Adding Voltage Compensation to a Generator Model
Generator Voltage Compensation
The input signal to Exciter models is a voltage signal (often labeled as either Vc or Ec on a block diagram). In the simplest form this is just the per unit voltage magnitude of the terminal bus of the generator. However, sometimes instead of using that voltage signal directly, the voltage is compensated by attempting to control a voltage looking either outward into the system or backward from the terminal bus. This can have benefits to system stability.
- Line Drop Compensation (LDC): This means the control is looking outward into the system in an attempt to control a voltage closer to the high voltage system. A generator may control a voltage looking out an impedance equal to between 50% and 80% of the step up transformer impedance for instance. Within a software tool, this gets expressed as a positive Rcomp+jXcomp. (Xcomp >0)
- Reactive Current Compensation (RCC): This means looking backward from the terminal bus. This type of control is fundamentally necessary when multiple generators are connected to the same electrical point. Without RCC the exciter controls will fight with each other because they are trying to control the same voltage (You would see exciters go to minimum and maximum limits in the software tool). Mathematically this is very similar to the concept of “droop” in governor MW control and thus this is sometimes referred to as “voltage droop control”. Within a software tool, this gets expressed as a negative Rcomp+jXcomp. (Xcomp <0)
The concepts of Line Drop Compensation and Reactive Current Compensation end up being the same inside software with the only difference being the sign of the Rcomp and Xcomp values. Within software data structures, this compensation sits right between the machine model and exciter model. This leads to some confusion as it would be possible to either add this feature inside the machine model, inside the exciter model, or even make a separate model. Historically all three choices have been used by different models and PowerWorld Simulator supports all three methods as follows.
- Voltage Compensator Model: Add a separate model such as COMP or IEEEVC. Examples include
- Machine Model: Include the Rcomp/Xcomp value as part of the input data for the machine model. This is done in PowerWorld Simulator on models such as GENROU, GENSAL, GENPTF, GENTPJ, and so on.
- Exciter Model: Include the Rcomp/Xcomp as part of the input data for the exciter model. Examples include
Looking at historic models, you will notice that PSS/E based models will use choice 1 (separate model), PSLF based models will use choice 2 (include in machine model), and BPA IPF program “Type F” exciters use choice 3 (include in exciter model). PowerWorld Simulator supports all these choices and handles them as follows. Within PowerWorld Simulator the voltage input signal is determined by going through the following hierarchy.
- If a Voltage Compensator Model exists then use the output of this model
- Else if the Machine Model has Rcomp/Xcomp use that complex impeance in combination with the terminal voltage and generator terminal current.
- Else if the Exciter Model has Rcomp/Xcomp use that complex impeance in combination with the terminal voltage and generator terminal current.
- Otherwise just get the terminal voltage instead
Coordinating Voltage Compensation Across Multiple Units
Negative values of Rcomp and Xcomp are used when multiple units exist at the same electrical point. This is done to make the system stable as each generator regulates a voltage “behind” their terminal and these represent independent control points. If however the Rcomp/Xcomp is a positive value (Line Drop Compensation), then having multiple units at the same elecrical point would create an unstable control system in real life which will manifest itself as numerical instability in software. This is because each generator will be regulating the same voltage point and the exciter controls will fight with each other because they are trying to control the same voltage. This would be similar to having governors configured with positive droop which would also lead to instability.
Because of the way the software models are configured however, it is very easy for a user to specify positive Rcomp and Xcomp which creates an unstable system. Within PowerWorld Simulator, if multiple generators are at the same electrical point and have Xcomp > 0 then the compensated voltage is always automatically calculated using the SUM of the generator currents at the bus instead of only the individual generator. This prevents generators fighting with one another that inevitably leads to one generator going to its maximum excitation while the other goes to the minimum.
LDC and RCC at the same time
It keeps getting more complicated. You may also have multiple units at the same electrical point that perform reactive current compensation AND line drop compensation simultaneously. To model this, there are separate Voltage Compensation models that allow you to coordinate between 2 units that are on RCC and LDC at the same time. These models are
Both of these models use complex arithmetic using complex impedances, complex voltages and complex current to calculate the compensated voltage.
Work done in the WECC Model Validation Working Group in 2015 however led to the development of a new model. The purpose of this new model was to not use complex currents as feedback, but instead use the reactive current only and also coordinate up to 4 generators together. This led to the creation of the CCOMP4 model as follows.
Investigation done by Pouyan Pourbeik (then at EPRI) found that at least three major excitation system vendors confirmed that they only use the reactive current as feedback; that is the imaginary component of the complex terminal current. The following is a WECC guideline from June 2015 which discusses this: Cross-Current Compensation Model Specification (CCOMP4)
October 19, 2016