HST has developed a simplified power flow model which can compute both real and reactive power flows throughout the plant. The user may choose between two power flow models when simulating the plant performance: Real Power Control Only and Real and Reactive Power Control. Both control modes are described in the section Plant Control Models.
Both power flow models take advantage of the fact that the HST software automates the sizing, positioning (and/or routing) of equipment on the project site. HST also utilizes an equipment database to retrieve equipment properties that are used within the power flow calculations. Thus, AC impedances can be estimated automatically between the inverter terminals and point of delivery.
HST utilizes the NREL System Advisor Model (SAM) to compute POA irradiance, PV module conversion and associated losses, DC system power flow and associated losses and inverter DC to AC conversion and associated losses. All power flow and associated losses between the inverter AC terminals and point of delivery (POD) are computed directly by the HST platform (outside of SAM).
Simplifying Assumptions
The HST power flow model does take advantage of some simplifying assumptions to speed and simplify the power flow calculations.
Unlike more complex electrical engineering software load-flow packages such as GE PSLF, the HST model makes the simplifying assumption that all node voltages throughout the plant are at nominal voltage. This allows the estimation of real and reactive power flow in each circuit (using the equations given here) without having to converge on an operating voltage at each node within the plant. This simplification produces fairly accurate results from a losses and power flow estimation standpoint (assuming voltages are close to nominal). However, the voltage drop (and rise) within the plant under varying generating and reactive power compensation levels is not considered as a constraint within the model. Similarly, the impact of a transformer tap changer is not considered either. Since it is typical for voltage drop (and rise) within the plant to be limited to approximately +/- 10% of nominal voltage, the actual reactive power set point limits within the inverter for a given project may need to be more tightly constrained than what is listed on the datasheet.
Additionally, the HST platform automates the site layout (including equipment positioning, cable and transformer sizing and cable routing and installation) which requires making design decisions that directly impact losses and power flows. The choice of alternate equipment placement, equipment specs, installation methods or circuit routing can alter the power flow results.
For these reasons, some caution should be taken when using the results of the HST power flow model. While it is sufficiently accurate for early stage feasibility designs (and even exceeds the capabilities of other PV modeling software) the results may not align perfectly with more complex electrical engineering load-flow models using detailed design conditions.
Definitions
Apparent Power (S) – This is calculated as S = SQRT(P^2 + Q^2) and is measure in kVA (KiloVolt Amperes).
Real Power (P) – This is the real power flow at the Point of Delivery. Real Power is measured in kW (KiloWatts). Negative values represent kW absorbed by the plant (i.e. inverter nighttime loads, transformer no-load losses and station power losses).
Reactive Power (Q) – HST computes the reactive power for each circuit and transformer (inductive and capacitive) in the plant and rolls them up into a single value at the Point of Delivery. Reactive Power is measured in kVAR (kilovolt Amperes Reactive). Negative values represent kVAR absorbed by the plant.
Power Factor – This is calculated as PF = P/S
Here’s a few considerations that affect reactive power flow:
- Cables, when an AC voltage is applied to them, generate reactive power and are therefore exhibit capacitive properties. The magnitude will depend on the voltage magnitude, cable properties and cable length. HST assumes 1.0 per unit voltage for each circuit when performing capacitance calculations. This is a simplistic approximation since the voltage may vary by as much as +/- 10% within the plant, however it is a reasonable approximation for early stage design.
- Cables and transformers, when an AC current flows through them, absorb reactive power and are therefore exhibit inductive properties. The magnitude will depend on the current magnitude, cable/transformer properties and cable length. Since the current varies with generation level, the amount of reactive power the cables and transformers absorb will vary with generation level.
- At 0% generation (i.e. nighttime), it is typical for real power to flow into the plant and reactive power to flow out of the plant. As generation levels increase, this reverses and real power begins leaving the plant while reactive power starts to become absorbed.