Solving Power Quality Issues from Large Motor Starts usingFast-Acting Capacitor Bank AssistanceJames (Toby) Landes, P.E.Valquest Systems, Inc.401 S. Sherman Ste. 303Richardson, TX 75081, [email protected] – Starting a large motor, even with modern softstart equipment, is frequently problematic. With ever increasing focus on power quality, an electric utility can experience voltage sag problems when an industrial customer installs a large motor (400 - 2500 HP). Other customers can beadversely affected as well as the company with the motor(s). Of course, the utility is generally blamed by both.Solving these problems is not trivial. Capacitor assistancecan be a viable and relatively inexpensive solution but it mustbe done correctly. This paper discusses the benefits (both forcustomers and for the utility), techniques, problems, relativecosts and savings, alternatives, and analysis of capacitor assisted motor starting. Examples of actual installations andcase studies will be shown.Index Terms – Motor start, capacitor assistance, soft start,voltage sag, voltage fluctuation, voltage surge, power quality,zero voltage closing.I.II. INTRODUCTIONThe need to start large motors is a fact of life. Theiroperation enriches our lives in countless ways. In order forthem to operate, of course, they have to start. But startinga large motor is frequently problematic. This is because,all too often, the distribution system is sized adequately forrunning the large motors but not for starting them. Problem severity can range from minor irritations to completedisruption of normal operation.This paper will examine the problems and their causes.It will look at some of the viable solutions with their relative costs, advantages and disadvantages. It will also showtechniques for analyzing and applying capacitor assistanceto motor starts and give some case studies to illustratethese techniques.NOMENCLATUREIII.Large motor – For the purposes of this paper a large motor is any induction ac machine which causes a drop involtage at its point of delivery of more than 6% duringstartup.Motor startup – The time, usually between 10 and 40seconds, required to bring the motor from 0 to full speed.Soft-Starter – A device that reduces and in some casesvaries voltage applied to the motor during startup.VFD (Variable Frequency Drive) – A device for controlling a running motor’s speed.PLC (Programmable Logic Controller) – A user programmable control device with a variety of inputs, outputsand timers. It is usually programmed using Ladder Logic.IED (Intelligent Electronic Device) – Sometimes called aRelay, it is a user programmable control device with relatively specific functionality. Like the PLC, it has inputs,outputs and timers. It is usually programmed by addingand modifying logic equations. For the purposes of clarityand brevity this paper will refer to it as a PLC. When PLCis used, it generally will mean that a PLC or an IED orsome other logic device may be used.ZVC (Zero Voltage Closing) – A technique employed insome capacitor controllers that causes the capacitorswitches to close precisely as the voltage waveform oneach phase is passing through the zero voltage point.PROBLEMSLarge motors frequently cause problems at startup for theowner, for other electric utility customers, and for the electric utility itself. Most of these are a direct result of thehigh current draw of the starting motor but a few are secondary in nature.A. Problems for the Motor Owner/OperatorMany times the first indications of startup problems arenoticed by the organization that has the motor. They caninclude: Perceptible annoying, voltage sags that occurwhen current is sufficient to cause significantvoltage drop between the source and the load. Interference with other equipment such as VFDs,electronic controls, etc. that can happen with devices that are sensitive to supply voltage. Long startups, repeated startups, and inability toachieve start. These are all due to excessive voltage sag. They are indications that the line is notsized properly for starting the installed motor. Motor overheating – usually the result of repeatedattempts to achieve start.
Lost production – naturally occurring when motorstart is delayed or not achievable.B. Problems for other Utility CustomersThird party customers, unlike the utility and the motorowner have no vested interest in whether the motor runs ornot. They just want good power quality. These problemsare complaint generators. Perceptible voltage sags – very annoying, particularly to residential customers. This is by far themost common complaint. Resetting of devices with clocks – can occur if thesag and or dv/dt is sufficient to interfere with sensitive equipment. Damage to electronic appliances – can occur ifstarting assistance capacitors remain energized after motor comes up to speed because of the resulting voltage surge.C. Problems for the Electric UtilityMost of the utility’s problems are political and financial in nature. Having to deal with customer’s complaints – bydefinition a problem. A large motor owner/operator that is difficult towork with – one of the most difficult issues. Loss of revenue that occurs if the motor cannot bestarted. Potential loss of customers.IV.CAUSESBefore addressing these issues and finding solutions,the causes need to be quantified and understood. The highcurrent draw from the motor start is due to the nature ofthe load placed on the line as the motor begins to turn andcomes up to speed. They can be placed into three majorcategories: Annoyance phenomena, difficulty starting theprimary machine(s), and electrical interference with otherequipment.A. Annoyance PhenomenaAccording to numerous studies  voltage sags orblinks which exceed 6% are objectionable to people. Fluctuations that happen less than once per hour and less than6% are reasonably acceptable. At a rate of less than oneper hour and 3% or less, they are virtually unnoticeable.Large motors are almost never started more than onceper hour unless they require multiple tries to achieve start.However, they can cause considerably more than a 6% dipin voltage. Clearly, this sag is a cause of customer irritation and thus complaints.The cause of the sag is the current drawn by the motoras it starts. Full voltage starting current is between 6 and 7times full load current. Even with a soft starter, peak current during start is at least 3 times full load current. Thiscurrent draw does not diminish significantly until the motor reaches around 90% full speed. The effect on the distribution line is worse when the motor load is far from thesubstation because of the increased conductor impedance.Since a large contributor to the line impedance is reactive,increasing conductor size is not usually a solution.B. Difficulty Starting the Primary MachineLong starting times or downright inability to start themotor is a direct result of the voltage being too low. Thereis always a voltage below which the motor will not start(usually around 60% of full voltage). If a soft-starter isalready reducing voltage, the combination with the distribution line drop can make for difficulty starting.Motor acceleration is proportional to the difference intorque applied by current in the windings and the loadtorque. This is complicated by the fact that a completelyunloaded motor still needs significant current (usuallyaround 33% of full load amps) . Torque at any givenspeed is proportional to voltage squared. Fig. 1 shows atypical set of torque curves for an induction machine.For each voltage curve in Fig. 1 the accelerating torqueis the difference between the solid line and the load torque(dashed) line. Note that for 50% voltage, while the motorwill start turning initially, at around 45% of synchronous,acceleration goes to zero. It can never reach full speed. At35% voltage, the motor will not start at all.C. Electrical Interference with other EquipmentMany types of industrial electronic controllers, particularly VFDs, are very sensitive to voltage excursions outside their normal operating power tolerance. This type ofsmall to medium size motor control equipment is frequently observed to experience self-imposed shutdowns when anearby large motor is starting.The main reason that current is so high and torque is solow during startup is that the locked rotor power factor isvery low, usually around 10%. This means that only asmall fraction of the current is actually producing real kW,i.e. torque. The rest is kVAr and only produces energy inthe distribution lines as I²R loss.V.SOLUTIONSIt usually (although not always) falls to the electric utility to find solutions to motor start problems. Solutionsmust be tailored to the particular combination of utilityconstruction and customer equipment. Most solutions involve the reduction of current required from the source.
They are almost always unique and usually require engineering analysis to produce good results. The use of plainold common sense cannot be overstressed.There are typically several alternatives when it comesto solutions. Those listed below assume that the motorowner is cooperative and is willing to do everything possible to mitigate the problems. This includes using a softstarter, employing engineering services, complying withrecommendations, etc.A. Using Higher VoltageThis solution involves an increase in distribution voltage level such as going from 12.5 kV to 25 kV, not juststepping the voltage regulators up a little. It is very effective because it cuts the current in half and usually involvesnew line. However, it can be very expensive, usually inthe hundreds of thousands of dollars. Normally the customer is also required to make expensive changes. It is notoften a viable option unless the higher voltage is alreadypart of a planned project.B. Rebuilding the Line with Larger ConductorLarger conductor is less effective than raising the voltage because, with any distance, the inductance of the linecontributes a large part of the impedance. This means thatit will not have as much effect on voltage drop as reducingcurrent will. This is rarely a viable option.C. Building a new Substation NearbyThis can be a very effective solution but is usually prohibitive from a cost standpoint unless it was already part ofa pre-existing plan. Substation cost is generally in the millions of dollars.D. Using Fast-acting Capacitor AssistanceBecause properly sized, localized capacitors compensate the poor power factor of the starting motor, the currentdraw from the substation can be drastically reduced duringstartup. The reduction can be up to 85% of uncompensated current. These solutions need to be designed andanalyzed before implementation. Very often small changes are required at commissioning.This is by far the least expensive option for improvingmotor start performance. With costs in the tens of thousands of dollars, savings over other methods are easily anorder of magnitude or more.VI.FAST-ACTING CAPACITOR CONTROL BASICSFast-acting capacitor assistance involves the installation of a switched capacitor bank or banks that can be en-ergized and de-energized in a timely manner. These areusually distribution voltage banks. The switching functions need to be fast-acting for three reasons:1. At the beginning of startup the capacitors must beenergized before the voltage has sagged toomuch.2. If multiple switched banks are used the banksmust be staged precisely according to predetermined conditions.3. The capacitors must be de-energized at the end ofstartup such that the voltage does not increase beyond tolerable limits.The fast switching actions require a logic device orcontroller with inputs, outputs, and timing capability suchas a small PLC or an IED. An algorithm must be developed for the controller. There is no universal algorithmbecause the equipment and solution needs are always different.The PLC’s algorithm acts upon received inputs and internal timers for its functions and decisions. These inputsinclude: Signals from the motor starting equipment Load current Line Voltage Some combination of the aboveSignals from the motor starting equipment can be sentvia wires, fiber optics, or radio. Load current and linevoltage may be measured directly from PTs, CTs and LinePost Sensors if it has that capability or it can receive digitalthreshold crossing outputs from a current and voltage preprocessor.Control of the capacitor bank(s) is accomplished byoperating mechanical relays or, in some cases, sendingsignals to a ZVC capacitor control. A ZVC control is recommended when capacitor switching might cause voltagetransients that would interfere with the operation of othersensitive electronic equipment. It is also recommendedwhen multiple closely spaced capacitor banks are used inthe motor start solution. This is because it eliminates highcurrent discharges between capacitors on the same phasewhen energizing consecutive banks. High current discharges between capacitors can damage capacitors as wellas the switches and they can blow capacitor fuses.VII.CAPACITOR ASSISTANCE SYSTEM DESIGNThe complexity of a fast-acting capacitor assistancesystem design varies with the nature and severity of theproblems being experienced or anticipated. Design of thesystem generally requires four major efforts: Analysis of the subsystem affected by the motorstartups Engineering a solution Implementing the solution
Modifying the implementation as necessary forsatisfactory resultsA. Analysis of the subsystem affected by the motorstartupsIt is very important to quantify the electrical parameters of the substation and the feeder that serve the customerwith the motor(s). The gathering of accurate data is crucialto engineering a solution. A step by step suggested process is presented:1. If possible, take substation feeder current and voltagereadings immediately before taking motor start readings.2. Using a portable power analyzer, take motor startreadings with the following conditions: Start the motor in normal configuration. Measurement point at a location to get the motorcurrent with no other significant load present. Include current and voltage for each phase withresolution of one cycle. Start the recording before the motor starts and endafter the motor is up to speed3. Obtain accurate information about the applicable subsystem. A suggested list follows: Transmission line primary voltage Substation Transformer size Transformer secondary voltage Total loading Capacitors Regulators / Tap changer Feeder Conductor size and type Sectional variations Overall length Distance to the motor(s) Load distribution Downline capacitors and regulators Motor load Transformer size and secondary voltage Soft-start equipment Motor size, # poles, brand and model number Efficiency and starting power factor Locked rotor amps and full load amps Other pertinent information as availableB. Engineering a SolutionOnce all the information detailed in sub-section A hasbeen collected, the design process can begin. Start by creating a model using the following suggested steps:1.Use Power*Tools by SKM Systems Analysis, Inc. orWindmill by Milsoft Utility Solutions to create themodel.2. Model at a minimum the following components: Transmission line Substation transformer Line sections Distributed loads Downline capacitor banks and regulators Motor transformer Soft-starter with bypass contactor Motor(s) Nodes for measurement data placed as neededAuto-transformers can be used in place of regulatorsand soft-starters if they are not available.3. Run the model Work with the components until the data produced with a model run closely approximates thevoltage and current readings taken in sub-sectionA. The closer the model matches the actual readings,the better the design will be. If the model is not within 2% of the readings,something is wrong. Check for errors. Make adjustments. Use common sense.Once an accurate model has been created, start to modify it as the first step toward engineering the fast-actingassistance system. There is no set way to engineer thissolution. Remember that these steps are only suggestions.4. Add capacitor banks to the model. A good starting rule of thumb for 12470 voltfeeders is:C 0.2 * M * dWhere:C Total kVAr of the bank(s)M Motor HPd distance from the substation in km Start with a single bank of that size. With thebank energized, run the model with the motor instart mode. Then run it with the motor in runmode. Observe how much voltages vary. Usecommon sense to determine if multiple bankswould be helpful. Try various combinations of two banks energizedat various times during the start process. Try a fixed bank at least 1.5 km upline.When the model seems to produce good results in allmodes of motor start and run, and given that timely energizing and de-energizing of the capacitors can be achieved,it is time to begin determination of the PLC algorithm parameters.If the issues with motor startup are at the motor location, focus should be on voltages there. If, instead, theproblems are mainly 3rd party annoyance related, work
should be concentrated to optimize power quality along thefeeder. Using a spreadsheet will help with calculations:5. Put the model data into a spreadsheet Run the model using soft-start voltages to produce ramping motor currents as they appeared inthe power analyzer readings. Bring on capacitor banks(s) at various points inthe current ramp so as to find the best capacitorswitching times. If a pre-start signal will be available from the motor start equipment, it is possible that energizingcapacitors before the motor starts may be beneficial. Observe model voltages at various points alongthe feeder. Record all this information in the spreadsheet. Include as many model settings for reference aspossible. This will allow reproduction of the datalater and can be very helpful when acting on analternate idea or trying something different. Use the data to make a graph of the modeledfeeder voltages during the startup. Use the model to generate graphs for high, normal, and lightly loaded feeder conditions. Work with the capacitor bank sizes and operationtimes to optimize the design.C. Implementing the SolutionOnce the solution has been engineered it is time tobuild the equipment that will implement the solution.The capacitor racks are fairly straight forward and willnot be discussed, other than to recommend the use of solenoid operated switches. This is important for timely energization unless capacitors will be energized prior to motorstart and for de-energization unless over voltage has beendetermined not to be a problem after the motor is up tospeed.The PLC needs to be programmed to embody the algorithm created in the solution. It is assumed that the designer has the resources to program the PLC or to have itdone.The fast-acting control will consist of multiple components. Recommendations based on the author’s experiencewith these systems are listed: Signals from motor start equipmento Freewave FGRIO radio pairo Multimode glass fiberso Wires not recommended because of vulnerability to lightning. Voltage and Current data measurementso SEL relay such as a 735 or 2411o Various other PLCo Omron ZEN series controller o SEL relayControl Deviceso Mechanical relayso ZVC control operated by radio or fiber.These can be found on the internet.D. Modifying the implementation as necessary for satisfactory resultsWith the new design in place, once again take readingswith the same power analyzer. If the solution is satisfactory, the work is done.Most of the time, however, there will need to be someimprovements. The vast majority of these can simply bedone on-site at commissioning time by modifying the algorithm slightly, moving some of the thresholds, or changingsome timings.VIII.CASE STUDIESA. Case Study #1Grayson-Collin Electric Cooperative, Texas6 km of 4/0 AWG from the substation12470 VoltsThree 50