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Load rejection relay is the part of the modern steam turbine control system which assists the turbine governor to ensure that the speed of the turbine does not reach to its maximum trip value during load rejection. A simple question may arise that why the speed of the turbine raises during load rejection. The answer is because of the mismatch in between the generation and consumption, there will be huge kinetic energy for the turbine when the load on the turbine reduces. This kinetic energy is due to 100% steam flow through the turbine. In this condition if the raised speed is provided as a feed back signal to turbine governor, it cannot respond as fast to limit the raise in speed. So to assist and to provide better control during this transient condition load rejection relay is placed in steam turbine control system.
Construction of Load rejection relay
The control fluid from the steam turbine controller modulates the control valve i.e. basically the pressure which regulates the valve moment. Now a solenoid valve is placed in between the controller and control valve with a drainage path to drain the control fluid. It s designed like when the valve is off, it allows the control fluid to reach the control valve and if the valve is off then it blocks the control fluid flow and drains the entire fluid through drainage path.
An electronic logic circuit is provided to generate a pulse to solenoid according to the negative step change in the load. I.e. if the step size increases the pulse width will be so high so that the valve remains close until the turbo generator speed comes to normal.
Operation of Load rejection relay
Let us assume that the power plant is feeding 100 % load demand with its maximum capacity. if suppose due to some uneven conditions the demand has reduced i.e. the load remand reduced to 20% then due to kinetic energy the turbine tries to speed up. before this happens the negative step decrease in load initiates a closing pulse (pulse duration is proportional to negative step) to the solenoid valve. The solenoid valve closes and drains the control fluid which ensures that total steam to turbine is blocked. This means the load rejection relay acts as feed forward controller on the turbine controller before the speed raises. Still the turbine speeds up due to locked up steam in the turbine inner stages but this raise will not reach the maximum trip limit. This limited speed raise is sensed by the electro-hydraulic controller as a feed back signal and reduces the speed controller demand signal to control valve which automatically reduces the control fluid pressure. After termination of electronic pulse to solenoid, it again opens the normal path and closes the drain path. Mean while the control fluid pressure is reduced to zero by the turbine controller. Once the turbine set has reached normal speed then the electro-hydraulic turbine controller opens the control valve to allow the minimum steam to the turbine to drive the 20 % load demand.
The load rejection relay works for negative step change only that is it works for sudden load reduction or for over frequency. It will not work for under frequency, because after termination of CLOSE pulse to solenoid it opens the solenoid valve to allow the control fluid to control valve. But mean while the electro-hydraulic controller opens the turbine control valve with high frequency error caused by the under frequency. This may results in speeding up the turbine to its maximum trip value. So to avoid this the load rejection circuit inhibited by a frequency monitoring circuit.
The proportional pulse width, timing of load rejection relay will be set during commissioning or during turbine strat-up.