135 amps = ( 112500 / 3) / 277
Where: acos is arccosine or inverse cosine
W is Watt
PF is power factor
VA is
k is kilo ( = 1000 )
Therefore, each generator set in this case should carry 113 amps per phase or half the 226 calculated line amps. The 113 amps includes 67.5 amps of reactive current, half of the calculated reactive current of 135 amps. The reactive droop should be adjusted until each generator set carries equal reactive current. The load sharing control should be adjusted so that real current and/or watts are shared equally as well.
If one generator set is larger than the other, it should be adjusted to carry proportionate current. For this example, if a 150 kW generator set is paralleled to a 75 kW generator set, the larger generator set would carry 90 amps reactive (135 * 2 / 3) and the other would carry 45 amps reactive (135 * 1 / 3). Adjust the reactive droop based on the ratio of the actual measured currents, not the calculated values.
VAR Control
VAR control is analogous to the reactive droop function described above. It differs in that it applies to utility paralleling applications. Because the utility represents a nearly infinite bus, the voltage at the load terminals is not controlled at all by the generator set, and it is impossible to compare the ratio of the generator set current to the utility based on its rated output. In this situation, the excitation control changes from voltage feedback to VAR feedback. More specifically, the excitation is controlled to maintain a certain VAR output rather than a voltage output. This is called VAR control and again is used only in utility paralleling applications.
The VAR adjust can be set to any value within the generator set’s rated capability. Because the VARs cause heating in the armature, any value beyond the generator set’s rating could damage the alternator. In most cases, the generator set will be adjusted to generate VAR (lagging PF) but could absorb VARs (leading PF) as well. However, the VAR setting is maintained regardless of the relative PF. If the particular load requires more VARs than the generator set setting, the excess is derived from the utility bus.
The term rated VARs is a bit obscure. In essence, it is a value derived from the rated kW of the generator set. For a typical standby rating, the full load of the generator set is defined to have 0.8 PF. This means that the kW load is
for a linear load may be calculated as the cosine of the angle between voltage and current. This relationship is based on the power triangle. Using this power triangle concept, it can be shown that the reactive power for a linear load is equal to the sine of the power angle. Then, using these trigonomic functions, it can be shown that for a PF of 0.8, the VARs are related similarly to the VA by a factor of 0.6. More explicitly, the power angle is equal to the inverse cosine (arccosine) of the PF. For a PF of 0.8, the power angle is 36.9 degrees (0.2 radians). The sine of this angle, sine (36.9 degrees) is 0.6. This is the factor for calculating rated VARs from the rated VA. The ratio of these two factors is 0.75 (0.6 / 0.8), which can be used to calculate rated VARs directly from the rated kW; rated VARs equals rated watts * 0.75.
When a generator set will be connected in parallel with the utility, VAR or PF control should be ENABLED. If multiple generator sets are in parallel as well, then reactive droop should be ENABLED also. Additionally, note that VAR control should be used only when the generator set is connected in parallel with the utility. Parallel connection with the utility requires the logical indication that the circuit breakers tying the generator set bus to the utility bus are closed. This indication is made by use of the programmable digital input for VAR/PF mode. If this input function is activated, the excitation control changes to the selected VAR or PF control. If the logical indicator is not present and the VAR or PF control is not enabled, the control will not switch to VAR or PF control. Because the active state for the digital input is a HI or open connection, the default for the digital input (VAR/PF Mode) is DISABLED (displays ENABLED NO). If the input is ENABLED by the user, it should be held low by a contact or jumper until the actual closing of the connecting circuit breaker(s). The proper control method, VAR or PF, must be ENABLED within the regulator’s configuration menu.
Power Factor Control
PF control is much like the VAR control above. PF control is used only when the generator set is paralleled to the utility grid. The difference is that the PF of the generator set current is held constant. The setting for the PF adjust determines the relationship of the current and voltage from the generator set. The PF is a term that defines the ratio of real watts to the
Appendix 205 |
