Differential protection method for partition inverse-time transformer by considering DC bias effects

Abstract

The invention belongs to the technical field of relay protection of power systems, and particularly relates to a differential protection method for a partition inverse-time transformer by considering DC bias effects. The method is characterized by comprising the following steps: judging whether a partition signal is sent out or not by detecting a DC of a neutral point; if a microprocessor protection device does not receive the signal, judging an internal fault or an external fault according to an original ratio braking characteristic, judging saturation of a current transformer by a time-difference method and blocking the external fault; if the signal is received, partitioning an action region of the ratio braking characteristic into two blocks, if a serious internal fault is generated, enabling the signal to drop into a no-delay action region and if a light internal fault is generated, enabling the signal to drop into an inverse-time action region; and if the external fault is generated, enabling the current transformer to quickly retreat from the inverse-time action region, returning an action instruction within a time delay, retreating and entering a brake region and blocking differential protection. After the fault is removed, a brake current is a load current; the minimum action current of the ratio braking characteristics is lifted on the basis of the partition signal; and a relatively high differential current after the fault is removed is evaded.

Description

technical field [0001] The invention belongs to the technical field of electric power system relay protection, and in particular relates to a method for differential protection of partitioned inverse time transformers considering the influence of DC bias. Background technique [0002] my country's AC-DC interconnected EHV / UHV grid is currently under construction. DC transmission plays an important role in the development of my country's power grid with its unique technical advantages. At present, several high-voltage DC transmission projects have been put into operation in China. However, the adoption of DC transmission technology has had a certain impact on the conventional AC power grid and its equipment. When the DC power transmission system adopts a single-pole ground loop operation mode, the DC in the ground flows into the AC grid through the grounded neutral point of the transformer, which will cause DC bias. The magnetic phenomenon has a great impact on the safe and s...

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Problems solved by technology

On the other hand, after the external fault is removed, the primary side current of the current transformer returns to the load current, and its demagnetization effect is not enough to make the magnetic flux of the current transformer core exit the saturation section, so it may still operate in the local hysteresis near the saturation point In the loop, the waveform of the primary and secondary sides of the current transformer is slightly distorted, so that the differential current is just greater than the minimum operating current of the ratio braking characteristic. At this time, the braking current is approximately equal to the load current, and it is easy to fall into the action of the ratio braking characteristic. in the district

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