Research And Application On Monitoring Device Of AC Intrusion In Substation DC System

The power supply system of substations includes AC system and low-voltage DC system.Among them,the low-voltage DC system provides power for relay protection devices,automatic safety devices,breaker control circuits,signal circuits,which is the guarantee for safe and stable operation of substations.In the daily operation process,It often occurs that AC flow into DC,due to complex circuits,poor operating conditions,poor cable quality,design defects and other reasons.It may cause malfunction of protection devices,trips of circuit breaker,damage to equipment and other extremely serious consequences,bringing a great threat to the safe and stable operation of substations.By analyzing the case,this paper briefly explains the causes of AC flowing into the DC system.Then the paper construct an equal circuit to analyze the harm and determinants of AC flowing into different parts of the DC system,taking the control circuit of a breaker as an example.Build an equivalent circuit model of the above faults by using simulation software.After that,simulate different types of AC flowing into the DC system and obtain the waveform characteristics of bus’ s voltage.Then reveal the basic principle of monitoring technology for AC flowing into the DC systems.Based on the basic principle,a design scheme for a monitoring device for AC flowing into the DC system of substations is proposed.This scheme adopts a modular design,which can monitor AC flowing into the DC system and output alarm signals.At the same time,the proposed scheme optimizes the traditional filtering and rectification circuit in the equipment,using a differential lifting circuit to eliminate the sampling error to improve sampling accuracy and anti-interference.The feasibility of the proposed differential lifting circuit is verified using simulation software.Eventually,this device is applied to a substation for actual test trials.The proposed monitoring devices can respond to faults of AC flowing into the DC system with different amplitudes between 2V and 311 V,and the alarm operates reliably.The error between the displayed sampling value and the actual value is no more than 5%,proving that the sampling accuracy meets the requirements.It has been verified that the functional indicators achieved the projected designed objectives,which can monitor AC flowing into the DC system of substations and improve the level of the grid’s safety operation.