The most common configurations of AC outputs are:
|Three Phase Data
|Three Phase Wye
||3-Wire or 4-Wire|
Symmetrical or non-symmetrical ground connections
may be specified in all of the above configurations.
Parallel Operation and Circulating Currents (Phasor Relationships) - Some
ac-output power supplies are designed for parallel operation. To avoid
degrading the reliability of the system, parallel operation should only be used
under two conditions. One occurs when the system loads exceed the power
capability of a single power supply. The other occurs when critical loads
cannot tolerate the power loss that would occur during the time required to
transfer loads between sources in the event of a source failure. When AC
power supplies are operated in parallel, there are several critical operational
issues to be addressed:
- The frequency of operation of the sources must be
- The output phase of each source must self-adjust to
ensure equal sharing of the real component of load power.
- The output
voltage of each source must self-adjust to minimize reactive currents that would
otherwise circulate among the parallel outputs.
- Master/slave configurations, in which one supply
assumes the role of master, should be avoided.
Total Harmonic Distortion (THD) - THD is used to describe the purity of the
output voltage. It is defined as the ratio of all parasitic harmonics
present on the output to the fundamental harmonic. Some requirements may
limit the individual harmonics.
DC Offset - Some AC power supply outputs contain a DC voltage
component. Excessive DC components may be harmful to the equipment powered
by the AC output power supplies. Maintaining the DC content of the AC
output under 0.05% of the output RMS voltage is recommended.
Output Ripple and Noise - Similar to DC power supplies, the output of the AC
output power supply contains high-frequency ripple and noise induced by internal
conversion. Several publications provide guidance for specifying limits
for these parameters. For ripple content and for noise, the appropriate
sections of MIL-STD-704, MIL-STD-461 and MIL-STD-1399 Section 300 should be
Paralleling and Redundancy - Some AC output power supplies are designed for
parallel operation. Similar to their DC counterparts, current sharing is
achieved through droop share or via forced regulation techniques. However,
in contrast to DC, the outputs cannot be steered through a diode for "seamless"
fault isolation. Relay cut-off or semiconductor switches are employed to
disconnect the faulty module. Other considerations include:
- The activation of the disconnecting devices is titne-finite, therefore the
fault disconnect delay will be seen on the output. The resulting delay may
be tolerable in some applications. Since the AC output crosses zero two
times per period, most equipment operating from an AC source will not be
affected by this momentary interruption.
- The specification for a
redundant AC power system should state the maximum allowable interruption due to
a single module fault. IEEE STD 446 provides a good reference for a
typical tolerance envelope acceptable by end user equipment.
- To ensure complete redundancy, the redundant AC
output power supply must be examined for single-point failure modes.
AC output power supplies operating in masterslave configuration are
vulnerable to the failure of the master module. Systems regulated by a
main controller are susceptible to shutdown in the event of the controller
Output Protection - The output of the AC power source must be protected from
the effects of shorted loads. Relying upon load-side breakers to isolate a
faulted load requires that the AC source be capable of supplying as much as 300%
of its rated current to ensure tripping the breakers. This can greatly
increase the cost and complexity of the source's output stages.
Furthermore, where a single source feeds multiple loads, or where multiple
sources are connected in parallel, this approach can lead to hazards to
equipment and personnel resulting from currents in excess of cable and load
device ratings. For example, three AC sources connected in parallel can
provide up to 900% of their rated current to a single faulted load, which could
be well in excess of the capacity of the wiring to that load. The
preferred approach is to use a load-side circuit breaker equipped with an
under-voltage or remote trip coil and allow the AC source to trip the breaker
open should the load current exceed the source rating.
Regenerative Load and Backfeeding - Some AC loads have the capacity to pump
energy back into the power supply. These loads are primarily motor drives
or the output of another AC source connected in parallel. This scenario is
often present in parallel systems or during a slow bypass transfer. Since
it is not easy to either return the backfeed energy to the input power source or
to dissipate it internally, most AC output power supplies provide shutdown
protection against this phenomenon. If backfeeding occurrences are
expected, the ability of the AC power supply to operate reliably under these
conditions must be specified.
External Filter Resonance - The harmonic content on the AC output may create
undesired effects when an external filter is used. Typically, commercial
filters are designed to perform at the rated fundamental frequency. The
filter impedance is rarely specified for harmonic behavior. An external
filter may contain circuitry that resonates at frequencies coincident with any
harmonic on the AC output thus amplifying the distortion. High resonant
harmonic content may cause excessive stress, audible noise, and potentially
unsafe voltage levels on filter output. If the system design requires
an external filter, the specification should include the
Fundamental Frequency Input Current Ripple Reflection - AC output power
supplies deliver time-varying periodic output power. The periodic power
variation is reflected on the input for supplies using DC input power. For
example, a single-phase DC/AC inverter supplying 60 Hz output will draw a 12OHz
component from its input DC source, Some AC output power supply designs have the
capacity to attenuate the reflected component from the input by supplying an
internal energy storage. If the AC content on the source lines is
problematic to system operation, the specification should include the limitation
for maximum allowed harmonic frequency content reflected to the input.
Inrush Current Support - An inverter may be required to power a load with a
large inrush current. The inrush presents two potential dangers to the
system: non-linear latch-up and multiple load reset. Both phenomena are
related to the current limit characteristics of the AC power source. The
non-linear latch-up is similar in principle to load recovery in DC power
supplies. An unfavorable current limit foldback may prevent the power
supply from recovering after an overload. AC loads, such as compressor
motors, will draw up to 500% of the rated current for several seconds at start,
thus presenting an opportunity for latch-up. Locked rotor machines can
draw even higher current. The inverter must be capable of supporting this
load and this requirement must be specified. A multiple load reset may
occur when the AC source is powering critical equipment as the load with large
inrush demand is introduced. Large inrush results in output foldback and
starves the critical equipment.