Environmental Engineering Reference
In-Depth Information
the system Thévenin complex impedance as a function of frequency. Such curves can be used
in conjunction with the known levels of generated harmonics from say, a converter, to predict
the likely distortion of the PCC voltage by a renewable energy embedded generator. Voltage
distortion will usually be worse if the PCC node fault level is low.
In theory, a poorly designed directly connected synchronous or induction generator could
cause signifi cant harmonics, but modern machines manufactured by reputable companies are
generally free of such defi ciencies. Harmonics from distributed generators usually arise from
their power electronics. Soft-starters for wind turbines are usually thyristor based and may
cause signifi cant harmonics, but only for the few seconds during start-up. This is unlikely to
be a problem overall. Systems where the power electronics are used all the time are more of
a concern. Some early wind turbines and PV systems used thyristor based converters and did
cause signifi cant harmonics.
Modern pulse-width-modulated (PWM) converters as applied to grid-connect PV and with
variable speed wind turbines (using MOSFETs and IGBTs) are very much better in this
respect but still have to be carefully designed and checked for suitability in each installation.
In particular, there is a trade off because a high switching frequency will reduce harmonics
but will increase switching losses and therefore reduce the converter effi ciency.
A particular Spanish region with a very high penetration of wind power was found to have
excessive harmonic distortion due to PWM converters with a rather low switching frequency.
There is concern that very high penetrations of PV could have a similar effect, though this
should be considered in the context of the rapid increase of power electronics in consumer
appliances. National and International Standards, for example EN 61000-3-2: A, specify strict
limits for the harmonic distortion allowed from small loads and generators, and network
power quality in this regard is not regarded as a particular concern.
6.4.3 Phase Voltage Imbalance
Ideally, single-phase loads are evenly distributed between the three phases. In practice, any
differences will cause a phase voltage imbalance on the network.
A directly connected three-phase generator will normally serve to reduce any existing
phase voltage imbalance. However, in achieving this, the generator itself will heat up due to
abnormal circulating currents within the windings. To avoid overheating, wind turbines
usually include
imbalance protection
. In some rural networks, this imbalance protection can
operate frequently and cause signifi cant loss of generation.
In small distributed applications of RE, e.g. domestic wind turbines or photovoltaic, single-
phase generators could increase phase voltage imbalance. However, if such applications were
to be adopted in large numbers, statistically they would be evenly distributed on average
among the three phases just as loads are and no problems should occur.
6.4.4 Power Quality
Voltage fl icker, steps, dips, harmonics and phase imbalance may all degrade what is loosely
described as
power quality
. Such undesirable occurrences can be caused just as much by
loads as by generators. In general, it is the responsibility of the operator of the load or gen-
erator not to unduly affect the power quality for other users in the area. However, the impact
