在伺服模塊中閥位給定電壓與反映油動(dòng)機(jī)行程的LVDT反饋電壓進(jìn)行比較,經(jīng)比例放大后輸出給伺服閥。伺服閥根據(jù)控制信號(hào)的大小及方向控制作油動(dòng)機(jī)開(kāi)啟或關(guān)閉的速度。油動(dòng)機(jī)行程到達(dá)給定值時(shí),伺服閥處于斷流位置,油動(dòng)機(jī)行程維持不變。
圖4:連續(xù)控制油動(dòng)機(jī)示意圖
對(duì)于中壓主汽閥門(mén)由兩位控制油動(dòng)機(jī)控制,只能使閥門(mén)定位在全開(kāi)(全關(guān))位置,在安全油建立期間,油動(dòng)機(jī)受電磁閥控制,電磁閥帶電時(shí)油動(dòng)機(jī)全關(guān),失電時(shí)全開(kāi)。
l DEH控制系統(tǒng)的特點(diǎn)
1) 一次調(diào)頻
電網(wǎng)一次調(diào)頻作用是電網(wǎng)頻率很重要的穩(wěn)定基礎(chǔ),發(fā)電機(jī)組的轉(zhuǎn)速有差調(diào)節(jié)反饋是實(shí)現(xiàn)電網(wǎng)一次調(diào)頻作用最有效的手段,機(jī)組轉(zhuǎn)速反饋同時(shí)也是機(jī)組運(yùn)行安全最有力的保障。
傳統(tǒng)DEH系統(tǒng),在升速階段一次調(diào)頻功能不起作用。在機(jī)組并入大電網(wǎng)后,而由于通常電網(wǎng)頻率較穩(wěn)定,也不能檢驗(yàn)一次調(diào)頻的動(dòng)態(tài)性能。有的在孤網(wǎng)狀態(tài)下甚至是不穩(wěn)定的。
與傳統(tǒng)DEH比較采用了快速一次調(diào)頻回路(如圖5),以提高一次調(diào)頻動(dòng)態(tài)響應(yīng)性能。一次調(diào)頻信號(hào)不加任何切換邏輯,直接作用到DEH的總閥位給定。機(jī)組在任何工況下,轉(zhuǎn)速反饋都存在,對(duì)機(jī)組及電網(wǎng)的安全運(yùn)行提供了有力的安全防護(hù)。在升速階段即可驗(yàn)證一次調(diào)頻的穩(wěn)定性。
另外,在機(jī)組并網(wǎng)帶負(fù)荷時(shí),若實(shí)際發(fā)電機(jī)并未并網(wǎng)。對(duì)于傳統(tǒng)DEH由于其一次調(diào)頻功能尚未投入,帶初負(fù)荷的指令會(huì)引起機(jī)組超速。由于此DEH一次調(diào)頻功能的調(diào)節(jié)作用,帶初負(fù)荷的指令只會(huì)使機(jī)組轉(zhuǎn)速升高15r/min左右。
圖5:DEH控制系統(tǒng)SAMA圖主回路
2) (2) 主汽門(mén)調(diào)門(mén)切換
DEH升速過(guò)程采用主汽門(mén)(TV)控制,當(dāng)升速到2950 r/min時(shí),切換閥門(mén),由主汽門(mén)控制切換為高壓調(diào)門(mén)(GV)控制,最后定速3000 r/min。
常規(guī)的閥切換經(jīng)常由于TV與GV的開(kāi)起和關(guān)閉速率配合不好而造成的汽機(jī)轉(zhuǎn)速波動(dòng)很大[1],為此,通過(guò)改進(jìn)閥切換控制方案:閥切換開(kāi)始時(shí),開(kāi)高調(diào)門(mén)系數(shù)由1逐漸變?yōu)?,并且轉(zhuǎn)速PID同時(shí)作用于TV和GV,即:
GV閥位輸出=開(kāi)高調(diào)門(mén)系數(shù)×100+(1-開(kāi)高調(diào)門(mén)系數(shù))×閥位給定(轉(zhuǎn)速PID輸出)
TV閥位輸出=(1-開(kāi)高調(diào)門(mén)系數(shù))×100+開(kāi)高調(diào)門(mén)系數(shù)×閥位給定(轉(zhuǎn)速PID輸出)
control system.
Even good security patches can cause trouble for control system operators. As we discussed earlier, most of them need to shut down the system and re run the production process. In addition, they may also remove the functions previously relied on the control system. For example, a vulnerability of Stuxnet virus attack is the hard coded password vulnerability of Siemens WinCC SQL database. At that time, many security analysts accused Siemens of failing to release a patch to remove the password as soon as possible, but later found that the "cure" was even more serious than the disease. Users who change their passwords manually soon find that many key control functions need to be realized by entering the account with the help of password.
To make matters worse, the patch installation process usually requires the presence of employees with special skills. For example, the vulnerability of Slammer virus attack in January 2003 actually