GE hydro turbine governor control


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What is a difference between governor position control, power control and ISOCH mode?

>What is a difference between governor position control,
>power control and ISOCH mode?

These terms aren't particular to hydro turbine controls--they are generally applicable to most generator-drive prime mover control systems (also called 'governors'). Generator-drive prime mover control systems (governors) primarily control the flow-rate of energy into the prime move based on one of several possible references--examples of possible references include position control, power control and ISOCH mode.

When a generator-drive control system is operating in position control it is typically trying to maintain a control valve position in accordance with a position reference. For example, if the position reference for a control valve is 44%, output(s) from the control system will move the control valve to the desired postion (44% in this example) and to maintain the control valve in that position. There is some kind of position feedback from the control valve (usually an LVDT (Linear Variable Differential Transformer)) to the control system to know where the control valve is operating (actual position).

When the turbine-generator is producing electrical power while operating in position control the operator is responsible for changing the position reference to change the power being produced by the unit (i.e., the power output is not automatically controlled when in position control.)

Position control is often used during starting and shutdown, when power is not being generated. It can also be used when just controlling the flow-rate through the turbine--and power output will just be proportional to the flow-rate. (Many turbines equate flow-rate through the control valve(s) (or wickets, or gates) to position, so 44% position might be equal to 50% flow-rate, for example.)

When a generator-drive control system (governor) is operating in load control, output(s) from the control system will move the control valve to whatever position is required to maintain a load setpoint. There will be an input signal to the control system which indicates the load the generator is currently producing (usually from a watt- or mega-watt transducer) and output(s) from the control system will move the control valve to whatever position is required to make the load input (feedback) signal equal to the load reference (usually set by an operator from the operator interface (HMI)). When load control is active (only when producing electrical power), the control system automatically adjusts the control valve position to make the actual load equal to the operator-adjustable load control setpoint.

There are also remote setpoints for load control--so some remote location (a regional control center, for example) can send a load control setpoint signal to the control system (governor) to control the load. Also, some sites can have on-site control systems (such as DCS (Distributed Control System), or PMS (Power Management System, etc.) which can be used to adjust the load control setpoint.

Finally, ISOCH mode refers to a speed control mode (also known as Isochronous speed control) that very tightly controls the speed of the prime mover so that the frequency of the generator remains as constant and stable as possible. This mode of operation is typically only used when the generator is driving electrical loads independent of other prime movers and generators, or when only a small number of generators and their prime movers are synchronized together to power an isolated load (sometimes referred to as "island load control").

When multiple generators are synchronized together supplying an isolated load it's typical for only one governor to be operated in Isoch mode--because if there are multiple governors operating in Isoch mode they will (violently) fight each other to control speed--which usually results in a black-out. There are "de-tuned" Isoch load sharing modes of operation which allow multiple generator-sets (prime movers and their generators) to be connected together while in Isoch mode, and there are some Isoch load-sharing and Isoch stand-by schemes in which multiple governors are connected together to provide frequency control of the electrical system.

Most prime movers and their generators, when synchronized to an electrical distribution system (grid) with other prime movers and generators operate in Droop speed control mode (even if they are operating in some form of load control--which is just another form of Droop speed control). Droop speed control allows multiple prime movers and generators to all produce power in a stable fashion (without fighting each other as multiple units operating in Isoch speed control mode would!). They all act as one generator together, and Droop speed control allows for smooth and stable operation--as long as the grid regulators properly control the amount of generation versus the load on the system (the load is the sum of all the motors and lights and televisions and computers and computer monitors).