A 67-Types of Transformers-Part 4

In parts 2-3 of the Types of Transformers series, we have seen a brief description of 26 types of transformers used in the power sector. Continuing the series, this section covers 13 types of transformers based on their application or use.

A 66 -Types of Transformers - Part 3

https://www.dhirubhai.net/pulse/66-types-transformers-part-3-p-ramachandran

27.????Isolation transformers

Isolation transformers are used to isolate sensitive loads from transients and noise coming from the mains. Isolation transformers provide total galvanic separation between input and output circuits. Isolation transformers usually have a grounding electro-static shield made of nonmagnetic foil, located between the primary and the secondary. Any noise or transient that comes from the source is transmitted through the capacitance between the primary and the shield and onto the ground and does not reach the load. This separation provides safety and protection for devices from unwanted high electrical surges and damaging transients.

28.????Pre- magnetising Transformers

Pre-magnetizing transformers are small transformers (5-15 kVA) put either on the secondary or primary side of the main transformer to mitigate the inrush currents during the switching on of the main transformer. They are connected through a series of resistors to magnetize the core. When they are put on the secondary side, their rated voltages are selected to create almost 90 % of the rated voltage at primary terminals. Then the voltage between primary breaker contacts will be very small resulting in negligible inrush when switching the main transformer. Later pre-magnetizing transformer will be bypassed. These are mainly used with marine transformers used in ships.

?29.?????Transformers for Short Circuit Testing

These Transformers ?are used in High Power test laboratories to give high magnitude of currents for short durations at high or low voltages to the test objects (transformers undergoing short circuit tests or switchgear breaking rated rupturing current). To maintain distortion-free voltage waveform at the recovery stage, the rated core flux density is kept below 0.9 T and air gaps are provided in the core to control the residual flux. Since the loading will be only for short periods, thermal aspects can be neglected and cooling radiators are not required for the transformer. But windings shall be stabilized and strengthened to withstand the mechanical impacts from the continual short circuit high current flow through the transformer. Impedances shall be very low to limit the voltage drop through the transformers during the test duty. When Generators are the source of the testing lab, these transformers ?are usually step-up transformers (eg 3x 800 MVA for one second 400/ 2x10/2x16 kV ?with 2MVA continuous;16 MVA 30 sec 66/11 kV with 2 MVA continuous)

30.????Station service Power voltage transformer

Station service voltage transformers are single-phase electromagnetic voltage transformers used to give power supply to isolated small communities. The power of the transformer is limited to 100-200 kVA with primary winding connected across the EHV line and ground ( up to 220 kV) and the secondary windings rated to consumer voltage of 120/240 V. A metering circuit (at 110V) is also incorporated as an additional?secondary winding.

?31.?????Transformers for High Voltage Testing

Testing transformers are used in high voltage laboratories to generate a high voltage, low current source to test the insulation of electrical equipment by applying a?high voltage for short periods ( usually one minute). As per IS 13956: Testing Transformers, the rated voltages of these transformers are 100-1200 kV in steps of 100-200 kV, with rated current varying from 0.1-3A ( most common 0.5-1?A) Maximum impedance voltages are limited to 10 % ( 500 kV) to 30 % ( > 1000?kV). A single two-winding transformer is employed up to a voltage of 300 kV. For still higher voltages, transformers are put in the cascade connection ?to generate the required high voltage.

Another type of high voltage testing transformer is the resonant transformer used to generate high voltages by putting the transformer reactance in series or parallel resonance with the load ( tested object) ?capacitance.

IS:13956-1994-Testing Transformers

32.????Coupling Transformers for STATCOM.

The?static synchronous compensator?(STATCOM) is a power electronic converter designed for shunt connection with the power grid for reactive power control by absorbing or delivering reactive power, thereby stabilising the grid voltage.

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The transformers used for connecting STATCOM (static synchronous compensators) to the high voltage grid are of fixed voltage ratio, two winding transformers. But the secondary voltage (usually nominal voltage 33 kV) and current will vary in broadband depending on the compensation VAR flow (magnitude and direction) from and to the STATCOM. Transformers are designed with low-rated flux density (1.3-1.5 T) to take care of voltage variation during the operation. A high percentage impedance is provided for supplying the reactive VAR. Higher BILs are usually adopted for both HV &LV sides. Extra cooling is provided for transformers to take care of the heating from harmonic content in load currents as in the case of HV DC Transformers.?

33.????Interphase Transformers

The interphase Transformer is used " to absorb the difference between the direct voltages of the individual systems and must be designed for the times integral of this voltage." ( Schaefer,1965) Interphase transformers are used in systems that have two rectifier systems being used in parallel. As an example, interphase transformers are used in a circuit with a rectifier transformer having both delta and wye secondaries which create a current imbalance. The interphase transformer corrects this imbalance. This is also connected between the neutrals of the double star parallel bridge arrangement as a single-phase unit.

Interphase Transformer in Double Star Connection

???????????IEC 61378-3-2015 Converter Transformer Application Guide. (Fig 5)

?34.????Pulse Transformers

?In Pulse transformers, the intermittent full load will come for a short period (some time for a ?few seconds) and then a no-load for similar periods. Windings will see continuous mechanical shocks in service.

35.????Transformers for EV Charging

Electric Vehicle charging transformers are single-phase or three-phase units with a power rating of 1-500 kVA, taking power at 240/415 V and with the secondary voltage feeding the converter units to give a DC output for battery charging. ?Primary currents are rated for 16, 32, 48 A, or 48 &63 A for three-phase. These are dry type, isolation transformers with additional capacity to handle harmonic loads (usually K-9 transformers)

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?36.?Transformers for computer loads (Data centres, Bit-coin mining)

In bitcoin mining (crypto-currency) application: the load side is a large number of computers used in digital currency mining (computer operation worldwide. Typically, there will be a large station transformer feeding many pad-mounted transformers. The LV voltage for these pad mounts is typically 480 or 415 volts and will be feeding hundreds of miners (digital currency operators). The application is somewhat similar to Data Center Applications which also deals with computer loads.

37.?Medium Frequency Transformers

Medium frequency transformers (>1 kHz) are used with DC-DC converters to reduce overall weight and dimensions. The main applications are in Locomotive transformers and solid-state transformers.

38.?Standby (spare) Transformer

A standby or spare transformer is an identical unit to the working transformers that can be used to replace any of the working transformers that turn defective. These can be a single-phase transformer of a bank or a whole three-phase transformer. Typical cases for spare transformers are generator transformers in power stations and the three-phase auto-transformer bank in interconnecting stations. In the latter case, the spare unit is kept warm by energizing it and can be switched on in any phase of the bank, in case of a fault in any phase of bank.

39.?Rotary Transformer (Variable Frequency Transformer)

The variable frequency transformer (VFT) is a new type of grid interconnection device. By integrating technologies of power transformer, phase shifter, hydro generator, doubly fed generator, DC drive, and other technologies, and with functions such as smooth adjustment of transmission power, suppression of low-frequency oscillation, and adjusting system frequency function, it can realize the interconnection of asynchronous power grids, supplying power to weak power systems and passive systems, and providing black-start power. The variable frequency transformer (VFT) is a controllable, bidirectional transmission device that can transfer power between asynchronous networks. Functionally, the VFT is similar to a back-to-back HVDC converter. It provides a new option for the interconnection of large power grids and has been experimentally applied in North America, and has achieved good results. Remember, it has only the name ’transformer’ but it is more a type of hydro-generator.

The core technology of the VFT is a rotary transformer with three-phase windings on both rotor and stator. A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT

Reference: 2004-00-Variable Frequency Transformers-Einar Larsen, Danielle McNabb (GE Energy &Hydro Quebec)?