Electric Power

1Electric Power
Electric Power
Professor’s name
Name
Date

2Electric Power
Harmonics in a Power System
In a power system design, a frequency of 50 or 60Hz are the usual operating frequencies. Even
so, some loads in a power system produce loads that are multiples of the frequencies stated
above. These frequencies produced by these loads causes pollution in a power system and they
are the ones known to as the power system harmonics. The concept of harmonics is not a new
concept, in the past, 1916, there was a man called Steinmetz, who was the first man to discover
the harmonics, third harmonics was the more rampant type of harmonics, and designed a system
that could curb harmonics. He came us with the delta connection of transformer for the purposes
of mitigating the third harmonics (Grady, 2012). Harmonics tends to distort the normal smooth
waveform of the electric current, and these loads that causes this distortion are called non linear
loads.
Most nonlinear loads, like computers, uses the switch-mode power supplies (SMPS) which
causes distortion. The SMPS is what aids in the conversion of AC voltage used for the usual
utility to DC voltage which is regulated for the internal usage by the electronics. The power
supplies, which are nonlinear, draw some significant amount of AC power in high amplitude at a
short pulse which is what creates distortion. This distortion is manifested in the waveform it
shows in the end, it is measured in Total Harmonic Distortion, THD. In simple terms, the cause
of harmonics is a result of nonlinear loads whose current is unproportionally to the voltage
produced. The voltage applied could be a perfect sinusoidal, the current on the other hand could
face distortion (Manish & Nisheet, 2014). This distortion has a tendency of reverting to the
source of power, which often affects the equipment connected at the connection point of the
equipment.
Harmonics are these days receiving a lot of attention from engineers, since the loads connected
to them are increasingly becoming non-linear loads. This concern and mitigation measures, have
helped alleviate the unreliability of the power systems. The harmonic element in a power system
is referred to as the sinusoidal part of the periodic waveform that has a frequency that equals an
integer that is multiple the basic frequency also known as the fundamental frequency that the
system has, which is given by Fh=h, on the formula, h is the number that is multiplied to the
fundamental frequency F. The harmonics that are even are symmetrical can get cancelled or but
the odd ones are cancelled by a use of compensation or filtering. The sources of harmonics are
brought by the supply systems that have high voltages, like the HVDC systems, static VAR
systems, and renewable sources of energy converters which have interconnections, there are
large industrial loads such as the equipment of traction, the speed drives, drives controlled by
Thyristor, induction heaters, capacitor banks. The final source of harmonics low voltage which
are experienced at the consumer end, uninterrupted power supplying devices, some domestic
appliances, chokes, light dimmers, etc.
Harmonics Effects in power system
Power system voltage and current are supposed to have a waveform that are ideally sinusoidal.
This is when the load attached at the end of a power system has a current flowing through it
being nonlinear and this relates to the applied voltage as well. In a circuit that contain linear

3Electric Power
elements, which are resistance, inductance, and capacitance, the current flowing is of the same
proportion as the voltage applied. The result is a sinusoidal flow of current. In an instance where
the load is a simple full wave rectifier, the current is permitted to flow once the voltage input is
more than the voltage stored in the capacitor reservoir. In this case in point, the waveform tends
to distort the sine wave formed and harmonics is caused (Yacarrdni, 1996). Harmonics is formed
in the loads that are nonlinear by current being drawn in abrupt brief pulses and not the smooth
sinusoidal way (Bhattacharya & Divan, 1995). Other effects of harmonics distortions at a higher
level causes many different effects like increased heating of transformers, generators, motors,
capacitors, electronic equipment operating falsely, meters giving incorrect readings, protective
relays not working well or they just give false operating modes, and even in the communication
world there will be interference. Harmonics distortion is caused by devices that have non-linear
features. Not all of the components in the power system causes some serious issues in terms of
distortion, these are transformer inrush, and saturation, neutral connection of transformer, MMF
distribution in the rotating machines of AC, fluorescent lights, power supplies of a computer
when in switch mode, variable frequency motor drives (VFD), television power supply,
inverters, AC sources with imperfections, and so on. The optimal levels performed by a power
system is necessary and required, these are impeded when harmonics occur since the equipment
in the power system becomes inefficient when operating. This makes an escalation in the current
production requirement, will further escalates the cost of installations and hence reduced
profitability. The effects of harmonics cannot occur without affecting the conductors or cables
used in installation. A current flowing in a cable at a 50Hz frequency produces losses like the I2R
and the distortion of current which brings the introduction of other losses in the cable. These
losses increases the effective resistances as the frequency increases too which results in skin
effect, this is where the flux linkages across the cable’s x-section that are not equal pushes the
AC current on the peripherals of the cable (Sankaran, 2013). When the frequency is high, the
tendency of skin effect is high and vice versa. Since the harmonic current and the fundamental
frequencies flows in a conductor, rating a cable for a proper current flow will be important. The
ampacity level needs to be determined by calculation, which shall require that the skin effect of a
cable is known. The skin depth of a cable is dependent on how the current penetrate in the
conductor and inversely varies with the root of the frequency.
How to Mitigate Harmonics
Reducing the effects of harmonics with the existing equipment loads can prove to be difficult
more often, unless the load equipment is not being operated well. For instance, if we have a
transformer that is overexcited, it can be normalized by reducing the applied voltage to get to the
normal range, devices that are arcing, and most converters of power electronic adopt
manufacturing features.
The PWM drives which are used in charging the capacitors of a DC bus system straight from line
without any deliberate impedance are exempted from this. when a line reactor is added, or a
transformer is connected in series can give a significant reduction to harmonics, at the same time
cater for transient protection. The system or a three phase transformer connections method or
technique, can be used in the reduction of harmonics (Edvard, 2011). One such technique is a

4Electric Power
phase shift of 30O of the 6-pulse converters to an estimated benefit of a 12-pulse loads by the
reduction of seventh and fifth harmonics. When a transformer is connection in a delta form, it
blocks the flow of the zero sequence, triplens, harmonics from the line.
Oversizing a die installation, this solution somewhat reduces the effects of harmonics, but does
not eliminate completely the harmonics currents. However, what is widely practices is making an
oversize of the neutral conductor. In an operating electrical installation, derating the distribution
equipment that is exposed to the harmonic currents appears to be a solution as well. Another way
of buffering the harmonics is when non linear loads are connected to the reactors in series.
Another simple way is by anticipation of the harmonics, therefore designing the equipment in
such a way that will minimize or eliminate completely the eff3cts of harmonics.
Manufacturers these days are installing the speed drives with an aim of minimizing harmonics, it
has been known that the variable speed drives escalate the harmonics capabilities and therefore,
many manufacturing companies have installed harmonic-reducing fitting in them (Smith, 2013).
The use of filters also has been widely applied as a way of reducing harmonics. These are
passive filters or active filters, for passive harmonic filters, there’s the use of capacitors and
inductors that helps in blocking or shunting the harmonics to the ground depending on how they
are configured. The frequency increases with an increase of an impedance from an inductance,
but that of a capacitor becomes less. When the loads are varying, the passive filters might not be
effective. This also can be applied by incorporating a power electronic equipment at the
rectifying stage of an equipment, there might be a use of UPS. Active filters are known to
monitor the current at the load, they will also ensure the fundamental frequency is filtered out,
gauge the frequency content, and then triggers and opposing current commensurate to the
harmonics so that they cancel out the individual harmonic. This cancellation goes up to
mitigating around 50th harmonics and ensures the distortion levels are reduced by around 5%.
Power System Topologies
For a system to survive, or to render service even as they are threatened by many faults that are
caused by a hostile or natural interruption, is a feature of a power system that is desired. In
systems like the power systems that are integrated in a ship, the ability to overcome a huge
damage that happens suddenly, is of importance. Behind a power system survivability are many
factors, but the main factor is the topology of the power system with multiple generators and
loads alike (Poroseva, 2010). Some of the faults that occur are difficult to repair within a short
time. The most common topologies used in a power system are:
Secondary selective arrangement also called the main-tie-main
Ring bus topology
Primary loop arrangement
Secondary selective arrangement or composite primary loop
Secondary selective arrangement.