What kind of Medium Voltage drive configuration do you need?

Think of a drive as a black box converter. On the input side you have the grid supply with constant sinusoidal voltage and frequency and on the output, you want again a perfect sinus with variable voltage and variable frequency. The ideal black box does not influence your grid supply and delivers a pure sine wave to your motor. Now, what does it take to deliver such superb power quality and why does it matter? This is where the various types of medium voltage drive configurations, called topologies, come into play.

Let’s get into some basics first:

Direct or indirect, current or voltage, 3-level, multi-level, multi-cell, …. Here is a short demystification diagram.

The topology of the variable frequency drive combined with its filters determines the power quality of the power required for and delivered by the drive. In the figure below you get an overview of the different electrical configurations of MV-drives. You can hover or click to see the basic diagram for the drives per topology. We will for now only focus on the voltage source inverter (VSI) within the indirect conversion branch (AC-DC-AC).

Before getting into the nitty-gritty of topologies, let’s get the basics right.
Below you find a basic diagram of a low voltage source inverter.
There are basically 3 building blocks to this kind of variable frequency drive: the rectifier (DFE= Direct Front-End), the intermediate circuit and the inverter. The voltage source inverter relies on capacitors in the intermediate circuit between rectifier and inverter. The capacitors create a decoupling between the rectifier and the inverter and function as an energy buffer. In the third building block, often referred to as inverter, the DC-voltage is recombined by the switching pattern of the IGBTs (Insulated Gate Bipolar Transistors) to a fundamental AC-component with adjustable amplitude and frequency.

In the “topology-slang” the terms pulses and levels are quite important. Back to the analogy of our drive as a black box, it is safe to say that the term “pulses” relates to the input side and the term “levels” refers to the output stage. In our next chapter we will focus on the “levels”.

“Great things are done by a series of small things brought together”(Vincent Van Gogh)

Take a second look at the figure above and notice that the output voltage will be created by connecting the plus and the minus voltage at a high switching frequency following a Pulse Width Modulation (PWM) pattern. This switching between two levels makes this a two-level drive.
With the figure below you get the picture. This is a 3-level drive.

Notice the phase-to-phase output voltage of the 3-level inverter is made out of 5 steps and is already looking much better than the two-level output voltage. Some manufacturers also offer also 5-level inverters. It must be noted that, if the 3- or 5-level inverters are to drive standard direct-on-line motors designed with standard insulation, these inverters will require a sine wave output filter to reduce common mode voltage and voltage stress on the motor. These supplemental output filters can be of a significant cost and might also introduce performance restrictions on the power drive system.
Moving further in our overview of topologies and adding in more levels, you discover the modular multi-level (M2C or MMC) topology.

Notice in the basic diagram of the modular multi-level inverter that the centralized DC link capacitors in the DC-circuit are no longer present. Instead, we find capacitors for each power-cel within the inverter part. In the simplest case, such a power-cell consists of two IGBTs and one capacitor bank. The low energy level stored in each cell results in improved fault behavior as compared to other drive topologies. Medium voltage levels are obtained by adding together the outputs of multiple power-cells. We will dive more into the technical bits later on. For now we can state that this drive delivers a superior sine wave output voltage and current, thereby eliminating harmonic heating and insulation stress on the motor and reducing significantly the torque pulsations (<1%).

Next in our topology overview is the 2 Level Series Connected-multi-level cascaded H Bridges cells (2L SC-HB); what a mouth full ;-).

The multi-level cascaded H Bridges cells topology adds up the outputs of multiple low voltage power-cells to obtain medium voltage levels. Each power-cell is a simplified version of a standard 2-level PWM low voltage drive (H-Bridge converter). Many applications, including low voltage drives, use low voltage IGBTs, manufactured in very high quantities. Medium voltage drives that utilize low voltage devices can take advantage of rapid innovation cycles, exceptionally reliable components, and a sustained availability of spare parts.

The result of adding up the output of multiple low voltage power-cells is a high-quality output voltage providing:

• less than 1% induced torque ripple
• no need for output filter for motor cable lengths up to 2 km.
• high reduction in common mode voltage
• no derating on new or existing motor
• no voltage spikes at the motor thanks to the small output voltage steps thereby allowing the use/re-use of a motor with standard insulation

The integral dry multi-winding isolation transformer provides for

• adaptability to available input voltage
• clean input power thanks to the cancellation of harmonics by phase displacement (=>multi-pulse input from 18 to 54)
• isolation from the grid avoiding potential resonances and reducing impacts from utility transients such as lightning and other equipment switching.

This architecture does not allow for regenerative operation or extremely quick deceleration. An active front-end version of this architecture is rarely economically viable. You can use this kind of drive for general purpose applications. For the more demanding applications with 4 quadrant and high dynamic response, you will need to investigate the more advanced topologies. In fact, those advanced topologies are quite often a combination of above-mentioned topologies. Just fill out our form for more advice.

Power quality, is it all about the pulses?

The rectifier bridge connected to the capacitor only conducts current at the peaks of the voltage waveform (when the AC-voltage is higher than the capacitor voltage). So, the AC input current waveform will contain bumps. In the “Basic diagram of a voltage source inverter” below you can see that we actually have 6 pulses in the rectified voltage (Udc) over one period of the mains voltage. In fact, when referring to 6 pulse drive, we relate to a standard rectifier bridge containing 6 diodes resulting in a 6 pulse DC voltage. (For a more exact and a bit more technical explanation, we should state 3 commutation groups; a commutation group is essentially a leg which may be comprised of several devices in series or parallel to block the voltage or handle the current. See dotted lines in figure below).

The 3-level inverter depicted in the figure below is made up of two 6 pulse rectifiers supplied by a 3 winding converter transformer (=12-pulse infeed). The secondary windings of these 3 winding transformers have a phase shift of 30°el, thereby cancelling harmonics (the fifth and the seventh) on the primary side. This 12-pulse infeed results in lower line harmonic distortions. The ac line input current of the 12-pulse drive looks indeed a lot better than the 6-pulse one.

The 12-pulse drive will generally not meet the requirements for voltage and current harmonic distortion imposed by current standards. Going for higher number of pulses such as 18, 24 or even 36 will further reduce the harmonic distortion, but this comes at a price (rectifiers and transformers). An alternative to the architecture characterised by the number of pulses is the Active Front-End (=AFE). Instead of a rectifier or Direct Front-End (=DFE) we use basically a “repeat” of the inverter. The active control of the input stage enables a much cleaner input power and provides even for 4 quadrant operation.  

As already mentioned above, advanced topologies are quite often a combination of above-mentioned topologies. Here below you will find modular multi-level combined with 3-level output stage. It is a more advanced version of AFE than the one shown on the figure “3-level AFE basic diagram”. This AFE guarantees an even cleaner input power thanks to its modulation technique with selective harmonic elimination.

Combining the modular multilevel version for AFE and for the inverter stage together realises a high performance drive with clean in- and output power.

If we lost you along the line, do not hesitate to contact us for a free video call

General purpose versus complex applications

Not surprisingly, most applications (>80%) are quite standard and do not demand high performance drives with high dynamic response and/or regenerative operation. Most medium voltage drive manufacturers will refer to these drives as general purpose or standard drives.  The topology of those voltage source inverters is most of the time 3 or 5 -level or multi-level cascaded H Bridges cells. The list of concerned machines is quite long, but most common machines classified within general purpose applications category are 2 quadrant applications such as fans and pumps, compressors, propulsion, extruders and even most crushers.
It becomes a bit trickier when the application requires high performance torque control, fast dynamic response and/or 4 quadrant operation. These drives are sometimes referred to as engineered drives or high-performance drives. The topology of those voltage source inverters can vary from a standard 3 level VSI, with an active front end either 3-Level or modular multi-level, up to a full 4 quadrant modular multi-level drive. Some typical machines in this category are test benches, conveyor belts, centre winders, shaft generators, rolling mills and grid convertors.
As mentioned in our chapter application, close collaboration of your project team with your machine manufacturer is preeminent.
You need to ask the right questions to your machine manufacturer and to your vfd manufacturer candidates. You can contact us to assist you in the dialogue and help you in the clarification of their answers.

In all cases above we would be glad to assist you. All it takes is that you contact us for a free consultation.
Further down the road, we will enable informed decision-making by offering you our independent advice and support for  following tasks:

• Develop the project plan presentation.
• Define and manage the requirements of the power drive system PDS (converter duty transformer, variable frequency drive vfd and AC-motor).
• Evaluate the composition of the power drive system.
• Set-up and coordinate internal and external pre-bid meetings.
• Set-up the invitation to bid with Engineering, Procurement and Contracting companies and/or vendors.
• Assess preferred manufacturers of transformer, inverter and motor.
• Verify the offers and exceptions in regard to your specs of your PDS.
• Help to select the vendor of transformer, variable frequency drive and motor.
• Clarify the order of your PDS.
• Set-up witnessed and non-witnessed Factory Acceptance Tests (FAT) requirements and test plan.
• Define Site Acceptance Tests (SAT) requirements.
• Validate the site acceptance tests for the components of your power drive system PDS on-site.