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8th 2013 Asia-Pacific International Conference on Lightning, Jun. 26-28, 2013, Seoul, Korea
Introduction of Lightning Protection System Design
Concept of the Coal-fired Thermal Power Plants in
Korea
Yeo, Kyeongkyu#1, Jeong, Gucheol#2
#1,2
Electrical, Instrumentation & Control Engineering Dept., KEPCO E&C
2350 Yonggudaero, Giheung-gu, Yongin-si, Gyeonggi-di, South Korea
[email protected]
[email protected]
Abstract— This paper introduces about the lightning protection
system for the coal-fired thermal power plant in Korea. Power
generation plant has lots of building or structures for turbine
and generator, boiler, coal handling system, FGD(flue gas desulfurization), etc. and also lots of electrical equipment installed
outside.
Each building should be protected from lightning and the
electrical equipment shall not only be protected from the direct
stroke of the lightning but also be protected from transferring
surge intruded by the lightning stroke away from the equipment
at the plant.
In this paper the characteristics of lightning protection system
for power plant buildings, structures, and electrical equipment
will be introduced. And also special check points for lightning
protection system of electrical equipment will be introduced.
I. INTRODUCTION
Considering the importance of the power plant in
Korea as industrial energy sources, their operational
continuity is very important. The electric power
consumption in Korea is at its highest level during
summer, and the thunder storms which can give a
disturbance to the equipment in the power plant are
also most frequent and in high magnitude in a year.
Coal-fired thermal power plants are supplying
electric power to the base loads with nuclear power
plants in Korea. So, the operational security of the
coal-fired thermal power plants is very important to
the national industry.
The appropriate lightning protection system of
the plant is critical to ensure the service security of
the power plants during thunderstorm weather.
In this paper the characteristics of the lightning
protection system of the coal-fired thermal power
plant in Korea and a specific requirement are
introduced.
II. CHARACTERISTICS OF THE COAL-FIRED THERMAL
POWER PLANT
Coal-fired thermal power plant consists of lots of
equipment for the electric power generation and
several buildings and structures for the installation
and the protection of the equipment. Normal
buildings for office are also included in the plant
area.
The major systems for the electric power
generation are boiler which generates steam to
rotate the turbine, turbine and generator which
changes the mechanical power to the electric power,
coal handling system which transfers the coal from
coal yard and supplies to boiler, FGD which
desulfurizes the exhausting gas of boiler and the
other system or equipment including electrical
equipment.
The main electric power system consists of the
main generator, step-up transformer, auxiliary
transformer and start-up transformer, isolated phase
buses, switchgears to supply the electric power to
the house loads, high voltage power cables and
other low voltage systems.
High voltage substation is included to the plant
area because the generated electric power shall be
transmitted through high voltage transmission lines
to the loads which are far from the plant. And the
power plant has lots of electronic equipment, too,
which is requesting a special consideration for
protection from lightning surges.
Buildings and structures for each system shall be
protected from lightning stoke to protect the
building itself and also the equipment installed in
the building. The lightning protection system for
buildings or structures shall also such designed to
include the electrical equipment installed outside,
e.g. power transformers, extra-high voltage
equipment, etc., to their protection area considering
the equipment insulation level.
The electrical equipment shall also be protected
from lightning surges transferred through
conducting paths to ensure the service continuity of
electrical equipment in the plant.
Lightning protection system for buildings or
structures of the coal-fired thermal power plants
and the specific design consideration for the
lightning protection of the electrical system are
introduced in the following sections.
- High structure: Stack;
- Substation.
Lightning protection systems for each categorized
building is introduced.
A. Normal buildings
The design method of the lightning protection
system for normal building is same as that for
commercial buildings. The lightning protection
system consists of air terminals, conductors and
down conductors and grounding system.
The air terminals are located on the roofs of the
buildings or top of each structure to include any
facility to the protection zone generated by rolling
sphere in accordance with NFPA 780. Each air
terminal will be connected each other and
connected to the plant grounding system through
down conductors.
The plant grounding system is designed to meet
the requirements of touch voltage and step voltage
in accordance with IEEE Std 80-2000, which is
quite different from the commercial building. The
ground fault current for the design of the grounding
III. LIGHTNING PROTECTION SYSTEM FOR BUILDINGS AND
STRUCTURES
system of the power plant is much higher than that
Lightning protection system for buildings at the of the commercial buildings.
Some power plant in Korea are installed with
coal-fired thermal power plants will be introduced.
non-Franklin
rod, e.g. DAS(dissipation array
The major buildings and structures considered in
system)
or
BDAT(bipolar
discharge-dissipation
the design of the lightning protection system are as
lightning air terminals) as air terminals, even
follows:
though those air terminals are not mentioned on
- Boiler;
NFPA. However, the layout of the non-Franklin air
- Turbine and generator building;
terminals at the power plants is designed with the
- Stack;
same principle of the Franklin rod in accordance
- FGD building and structures;
- Structures and buildings for coal handling with NFPA 780.
system;
B. Stack
- Structures and buildings for limestone
The stack is the highest structure in the coal-fired
handling system;
thermal power plant so that it intercepts the
- Substation;
lightning frequently. According to the layout of the
- Other structures and buildings.
plant lots of equipment is installed near the stack.
The above building and structures can be divided And lots of the failures of the electronic equipment
to three categories considering the design installed on and near the stack during the lightning
requirements;
stroke to the stack have been reported. The special
- Normal building: Boiler, Turbine and efforts to prevent such failures are paid during
generator building, FGD building and power plant design stage of the lightning protection
structures, Structures and buildings for coal system of the stack.
handling system, Structures and buildings for
The design requirements of air terminals and
limestone handling system, Other structures conductors are mentioned on Chapter 6 of NFPA
and buildings;
780 and Annex A of IEC 62305. The major
difference in the design principle of the lightning
protection for stacks or high structures between 780
and IEC is the installation method of the air
terminals at the intermediate level of the
stack.[1][2]
The design method in NFPA 780 is applied for
the coal-fired thermal power plant in Korea. The
height of the air terminal rod is limited in
accordance with NFPA requirements. The air
terminals are connected to form a ring connection
and connected to the plant grounding system by at
least two down conductors. There are such power
plants to be protected by Non-Franklin rods, as
shown on Fig. 1.[3]
Fig. 1 Example of non-Franklin rods installed on the top of the stack
The electronic equipment on the stack is
protected by SPD. The grounding system near the
stack is supplemented with special grounding rods
developed to enhance the surge current
characteristics of the grounding system. Sometimes
supplementary grounding mesh is installed near the
stack for such power plant expecting high level of
lightning stroke rates.
C. Substation
Gas-insulated substation(GIS) is normally
installed for the power plant in Korea. Because the
gas-insulated buses are consisted of enclosures
having high conductivity and the ground resistance
of the substation is normally very low, no additional
lightning protection is necessary except for the
equipment regarding take-off tower, which is
protected from the direct lightning stroke by
grounding wires on the tower top.[4]
The generator step-up transformer and the other
transformers are installed near turbine building and
included to the protection zone of the lightning
protection for turbine building.
IV. LIGHTNING PROTECTION SYSTEM FOR ELECTRICAL
EQUIPMENT
The electric system configuration of the power
plant shall be thoroughly checked for the
appropriate design of the lightning protection. The
usual intruding way of the lightning surges
affecting the plant equipment is the back flashover
of the lightning stroke to the top of the transmission
tower. Because the transmission lines are protected
from the direct stroke by the overhead wires, the
lightning current level stroking the lines directly is
usually lower than that of tower top.
The magnitude and the waveform of the surge
transferred to the electric equipment at the power
plant depend on the magnitude and on the location
of the stroke and the characteristic impedance of
each electric system component.
The major electrical equipment of the coal-fired
thermal power plant affected from the transferred
lightning surges through the transmission lines and
the power plant equipment is the main generator,
the generator step-up transformer, the auxiliary
transformer, and the start-up transformers.
Surge arresters are usually connected for the
protection of the electrical equipment from
lightning surge. The protective level and the
continuous operating voltage of the surge arresters
regarding the protected equipment and its location
are critical to the proper protection of the
equipment. The design criteria regarding the
selection of the surge arresters for the equipment in
the substation are standardized by KEPCO in
Korea.[5]
However, the design criteria for the protection of
the equipment at the power plant are not
standardized.
The important check points regarding lightning
surge protection are determining the location of the
surge arresters for the protection of the generator
step-up transformer and the characteristics of the
surge arresters for the protection of the main
generator.
The generator step-up transformers are connected
to high voltage GIS either by high voltage cables or
overhead lines. In case of cable connection, the
surge arresters to protect the generator step-up
transformer can be installed either at GIS side or at
the transformer side according to the lightning
surge voltage at the terminal of the transformer.
Better protection margin can be expected if the
surge arresters are installed at the transformer with
additional cost of the supplementary gas insulated
buses.
If the installation of the surge arresters at the
transformer is not allowed due to the space or
economic reasons, those shall be installed in GIS
side after the careful study regarding the lightning
surge at the terminal of the transformer. Fig. 2
shows an example of the main electric power
system configuration of typical coal-fired thermal
power plant.
Even though the lightning surge voltage is
reduced to the protective level of the arrester at the
GIS, the surge voltage at the transformer can be
raised to a dangerous level due to the characteristic
impedance difference between GIS and the power
cable. [6]
Even though the main generator is installed in the
turbine building and connected to the transmission
line through step-up transformer, the generator is
usually protected from the lightning surges by surge
arrester due to the transferred surge through the
transformer and the importance of the generator in
the power plant. The arresters shall be selected with
high attention regarding the protective level and the
continuous operating voltage of the arresters.
The insulation level of the generator is lower than
that of the transformer for the same voltage, so that
the lower protective level is required for the
generator.
However, the voltage on the un-faulted phase
during ground fault is higher than that expected on
the effective grounding system due to the high
resistance grounding of the generator. So, the
higher continuous operating voltage is required for
the arresters to protect the generator than that for
the transformer. It is difficult to find an appropriate
surge arrester to meet the above requirements
simultaneously for some type of generator.
1) One of the surge arresters can be eliminated according to the lightning
surge voltage study.
2) Special attention necessary regarding the protective level and the
continuous operating voltage of the surge arrester
Fig. 2 Example of the electric power system configuration of coal-fired
thermal power plant regarding surge arresters
V. CONCLUSION
The lightning protection system for the coal-fired
thermal power plant in Korea is introduced. The
specific difference of the protection system
compared to normal commercial building is based
on the importance of the plant and the high
structure, such as stack. And the other specific
protection system is regarding the main electric
power system. Even though the lightning protection
requirements for the substation equipment are
standardized by KEPCO in Korea, those for the
plant equipment are difficult to be standardized
because of the difference of the characteristics of
the main equipment of each power plant. Detail
lightning surge analysis regarding the generator
step-up transformer is required if the arrester cannot
be installed at the transformer because of the layout
or economic reason. And high attention is requested
to determine the protective level and the continuous
operating voltage of the arresters for generator.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
Standard for the Installation of Lightning Protection, NFPA 780, 2011
Edition.
Protection against lightning – Part 3: Physical damage to structures
and life hazard, IEC 62305-3, Edition 2.0, 2010.
M. D. Robinson, “Grounding and lightning protection”, EL- 5036,
Power Plant Electrical Reference Series Volume 5, EPRI, 1987
“A Study on the Insulation Coordination of 765kV System”, KEPRI,
South Korea, 1995
Selection of Arresters, DS-2531, KEPCO, South Korea, 2008
“Lightning Protection Design Guide-Book for Power Stations and
Substations”, CRIEPI , Japan, 1976