The
electric transmission grid represents a network that serves to the transmission
and the interregional exchange of electric energy.
Within
the transmission network is made an energy balance that takes into account the
network losses.
System
losses are:
• Power
losses (ΔP) represent the consumption of electric power necessary for the
transmission of electricity and for the distribution process;
• Energy
losses (ΔW) represent the consumption of electric energy necessary for the
transmission of electricity and for the distribution process;
If we define the contour of an electric network like an imaginary
line which closes the ensemble of electric installations and which provides the
energy exchanges between the neighboring systems, then we can calculate the
losses based on the next formula:
∆W=Wai – Wae
Where: - Wai - active
electrical energy entering in contour
Wa - active electrical energy
leaving the contour
In a national energy system the losses don’t
have a constant value because there are a lot of factors that influence this
value, factors like:
- The level of energy
consumption;
- The
production structure regarding the consumption areas;
- The changes
on the interconnection lines;
- The voltage
control;
- The reactive
power conditions;
- The losses
caused by the corona phenomenon;
THEORETICAL
ASPECTS
The corona
phenomenon is an autonomous incomplete discharge around the conductor, followed
by acoustic phenomena. The corona phenomenon appears when the value of the
electric field around the conductor reaches a critical limit of 21, 1 kV/cm.
The influence
of the corona discharge manifest through:
- The
increasing of the losses of power and energy in electrical grids;
- The shortening of the
life period of the conductors, fittings, clamps through corrosion process;
- The
generation of strong high-frequency disturbances which interrupt radio and TV
transmission
- The
generation of acoustic noise disturbing
To avoid the
appearance of corona phenomena it is necessary increasing the value of Ucr:
- By enlarging
the conductor's radius; this measure leads to difficulties in assembling and
operating electric lines;
- By using
bundle conductors who decrease the value of the electric field near the
conductor’s surface;
In what
concerns the underground cable, the power losses appear due to the ionization
phenomenon in the cable dielectric and due to the imperfection of the cable’s
insulation. At the underground cables used at 110 kV and 220 kV, the power
losses in insulations can reaches values of 5¸10 kW/km.
The presence of
corona discharge in high voltage grids leads to energy losses that depend on:
Constant
factors like: the pole type, the bundle conductor's surface, the distance
between conductors and phases;
Variable
factors like: transmission voltage level, weather, the conductor's surface,
clamps and armatures, the type and pollution rank of insulations;
The corona
losses don’t depend on the grid’s transmission power and represents few
percents from the line transmission capacity.
THE CORONA
PHENOMENON AND ITS EFFECTS ON LOSSES
The Romanian
transmission grid is a structure well developed with interconnection lines,
protections and automations systems which ensures a high level of security for
the power system. No matter how much would be developed the part that concerns
the equipments, the installations and the protections, in an electrical grid
will always exist power losses that, at most, can be reduced. From this point
of view, the corona losses are a grid characteristic and are strongly influenced
by the meteorological conditions and the voltage level of that electric
network. As a Transmission System
Operator, one of our purposes is to decrease the corona losses and that’s why,
starting with the year 2010, we began to realize a forecast of the losses.
Practically, the Romanian Power Grid Company has the benefit obtained through
reducing the costs with the imbalances. Nevertheless, it has been observed the
fact that not always it can be realized a good forecast. The contrary, the
differences that appear are bigger and for this reason the economical losses
grow.
Analyzing the
aspects mentioned above, we get to the conclusion that the reducing of corona
losses can be achieved following a good forecast of losses – a risky situation
and sometimes with negative effects. So, the investments in transmission grid
and in its components are the safest solutions.
Growing the
number of the conductors on each phase of the electrical line, purchasing and
replacing the old transformers, the installation of equipments with role in
adjusting the voltage level , are some of the measures that can be taken to
reduce the losses provided by corona phenomenon.
Corona
Calculations
The following
corona calculations are from Dielectric Phenomena in High Voltage Engineering,
F.W. Peek, 1929
For
Concentric Cylinders in Air
• Corona will
not form when RO / RI < 2.718. (Arcing will occur instead when the voltage is too high.)
For Parallel
Wires in Air:
• Corona will
not form when X / r < 5.85. (Arcing will occur instead when the voltage is
too high.)
For Equal
Spheres in Air:
• Corona will
not form when X / R < 2.04. (Arcing will occur instead when the voltage is
too high.)
• Arcing
difficult to avoid when X / R < 8
Where
• RO = Radius
of outer concentric sphere
• RI = Radius
of inner concentric sphere
• R = Sphere
radius
• r = wire
radius
• X = Distance
between wires or between spheres
Corona
Prevention
Corona can be
avoided by minimizing the voltage stress and electric field gradient. This is accomplished by using utilizing good high voltage design practices,
i.e., maximizing the distance between conductors that have large voltage differentials,
using conductors with large radii, and avoiding parts that have sharp points or sharp
edges. Corona inception voltage can sometimes be increased by using a surface treatment,
such as a semiconductor layer, high voltage putty or corona dope. Also, use a good,
homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well. If you are limited
to using air as your insulator, then you are left with geometry as the critical
parameter.
Finally, ensure
that steps are taken to reduce or eliminate unwanted voltage transients, which can cause corona to start
CONCLUSIONS
The corona
phenomenon is a complex one because at high alternative voltage this phenomenon
produces energy losses – that’s why we take special measures to restrict the
appearance of the phenomenon. At high voltage pulse and at high continuous
voltage the corona phenomenon is useful and it has multiple applications like:
the flattening of the overvoltage pulses, industrial electrofilters, the Xerox
multiplication process etc
The most
important conclusion that the active power losses caused by the corona
phenomenon have high values on electric lines with one conductor and with frost
formation. The solutions to this problem were mentioned above and concern in
making investments in the components of electrical grid, like: changing the
conductors because the losses will decrease if we use bundle conductors or if
we increase the number of conductors per phase.
Romania is a
country situated in a geographical area that favors the appearance of fog and
the deposit of frost, phenomena that have the biggest impact on the losses. We
have not the power necessary or the possibility to change the nature or the
geographical area to obtain a smaller value of the losses. The only solution
and the most plausible both of the technical and economical point of view, is
to decrease the costs with the imbalances. Practical, we can take measures to
obtain a better adjustment of the voltage, to avoid the congestions what
presumes to
REFERENCES
[1] Alina Andreea NEAGU -The corona phenomenon and its effects on the
losses in the High-Voltage Transmission lines
[2] Miga-Papadopol M., Nota
privind CPT-ul inRET, Bucuresti, 2009
[3] Ungureanu M., Metodologie
privind abordarea statistica (meteo, relief, tensiuni) a pierderilor corona
pentru liniile de 220 si 400 kV, Bucuresti 2002
[4] Marudava Sarama P., Corona England
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