Transformer introduction - Load ratio | dry goods share

When choosing a transformer, the load ratio is a very important technical indicator. When the load rate is too high, the load loss of the transformer increases sharply, causing the transformer temperature to rise, and great harm to the operation and life of the transformer. The load rate is too low, resulting in a waste of capacity, but also increase the operating cost. Therefore, in the design of the project, it should be combined with the project background, in accordance with the relevant provisions, to determine a reasonable load rate, not only to avoid the appearance of "big horse car", but also to avoid overload, affecting the service life of the transformer.

1. Definition of transformer load ratio

The transformer load ratio is the ratio of the transformer's apparent power output to the rated capacity of the transformer, which can be calculated by the following formula.

Beta = I2 / I2N = P/SNcos phi

β, transformer load ratio;

I2, transformer load current, A;

I2N, secondary rated current of transformer,

A; Transformer load side output power, kW;

S, transformer rated capacity, kVA;

cosφ, load power factor;

2. The load rate of the transformer when the minimum power loss is achieved

The basic formula of transformer power loss and loss rate is as follows:

Δ P = P0 + beta squared by PK

ΔP%=ΔP/ (β×SN×cosφ+P0+β²×PK)

According to formula (1) and (2), two curves can be drawn as shown in the following figure

It can be seen from the loss rate curve of the transformer in the figure that when the load factor β reaches a certain value, the loss rate ΔP% is the smallest. Formula (2) takes the first derivative of β and makes it zero. The conditions for generating the minimum loss rate are as follows:

β=√ (P0/PK)

Therefore, when the transformer is running, ΔP% can be reduced by adjusting the transformer load ratio, and the P0/PK of the general transformer is ¼~⅓, so the lowest loss rate generally occurs in the load ratio β=50%~60%.

However, taking into account economic factors such as capacity and electricity costs, when choosing transformers, the load rate is often higher than the load rate at the lowest loss rate, and the requirements are not the same in different application scenarios and different regions.

3. Specifications, design manuals and landmarks for transformer load ratio provisions

3.1 Code for Electrical Design of Civil Buildings JGJ16-2008

Article 4.3.2: The long-term working load rate of the distribution transformer should not be greater than 85%.

This code is applicable to the electrical design of new, rebuilt and expanded civil buildings in cities and towns, and is not applicable to the electrical design of civil air defense projects, gas pressurization stations, and automobile gas stations. In the absence of other provisions by the competent authorities, this article may be followed.

3.2 Engineering Technical Standard for Distributed Charging Facilities for Electric Vehicles GB/T 51313-2018

Article 5.2.2: The existing parking space should be equipped with charging facilities should be based on transformer capacity, transformer load rate during peak hours, etc., the connection mode should be selected. When using a single bus connection, the load rate should not exceed 100%; When using a single bus segment connection, the load rate should not exceed 60%.

This specification applies to the planning, design, construction and acceptance of distributed charging facilities for electric vehicles. In this specification, when using a single bus, the load rate should not exceed 100%, expressed doubts. Such a high load rate cannot guarantee the economic operation of the transformer. You can also leave a comment to share your thoughts.

3.3 Code for Electrical Design of Residential Buildings JGJ242-2011

Article 4.3.1 stipulates that energy-saving transformers should be selected for residential buildings. The wiring of the transformer should be D, yn11, and the load rate of the transformer should not be greater than 85%.

This code applies to the electrical design of new, rebuilt and expanded residential buildings in cities and towns, and does not apply to the electrical design of air defense basement projects attached to residential buildings. In the absence of other provisions by the competent authorities, this article may be followed.

3.4 Article 1.11.2.1 of the Fourth edition of the Industrial and Civil Distribution Design Manual stipulates that the transformer load rate should be 75%~85%, taking into account the error of load calculation.

This manual applies to the design of industrial and civil power distribution of 110kV and below, and can be implemented in accordance with this article when the competent authorities have no other provisions.

3.5 Rules for landmarks

Local authorities have different requirements for the load ratio of distribution transformers and substation main transformers. Take Beijing as an example: when there are two or less 10kV distribution transformers, it is calculated according to 60%; For a single distribution transformer, the value is 80%. Meets the energy saving requirements of distribution transformers.

4. Case study Examples:

A new 110kV substation, the known calculated load is 16MVA, of which the primary load is 5MVA, the secondary load is 6MVA, the overload capacity of the transformer is considered by 1.2 times, the transformer load rate is considered by 60%, and the number of transformers should be installed and the minimum capacity is what?

Answer:

1, according to the "35~110kV substation design Code" article 3.1.2, two main transformers should be selected. 2, according to the "35~110kV substation design Code" article 3.1.3, set each main transformer capacity is S, there are 2×S×60%≥161.2×S≥ (5+6) S≥13.33MVA, so the main transformer capacity is at least 16MVA.