Adjustable frequency drives for improved energy efficiency and parallel, multiphase pumps
Maximizing uptime and minimizing energy consumption (and costs) is key in pumping applications. As facilities upgrade and new ones are built, system design must maximize uptime and minimize energy costs. Today's medium-voltage adjustable frequency drives (AFD) are a part of the design solution and are better able to help keep systems efficient while minimizing energy costs. Even in applications with multiple pumps, one drive can provide reliable, efficient control.
Uptime and Energy Efficiency Matter
Making sure that systems and processes are up and running while improving energy efficiency (and cost) is crucial in pumping applications. Maximizing system efficiencies and protecting valuable assets is part and parcel of the overall goal—driving down energy consumption and improving system reliability. Increased awareness of the environmental and economic benefits of sustainable operations is driving changes in industry regulations. As a result, it is critical to be able to quickly adapt to evolving industry regulations and realities by adopting new technologies to control costs and reduce energy requirements.
Solutions that help manage power more efficiently, effectively and safely are sought to meet both operational goals and government regulations. That said, best practices within pump applications today involve energy conservation, as well as process efficiency and reliability, with a strong emphasis on long-term benefits.
State-of-the-art AFDs combine innovative technology with reliable design and construction.
Control Power, Reduce Cost
By matching power consumption to changing system requirements, AFDs are relied upon to provide steady, efficient power for variable speed pump applications. AFDs also protect motor and pump assets by controlling power and minimizing the mechanical stress placed on the system by starting and stopping pumps. As a result, municipalities can often calculate the return on investment (ROI) for medium-voltage AFDs in terms of months, not years.
AFDs are among the most technologically sophisticated methods of motor control and have benefited from years of evolution in motor starting technology. Traditional across-the-line starters apply full voltage to motor terminals, which can sometimes generate high inrush currents that cause stress to mechanical equipment.
Reduced-voltage starting has evolved to help manage inrush currents and peak voltages and is required by some utilities to prevent excessive voltage drops in the supply grid. Solid-state soft starter technologies have also emerged to further eliminate shock to mechanical equipment, because they reduce the load and torque applied to the motor powertrain during startup.
Using silicon-controlled rectifier (SCR) technology, soft starters provide a greater degree of control for reduced voltage starting to help avoid motor coupling and shaft damage, prevent rotor and winding failure and stop drive belt squeal and breakage. Additionally, reduced voltage soft-starters offer a wide range of current limit settings, providing greater control flexibility. For pumping processes, in particular, soft starters also help avoid “water hammer” in pipes by reducing the line pressure so that valves can close gently and prevent a surge wave. Today, a wide range of motor starters offers high system configurability and flexibility for control gear design.
Medium-voltage AFDs have become more prevalent in variable speed applications. AFDs offer all the protective features of reduced voltage and soft starters, while variable speed control allows the process to match energy consumption with process demands. With a typical motor duty cycle in a variable speed pump or fan application, the resulting energy savings using an AFD in place of a traditional starter with mechanical speed control (valves, mechanical braking, etc.) can be significant. By matching power consumption directly with process requirements and maintaining optimal operating parameters at all times, the energy savings realized by using AFDs can generate investment payback periods of less than two years.
Additionally, advances in medium-voltage AFD design and application has increased the long-term reliability and capital efficiency of medium-voltage drives. Increases in semiconductor device ratings have allowed drive designs to produce comparable levels of output power with lower device counts. The emergence of fully-integrated drives has led to smaller equipment footprints and more efficient use of facility floor space. These factors have driven additional increases in the long-term economic benefits of AFDs beyond energy savings.
Maximizing Drive Efficiency
Effective drive specifications will use the advantages of today's technology to maximize system efficiency and the economic benefits of a drive system over the equipment's life. Taking stock of transformers, power factor insulated gate bipolar transistor (IGBT) devices, voltage source inverter topology and other aspects of the drive system is critical to maximize efficiency.
Transformers—More efficient transformers generate less heat and have lower temperature rises. Dry type transformers for use in medium-voltage AFDs should have a 220-degree-C insulation system to operate at 115 degrees C rise at full-load conditions. Input or isolation transformers should be of a high efficiency type with full-load losses of no greater than 2 percent.
Power factor—Low power factors will draw higher currents for the same amount of power output, leading to higher energy losses in the drive system. Medium-voltage AFDs should include built-in power factor correction equipment to maintain a constant input power factor of at least 0.97.
IGBT devices—Gating circuitry requires electronic signals to turn devices on and off, which lead to switching losses in semiconductor devices. Silicon-controlled rectifiers (SCRs), gate commutated thyristors (GCTs) and symmetrical gate commutated thyristors (SGCTs) typically require relatively high turn on and off gate currents. IGBTs, however, can be used in conjunction with external low-power gate drivers and require lower power losses compared to other semiconductor devices. Lower power losses inherent to the IGBT design make them an efficient semiconductor device option.
Voltage source inverter topology—The two major drive types are known as a current source inverter (CSI) and a voltage source inverter (VSI).
CSI drives use silicon-controlled rectifiers (SCRs), gate commutated thyristors (GCTs) or symmetrical gate commutated thyristors (SGCTs) in what is typically known as an active rectifier or active front end (AFE). The input to a CSI design is similar to a low-voltage drive six-pulse input. At higher horsepower, a six-pulse AFE creates high system harmonics and poor power factor, especially at low operating speeds. Input transformers, reactors and harmonic filters are required to reduce the detrimental effects of drive harmonics on the power system and lower drive efficiency.
VSI designs use a diode rectifier to convert AC voltage to DC without the use of electronic firing. Capacitors in the DC link regulate DC bus voltage ripple and store energy for the system. Furthermore, if the input transformer and diode rectifier bridge are of at least a 24-pulse design, IEEE519 requirements will be met. Diode rectifier bridge efficiency and the benefits of medium-voltage IGBTs described above make the VSI design using IGBTs the most efficient drive design today.
Other components—A fully-integrated drive should include the drive isolation transformer, converter, DC ink, power factor correction, inverter and all drive auxiliaries within the drive enclosure.
Synchronous Transfer Control
In systems with more than one pump, a single drive can be configured to start any number of pumps, depending on the size and layout of the system, providing efficient, reliable pump control and delivering energy efficiencies across the entire system. Note that the AFD can control only one pump at a time.
A synchronous control system involves a single AFD, a synchronous transfer controller, bypass contactor and motor select contactor per motor and pump. Additionally, an AFD output contactor can feed the AFD Bus.
The idea is to adjust the AFD output voltage, frequency and phase to match the utility so that the system can transfer the pump in a “bump-less” manner. Once the synchronization is locked, the bypass and motor select contactor are closed and opened so that paralleling is minimized. The AFD waits for another “start” command from the programmable logic controller for the next pump, while the first pump runs on bypass.
Today's drives can save up to 40 percent in annual energy consumption in typical medium-voltage applications. The ROI can be calculated in months, not years.
Pumps & Systems, December 2011
