33KV Oil Immersed Electrical Distribution Transformer

For many years, Cotenele has provided customers with a large number of 33KV Oil Immersed Electrical Distribution Transformer in many substations, power plants, wind power, solar power and other projects, among which the representative one is from Rwanda Energy Development Corporation Limited (EDCL) Kigoma, Birembo, Rukarara, Kibuye, and Rwinkwavu Power station project. This 33kV (35KV) oil-immersed distribution transformer high-voltage side rated voltage is 33 KV (35 kV) and which uses transformer oil as the insulating and cooling medium. It is a static electrical device that, based on the principle of electromagnetic induction, converts one AC voltage (current) value into another (or several) AC voltage (current) values of the same frequency but different magnitudes.

Model and meanings

What is a 33KV oil-immersed distribution transformer?

A 33 kV oil-immersed distribution transformer is a distribution transformer whose high-voltage side rated voltage is 35 kV and which uses transformer oil as the insulating and cooling medium. It is a static electrical device that, based on the principle of electromagnetic induction, converts one AC voltage (current) value into another (or several) AC voltage (current) values of the same frequency but different magnitudes.

According to relevant standards, the rated frequency of a 35 kV oil-immersed distribution transformer is 50 Hz, and the rated capacity typically ranges from 30 kVA to 2500 kVA (within the distribution transformer category). Some larger-capacity units may fall into the power transformer category, with capacities up to 31,500 kVA and above.

Standard Environmental Requirements

1. Ambient Temperature:

- Maximum value: +40 °C

- Minimum value: -25 °C

- Monthly average: not to exceed +30 °C

- Annual average: not to exceed +20 °C

2. Altitude: The installation site shall be at an elevation no greater than 1000 m above sea level.

3. The supply voltage waveform shall be essentially sinusoidal.

4. The three-phase supply voltages shall be approximately balanced.

5. The total harmonic distortion (THD) of the load current is limited to a maximum of 5% of the rated current.

6. Installation Location: The transformer may be placed either inside a building or in the open air.

Standards

International standards:

- IEC 60076-1:2011 - Power transformers - Part 1: General provisions

- IEC 60076-2:2011 - Power transformers - Part 2: Temperature rise for liquid-immersed types

- IEC 60076-3:2013+AMD1:2018 - Power transformers - Part 3: Insulation levels, dielectric tests and air clearances outside

- IEC 60076-5:2006 - Power transformers - Part 5: Short-circuit withstand capability

- IEC 60076-10:2016 - Power transformers - Part 10: Measurement of sound pressure levels

National standards:

- GB/T 1094.1-2013 - Power transformers - Part 1: General requirements

- GB/T 1094.2-2013 - Power transformers - Part 2: Temperature rise of liquid-immersed transformers

- GB/T 1094.5-2008 - Power transformers - Part 5: Short-circuit withstand performance

- GB/T 1094.10-2003 - Power transformers - Part 10: Determination of acoustic noise levels

- GB/T 1094.3-2017 - Power transformers - Part 3: Insulation levels, dielectric tests and external clearances in air

Technical parameters of 33KV oil-immersed transformers

S13-35kV Technical parameters of non excitation voltage regulating transformer (0.4kV)

S13-35kV Technical parameters of non excitation voltage regulating transformer (3.15~15kV)

S13-35kV Technical parameters of non excitation voltage regulating transformer (6.3~15kV)

Product Structure of 33 kV power distribution transformers

A 33 kV oil-immersed power transformer mainly includes the following core components:

1. Core (Magnetic Circuit): The 33kV power transformer magnetic circuit is composed by cold-rolled silicon steel laminations with high magnetic permeability and grain orientation. The core employs a multi-step lapped joint structure to reduce no-load losses and noise. In a three-phase transformer, the vertical parts of the core are called core legs (on which the windings are placed), and the horizontal parts are called yokes (which complete the magnetic circuit).

2. Windings (Coils): The main components that accomplish electrical energy transmission and conversion. Except for small-capacity units where copper wire is used, the low-voltage windings are generally of a cylindrical foil-wound type using copper foil; the high-voltage windings adopt a multi-layer cylindrical structure. These designs ensure balanced ampere-turn distribution, low leakage flux, high mechanical strength, and strong short-circuit withstand capability.

3. Tank: The shell of a 33kV power transformer contains an iron core and windings inside. The fuel tank adopts a corrugated heat sink structure, which has good breathability and can compensate for changes in oil volume caused by temperature fluctuations. Some stores also use bell-type fuel tanks or fully enclosed fuel tank structures for voltage transformation.

4. Conservator (Oil Conservator): The conservator is connected to the tank through a pipeline, used to store oil and compensate for changes in oil volume. In some modern power transformers, corrugated tanks have replaced the function of Oil Conservator.

5. Insulating Bushings: The power transformer insulation bushing is an insulating component which located between the conductive rod and the oil tank cover, it also provides both insulation and mechanical support for the conductive rod.

6. Tap Changer: Tap changers are mainly used for voltage regulation of power transformers, and there are two types: off-circuit (de-energized) and on-load (energized).

7. Other Accessories: Including several accessories such as gas relays (Buchholz relays), breather (silicone respirators), pressure relief devices, thermometers, oil filters, etc.

Classification of 33kV power transformer

Product Advantages of 33kV power transformers

The 35 kV oil-immersed distribution transformer offers the following main advantages:

1. Low Losses: Using high-quality grain-oriented high-permeability silicon steel and optimised core and winding designs, no-load losses can be reduced by more than 20% compared to conventional transformers, and load losses by more than 10%. The S22 series achieves 15-20% lower no-load losses than the S11 type.

2. Low Noise: By selecting high-quality core materials, appropriate magnetic core density, and segmented laminated bonding technology, noise can be effectively reduced. For 1000 kVA equipment, the noise level can be controlled below 50 decibels.

3. High Short-Circuit Withstand Capability: Using dynamic analysis method for short-circuit mechanical force calculation; The winding is wound through crimping process to ensure high mechanical strength. This transformer can withstand surge currents up to 32 times the rated current through short-circuit testing.

4. Low Temperature Rise: Oil flow distribution is calculated and optimised to reduce hot-spot temperatures and average winding temperature rises.

5. Fully Sealed, Maintenance-Free: Our power transformer adopts a fully enclosed oil tank, which can isolate the oil from the air and slow down insulation aging. Corrugated-fin tanks replace the conservator, effectively preventing moisture and oxygen ingress. The maintenance-free period can exceed 20 years.

6. Easy Installation and Maintenance: Relatively compact size and low cost. A non-lifting-core design allows the transformer to withstand transport vibrations.

7. Smart Functionality Available: Some transformers models can be equipped with "remote signal, remote measurement, remote adjustment, and remote control" (four remote) functions to achieve unmanned substation operation.

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