Industry

# The basic principle of transformer differential protection

1. Working principle of transformer differential protection

The principle is the same as that of the line differential protection, which is to compare the phase and value of the current on each side of the protected equipment.

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2. The difference between transformer differential protection and line differential protection:

Because the rated currents on the high-voltage side and low-voltage side of the transformer are not equal, plus the phases of the currents on each side of the transformer are often different. Therefore, in order to ensure the correct operation of the longitudinal differential protection, the transformation ratio of the current transformer on each side and the compensation of the current phase on each side must be appropriately selected so that the secondary currents on both sides are equal during normal operation and external short-circuit faults.

For example, the double winding transformer in the figure below

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3. Features of transformer longitudinal differential protection

1. The characteristics of the excitation inrush current and the method to overcome the excitation inrush current

(1) Excitation inrush current:

In the case of no-load input of the transformer or restoration of power supply after removal of an external fault, etc., in the case of no-load input of the transformer or restoration of power after the removal of an external fault, the value of the transformer excitation current can reach 6 to 8 times the transformer rated current. The transformer excitation current is usually called Excitation inrush current.

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(2) Causes of inrush current

Because the magnetic flux in the iron core should lag behind the applied voltage by 90° in a steady state, and when the voltage instantaneous value u=0, the magnetic flux in the iron core should be -Φm. However, since the magnetic flux in the iron core cannot change suddenly, a non-periodic component of magnetic flux +Φm will appear. If the remanence Φr is considered, the magnetic flux in the iron core will reach 2Φm+Φr after half a period, and its amplitude is As shown in the figure.

At this time, the transformer core will be seriously saturated, and the value of the transformer's excitation current will become very large at this time, reaching 6-8 times the rated current, forming an excitation inrush current.

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(3) Characteristics of magnetizing inrush current:

① The excitation current has a large value and contains obvious non-periodic components, which makes the excitation current waveform obviously skewed to one side of the time axis.

②The excitation inrush current contains obvious high-order harmonics, among which the excitation inrush current is dominated by the second harmonic.

③The waveform of the magnetizing inrush current has a discontinuous angle.

The following table shows an example of excitation inrush current test data

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(4) Measures to overcome the influence of inrush current on transformer longitudinal differential protection:

① Use differential relay with fast saturation converter to form differential protection;

② Differential protection based on the principle of second harmonic braking;

③ Transformer differential protection based on the principle of discontinuous angle;

④The transformer differential protection is composed of the fuzzy identification blocking principle.

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2. Causes of unbalanced current

(1) Unbalanced current in steady state

①The current phases on both sides of the transformer are different

Transformers in power systems often use Y and d11 wiring methods. Therefore, the phase difference between the currents on both sides of the transformer is 30°. As shown in the figure below, the current on the Y side lags the current on the △ side by 30°. If the current transformers on both sides use the same With the connection method, the secondary currents of the corresponding phases on both sides are also different by about 30°, which generates a large unbalanced current.

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②The calculated transformation ratio of the current transformer is different from the actual transformation ratio

Due to the standardization of the transformation ratio, the actual transformation ratio is inconsistent with the calculated transformation ratio, resulting in unbalanced current.

③Different current transformer models on each side of the transformer

Due to the different voltage levels and rated currents on each side of the transformer, the current transformers on each side of the transformer have different models, and their saturation characteristics and excitation current (reduced to the same side) are also different, resulting in a larger difference in the differential circuit. Unbalanced current.

④ Transformer on-load adjustment tap

Transformer load adjustment tap is a method of voltage adjustment in the power system. Changing taps means changing the transformation ratio of the transformer. In the setting calculation, the differential protection can only be set according to a certain transformation ratio, and the appropriate balance coil is selected to reduce or eliminate the impact of unbalanced current. When the differential protection is put into operation, it is generally impossible to re-operate the current loop of the differential protection when the tap change of the voltage regulation, so a new unbalanced current will appear. The magnitude of the unbalanced current is related to the voltage regulation range.

(2) Unbalanced current under transient conditions

Characteristics of unbalanced current in transient process:

① The transient unbalanced current contains a large number of non-periodic components, which deviate from one side of the time axis.

② The time when the maximum transient unbalanced current appears lags the time of the maximum current on the primary side (according to this feature, the delay of the protection to avoid the transient unbalanced current will inevitably affect the rapidity of the protection, and even make the transformer differential protection impossible accept).

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3 Measures to reduce unbalanced current

(1) Reduce the unbalanced current under steady-state conditions

The current transformer used on each side of the transformer differential protection is a Class D current transformer dedicated for transformer differential protection; when the external maximum steady-state short-circuit current is passed, the secondary load of the differential protection circuit must meet the 10% error Claim.

(2) Reduce the secondary load of the current transformer

This is actually equivalent to reducing the terminal voltage on the secondary side and correspondingly reducing the excitation current of the current transformer. Common methods to reduce the secondary load are: reduce the resistance of the control cable (appropriately increase the conductor cross-section, and shorten the length of the control cable as much as possible); use current transformers for weak current control (secondary rated current is 1A) and so on.

(3) Use current transformer with small air gap

The remanence of the core of this current transformer is small, and the current transformer is not easy to saturate when the primary current is large. Therefore, the excitation current is small, which is beneficial to reduce the unbalanced current. At the same time, the transient characteristics of the current transformer are also improved.

(4) Reduce the unbalanced current caused by the different current phases on both sides of the transformer, adopt phase compensation

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If the transformer is Y, d11 wiring its phase compensation method is to connect the current transformer on the star side of the transformer into a triangle, and connect the current transformer on the triangle side of the transformer into a star, as shown in (a), to compensate for the 30° Phase difference.

b) is the primary current on the star side and the primary current on the delta side, and their phase relationship. After using phase compensation wiring, the currents in the differential arm of the secondary circuit side of the transformer star-side current transformer are respectively (red in the upper right figure), and they are just in phase with the current in the secondary circuit of the triangular-side current transformer, such as c ) Shown. In this way, the phases of the currents on both sides of the differential circuit are the same.

② Numerical compensation

Transformer star side current transformer ratio

Transformer delta side current transformer ratio

③Software calibration

Software for phase correction in microcomputer protection

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(5) Reduce the unbalanced current caused by the calculated transformation ratio of the current transformer due to the difference between the standard transformation ratio and adopt numerical compensation

① Use auto-converter.

②Using the balance coil in the BCH type differential relay.

③The compensation coefficient is used in the software of the transformer microcomputer protection to minimize the unbalanced current of the differential circuit.

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(6) Unbalanced current caused by different types of current transformers on both sides of the transformer

Take it into consideration in the setting calculation of the differential protection.

(7) Unbalanced current generated by the transformer's load adjustment tap

Considered in the setting calculation of transformer differential protection.

In the steady state, the unbalanced current of the differential protection of the transformer can be determined by the following formula

(8) Reduce the impact of non-periodic component current in the transient process

①The differential protection adopts an intermediate converter with fast saturation characteristics,

②Choose differential relays with braking characteristics or differential relays with discontinuous angle principle, etc., and use other methods to solve the problem of non-periodic component current in the transient process.

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4 Sum differential ratio braking differential protection principle

1. The differential protection principle of dual-winding transformer ratio braking.

(1) Action criterion of sum-differential ratio braking

①Differential current:

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②Brake current:

③The first criterion of differential protection action:

④Brake ratio coefficient:

⑤In the event of an external fault, the protection reliably does not operate. The following criteria should be met:

⑥ The second criterion of differential protection action

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2. Setting of ratio braking characteristics

(1) Minimum starting current Iact0

(2) Inflection point braking current Ibrk0

(3) Maximum braking coefficient Kbrk.max and braking characteristic slope S can be selected

①Maximum braking coefficient ②The ratio braking characteristic curve is as follows

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③The setting value D of the ratio braking coefficient is 0.3～0.5

④The slope S of the ratio braking characteristic can be seen from the figure above

When Ibrk0<<Ibrk.max and Iact0<<Ibrk.max, the above formula can be obtained

That is, the broken line BC of the proportional braking characteristic crosses the origin of the coordinate and has the same braking coefficient under any braking current.

(4) Internal fault sensitivity check

In the minimum operating mode of the system, calculate the minimum short-circuit current (period component) of the metallic short circuit at the transformer outlet, and calculate the corresponding braking current at the same time. The sensitivity coefficient of the corresponding starting current is determined by the corresponding ratio braking characteristic. Ksen> 2.0

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3. The principle of differential protection of three-winding transformer ratio braking.

For three-winding transformers, the principle of differential protection is the same as that of double-winding transformers, but the differential current, braking current and maximum unbalanced current should be changed accordingly. The differential current and braking current are respectively

In some transformer differential protection, the maximum current of the three sides is directly used as the braking current, namely

The calculation formula of the maximum unbalanced current is as follows:

In the microcomputer protection, the error is very small after considering the numerical compensation coefficient Δm≈0, then the above formula is

4. Principle of magnetizing inrush current blocking

Adopt second harmonic braking principle

The transformer magnetizing inrush current contains a large amount of second harmonic components, which generally account for more than 40% of the fundamental component. Using the ratio of the second harmonic in the differential current as the braking coefficient, the magnetizing inrush current when the transformer is closed at no load can be identified, so as to prevent the protection from malfunctioning when the transformer is closed at no load.

In the differential protection, the amplitude of the second harmonic of the differential current is expressed by, and the ratio of the second harmonic in the differential current can be expressed as the following formula:

If the second harmonic restraint coefficient is selected as the fixed value D3, as long as it is greater than the fixed value D3, it can be considered that the magnetizing inrush current has occurred and the protection should not act. The protection action is only allowed when the value is less than D3 and other criteria of ratio differential are met.

The second harmonic restraint coefficient D3, there are three coefficients of 0.15, 0.2, 0.25 to choose from

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5. Differential quick-break protection

(1) Reasons for using differential quick-break protection

Under normal circumstances, the differential protection based on the ratio braking principle can be used as the main protection of power transformers, but in the case of severe internal faults and large short-circuit currents, the severe saturation of TA will seriously deteriorate the AC transient transmission. The secondary side of TA The fundamental wave current is zero, the higher harmonic components increase, and the criterion for reflecting the second harmonic is wrong. The differential protection of the ratio braking principle is shut down. It cannot reflect the short-circuit fault in the area. Only when the transient process has passed a certain period of time Only when TA exits the differential protection of the transient saturation ratio braking principle, it affects the rapid action of the ratio differential protection. Therefore, the differential protection of the transformer ratio braking principle should also be equipped with differential quick-break protection as an auxiliary protection. Speed up the action speed of the protection in case of serious internal failure. Differential quick-break protection is instantaneous quick-acting protection for differential current over current.

(2) The setting value of the differential quick break is set according to the maximum unbalanced current and the magnetizing inrush current.

6. Logic block diagram of transformer ratio differential protection program

(1) Logic block diagram of transformer differential protection program

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2) Logic principle of transformer differential protection program

In the program logic diagram, D1=Iact0, D2=KrelId/Ibrk are the setting values of the ratio braking coefficient, and D3 is the setting value of the second harmonic braking coefficient. It can be seen that the three criteria for the action of the ratio differential protection are the "and" relationship, which must be met at the same time to be able to act on tripping. The differential quick-break protection is used as the auxiliary protection of the ratio differential protection. Its fixed value is D4=Iact.s. When the ratio differential protection cannot quickly reflect the fault in the serious area, the differential quick-break protection should quickly exit the trip without time delay. Therefore, these two kinds of protection are logically related to "or". The ratio differential protection will produce a large differential current and malfunction when the TA secondary circuit is disconnected. Therefore, it must pass through the NO gate locked by the TA disconnection and then pass through the AND gate Y3 to exit the action. When TA is disconnected, the AND gate Y3 is locked and cannot exit.

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