Unit System and Conventions

Signing Convention

Loads

Active and reactive power, $P$ and $Q$, are always positive for loads.

Generators

Active and reactive power, $P$ and $Q$, are always positive for generators.

Power Factor

The power factor is defined as

$$pf=cos(\varphi )=\frac{P}{|S|}$$

where $cos(\varphi )$ is the power factor, $\varphi$ is the phase angle between current and voltage, $P$ is the real power and $|S|$ is the absolute value of the apparent power. It holds that $0\le pf\le 1$ since $P\ge 0$.

The power factor $\cos (\varphi )$ and the reactive power $Q$ can be defined as

• Inductive
• Capacitive

Inductive reactive power consumes reactive power and decreases the voltage. Examples: motors, air conditioners, water pumps

Capacitive reactive power produces reactive power and increases the voltage. Examples: capacitor banks, underground cables

Power Flow

$P$ over a branch is always positive. The direction is defined by arrows:

• -->: $P$ flows from Bus 1 to Bus 2
• <--: $P$ flows from Bus 2 to Bus 1

Frequency

The standard frequency is 50Hz. It can be changed in the Grid Editor.

Power in three-phase systems (balanced)

The power in three-phase systems is the sum of the power per phase. In balanced three-phase systems the three-phase power is 3x the single phase power:

$$S_{3\text{ph}}=3\cdot V_{\text{R}}\cdot I_{\text{R}}$$

where $S_{3\text{ph}}$ is the three-phase apparent power, $V_{\text{R}}$ is the phase to ground voltage and $I_{\text{R}}$ is the phase current. In the above equation the phase to ground voltage can be replaced by the phase to phase voltage with

$$V=\sqrt{3}\cdot V_{\text{R}}$$

where $V$ is the phase to phase voltage. Substituting the second equation into the first we get

$$S_{3\text{ph}}=\sqrt{3}\cdot V\cdot I_R$$

The following conventions apply:

• Voltages entered and displayed are phase to phase, i.e. $V$.
• Powers entered and displayed are aggregated three-phase power.
• Currents entered and displayed are phase currents.

Power in 3-phase systems (unbalanced)

In unbalanced three-phase systems the power per phase is given as:

$$S_{\text{ph}}=\frac{V\cdot I_{\text{R}}}{\sqrt{3}}$$

The following conventions apply:

• Voltages entered and displayed are phase to phase, i.e. $V$.
• Powers entered and displayed are defined per phase.
• Currents entered and displayed are phase currents.

Per Unit System

Bus voltages are given as absolute or per unit values. The per unit values are relative to the phase to phase base voltage $V_{\text{nom}}$ defined for each Bus.

The default base apparent power is $S_{\text{nom}}=100\text{MVA}$. It can be changed in the Grid Editor.