.Spot.UsersGuide.Introduction.Units

Information

Units SI and pu

Electric parameters can be defined either in SI or in pu ('per unit') units. Meters allow this choice for output-signals.
Note that 'pu' only affects input-output and precalculation whereas the model equations always refer to SI.

The relation between values in SI- and pu-units is given by:

  value_pu = value_SI/base_SI

Base values are defined by nominal values in case of pu-units.
As independent base-values (for electrical quantities) we use voltage and apparent power

  V_base, S_base

from which other values are deduced according to the relation

  I_base*V_base = S_base
  R_base*I_base = V_base

The relation between inductance L and impedance X or capacitance C and susceptance B involving a frequency according to

  omega*L = X
  omega*C = B

is always based on the nominal angular frequency

  omega = omega_nom

Norms, amplitudes and effective values, an example (voltage 400 | 230 V)

abc-representation:

  v_abc  = {v_a, v_b, v_c}    voltage vector, phase to neutral
 |v_abc| = sqrt(v_abc*v_abc)  voltage three-phase norm

dqo-representation:

  v_dqo  = {v_d, v_q, v_o}    voltage vector, phase to neutral
 |v_dqo| = sqrt(v_dqo*v_dqo)  voltage three-phase norm

relation abc to dqo:

  v_dqo  = P*v_abc            P: orthogonal transform abc to dqo
  vpp_dq  = P*(v_b - v_c), .. definition phase to phase voltage dq
 |v_abc| = |v_dqo|            as P orthogonal

up