Low Voltage and Medium Voltage. Variable Frequency Drives: Application Considerations

Transcription

1 Low Voltage and Medium Voltage Variable Frequency Drives: Application Considerations PREPARED FOR 5 th Eastern Canadian Electrical and Instrumentation Conference for the Pulp, Paper And Sawmill Industry September 16-18, 1997 Sudbury Robert A. Hanna Ph.D., P. ENG., FIEE

2 1.0 INTRODUCTION Variable Frequency Drives (VFDs) have provided the industry significant advantages in improved process control, higher efficiencies and energy savings. The reliability of low voltage and medium voltage VFDs have improved significantly in the last decade due to improvements in switching devices, cooling system, harmonic mitigation, design of converters/inverters and control systems. VFD technology has matured and the cost of adopting it has become more economical as is evident by the increased number of drives put in service over the last few years. This presentation covers the application of VFD to variable torque, constant torque and constant horsepower type load. It describes the principles of VFD, the use of semiconductor switching devices for power conversion and the basic types and key features of low voltage and medium voltage VFDs. It also highlights the harmonic problems encountered when using VFD for new or retrofit applications, mitigation techniques to suppress harmonics and reviews IEEE standard for harmonic guidelines. 2.0 PRINCIPLES OF VARIABLE FRQUENCY DRIVES Variable frequency drives used with AC induction motors are required to produce variable frequency / variable voltage to control the speed of the motor. Frequency is controlled in order to vary the speed of the motor according to the following formula: Speed = 120 x Frequency / No. Of Poles Voltage is varied along with the frequency so that the flux density in the air gap between the rotor and stator, and therefore, the torque produced by the motor can be controlled. A constant relationship between voltage and frequency (volts/hertz) must be maintained, see Figure 1, to optimize motor utilization and keep constant rated flux. Higher or lower volts/hertz ratio would result in overexcited or under-excited motor respectively, and improper motor utilization. For example, for 575V motor the volts/hertz ratio is 9.6 and for 4160 motor the ratio is across the operating speed up to 60 Hz. Above 60 Hz the voltage is kept constant at rated value and only frequency is linearly increased, and this results in constant horsepower. The torque, in this case, is inversely proportional 2

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4 to speed, but their product is equal to constant horsepower. Figure 2 shows the torque speed characteristics of an induction motor at different supply frequencies and constant volts/hertz ratio. 3.0 SEMICONDUCTOR POWER SWITCHING DEVICES FOR VFDs Power conversion in low and medium voltage variable frequency drive is typically achieved by using one of the following four semiconductor power switching devices in inverter circuit. Silicon-Controlled Rectifier (SCR) Gate turn-off Thyristor (GTO) Bi-Polar transistor Insulated gate Bi-polar transistor (IGBT) An ideal switching device for VFDs would include the following features, all of which will permit a simpler, more reliable, low cost VFD design at higher horsepower: Internal losses would be very low to provide a high operating efficiency, leading to small size and low cost. Switching controls would be simple. The device would be easy to switch on or off, keeping the logic controls and associated power circuits to a minimum for low cost and reliability. Switching frequency capability would be fairly high. The device could be switched on or off at high frequencies, allowing the inverter to produce a variable frequency sine wave with minimum harmonics to minimize motor heating. Surge rating would be high. The device would have ample overcurrent and overvoltage capacity to improve reliability and to simplify protection. Current and voltage ratings would be high. The device could be manufactured easily and reliably for high-horsepower applications. 4

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6 The choice between the above switching devices depends largely on the manufacturer s experiences and drive size. For VFD application, one can essentially classify all semiconductor devices into two broad categories: 1. Those devices which only require an impulse on their gate to be turned on. These are SCRs type devices (also called thyristor) and they were the first to be introduced. Due to their high voltage and current ratings, they are still employed particularly for large VFDs up to HP. Their primary limitations have traditionally been their low switching frequency and switching losses. In addition, they may require some rather sophisticated external control circuitry to allow SCRs to be turned off. The gate turn-off thyristor (GTO), featuring a second gate, was introduced in the 80s to address these issues. The second gate allows GTO to be conveniently turned off without power commutating circuits. GTO devices have moderate switching frequency and moderate voltage and current ratings. GTO devices have been used in medium voltage VFDs ranging 400HP HP. 2. Those devices that require a permanent current at their gate to keep them in the conducting state. These are IGBT and a bi-polar transistor type devices and require no external circuitry for turn on or turn off purposes. However, they have limited voltage and current ratings when compared to SCR to be widely used in medium voltage VFDs for high HP ratings. The IGBT is currently the most popular switching device in low voltage 480V and 575V VFD technology and for rating up to 1000 HP. This is because of its higher gain, lower losses and fast switching up to 20 khz. In the last two years, IGBT devices have been employed in 2300V and 4160V medium voltage VFDs up to 5000 HP ratings and their acceptance is growing. IGBT produces very steep voltage waveform due to its fast switching and extra precaution should be taken for motor insulation protection particularly in retrofit applications and long run cables. 6

7 The voltage and current ratings for the switching devices described here should only be used as a guideline since this technology is rapidly changing. The drive manufacturer must always be consulted for the most up-to-date drive current and voltage ratings. 4.0 BASIC TYPES OF LOW VOLTAGE VARIABLE FREQUENCY DRIVES The basic components of a VFD are a line converter (rectifier), DC link and load converter (inverter). The rectifier changes constant AC input voltage to constant or variable DC output voltage. The inverter alters the DC to variable voltage/frequency AC power fed to motor. The rectifier performs the power conversion using diodes (uncontrolled DC voltage) or SCR (controlled DC voltage). The switching power devices typically used in an inverter circuit to achieve variable voltage/frequency are those described in the previous section. It should be noted that all types of VFDs described here have soft start capability to control motor starting current and acceleration time. The basic topologies used to produce low voltage VFD are voltage source inverter (VSI, also called variable voltage inverter VVI), current source inverter (CSI) and pulse width modulated (PWM). When low voltage VFD is applied to 2300V or 4160V motor an output step-up transformer is used to match the inverter voltage to motor voltage. 4.1 VSI Drives The VSI drive, Figure 3, consists of a controlled rectifier (SCRs) to convert the incoming AC voltage to variable DC voltage. The frequency of the output is controlled by sequentially switching the transistors or SCRs in the inverter section in six discrete steps to produce the output voltage shown in Figure 3. This inverter concept was one of the firsts to be introduced in the 70s in low voltage VFDs and is not widely used at present. The six-step inverter circuit is characterized by the use of capacitors in the DC link. 7

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9 The inverter output is controlled independent of load, so any squirrel cage induction motor, or even a group of motors, within the rating of the inverter may be driven by a VSI drive. Harmonic currents are proportional to the imposed harmonic voltages in the output wave and inversely proportional to motor leakage reactance. Motors designed with large leakage reactance reduce the harmonic currents, motor heating and pulsation torque. Regeneration, if required, is possible but with additional converter section. The main features of VSI drives are: Power rating up to 1,000 HP Speed reduction 10 : 1 Suitable for variable torque and constant torque load applications. Efficiency range % Poor input power factor at low operating speeds Inverter power and control circuit is reasonably simple For retrofit application, motor derating might be necessary to account for harmonics and reduced cooling. 4.2 CSI Drives Current source inverter drives can be identified by the large reactor in the DC bus, see Figure 4. Most current source drives require that a motor be connected before the drive has the capability to commutate. Motor inductance characteristics, in addition to capacitors in the drive, are part of the commutation circuit. This makes it difficult to retrofit these drives to existing fixed speed motor. The most common designs of CSI drive do create high voltage spikes during commutation. This could be a factor in selecting the drive in the higher voltage drives (2300 V and above) to assure that the insulation on the motor will not be damaged by the voltage spikes. An alternative design to this approach is one, which includes capacitors on the output to minimize the voltage spikes. The harmonic currents are determined by the harmonics in the output wave while the harmonic voltages generated in the motor are proportional 9

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11 to leakage reactance. Motor heating is less in motors with low leakage reactance. The main advantage of the CSI drive is its ability to have complete control of motor current, which results in complete torque control. However, this current controlling characteristic necessitates a large filter inductor and a semi-complex regulator due to the difficulty of controlling the motor solely by current. The main features of low voltage CSI drives are: Power rating 50 1,500 HP Speed reduction 10 : 1 Suitable for variable torque, constant torque load applications. Efficiency range % Poor input power factor at low operating speeds Inverter power circuit is simple, but control circuit is semi-complex. Not suitable for retrofit applications Regeneration capability It should be noted that CSI design concept is hardly used at present for low voltage VFDs but it is employed with some modifications for medium voltage VFDs. 4.3 PWM Drives PWM inverter is the most popular topology used today in the 480V and 600 V levels and has become almost the standard in the low voltage VFD industry. The PWM drive, see Figure 5, utilizes a diode rectifier to provide a constant DC voltage. The inverter section in this type of drive is required to control both voltage and frequency. This is done by varying the width of the output pulses as well as the frequency in such a way that the effective voltage is approximately sinusoidal. Almost universally, all low voltage VFD manufacturers employ IGBTs switching devices in the inverter circuit. The main feature of IGBT device is its capability to be switched on or off at high frequencies, allowing the inverter to produce a variable frequency sine wave with minimum harmonics to minimize motor heating. Typically the switching frequency ranges between 2 khz to 11

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13 20 khz and voltage rise time is less than 0.2 microseconds. The higher the switching frequency the higher is the output voltage rate of rise and this could provide problems to the motor insulation. A steep wave front pulse does not allow the voltage to distribute itself evenly across all the mesh stator windings. The first few turns experience most of the initial voltage rise and this repeatedly exert voltage stress and ultimately could result in winding failure. Experience has shown that this problem is more profound when the motor is located over 50 ft from the VFD. The combination of motor inductance and cable capacitance could cause the pulse voltage to be as high as 2 times the DC bus voltage, namely, for 575 V VFD the DC voltage is 800 V and the spike voltage across the motor to be 1600 V. In the last 2-3 years, there have been several incidences reported for motor winding experiencing insulation failures in the first or last few turns when operating with IGBTs inverters. Motor manufacturers are now offering inverter duty motors that have improved insulation system. NEMA MG1 Standard has added a new section part 31 that specifies limitations for definite purpose inverter-fed motors. It calls for motors rated 600 V to withstand voltage spike up to 1.6 KV with a rise time greater than 0.1 microseconds. Many motor manufacturers are now building an inverter duty motor that complies with this new standard. In addition, it is become fairly common to install 3%-5% reactor between the inverter and motor when feeder cable is greater than 50 ft to mitigate the voltage spikes imposed on motor. It should be noted that this kind of drives are also sensitive to input line disturbances and this result in nuisance tripping particularly during utility capacitors switching. Input line reactors with 3% value are typically installed to mitigate this problem. The main features of low voltage PWM drive are: Power rating 5 1,500 HP Speed reduction 30 : 1 Suitable for variable torque and constant torque load applications. 13

14 Efficiency range % Input power factor is near unity Inverter power circuit is simple but control circuit is complex Motor voltage stress is increased at higher switching frequency Susceptible to incoming line disturbances mainly due to capacitor switching Noisy when operating at lower carrier frequency Suitable for retrofit application and less motor derating is required, because harmonic currents are reduced. In certain applications where high starting torque (above 150 %) is required at 150 % starting current, conventional PWM drive could be modified to include flux vector control. In this case, the magnetizing current component and torque-producing component of the stator current are both independently controlled. This type of drive controller requires and encoder mounted on motor shaft to measure the actual speed. The flux vector controllers are also used when automatic reacceleration (after a momentary loss of power) is specified for high inertia load such as induced draft fan. The controller is capable to almost instantaneously acquire synchronism with motor (catch on fly) upon power restoration thus avoiding process upset. Flux vector control can be retrofitted on existing PWM drives. 5.0 BASIC TYPES OF MEDIUM VOLTAGE VARIABLE FREQUENCY DRIVES The basic components of medium voltage VFDs are identical to low voltage VFDs, namely rectifier front end, DC bus and inverter. Medium voltage VFDs operating voltage is at 2300V, 4160V or 6600V but the majority are designed at 4160V. Medium voltage VFDs size ranges between 400HP to 20000HP. The highest speed for which a VFD was built is 11000rpm for compressor application at 3500HP rating. First medium voltage drive for induction motor was introduced to the market in Presently, there are over 1500 medium voltage VFDs in service in North America, and over 15 in Ontario ranging from 700 HP to 3500 HP. The use of medium voltage drives is increasing and over 400 drives were 14

15 sold in 1996 in North America and this number is expected to reach 500 in There are four manufacturers of medium voltage VFDs in North America and this number is expected to increase to six by the end of All manufacturers offer both air and liquid cooled VFDs depending on size and customer requirements. Typically, drives rated up to 2500 HP are air-cooled and above that are liquid cool to reduce the requirements for air-conditioning system. Heat loss produced by air cool drives is typically 2% KW/HP rating, namely, for example, for 2500 HP rating the heat loss is 50 KW. All variable frequency drives manufactured today produce at least some harmonic current and voltage, which will be impressed on the motor causing additional heating and voltage stress. The impact of VFD on motor or power system depends on drive topologies and switching devices. Generally, the basic topology used to produce medium voltage VFDs are current source inverter pulse width modulated (CSI-PWM), three level voltage source inverter pulse width modulated (VSI PWM) and multipulse pulse width modulated. 5.1 CSI PWM Drives This drive concept was introduced in the market in 1993 and since then there are several hundreds of drives in service for pumps, fans and compressors applications. The CSI PWM drive, see Figure 6, consists of rectifier circuit utilizing SCRs in to convert 60 Hz input supply to variable DC supply, large reactor in the DC link and PWM inverter using GTOs coupled to an output capacitor for high order harmonic reduction. PWM inverter uses Selective Harmonic Elimination (SHE) techniques to eliminate lower order harmonics mostly the dominant 5 th and 7 th components. The key features of CSI PWM drives are: Power rating up to HP Suitable for variable torque and constant torque application Input isolating transformer is required for retrofit applications and is optional for new applications. 15

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17 Input converter (rectifier) can be configured in 6 pulse or 12 pulse for harmonic reduction. Input power factor reduces linearly with speed and input filter could be required for power factor correction and harmonic reduction Regeneration capability for motor braking Air cooled or liquid cooled with redundant pumps Bypass option Redundancy in switching components Vector control for high torque, low speed application (extruder) Levels Voltage Source PWM Drives At present, this drive topology is not adopted by VFD manufacturers in North America but mostly used in Europe. The drive, see Figure 7 has diode rectifier input, a large DC link capacitor and PWM inverter that normally uses GTOs arranged in three levels. Typically, these drives are build at 3300V thus limiting their use for retrofit applications with motors rated 4160V or for new applications having bypass. The key features of VSI PWM drives are: Power rating up to 5000 HP Suitable for variable torque and constant torque application Input isolating transformer is required for new and retrofit applications Input converter (rectifier) can be configured in 6 pulse or 12 pulse for harmonic reduction. Input power factor is greater than 0.95 throughout the speed range Mostly liquid cooled with redundant pumps 17

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19 5.3 Multi- Level Voltage Source PWM Drives This drive technology was first introduced to the market in early 1995 for use with medium voltage induction motors. The drive is characterized by the use of multi-winding indoor input isolating transformer, multi-pulse diode rectifier and multi-pulse IGBTs PWM inverter, see Figure 8. The drive converter consists of several cells connected in series to produce the required motor voltage, namely, three cells in series per phase for 2300V and five cells in series for 4160V. Each cell design is identical to low voltage 480V PWM circuit. The main features of this drive is as follows: Up to 5000 HP rating built and capability up to HP Suitable for variable torque and constant torque application Input indoor isolating transformer is required for new and retrofit applications Input converter (rectifier) and load converter (inverter) is configured in 18 pulse, or 24 pulse or 30 pulse for harmonic reduction Input power factor remains above 0.95 across operating speed range Meet IEEE harmonic requirements for current and voltage total harmonic distortions at drive input and output without the use of harmonic filters Air cooled with optional redundant fans or liquid cooled with standard redundant pumps. Bypass option Cell bypass Vector control for high torque, low speed application (ex: extruder) 19

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21 6.0 APPLICATION CONSIDERATION OF VFD VFDs are extensively used in industry for various applications. They are normally divided into three categories depending on their torque requirement and these are constant torque, variable torque and constant horsepower. 6.1 Constant Torque Application A constant torque load is characterized as one in which the horsepower requirement is directly proportional to the operating speed. The horsepower torque relationship is defined by the following formula: Torque (lb.ft) = (HP x 5250) / RPM This characteristic is shown graphically in Figure 9. Constant torque is achieved by maintaining the motor current constant at all specified operating speed range. Horsepower requirements is linearly proportional to speed and hence energy saving is less when compared with variable torque application. Therefore, in general, it is difficulty to justify the use of these drives on energy basis alone without considering the other process requirements. For constant torque application, the motor experiences considerable temperature rise when operating at minimum specified speed, because of reduced ventilation and constant current requirement. For self ventilated motor, temperature could rise by up to 20 C above its sine wave rating when running at rated load and 50 % speed. Figure 10 shows a typical motor continuous torque capability versus speed for self-ventilation. In general, the motor is capable to safely operate at rated torque and current between 100 % to 50 % speed particularly if it is designed with 1.15 service factor and class F insulation. Below 50 % speed operation, motor performance should be carefully reviewed to avoid premature winding failures. 21

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24 Typical examples of constant torque applications are reciprocating compressors, reciprocating pumps, conveyors, screw type mixers, extruders, etc. It is recommended to carry out torsional analysis in large drive constant torque applications especially for two poles motor to ensure that the natural resonance of the mechanical components are not excited by the drive harmonics within the operating speed range. 6.2 Variable Torque Application For variable torque load, the torque varies directly with speed squared, and horsepower varies directly with speed cube, see Figure 11. Harmonic losses are maximum at rated speed. Considerable energy saving is achieved with this application even at slightly reduced speed. For example, at 80 % speed the horsepower requirement is almost 50 % rating. Typical applications are centrifugal pumps, fans and compressors. Motor should be specified with 80 C rise above 40 C ambient when connected to VFD and at rated speed to properly allow for harmonic losses. Class F insulation is recommended. Motor derating is minimal for retrofit variable torque application. Continuous torque capability without motor overheating is from 25 % to 100 % speed for self ventilated machine. 6.3 Constant Horsepower Application For constant horsepower load, torque is a function of speed in the constant horsepower range, such that, as speed is increased, torque will decrease inversely, and horsepower will remain relatively constant, see Figure 12. Typical examples of constant horsepower loads are grinders, lathes and winders. Typically constant horsepower loads are operated above base speed. 24

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27 7.0 HARMONICS IN VARIABLE FREQUENCY DRIVES Harmonics currents and voltages are produced whenever VFDs are used. Their operation, as described earlier, require converting AC to DC and DC to AC and this results in generating harmonics both into board and power supply. Common sources of harmonics in industrial electrical system are as follows Variable Frequency Drive Rectifiers DC Motor Drives UPS Arc Furnaces Static Var Generator Cyclo Converter Static Motor Starter The presence of these devices in a plant does not necessarily indicate that there is a harmonic problem. At one extreme, these harmonics may be of low magnitude and, therefore, harmless, or they may be high enough to cause problems such as motor overheating, capacitor s failure and telephone interference. It has been suggested that when 20 % or more of the plant load consists of harmonic producing equipment, then a harmonic study should be considered. Harmonics are currents or voltages which have frequencies that are integer multiples of the fundamental power frequency. For example, for 60 HZ supply the fifth harmonic is 300 HZ. A 3-phase power converter generates harmonic current the order and magnitude of which are given the following equations: h = KP ± 1 Ih = I1 / h h : order of harmonic 27

28 P : number of pulses of the converter system K : any integer 1, 2, 3, --- Ih : harmonic current I1 : fundamental current It can be seen that the magnitude of harmonic current is inversely proportional to its harmonic order, and therefore, the magnitudes of high order harmonics diminish rapidly. A six pulse converter system (P = 6), see Figure 13, would produce harmonic currents of the order 5 th, 7 th, 11 th, 13 th, 17 th, 19 th, 23 rd, etc. For a 12 pulse converter configuration, as shown in Figure 14, the harmonics generated are 11 th, 13 th, 23 rd, 25 th, etc. Therefore, a 12 pulse converter provides a significant reduction in the voltage distortion and, equally important, it eliminates, assuming balanced conditions, the lowest order harmonics of 5 th and 7 th which are typically of most troublesome. The 12 pulse configuration is typically used with drives rated say 2,000 HP and above. 8.0 HARMONIC GUIDELINES In order to compare levels of harmonic distortion in a power system, the Total Harmonic Distortion factor is used, and is defined in IEEE Standard (Recommended practices and requirements for harmonic control in electrical power system) as: ½ THD = [ sum of squares of amplitude of all harmonics ] square of amplitude of fundamental X 100% This standard specifies two criteria to evaluate harmonic distortion: 1. A limitation in the harmonic current that a use can transmit into the utility system. 2. The quality of the voltage that the utility must furnish the user. The first criteria puts the responsibility on the user to limit harmonic current injected back into power system and these limits are given in Table 1. These 28

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32 harmonic current limits are based on the size of the user with respect to the size of the power system to which the user is connected. The smaller the ratio of VFD load to available short circuit level, the larger is the harmonic current allowed to be injected back into utility. The second criteria put the responsibility on the utility to furnish the user with a good quality voltage. Table 2 shows the permissible harmonic voltage level at the incoming power supply. Ontario Hydro voltage THD requirements are, in general, more stringent than those given in Table 2. For example at 27.6 KV level Ontario Hydro voltage THD requirement is 3%, whereas IEEE recommends 5%. Another harmonic limitation which is normally very difficult to meet, and is sometimes overlooks is the IT products. This represents the harmonics interference with telephone lines when running in the proximity or power lines. Table 3 shows the IT limits as specified by IEEE Ontario Hydro IT products limits are more stringent than IEEE guidelines and is 5,000A at 27.6 KV voltage level. It is extremely important to consult with the local utility as their harmonic guidelines could have a significant impact on the filter requirement and overall VFD cost. 9.0 CONCLUSIONS The use of medium voltage VFDs is growing rapidly based on the number of drives sold over the last few years. More manufacturers are entering the market to build medium voltage VFDs and this trend is expected to continue. The users have accepted this technology and are employing it to control critical processes. When selecting a VFD for a particular application several factors need to be carefully examined at the initial phase. These include type of VFD, torque requirements at all operating speed range, power quality issues, torsional analysis, impact on existing motor and mechanical equipment, space availability, HVAC requirements, bypass option and redundancy. 32

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35 For low voltage VFDs, measures should be incorporated to reduce drive susceptibility to incoming line transients as well as impact of output voltage spikes on motor winding. For medium voltage VFDs, the users should become familiar with the various inverter technologies offered by the manufacturers to make a selection that best meets the project requirements. An overall system evaluation should be conducted that includes harmonic filters, motor features, VFD configuration (6 - pulse or 12 - pulse or multi - pulse), bypass option, air versus liquid cool, space requirement, torsional analysis, etc. It is inevitable that harmonics will be generated whenever a VFD is used. The order and magnitude of these harmonics greatly depends on the drive arrangement and system impedance. The principle effect of harmonics on electrical equipment is overheating and possible excitation of the combination of the existing power factor correction capacitors and system impedance. For constant torque application it is recommended to specify a motor with 80 C temperature rise at rated torque and minimum operating speed. For variable torque application, 80 C temperature rise is recommended at rated load and rated speed because harmonic losses are maximum. Class F insulation should always be specified as minimum for constant or variable torque applications. For large VFD installation, harmonic study is recommended to ensure compliance with the utility requirements and that there is no resonance available with power factor correction capacitors. 35