Matthew,

I will wait for a power station engineer to answer your query about leading under excited and lagging power factors which regulate the stator  winding DC voltage before trying to explain how the 3D magnoflux actually moves inside the conductors.

https://en.wikipedia.org/wiki/Voltage_control_and_reactive_power_management#:~:text=In%20a%20typical%20electrical%20grid,terminals%20within%20the%20target%20range.

Wiki has some info see link.

Hi Matthew. I was taught in simple terms that  reactive power is like a balancing act for AC circuits. It helps to keep the voltage, the magnetic force, and the electric force just right. But if there is too much or too little of it, things can go wrong. Too much reactive power can make the voltage go up and damage the equipment. Too little reactive power can make the voltage go down and cause a outage. Also the beer analogy helped me understand power factor better. You’ve probably seen it, the beer represents the useful power that does work, the foam represents the wasted power that causes losses, and the mug represents the total power that is supplied. The power factor is the ratio of the beer to the mug, and the higher it is, the more efficient the system is.

Hi Matthew, as AMK says, reactive power helps keep the voltage to the nominal levels for the system (400kV, 275kV, 132kV…) if there is too much or too little the voltage can stray above or below the target voltages. In the UK, the grid stability varies depending upon where about in the country you are. I’m in the north of Scotland where the grid stabiltiy is weak due to a number of factors, but mainly due to being far away from large generators that as well as producing electricity, help control the voltage. 
Further down into England where there are large pools of industry, which would be starting up at the begining og f the working day and shutting down at the end can also have an effect on the voltage level and the reactive power. Lots of motors all starting around a similar time, lots of people waking up and starting their day all has an effect on the demand and how the voltage stabiltiy is maintained. 

There are also other factors that can affect the voltage such as demand and supply but these are less influenced by reactive power availability. 

Hi Ollylee21, interesting discussion. I am curious to know more about how you and your colleagues manage the voltage levels and the reactive power in the north of Scotland, and what kind of solutions or innovations you are working on or planning to implement. 

At grid level, the impact of reactive power is crucial for maintaining the system stability and reliability. The simplest way this can be understood is via understanding Q-V and P-F control concept. where...

Q - Reactive power

V - Voltage

P - Active power

F - Frequency.

Q-V Control: This is also known as reactive power-voltage control. The automatic voltage regulator circuit is used for voltage control. This control loop regulates the generator voltage and output power. The terminal voltage and reactive power are also met. Devices used for system voltage control include Static VAR compensator, Synchronous condenser, Tap changing transformer, Switches, Capacitor, and Reactor¹

P-F Control: This is also known as active power-frequency control. The power system is basically dependent upon the synchronous generator and its satisfactory performance. The important control loops in the system are frequency control, and automatic voltage control. Frequency control is achieved through generator control mechanism¹

These control structures are essential for power system operations and control, which describes actions taken in response to unplanned disturbances (e.g., changes in demand or equipment failures) in order to provide reliable electric supply of acceptable quality².

If the Grid has more inductive load, the voltage starts to drop and the generators need to inject capacitive reactive power in order to balance it. The value of reactive power required would depend on the Q-V control equation defined for the environment and the same is applicable in the vice-versa case

On the other hand if the load on the grid (active power) demand is increasing, the grid starts to loose frequency and the generators need to add more active power to balance it. The value comes through the P-F control equation defined for the environment and the same is applicable for vice-versa case.

Hope this helps

It is worth noting that the reactive power measurement (i.e. current and voltage phase offset) as an indication of grid health and if things should be speeded up  up or slowed down, only works properly with generation from  rotating machines that vary between generating and 'motoring' when they are trying to run fast or slow  relative to the frequency of the rest of the grid.

With increasing amounts of inverter generation from solar panes and windfarms and so on that has no physical  inertia, this effect has to be emulated quite carefully to work with the current control mechanisms. In an inverter it is more arbitrary and by altering the switching instants to a different part of the cycle  the effect of a quite un-physical capacitor or inductor can be generated more or less at the press of a button,  Also inverters do not drop in frequeny when overloaded, so the trigger for automatic load balancing/load shedding decisions needs to be adjusted,

Mike

Hi AMK, in the north we use various plant to try and regulate the voltage, mainly STATCOM/SVC, Reactors and Quad boosters. 

Hi Matthew, I will help explain the practical application of reactive power in managing grid and voltage stability.

Fundamentally, as I think it's been explained in the earlier replies, reactive power is key to managing the voltages on the power grid. Too much reactive power and voltages increase, and too little and voltages drop. Therefore, reactive power on a power grid needs to be actively monitored and controlled to ensure voltages remain stable and within limits. In GB, the voltages are managed on the transmission network to within the Security & Quality of Supply Standards (SQSS) limits.

To ensure reactive power (as denoted by "VAr") can be controlled, and therefore voltages can be managed, requires a mix of equipment and technologies, some of the key ones include:

- Shunt Capacitors (these inject reactive power).
- Shunt Reactors (these absorb reactive power).
- SVC or STATCOM or Synchronous Condensers (these absorb and inject reactive power, responding dynamically to the need of each to help maintain steady voltages).
- Transformers (tap changers enable the control of voltages at the LV/MV terminal).
- Generators (while generators tend to be thought of as just MW producers, they play an important role in helping to manage voltages locally to their connection point - at transmission level they are required to be able to achieve at least 0.95 power factor (lead / lag)).

Furthermore, voltage can also be controlled to a certain extent by controlling the power flow on the grid. This is also crucial to ensuring voltage stability following an outage / contingency. You can observe how voltage on the grid changes under varying power flow scenarios, due to the relationship between voltage drop and current level, length & type of transmission line. Examples of equipment to help direct/redirect power flow include Quadboosters, Series Reactor/Capacitor, and Static Synchronous Series Compensators.