Medium Voltage Variable Frequency Drives (VFDs) are key components in the realm of industrial automation. They provide precise control and efficiency in the management of electric motors. In this article, we delve into what Medium Voltage VFDs are, key manufacturers in the industry, applicable specifications, and how to select an appropriate VFD.
Understanding Medium Voltage VFDs
At their core, Medium Voltage VFDs operate on the principle of variable frequency drive technology. They control the speed and torque of electric motors by adjusting the frequency and voltage supply to them. This dynamic control enables seamless adaptation to varying load conditions, optimizing energy consumption and prolonging the lifespan of equipment. Generally, the design of Medium Voltage VFDs is beneficial to motors ranging between hundreds to several thousands of horsepower. These drives operate at voltages from 2300V, which is classified as medium voltage according to ANSI C84.1. While in some heavy-duty applications, voltages could be as high as 69kV AC. VFDs can either be of the current source inverter (CSI) type or voltage source inverter (VSI). However, the VSI is more popular due to its high reliability and low harmonic distortion.
Common Manufacturers of Medium Voltage VFDs
There are a host of Medium Voltage VFD manufacturers in the industry, but this section reviews some of the most common.
ABB
ABB is a global leader in power and automation technologies, offering a diverse range of medium-voltage VFDs. Drives range between 200 kW to 150 MW, and even more, depending on project demand. VFDs from this manufacturer are tailor-made to meet demands for applications in several industries including marine, chemical, power, water, mining, as well as oil and gas. Common features of VFDs from this OEM are:
- Arc-resistant design.
- Fuseless design.
- Combination of air and water cooling systems.
- Softstarter.
- Constant network power factor across the entire speed range.
Siemens
Siemens has established itself as a prominent player in the medium voltage drive market, delivering innovative solutions via its SINAMICS Perfect Harmony product line. Some features associated with this product line include:
- Advanced Cell Bypass: Can bypass multiple failed cells in less than a quarter of a second to maintain a balanced output voltage.
- Synchronous Transfer: This serves to soft-start multiple motors in a series. In addition, it efficiently transfers them across the line without stressing the power grid. Synchronous transfer increases energy efficiency, as well as protects motors and equipment from excessive torque transients.
- Clean Power Input: SINAMICS Perfect Harmony VFDs meet the most stringent IEEE-519-2014 requirements for voltage and current harmonic distortion. It utilizes an integrated sinusoidal converter to eliminate the need for harmonic filters and power factor correction capacitors.
- Process-Tolerant Protection Strategy (ProToPSTM): Unlike typical systems that trip the drive and shut down automatically due to a malfunction, ProToPSTM offers proactive control for applications where failure avoidance is critical. With a proven record of 99.99% process uptime, it protects systems from faulty sensors or data.
Schneider Electric
Schneider Electric provides medium voltage VFDs, ranging between voltage classes of 2.4kV to 6.6kV, while the maximum power rating ranges from 241 hp to 2279 hp. VFDs operate within a wide temperature range from -25℃ to 70℃. They can also operate at altitudes up to 3280.84 ft without derating, and up to 6561.68 ft with 1% current derating per 328ft.
Toshiba
Toshiba is an industry leader in providing Medium Voltage VFDs with features such as multi-level Pulse Width Modulation (PWM) with Neutral-Point Clamping (NPC). This enables a smaller footprint, lesser number of components, and lower cost than many competitors. Its designs are also robust, with the MTX being the world’s first drive specifically for outdoor mounting in remote applications.
How to Select Medium Voltage VFDs
Selecting an appropriate VFD for an application ensures maximum long-term payback and minimum initial cost. To get the selection process right, there are key factors that are divided into two major groups: Electrical Factors and Design Installation Factors.
Electrical Factors
- Process Loads: In VFDs, process loads are classified into constant torque and variable torque. Constant torque (CT) loads include equipment such as conveyors, crushers, and grinders. While variable torque (VT) loads are typical of pumps and fans. Generally, VFD suppliers provide 10% additional overload capacity above full torque and ampere rating for VT loads. For CT loads, there is usually 50% additional capacity, with both additional capacities limited to a time of 60 seconds. Other factors to consider include the load profile, duty cycle for the process to be powered, and regeneration requirements.
- Drive System: For the drive system, it is necessary to define the system voltage, motor voltage, and VFD waveform. Inform the supplier if the motor to be powered is new or existing, as this will determine the drive input voltage. Although Medium Voltage VFDs without isolation transformers are cheaper, they are prone to malfunction and damage from lightning and line voltage surges. So, it is advisable to insist on having isolation transformers.
- Power System: When assessing power requirements, there are two roles that VFDs play. One is a power consumer, so the power supply in the facility must enable the VFD to operate reliably. The second role is as a neighbor to other equipment in the facility such as switchgears and switchboards. When VFDs draw power, they create harmonics – distortions in voltage and current. These harmonics could cause malfunction and extra heating of other equipment connected to the same power system. Choose a VFD that keeps harmonic values to a minimum. IEEE 519-2014 recommends 5% or less of fundamental current, and 5% or less distortion on voltages produced by this current. Review the VFD supplier’s power factor impact on the facility’s power system, and take advantage of modern power factor correction technology.
Design Installation Factors
- Physical Environment: Most medium voltage VFDs operate indoors in a clean and corrosion-free environment. However, they could also function outdoors where the conditions often become extreme. Depending on the location and conditions, the influence of derating should be applied to its capacity. Air-cooled VFDs typically require about 0.85 tons of air-conditioning per 100 hp of load. As load requirements increase, it may be profitable to shift to water-cooled VFDs, especially if cooling water is available. It is necessary to prevent dust from accumulating on electronics and heat sinks on the equipment.
- Auxiliary Equipment: Some applications require additional equipment to ensure reliability. For example, motor-mounted tachometers are a requirement when applications need to reach values higher than 150% of torque rating. It is necessary to review the possibility of including such auxiliary equipment with the supplier.
- Maintenance and Troubleshooting: Currently, computer tools for maintenance and troubleshooting have many desirable features. Some of these include built-in trending, fault logging, and email notifications when trouble occurs.
- Connections: There are a variety of communication protocols available when connecting control systems to a VFD. The supplier needs to clearly define interface requirements.
- Restrictions and Requirements: Usually, medium voltage VFDs are tolerant of input and output connections and cables. Manufacturers recommend EMI shielding on output cables and proper grounding of the equipment.