1. Introduction
A well-known domestic chemical enterprise, in response to the national "Carbon Peak and Carbon Neutrality" goals and considering its production status, planned to implement an energy-saving, emission-reduction, and capacity-expansion project for its polycarbonate production unit. This included converting the originally steam-driven MVR steam compressor to electric drive.
The MVR steam compressor system was originally driven by a high-speed steam turbine (referred to as the steam turbine), directly connected to the drive shaft of the high-speed centrifugal compressor, as shown in Figure 1a. The main challenges during the "steam-to-electric" retrofit of the compressor were as follows:
(1) Implementing an electric drive conversion using a traditional motor would require adding components such as a speed-increasing gearbox, lubrication station, and turning gear (as shown in Figure 1b below), necessitating demolition and retrofit of the existing steam turbine foundation. This would require redesign and verification of the torsional vibration system, resulting in a long retrofit cycle, high construction intensity, and issues such as low transmission system efficiency, high energy consumption, and high maintenance costs.
(2) Adopting the model of "High-speed motor + High-frequency medium voltage drive" (as shown in Figure 1c) could utilize the existing steam turbine foundation for retrofit, making project implementation less difficult. However, this technical route poses significant challenges for feasibility assessment.
(3) Large-capacity high-speed direct-drive systems represent an advanced technological direction in the field of electrical drives. The structural strength of high-speed motors, the high-frequency drive performance of medium voltage drives, safety and reliability are all at the forefront of industry technology.
Figure 1 Diagram of the MVR steam compressor drive structure before and after retrofit
As a crucial technical renovation project for energy saving, carbon reduction, capacity expansion, and efficiency improvement, both the traditional "Medium voltage drive + Standard motor + Speed-increasing gearbox" and the advanced "High-speed motor + High-frequency medium voltage drive" routes presented technical or implementation difficulties.
In light of this, after extensive investigation, the implementing organization ultimately decided to adopt the advanced "High-speed permanent magnet synchronous motor + High-frequency medium voltage drive" technical route. The parameters of the supporting equipments are shown in the table below:
2. Analysis of Key Technical Difficulties
Typically, the output frequency range of medium voltage drives is 0-120Hz, whereas the rated frequency of the high-speed motor supporting this project is 253.33Hz, with a maximum operating frequency of 260Hz. This means the medium voltage drive needs to possess the driving capability for high-frequency output at 260Hz and above. This is not merely a simple frequency doubling of the output; it places higher demands on the drive's signal sampling rate, output carrier frequency, system communication speed, main control computing power and control algorithms, suppression of power device high-frequency switching losses, and thermal simulation optimization of power cells.
2.1 Permanent Magnet Synchronous Motor (PMSM) Rotor Position Detection at Standstill
When a permanent magnet synchronous motor rotor rotates, an induced EMF is caused on the stator side. The drive can obtain the real-time rotor position angle through phase-locking of this induced EMF. However, when a PMSM is at standstill, the correspondence between the rotor magnetic pole position and the stator magnetic poles is random (as shown in Figure 2).
Without performing rotor standstill position detection, directly applying a three-phase voltage to the stator, while the stator magnetic field and rotor magnetic field are random, this could cause the motor to start in reverse, and in severe cases, lead to drive output overcurrent and damage to the mechanical transmission. To avoid these situations, when starting a synchronous motor, the initial position of the PMSM rotor must first be detected. After obtaining the rotor's initial position, a three-phase voltage in phase with the rotor angle is applied to start the motor.
Figure 2 Uncertainty of rotor magnetic pole position relative to stator magnetic poles at standstill
To address this, Nancal Electric developed rotor position detection at standstill technology for PMSM in speed sensorless control scenarios. The difficulty of this technology lies in the software requiring a foundation based on "vector control algorithms," and the hardware requiring the drive to possess high-precision output-side voltage and current sampling and processing capabilities to accurately identify weak variations in the motor-side sampling signals.
This technology achieves a rotor initial position detection accuracy error of <3°. The initial voltage vector angle of the drive output starts from the rotor position angle, preventing startup reversal and overcurrent. The figure below shows the voltage, speed, and current waveforms during startup using this technology. As can be seen, the frequency drive drives the motor smoothly, without voltage or current overshoot.
Figure 3 Voltage, speed, and current waveforms during startup
2.2 Ultra-High Frequency Drive High-Speed Sampling Technology
When a high-speed motor operates at 260Hz, each cycle is only 3.85ms (T=1/260≈3.85ms). Compared to the driving condition of a general-purpose medium voltage drive at 50Hz (20ms per cycle), achieving the same control performance requires the sampling cycle to be more than 5 times faster. Therefore, compared to motors with a conventional rated frequency of power frequency, driving high-frequency motors requires higher frequency sampling and control to ensure the dynamic response speed of the drive output.
Meanwhile, the voltage and current sampling components of the drive must possess high bandwidth (>100kHz) and low latency (<1μs) characteristics; the ADC conversion time and signal conditioning circuit delay of the main control system must be strictly controlled; the control algorithms of the main control system must complete Park/Clarke transforms, PI regulation, PWM generation, etc., within a single control cycle, placing extremely high demands on real-time performance.
2.3 High-Frequency Output PWM Carrier Modulation Technology
When driving a motor at 260Hz, if the carrier ratio (carrier frequency / operating frequency) is too low (e.g., <20), it will lead to a significant increase in current harmonics, affecting torque smoothness and efficiency. To reduce current ripple and improve control accuracy, the PWM carrier frequency needs to be increased. However, increasing the carrier frequency results in increased power device switching losses and du/dt stress. Therefore, rationally selecting the PWM carrier frequency and implementing output modulation across different operating frequency ranges is one strategy to address high-frequency output control of drives.
The NC HVVF series medium voltage drives, with their cell series structure, achieve high-frequency carrier output on the motor side through two technologies: cell PWM carrier frequency and cell series phase-shifting superposition. This meets the control accuracy requirements under high-frequency output conditions of the drive and effectively suppresses the switching losses of power devices, satisfying application scenarios for 120-260Hz high-frequency drive operation.
2.4 Impact of Power Cell and Main Control Communication Speed on High-Frequency Drive
The communication speed between the drive's main control system and the power cells is also a crucial factor for medium voltage drives for high-frequency motors. High-frequency PWM carrier frequency requires the main control system to provide high-resolution IGBT switching command signals to all power cells via high-speed communication. This ensures that the IGBT inverter bridge circuit of each power cell can switch on and off precisely.
Compared to the 1-2MHz communication speed of general-purpose drives, medium voltage drives applied in high-frequency motors need to adopt high-speed communication fiber optic links with speeds above 10MHz and high-speed FPGA processing capabilities. Through carrier modulation technology, dead-time compensation technology, carrier phase-shifting and other technologies, high-speed transmission of drive signals for each power cell is achieved, effectively avoiding issues such as unsynchronized cell outputs caused by large communication delays or insufficient bandwidth, which could result in circulating currents and voltage imbalance.
2.5 Demands of High-Frequency Drive on Main Control Computing Power
High-frequency drive medium voltage drives not only place higher demands on the drive's output sampling, IGBT drive carrier frequency, and cell fiber optic communication but also impose higher requirements on the processing speed and floating-point computing power of the medium voltage drive's main control system.
The main control hardware platform of the NC HVVF series medium voltage drives adopts an integrated "DSP + FPGA + ARM" triple-core main control board, overcoming the limitations on processing performance imposed by the backplane bus architecture of traditional rack-mounted main control cabinets. The main control chip features high clock frequency (≥500MHz) and a hardware Floating-Point Unit (FPU), enabling it to better adapt to and meet the control demands of high-frequency drives.
3. Application Results
The NC HVVF series medium voltage drives provided by Nancal Electric delivered excellent high-frequency drive performance and system safety control performance for the high-speed PMSM in the MVR steam compressor "steam-to-electric" retrofit project of a polycarbonate production unit, providing highly reliable and sustainable technical support for the project retrofit. Since being put into operation in June 2023, they have continuously and stably operated for over 20,000 hours without failures or unplanned shutdowns, playing a significant role in helping the customer achieve energy savings, efficiency improvements, and ensuring production benefits.
The drive's operation data monitoring is shown in the figure below:
Figure 4 On-site drive operation data monitoring screen
4. Conclusion
From the above analysis, it is evident that medium voltage drives for high-frequency motors differ significantly from general-purpose medium voltage drives in terms of hardware, communication, carrier control, and algorithms. High-frequency drives place higher demands on the product's hardware development platform and software computing power in key technologies such as the main control platform, signal sampling, power electronics drive, cell communication etc. For high-speed synchronous motor applications, comprehensive requirements of "high precision, high bandwidth, high synchronization, and high computing power" are placed on medium voltage drives.
The key technological breakthroughs lie in:
(1) The rotor's initial position angle detection at standstill;
(2) High-bandwidth, low-latency sampling and communication architecture;
(3) Optimal balance between PWM carrier frequency and switching losses;
(4) Vector control algorithms tailored for high-frequency PMSM.
To meet the application demands of high-frequency medium voltage drive technologies for high-speed motors, system-level collaborative optimization is required, integrating high-power power electronics drives, a triple-core integrated main control platform, and high-bandwidth dynamically optimized vector control algorithms. This enables precise starting, stable control, and reliable operation of high-speed PMSM in medium-voltage and high-power scenarios.