Most common medium voltage VFD topologies in comparison
Abstract
Medium voltage drives consist of several topologies. While some of them are seldom found in industrial applications, others are very popular and create integral part of the portfolio of most global VFD manufacturers. In this article we look at the most popular medium voltage VFD topologies along with their short characteristics.
Introduction
Industrial electric motor drives increasingly feature variable speed control by means of variable frequency drives (VFD). Looking at the portfolio of global VFD manufacturers we can find classical technology used since decades as well as modern topologies introduced to the market relatively recently. There are multiple factors determining popularity of a VFD platform: versatility, reliability, motor friendliness, compact design, modularity, production cost etc.
Medium voltage VFDs in general
Medium voltage VFDs include multiple different topologies. Nevertheless, some of them emerged due to their superior characteristics. Figure 1 shows one possible classification of medium voltage VFDs (depending on literature source you may find a slightly different categorization). The topologies discussed in this article are highlighted in red.
Voltage Source Inverters
Voltage source inverters (VSI) contain capacitor bank in the dc link. The purpose of the dc link is a decoupling of grid side and machine side converters, smoothening of dc voltage and a temporary energy storage (however with very short time constant, see [1]). For a very short time interval between few inverter switching instants the dc capacitor voltage can be assumed as (nearly) constant thus giving the inverter the character of a voltage source.
3-Level Neutral Point Clamped (3-L NPC) Inverter
3-level neutral point clamped (3-L NPC) is a popular medium voltage VFD topology available since late 1980s. The nominal output voltage is typically 2.3 kV, 3.0…3.3 kV and 4.16 kV. Output phase voltage has 3 levels, line to line voltage consists of 5 levels.
Topology
3-L NPC inverter is derived from the basic 2-level inverter used in low voltage VFDs. When increasing the nominal output voltage, using just two levels becomes troublesome (large common mode voltage, high insulation stress for the motor). Thus, at least 3-level inverters are used. 3-L NPC consists of four active semiconductor switches in each inverter phase. The active switches in the middle are connected to the neutral point of the VFD through clamping diodes → therefore the name neutral point clamped.
Characterstic
3-L NPC is a widely used VFD topology. It allows easy modularization and power scalability. It is a versatile platform with many design options such as integrated/external transformer, diode front end/active front end, with/without braking chopper, small dv/dt output filter/bulky output sine filter etc.
Variants
3-L NPC is a widespread topology dominating the 3 kV and 4 kV motor drives. The rectifier can be either diode front end (DFE, usually 12-pulse or 24-pulse) or an active front end (AFE).
The output circuit has a major impact on the characteristics of the VFD. Small dv/dt output filter is used for high performance drives. The inverter-motor combination reaches excellent dynamics. However, the inverter is less motor friendly and the motor shall be specifically designed for inverter duty type of supply. An output sine filter provides the exact opposite character of the VFD: high level of motor friendliness (‘standard’ motor) and limited dynamics.
3-L NPC is also available in a multidrive configuration, i.e. having multiple inverter modules connected to the same dc bus and supplying several motors. Speed and torque of each motor can be controlled individually.
Advantages
Among advantages of 3-L NPC topology belong the relative simplicity of the hardware, low parts count (several megawatt of power possible with just 12 active switches inside the inverter), high power density resulting is acompact design and small footprint, rectifier flexibility (AFE or DFE), possibility to have…
Limitations/drawbacks
3-L NPC is limited in output voltage. The topology is widely used in “3 kV” class, many products support also 4.16 kV. Higher voltages are generally not available. While this voltage level is a good fit in low/medium power range, it gets too low at high-power range resulting in high inverter currents.
Sine filter with superior output quality is available only up to …
Market presence
3-L NPC topology is included in the portfolio of many global VFD manufacturers, such as ABB (ACS1000, ACS6080), Ingeteam (MV 100, MV 500), Innomotics (Sinamics GM150, Sinamics SM150) or General Electric (MV 7000).
Cascaded H-bridge (CHB, multicell)
Another widely used VFD topology is a cascaded H-bridge (CHB) design, also called multi-cell as it uses power cells as fundamental building blocks.
Topology
The topology is CHB is schematically shown in Figure 3. The depicted variant is the smallest one in terms of number of cells. Input transformer is a dry-type multi-winding phase-shifting transformer, integrated in a common enclosure together with the VFD. Number of transformer secondary windings equals number of power cells.
Characteristic
Cascaded H-bridge is a very popular VFD topology, available on the market since late 1980s. The technology was pioneered by US company called Robicon. Although there are many VFD manufacturers having CHB in their portfolio, some people may still use the brand Robicon (acquired by Siemens, nowadays Innomotics) as a synonym for CHB. The topology is also known under the name “multicell” (slang) referring to the number of series connected power cells in each inverter phase. CHB scales the output power with the voltage. The more power is required, the more power cells per motor phase are used.
Variants
The topology is available for wide range of nominal output voltages, typically from 2.3 kV up to 13.8 kV. Input transformer is always integrated (built in the VFD) and the power cells have the same basic design. Frequent option is a bypass switch – it can be either manually operated or automatic.
Advantages
One key advantage of CHB is low production cost due to highly standardized power cells with high repeatability (exact same power cell can be used in VFD across wide range of power and voltage). Another strength is the motor friendly output waveform…
Limitations/drawbacks
The input transformer is always integrated inside the VFD due to the high number of secondary windings and large quantity of interconnecting cables. The integrated transformer increased the VFD footprint, adds losses into the electric room and is limited in the maximum input voltage. Due to its complex topology, an effective…
Market presence
CHB based VFDs are manufactured by practically all global manufacturers, among others ABB (ACS580MV), Hyundai (N5000), Innomotics (Sinamics GH180), Nidec (Silcovert TH) or Rockwell (PowerFlex 6000).
Modular multilevel converter (MMC, M2C)
The idea of a modular multilevel converter (MMC) is probably inspired by the cascaded H-bridge topology. Motivation to develop the MMC was to preserve the strengths of CHB (such as high-quality output waveform) while removing some of its main disadvantages (such as complex input transformer that can’t be separated, non-existent AFE variant etc). MMC is very perspective topology for high power converter applications. Original target was mainly the high voltage DC (HVDC) transmission. At least one leading manufacturer is marketing
MMC topology also as an industrial VFD.
Topology
Each phase of MMC converter consists of a string of power cells. They are similar to those of CHB. However, power cells of MMC converter do not have the input diode rectifier. They consist of cell capacitor(s) and Hbridge inverter. In principle, the MMC can be combined with other topologies, for example:
- Diode rectifier + MMC inverter (e.g. when no dynamic braking required)
- MMC rectifier + 3-L NPC inverter (e.g for high dynamics)
- MMC rectifier + MMC inverter (“back-to-back”)
Characteristic
MMC, as the name says, utilizes a modular design. The power is generally scaled with voltage. To reach higher voltage, more power cells are connected in series. The manufacturer may have one or a few designs of power cells (often just one design for highest repeatability and lowest cost). As for the CHB topology, the platform has high quality of output voltage waveform which further improves as the number of cells per phase increases.
Variants
MMC platform exists in two variants that are quite different. The common variant is an indirect MMC having a separate grid side converter and machine side converter. This is a preferred option for an industrial MMC based VFD. As described above in the chapter “Topology”, several combinations are possible: e.g. MMC inverter combined with a diode rectifier or MMC based active front end combined with 3-L NPC inverter.
The other variant…
Advantages
MMC achieves excellent quality of the output waveform…
Limitations/drawbacks
MMC topology at medium voltage and power typically leads to larger footprint…
Market presence
MMC as a VFD is primarily offered by Innomotics (Sinamics GH150).
Current Source Inverters
Current source inverters (CSI) have an inductor in the dc bus. The purpose is exact same as in the VSI drives: to decouple the grid and machine side, to smoothen the direct current (dc) and to provide time-limited energy storage. Inductor limits the rate of change of the throughput current, i.e. dc current cannot change step-wise.
During the short time between inverter commutations the current can be considered as (nearly) constant thus providing this type of inverter the current source behaviour.
Self-Commutated CSI (PWM CSI)
First representative of current source inverters is a pulse-width modulated current source inverter (PWM CSI). It uses self-commutated power semiconductors in the inverter section (and the AFE variant also in the rectifier section).
Topology
The topology, developed in late 1980s, is using self-commutated reverse blocking power semiconductors inside the inverter. The rectifier uses active switches as well – either thyristors (multi-pulse rectifier to comply with harmonic standards) or self-commutated semiconductors constituting an active front end (AFE). The rectifier and inverter are decoupled by a dc link consisting of a dc reactor.
Characteristic
This platform is a current source inverter with self-commutated power switches inside the inverter that allows modulation such as PWM or a pulse pattern / selective harmonic elimination. It produces low common mode and is therefore motor friendly.
Variants
The variants differ in the type of rectifier while the inverter remains unchanged. The user can choose between multi-pulse thyristor rectifier (e.g. 18-pulse offered by the main manufacturer of this topology) supplied from a multi-winding phase-shifting transformer or an active front end rectifier.
There is also a transformerless (“direct-to-drive”) sub-variant of the AFE which eliminates the input isolation transformer. Pre-condition is a suitable nominal grid voltage.
Advantages
This topology supports both asynchronous (induction) and synchronous machine drives. Thus, it is competitive also at lower power. Integrated transformer variant is also offered in the lower power range. Bi-directional power flow…
Limitations/drawbacks
Commutation capacitors are required on the motor side. Power scalability…
Market presence
Global manufacturer of PWM CSI drives is Rockwell (PowerFlex 7000).
Load-commutated inverter (LCI)
LCI is a well-known high-power drive topology. A lot has already been written in our blog. Here a brief description in context of other topologies is provided.
Topology
Load-commutated inverter (LCI) consists of thyristor bridges that form the line side and machine side converters. The thyristors are switched on by current impulse injected through the gate. However, the switch off process is driven by the external voltage. On the grid side the thyristors are line-commutated (by line voltage) and on the machine side they are load-commutated (by machine voltage). That characteristic gives the topology its name.
Characteristic
LCI is on the market since around 50 years. The topology is simple and straightforward. It is well-known for its robustness and reliable operation. High power density leads to the most compact design in the multi-megawatt range. Three basic variants of LCI exist (see next paragraph).
Variants
LCI distinguishes three different variants based on their purpose:
- Soft starter
- Gas turbine starter
- Full drive
LCI soft starter is used to start large fix-speed synchronous machines when the direct on-line starting current is not acceptable. Soft starter is normally designed for short time duty and may actively utilize thermal overloadability of the power electronic converter. The soft starter system often includes input and output transformer. The converter has a synchronization equipment that allows a smooth transition from inverter supply to the grid supply. Minimum configuration is a 6-pulse line converter and 6-pulse machine converter. Larger soft starters may be 12-pulse on the line side.
Gas turbine (GT) starter is used to start a gas turbine and bring it to the minimum speed where the combustion is started. From there the gas turbine acts as a mechanical driver and accelerates on its own. Thus, GT starter does not require any synchronization equipment.
LCI full drive is used to continuously control the speed or torque of the synchronous motor. The full drive configuration typically consists of a 12-pulse line converter and 12-pulse machine converter feeding a dedicated dual three-phase machine. Full drive usually adds a grid filter to minimize harmonic distortion and compensate the reactive power (maintain input power factor close to unity).
Advantages
LCI is a very robust VFD platform due to combination of thyristors and…
Limitations/drawbacks
The inverter requires an overexcited synchronous machine to commutate…
Market presence
LCI is actively manufactured by ABB (MEGADRIVE-LCI), Innomotics (Sinamics GL150) and General Electric. Few smaller (regional) manufacturers also produce LCI. Some companies are active in control upgrade business.
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You can also purchase our webinar “VFD topologies in comparison – One size does not fit all” that handles the exact same topic. In approx. 90 minutes we will walk you through the most common medium voltage VFD topologies. You can watch this on-demand webinar whenever the time suits you through the WebinarJam platform.
Literature/Reference:
[1] Bin Wu, Mehdi Narimani: High-power converters and AC drives, second edition, IEEE Press, 2017, ISBN: 978-1-119-15603-1
[2] Product brochures and catalogs of the respective manufacturers
[3] University classes
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