PWM DRIVE OVERVOLTAGE TRIPS IN ELECTROCHEMICAL PLANTS

Transcription

1 PWM DRIVE OVERVOLTAGE TRIPS IN ELECTROCHEMICAL PLANTS Copyright Material IEEE Paper No. PCIC Paul Buddingh, P.Eng. MEMBER, IEEE ANDRITZ AUTOMATION Ltd International Place, Suite 100 Richmond, BC V6V 2X8, Canada Jack St. Mars, A.Sc.T. MEMBER, IEEE CANEXUS Chemicals Canada Inc. 100 Amherst Avenue North Vancouver, BC V7H 1S4, Canada Abstract Case study application paper for a recurring problem the authors have recently experienced in electrochemical plants using low voltage PWM ASD's. Low voltage ASD's are tripping on 1.3 per unit voltage switching transients lasting 1-3 cycles. The ubiquitous ASD has been so successful that it is now used on critical plant processes that cannot tolerate outages. Most power system components are built to operate with a significant level of transient voltage resilience. This does not appear be the case for some of the low voltage ASD's in use today. This paper examines this issue and provides a specifying methodology for new ASD's, field proven cost effective corrective measures for installed ASD's, and a case study. Index Terms ASD, overvoltage trip, transient voltage, switching transient, capacitor bank, harmonic filter. PWM, VFD I. INTRODUCTION Due to the inductive and capacitive nature of power systems, transient voltages are a normal part of any power system. Electrochemical plants typically utilize large multistage passive LC harmonic filters to compensate for the reactive power required and correct the harmonic current produced by high power rectifier systems. These multi-stage filters are typically switched on stage by stage as the rectifier load increases. The large capacitance of the filters interacts with the supply system inductance to produce short term low level voltage disturbances in the pu voltage and 1-3 cycle range. A contributor to this problem is the filter size ratio to rectifier load and the relative fault levels of the power system. These factors can produce a larger transient voltage than the 1.2 per unit overvoltage limit of some ASD's. The large size of the filters also precludes a transient suppression solution using pre-insertion resistors for economic reasons. In addition, Metal Oxide Varistors (MOVs) are not normally useful at these low per unit voltages. With the increasing use of capacitors and harmonic filters in power systems, it is prudent to consider these effects when applying and specifying ASD s found not only in electrochemical plants but in power systems in general. II. TRANSIENT NATURE OF POWER SYSTEMS Transient overvoltages are due to natural and inherent characteristics of power systems [1]. In general, there are two types of overvoltages: lightning generated overvoltages and switching overvoltages. This paper will address the switching overvoltages caused by capacitor switching as elements in harmonic filters. When an electrical circuit only has resistance (R) which dissipates energy, there are no transient overvoltages. Power systems include energy storage elements capacitance (C) and inductance (L), which create transient overvoltages. Sudden switching of an energy storage element cannot change the voltage and current relationship instantaneously, as in a resistance only (energy dissipative) element. The time between the initial switching of an energy storage element and its final value, after some time, is referred to as its transient nature. When combined in a practical power system, some level of oscillation and overvoltages occur when switching, R L and C elements. Normally, by design, power system equipment can tolerate a moderate amount of temporary overvoltage. The implementation of electrical insulation standards are one method of meeting this need. Surge arresters and other methods are used to protect equipment from excessive equipment damaging overvoltages in the > 2 pu range. This paper addresses temporary overvoltages in the lower pu range. III. THEORY Without any resistance to damp the circuit, the maximum instantaneous capacitor voltage will be two times the crest value of the line to neutral voltage [1]. Due to the effects of resistance in a practical circuit, this will be slightly less. These are based on a clean system free of pre-ignitions or restrikes or existing switching events. Under these extreme conditions, overvoltages in excess of 2 pu are possible. Reference [2] provides a useful and practical guide for further information. IV. TYPICAL ELECTROCHEMICAL POWER SYSTEM Typical electrochemical plants are supplied by high voltage, high capacity transmission lines, designed as stiff high fault level systems to tolerate the demands of electrochemical loads. A power transformer is used to step down this voltage to the 11 to 25 kv range. Large rectifiers of MW power the electrochemical process. To supply reactive power to the rectifiers and correct for the harmonic current produced, large multi-stage harmonic filters are used. These filters are typically in the 3-7 Mvar range with 5 th, 7 th, 11 th and 13 th harmonic elements. 1

2 V. CASE STUDY Fig. 1 Single-Line Drawing An electrochemical plant circuit utilizing rectifiers and a 5 th and 7 th harmonic filter supplied from a 12 kv circuit is the case study presented here. Upgrades to a critical plant process included a 300 hp ASD. Harmonic filter switching occurs once the rectifier reaches 20% load to maintain stability. The owner has successfully utilized this method at several similar plants. However, in this particular case, the ASD would consistently trip with a DC bus overvoltage alarm during filter energization. By connecting a high-speed data recorder to this ASD, transient overvoltage readings were collected. Measurements were taken at the primary of the 3% input reactor. The recorder triggered on overvoltage events and captured 12 electrical cycles of data (approximately 200 ms). On the ASD DC bus, a recording oscilloscope was used. Energizing the filter resulted in the increase of the AC steady state voltage at the reactor terminals by 4% in accordance with the design. The steady state DC bus voltage also increased proportionally by 4%, as expected. To collect the required transient data, a series of switching tests were performed. The high-speed recorder revealed peak voltages of 1.3 pu. Decay of the transient voltages to normal (<110% of nominal voltage) consistently occurred in less than 3 cycles. Fig. 2 Pre Transient and Transient Data Capture 2

3 VIII. POWER SYSTEM SOLUTION A technically effective method to deal with low level transient causing ASD tripping is to incorporate pre-insertion resistors or reactors to mitigate capacitor-induced overvoltages. An auxiliary switching device temporarily connects the resistors to the power system only during the switching transient phase. The resistive element damps the overvoltage. Due to the high cost of this solution, it is normally used only on very small equipment installations or in large utility banks that have significant pre-ignition, restrike, back to back, or other serious switching problems and serious overvoltages greater than 2.0 pu. IX. ASD SOLUTION Fig. 3 Transient and Post Transient Data Capture VI. ANALYSIS The recorded waveforms show a predictable energization of a single capacitor bank (harmonic filter), with upstream system inductance consistent with the reference literature [2]. The low impedance of the uncharged capacitor results in an immediate dip in one of the phase voltages. After the initial dip in voltage, the phase voltage recovers with temporary oscillations. The frequency of the oscillations is primarily a function of the source inductance and filter capacitance. Fn = 1/(2π LC) in Hz = Xc/XL = (MVAsc/Mvar) [1] Damping of the oscillation is a result of system resistance and the surge impedance of the downstream system. The voltage drops, transitory oscillations occur, and the waveform returns to nominal steady state pre-switching voltage after 3 cycles. VII. STANDARDIZED WORLD ASD S The drive used in this case study is a global 400-volt ASD developed for 380 to 480-volt systems. The case study system voltage is 480 volts. The ASD has a fixed DC bus trip voltage of 825 Vdc designed to protect the ASD electronics. The normal steady state DC bus voltage is 1.4 times the applied AC rms voltage. 825/(400*1.4) = 1.47% overvoltage tolerance (1) When the system voltage is less than 400 volts, the overvoltage capability is >1.5 pu. If this same drive is used for 480-volt service, the maximum overvoltage drops to 1.2 pu of the system voltage. Note that in most cases, a 1.5 pu overvoltage capability will provide sufficient headroom to eliminate the overvoltage trip problem. The most frequently recommended hardware solution is to increase the impedance of the ASD input reactor, thereby reducing the incoming voltage transient. Given that the inductive impedance of the reactor increases with frequency, it is well suited to mitigating the higher frequencies of the voltage transient. Modeling of the drive connected to the power system demonstrates the importance of the size of the DC bus capacitors. Higher capacitance will reduce voltage rise on the ASD DC bus. Installing a regenerative resistor connected to the DC bus prior to start-up can also reduce overvoltage. Another solution used by the authors is to use an ASD with a nominal voltage of the next class, for instance, a 600-volt drive on a 480-volt system. With this approach, consideration must be given to the lower current vs. horsepower rating of the higher voltage ASD to ensure that the motor load and power requirements can be accommodated. Also required is careful consultation with the vendor to ensure that control voltages, undervoltage protection setting, software parameters and the like are consistent with the lower operating voltage. Where the process load can tolerate momentary deenergizing, a software solution is also an option. Upon sensing an overvoltage and tripping, the ASD shortly thereafter restarts, synchronizes with, and reconnects to, the spinning motor, without upset. With many types of loads, this is a useful and effective solution. X. SPECIFICATIONS The authors have used and recommend the voltage tolerance specification provided in Table 1, which is based on the Computer Business Equipment Manufacturers Association (CBEMA) curve and is addressed in ANSI/IEEE std This voltage envelope is intended for electronic computer equipment connected to AC power systems. It is reasonable to expect a robust industrial duty ASD to be capable of meeting the same voltage envelope criteria. 3

4 TABLE 1 VOLTAGE ENVELOPE SPECIFICATION Steady State RMS voltage ±10% nominal system rms tolerance voltage Line Voltage Swell +120% 0.5 s rms Low frequency Ring wave +130% peak ms (1000 Hz<Vpeak> 200 Hz +150% peak 8.3 ms High Frequency Impulse 200% peak ms (80 joule minimum transient immunity) Voltage Sag 80% nominal rms 10 s 70% nominal rms 0.5 s XI. SOLUTION USED Increasing the reactor impedance will not lower the transient voltage sufficiently on this drive. Increasing the bus capacitance is not an option. A regenerative brake would not fit the existing drive footprint. As a result, the project team obtained proposals for three 600 nominal voltage ASD s using the voltage envelope specification listed above. Proposal 1: 400 hp 690-volt 335-ampere ASD. Key parameters of DC bus trip voltage, reactor size and capacitor size were not available. Proposal 2: 600-volt chassis 402-ampere ASD. The DC bus trip voltage was 1,035 Vdc (bench tested), had a 5% reactor and 22,000 uf of capacitance. Proposal 3: 600/690-volt chassis. 416-ampere ASD. This ASD has a 1,200 Vdc bus trip voltage, a 6.8% impedance reactor and 7,500 uf of capacitance. XII. SELECTING THE REPLACEMENT ASD The selection criteria included DC bus trip voltage, ability to operate with minimal changes on the 480 V system, reactor impedance, and DC bus capacitor size. To establish suitability, modeling the response of the ASD to voltage transients requires input reactor and capacitor size information. This eliminated Proposal 1 from consideration. Proposal 2, with an actual bench tested DC trip value of 1,035 Vdc, provided a measure of confidence. The large 22,000 uf capacitance used and 5% reactor limited the overvoltage to 930 Vdc or 1.3 pu during simulations with a 105% voltage input. This provided a 10% margin to the trip voltage. Proposal 3 had a much higher DC bus trip voltage of 1,200 Vdc. However, the smaller capacitors 1/3 the size of Proposal 2 resulted in a higher DC bus transient voltage of 1,050 Vdc with a 5% reactor. The input reactor also has a 6.8% impedance tap. With the higher impedance reactor, the overvoltage is less than 950 volts. This provides a margin of 20%. Proposal 3 also provides the option of adding capacitance that will further reduce the overvoltage and provide an additional 5% margin. Fig. 4 Transient DC Bus Voltage with Option 3 Drive XIII. CONCLUSIONS Proposal 3 met the criteria of the voltage envelope requirements with a generous safety margin. It offers the option of adding capacitance as a contingency if system parameters change and transient overvoltages increase slightly. It is expected to operate without upset during harmonic filter switching and result in an effective solution to the problem. The electronic PWM ASD has evolved from a specialized device that required the knowledge of experts to apply, to the commonplace devices of today used with little fanfare. Part of this evolution is the standardization across world markets that has driven down costs and made these machines universally available. Nevertheless, selecting and applying ASD s require particular knowledge of the connecting power system. This paper demonstrates that not all ASD s are created equal. Suppliers must be informed of the circumstances and details of the system installation in order to supply the appropriate equipment. Although this concern is certainly not unique to the electrochemical industry, it demonstrates why experience and knowledge are key when applying equipment, particularly power electronics to electrochemical power systems. Adequate engineering is necessary to provide a suitable application of ASD s to real world power systems. 4

5 XIV. REFERENCES [1] Red Book IEEE standard 141 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, [2] Allan Greenwood, Electrical Transients in Power Systems, 2 nd Ed., Wiley, [3] J, Aidala & L, Katz, Transients in Electric Circuits, Prentice Hall, [4] R. B. Standler, Protection of Electronic Circuits from Overvoltages, Wiley, [5] R.P Oleary & R.H. Harner, Evaluation of methods for controlling the overvoltages produced by the energization of a shunt capacitor bank, CIGRE Int l conference on large high voltage electric systems, Aug-Sept [6] J.C. Das, Analysis & Control of Large-Shunt-Capacitor- Bank Switching Transients, IEEE Transactions Nov-Dec [7] P.C. Buddingh, Even harmonic resonance-an unusual problem, IEEE Transactions on Industry Applications, Volume 39, Issue 4, pp , July-Aug [8] J.C. Read, "The Calculation of Rectifier and Inverter Performance Characteristics," Proceedings of the leee, Vol. 92, Part 2, No. 29, pp , October [9] IEEE IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. XV. VITA Paul Buddingh, P.Eng. is a consulting engineer with more than 20 years experience designing specialized power systems for advanced technology equipment. Paul is a graduate of Lakehead University in Thunder Bay, Ontario, Canada where he earned a degree in Electrical Engineering. Upon graduation, Paul spent several years in Toronto, ON, Canada as a consulting engineer working in heavy industry. He later co-founded a company that successfully developed a novel magnetic approach to solving zero sequence harmonic problems. Paul later moved to Vancouver, BC Canada and joined Universal Dynamics (now ANDRITZ AUTOMATION Ltd.), where his present role is Principal Consultant, Engineering. Paul s technical work centers on designing high reliability power systems for difficult loads, as well as power converter issues, alternative energy sources and resolving complex power system problems and energy issues internationally. Paul is a registered Engineer in the provinces of Ontario, Manitoba and British Columbia, and the author of several IEEE papers. He is past Chair of the electrochemical subcommittee of the IEEE PCIC and is active on several IEEE Standards working groups. ( pbuddingh@ieee.org) Jack St. Mars (M 88) is a native of Vancouver, BC, Canada. Jack received his Diploma of Technology in Electrical Power Engineering from the British Columbia Institute of Technology (BCIT), Burnaby, BC, Canada in Following graduation, Jack worked for the period of one year in electrical consulting on commercial building electrical system design. In 1972, he began his extensive career in the electrochemical industry in the Plant Engineering Department of Hooker Chemicals Plant, now Canexus Chemicals Canada Inc. in North Vancouver, BC, Canada. During his 39 years with the company, Jack s focus has been on the testing, maintenance and upgrading of high voltage, power distribution and specialty rectifier and harmonic filter systems. Currently, Jack works in a corporate role where he continues to provide electrical engineering support for the power systems at his employer s six electrochemical plants. 5