Dry Type Transformers

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Content

What is a dry transformer? �

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• A dry transformer does not utilized any kind of liquid for cooling, it has windings encapsulated under Vacuum, in epoxy resin reinforced with glass net.
• It is the most technologically advanced design for extreme conditions.
• These transformers meet strict parameters with respect to electrical system demands, are virtually maintenance free and are manufactured in accordance with industry and international standards including, IEC 60076-11.

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Main advantages: safe and environmental friendly

• Reduced environmental contamination.
• Zero risk of leakage of flammable or contaminating substances.
• Environmental safe in production.
• Well suited to damp and contaminated areas.
• No fire hazard.
• Transformers are non flammable and self-extinguishing.
• High resistance to short circuits.
• High capacity to support overloads.
• High performance in dealing with seismic phenomenon.
• Capable of withstanding the most severe of rolling and vibrating conditions.

Main advantages: The most economical

• Less space needed.
• Less civil work needed.
• No special safety features required. (fire detection)
• Maintenance free.
• Longer transformer life due to low thermal and dialectic ageing.
• Can be installed closer to the point of consumption reducing load cable losses.
• Optimal design subject to constant improvements in design as new materials become available.

Environmental, climatic and fire classes

Environment

• E0 Normal indoor installation, no condensation, no considerable pollution.
• E1 Limited pollution, occasional condensation eg off circuit periods.
• E2 ✔ Heavy pollution, frequent condensation.

Climate

• C1 Lower ambient temperatures:
• Operation -5 °C
• Storage and transport -25 °C
• C2 ✔ Lower ambient temperatures:
• Operation -25 ºC/-25 ºC
• Storage and transport

Fire

• F0 No special requirements except typical characteristics for dry type transformers.
• F1 ✔ Increase demands.
• All material practically free of halogens
• Limited formation of fumes
• Limited contribution with calorific energy to the source of fire
• Self extinguishing transformer fire

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Dry transformers VS Oil transformers

The best alternative due to:

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• Suitable to operate in humid or heavily polluted environments (E2).
• Suitable for operation, transport and storage at ambient temperatures down to -25 ºC (C2).
• Restricted flammability, self fire extinction (F1).
• Emission of toxic substances and opacity of fumes is minimized (F1).
• Withstand the same impulse test even for exposed situation.
• Have more ability to withstand thermal and dynamic short circuit effects.
• Mechanically stronger, safer against vibrations, earthquakes, etc..
• No liquid, neither leaks and no risk of polluting spills.

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Types and Designs

• Types:
• Standard three phase transformer.
• Multi winding transformer.
• Low Voltage transformer.
• Autotransformer.
• Drive systems transformers.
• Excitation systems transformers.

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• Standard designs:
• High Voltage winding material: foil disks.
• Low Voltage winding material: aluminum/copper foils.
• Insulation class: F (155°), H (180°).

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Dry Transformer Components

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Wheels

Foundation

Lower clamp

High-voltage connecting rod

Insulating cylinder

iron core

Neutral bar

Low-voltage winding

High-voltage winding

Upper clamp

Dry Transformer Components

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1. Magnetic Core:

The magnetic core is made from laminations of grain-oriented silicon steel insulated with 0.25- 0.3 mm thickness. The choice and grade of steel and the 45 degrees cutting pattern and method of step-lap assembly minimize the loss level and the no-load current with the effect of a very low noise level.

Dry Transformer Components

2. Low Voltage Winding:

The low voltage winding is made of aluminum or copper foil to achieve zero axial stresses under short circuit conditions; the foil is insulated by a class F or H inter-layer film, pre-impregnated with heat-activated, epoxy resin.

The ends of the winding are protected and insulated using a class F or H insulator.

The whole winding is polymerized by being placed in an autoclave for 2 hours at 130°C which guarantees:

• Outstanding resistance to industrial atmosphere aggression,
• Excellent dielectric withstand,
• Excellent resistance to radial stresses under bolted short circuit conditions.

Each LV winding terminates in a tin-plated aluminum or copper connection point, enabling connections to be made without using a contact interface (grease, bi-metal strip). Assembly is carried out according to current practices, including using spring pressure washers under nuts and screw heads.

Dry Transformer Components

3. High Voltage Winding:

The high voltage winding is usually made and wounded by series aluminum foil. This kind of wounding method is because of the high turn numbers and small cross-section in high voltage winding.

These methods are used to obtain very low stress levels between adjacent conductors. This winding is cast and moulded under vacuum in a class F or H loading and fireproofed resin

Dry Transformer Components

Spacer strips (LV)

• Glass random non-woven with B-stage epoxy resin impregnation
• Polyester non-woven with B-stage epoxy resin impregnation 

Support and spacers (LV)

dog bones

Layer insulation (LV)

Multi-layer insulation with B-stage epoxy resin coating

• Glass fabric with B-stage epoxy resin impregnation
• NOMEX with epoxy resin coating

Outer bandings (LV)

• Thread reinforced polyester non woven with B-stage epoxy resin impregnation  

Insulation cylinders

• PET Folie 
• NOMEX 
• PET Vlies 
• Polyester folien-Laminat 

Cast-resin reinforcement (HV)

• Glass roving fabric with cured epoxy resin

Dry Transformer Components

4. Voltage regulation

Some tap voltages are designed at high voltage side to control the voltage at the low voltage side and keeping it constant.

The variation step voltages are ±2 × 2.5% according to the rated voltage.

The variation of tap voltages is done via a piece of brass or copper at the high-voltage side.

The variation of tap voltages in dry transformers is off-circuit type, and the transformer must be turned off.

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Dry Transformer transportation guidelines

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Handling

• lifting with slings

Lifting is carried out using the 4 lifting holes for a transformer without an enclosure and by 2 lifting lugs in the case of a transformer with an enclosure. The slings should not form an inside angle greater than 60°.

• lifting with a fork lift truck

The lifting capacity of the fork lift truck should first be checked. If suitable, the forks should be inserted inside the base channels after removing the rollers.

• towing.

Towing the transformer with or without enclosure should be done from the under base. For this purpose hole of 27 mm diameter are provided on every side of the under base. owing can be done in two directions : in the axis of the under base and perpendicular to that axis.

Dry Transformer Maintenance guidelines

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Storage

The transformer should be protected in storage from water drops and dust generating work (masonry, sanding, etc.).

1. The transformer can be stored at a temperature down to - 25°C.
2. The place of transformer storage should be protected from water drops, and if the transformer is delivered with a plastic cover, it should be kept over the equipment while it is in storage. It is better to place a dehumidifier such as silica gel close to the windings in humid environments and provide proper ventilation.
3. The dry transformer is resilient against fire. It means that no anti-fire precautions are necessary.

Dry Transformer Installation

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Installation and Commissioning

It's not the dry transformer manufacturer's responsibility to install it basically, but it's recommended for commissioning to take advice from the manufacturer's professionals.

For installation, the following guidelines are followed :

• The transformer should be installed on a flat surface and indoor (IP 00), and for outdoor installation, use an enclosure with appropriate IP (eg. IP 35).
• the altitude should not be above 1000 meters unless a higher altitude is specified at the time of enquiry ;

the ambient temperature for the transformer to be within the following limits :

-minimum : – 25°C ;

-maximum : + 40°C (unless a higher temperature is designed for based on information provided at the time of enquiry).

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Dry Transformer Installation

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Installation and Commissioning

For an altitude and a daily average temperature higher than 1000 meters above the sea and 30° C respectively, the dry transformer must be de-rated according to IEC 60076 and IEEE C57.91:

Installation and Commissioning

• Ground the transformer and all metal parts including the enclosure.
• Check the terminal connections and the similarity of all taps in three phases
• Check the connection of thermal sensors via ohmmeter from the terminal box
• All steel and brass bolts should be tightened with the recommended torques below.

Dry Transformer Installation

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Installation and Commissioning (forced ventilation )

In the event of temporary overloading, to avoid overheating of the windings, it is possible to install forced ventilation.

For powers equal or greater than 1000 kVA, it is possible to install forced ventilation to achieve a temporary increase in power of 40%, without any special modification.

However, if an increase in power is requested, account must be taken of the impact of this choice on the following points :

-Sections of cables and of Prefabricated Busbar Trunking (PBT),

-The rating of the transformer's protective circuit breaker,

-The size of inlet and outlet openings for air in the transformer room,

-The life span of fans in service, which is considerably shortened compared with that of the transformer

Dry Transformer Installation

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High voltage side cable connection

The cable connection holes are reserved at the upper and lower ends of the connecting rod on the high voltage side of the transformer. Taking the delta connection as an example, the phase sequence at the upper end is U-V-W and the phase sequence at the lower end is V-W-U. Attention should be paid to correct phase sequence when connecting high voltage cables. Fasteners shall be used during the connection, and loosening preventive measures shall be taken to fix the cable at the terminal of the high-voltage connecting rod of the transformer.

Dry Transformer Installation

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Low voltage side bus-bar or cable connections

The low-voltage terminal of the transformer has reserved cable connection holes, and fasteners with loosening preventive measures are required to fix the cable or bus bar. If it is required to be equipped with copper-aluminium composite foil (cupal) as specially specified in the contract, the copper-aluminium composite foil (cupal) shall be installed between the low-voltage terminal of the transformer and the cable, or between the low-voltage terminal of the transformer and the bus bar. Please refer to the figure.

• In the installation of copper-aluminum composite foil, the copper surface should be in contact with the copper bars, and the aluminum surface should be in contact with the aluminum bars.

Dry Transformer Installation

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Cable installation layout (with enclosure)

For the transformer with protective enclosure, cables in its high-voltage side and low-voltage side can be imported to internal part of the enclosure through the top or bottom of the enclosure, which can be connected to the corresponding wiring site of the transformer. •

The bending radius of the cable needs noting, to prevent force on end of cable connection.

Dry Transformer Installation

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Cable installation layout (without enclosure)

The high voltage side cable can be connected to the transformer after layout in the bottom cable tray or after layout the top cable tray. Cable or bus bar can used for the low-voltage side connection. If the bus bar connection is used, soft connection transition is recommended to avoid mechanical impact on the low-voltage winding and reduce noise level due to structural reasons.

• High-voltage cables are not allowed to pass through the high-voltage connecting rod (delta connection) of the transformer.

Dry Transformer Installation

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Electrical safety clearance

Sufficient space must be ensured around the transformer to ensure a smooth connection of the cable and the necessary electrical distance. The minimum clearance value for flashover protection is as below.

• These electrical distances are for installation equal to or below 1000 m above the sea level, and for each 500 m increase above the 1000 m, the mentioned distances must increase by 10%.
• After the transformer is installed, ensure that the transformer body and enclosure are reliably grounded.

Dry Transformer Installation

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Commissioning of temperature control system

The transformer may be equipped with a temperature control system to monitor the temperature of the winding to prevent abnormal temperature rise of the winding, thereby protecting the transformer.

The temperature control system includes a temperature controller and a temperature sensor (PT100). The temperature sensor (PT100) is placed in the three-phase winding of the transformer. When the temperature reaches the set value, the temperature sensor (PT100) feeds back the corresponding temperature change to the temperature controller. After the temperature controller receives the corresponding feedback signal, it corresponds to different functional responses. The over-temperature alarm and over-temperature trip functions need to be connected with the relay protection system of the front-end equipment so that the higher-level equipment can be linked when the temperature controller acts.

Dry Transformer Installation

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Ventilation of the Transformer Room

• In the case of natural cooling(AN):

the ventilation of the substation or of the enclosure must ensure by natural convection the dissipation of the heat produced by the transformer’s total losses.

• In the case of a sufficiently ventilated substation, appropriate ventilation will consist of a fresh air intake opening of S section at the bottom of the substation and an outgoing air opening S’ located above on the opposite wall at height H metres above the intake opening.

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• To ensure efficient cooling of the transformer and sufficient air circulation, it is essential to maintain a minimum height of 150 mm under the live section, by installing rollers or an equivalent booster.
• It must be noted that restricted air circulation reduces the transformer’s continuous and short term overload capacity.

Dry Transformer Installation

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Ventilation of the Transformer Room

• In the case of Air cooling(AF):

Forced ventilation of the substation is necessary for ambient temperatures above 20°C, or small or badly ventilated rooms for applications with frequent overloads.

The fan can be thermostat controlled and operate as an extractor in the top part of the room.

Dry Transformer Installation

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Ventilation (AN) of the Transformer Room (Example)

- Transformer 1000kVA,

-Po = 2300 W ,Pcc at 120°C = 11000 W,

i.e. P= 13.3 kW. If the distance between the grills = 2 m, then S = 1.7 m² of net surface area necessary. If we imagine a grill obstructing the air inlet by 30% ; the air inlet grill surface area should then be 1.5 m×1.5 m, and that of the air outlet should be 1.5 m×1.6 m.

Pre-commissioning test

• Voltage ratio test: Measure the voltage ratio of the winding under all taps and the connection group. The allowable deviation of rated tap voltage ratio shall not exceed ± 0.5%, except for special transformers.

Tips:

• Earth the transformer

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Dry Transformer Commissioning

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• Wingding resistance test: Measure the DC resistance of the winding at all taps and records the winding temperature. (Measure the DC resistance shows the accuracy of transformer connections)

Tips:

• DC current for low voltage winding: 10 A
• DC current for high voltage winding: 1A
• Maximum DC current: 10% rated current
• Minimum DC current: 1.2 times the maximum of no- load

HV winding

LV winding

200 V- 380 V

Pre-commissioning test

• Insulation resistance test: Measure the insulation resistance values of the windings and cores and record ambient temperature and humidity.

Tips:

• The insulation resistance is measured at ambient temperature of -25 º C ~ 50 º C, humidity ≤ 90%
• If the insulation resistance is not less than 2 MΩ, a voltage can be applied to the low-voltage side of the transformer and the transformer can be dried by no-load heating.
• If the insulation resistance value is less than 2 MΩ, it can be dried by air drying or baking with heat lamp.

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Dry Transformer Commissioning

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1. HV Winding
2. LV winding
3. Body
4. Megger

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I. Voltage regulation

If the actual grid voltage deviates from the rated voltage of the transformer, the tapping gear of the transformer can be adjusted appropriately to control the output voltage value.

• For no-load voltage regulation, the transformer gear tap shall be adjusted to the appropriate gear to keep in line with the grid voltage.

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Tips:

• The gear of transformer is set at rated gear (4-7) by default when it leaves factory. Make sure that the transformer is powered off before adjusting the gear.
• The three-phase tap must be adjusted at the same time. The rear gear bolt must be tightened after adjustment and the torque value must be restored.

Dry Transformer Operation

II. No load operation

When large capacity no-load transformers are put into operation, visible sparks may be generated at the joints of external components (especially at the core and clamps), but this phenomenon will soon disappear. This is due to physical reasons, and will not affect the safe operation of the transformer, so it is not a product defect.

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III. inrush current

There is inrush current when the transformer is switched on, and the peak value can reach 6-8 times of rated current. The setting value of current quick protection for the transformer shall be larger than the peak value of inrush current, and the time limit shall be ≤ 0. 5 s. Therefore, no-load inrush impulse test shall be carried out before operation to check whether the relevant protection of the transformer is malfunction or not. If the protection value is set improperly, it will cause the tripping phenomenon in the later closing.

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IV. long-term storage or overhauled

For long-term storage (storage time more than 3 months) or overhauled transformers or transformers to be used at ambient temperature below 0 ℃, it is recommended to operate without load for 12 hours before putting the transformers into load, and the load shall be gradually increased.

Dry Transformer Operation

V. overload operation

The transformers are designed to operate at rated power at ambient temperature defined by IEC 60076 :

• 40 °C maximum
• 30 °C monthly average of the hottest month
• 20 °C annual average
• If operated normally, the transformer should attain its expected lifetime consumption. In particular, the average annual temperature and the load significantly affect the lifetime consumption. Environment temperatures differing from the annual average have an effect on the system’s load capacity.
• The three-phase voltage, current and temperature values shall be recorded periodically during the operation of the transformer so that the historical data of the transformer can be consulted in case of overhaul or fault.

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Dry Transformer Operation

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Maintenance

Cast-resin transformers are maintenance-free. Routinely inspect and clean the windings, bolt connections, alarm devices and all fan functions once a year. If the transformer is in dusty environment or close to the pollution source, it is recommended to clean the equipment every six months. It is recommended to fully inspect and clean the transformer every 5 years. Before maintenance, the transformer must be powered off and all terminals must be short-circuited and grounded.

• Transformer cleaning

Blow off the transformer with dry compressed air or wipe with dry cloth and alcohol (concentrations above 85%) to prevent the formation of creepage paths and blockage of cooling air passages.

Dry Transformer Maintenance

• Cleaning method 1

Use compressed air to blow the transformer. The compressed air must be oil-free and water-free, and the air pressure ≤ 6 bar. Vacuum cleaners can also be used instead of compressed air for dust collection.

Cleaning method 2

Wipe with a dry rag and alcohol (concentrations above 85%).

Dry Transformer Maintenance

Dry Transformer Maintenance

Dry Transformer Maintenance

* Applied withstand voltage test: The withstand voltage strength of the transformer is tested. The on-site tested withstand voltage value is 80% of the factory test voltage value. The factory test withstand voltage value can be shown in the factory test report and nameplate.

Dry Transformer Maintenance (Frequently asked questions guidelines (chart))

*De-rating the Transformers according to non sinusoidal currents (Slide 44)

Dry Transformer Maintenance (Frequently asked questions guidelines (chart))

Loading transformers with non sinusoidal currents

• All ratings for transformers are based on sinusoidal rms values, rated power, rated voltage, losses, temperature rise, etc.

Dry Transformer Operation

• Load losses and temperature rise are based on sinusoidal rms rated current.

All losses referred to the reference temperature.

When transformer supply a non sinusoidal load current with the same rms value than rated current, DC losses remain constant but winding eddy losses increase due to the higher frequency of the harmonics, as a result, winding temperature rise increases and temperature limits can be exceed.

Loading transformers with non sinusoidal currents

• A transformer requested to supply non sinusoidal loads, shall be oversized in order to guarantee that windings temperature limits are not exceed in service.

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• Total Harmonic Distortion:

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Dry Transformer Operation

It is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

• K- factor:

K-factor defines the non-linear load a transformer can tolerate without overheating or damage. The basis for K factor is seen in the ABB Document (Loading transformers with non sinusoidal currents). This document identifies the method for correctly de-rating transformers for non-linear loads.

Loading transformers with non sinusoidal currents

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Dry Transformer Operation

* Remark: this equivalence list is valid up to 3150kVA transformers

Loading transformers with non sinusoidal currents (Example)

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Dry Transformer Operation

• Fundamental Power – 2MVA
• Net harmonic content – K-17 => Factor K equals to 1.28

• Equivalent Power – 2000 × 1.28 = 2560kVA

• As a result of this, the transformer shall be physically sized as 2560kVA rated power and

the temperature rise limit shall be guaranteed at such equivalent power as well.

Therefore, should the temperature rise test is requested, during the full load part of the test the current must be equal to the rated current multiplied by the proper Factor-K, i.e. assuming the no load voltage level on LV side is 417V (Dyn11), rated current will be

2000/ (417×√3) = 2.77kA, however test must be run at 2.77×1.28= 3.54kA (corresponding

to 2560kVA).

Excitation Transformer

Dry Transformer

Excitation Transformer

Dry Transformer

Electrostatic shielding

• Electrostatic shielding is a metallic barrier (aluminum or copper foil ) built into a transformer between the primary and secondary winding. The shield is grounded.

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1. Providing an electrostatic shield between the primary and the secondary windings thus avoiding transfer of surge/impulse voltages passing through inter-winding capacitance.
2. The stray capacitance between windings can generate asymmetry currents and could lead to high common mode noise and is found to emphasize the noise transmission in a converter,.

Excitation Transformer

Dry Transformer

Excitation Transformer

Dry Transformer

Dry Transformer

The First Indoor 110KV/35KV 31.5MVA Cast Resin Transformer (CRT) and GIS Substation Layout

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