Thinking ahead: Predictive control revolutionizes variable-speed drives

Industrial electric drives can be transformed by ABB’s Model Predictive Pulse Pattern Control (MP3C) – an advanced motor control principle that ensures efficient, precise and reliable process control and is used by the ACS6080 and the new ACS8080 medium-voltage variable speed drives.

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_{Tobias Geyer, ABB Medium Voltage Drives, Turgi, Switzerland, tobias.geyer@ch.abb.com
Jess Galang, ABB High Power Medium Voltage Drives, Turgi, Switzerland, jess.galang@ch.abb.com
Aleksandra Wozniak,} ABB Medium Voltage Drives, Aleksandrow Lodzki, Poland, aleksandra.wozniak@pl.abb.com

Each day, heavy industries across the globe are faced with a delicate balancing act: extract the best performance from equipment while reducing carbon emissions and costs and improving energy efficiency. To help achieve that balance, industries utilize variable speed drives (VSDs). A VSD is a power electronics converter that controls motor speed by varying the supplied frequency and voltage.

A significant advance in VSD motor control came with ABB’s innovative direct torque control (DTC), first developed in the 1980s. Because DTC uses a high sampling frequency (about 40 kHz) and relies on hysteresis controllers with a switching table to control the electromagnetic torque and the magnetization of the electric machine, torque response is fast and precise. The result is a superb dynamic performance of the drive and a system resilient against voltage fluctuations and other disturbances.

Challenges faced by VSDs

An underlying challenge faced by VSDs is the minimization of distortion of the output waveforms and machine currents. This minimization can be achieved to some extent by increasing the switching frequency of the semiconductor switches. However, in typical MV drive scenarios, the switching frequency cannot be increased above a few hundred Hertz per semiconductor due to device thermal constraints.

A more suitable way to minimize distortion is offered by synchronous optimal modulation. Here, offline-calculated optimized pulse patterns (OPPs) can result in values of total demand distortion (TDD) of the machine current of under 5 percent over the range of modulation indices. Individual current harmonics of non-triplen order (5th, 7 th, 11th, 13th, etc.) are reduced and interharmonic components are avoided

However, using OPPs in a closed-loop control system is difficult because discontinuities in the switching angles can introduce harmonic excursions. A solution is offered through trajectory tracking of the stator flux. This technique is preferred since it does not require estimating the leakage inductance of the machine in real time [1]. But for flux trajectory tracking to be effective, real-time assessment of the stator flux trajectory fundamentals separately from the flux harmonics should be avoided.

A recent solution that addresses this real-time requirement of flux trajectory tracking is model predictive pulse pattern control (MP³C) [2], in which the stator flux control problem is addressed via model predictive control (MPC). MPC has gained popularity in the field of power electronics because of its high performance and ability to control multiple input and output signals, coupled with the powerful controllers available in modern converters that have the computational muscle to execute complex algorithms in real time. MPC is based on a mathematical model of the system and operates by real-time minimization of an objective function – namely, the deviation of the stator flux vector from its optimal trajectory within a limited-time prediction horizon. The MPC approach achieves a motor friendliness that minimizes motor temperature, noise and vibration and delivers superior harmonic and dynamic performance »01.

OPPs and MP³C take VSD control to a new level

In OPPs, the fixed modulation interval of classic pulse-width modulation (PWM), with two switching transitions per phase and modulation interval, is abandoned. Instead, OPPs are computed offline by calculating the optimal switching angles and switching transitions needed to minimize the current distortions for a given switching frequency. Quarter- and half-wave symmetry is typically imposed. OPPs are stored in a look-up table for use in real-time operation.

Assuming an inductive load, the current distortions are proportional to the sum of the squared differential-mode voltage harmonics divided by the harmonic order. The optimal switching angles and switching transitions can then be calculated for a given modulation index and pulse. The modulation index corresponds to the output voltage and the pulse number relates to the switching frequency. The reference trajectory of the stator flux vector results from the integration of the stator voltage of the OPP over time. Per definition, this trajectory is optimal; by tracking the trajectory tightly, the TDD of the stator currents is minimized by modifying the time instants of the switching transitions accordingly – essential in the presence of significant disturbances, eg, a strong DC-link voltage ripple. By tightly tracking the stator flux reference trajectory, MP³C achieves a close-to-optimal harmonic current spectrum and minimizes the adverse impact of any DC-link voltage ripple on the stator current TDD. The result is minimal TDD of the produced currents – even in applications that employ direct-on-line (DOL) machines, which typically feature very low leakage inductances.

Delivering dynamic performance with the ACS8080 and MP³C

Launched at the end of 2024, ABB’s ACS8080 MV drive set new standards in drive technology and safety and extended the range of capabilities that were already offered by the market-leading ACS6080 MV drive with MP³C »02. Designed for almost any industrial application in the 300 to 6,700 kW range, the modular ACS8080 integrates easily into existing systems with unmatched compatibility and flexibility, delivering maximum efficiency, decreased environmental impact and reduced total cost of ownership over the long term. The drive is equipped with an array of safety features, including arc prevention and fast arc elimination, a DC grounding switch and functional safety mechanisms to help protect personnel and equipment from unpredictable and unsafe situations. Long-term reliability is assured with a low parts count, high-quality components, a reduced number of potential points of failure and minimized maintenance needs. The ACS8080 builds on well-proven ABB MV drives technology, enhancing its capabilities with the latest digital innovations and fully leveraging power electronics with easy-to-use controls. Prominent amongst the software innovations employed by the ACS8080 is MP³C, which allows the drive to achieve unparalleled system efficiency, delivering maximum power output while minimizing losses.

MP³C in action

MP³C has proven its worth in a recent ABB collaboration with industry partners to deliver equipment to a customer water transmission system project involving 10 5.8 MW induction motors, each driven by an ABB ACS6080 VSD »03. Here, MP³C enabled ABB to go with one fewer inverter unit in each of the 10 subsystems and thus offer a significantly reduced cost of the converter. By having fewer inverter units, MP³C delivered not only a lower customer CAPEX but also a reduced footprint.

Driving into the future

The combined strengths of OPP technology and MPC in MP³C deliver the most efficient, high-performance motor control currently available. OPPs provide minimal harmonic distortions per switching frequency and MPC enforces the ideal stator flux trajectory of the OPP by manipulating the OPP’s switching instants. A high sampling frequency of 40 kHz with a receding time-horizon policy results in a fast dynamic response and superior disturbance rejection.

With MP³C, ABB has attained a good balance point – where enhanced dynamic performance meets excellent harmonic performance. Simulations, laboratory and field results demonstrate the superiority of MP³C compared to conventional methods. For instance, the trade-off between the inverter losses and the harmonic losses of machines when varying the switching frequency shows that MP³C performs better than the frequently used field-oriented control (FOC) method in terms of losses, delivering substantial annual electricity savings. Similarly, reduced air gap torque TDD results in lower mechanical and thermal stresses, extending equipment lifetimes and lengthening maintenance intervals, ultimately translating to lower operational expenditure.

Increasing the fundamental frequency is possible thanks to MP³C’s low harmonic distortions at low pulse numbers. This aspect opens up the system design space for applications such as compressors, which require high-speed motors. By increasing the output speed of the drive, the size and weight of an additional gearbox can be decreased, or the need for one obviated, leading to a lower initial investment.

MP³C combined with the proven ACS6080 and the new ABB ACS8080 drives makes for a winning team that will literally drive performance, efficiency, and safety across a wide range of industries – such as minerals and mining, oil and gas, marine and pumped-storage power plants – with best-in-class motor control

References
[1] J. Holtz and N. Oikonomou, “Synchronous optimal pulse-width modulation and stator flux trajectory control for medium-voltage drives,” IEEE Transactions on Industry Applications, vol. 43, no. 2, pp. 600 – 608, March/ April, 2007.
[2] T. Geyer et al., “Model predictive pulse pattern control,” IEEE Transactions on Industry Applications, vol. 48, no. 2, pp. 663 – 676, March/April. 2012.

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