Back pressure control for an induction generator based system consists of the steam inlet control valve, which is used to control the back pressure to the process, and a back pressure transmitter.
The pressure transmitter takes the pressure sense from the exhaust of the turbine and sends a 4-20 MA signal to the PLC. If the process requirement of steam drops then the back pressure increases which is communicated to the control valve through the PLC and the control valve throttles the inlet steam to maintain the back pressure. Similar procedure is followed when the withdrawal of process steam increases. In this case the control valve opens to allow more flow to make up the pressure.
A scheme of the full operating system of an Induction Generator based control for back pressure is depicted to the right for clarity.
This system does not require a governor as the speed is controlled by the grid frequency. When the IG is locked on to the local grid using the reactive power of the grid, the voltage, frequency and the phase angles, are matched to the grid levels. Hence the need of a governor is eliminated. Moreover, this is most efficient way of harnessing power from steam as the generation continues as long as steam flows through the turbine. The amount of steam flowing through the turbine that is used for the process, governs the generation of power. This is in contrary to the scheme followed in an alternator based system, where the governor controls the speed based on electrical load on the alternator and controls the steam flow into the turbine and process. This could affect the process since steam flow depends on the electrical load.
ETGroup has a unique way of hooking the induction generator on to the local grid, which we have fine tuned for years. The turbine is first ramped up in speed and the grid frequency is measured and compared with the speed generated by the turbine. The turbine speed is then matched up to the speed required as per the grid frequency. The PLC then gives a signal to the contactor to latch the system to the grid. This system is highly reliable and requires very low latching current, which is highly beneficial to the power pack and of course also for the customer.