75 MVA converter duty transformer
After publishing several theoretical articles on VFD duty transformers it is time to have a closer look at some real designs. In today’s post we present a brilliant example: 75 MVA converter duty transformer designed according IEC 60076 standard. This transformer is supplied from 110 kV distribution line, has four secondary windings feeding a 24-pulse active rectifier and in addition a tertiary winding to connect a harmonic filter. The entire construction is optimized for low level of acoustic noise. Does it sound interesting? Then continue reading this post!
Input transformer for a high-power drive
The subject transformer, manufactured by ETD Transformatory [2], is used to supply power for a multi-megawatt variable frequency drive (VFD) that is then controlling speed of a 2-pole synchronous turbo-motor (60 MW, 2930 rpm). The VFD technology is a load-commutated inverter (LCI). Refer to our ‘LCI versus VSI’ series [3] where we have thoroughly compared these two technologies used in large capacity drives. LCI offers unmatched power density and excellent reliability track. On the other hand, input power factor of LCI is low at low motor speed. Therefore, an input filter is used to (a) compensate the reactive power, (b) minimize characteristic harmonics. This is the reason for having an additional 30 MVA tertiary winding on the transformer. Due to water forced cooling (OFWF) the transformer is quite compact despite the high power and high voltage (see fig. 1).
Topology of 75 MVA converter duty transformer
Transformer has four secondary windings. Each winding supplies one 6-pulse bridge of thyristor rectifier. The secondary windings have a relative displacement of 15° providing the drive a total 24-pulse reaction towards the high voltage grid. Such configuration provides relatively low harmonic footprint. A dedicated tertiary winding allows the connection of a multi-stage filter used for harmonic filtering and reactive power compensation. Electrostatic shield is applied between the windings.
Challenging design
Design and construction of this 75 MVA converter duty transformer was quite a challenge. Reason for that is a combination of several important requirements. Besides general high power rating and high voltage on primary side, the multi-winding design with corresponding phase shifting comes into picture. In addition, there is a tertiary winding dedicated for a harmonic filtering and power factor compensation. Furthermore, to make everything work, a rather strict impedance requirements must be fulfilled. We don’t talk about just one value of a short circuit impedance but about a complex impedance matrix (see below). Also, the transformer shall be short circuit proven and high value is put on the quality of the tank. Finally, the design shall respect strict requirements on maximum acoustic noise.
Impedance considerations
One of the functions of VFD transformer, apart from voltage adaptation, is the limitation of fault current on the converter side. For such purpose the VFD manufacturer specifies the transformer voltage impedance to be in certain range. In fact, a complex transformer, such as the one described, does not have just one impedance value. An impedance matrix is used to characterize the transformer. These impedances are important for proper operation of LCI. Minimum impedance between primary (HV) and each secondary (LV) winding guarantees a short circuit proof design. On the other hand, impedance between secondary (LV) winding and filter winding (FV) shall not be too high in order to keep reasonable commutation length. To make it even more complex a knowledge of impedance behavior at higher frequencies is needed. A proper transformer design is an iterative process between transformer and VFD manufacturer in order to find the balance.
Filter considerations
Depending on the operation philosophy the filter branches are all connected and energized before the drive starts the operation or they are connected later on at defined minimum load. The selected approach also impacts the transformer design as it shall withstand increased voltage without core saturation. The branches of the filter are tuned for the characteristic harmonics produced by the VFD and the filter winding is exposed to a distorted current waveform (in this case less of an issue due to 24-pulse configuration).
Transformer data in a nutshell
Rated power 75.2 MVA / 4 x 18.8 MVA / 30 MVA (primary/secondary/tertiary)
Primary voltage: 110 kV
Insulation primary: LI 550 kV / AC 230 kV
Secondary voltage: 4 x 4.2 kV
Tertiary voltage: 13.8 kV
Regulation: De-energized tap changer ± 2 x 2.5%
Winding material: Copper
Insulation liquid: High grade mineral oil
Cooling: OFWF (Oil Forced Water Forced)
Tank construction: Conservator type, vacuum proof
Redundancy: Redundant heat exchangers and pumps (2 x 100%)
Dimensions (total): 5’250 x 3’850 x 5’450 mm
Total weight: approx. 100 t
Transport weight: approx. 76 t
Design lifetime: > 30 years
Low noise design
In addition to above technical requirements there are strict limits on maximum audible sound level on site. At the same time, the typical noise level of a transformer increases as the rated power goes up (see experimental formulas in IEC 60076). Moreover, additional noise is expected due to converter operation and non-sinusoidal harmonic currents. Therefore, a special low-noise transformer design is required. The manufacturer implemented several measures to reduce the noise level to a target value specified by the end user. Those measures include the use of high quality steel sheets, reinforced mechanical clamping etc.
Cooling redundancy
The cooling circuit is fully redundant. It consists of two pumps and two water-to-oil heat exchangers (always just one in operation). Due to a cross-connection with corresponding valves each pump can be used with each heat exchanger allowing maximum availability of the cooling system. The heat exchangers can be of tubular shape like in this case or of a plate type.
Outline drawing
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Accessories and protection elements
The transformer is equipped with all “standard” accessories that are used for equipment of this type. These include oil and winding temperature indicators, oil level indicator, Buchholz relay, pressure relief device, dehydrating breather, oil flow indicator, auxiliary control box, ladder etc. All protection and control equipment originates from renowned manufacturers. The converter side windings are equipped with plug-in connectors.
Summary
High power multi-winding VFD transformer is an engineering masterpiece. Practically every design is unique due to project specific requirements. This article presented a 75 MVA converter duty transformer. It belongs to designs with highest complexity. Combination of high power rating, high input voltage, four secondary windings with phase shifting, additional tertiary winding and very specific impedance requirements is very demanding and only few manufacturers are able to build such units. Above transformer successfully passed all routine and type tests and was shipped to its final destination. Transformer manufacturer provided an excellent support during installation and commissioning which is quite critical for such large equipment. The commissioning is scheduled to be completed in Q1/2022.
The described 75 MVA converter duty transformer is a great example of the theoretical topics discussed in previous posts.
References
[1] VFD transformers – entire series, https://mb-drive-services.com/category/vfd_transformers/
[2] ETD Transformatory, http://www.etd-bez.cz/en
[3] LCI versus VSI series, https://mb-drive-services.com/category/lci-versus-vsi/
[4] MEGADRIVE-LCI, https://new.abb.com/drives/medium-voltage-ac-drives/megadrive-lci
[5] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
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