Internal Fault and Differential Protection of Transformer

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Internal Fault and Differential Protection of Transformer

Time: 12-27 2022    Auther: Ztelec group

The common faults of transformers are divided into internal faults and external faults. Internal faults are variou...

The common faults of transformers are divided into internal faults and external faults. Internal faults are various faults that occur in the transformer oil tank. In order to monitor different faults or abnormal working conditions, we have set up different protections, which are divided into main protections. Unlike the backup protection, the main protection has snap-action characteristics.

1. The basic principle of differential protection

The longitudinal differential protection is one of the main protections of the transformer, and the protection operates instantaneously to trip the switches on each side. The protection area is the part between the current transformers on each side of the differential protection, including the transformer body, the lead-out lines between the current transformer and the transformer. The existing transformer longitudinal differential protection adopts microcomputer protection device, and the current of each phase enters the protection device respectively, and the longitudinal differential protection is realized by software algorithm. We take one phase as an example to illustrate the basic principle of longitudinal differential protection.
The differential current "felt" by the protective device is the vector sum of the secondary currents of the two coils. As shown in Figure 1, when the system is operating normally or is short-circuited externally, the secondary currents of the two coils are the same in size and opposite in polarity, and the differential current is 0, and the protection does not operate at this time. As shown in Figure 2, when a ground fault occurs within the protection range, the secondary currents are equal in size and polarity, and the differential current is the sum of the secondary currents. When the differential start value is reached, the protection operates.
Figure 1 Schematic diagram of transformer normal operation/external short circuit differential current
Fig. 2 Schematic Diagram of Short Circuit Differential Current in Transformer Area
On the basis of the rheological secondary coil connection method above, the longitudinal differential protection adds phase adjustment, zero-sequence current elimination, and amplitude conversion to the current vectors on different sides to form a differential current calculation method, and then introduces the ratio braking characteristic curve. Constitute the basic logic of protection.
Taking YN-d11 wiring as an example, the wiring diagram and current phasor diagram are shown in Figure 3. It can be seen that because there is a 30° angle difference between the high and low side phasors in the same phase, the vector sum of the two currents is not 0 during normal operation, and the phase conversion is required first. After the conversion, the high and low sides of the same phase have the same phase.



Figure 3 YN-d11 wiring diagram and current phasor diagram

There are two methods of phase conversion, one is based on the Y side, so that the current phase of the d side is consistent with the Y side current phase, referred to as "angle rotation star"; the other is based on the d side, so that the Y side current phase is consistent with the Y side current phase The side d is the same, referred to as "star rotation angle", and most of the existing protection devices adopt the star rotation angle method.
The purpose of zero-sequence current elimination is to prevent misoperation of longitudinal differential protection. For YN-d wiring, when a ground fault occurs outside the high-voltage side, zero-sequence current flows on the high-voltage Y side, but there is no zero-sequence current on the low-voltage d side, and the zero-sequence currents on both sides cannot be balanced, so the differential protection will malfunction . In the "star rotation angle" conversion mode, the difference between the two currents after the phase shift on the Y side has filtered out the zero current, so no measures need to be taken. In the "angle to star" conversion mode, zero-sequence current compensation is performed on the Y-side current vector.
The three-segment broken-line ratio braking curve is shown in the right figure in Figure 4, which can be further subdivided into two types, one type of AB segment is a horizontal line, and the other type of AB segment is an oblique line.

2. Auxiliary components of differential protection

In order to make the differential protection more reliable, the protection logic also involves components such as start, quick break, and lock:
1) Starting element: The starting variable includes the maximum value of three-phase differential current, the amount of current mutation, etc. When the starting variable is greater than any starting value, the protection device opens the differential protection.
2) Differential quick-break element: At a high short-circuit current level, due to the saturation of the current transformer, the second harmonic generates a huge braking torque and the differential element refuses to move. In order to avoid protection refusal, a differential quick-acting element is installed in the device. When the short-circuit current reaches 4~10 times the rated current, the quick-acting element quickly moves to the outlet. In addition, in order to prevent incorrect action of the protection due to inconsistent transient characteristics of the current transformers on each side when there is a large short-circuit current, the relevant characteristics of the current transformers on each side of the differential protection of the main equipment including transformers should be consistent. The high voltage side of No. 2 main transformer of a certain substation is connected to the second string of 2/3 wiring, and this string is connected to another tie line.
3) Exciting inrush blocking element: When the airdrop transformer and the short circuit outside the transformer area are cut off, a huge exciting inrush current will be generated. In order to prevent the differential current caused by the exciting inrush current from causing the device to malfunction, an inrush current blocking element is set up for the longitudinal difference protection, using waveform distortion ( Intermittent or asymmetric differential current waveform), harmonic component identification (second or third harmonic content), fuzzy identification identification of excitation inrush current. However, when the transformer is actually airdropped, especially for the first time, the differential protection will still malfunction due to insufficient degaussing of the main body, and the harmonic content of the airdropped differential current may be lower than the harmonic component blocking threshold. In order to fundamentally eliminate the influence of residual magnetism on transformer empty charging, we can take degaussing measures and perform another empty charging, or temporarily lower the second harmonic blocking threshold to ensure the normal operation of the main transformer.
4) CT disconnection element: When the secondary phase of the CT is disconnected, the differential current is the load current of the disconnection phase, and the protection may malfunction. At this time, the zero-sequence current, phase current changes, abnormal sudden drop of phase voltage, etc. can be used to judge CT disconnection.
5) CT saturation blocking element: When an external fault occurs, CT saturation will cause differential protection to malfunction, so the protection device is equipped with a CT saturation blocking detection element. When the CT is saturated, the differential current occurs after the CT is saturated for a period of time, so the device uses the timing consistency of the braking current and the differential current to judge whether the CT is saturated. In addition, in order to minimize the impact of CT saturation on transformer longitudinal differential protection, the current transformers of each branch of transformer differential protection should be preferentially selected with current transformers with higher accuracy limit factor (ALF) and rated inflection point voltage.

3. Other differential protection

1) Split side differential protection
The split-side differential protection is a differential protection that takes the Y-side winding of the transformer as the protected object, and is composed of the first and last CTs of the windings on each side according to the phase. Taking the self-coupling phase A in Figure 5 as an example, the protection is composed of TA1A, TA2a' and TA3A. According to Kirchhoff's current law, there is no electromagnetic coupling relationship between the currents at both ends, so the protection does not require inrush current blocking elements, differential snap-action elements and over-excitation blocking elements. In addition, the fixed value of the operating current of the split-side differential protection is lower, and the sensitivity is higher than that of the longitudinal differential protection. But the disadvantage of this protection is that it cannot protect inter-turn short circuit.
2) Split-phase differential protection
Phase-splitting differential protection is a differential protection that takes each phase winding of the transformer as the protected object, and is composed of CTs on each side of each phase winding. example). This protection can reflect all faults of a certain phase of the transformer except the low-voltage side leads, but an inrush current blocking element is required.
3) Zero sequence differential protection
The zero-sequence differential protection consists of zero-sequence current transformers on the neutral point side of the transformer and zero-sequence circuits of the current transformers on the star side of the transformer. Figures 6 and 7 are the current loops when ground faults occur outside the zone and inside the zone, respectively. Similarly, the secondary currents of this protection have no electromagnetic coupling relationship, so the protection device does not need an excitation inrush current blocking element or an over-excitation blocking element; at the same time, it is more sensitive to the grounding fault of the transformer winding. However, the zero-sequence differential protection can only reflect the internal ground fault of the high and medium voltage side, and cannot protect the inter-turn short circuit.


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