Note 1 to entry: The power available is depicted by a region on the P/Q power plane. The boundaries of the region represent the operating limits of the EES system. In Figure 4, the power capability chart is divided in four quadrants by the P/Q axes (the producer reference frame as described in IEC TR 61850-90-7:2013 is adopted):

1. in the quadrant Q1, the EES system is discharging to the electric power system and its behaviour is like that of a capacitor;
2. in the quadrant Q2, the EES system is charging from the electric power system and its behaviour is like that of a capacitor;
3. in the quadrant Q3, the EES system is charging from the electric power system and its behaviour is like that of an inductor;
4. in the quadrant Q4, the EES system is discharging to the electric power system and its behaviour is like that of an inductor.

X

Key

PCR	rated active power during charge
PDR	rated active power during discharge
QIR	rated inductive reactive power
QCR	rated capacitive reactive power

Figure 4 – Illustrative example of a power capability chart

Note 2 to entry: The watt (W) and the var are the SI coherent derived units for active power and reactive power, respectively; other units can be chosen for convenience as well (e.g. MW, Mvar).

Note 3 to entry: If restrictions are not declared, the power capability chart is normally valid for the entire service life of the EES system.