影响DCS安全运行的因素及对策

使用无线通讯设备，不正确接地及电源系统配置的不完善，均会影响ＤＣＳ的安全运行。克拉玛依炼油厂进行的对讲机对仪表和ＤＣＳ的干扰实验表明，干扰严重者会导致ＤＣＳ数据丢失，退出运行程序，引起设备误停车。其预防措施是加强管理，使干扰设施远离ＤＣＳ。在设置接地体时应防止由保护接地引起的干扰，既要满足仪表防爆和抑制干扰的要求，又要达到电位平衡的目的，一般应与全厂统一接地网连在一起，仅在统一接地网不能满足要求时才独立接地。ＤＣＳ的电源接地采用三相四线制时，输出中线应接地；而采用单相电源时，若ＤＣＳ与电力设备为公用接地，则 ＤＣＳ电源零线不接地；若 ＤＣＳ采用独立接地，ＤＣＳ电源零线应接地。将工作接地电阻一概定为 ４Ω ，保护接地电阻５Ω，或工作接地电阻１Ω，保护接地电阻４～５Ω，均是一种误区，应区别对待。对不需要公共线的系统，接地电阻５Ω，采用二极管安全栅时，工作接地电阻应高于保护接地电阻，同时要避免多点接地。 ＵＰＳ应有主备电源切换，ＵＰＳ内的充电器和逆变器应冗余配置，性能参数应满足ＤＣＳ的要求。

FACTORS AFFECTING SAFE OPERATION OF DCS SYSTEM AND COUNTERMEASURES

DCS operation will be effected by RFI from radio communication equipment, misgrounding and inappropriate power supply system. The experiment of RFI interference to the DCS system and instruments conducted by Kramayi Refinery showed that the RFI interference could lead to the loss of data which resulted in the interruption of the DCS operation, causing the unexpected plant shutdown. The preventive measure is to strengthen the management and keep the interference source equipment far away from the DCS system. The grounding interference should be taken into consideration in the design of the protection grounding system. The requirements of explosion proof and antiinterference of the instruments should be met while the potential should be balanced. The DCS safe grounding should be connected to the overall plant grounding system. An independent DCS POwer grounding is practiced only when the overall plant grounding system can not meet the requirements. The DCS power grounding should be: when three- phase and four-wire system is adopted, the neutral wire should be grounded; when single-phase system is adopted and if the DCS system is connected to the overall plant grounding system, the zero wire of the DCS should not be grounded. If the DCS has an independent grounding system , the zero wire should be grounded. That when Master Reference Grounding (MRG) is 4 ~ 5 ohm, the grounding resistance is 5 ohm, or when MRG is 1 ohm, the grounding resistance is 4 ~ 5 ohm is misconception. The good MRG of DCS should be: for non-common line single system, the MRG is 5 ohm; when diode safety barriers are used, the MRG should be higher than that of the protection grounding. Multipoint grounding should be avoided. The UPS shall be provided with automatic back power switching system. The charger and inverter should be configured redundantly to assure the safe operation of DCS system.