Transformer differential protection with load test - Solutions - Huaqiang Electronic Network

LC03-3.3 SOP8 TVS Static Protection 3.3V

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

1 Introduction

The principle of differential protection is simple, the use of electrical quantity is simple, the protection range is clear, and the action does not need to be delayed. It has been used for transformer main protection, and its operation is directly related to the safety of the transformer. How do you know the operation of differential protection? How do you know that the differential protection is set and wired correctly? Only useful load current test. But what quantity to measure when testing? How is the measured data analyzed and judged? Let's discuss some of these issues below.

2 Brief principle of transformer differential protection

Differential protection works by using the Kirchhoff current theorem. When the transformer works normally or is out of zone, it is regarded as an ideal transformer, and the current flowing into the transformer and the current flowing out (the converted current) are equal, and the differential relay No action. When the internal fault of the transformer occurs, the two sides (or three sides) provide short-circuit current to the fault point, and the secondary current sensed by the differential protection is proportional to the current at the fault point, and the differential relay operates.

3 Importance of transformer differential protection with load test

The principle of transformer differential protection is simple, but the implementation is complicated, and various differential protections are different in the details of the implementation. It also increases the complexity in the specific use, making the probability of human error increase and correct action. The rate is reduced. For example, Xu Ji's microcomputer transformer differential protection calculation Y-â–³ wiring transformer Y-side rated secondary current is not multiplied, and the protection of NARI is multiplied. These small differences, design, installation, and setting up personnel are easy to neglect and confuse, resulting in protection from misoperation and refusal. In order to prevent this, it is necessary to carry out the load test when the transformer differential protection is put into operation.

4 Transformer differential protection with load test content

To eliminate omissions in the design, installation, and tuning process (such as wire connection error, polarity reversal, balance factor error, etc.), it is necessary to collect sufficient and complete test data.

1. Differential current (or differential pressure). Transformer differential protection relies on CT secondary current and differential current on each side. Therefore, differential current (or differential pressure) is an important part of differential protection with load test. Differential relays with current balance compensation (such as LCD-4, LFP-972, and CST-31A differential relays), which are used to measure phase A, phase B, and phase C differential currents with a clamp-type phase meter or a microcomputer-protected liquid crystal display. And recording; magnetic balance compensation differential relay (such as BCH-1, BCH-2, DCD-5 type differential relay), using phase 0.5 AC voltmeter to sequentially measure the A phase, B phase, C phase differential pressure, and recording.

2. The magnitude and phase of the current on each side. It is not sufficient to judge the correctness of the differential protection only by the differential current, because some wiring or small errors of the ratio often do not produce obvious differential current, and the differential current changes with the load current, the load is small, and the differential current becomes smaller. Therefore, in addition to testing the differential current, the amplitude and phase of the A-phase, B-phase, and C-phase currents on each side of the transformer are sequentially measured by the clamp-type phase meter in the protection screen terminal row (phase is a phase PT secondary voltage) Make a reference) and record. It is not recommended to measure the current amplitude and phase through the microcomputer protection LCD.

3. Transformer currents. Record the current on each side of the transformer by controlling the current, active, reactive power meter on the screen, or monitoring the current, active and reactive power data on the display, or the current, active, and reactive power telemetry data at the dispatcher. The reactive power magnitude and flow direction lay the foundation for CT ratio and polarity analysis.

How big is the load current? Of course, the bigger the better, the larger the load current, and the more obvious the various errors are reflected in the differential flow, the easier it is to judge. However, in actual operation of the transformer, the load current is limited by the network and will not be large, but at least it should meet the accuracy requirements of the test instrument used, and the comparability of the differential current and the load current. If the secondary load current is only 0.2A and the differential current is 65mA, it is quite difficult to judge the correctness of the differential protection.

5 Transformer differential protection with load test data analysis

After the data is collected, it is the analysis and judgment of the data. Data analysis is the most critical step with load testing. If the sloppy, or the principle and implementation of the transformer differential protection is not enough, it will cause one error to slip away and draw a wrong conclusion. So what should we start with for the measured data?

5.1 See current phase sequence

Under the correct wiring, the currents on each side are positive sequence: phase A leads the phase B, phase B leads the phase C, and phase C leads the phase A. If this does not match, it is possible:

a. The secondary current loop phase in the terminal box does not correspond to the primary current. For example, the cable core defined as the A-phase current loop in the terminal box is connected to the C-phase CT. This is the case when the primary device is switched. It is easy to happen.

b. The cable core from the terminal box to the protection screen is reversed. For example, a cable core is connected to the A-phase current loop in the terminal box, and the B-phase current input terminal is connected to the protection screen. This situation is generally caused by the installer's sloppy.

5.2 Look at the symmetry of the current

The amplitudes of the A-phase, B-phase and C-phase currents on each side are basically equal, and the phases are 120° out of phase, that is, the A-phase current leads the B-phase by 120°, the B-phase current leads the C-phase by 120°, and the C-phase current leads the A-phase by 120°. If the deviation of the amplitude of one phase is greater than 10%, it is possible:

a. The transformer load is asymmetrical in three phases, one phase current is too large or one phase current is small.

b. The transformer load is three-phase symmetrical, but the fluctuation is large, causing a large load when measuring the amplitude of one phase current, and a small load when measuring the other phase.

c. The phase-to-phase CT ratio is wrong, for example, the phase CT secondary winding tap is connected incorrectly.

d. There is a parasitic loop in a phase current. For example, a cable core is insulated and damaged when the cable is peeled off, and a leakage current is formed to the cable shielding layer, so that the current flowing into the protection screen is reduced.

If the phase deviation of a two phase is greater than 10%, it is possible:

a. The transformer load power factor fluctuates greatly, causing a large power factor when measuring the phase of one phase current, and a small power factor when measuring another phase.

b. A parasitic loop exists in a phase current, causing the phase current to phase shift.

5.3 Look at the current amplitude of each side and verify the CT ratio

By dividing the primary current on each side of the transformer by the secondary current, the actual CT ratio is obtained, and the ratio should be substantially the same as the setting ratio. If the deviation is greater than 10%, then it is possible:

a. The primary line of the CT is not connected in series or in parallel according to the setting ratio.

b. The secondary line of the CT is not connected to the corresponding tap according to the setting ratio.

5.4 Look at the two (or three) side phase phase current phase, check the correctness of the differential protection current loop polarity combination

Here, the two types of wiring should be treated separately. One is to connect the Y-side CT secondary winding of the transformer to △, and the other is to make the CT secondary winding on each side of the transformer connected to the Y-type. For the former wiring, the secondary current phases on both sides should be 180° out of phase (three-turn transformers can be operated on both sides to check the correctness of the polarity combination of the differential protection current loop), and for the latter wiring, The phase difference angle of the secondary current on both sides is related to the wiring mode of the transformer. For example, a transformer is YY-△-11 wiring. When its high and low voltage sides are running, the secondary current of the high voltage side should exceed the low voltage side (11-6)×30°, while when it is running on the high and medium pressure side. The secondary current and the medium voltage side current on the high voltage side are still 180° apart. If the phase difference of the phase currents of the same name on both sides does not meet the above requirements (deviation is greater than 10°), then it is possible:

a. When the CT secondary winding is combined into â–³, the polarity is wrong or the phase is wrong. For example, when the YY-â–³-11 transformer is combined with the Y-side CT secondary winding, the combined A-phase current should be in the A-phase CT pole. The end of the non-polar end of the B-phase CT (or the non-polar end of the A-phase CT and the polar end of the B-phase CT) is taken out, but not at the A-phase CT polarity end and the C-phase CT non-polar end. (or the junction point of the A-phase CT non-polar end and the C-phase CT polar end) is taken out.

b. One side of the CT secondary winding is reversed in polarity. When installing CT, if the polarity is not placed on the drawing for some reason, the secondary polarity should be reversed accordingly. This will happen if the secondary polarity is not reversed.