ABB Relion 670 Series Applications Manual page 510

Section 17
Monitoring
Voltage harmonic distortion levels can vary drastically, depending on the configuration of system.
These voltage harmonics can damage the equipment as they are designed to operate for certain
range of voltage inaccuracy.
Moreover, in four-wire distribution systems (three-phase and neutral), the currents in the three
phases will return via the neutral conductor, a 120 degree phase shift between corresponding
phase currents that causes the currents to cancel out in the neutral, under balanced loading
conditions. When nonlinear loads are present, any 'Triplen' (3
currents does not cancel out. However, they will be added cumulatively in the neutral conductor,
which can carry up to 173% of phase current at a frequency of predominately 180 Hz (3
harmonic).
In case of electric traction systems, it generates various power quality problems that have an
important impact on its distribution network. DC traction loads, fed through AC/DC rectifiers,
generates non-linear voltages and currents on the AC system, that will result in harmonic voltage
distortion of the power supply system. Traction power supply system creates power quality
problems to the corresponding grid, which can cause:
• Poor power quality
• Increase in operational cost due to less productivity
• Damage to sensitive equipment in nearby facilities.
Maintaining high power quality in traction system is very complex. The presence of non-linear
loads reduces the capability of the existing harmonic mitigation techniques. However, it is
essential to minimize the issues like harmonics, voltage sags and flicker to protect sensitive
equipment affected by the aforementioned issues produced by traction systems.
Various practical conditions which have dynamic characteristics like the speed of locomotion, load
and line condition will make this problem even worse. Harmonic current increases the heat
dissipation due to hysteresis and eddy currents, which causes stress on insulation materials.
Harmonic current increases transmission loss and the voltage drops.
In general, harmonics can cause reduced equipment life if a system is designed without
considering the harmonics and if the equipment is not designed to withstand harmonics. Hence, it
is important to measure and monitor harmonics in power systems. Harmonic voltage distortions
on 161 kV power system and above is limited to 1.5% of total harmonic distortion (THD), in with
each individual harmonic is limited to 1.0%.
Current harmonic limits vary based on the short circuit strength of the corresponding system they
are injected into. Harmonic current limit defines the maximum amount of harmonic current that
can be inject into the utility system. The difference between THD and TDD is used to calculate the
harmonics level during light load conditions.
17.13.3 Setting guidelines
The recommended limits for total harmonic distortion and individual harmonic distortion are
available in the IEEE 519 standard. The limits are based on measurements which are done at the
point of common coupling. It should not be applied to either individual pieces of equipment or at
locations within a user's facility. In most cases, harmonic voltages and currents at these locations
could be found to be significantly beyond recommended limits at the PCC due to the lack of
diversity, cancellation, and other phenomena that tend to reduce the combined effects of multiple
harmonic sources to levels below their algebraic summation.
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