IEEE Power & Energy Magazine - March/April 2020 - 55

- ecause these towers need guy wires, they are feasible
B
in plains and hilly areas but not in mountainous terrain. Guyed towers were chosen often in the past when
land-usage costs were lower. The manufacturing cost
of a guyed tower is relatively low. In recent years, China's land-use expense has increased; therefore, the total cost of a guyed tower is becoming higher than that
of a self-supporting design. As a result, guyed-tower
construction is used less frequently than previously.
2) Single-circuit, self-supporting tower: Cup- and cathead towers are typical types of single-circuit-suspension, self-supporting structures. Cat-head towers are
commonly used in narrow line corridors. In wide corridors, cup-type towers are preferred. For the 1,000-kV
Southeast Jin-Nanyang-Jingmen UHV ac project,
both types were used.
3) Double-circuit tower: A double-circuit tower has three
or four crossarms to vertically support three-phase conductors. This design is used for double-circuit transmission lines at all voltage ratings, including 1,000 kV.
Besides the tower, the conductor accounts for a large portion of the price to build a UHV ac overhead line. Constrained
by cost, the conductors are selected to most effectively meet
the requirements for the transmission capacity, electromagnetic (EM) environment, mechanical strength, radio interference, and audible noise. Three types of conductors have
been used in UHV ac transmission-line projects for different
scenarios. The aluminum conductor steel-reinforced variety
is common. The aluminum-alloy-conductor, steel-reinforced
version is employed in icy areas, while aluminum-conductor, extra-high-strength, steel-reinforced models are widely
used for long spans, such as river crossings.
The strong electric field of the UHV ac transmission line
triggers a corona, resulting in power losses. In addition, the
corona causes EM interference, audible noise, and electrical
corrosion on the surface of the wire, reducing the life of the
transmission line. To suppress the corona, bundled conductors are adopted. While 500-kV ac transmission lines use
four LGJ-300 or LGJ-400 four-bundle conductors, a 1,000-kV
ac transmission line adopts eight LGJ-400, LGJ-500,
LGJ-630 eight-bundle conductors, typically with a 30-mm
wire diameter and 40-cm splitting distance. For example,
the Southeast Jin-Nanyang-Jingmen line uses eight LGJ500 wire conductors, and the Huainan-Shanghai line adopts
eight LGJ-630 wire types.
UHV ac transmission lines connect power-generation
centers in the west and north to load centers in eastern and
southern China. They are built at high altitudes. To ensure
voltage insulation, the altitude-correction factor is measured
and applied to adjust the withstand voltage of the external
insulation. At high altitudes, the flashover voltage is less
than at low altitudes. The effect of altitude on the flashover
voltage is more dependent on the type of insulator than the
length of the string. At high altitudes, icing flashover is an
issue that needs to be addressed. The icing-flash test shows
march/april 2020

that the icing-flashover voltage is more dependent on the
length of the string. V-insulators and composite insulators
with ice barriers are recommended.

UHV Substation-Equipment Technologies
The equipment in a 1,000-kV substation includes transformers; shunt reactors; switchgear such as gas-insulated
switchgear (GIS) and hybrid GIS; metal-oxide surge arresters; capacitive voltage transformers; grounding switches;
post insulators; and low-voltage, reactive-power compensation devices. Except for the associated low-voltage,
reactive-power compensation devices, all UHV substation
equipment is difficult to design and manufacture. As part of
the UHV ac transmission project, engineers developed and
manufactured the first 1,000-kV/1,000-MVA UHV transformer and one of the largest UHV shunt reactors in the world.
The rated voltage of the 1,000-kV/1,000-MVA transformer is 1,000 kV for the primary winding, 500 kV for
the secondary winding, and 110 kV for the tertiary winding. The low-voltage, reactive-power compensation devices
are installed on the 110-kV winding. The 110-kV winding
capacity is designed to be higher than the installed capacity of the low-voltage reactive power-compensation devices.
For example, the Southeast Jin-Nanyang-Jingmen project
requires 840 Mvar of capacitive and 960 Mvar of inductive
reactive power compensation, but the rated capacity of the
tertiary winding is designed for 1,000 MVA. Shunt-reactor
and shunt-capacitor banks are installed in UHV ac substations for voltage control. To avoid resonance, the designs
call for using less than 100% of the shunt compensation. In
the Changzhi, Nanyang, and Jingmen substations, the shunt
capacitors are rated to provide a compensation of 85-88%.

Smart UHV Grid
Smart simulation, control, and operation technology is a
goal of the State Grid's UHV-system deployment. To facilitate large UHV ac-grid planning and design, a digital/analog
hybrid simulator was developed to model the effect of UHV
devices on the UHV grid. A new-generation EMS is under
development to model, monitor, and operate the UHV grid
from a control center.

Simulation Center
With the highest number of HVdc projects and the largest
amount of renewable generation, the State Grid is one of the
most complex power networks in the world, making it difficult to simulate, analyze, and control. To improve situational
awareness, a simulation center (Figure 1) was built at the
China Electric Power Research Institute (CEPRI), Beijing,
using state-of-the-art technology. The simulation center
houses two systems: a digital-analog hybrid simulator and
fully digital one.
The digital-analog hybrid consists of a large-scale,
power-system real-time simulator, and, to date, it is the largest of its kind, modeling more than 20 HVdc controls and
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IEEE Power & Energy Magazine - March/April 2020

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