A 66 -Types of Transformers - Part 3

In part 2 of this series, we have seen brief descriptions of various types of transformers used in T&D and utilization areas.

A 65 -Types of Transformers - Part 2

https://www.dhirubhai.net/pulse/65-types-transformers-part-2-p-ramachandran

We will discuss some more types of transformers in this part of the article.

?15 .??Photovoltaic Solar inverter Transformers

?Inverter transformers are used in solar parks for stepping up the AC voltage output (208-690 V) from solar inverters (rating 500-2000 kVA) to MV voltages (11-33 kV) to feed the collector transformer. These are usually star (LV) /delta (HV) connected three phase units with DETC on HV side and with multiple LV circuits (2 or 4) for connecting to individual inverters- Transformer ratings up to 5 MVA are with double LVs and up to 16 MVA are with quadruple LV circuits. LV side of transformer will see voltage polarity reversals, pulsation and heavy harmonic voltage/current content from the solar inverters. Usually, a grounded electrostatic shield is provided between LV and HV windings to filter out these harmonics reaching HV side. Mineral oil or ester is used as insulating liquid in these transformers.

IEEE Std C57.159-2016 Guide on Transformers for Application in Distributed Photo Voltaic (DPV) Power Generation Systems

?16.????Collector Transformer for Solar & Wind farms

Collector transformers are used at solar or wind power farms to pool the output from individual solar or wind turbine transformers and to feed to the power grid. These are two winding, three phase step-up transformers with OLTC on HV side. The outputs from the HV side (11-33 kV) of individual solar inverter or wind turbine transformers are pooled and stepped by the collector transformer to the transmission line voltages (66-400 kV). The MVA rating of these collector transformers is limited by the transformer MV breaker rupturing capacity. At 36 kV, maximum transformer MVA that can be handled by breaker is around 160 MVA with normal transformer impedances. So, the maximum size of collector transformer is limited to 315 to 400 MVA with double LV circuits at 33 kV. Maximum collector transformer rating is limited to 400 MVA, 33-33/ 400 kV ,three phase, YNyn (or YNd) connected, three winding transformers.

CEA, India, Publication - Standard Technical Specifications of Power Transformers for Solar Park Pooling Stations”, 2021

17.????Motor Starting Transformers

Motor starting transformers are small, three phase auto-transformers with variable output voltage taps for starting large induction motors at reduced voltages. These are used to reduce the motor starting currents. Since the load on these transformers is intermittent and of short duration, high current density is used in windings keeping the temperature rise within permissible limits of insulation. Core flux density is kept below 1.6 T to reduce the transformer inrush current from saturation. Sometimes air gap is provided in core to reduce the voltage change at motor terminals. These are available in dry type or liquid immersed types.

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????????????Connection Diagram and terminal markings of Motor Starting Transformer

18.???Booster Transformer (in USA, Series Transformer)

A transformer of which one winding is intended to be connected in series with a circuit in order to alter its voltage and the other winding is an energizing winding.(IEC Pub 50(421)-421-01-12).

One application of booster transformers is in electric railway catenary feeders to collect the return current from the rail to the return conductor. These are single phase units with 1:1 ratio and low impedances. Primary voltage 25 kV or 15 kV with rated currents of 200-800 A. Another application is in power distribution circuits, by using +_10 % voltage output transformers with OLTC, to maintain constant supply end voltage.

19.??HV DC Transformer (Back-to-Back & Line Transformers)

HV DC converter transformers are used in HV DC transmission systems at sending and receiving end stations for converting voltage from AC to DC and vice versa. It will also be required in HV DC back-to-back stations where galvanic separation between two power grids is achieved by AC- DC- AC voltage conversion. An OLTC with a fairly large tapping range is provided on HV side for optimum operation.

HV DC transformers are built with two valve windings of equal power and voltage ratings. One of the windings is connected in star and the other in delta. With this connection arrangement the dominant harmonics from the converter will be cancelled out. These transformers can be built as three phase units or single-phase units depending on transport limitations. In single phase transformer design, the two valve windings are built on the same transformer but on different limbs of core with 2 or 4 limbed constructions. When built as three phase transformers, there is generally one unit with the valve winding arranged in star connection and the other unit in delta connection.

HV DC transformer load current contains high harmonic content from converters. The valve windings should with stand the HV DC voltages during the polarity reversals in converters. Hence valve windings require a different insulation build up than the system used in conventional AC transformers.

??IEC 61378-2 ed1.0 :2001/ IEC/IEEE 60076-57-129-2017/ IEEE C57.129- 2008

?20.??Transformers for Electrostatic Precipitators (ESP)

ESP Transformers are small high voltage rectifier transformers, supplying power to Electrostatic Precipitators used for removing suspended particles from flue gases from boilers in thermal power stations These are single phase transformers with primary AC supply at 415 V. They generally form an integral part of rectifiers to provide a DC output of 30-130 kV and 200-1600 mA for Electrostatic Precipitators.

Section O of CBIP Manual No.317 Manual on Transformers – 2013

?21.????Rectifier Transformer (for Smelters)

These are MV converter transformers with very high LV currents used for feeding power to aluminum electrolysis, copper refining, DC arc furnaces, graphitizing furnaces, traction substations etc.

IEC 61378- Converter Transformers 1,2 &3 Parts. Part 3 Application Guide

?22.????Auto-Regulating (full voltage range) Transformer, feeding Rectifier Transformer

Supply voltage to rectifier transformers used in aluminum smelters need to be continuously varied over a large voltage rage (say 2- 115 % of rated voltage) by using multi-coarse and fine regulating windings. Special three phase auto-transformers are used for this purpose. A typical application connection diagram is shown below. Several other possible connections are given under Fig 7-13 in IEC 61378-3-2015 Converter Transformer-Application Guide. Earlier days a DETC (de-energised tap-changer) was used for selecting the coarse taps and OLTC was used for selecting the fine taps. Today special vacuum OLTC (with on-load taps up to 106 taps) is available, incorporating several coarse taps and fine taps together in an on-load tap-changer. Auto-regulating transformers are available from 72.5 kV to 420 kV primary voltage. Since the impedance of these transformers is very low, usually these units are installed as integral with rectifier transformer of smelter.

?? Auto-Transformer with closed phases and Multi-coarse Regulation for 106 tap OLTC

?IEC 61378-3-2015 Converter Transformers Part 3 Application Guide- Clause 6.2.3

?23.???Transformers for VFD -variable frequency drives (alternate term, VSD - variable speed drives)

These are converter transformers used in applications where a variable speed is required on the motor shaft. VFD is a power conversion device that converts fixed frequency AC power supply to an AC supply of variable frequency and voltage through an AC-DC-AC converter. This output voltage is used for driving induction motors at variable speeds. ?VFD transformers are used in a wide variety of applications such as rolling mill drives, ship propulsion system, mine hoist drives, wind tunnel drives etc. Transformer windings are designed ?to suit 6,12,18,24,30,36,42,48 or 54 pulse rectifiers. There are a large number of connections and winding arrangements to suit the voltage shifts between windings required with such rectifiers. Even though VSD transformers are generally small MV transformers (6 MVA 6.6-36 kV primary and 0.4- 4 kV secondary), ratings may reach up to 100 MVA 170 kV.?As with rectifier transformers, load on these converter transformers is distorted currents with heavy harmonic content.

?VFD transformers are also equipped with a tertiary winding which is connected to ?????a filter. Filter reduces the harmonic voltage distortion in supply voltage, Tertiary is also used for connecting power capacitors for power factor improvement. ?

?24.?Voltage Regulators

Voltage regulators are single phase or three phase voltage boosting auto- transformers with OLTC. The nominal voltage is regulated by +-10 % in steps of 16 or 32 steps by auto-connected buck-boost tap winding arrangement. When used in MV voltages, transformer ratings up to 8 MVA is common, but at LV voltages maximum rating is limited to 250 kVA

?25.?K-rated transformers

K-rated transformers are specially designed to take care of extra losses and heating from the harmonic content in the load. With today’s technology, several products having power electronics (switching power supplies, variable frequency drives) introduce non-sinusoidal components in the load current resulting in extra eddy losses and heating in transformers. To take care of these harmonic components, transformers catering to such loads are to be oversized compared to nominal rating as per rating plate. So, they will be capable to handle 100 % rated current as per fundamental frequency (50 Hz) + non-linear harmonic content in load as specified according to K-factor. ?K-factor is a weighing of the harmonic load currents according to their effects on transformer heating, as per ANSI/IEEE C57.110-2018.

A K-Factor of 1 indicates a purely linear load with no harmonics, while a K-factor of 40 would indicate a severely non-linear load very rich in harmonics. The higher the K-factor rating on a transformer, the larger the nonlinear load it is designed to handle without overheating. The neutral of K-rated transformers is oversized for 300 % of phase line current. Usual K-factor ratings are K-1, K-2, K-4, K-13, K-20, K-30 & K-40. K-factor of 4 can take third harmonic content of 16 % of fundamental rating,110 % of fifth and 7 % of 7th harmonic etc. K-9 can take 163 % of harmonic and K-13 will handle 200 % harmonics.

IEEE C57.110-2018-Practice for establishing transformer capability for non-sinusoidal load current -Annexure D

26.?Control Transformers

Control transformers supply the required power (voltage and current) to operate the industrial control units such as relays, solenoids, PLCs and other control components. They isolate the control components from power surges and lighting circuits. During inrush current conditions, they keep the steady voltage at control element terminals.?