Northeast China Power Grid
Northeast Interconnections or power grid covers the northeast part of China, which includes Liaoning, Jilin, and Heilongjiang provinces and East Inner Mongolia Autonomous Region, with a total installed capacity of 147 GW and 120 million people served.
The maximum generation of Northeast Grid is 70 GW; the all-time peak load is 64 GW. Northeast Grid has 28 GW of wind and 10 GW of solar PV. The ratio of renewable energy installed capacity to total capacity is 33%. In 2018, the annual renewable energy penetration was 19% and the record wind plus solar PV output was 19 GW, for an instantaneous penetration of 35%. The generation mix still relies heavily on coal fired power plants, but the share of renewable energy has increased steadily in recent years.
An important role for CHP in heating and electricity generation, but a challenge to renewable energy integration
Northeast China is a severely cold region, with about a 150 to 250 day heating period per year. In January, the average temperature reaches -15 to -20°C in Heilongjiang province. The overlap of low temperature and high wind speed is the major challenge of renewable energy integration in Northeast China. In winter, the CHP units are forced to serve as the source of district heating, and they are usually operated in the minimum condensing steam state. This minimum condensing steam state increases the minimum power output, reducing the flexibility for load-following. It is estimated that 100 ton/hr increase in steam extraction will raise the minimum power output by 28.2 MW for a 330 MW extraction steam turbine unit. About 70% of renewable energy curtailment occurs during the heating period. Even during times of full wind and solar curtailment, the total minimum generation from CHP power plants still exceeds the consumption in the load valley.
CHP has high energy efficiency in producing electric power and heat. It’s a national strategic policy to develop CHP in urban areas for district heating and electricity. CHP capacity has increased at an average annual rate of 13.35% since 2006, accounting for 71% of total thermal capacity by the end of 2018 and 90% of on-line thermal capacity during the heating season.
Renewable energy curtailment before 2014 was a serious problem, see Figure 1. Major measures in Northeast China have been taken to mitigate the curtailment situation.
- Banning new investments in wind and solar (red flag warning mechanism) in regions with high curtailment.
- A UHVDC (10 GW capacity) connection project was put into operation, connecting Northeast China Grid with North China Grid.
- Strengthening of grid connections and reduction of bottlenecks in the transmission grid.
- Launch of a load regulation ancillary service market in Northeast China Grid to encourage flexible operation of thermal power plants.
- Pilot projects involving investments in retrofitting of existing coal power plants for flexibility, particularly combined heat and power (CHP) plants in the northern regions.
Flexible CHP and How?
There were 27 coal-fired power plants that had completed flexibility retrofits in Northeast China by the end of February 2019. Eight coal-fired power plant flexibility retrofitting projects are under construction, anticipated being put into operation by the end of 2019. Different technical solutions are adopted in those 27 power plants. With respect to the power plants that have completed their retrofitting, they mainly utilized 3 different technical solutions:
- Electric Heater + Heat storage/ Electric Boiler Heat Accumulator
- Optimization of Turbine Operation Mode /Turbine LP Cylinder Bypass
- Systematic retrofit of condensing unit
These retrofit projects are aiming to provide down regulation flexibility, motivated by substantial revenue from the load regulating ancillary service market. Since the opening of the market in October 2014, 54 coal fired power plants received 5.8 billion RMB in revenue from selling their flexibility capabilities. Highest revenue for a single power plant is 465 million RMB.
42 TWh of wind generation has been saved from curtailment thanks to the market. The cost for every MWh of retained wind generation is 63 RMB, while the annual average price of wind generation is 500 RMB/MWh.
The core idea of this market is to encourage thermal power plants to reduce their output when there is a generation surplus due to high penetration of renewable energy. A baseline operating point for down regulation capability is drawn, which in the northeast region is 50%. Power plants operating above the baseline when the system has a generation surplus would pay power plants operating under the baseline. The dispatching centre (system operator) hosts a day-ahead auction of down-regulation service. Thermal power plants capable of going under the baseline can bid in with a price and possible down-regulation capability. During real-time operations, the dispatching centre will activate the units starting with the bid price from the lowest bidder. The last unit activated will establish the uniform price, and all power plants will receive payment based on this uniform price. The settlement is carried out on a 15-minute basis. The total cost is allocated proportionally to those power plants that operate above the baseline during that time period.
CHP as a resource of flexibility is practical, profitable and effective, especially for renewable energy integration
Since 2015, flexible CHP caused a significant drop of coal fired power plants minimum load operating point in the valley period, from 58% to 50%, see Figure 2, dropping the curtailment ratio from 17% to 3%. The main cause of curtailment has shifted from system balancing to grid congestion in the last four years. The percentage of curtailment caused by system balancing issues has decreased from 70% in 2015 to 32% in 2018, see Figure 3. Wind integration in the load valley period has been improved by 10% of wind installed capacity since 2015, see Figure 4.
With a well-designed market, CHP units can provide a significant source of flexibility. CHP has proven itself as a practical, profitable and effective flexibility resource. With the trend of reducing coal consumption and retirement of coal fired power plants, flexibility from CHP units may provide a major contribution to plant revenue.
China’s power sector has been experiencing major reforms in recent years, especially in electric power markets. A spot market is within reach in 3 to 5 years. The load regulating ancillary service market will be integrated into the spot market eventually, and we expect that spot markets will contribute significantly to our power system flexibility.
Yu Jun & Du Shibo
Dispatching and Control Center of Northeast Branch of State Grid Corporation of China