I am getting familiar with the power system and I need some help to clarify the following questions regarding voltage control in the power grid:
i) How is the voltage control made in transmission and distribution? What variables are measured?
ii) Is there any kind of coordination from a central entity to decide the control actions along the grid?
iii) Which model is used to take control actions? Does it have into account any dynamic stability criteria, e.g., lyapunov based or similar?
iv) Does the control have into account transients produced by external sources, is it robust?
In power grid, efficiency and stability point view, 0.8 lag is the best power factor Active power is supplied by alternator, depending on torque angle (delta), is depends on the mechanical energy supplied(as load demand increase dela will increase).Power factor is controlled by reactive power or excitation of alternator. Hence reactive power control is the main source to control the voltage(power factor) of grid. Power factor of grid depends on other factor like load type, transmission system configuration etc, exact reactive power demand for voltage control is obtained by load flow analysis of system, and can be controlled to obtain required voltage(power factor).
Modern day, power system efficiency can be improved, by proper load management, from load dispatch center,by computer control, of power grid,used online,"computer is intelligent, as intelligent the master."
Total generation, can computed, using equal incremental cost and penalty factor consideration for system would be more simple and accurate, hence available generation would be possible more economically, and efficiency can increased tremendously.
In transmission networks, the voltage is controlled by the excitation system of the generators based on the load conditions. In case of low loading the reactive power generated by the lines will be more than that which is absorbed so some generators might also have to absorb reactive power in order to keep your voltage in check. Alternatively, shunt reactors are used to control the voltage. So, depending on the load excitation systems need to be controlled to regulate the voltage. An improvement on shunt reactors are FACTS which can provide either capacitive or inductive compensation to allow a wider range of voltage control.
AVRs are used to control the voltage at the terminals of the generators. I recommend the text book by Prabha Kundur to understand the basic modelling of AVRs. The control blocks are mentioned in the book.
In case you are planning to use FACTS devices the control strategy will change and depending on the device you are designing the control variables will also change.
On the distribution side voltage can be controlled by using tap changing transformers and voltage regulators. As the contribution from the shunt capacitance is almost negligible, the connected load will determine the drop towards the tail end (considering a radial topology).
For modern computerized systems,in practice, the implemented fastest and easiest, control is" load frequency control", for voltage control, normally done from load dispatch centers, of load areas, by monitoring the frequency continuously, in industrial(inductive/ mechanical) loading, little change in frequency changes(rotating field/speed) the demand (reduces considerably by little fall of frequency), if frequency reduce below limit, the explained steps are implemented, as required.
there are many devices and tools to control voltage. Voltage magnitudes and angles at every node in the system are measured on continuous basis. Of course, central authority and dispersed control centers coordinate on controlling voltages throughout the system. Stability criteria is certainly considered to avoid voltage collapse and other problems. Fairly robust.... but who really knows what external events would really happen....