The primary guide of delta star transformer

Do you know anything about star Delta transformers? This article will introduce the definition of delta star transformer, advantages and disadvantages, application, calculation formula and the reason for using this connection.

 

Daelim Transformer is a professional transformer supplier located in China. The professional technical team can design a 50kva transformer that fully complies with ANSI/EEE, DOE, CSA, AS/NZ, and other international standards. It has SGS, CESI, IEEEC 57, IEC60076, CSA C88, and other transformer test reports, as well as UL, and cUL certificates.

Table of contents

1. The difference between star connection and delta connection 2.Why do we use delta-star in transformer? 3.Where are delta-star transformers used? 4.Delta star transformer application 5.Delta-star connection advantages and disadvantages 6. Delta star transformer advantages 7. When does delta star transformer work satisfactorily 8.Delta star transformer calculations 9.Delta star transformer manufacturer 10.Why delta star transformers are used for lighting loads

 

1.The difference between star connection and delta connection

The common transformer winding has two kinds of connection, namely "delta connection" and "star connection".

 

The difference between star connection method and triangle connection method of transformer: whether there is neutral point, third harmonic and zero sequence current in the fluidity of triangle, different current voltage, different applicability.

 

1. There is no neutral point:

Star connection (Y) connection can be derived from the neutral point; The triangle (D) connection cannot lead to a neutral point.

 

2. Third harmonic and zero sequence current flow in triangle:

Star connection (Y) connection third harmonic and zero sequence current can not flow in the internal; Triangle (D) connects the third harmonic wave and the zero-sequence current can flow in the triangle, and has no influence on other coils.

 

3.Current and voltage

Star connection (Y) connection method: line current and phase current equal; The line voltage is square root of 3 of the phase voltage; There are two kinds of voltage values, there are zero lines, such as 380V and 220V two kinds of voltage.

 

Triangle (D) connection: line voltage is equal to phase voltage; The line current is square root of 3 of the phase current; There is only one voltage, no zero line.

 

4. Applicability:

Star (Y) connection: generally used for distribution network output, easy for users to select voltage.

 

Triangle (D) connection: generally used for power network transmission.

 

Learn more : Basic Guide Of Delta Transformer

 

2.Why do we use delta-star in transformer?

The low voltage side of the main transformer is triangulated to eliminate the third harmonics. Prevent a large number of harmonics to the system, resulting in power grid voltage waveform distortion. An important feature of the third harmonic is the same phase, which can form a circulation in the triangle side, so as to effectively weaken the harmonic transmission to the system and ensure the quality of power supply. There are also zero-sequence current can also form a circulation in the triangle connection, because the neutral point is directly grounded on the high voltage side of the main transformer, to prevent the failure of the low voltage side, zero-sequence current into the high voltage side, so that the zero-sequence protection of the superior power grid misoperation. The high voltage side star of the main transformer is to reduce the loss of the line and reduce the current of the line and reduce the non-ferrous metal and improve the neutral point grounding. The reason why the low voltage side is connected to the triangle is that the triangle has third harmonic attenuation.

 

In the transformer, one side of the primary and secondary sides is expected to be connected to a triangle, so that one side can provide a loop for the third harmonic current so as to ensure that the induced potential is a sine wave and avoid distortion. The triangular-connected winding plays the same role on the primary side or the secondary side.

But in order to save insulation materials, in fact, the high voltage side is always star connection, the low voltage side is triangular connection.

 

1. Because the high voltage side is under a constant line voltage, its phase voltage is only 1/↓3 of the line voltage, and insulation is usually designed according to the phase voltage, so less material is used. The insulation layer is not as thick as the bag (no. Then, the wire length should increase with the same winding number). Correspondingly, the core does not have to be larger because of the winding and resting product, and the main system is a high-current grounding system. Only the high-voltage side star connection mode can be used. For the wiring mode of three-phase transformer group, if the star/star wiring can cause serious distortion of phase potential waveform, may cause insulation breakdown high voltage side Y connection, low phase voltage, can reduce the cost of improving insulation;

 

2, Low voltage side Angle connection, low phase current, can reduce the winding cross-sectional area, reduce the cost; Protection against third harmonics. Low power transformer high voltage side connection triangle is to prevent the third harmonic into the low voltage side, the harm to electrical equipment.

 

3, Excitation transformer hv side joint as a Y type, low voltage side into a triangle, reason: export high side voltage of the generator voltage, excitation transformer hv winding into Y type, 1 / V3 phase voltage as the line voltage, the high side of the transformer can do it according to the phase voltage, if high side into a triangle, the generator transformer high voltage winding through the line voltage, Costs have gone up a lot; Low voltage side connected into a triangle: the excitation low voltage side of the general voltage is low, mostly not more than 1000V, normal operation, transformer low voltage side excitation current is very large, connected into a triangle, phase current is 1/ V3 of the line current, the cross-sectional area of the winding wire is small, easy to manufacture, winding manufacturing cost can be reduced a lot. In addition, it also forms a loop for the third harmonic to protect the generator.

Try for free: Pad Mounted Transformer

 

Why do generators have to star?

One is to eliminate high harmonics; Second, if connected into a triangle, when the internal failure or the wrong winding caused by three-phase asymmetry will generate circulation, the generator burned. The most important component of high order harmonics is the third harmonic, which is caused by the discontinuous distribution of magnetic field between slots. The fundamental frequency is 50Hz, the third harmonic frequency is 150Hz, one period of the fundamental wave is three periods of the third harmonic, that is, 360 degrees of the fundamental wave is equal to 3x360 degrees of the third harmonic. Since the fundamental waves are 120° apart in phase, for the third harmonics it is 3x120° =360°, 360° difference in Angle equals no phase difference, they are in the same direction. If the generator is connected to a triangle, there will be a circulation, and if the generator is connected to a star, they will cancel each other out.

Get it now：Basic Guide to kVA Transformer

 

3.Where are delta-star transformers used?
 

Delta-star starting can only be used for starting the occasion of small load, because the torque when the motor is connected to the star connection is only half of the triangle connection, if the starting load is heavy, the motor may not be started, resulting in burning. Can be used in such as: water pumps, fans, air compressors, and other starting equipment can be empty load operation. For example, the ball mill and other equipment with heavy starting load cannot be used.

Try for free：The Ultimate Guide to Three Phase Transformer

 

4.Delta star transformer application

Now generally, the high pressure side above 35KV system is star-shaped connection, the low pressure side is triangular connection, the high pressure side below 10KV system is triangular connection, and the low pressure side is star-shaped connection. Transformers with high and low voltage sides are also star-connected.

 

Read more about：Basic guide for transformers in Substation

 

5.Delta-star connection advantages and disadvantages

1). Advantages of Y connection:

 

The most economical for high voltage winding;

 

Can there be a midpoint to use;

 

Allow direct grounding or grounding through impedance;

 

Allow to reduce the insulation level of the midpoint (i.e. graded insulation);

 

A tap joint can be provided at the midpoint of each phase, and the tap switch can also be located at the midpoint; Single-phase load is allowed, and the midpoint can carry current.

 

Disadvantages of D connection: reduced motor power.

 

2).Advantages of D connection:

 

Most economical for high current low voltage winding;

 

Zero sequence impedance can be reduced when used with Y connection winding.

 

Disadvantages of D connection: High insulation requirements.

Read more : 750 kVA Transformer Introduction

 

6.Delta star transformer advantages

1.A transformer, the same wind can be installed in Y type winding, the wind can also be installed in the delta type winding;

2. The same transformer, the installation of winding delta type, wire section is small, series turns, high voltage phase, phase current is low;

3.The same transformer, the installation of Y winding, wire section is large, the number of turns in series is small, the working phase voltage low, high phase current;

4.Delta type winding requires three-phase symmetry, power symmetry is also high, so that there will be no circulation, otherwise it will heat up, increase the loss; 5, Y winding in the three-phase symmetry is not good, power symmetry is not high, there will be no circulation, but there will be zero drift, three-phase work serious asymmetry;

5.In use, the type winding can be started by Y-delta start mode, and the type Y winding can not be started by Y-delta start mode;

6. because the resistance heat loss is proportional to the square of the current, so the same transformer, the installation of winding delta type heat loss is small.

 Keep reading: Lets know more on construction of a three phase transformer

 

7.When does delta star transformer work satisfactorily

Star delta connection refers to a kind of three-phase ac transformer step-down starting way, generally for three-phase ac starting current transformer can reach the 5 to 7 times the rated current, especially high power or overloading the start equipment the starting current and higher, so the ability to withstand the impact of large start current for the transformer itself and provide it with the power of the power supply system, As well as other electrical equipment in this system is a test, so people usually try to reduce the transformer starting current in some way. Star delta step-down start is a very common way of them, it is used in the starting current is bigger, or the capacity of the power supply system is not very big, will be three winding the stator three-phase ac transformer through the external contact and to realize the change of the connection mode, which is at the time of transformer start using star connection mode, When the starting process ends and turns to normal working state, the triangle connection can effectively reduce the starting current in the starting process, which can not only protect the transformer but also reduce the influence on the power supply system and other electrical equipment.

 

Read on: The Ultimate Guide to Dead Front Transformers

 

8.Delta star transformer calculations

Determination of starting time of star triangle

 

Calculation formula:

 

(Capacity squared ×2) +4 (seconds)

 

A, formula

 

Motor starting star triangle, starting time is easy to set;

 

Capacity square root times two, total plus four units per second.

 

Motor starting star triangle, overload protection thermal element;

 

Set current phase current, capacity times eight divided by seven.

 

Second, capacity calculation

 

When the generator capacity is 200KVA, calculate KW, 200×0.8 = 160KW 0.8 is the power factor (range 0.7-0.9, usually 0.8), namely COS φ = 0.8

 

(1) Calculate the motor capacity

 

Power KW÷ Power factor 0.8,

 

For example, 37KW÷0.8 = 46.25kVA

 

(2) Calculate the star triangle start conversion time

 

6.801×2+4 = 17.602≈17.6 secs 6.801×2+4 = 17.602≈17.6 secs

 

Keep on reading: Basic Guide To High Voltage Power Transformers

9.Delta star transformer manufacturer

Daelim is a manufacturer of star triangle transformers, if you want to know more information about star triangle connection mode and star triangle transformers, please consult Daelim manufacturers

 

Rread my article on：Basic introduction of electric furnace transformer

10.Why delta star transformers are used for lighting loads

When asymmetrical triangular load makes star connection, three-phase four-wire connection method must be adopted, and the middle line must be firmly connected to ensure that the voltage of each phase of the three-phase asymmetric load remains symmetrical and unchanged. If the middle line is disconnected, it will lead to the asymmetry of the three-phase load voltage, which causes the phase voltage of the light load is too high, so that the load suffers damage; One phase voltage in the load is too low, so that the load can not work normally, especially for three-phase lighting load, all adopt the star connection method unconditionally.

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