the presence of the harmonic distortions, the total system losses can be calculated by the summation of the individual losses at every harmonic frequency as follow: I 2. Z = [I 1 ] 2.Z 1 + [I 3 ] 2.Z 3 + [I 5 ] 2.Z 5 + [I n ] 2.Z n (2) Where, I 1 : Fundamental current I n : Harmonic current at harmonic order (n) Z n : System impedance at harmonic order (n) n: Integer = 1,2,3,. Due to the fact that most of the electrical equipment are specified and designed based on 50 or 60 Hz, the addition of harmonic losses lead to additional temperature rise in the equipment which can (de-rate) or limit the ability of the equipment to work at the rated conditions for its specified lifetime. To clarify these harmonic losses, the following example is presented. Harmonics losses can be considered as the difference in heat dissipation between two parallel loads of the same kw rating, one linear and the other nonlinear, when they are fed from the same source [3]. As shown in Fig. 1, (I h ) is the harmonic current that produces the additional losses. There is a linear load at the left of the figure and a nonlinear load at the right. The nonlinear load is assumed to be a VFD. As explained by the theory of operation of the VFD, it distorts the source side current waveform because it draws current in portions during its operation. The decomposition of the current waveform using Fourier analysis provides the spectrum of the harmonic currents that can be used to calculate the contribution of every harmonic component to the total system losses. (1) Fig. 1 Harmonic current flow and relevant losses in electrical network This example is intended to illustrate in a simple way how the harmonic losses take place in the presence of two similarly sized but different types of loads. If a shunt passive harmonic filter (F h ) exists it will absorb the harmonic current, for which it is tuned, from the nonlinear load and accordingly the upstream network will not be subjected to these additional harmonic losses, accordingly a noticeable electrical energy saving is achieved in the electrical distribution networks. 2

3 III. HARMONICS EFFECTS ON VARIOUS ELECTRICAL EQUIPMENT A. Harmonics Effects on Generators: Power generators are basically designed to feed linear loads. However, when the majority of the plant loads are nonlinear, the generators should comply with certain requirements that allow them to operate continuously without being exposed to overheating, torsional torques and excessive vibrations [4]. In the presence of nonlinear loads, harmonic voltage distortions will be present at the generator terminals which cause the following consequences on the generator s operation: Production of positive and negative sequence currents cause torsional torques and vibrations on the generator shaft. These torsional torques and vibrations can wear-out the shaft bearings and cause a premature mechanical failure to the generator The presence of excessive negative sequence currents leads to increased generator voltage unbalance High frequency harmonic currents (> 2.5 KHz) may cause malfunctions to the generator protection relays and automatic voltage regulator (AVR) systems B. Harmonics Effects on Transformers: Transformers are designed to deliver the required power to the connected loads with minimum losses at the fundamental frequency. Harmonic distortions will significantly cause excessive heating losses and accordingly a considerable de-rating to the transformer [5]. There are three main effects that cause increased transformer heating when the load current includes harmonic components these effects are summarized as follow: Increased RMS current, usually the transformer is sized only for the KVA requirements of the loads, but when harmonics are present, the RMS current may be higher than the transformer current rating. This increase in the RMS current results in increased winding losses. Additional eddy current losses, the eddy currents are induced currents caused by the flow of the magnetic fluxes in the transformer core. These eddy currents losses causes additional transformer losses called (eddy losses), these losses increase with the square of the frequency. Therefore, these losses increase considerably in a harmonic polluted network. C. Harmonics Effects on Induction Motors: Rotating machines like induction motors can be significantly impacted by the power system harmonics. Harmonic voltage distortion at the motor terminals is translated into harmonic fluxes inside the motor. These fluxes do not contribute significantly to the motor output torque, but rotate at a frequency different from the rotor synchronous frequency. These harmonic fluxes induce high frequency currents in the rotor which lead to various problems for the induction motors [6]. These harmonic problems to the rotating machines can be summarized as follow: Overheating for the motor, if the incoming supply voltage to the motor contains excessive harmonic distortions, the performance of the motor will be significantly impacted. The effect of the distorted supply on motors has been studied before in [1]. In Ref. [7], laboratory tests have been carried out to examine the impact of harmonic distortions on the motor heating. The results of these tests are summarized in Fig. 2 which shows the temperature rise on the enclosure of an 11 kw three-phase induction motor fed from different supply sources, a quasi-square wave (QSW) inverter and a pulse-width modulated (PWM) inverter. These two different supplies were used along with a standard induction motor. The influence of the supply distortion on motor heating at full load conditions can be seen by comparing curves (c) and (d). With the higher level of distortion of the quasi-square wave supply, the motor enclosure temperature settles down at 59 C which is around 10 C higher than the settling temperature of the motor when it is fed from a PWM inverter. Fig. 2 Effect of distorted waveform supplies on motor enclosure temperature 3

4 From Fig. 2, it is concluded that the more harmonic distortion in the supply networks the more temperature rise in the motor. Oscillating torque problems, the flow of the fundamental current in the motor induces fundamental magneto-motive force (MMF) which produces the steady state motor output torque. In the presence of harmonics, the positive and negative sequence harmonic currents produce oscillating (MMF) and accordingly oscillating torques on the motor shaft. As shown on Fig. 3, the interaction between the steady motor torque and the oscillating torque leads to: Shaft early cracks and failures Increased motor vibrations Increased noise Fig. 3 The harmonic currents in the rotor and relevant oscillating torque opposing the normal steady motor torque Harmonic effects on explosion-proof motors, a very important application of the AC induction motors in the petrochemical plants is the explosion-proof motors (or sometimes called flame-proof motors) which are induction motors suitable to be installed in hazardous environments. These types of motors are designed on the principle that no matter what happens inside the flame-proof enclosure (even an internal explosion), no spark can be transmitted to the surrounding hazardous area. In oil and gas applications, the explosion-proof electrical equipment are mandatory because if any spark occurred in a hazardous area, it will lead to a great disaster [4, 7, 8]. This special motor design relies on the flame-proof enclosure and the shaft mounted flame-proof seals to contain any internal explosion. However, in the presence of harmonics, the rotor may be overheated and subjected to excessive vibrations. These harmful effects lead to premature deterioration to the flame-proof seals, and accordingly any internal spark may be transmitted to the external hazardous environment. Explosion-proof electrical equipment are also defined with a term called T class which is the maximum allowable enclosure surface temperature. This temperature must be lower than the ignition temperature of the gas present in the hazardous area environment in which the equipment is installed. Depending on the magnitude of the voltage distortion, the rotor temperature may exceed the motor's surface temperature class T class (for example, 200 C for T3) which can be very dangerous in the petrochemical plants. The design, testing and certification of the explosion proof motors are performed based on pure sinusoidal supplies. According to IEC [9], explosion-proof categories Ex-d (flameproof), Ex-e (increased safety) and Ex-p (pressurized) are permitted and certified only to operate on 2% voltage distortion supplies otherwise they are described as operating outside the conditions settled when they were certified and accordingly, their certifications are no longer valid. The conclusion is that where an explosion-proof motor is driven by a VFD or subjected to excessive harmonic distortions, it must be suitably designed to operate continuously under these conditions without being overheated. D. Harmonics Effects on Capacitor Banks: Capacitor banks for power factor correction applications may be overstressed and even damaged due to excessive harmonic distortions. The effects of harmonic problems on the capacitor banks can be summarized as follow: Overstressing on the capacitor banks, overvoltage due to the voltage harmonic distortions can overstress and shorten the lifetime of capacitor banks. Voltage, temperature and current stresses are the main problems that lead to dielectric breakdown of the capacitor banks' insulations. The output reactive power from a capacitor bank varies with the square of the voltage, as described by the following equation: V 2 Qc X (3) c Where, Q c : Reactive power injected by a capacitor bank V: System voltage X c : Capacitive reactance of the capacitor bank 4

5 From equation (3), a 5% increase in the nominal voltage of a capacitor would cause it to deliver (1.05) 2 = 1.1 or 110% of its rated reactive power. This overloading is a serious condition and may lead to capacitor failure. The IEEE standard [10] states the allowable operating limits for the capacitor banks. These limits are summarized in Table 1. These limits should be taken into consideration in the design of the capacitor banks and harmonic filters. TABLE 1 ALLOWABLE OPERATING LIMITS FOR SHUNT CAPACITOR BANKS Capacitor parameter Maximum allowable operation limit (% of the nominal ratings) Reactive power 135 % Peak voltage 120 % RMS current 135 % RMS voltage 110% Resonance, resonance is a condition at which the capacitive reactance of a capacitor bank equals the inductive reactance of the distribution system at a specified frequency. There are two types of resonance, parallel and series resonance. In series resonance the total impedance at the resonant frequency is reduced exclusively to the resistive circuit component and if this component is small, very large values of current at the resonant frequency will be developed. In the case of parallel resonance, the total impedance at the resonant frequency is very large (theoretically tending to infinity). This condition may produce large over voltages among the system elements. Therefore, resonance may cause severe problems for the capacitor bank and the other power system elements as well. Capacitor banks' fuses failures, as mentioned earlier in section (II), the RMS currents may increase under harmonic distortions, this can cause undesired malfunction of the fuses that protect the capacitor banks. Capacitor banks can be more stressed under the failure of a fuse on one of the phases, which leaves the remaining units connected across the other phases. Thus, they are subjected to an unbalanced voltage condition that can produce additional overvoltage on the capacitors. E. Harmonics Effects on Switchgears: Low voltage and medium voltage switchgears may suffer from the harmonic problems in various ways including [5]: Increasing the heating losses inside the switchgear Reducing the steady-state current carrying capacity of the bus bars Shortening the life of some insulating components Malfunction and nuisance operation of protective relays Interference to the communication systems inside the switchgears F. Harmonics Effects on Power Cables: If power cables are subjected to excessive levels of harmonic currents, it will suffer from various problems such as [5]: Overheating of the conductors due to the increase of the RMS currents Increasing the apparent resistance of the conductors with the increase in frequency due to the skin effect. As a result, the effective alternating current resistance (R AC ) is raised above the direct current resistance (R DC ), especially for larger conductors. Thus, the power losses of the conductors increase Cables involved in system resonances may be subjected to voltage stresses which may lead to insulation failures G. Harmonics Effects on Transmission Lines: Harmonic currents increase the losses in the transmission system due to skin effect losses. Thus, the transmission efficiency decreases. Also excessive harmonic distortions can lead to the failure of the single phase auto reclosure system, or it can't automatically reclose at the predefined closing time. 5

6 H. Harmonics Effects on Telecommunication Systems: Telecommunication systems may suffer from the harmonic problems in various ways including [4, 5]: Telephone interference, the high frequency harmonic currents (> 2.5 KHZ) induce harmonic currents in the communication circuits that are running parallel to the power circuits. When these induced currents flow in the communication circuits they produce noise and interfere with the communication signals. To avoid these problems caused by harmonics, radio frequency interference (RFI) filters and shielded cables may be used to minimize the effects of the harmonic problems. Commutation notching effects, commutation notches cause harmful effects on the communication systems. Notching is simply an overlap occurs during the operation of the power electronic switches when two phases are short circuited. This notching condition can produce high frequency harmonics that cause excessive noise to the communication circuits. Good practice is to isolate communication circuits from power circuits. I. Harmonics Effects on Sensitive Electronic Equipment: Sensitive electronic devices such as electronic measurement devices and special medical instruments may suffer from the harmonic problems in various ways including [4, 5]: Some electrical measuring devices that are dependent on the accurate determination of the voltage zero crossings or other aspects of the voltage waveform are subjected to malfunctions and erratic operations in the presence of harmonic distortion because the harmonic distortions result in shifting the voltage zero crossing and distorting the sinusoidal waveform of the supply voltage. Computers and programmable controllers usually require to be supplied from AC sources that have less than 5% voltage harmonic distortion. Higher levels of harmonic distortions result in erratic and malfunction of these equipment, which can have serious consequences according to the application of these equipment. Perhaps the most serious problems are the malfunctions of the medical instruments that may threaten the patients lives. Accordingly, most of the medical instruments are provided with built-in harmonic filters. IV. ECONOMIC ASPECTS FOR THE HARMONIC MITIGATION TECHNIQUES To avoid the harmful effects of the power system harmonics detailed in section (III), a harmonic mitigation technique should be installed in the electrical network to mitigate these harmonics. Nowadays, there are various harmonic mitigation techniques in the market each with specific technical advantages and disadvantages. The selection of the optimum technique is a compromise between the technical and economic aspects. Thus, the optimum technique is the one that offers the best cost \ benefit ratio [11]. The optimum economic mitigation solution is the technique that satisfy the following requirements: Reduce the project capital expenses (CAPEX) Reduce the project Operational expenses (OPEX) by reducing the electrical energy consumed and minimizing the equipment failures and relevant maintenance costs. Satisfy the technical aspects required by the design engineers and consultants Satisfy the plant owner requirements Due to the relatively high cost of the harmonic mitigation techniques, it may be a difficult task to convince the project consultants and plant owners to install a harmonic mitigation technique. Thus the sales engineers of the harmonic mitigation products' manufacturers must support their offers to the consultants and plant owners with the following: Highlight the possible harmonic problems that shall be avoided if the appropriate harmonic solution is installed Highlight the energy savings achieved after reducing the harmonic currents within a plant Highlight the enhancement in the system reliability and supply continuity that shall be achieved if the harmonic solution is installed Highlight the payback period of the installed harmonic solution The following section summarizes some useful economic aspects which are needed to select the optimum economic harmonic mitigation technique. 6

7 1. Reduction of the Capital Expenses (CAPEX) and relevant Energy Savings: Reducing the equipment initial cost or (CAPEX) without affecting the required technical specifications is the main concern of the design engineers. In addition to reducing the initial cost of the equipment, there are other factors that may affect the CAPEX such as footprint saving (space inside substation buildings). The following simple example in Table 2 shows how the harmonics mitigation reduces the RMS currents and accordingly reduces the size of cables, the rating of circuit breakers and contactors. TABLE 2 EFFECT OF THE HARMONICS MITIGATION ON CAPEX SAVING AND RELEVANT ENERGY SAVINGS Equipment Without mitigation With mitigation CAPEX saving VFD input current 60 A 38 A - Transformer rating (upstream transformer to many VFD s) 800 KVA 630 KVA 11 % Cables 35 mm 2 16 mm 2 43 % Circuit breaker 80 A 50 A 9 % Contactors 80 A 50 A 20 % Total CAPEX saving 15 % From Table 2 it is concluded that mitigating the power system harmonics leads to reducing the project initial cost due to the elimination of the harmonic currents in addition to saving the electrical energy supplied to the plant. 2. Reduction of the Operating Expenses (OPEX) Harmonic mitigation generally contributes to reducing the losses in transformers, motors, cables etc. In addition, harmonic mitigation leads to reducing the annual consumed power by the plant thus reducing the energy bill. In most of the cases, energy savings achieved as a result of harmonic mitigation could reach up to 10% of the electricity bill. In addition, power system harmonics mitigation leads to minimizing the equipment failures and accordingly save the relevant maintenance costs. 3. Improving the Business Performance and Reducing of the Plant Downtimes: The power system reliability and supply continuity are very important and critical factors in the petrochemical applications because any short shutdown causes severe economic losses. A recent study in the U.S.A. showed that the power quality problems (especially the harmonic distortion problems) cost the U.S. industrial business more than 15 billion dollar per year. Accordingly the power system harmonics should be mitigated within the critical industrial applications to minimize the unwanted process interruptions and plant shutdowns [11]. Table 3 shows the criticality of the supply continuity in the industrial applications by highlighting the financial losses due to undesired plant shutdowns for various industrial applications per each shutdown event. TABLE 3 FINANCIAL LOSSES DUE TO UNWANTED PLANT SHUTDOWNS Industry Financial losses per event Semiconductor ''wafer processing'' 3,800,000 Financial company 6,000,000 per hour Internet an data center 750,000 Telecommunication network company 30,000 per minute Steelworks plant 350,000 Glass industry 250,000 7

8 V. ECONOMIC MARKET SURVEY FOR THE HARMONIC MITIGATION TECHNIQUES An extensive economic market survey for the available harmonic mitigation techniques has been performed to enable the design engineers, consultants and plant owners from selecting the optimum economic mitigation technique. The results of this market survey are summarized in the following section. 1. Economic Comparison between the Various VFD Configurations: Selecting the optimum VFD is a difficult task for the design engineers because the lower the harmonic distortion caused by the VFD the higher the VFD cost. In Table 4 and Fig. 4, an economic comparison is performed between the various types of the VFD systems in the market for the same load rating. The costs of the various VFD types are relative to the cost of the 6 pulse VFD without reactor [12]. TABLE 4 ECONOMIC COMPARISON BETWEEN THE VARIOUS VFD CONFIGURATIONS VFD configuration CAPEX cost 6-pulse VFD without AC line reactor 100 % 6-pulse VFD with AC line reactor 120 % 12-pulse VFD with Zig-Zag transformer 200 % 12-pulse VFD with double secondary transformer 210 % 24-pulse VFD with two, 3-winding transformers 250 % to 300% Fig. 4 Economic comparison between the various VFD configurations There are other factors that should also be taken into consideration when comparing various types of the VFD systems and their harmonics emissions which are: Complexity of installation Complexity of maintenance and troubleshooting Drive dimensions and physical space (foot-print) requirements Market availability for the various power (KW) ratings. Fig. 5 represents a comparison between the various VFD configurations. The horizontal axis represents the complexity of installation and maintenance while the vertical axis represents the injected current harmonic distortions of each VFD. 8

9 Fig. 5 Comparison between the various VFD types from the complexity of installation and maintenance point of view From Fig. 5, it is concluded that the lower the harmonic currents emitted from a VFD the more complex and difficult installation of the VFD. Accordingly the design engineers and consultants should consider all these aspects together to be able to select the optimum VFD within a plant. 2. Economic Comparison between the Various Harmonic Mitigation Techniques: This section provides an economic market survey for most of the harmonic mitigation techniques available in the market. In Table 5 and Fig. 6, the results of this market survey are summarized. TABLE 5 ECONOMIC COMPARISON BETWEEN VARIOUS HARMONIC MITIGATION TECHNIQUES Harmonic mitigation technique Cost ($ /KVA) AC line reactor 3 LV capacitor bank (fixed, uncontrollable) 12 K-rated transformer 20 LV capacitor bank (switched in steps) 25 LV passive filter 35 Active harmonic filter 150 Fig. 6 Economic comparison between the various harmonic mitigation techniques From Fig. 6, it is obvious that the active harmonic filter is the most expensive mitigation technique in the market. 9

10 VI. CONCLUSION Mitigating the power system harmonics represents a great importance in the petrochemical plants in order to increase the power system reliability, enhance operation economics and avoid unwanted equipment failures and process downtimes. There are at least ten harmonic mitigation techniques, each having many pros and cons. Previous articles focused on the technical aspects of the harmonic mitigation techniques. This paper focuses on the economic impacts of the power system harmonics and the electrical energy savings achieved through harmonics mitigation in the petrochemical sector and it provides comprehensive economic comparisons between most of the harmonics mitigation techniques supported by an extensive market survey. Accordingly this paper is considered a helpful guide that enables the design engineers, consultants and plant owners of selecting the optimum economic harmonic mitigating technique for their plant. REFERENCES [1] Mohamed Zaki El-Sadek, Power System Harmonics, 2 nd edition, 2006, Mukhtar Press, Egypt [2] Mohamed Zaki El-Sadek, Power System Harmonic Filters, 1 st edition, 2007, Mukhtar Press, Egypt [3] Francisco C. De La Rosa, Harmonics and Power Systems, 1 st edition, 2006, CRC press [4] Roger C. Dugan, Electrical Power System Quality, 2 nd edition, 2004, Mcgraw-hill. [5] IEEE, Recommended Practice for Industrial and Commercial Power Systems, ANSI/ IEEE , Chapter 10, PP [6] A.Y. Abdelaziz, S.F. Mekhamer, Sherif M. Ismael, Sources and Mitigation of Harmonics in Industrial Electrical Power Systems: State of the Art, the Online Journal on Power and Energy Engineering (OJPEE), Cairo, Egypt, October 2012, Vol. 3, Issue 4, PP [7] R. Yacamini, '' Power System Harmonics, Part - 3, Problems Caused by Distorted Supplies'', IEEE, Power Engineering Journal, 1995, PP [8] S. Kanerva, et al., Motor Design Considerations for Medium Voltage Adjustable Speed Drive Systems in Hazardous Areas, IEEE Petroleum and Chemical Industry Committee, PCIC , Dec [9] IEC, Rotating Electrical Machines Part 1: Rating and Performance, IEC , 2010 [10] IEEE Standard for Shunt Power Capacitors, IEEE [11] Schneider Electric, Harmonics Solution Handbook, 1 st edition, June 2009 [12] ABB, VFD Technical Guide no.6 - Guide to Harmonics with AC Drives, Revision B,