Fu Hongqin

(State Power Investment Corporation of Ningdong Aluminum Energy Branch of Ningxia Energy Aluminum Company,Ningxia,Yinchuan,750411)

Abstract:This paper mainly illustrates the importance ofDCpower supply of electrolytic aluminum rectifier cabinet to smooth operation of rectifier unit by posing a case of the rectifier cabinet and common trip circuit lost DC power,then the whole electrolytic series cannot operation due to misoperation.And by analyzing this case,the author found the design defectsof the rectification cabinet DC power supply circuit,and then the author proposed improvement measures and took concrete actions to eliminate the hidden dangers of power supply circuit.This case can be used as a training material for telling accident experience to relevant units in the same industry.

Keywords:misoperation;electrolytic aluminum series;rectifier unit;trip

I.Introduction

On August 26,2017,when the power supply workshop of State Power Investment Corporation of Ningdong Aluminum Energy Branch of Ningxia Energy Aluminum Company was dealing with the grounding fault of DC system of rectifier units,7 rectifier units of 350kA electrolytic aluminum series were tripped due to misoperation.Through finding out the cause of the incident,it was found that there were design defects in the common trip DC power supply circuit of the 7 rectifier units.This paper will mainly explain the trip causes of the system and propose the modification scheme of the power supply circuit of the common trip cabinet of the rectifier cabinet.

II.Overview of the Incident

At 08:00 on August 26,due to rain and humid weather, the background machine in the central control room of the rectification office of the power supply workshop reported the signal of“DC screen system 220V bus differential pressure alarm”and fault information such as the bus positive voltage 175V,negative voltage 57 V,insulation resistance 3KΩand So on.Operators on duty in the workshop checked the equipment and found that there was a grounding fault in the DC system,which needs to be eliminated immediately[1].

In order to prevent the system protection from refusing to operate or misoperation due to the reduction of DC voltage, the workshop director arranged maintenance personnel to conduct troubleshooting by adopting“the method of stopping power.” When the maintenance personnel disconnected the DC power switch of the rectifier unit common trip cabinet,the rectifier units 1 to 7 tripped,the 350kA electrolysis series was completely power off, and the power grid was unloaded to 410MW.The workshop immediately reported to the superior leaders according to the emergency disposal procedures, and organized people to resume the power transmission of rectifier unit after checking and determining the cause of equipment trip according to the on-site disposal plan.

At 08:33 on August 26,with the consent of the power dispatching,the workshop closed the switches of rectifier units 1 to 7,downshifted and load up the on-load switches.At 08:50,the on-load switch rises to the 18th gear of normal operation,and the load of 350kA electrolysis series rises to full.After the 350kA electrolysis series returns to normal operation,the workshop organized maintenance and relay protection personnel to continue to search for the grounding fault of the DC screen system.

III.Analysis of the Incident

After searching,the workshop found that the common trip circuit relays of 350kA electrolysis series rectifier units 1 to 7 all adopt normally closed contacts,which is different from the normally open contacts adopted in previous designs.After reading the design drawings,the trip reason was analyzed as follows:

Figure 1 Electrical Schematic Diagram of Communal Trip Cabinet of Rectifier Unit Before Modification

Figure 1 shows that under normal conditions,the coils of common tripping relays K621 and K622 of rectifier cabinet are energizing,and their normally open contacts K621 and K622 are closed.Tripping relays K631,K632,K633,K634,K635,K636 and K637 of No.1 to No.7 rectifier units are energizing,and all normally closed contacts of tripping relays of rectifier units are open.The workshop adopts“the method of stopping power”to check DC ground fault.When the DC 220V power supply of the common trip circuit is disconnected,the coils of the common trip relays K621 and K622 lose their power,and their normally open contacts K621 and K622 open,causing the coils of the No.1 to No.7 rectifier unit trip relays K631,K632,K633,K634,K635,K636 and K637 to lose power.The normally closed contacts of the trip relays of each rectifier unit are closed,and the rectifier cabinet PLC demands the No.1 to No.7 rectifier unit to trip,which causing the trip from No.1 to No.7 rectifier unit.

IV.Problems

(i)The design of the common trip circuit of the rectifier cabinet of 350kA electrolysis series rectifier unit is unreasonable.The design adopts normally closed contacts.There are too many tripping conditions for the trip circuit to be connected in series,resulting in too many factors affecting the start-up of the common trip circuit,which easily leads to misoperation and tripping of the rectifier unit.

(ii)Another unreasonable design is that the disappearance of DC power supply in the rectifier control cabinet of a single rectifier unit will cause the public trip circuit to trip,which will result in a series of power off.

(iii)Using“the method of stopping power”to find the insulation fault of DC screen, the consequences caused by the disconnected power supply circuit are not considered enough[2].

(iv)Long-term electrification of internal relays cause communal trip cabinet of rectifier unit trips,and when the relays power off,it cause rectifier unit trips.Thus the communal trip cabinet cannot be powered off for overhaul for a long time,and during this time,it is normal for the rectifier series to trip,which is caused by failure of intermediate relay.

V.Solutions to the Problems

Considering that the rectifying load of electrolytic aluminum is Class I load,and the load is relatively large,so frequent series trips will cause great impact on the power grid system.As the fluctuation of DC system may cause power failure of the entire electrolytic series,it is necessary to reform the public trip cabinet.See Figure 2 for the modification scheme.

Modification scheme:the public trip emergency stop button and fu are changed from series connection to parallel connection,and change the normally closed contacts to normally open contacts;The normally closed contacts of tripping relays K631,K632, K633, K634, K635, K636 and K637 of common tripping circuit No.1 to No.7 rectifier units are changed to normally open contacts.

During normal operation,the-K621,-K622 relays are not charged,and the-K631 to-K637 relays of rectifier units 1 to 7 are also not charged.Thus, after the 220V DC power supply of the common trip cabinet is suddenly power off, the normally open contacts of the relays K621,K622,K631,K632,K633,K634,K635,K636,and K637 will not close,and the trip circuit of rectifier units 1 to 7 will not be connected,which will not cause the rectifier unit to trip.The DC power loss signal can be uploaded to the background machine in the central control room through PLC,which is convenient for operators on duty to check the reason of power off.

When the emergency stop of 350kA electrolysis series is required due to the fault of the rectifier unit,the operator on duty can manually press the-S620 public trip emergency stop button,the relay coil-K621 proceeds self-locking after charging, the relay coil-K631 to-K637 of rectifier units 1 to 7 retains power and the normally open contacts of relays K631 to -K637 are connected.Thus the tripping circuits of rectifier cabinets of rectifier units 1 to 7 are respectively connected,and then the 7 rectifier units are tripped.

The modification scheme adopts the parallel trip and double relays of-K621 and-K622 to prevent the relay from operating and improve the reliability of the trip circuit

VI.Conclusion

By reforming the DC power supply circuit of the common tripping cabinet of rectifier units,it can not only improves the safety and reliability of rectifier series operation,but also eliminates the risk of all 7 rectifier units tripping at the same time,which can be caused by the misoperation of DC power supply by maintenance or operation personnel on duty.In addition,after using this modification scheme,when the single unit of the public trip cabinet is powered off for maintenance, it would not the affect the operation of other rectifier units.