Breaker opening

As previously mentioned in Chapter 5, one of the most severe disturbances for the power recovery train (PRT) is a generator breaker opening. This event often causes the PRT to trip on overspeed or other process or machine conditions. A control solution has been developed to keep the PRT and the process under control during breaker opening. 

When the breaker opens, die load (or a portion of it) on die expander is suddenly shed. This event is deteeted by die breaker status before die speed inerease is deteeted by die eontrol system. Therefore, a feedforward approaeh is employed to antieipate die need for eontrol aetion before feedbaek indieates diis need. 

Breaker opening eauses the expander inlet valve to elose. This, however, would disturb the differential pressure between the regenerator and the reaetor stripper. To keep this pressure eonstant, the bypass valve needs to be opened to keep pressure, P, upstream of the inlet and bypass valves eonstant. Again, this needs to be done earefully. Opening the bypass valve too mueh ean eause the pressure to drop to sueh a level that eatalyst enters the expander. This must be prevented under all eireumstanees. 

To follow through with the open-loop approaeh, there must first be a signal that warns the speed eontrol funetion of load loss (or breaker opening) and impending speed inerease. This is provided by a digital eontaet that indieates motor/generator breaker status. Next, a load loss estimate is needed. This is provided by a kilowatt transdueer on the motor/generator. 

Step 1 Calculate actual expander power before breaker opening. To determine the required power from the expander after the breaker opening, it is neeessary to know the aetual power developed by the expander before breaker opening based on readily available and reliable measurements on the expander. Typieally these are Pj, Tj, Pj, Tj, and N. The PRT speed ean be ignored beeause it is nearly eonstant before and after the breaker trip and the expander performanee eurve is typieally a weak funetion of rotational speed. 

Step 2 Calculate required expander power after breaker opening. The power required from the expander after the breaker opening can be calculated as the expander power produced before the breaker opening minus the electrical power produced by the generator immediately prior to the breaker opening. This can be expressed as ... 

Calculating power developed by the expander before breaker opening, is described in Step 1. The generator power, can be measured by using a kilowatt transducer. The kilowatt transducer (or current transducer) is typically available in the motor/generator control panel. If not, it can be added to the machine at a low cost. Knowing both J(,gf and Jg, can be calculated. 

Step 3 Calculate new inlet pressure, P afp/ expander. As the breaker opens, the PRT will start to accelerate if immediate action is not taken. Therefore, the inlet valve is going to be closed by a step. 

Closing the inlet valve will potentially ehange the expander inlet and outlet parameters. The parameters that inlluenee the expander power ealeulation are Pj, Pj, and Tj. Therefore, it is important to eonsider what happens to these three parameters during breaker opening. 

Equation 7-3 can be rewritten for conditions after the breaker opening ... 

Fig ure 7-6. Breaker opening causes the expander iniet vaive to ciose. 

As ean be seen from Equation 7-9 and Equation 7-13, it is now possible to ealeulate the redueed expander flow, if the expander pressure ratio is known. To ealeulate pressure ratio, Pj and Pj are needed. In this situation, this ealeulation is for eonditions after the breaker opening. Sinee Pi and P2aft known, it is possible to ealeulate q exp aft- This is elearly illustrated in Eigure 7-7. 

Combining Equations 7-21 and 7-22 and defining the formula for the eonditions after the breaker opening yields ... 

Step 6 Calculate step on the inlet valve. Now that the new inlet valve e after the breaker opening has been determined, this ean be translated into a valve position. This is a simple proeess using the valve eharaeteristie of the manufaeturer. Eigure 7-8 shows a typieal butterfly valve eharaeteristie. This eurve shows the ealeulated Using the valve eurve, the new valve position is determined. 

Step 7 Calculate the differential in Ae One PRT eontrol objeetive is to maintain the differential pressure between the regenerator and the reaetor stripper. At the time of the breaker opening, it is assumed that the reaetor stripper pressure will not vary. Therefore, to keep the differential pressure eonstant, the regenerator pressure needs to also remain eonstant. Eor the expander, this means that must remain eonstant. To keep P eonstant, the mass flow before and after the breaker opening must remain eonstant (Equations 7-7 and 7-8). This implies that whatever mass flow is redueed on the inlet valve must be rerouted over the bypass valve. 

The eriterion of equal mass flows before and after the breaker opening is the same as keeping the total redueed flow through the inlet and bypass valves equal. This is only true when valve inlet eonditions are the same, whieh they are in this ease. 

I kept winding the hand-wheel and then, quite suddenly, I heard some bangs as various valves closed and electrical breakers opened, and the 3000 revolutions per minute tone of the huge 250 MW spinning main turbine suddenly lowered in pitch as the turbine began to slow dovra. With a sinking feeling, I realized the whole plant had tripped. 

If the pressure in the outer containment exceeds the pressure in the drywell, then vacuum breakers open to equalize the pressure. Long-term containment heat removal can be accomplished with suppression pool cooling or by containment sprays (with appropriate circulation of the water through heat exchangers) in the outer containment. 

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