The Ultra-high Voltage (UHV) transmission has been employed in large measure in China to connect far off load centres with power generation centres. The UHV is also a key component of the internet of energy with a substantial renewable component. The operation of steam turbines supplying to the UHV transmission has come under some stress.
Below are excerpts from the paper, “The analysis of UHV transmission’s impact on steam turbine operation” by Zhi-hao LUO, Bo CHENG and China Feng YIN of Electric power research institute of state grid at the Zhejiang electric power company; Jin-long LIAO, Zi-tao Yu, Wei LI and De-ren SHENG of Institute of Thermal Science and Power System, Zhejiang University presented at the 2017 ASME Turbo Expo.
As a result of the UHV transmission, utilization time of the steam turbine in local power plants can reduce, and some machines may have to work in low load for prolonged periods. The change of the generating power is affected by access to UHV transmission.
Frequency is one of the important indicators of monitoring power quality and security of the grid. The units use Primary Frequency Control to keep frequency stable when a random load is produced in the grid. High-power units can work in full load and sources such as wind and hydro power can use PFC to keep frequency stable. But the situation changes after UHV power transmission accesses the local grid.
The stability of voltage and electric current of the UHV should be kept because of the UHV power transmission’s long distance and high parameters. Power generating units should undertake the heaviest tasks of PFC to keep the grid stable.
Percentage speed variation reflects the ability of the PFC. It is the most important indicator in regulating the system. A large percentage speed variation would mean, the PFC of the units is weak. The value should be set big to decrease the influence of the change of frequency for the units undertaking most of load in the grid. The value should be set small for peaking units to share the random load when the frequency changes.
PFC dead band is set to prevent the steam turbine valve from moving frequently when the frequency fluctuates on a small scale. PFC will fail to adjust frequency if dead band is set too large. If it is set too small, the units will not be stable.
Response lag time reflects the rapidity of PFC. Settling time is the time needed to achieve the desired values when the frequency is constant in the PFC. The response lag time and settling time should conform to the relevant provisions.
In China, response lag time needs to be less than three seconds and settling time should less than 60 seconds. The PFC parameters should be adjusted to keep PFC response rapid and keep frequency stable according to the characteristics of the UHV transmission and the PFC.
Grid and units in different areas are contacted by the UHV transmission. The dynamic stability of the grid will increase because of the energy net. Low frequency oscillation of grid will be decreased. Then it can increase the stability of the local units. The requirements of the units’ stability can be reduced slightly in PFC.
According to the provisions of “Power Grid Operation and Management in East China”, new 1,000 MW units should participate in the PFC to keep the safety of grid and economics of power generation. The percentage speed variation should change between 4% and 6%, the PFC dead band should be± 0.05Hz and the load variation should be ±6%.
The specific ability of the PFC should be measured according to the actual operation of the units. The ability of the PFC depends generally on the boiler, heat storage and steam turbine control valve. The units should select other methods to increase the load for once-through boilers due to their limited heat storage capacity. The capacity of the PFC can be assessed from the control valve opening, condensate throttling, feed water bypass and removing high-pressure heater.