Capítulo 2: Evaluación de procesos
2.3 E STRUCTURA DEL M ODELO DE E VALUACIÓN DE P ROCESO E JEMPLAR
VFD Concepts
Electric AC motors are the most common source of process industry drivepower due to their reliability and low cost. An additional feature of electric motors is that shaft speed or torque can be controlled by varying the electrical supply at the motor terminals. This feature is exploited by a separate piece of power electronic hardware known as the Variable Frequency Drive (VFD), some of which are pictured in the figure below.
Figure 2.31 - Torque-speed curves for idealized VFD, (ABB Drives 2002)
VFD Energy Efficiency Potential
The main benefits of applying VFDs come from matching the input power to the needs of the process. These benefits are magnified in cube-law applications such as pumps and fan systems, where power requirements increase with the cube of speed, and the savings are typically between 30 and 50%. The following sections introduce the technical features of VFDs with the greatest potential for energy efficiency.
Speed Control instead of Throttling
Of the estimated 65 percent of industrial energy used by electric motors, some 20 percent is lost by wasteful flow throttling mechanisms. The mechanics of flow throttling is described in the fan and pump sections of this handbook. Additional energy is wasted in inefficient variable speed coupling methods. VFDs allow application speed to be controlled without energy-wasting flow throttling or inefficient belts and hydraulic couplings.
VFD controlled pumps and fans can respond faster and more reliably than flow throttling valves or dampers, especially at flow extremes when control valves become highly non-linear. VFDs avoid the problem of poorly-sized control valves which may be unstable for control at less than 10% opening. In fan systems, the use of a VFD for speed control avoids the deadband and hysteresis found in mechanical dampers or inlet guide vanes control methods. Damper leakage is also avoided when using VFDs for flow control. Compared to throttling control methods, VFDs have lower inspection & maintenance costs. Any reduction in speed achieved by
using a VFD has major benefits in reducing pump wear, particularly in impellers, bearings and seals.
There are some additional side-benefits for VFD-flow control in pumping systems.
With a VFD receiving a signal from a pressure transmitter, it is possible to monitor the pressure of the incoming pipeline and configure the converter to take steps if the risk of cavitation is high; i.e. if pressure falls below NPSHR plus a margin. Water hammer is caused by rapid changes in flow. These flow changes are followed by rapid pressure transients that cause pipes, pipe supports and valves to be damaged causing leakage. VFDs allow the user to gradually ramp pump acceleration at a safe rate to avoid hammering.
Despite the potential for 60% savings in many cube-law applications, only 5%
of all industrial motors are controlled by VFDs, and it is estimated that 30% of existing motors can be cost-effectively retrofitted with VFDs (ABB Drives, 2006).
The potential is larger in smaller applications (under 2.2 kW) where 97 percent of all motors have no form of speed control at all, equating to some 37 million industrial motors sold annually worldwide (ABB Review, Special Report Motors & Drives, 2003). 40 percent of the value (and 90 percent in units) of all drives shipped are rated at less than 40 kW.
Improved Motor Efficiency
Another benefit of VFDs is the improved energy performance of the motor itself. The VFD control allows the motor to operate closer to its best efficiency point. The drive can generate a family of characteristic curves for the motor, in the same way that a speed-controlled motor generates a family of characteristic pump curves. In both cases, the intersection with the load characteristic curve gives an operating point which is near to the area of optimum efficiency for the equipment.
VFDs can correct for oversized motors by running them at reduced speed. The operational benefits of such correction can justify a VFD even in applications which do not call for speed control.
Compared to most other speed control methods described in the Motors and DriveTrains section, with a VFD there is no loss due to mechanical slip between motor and load.
Two and Four Quadrant Operation
Quadrant operation is the term used to describe a drive’s ability to provide braking, reversing (2-quadrant) and regenerative (4-quadrant) power. The ‘quadrant’ refers to the space these modes occupy on a torque-speed axis shown in the figure below.
For loads which generate power for only a short time, then it is typical to offer only a braking resistor where the power generated is dissipated as heat losses.
T Figure 2.32 – Four quadrants of VFD operation
Active rectifiers will allow these losses to be regenerated into electrical power for the network. The type of rectifier will determine if power can flow in both directions. A conventional passive diode rectifier only supports motor loads where the power flow is only in one direction.
Motor Soft-Starting
VFDs provide the same functionality as a dedicated soft-starter - a power-electronics device installed in-line with the motor that slowly ramps up the voltage for a smoother startup. Soft starting is mainly used to limit large motor starting currents and to better manage power factor during startup so that the electric power system is not unduly stressed. Soft-starting therefore improves voltage bus stability, reduces currents and allows reduction in transformer size, and other power system hardware. There is also reduced risk of process disturbance due to voltage drops; and fewer trips of other electrical devices connected to the same bus. Most motors experience startup in-rush currents that are 5 to 6 times higher than normal operating currents, which is reduced to 1.5 with VFDs, thus reducing wear on the motor. Soft starting reduces heat load and allows a greater number of starts within a given time period, increasing the flexibility of the control system to optimize the process. (Motors not equipped with a VFD or other form of soft startup power electronics are referred to as ‘Direct-On-Line’
(DOL) motors. ) Note that the load itself may also benefit from a smooth ramping up of torque and speed. The sections on Motor Starting Conditions and VFD Starting Torque Conditions discuss design criteria for startup conditions.
Power Factor Correction
The internal design of a VFD helps to improve a motor’s power factor (PF). When a VFD is installed, the reactive power that is the source of low PF circulates mainly between the DC link and the motor, and therefore it cannot affect the input AC electric power supply system. Higher power factor will reduce I2R losses in the power network transformers and cables. A VFD-equipped motor system will typically have high PF of 0.98 compared to 0.83 for a typical DOL motor. The PF when using a VFD does not degrade with decreased speed, as is the case with DOL motors.
The VFD’s rectifier unit can optionally be of the ‘active’ type that performs leading and lagging PF correction, which is to the benefit of the entire input side of the electric power system. The potential for such an Active Rectifier Unit (ARU), also known as ‘active front end’ is discussed in the handbook section on reactive power compensation.
Slip Compensation
Without a VFD, when the motor load torque is increased, the speed of the motor will decrease as shown in the figure below. To compensate for this slip, the torque vs. speed curve is modified with use of a VFD so that the torque increase can be accomplished while maintaining a constant speed (ABB Oy. Drives, 2002). This slip-compensation is a standard function on most VFD drives and is especially useful in applications which require constant speed but varying torque.
Figure 2.33 – Slip compensation of a VFD shown on motor torque vs. speed curve, (ABB Oy. Drives, 2002)
Speed Setpoint Control
The VFD drive is more than just a speed control actuator - most VFDs are also equipped with programmable PID loop control functionality to maintain a desired
motor speed or torque setpoint. Having the PID controller onboard the drive reduces loop latency to a minimum and allows faster response to process changes when compared to central speed control. The extra responsiveness is useful in a few applications with highly varying load torques, where precise speed control is a requirement. VFD control can hold the process at a steady set point, instead of exhibiting cycling that may occur with other means of speed control.
VFD programming tools are equipped with libraries of application-specific control functions for pumps, fans, cranes etc. The control blocks in these libraries have pre-defined Input-Output characteristics and are optimized for these applications
Regenerative Braking
Some VFDs are able to ‘harvest’ wasted energy through motors acting as generators. This is also known as 4-quadrant operation. The LCI and VSI drive types (described below) can be ordered with inverter-equipped front-ends that are capable of such 4-quadrant operation. In conveyor systems with uphill and downhill conveying, some VFD drives can manage energy sources and sinks using a common bus principle, with optimum efficiency and energy flow between motors of motoring and generating mode (ABB Switzerland, MV Drives, 2002 p.16, Fact Pack #2).
Inherent Motor Monitoring and Protection
VFDs have built-in power functionality needed to protect a motor from abnormal conditions in the power supply or within the motor itself. Most critically, VFDs have acceleration and deceleration ramp limiting functions and motor current limiters to prevent driving torques which cannot be sustained by the motor. VFDs isolate motors from the electrical supply line, which can reduce motor stress and inefficiency caused by varying line voltages, phase imbalance and poor input waveform.
Many of these vital protective functions would otherwise require the installation of separate devices. The most common protective functions are: short-circuit, over current, unbalanced phase, ground fault, motor temperature, and motor stall. VFDs also monitor for underloaded situations, as in the activation of safety release mechanisms in the driven equipment, and will also detect phase loss on the incoming side. If a problem is detected, the VFD will send a warning or an alarm to operators, and initiate a drive trip.
Built-in monitoring of motor current and temperature, plus many other parameters allows the VFD to detect, warn and even take action to mitigate the effects of certain drive system problems. For example, if the motor current is oscillating, this may indicate a problem at the couplings. No extra current transducer is required for this monitoring. VFDs may also be programmed to take action when a faulty gearbox or bearing sends a high-temperature signal when operating at full load, but no
such warning at lower loads. In this situation, the VFD can decrease the load until an appropriate time for maintenance on the faulty equipment. These monitoring functions can improve the uptime of a plant, with benefits as described in the section on Lifecycle Costing Calculations - Energy cost of a trip
Overspeed Capability
By controlling the frequency and voltage, VFDs can achieve higher motor speeds than otherwise possible from only a fixed electric power supply. This overspeed capability may be sufficient to handle modest production capacity increases without the investment needed to replace the motor or actuator. Speed increases of 5-20 percent are not a problem for VFDs on most motors. This extra capacity is within the ratings of most installed motors, which are specified with generous margins plus a service factor. Margins and more details on over-speeding can be found in the NEMA MG-1 standard.
Variable torque applications, such as pumps and fans, require increasing torque with speed. If the intention is to run above the motor’s nominal speed, then it may be necessary to oversize the motor due to the reduced torque in the field weakening region.
Voltage Fluctuation Ride-Through
This feature is especially useful on weaker networks (for example, in industrial boiler applications) with transient voltage dips; VFDs allow the motor to ride through such dips thus avoiding a trip. Dips may last from a few cycles to a few hundred milliseconds. See the section on Lifecycle Costing Calculations - Energy cost of a trip
Voltage Boost and IR Compensation
At low speeds during startup (<20 Hz), a VFD can provide a temporary increase in voltage to help motors achieve a higher breakaway torque, without the high currents associated with DOL starting. One pitfall with voltage boost is the risk of overheating of very lightly loaded motors at speeds under 10 Hz (Bezesky 2001). Voltage boost applies a fixed extra voltage, but with IR compensation the extra voltage is proportional to the motor current, as shown in the figure below. IR compensation will therefore only provide extra voltage for motors under load and drawing current, reducing the risk of saturation and overheating.
Both of these features apply only to voltage/frequency (scalar) drives, or vector drives operating in scalar mode. Vector drives by design can generate higher starting torques and do not need a voltage boost feature; these drives can provide several hundred percent higher than nominal torque for motor starting. The ability of
drives to boost starting torque allows engineers in some cases to avoid oversizing or selecting less-efficient high-torque motor designs.
Figure 2.34 – Voltage boost at low speed, ABB VFD Programmable Features
Figure 2.35 - Small (LV) and large (MV) VFD hardware (ABB Drives, 2008)
VFD Types and Applications
Industrial VFDs cover a wide power range, from 100 hp (75 kW) to more than 30,000 hp (22 MW). Most large industrial plant VFD applications require a medium voltage AC drive, which are normally in the power range from 500 hp (380 kW) to 7000 hp (5 MW) and operate at bus voltages from 2.3 to 6.9 kV.
There are several different drive types, and the choice of drive type depends on many factors, starting with the basic motor type: Induction motors are typically controlled by Voltage Source Inverter (VSI) or Current Source Inverter (CSI) system
topologies. Synchronous motors are typically controlled by Load Commuted Inverters or Cycloconverters, all described in the following sections.
The internal converter control method also varies between topologies, and there may even be more than one control method to choose from for any given topology. See the ABB Guide to Standard MV VFDs (‘Fact Packs #1 and #2)
VFD Internal Design
VFDs are power electronics devices that convert a constant input frequency and voltage into variable frequency and voltage output for the motor. A complete VFD system may consist of all these elements: cabling, an input isolation transformer, power factor correction equipment, harmonic filters, the frequency converter itself, output filter, and an electric motor. The combination of all the above elements is the physical ‘topology’ of the AC drive system. The converter element’s power switching function is the ’motor control platform’, which should not be confused with a low-voltage Motor Control Center described elsewhere in this handbook.
The physical topology provides the electrical ‘hardware’ and the motor control is the
‘software’ which performs the power switching routine.
3 Load
Rectifier
Supply Side Motor
Inverter Intermediate
DC Link Converter Variable Speed Drive
Figure 2.36 – Diagram showing VFD nomenclature, (ABB Switzerland, MV Drives, 2002)
The most common converter topology is composed of three main equipment stages:
− A ‘front-end’ rectifier unit on the 3-phase supply side to convert AC to DC power and to ensure that harmonics drawn from the network are kept within tolerance.
− A DC link composed of one or more capacitors and inductors which filter and smooth the DC voltage
− An inverter unit, using one of the power electronics technologies described in the sections below, performs the inversion and modulation of the voltage which is applied to the motor windings.
The effect of these stages on the sinusoidal input AC supply is shown in the figure below:
Figure 2.37 - VFD waveform at different stages, using PWM-type drive as an example (ABB Inc. 1998)
VFD Power Electronics
The switching elements at the heart of much power electronics are thyristors, also known as Silicon Controlled Rectifiers (SCRs). These are turned on (conduct power) by a control signal and will continue conducting until current drops near zero, a feature known as ‘self-commutation’ (Competitek 1996). The number of switching elements (or ‘pulses’) in the converter is a measure of the ‘resolution’ of the drive in supplying output to the motor. The number of pulses will roughly determine the amount of VFD harmonics induced in the motor and drawn from the power supply.
The power electronics switching technologies that are in use today are:
− IGCTs (Integrated Gate Commutated Thyristor) offer fast switching (like a transistor) and inherently low losses (conducts like a thyristor); in other words, it combines the best of both technologies above. IGCT technology is the most common switch in MV drives sold today.
− IGBTs (Insulated Gate Bipolar Transistor) are fast switching and low cost, but losses are high at medium to high voltage levels. Used in LV applications.
− GTOs (Gate Turn-Off Thyristor) are thyristors with a controllable ‘off’ switch, additional circuitry that lowers overall reliability, slows the switching and increases expense. GTOs are used in high-current, high-power applications, but have relatively high MV losses.
From an energy efficiency perspective, a semiconductor drive technology that has low losses means greater energy efficiency, less cooling equipment, and greater reliability of the VFD device.
VFD Topologies
The main drive topologies which are in wide usage today are listed below, based on descriptions from the ABB Drives Fact Pack #1, 2005.
Voltage Source Inverter (VSI)
This section is based on information in (Competitek 1996), (IEEE, 2007), and (ABB MV Drives, 2002)
The VSI drive is sometimes referred to as a Variable Voltage Inverter (VVI) because it achieves speed control by varying the voltage fed to the motor. In its simplest and earliest form, the VSI inverter’s switching elements produced a 6 (or 12 or higher) step voltage waveform output for the motor. The design of the VSI topology is shown in the figure below. A common topology and control combination for low to medium horsepower AC machines today is VSI hardware using PWM inverter control. The high-speed PWM switching converter provides fast response, precise speed control and maintains high PF over the entire speed range.
Control electronics
Figure 2.38 - VSI design schematic ABB (ABB Switzerland, MV Drives, 2002)
The main characteristics of a VSI drive with PWM are summarized below.
− power range : up to 8,000 kW (for 3-level inverters)
− motor voltage levels : 2.3 to 6.9 kV
− speed range : 0 – 100%
− has low starting current
− does not contribute short-circuit current
− not capable of regenerative operation,
− high ac input power factor, near unity for entire speed range
− converter efficiency >= 98%
− PF is high and constant over entire speed range
− low network harmonics, voltage reflections & voltage stress
− full torque even at standstill & very low speed
− standard drives for pumps, fans, compressors, conveyors
Although PWM is the most common converter control method, other methods such as Flux Vector can also be used together with the underlying VSI topology.
Current Source Inverter (CSI)
This section was adapted from these sources: (IEEE, 2007) and (ABB Switzerland, MV Drives, 2002)
The CSI drive system controls current output to the motor using a gate-turn-off-thyristor current source inverter on the load side. The inverter is operated in a self-commutating mode over the full speed range of the drive; they turn off by themselves every time current passes through zero. The amplitude of the motor current is adjusted by the supply-side rectifier, whereas the frequency and thus motor speed
The CSI drive system controls current output to the motor using a gate-turn-off-thyristor current source inverter on the load side. The inverter is operated in a self-commutating mode over the full speed range of the drive; they turn off by themselves every time current passes through zero. The amplitude of the motor current is adjusted by the supply-side rectifier, whereas the frequency and thus motor speed