How Do Petrochemical Companies Choose Main Transformers?

The improper selection of the main transformer has become one of the main factors hindering the development and expansion of the power supply and distribution network of all petrochemical enterprises.

The general substation of petrochemical enterprises provides a reliable power guarantee for the long-term, continuous operation of process production equipment.

The substation is an important production facility of petrochemical enterprises and occupies an important position in the whole plant.

Whether the main transformer of the main substation of a petrochemical enterprise is properly selected or not directly affects the safety and reliability of the power supply and use of the whole plant.

This article discusses and discusses in detail the main transformer selection of petrochemical enterprises, and gives reasonable suggestions.

Selection of Power Distribution Main Transformer in 110kV Substation Design

Pad-mounted Transformer

We can provide you single phase and three phase pad mounted transformer

Dry-type Transformer

Type:Cast resin; Rated Capacity: Up to 25MVA; Rated Voltage: Up to 36KV;

Pole Transformer

TypeCSP type Frequency: 50/60Hz; Rated Power: 5~167kva

Dry-type Transformer

Frequency: 50/60Hz Rated voltage:10kv, 20kv,30kv Rated Power: 400~2500kva

The Ultimate FAQ Guide

What is The Selection Principle of The Main Transformer?

Two main transformers should be installed in the main substation with two power supply lines and the main wiring is a sectional single bus connection mode;

For a total substation with two power supply lines and a double-bus connection method, the main transformer capacity should be set as two when the capacity of the main transformer is 40 MV·A and below;

The capacity of the main transformer is greater than 40 MV·A. For the main substation with a three-way power supply line and the main wiring is three-section single-bus connection mode, the main transformer should be set as 3 sets;

For the main substation with two or three power supply lines, the main wiring is double bus wiring. When the capacity of the main transformer is 40 MV·A and above, 4 units should be set so that the capacity of the main transformer is not more than 40 MV·A.

How to Install a 500kV Main Transformer?

How to Choose the Main Transformer Capacity?

main transformer
main transformer

Determination of the capacity of the main transformer: greater than the total calculated capacity of the primary and secondary loads in the development plan of the whole plant;

It meets the requirements of the electrical quality of the power distribution device busbar and the terminal when the large-scale unit does not change the wiring operation mode (as far as possible in the minimum operation mode) when starting directly or at a reduced voltage.

At the same time, it will not have a big impact on other electrical loads and affect the power system so as to change its operation mode.

When the main transformer is set with two sets, except for the production equipment of the whole plant during the start-up and shutdown period, each set shall meet the continuous operation with all primary and secondary loads;

When the main transformer is set up with 3 or more units, it should be ensured that when one unit is out of operation, the rest can carry the requirements of long-term continuous operation with a full load, and there is still a margin;

For the dedicated large-scale unit, the line-the capacity of the transformer unit satisfies the load capacity, the busbar voltage level when starting, and the terminal voltage level; the normal operating load rate is at the best point of energy consumption and economy.

Main Electrical Wiring of 110kV Substation

What is the Main Transformer Winding Rule?

Winding of 10 kv transformer

According to the requirements of the main wiring, determine the two-winding or three-winding type; winding group wiring, 110 kV windings use Y0 wiring.

According to the requirements of system operation and dispatch, use direct grounding or non-grounding method, or implement grounding joint jump method with another main transformer;

When 35 kV voltage level is used as the main power supply network of the whole plant, and the grounding capacitance current is close to or expected to be greater than the value required by the specification, use YN, yn0, d11 wiring;

When the 35 kV voltage level of the main substation may be connected to the 35 kV power system or run in parallel, the connection mode of the 35 kV winding of the transformer should be consistent with the system.

Connection Mode of Working Winding of Three-phase Transformer

How to choose the impedance voltage of the main transformer?

For a two-winding transformer with a capacity greater than or equal to 6.3 MkV·A, 110/6 kV usually chooses 10.5% impedance voltage; 35/6 kV usually chooses 8%.

The selection of the impedance voltage of the main transformer should take into account the influence of the following factors: the short-circuit level of the medium voltage side and the low voltage side in the maximum and minimum operation modes of the system;

The breaking current and impulse peak current of switching appliances;

The minimum cross-sectional area of the cable that meets its thermal stability;

The starting mode, starting time, main wiring mode, bus voltage level, and terminal voltage level of the large-scale unit motor;

For the transformer-motor unit, the impedance voltage of the transformer should be selected according to the requirements of meeting the starting conditions.

Preventive measures for tripping of three-side switch of 110kV main transformer

How to Choose the On-load Voltage Regulation of the Main Transformer and Its Taps?

Distribution Transformer Capacity test

Set at the neutral point of the high-voltage side of the transformer and the star-connected winding;

For the main transformer (step-down transformer) of the 35~110 ​​kV general substation, when the power is supplied from the regional substation or power plant and the power system exclusively owned by the petrochemical complex, the on-load tapping transformer should be used;

When the power supply line is connected to the power system, or to the regional power substation or power plant dedicated to the petrochemical complex, the on-load tapping transformer should also be used;

The voltage ratio of the main transformer should be selected according to the calculation results of the power flow distribution.

What is the Insulation Strength of the Main Transformer Winding?

For systems where the neutral point is not grounded or is grounded through the arc suppression coil, each winding of the main transformer should be considered as full insulation;

For the neutral point directly grounded system of 110 kV and above, the windings of the main transformer can be considered semi-insulated.

When set to on-load voltage regulation, the insulation of the on-load voltage regulation tap changer should be the same as the insulation of the transformer.

Regarding the transformer neutral point voltage regulation method, the insulation of many transformer neutral points in the power system is semi-insulated.

The insulation level of the tap changer is consistent with that at the neutral point, and special attention should be paid when selecting the tap changer.

Atmospheric overvoltage is harmful to the neutral point of the graded insulation transformer and needs to be protected.

However, for the neutral point of a fully insulated transformer, and for a transformer with a single incoming line in a multi-thunder zone, the probability of intrusion of lightning waves at the same time is about 10%, which is also very dangerous. It is advisable to install a lightning arrester for protection.

The insulation level requirements of the neutral point of the transformer are shown in the following table.


The design of the general substation of a petrochemical enterprise is very different from the design of the general substation of the power system and other industries. This article follows the eight principles of main transformer selection to show the difference.

For reference of designers and operators. Of course, this article is aimed at the 110 kV main substation, and it can also be used as a reference for the design of the 220 kV main substation.

About Bin Dong

Hello, I am Bin, General manager of Daelim which is a leading transformer manufacturer. As a expert, I am glad to share my experience in transformer to you. I believe our professional team can always provide you with high quality service and we are ready to provide you with one stop solution. If you have problems when you are looking for the equipment, what you need to do is tell us.

Custom Main Transformer

If you find that the existing main transformer cannot meet your requirements, please contact us. We have an experienced team of designers. They can design and produce high-quality main transformers according to your specific needs. At the same time, the stability and high efficiency of the main transformer are ensured.

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