Preliminaries - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 63

Table 2.1 Common terms to describe power quality

Term Meaning

swell The RMS voltage exceeds the nominal value by 10% to 80% for

0.5 cycle to 1 minute.

dip or sag The RMS voltage is below the nominal value by 10% to 90% for

0.5 cycle to 1 minute.

flicker Random or repetitive variations in the RMS voltage between 90%

and 110% of the nominal value, which cause rapid visible
changes of light level in lighting equipment.

spikes, impulses, or surges Abrupt, very brief increases in voltage, which are generally caused

by large inductive loads being turned off.

under-voltage The voltage drops below 90% of the nominal value for more than

1 minute.

over-voltage The voltage rises above 110% of the nominal value for more than

1 minute.

brownout An apt description for voltage drops somewhere between full

power (bright lights) and a blackout (no power - no light).

harmonics The wave shape is distorted.

of loads today are non-linear and generate harmonic currents when a purely sinusoidal voltage
supply is provided. These harmonic currents then cause harmonic components in the voltage
because of the impedances in the distribution network and, also, inside the voltage sources. Of
course, harmonic voltages then cause harmonic currents as well. The odd multiples of the 3rd
harmonic (3rd, 9th, 15th, 21st ...), i.e. the 6 n 3 harmonics, are called triplen harmonics. These

currents on a three-phase system are zero-sequence harmonics, which are additive in the neutral
line and cause particular concern. The 6 n

−

1 harmonics are negative-sequence harmonics
and can cause problems to electrical machines because these harmonics create a negative
torque and attempt to drive the machines in reverse. Harmonics are not desirable because
they cause overheating, increased losses, decreased power capacity, neutral line overloading,
distorted voltage and current waveforms, etc. It has become a very serious issue in modern
power systems. Hence, stringent regulations have been put into place (Hornik and Zhong,
2011; Yousefpoor et al. 2012). The total harmonic distortion (THD) of voltages and currents
needs to be maintained low, often below 5%. Table 2.2 shows the maximum THD allowed
in the currents fed to the grid (Blooming and Carnovale, 2007). Since harmonics tend to
be cumulative in power systems (Irwin, 1996), the controllers used should have very good
capability in harmonic rejection, in order to meet the operator's requirements.
A natural solution is to introduce some controllers and, as a result, several feedback control
schemes are available to reduce the voltage THD of inverters. In principle, this is a tracking
problem with a sinusoidal reference while rejecting other harmonic components. Deadbeat

−

Table 2.2 Maximum THD allowed in currents fed to the grid

<

11 th 11 th -15 th 17 th -21 th 23 rd -33 rd

>

33 rd Odd harmonics

Maximum current THD < 4% < 2% < 1 . 5% < 0 . 6% < 0 . 3%