The static transfer limit for 1 p.u. active power in transmission (X>>R) is around SCR = 0.9 not SCR = 1. This is determined by Saddle Node Bifurcation, insolvability of power flow operating point. The dynamics that ultimately prevent the exploitation of static limit is the voltage sensitivity in respect of reactive power, but VSC active damping can overcome that by manipulating EMT control.
When you claim that SCR is less than 1, you should properly formulate your definition of SCR. If the VSC power capacity instead of only active power is used to calculate SCR, then reactive power control will have very limited impact on the minimum SCR.
@@thomasbright5126 Calculating SCR and the determination of maximum power transfer (in other words the minimum SCR) are different things. When calculating the SCR, it is the nominal active power on the denominator, (if apparent power were used then mathematically there would be no SCR below 1 to deliver 1 p.u. active power anymore). Obviously, in most of the cases and literature, nominal active power are used in the dominator. I suppose this is because originally VSC was not designed to supply reactive power for economical concern and a presumed strong grid to connect. However, the determination of minimum SCR is about the transfer of active power across network impedance. It is the net power that should be counted, not just generation. And this mathematical constraint can be extended to bidirectional power. As I cite previously, it is a problem of solvability of power flow within bounded voltage limits. A simple numerical example. When SCR =1, the transferred reactive power cannot stay at zero to send 1 p.u. active power, given both sending and receiving ends voltages are close to 1 p.u.. With an assumed infinite bus on the receiving end, the voltage of sending end will collapse when the active power is 60%-70%. This has nothing to do with PLL, GFL, or Grid forming. It is simply about the existence of a solution to equilibrium. Another dimension is that even though an equilibrium is secured with appropriate reactive power, if the system has adequate damping or not. This has something to do with PLL, but far from a complete story. It is more about the interaction between the overall control strategy of the system and the dynamics of voltage sensitivity in respective of reactive power. This dynamics always exist, regardless of the operating reactive power. It is time-variant and non-linear.
@@chendong2917 Maybe I am wrong, but the definition of SCR is about the MVA capacity of a bus over the rated active power of a power generation device. let say the bus capacity is 1.0p.u. When you already export 1.0p.u active power and then you added more reactive power, you have some apparent power large than the MVA capacity of the bus. In this case, can we call this VSC is operating in the grid strength of SCR being 1?
@@thomasbright5126 Yes. That is my point. In most cases, the denominator of SCR is defined as the rated active power, which is a constant before you start the analysis. Once the denominator is chosen, it does not change with the operating condition. - The question becomes, for which SCR, an active power of 1 p.u. can be transferred regardless of operating reactive power, which is easy to evaluate. The other option is to define the denominator of SCR as the rated apparent power. Then, for any SCR condition requiring a minimum reactive power, the active power cannot reached rated apparent power. And this threshold to disable 1 p.u. active power is way above SCR = 1. In either case above, the singular point is not SCR = 1, I am afraid. The electrical constraint is not the capacity of the bus, but the power carried by the network impedance. Before we proceed to EMT damping, this is a classical voltage stability issue.
@@chendong2917 The problem is not the denominator, but it is about the numerator. When you have 1.0.pu active power plus some reactive power, it means the MVA capacity of bus is larger than 1.0p.u, then you wlll have SCR>1. If we can ignore the reactive power in the calculation, then the larger capacity of a converter is always helpful for stability because of more freely controlled reactive power.
Thank you so much, please if you can improve the sound quality. ❤
The static transfer limit for 1 p.u. active power in transmission (X>>R) is around SCR = 0.9 not SCR = 1. This is determined by Saddle Node Bifurcation, insolvability of power flow operating point.
The dynamics that ultimately prevent the exploitation of static limit is the voltage sensitivity in respect of reactive power, but VSC active damping can overcome that by manipulating EMT control.
When you claim that SCR is less than 1, you should properly formulate your definition of SCR. If the VSC power capacity instead of only active power is used to calculate SCR, then reactive power control will have very limited impact on the minimum SCR.
@@thomasbright5126 Calculating SCR and the determination of maximum power transfer (in other words the minimum SCR) are different things.
When calculating the SCR, it is the nominal active power on the denominator, (if apparent power were used then mathematically there would be no SCR below 1 to deliver 1 p.u. active power anymore). Obviously, in most of the cases and literature, nominal active power are used in the dominator. I suppose this is because originally VSC was not designed to supply reactive power for economical concern and a presumed strong grid to connect.
However, the determination of minimum SCR is about the transfer of active power across network impedance. It is the net power that should be counted, not just generation. And this mathematical constraint can be extended to bidirectional power. As I cite previously, it is a problem of solvability of power flow within bounded voltage limits.
A simple numerical example. When SCR =1, the transferred reactive power cannot stay at zero to send 1 p.u. active power, given both sending and receiving ends voltages are close to 1 p.u.. With an assumed infinite bus on the receiving end, the voltage of sending end will collapse when the active power is 60%-70%. This has nothing to do with PLL, GFL, or Grid forming. It is simply about the existence of a solution to equilibrium.
Another dimension is that even though an equilibrium is secured with appropriate reactive power, if the system has adequate damping or not. This has something to do with PLL, but far from a complete story. It is more about the interaction between the overall control strategy of the system and the dynamics of voltage sensitivity in respective of reactive power. This dynamics always exist, regardless of the operating reactive power. It is time-variant and non-linear.
@@chendong2917 Maybe I am wrong, but the definition of SCR is about the MVA capacity of a bus over the rated active power of a power generation device. let say the bus capacity is 1.0p.u. When you already export 1.0p.u active power and then you added more reactive power, you have some apparent power large than the MVA capacity of the bus. In this case, can we call this VSC is operating in the grid strength of SCR being 1?
@@thomasbright5126 Yes. That is my point.
In most cases, the denominator of SCR is defined as the rated active power, which is a constant before you start the analysis. Once the denominator is chosen, it does not change with the operating condition.
- The question becomes, for which SCR, an active power of 1 p.u. can be transferred regardless of operating reactive power, which is easy to evaluate.
The other option is to define the denominator of SCR as the rated apparent power. Then, for any SCR condition requiring a minimum reactive power, the active power cannot reached rated apparent power. And this threshold to disable 1 p.u. active power is way above SCR = 1.
In either case above, the singular point is not SCR = 1, I am afraid.
The electrical constraint is not the capacity of the bus, but the power carried by the network impedance. Before we proceed to EMT damping, this is a classical voltage stability issue.
@@chendong2917 The problem is not the denominator, but it is about the numerator. When you have 1.0.pu active power plus some reactive power, it means the MVA capacity of bus is larger than 1.0p.u, then you wlll have SCR>1. If we can ignore the reactive power in the calculation, then the larger capacity of a converter is always helpful for stability because of more freely controlled reactive power.