Chapter 6
Wind Power Peak-Valley Regulation and Frequency Control Technology
Kun Ding, and Jing Zhi
Abstract
This chapter introduces wind power's demand for peak-valley regulation and frequency control and suggests several measures such as utilization of thermal power generator, energy storage, and demand response.
Keywords
Demand response; Demand-side management; Energy storage; Frequency control; Measure; Peak-valley regulation; Thermal power generator6.1. Peak-Valley Regulation and Frequency Control Measures Adopted by Large-Scale Wind Power Bases
The connection of Jiuquan Wind Power Base with the power grid can be described simply in Figure 6.1. It can be seen from the figure that relevant peak-valley regulation and frequency control measures can be classified into the following three aspects: (1) reducing the peak-valley regulation and frequency control demand of wind power; (2) strengthening peak-valley regulation and frequency control power source construction and application; and (3) reducing the demand of peak-valley regulation and frequency control for load or even using load to regulate peak load and control frequency for wind power.
6.1.1. Reducing Peak-Valley Regulation and Frequency Control Demand of Wind Power
There are three main measures for reducing peak-valley regulation and frequency control demand: (1) improving the performance of wind turbines and strengthening wind farm monitoring and management; (2) strengthening wind power forecasting system construction and improving forecasting accuracy; and (3) encouraging wind farms to prepare peak-valley regulation and frequency control power sources by themselves.
6.1.1.1. Improve the performance of wind turbines and strengthening wind farm monitoring and management
The primary means of reducing the peak-valley regulation and frequency control demand of wind power bases is to improve the performance of wind turbines and strengthen wind farm active power control and management. Existing wind farm integration regulations such as Technical Regulations on Connecting Wind Farms with Power Grid include relevant technical requirements for wind farm active power control.
Most foreign wind farms have the capability of active power regulation and can accept the instruction of the power dispatching department to take part in secondary frequency control of the power grid. For example, with an installed capacity of 160 MW, the Horns Rev Offshore Wind Farm in Denmark has eighty 2-MW wind turbines. Installed with the control system produced by Vestas, the wind farm can continuously regulate and smoothly control its power through effective control of wind turbines and other electric equipment within it.
The wind farm monitoring and management is mainly realized through the wind farm comprehensive control system. The wind farm comprehensive control system has input signals including dispatch instructions, wind speed and active power, and reactive power and voltage of points of common coupling, and its control objective is to keep the active power, reactive power, and voltage of the wind farm changing within the reasonable range. The architectural structure of the system is shown in Figure 6.2. Normally the power grid regulates the automatic generation control device of some frequency power plants based on the wind farm's output power in order to keep the power balance of the system. In emergencies, the dispatching center issues instructions to the wind farm based on the operation of the power grid and makes requirements for the active power and reactive power of the wind farm. The wind farm identifies the power output based on wind speed and voltage and issues instructions to all wind turbines.
6.1.1.2. Strengthen forecasting system construction and improve forecasting accuracy
The significance of the wind power forecasting system mainly lies in: (1) Optimizing the generation of conventional wind turbines based on the forecasted wind farm generation curve to reduce operation cost. When the wind power enters the electricity market, the forecasted wind power of the wind farm is involved in the market bidding. (2) Mastering the wind power generation change laws can reduce uncertainty and enhance the security and reliability of the system. (3) Wind farm power forecasting is conducive to its operation and maintenance—the wind farm can choose maintenance downtime when the wind speed is low to reduce its power loss; it can take preventive and protective measures in advance when there is the possibility of strong winds that can damage the wind farm; it can adjust its operation in time to reduce internal loss and improve the integrated wind power quality.
6.1.1.3. Encourage wind farm to prepare peak-valley regulation and frequency control power sources
The most available means of the wind farm preparing peak-valley regulation and frequency control power sources is energy storage facilities. Presently, there are four energy storage methods: (1) mechanical energy storage including flywheel energy storage, pumped storage, and compressed air energy storage; (2) electromagnetic energy storage including capacitor energy storage and superconducting energy storage; (3) chemical energy storage including battery energy storage, fuel cell energy storage, and supercapacitor energy storage; and (4) phase change energy storage such as ice energy storage. The pumped storage is a traditional energy storage method.
6.1.2. Construct or Strengthen the Use of Peak-Valley Regulation and Frequency Control Power Sources
6.1.2.1. Construct on-site supporting peak-valley regulation and frequency control power sources
Constructing on-site supporting peak-valley regulation and frequency control power sources refers to building a pumped storage and thermal power generator system in the Hexi Corridor area.
6.1.2.1.1. Pumped storage
In the Hexi Corridor area in Gansu province, sites suitable for constructing pumped storage power stations include Dalong Mountain and Changshiti River in the Heihe River basin and Liugouxia area in the Shule River basin in Jiuquan (Table 6.1). These three sites have the following characteristics: (1) the largest total construction scale of these three sites is weekly regulation of 1200 MW and daily regulation of 3200 MW, and they are unqualified for constructing seasonal pumped storage power stations; (2) the total regulating storage capacity of these three sites is 20.92 GWh, and the designed full power utilization hours are 5–6 h; (3) they are far from wind power bases; (4) Dalong Mountain and Liugouxia are located in the experimental area of the national nature reserve and the project development needs to be approved by relevant departments; and (5) the total annual evaporation of the three sites reaches 19.62 million m3.
Table 6.1
Main Parameters of Pumped Storage Power Station Sites in the Hexi Corridor Area in Gansu
| Main Parameters | Pumped Storage Power Station | ||
| Dalong Mountain | Changshiti River | Liugouxia | |
| Type | Daily regulation pumped storage power station | Daily regulation pumped storage power station | Daily regulation pumped storage power station |
| Installed capacity (10 MW) | 120 | 120 | 80 |
| Regulating storage capacity (10,000 m3) | 612 | 763 | 717 |
| Regulating storage capacity (GWh) | 7.33 | 7.72 | 4.26 |
| Average water head (m) | 587 | 496 | 291 |
| Distance and height ratio | 6.21 | 12 | 6.17 |
| Construction term (month) | 72 | 72 | 67 |
| Static investment (yuan/kW) | 3857 | 4307 | 5013 |
| Linear distance from the wind farm cluster (km) | About 300 from Yumen | About 300 from Yumen | About 80 from Yumen |
| About 450 from Guazhou | About 450 from Guazhou | About 200 from Guazhou | |
Due to the limited installed capacity, pumped storage power stations alone cannot regulate wind power regulation. However, with the existing power grid conditions, in extreme cases pumped storage power stations with reasonable capacity can greatly relieve the power grid of regulation and transmission pressure. In addition, they have many functions including providing powerful reactive power support and serving as a black start power source, which is very conducive to improving the regulating performance of the whole power grid.
6.1.2.1.2. Coal-fired thermal power generator system
At present both in China and other foreign countries the minimum stable load without auxiliary fuel support of a supercritical coal-fired thermal power generator system is mainly 30–40%; the depth of peak-valley regulation can reach 50% and above; and the generation regulation rate is about 3–5% per minute. Supercritical power generator systems produced in China adopt the composite sliding pressure operation mode and can meet the requirement for bearing periodic load of the power grid. Since the peak-valley regulation depth of thermal power generator systems exceeds 50%, installing thermal power generator systems with 1.6–2 times of installed wind power capacity can meet the peak-valley regulation demand when wind power fluctuates.
The generation regulation rate of coal-fired thermal power generator systems is also affected by the load level of the thermal power generator systems. The generation regulation rate of turbines produced by the three largest turbine manufacturers in China—Shanghai Turbine Plant, Harbin Turbine Co., Ltd, and Dongfang Turbine Co., Ltd—is within the reasonable peak-valley regulation range of thermal power generator systems (50–100%) and the generation regulation rate can reach 5%. When the generation ranges from 30% to 50%, the generation regulation rate can reach 3%.
Thermal power generator systems can adopt the method of combining primary frequency regulation with secondary and tertiary frequency regulation to regulate the peak load of thermal power. The wind power generation change in a small time range can be automatically regulated through the primary frequency regulation of thermal power generator systems. Wind power generation change in a wider time range can be regulated through secondary and tertiary frequency regulation.
6.1.2.1.3. Gas turbine
Gas turbine generator systems have the following technical characteristics: (1) with quick start and good mobility small generator systems can usually change from cold start to full load operation within 15 s to 2 min, and 50 MW generator systems need only 5–8 min; (2) with reliable operation and sound economy, the efficiency of generator systems can be maintained at a high level within a large generation range. Due to the above characteristics, gas turbine generator systems regulate peak load and serve as emergency backup in the power system. If the capacity of the gas turbine generator system accounts for 15–20% of the total capacity of the power system, then it can generally meet the peak-valley regulation demand of the power system so that generator systems bearing basic load in the power system can maintain economic operation for a long time.
6.1.2.2. Make full use of peak-valley regulation and frequency control power sources in Gansu Power Grid
Strengthen the management of thermal power generator systems
When the peak-valley regulation in the power grid is difficult, thermal power generator systems participate in load regulation with 50% and even more of their capacity. In recent years thermal power generator systems that have entered production in the Gansu Power Grid are mainly 300-MW and 600-MW supercritical and ultra-supercritical thermal power generator systems. The minimum stable load of supercritical thermal power generator systems can be as low as 30–40% of the rated power. Compared with the existing thermal power generator systems, there is a lot of room for improvement in terms of peak-valley regulation capability.
6.1.2.2.1. Give full play to hydropower peak-valley regulation capability
Reasonable arrangement of the start-up mode of hydropower and thermal power generator systems in the power grid can give full play to the peak-valley regulation capability of the hydropower and thermal power generator systems.
1. The installed hydropower capacity in Gansu Power Grid is mainly concentrated in main streams of the Yellow River, Bailong River, and Tao River, which is a tributary of the Yellow River. Liujiaxia Hydropower Station, Bikou Hydropower Station, and Jiudianxia Hydropower Station with relatively large storage capacity and peak-valley regulation capability are located on the above-mentioned three rivers, respectively. Located on the lower reaches of Bailong River, Bikou Power Plant cannot give full play to cascade hydropower optimization and dispatching. However, hydropower stations on the main streams of the Yellow River can rely on the Liujiaxia Power Plant and large reservoirs in Qinghai for cascade hydropower optimization and dispatching and greatly improving their peak-valley regulation capability.
2. The peak-valley regulation capability of hydropower generator systems is better than that of thermal power generator systems. In reasonably arranging the start-up mode of hydropower and thermal power generator systems, we should pay more attention to giving full play to the peak regulation capability of hydropower in preventing hydropower generator systems from bearing the base load.
6.1.2.2.2. Strengthen the management of direct supply generator systems and captive power plants
Included in the regulation and management range of Gansu Power Grid are thermal power generator systems and captive power plants providing direct power supply for key accounts. These thermal power generator systems usually keep operating at a high load rate and have limited peak-valley regulation capability. In 2010, the annual direct power supply for key accounts in Gansu was about 4.5 billion KWh, the equivalent of the power produced by two 300-MW generator systems operating at a high load rate. However, with the decrease of the start-up capacity of thermal power generator systems not providing direct power supply for key accounts, the total peak-valley regulation capacity of thermal power generator systems in Gansu Power Grid decreased. In the future, these thermal power generator systems must participate in the system peak-valley regulation operation to reduce the peak-valley regulation pressure of the whole Gansu Power Grid.
6.1.2.2.3. Improve wind power dispatching technical level
In October 2009, State Grid Gansu Electric Power Company decided to initiate a project to develop an active power intelligent control system for large clusters of wind power in the power grid, aiming to provide more wind power and ensure the stability of the power grid. This system went live on March 12, 2010. The development of an active power intelligent control system for large clusters of wind power can maximize power generation in wind farms under normal operation based on the architecture of Gansu Power Grid and the current actual wind power operation and on the condition of ensuring the reliable operation of the power grid in various modes of operation and in cases of fault. In addition, it ensures the minimization and optimization of wind turbines when accidents happen to the power grid. It can not only improve the power transmission capability of the power grid but also ensure the generation of wind farms and realize the goal of making full use of wind power resources.
6.1.2.3. Make full use of the peak-valley regulation and frequency control power sources in the Northwest China grid
It is insufficient to only rely on the peak-valley regulation and frequency control power sources in Gansu Power Grid; it is also necessary to rely on Northwest China Grid for assisting with peak-valley regulation based on further optimization of the hydropower and thermal power operation mode in Gansu, exploitation of hydropower and thermal power peak-valley regulation potentials, and enhancement of thermal power peak-valley regulation.
Due to the great fluctuation of the peak-valley difference, Shaanxi Power Grid has a great demand for peak-valley regulation capability. In addition, since thermal power is its major power source, it has a small peak-valley regulation margin and is weak in peak-valley regulation. Compared with Shaanxi Power Grid, Ningxia Power Grid has a smaller demand for peak-valley regulation and a stronger peak-valley regulation capability. However, with the development of wind power and solar power in Ningxia, in the future it will also face the problem of insufficient peak-valley regulation capability. Hami Wind Power Base in Xinjiang has a huge demand for peak-valley regulation. As a result, its peak-valley regulation capability is also inadequate. In Qinghai the hydropower and thermal power generator systems account for a large proportion and the peak-valley load difference is quite small. Although large-scale solar power generation has been planned, due to technical limits, Qinghai Power Grid will still have limited installed capacity in the short term. As a result, Qinghai has a large surplus of peak-valley regulation capacity and is suitable for being used as the peak-valley regulation power source of wind power in Gansu.
6.1.2.4. Consider making use of transregional peak-valley regulation power sources
The Chinese central government has already approved the Jiuquan-Zhuzhou ±800 kV UHVDC (ultra-high-voltage direct current) transmission project. Although the DC interconnection tie line cannot frequently adjust transmission power, it is technically feasible to make use of transregional peak-valley regulation power sources. Due to its huge installed hydropower capacity, it is possible for Central China Grid to provide peak-valley regulation services for transmission power grids. Since in Central China the peak-valley regulation capability is greatly weakened in the flood season, it is better to regulate peak load in the drought period.
6.1.3. Reduce Peak-Valley Regulation and Frequency Control Demand or Use Load to Regulate Peak Load and Control Frequency
We can reduce the peak-valley regulation and frequency control demand through the demand-side management and even use load to regulate the peak load and frequency of fluctuating power sources to improve the power grid's overall peak-valley regulation and frequency control capability and enhance its acceptance of fluctuating power supply.
6.1.3.1. Locally construct load that can bear fluctuating power supply
Locally constructing a load that can bear fluctuating power sources, or directly supply wind power, is also one kind of side demand management. With the special characteristics, the peak-valley regulation and frequency control of fluctuating power supply will be discussed in detail below.
Jiuquan Wind Power Base can directly supply power for environmental protection industries such as garbage disposal and sewage treatment and high energy-consuming industries such as electrolytic aluminum and chlor-alkali chemical industry. In this way, the power generated can be consumed locally. Unstable wind power current can only make the output of industries such as garbage disposal, sewage treatment, electrolytic aluminum and chlor-alkali chemical industry fluctuate greatly, but it does not affect the effect of electrolysis. Through sewage treatment the precious water resources in Jiuquan can be recycled and reused. In this way we can protect the fragile ecological environment in the Hexi Corridor area and build our green homeland with green energy. Industries such as electrolytic aluminum and chlor-alkali chemical industry are traditional advantage industries of Gansu province. When the wind power cannot be totally sent out, by constructing industries such as electrolytic aluminum in Jiuquan we can achieve a win–win solution for both economic development and wind power accommodation.
6.1.3.2. Demand-side management
Demand-side management (DSM) refers to the management activities of adopting effective incentive and guidance measures to improve the power-consuming efficiency of terminals and change the power-consuming modes and reduce power consumption and power demand in meeting the same power consumption functions. This can be done with the support of relevant laws, regulations, and policies made by the government and through the joint efforts of power generation companies, power grids, energy service companies, social intermediary organizations, product suppliers, and electricity consumers so as to save resources, protect the environment, achieve the best social benefits, benefit all parties, and minimize energy service costs.
DSM has become an advanced international energy management activity and an important means for developed countries to implement sustainable development strategy. It has been successfully implemented in more than 30 countries and regions including France, Germany, Korea, the United States, and Canada and attracted increasing attention. The report issued by International Energy Agency (IEA) in 2004 indicates that since the oil crisis developed, countries have managed to reduce energy consumption per unit GDP by about 50% by taking various measures, including DSM. For example, in 2000 the per capita energy consumption is almost identical to that in 1973, but the per capita GDP increased by 74%. France has increased its daily load rate from 73% to about 85% by taking DSM measures such as electric power load monitoring, which greatly reduces peak-valley regulation demand and accordingly reduces power generation capacity by 19,000 MW. Britain has raised and invested $165 million in more than 500 projects aimed to improve energy efficiency. In this way it has saved 6.8 billion kWh power, the equivalent of annual power consumed by 2 million families.
In the Pennsylvania–New Jersey–Maryland (PJM) electricity market in the United States, where the power supply is mostly with 45,000 MW, the time when power demand is with 75,000 MW is accounted for 2% of the annual time (less than 200 h). If consumers participate in the demand response in this period of time, this can reduce the construction of power plants by 15%. According to Baltimore Gas and Electric (BGE), the price of demand response resources is $165 per kW, which is one-fourth to one-third lower than the cost of newly constructed peak load power generation.
6.2. Thermal Power Generator System In-depth and Rapid Peak-Valley Regulation Technology
6.2.1. Thermal Power Generator System In-depth Peak-Valley Regulation
The in-depth peak-valley regulation performance of thermal power generator systems can be improved in the following three aspects:
1. For distributed control systems (DCS), make full use of DCS resources, further optimize its control functions, and tap the system functions to adapt to the peak-valley regulation requirements.
3. For thermal power generator systems participating in peak-valley regulation, develop and use more security monitoring systems to provide comprehensive means of monitoring and protecting thermal power generator systems.
The in-depth peak-valley regulation of thermal power generator systems is a kind of work requiring all-around efforts. In analyzing and studying the security and economic efficiency of primary equipment, we should improve the thermal power control system, enhance technical transformation, replace old equipment or equipment that needs to be eliminated with advanced control systems, and update peripheral detection and execution devices improper for in-depth peak-valley regulation so that thermal power generator systems can participate in power grid peak-valley regulation in a safe, stable, and economic way.
6.2.2. Rapid Peak-Valley Regulation Technology of Thermal Power Generator Systems
Regarding the current situation in which thermal power generator systems respond to load slowly and have large pure time delays in automatic generation control (AGC), optimized control plans based on intelligent judgment and decoupling control can be adopted.
For large-capacity thermal power generator systems used in the direct-fired pulverizing system, there exists large pure time delay from changing coal quantity to steam flow. For 300-MW thermal power generator systems, the time delay generally ranges from 1.0 to 2.5 min while for 600-MW thermal power generator systems, it will be longer. In addition, when thermal power generator systems are in sliding pressure operation, the main steam pressure also changes. It will take longer time for the variation of steam flow to accumulate to a certain amount so that it can enable the main steam pressure to effectively change. Therefore, in the conventional coordination and control mode, in order to prevent the main steam pressure from deviating in the opposite direction, we have to conduct delayed time processing on turbine instructions and loosen the allowable range of steam pressure controlling deviation so that thermal power generator systems are in the slow uncontrolled state.
Shown in Figure 6.3 is the process of adjusting all major parameters in the conventional coordination and control mode. Due to the impact of the ④ valve action, the characteristics of ② the main steam pressure object become complicated and the conventional proportional integrate differential (PID) cannot normally realize its regulation functions. The steam pressure object itself is a great time delay link, and in the regulation process its deviation with the set value ⑥ is quite large. As a result, it has poor adaptability to the complicated operation situation in the AGC mode, with frequent changes of load instructions. In addition, the greatest drawback of the mode lies in that its load and the steam pressure response are synchronous, and the steam pressure's demand for and rejection of heat when load variations are also always homodromous. In the initial stage of changes, they restrict each other and produce a great time delay, and then approaching the target load they encourage each other and cause a large overshoot. In this mode the system has difficulties in tuning and poor regulation quality. Therefore, it cannot meet the sliding pressure control requirements.
However, when the amount of fuel makes rapid changes approximating to step changes, the steam pressure response time will be greatly reduced. The reason lies in that when the instantaneous change of the amount of fuel reaches a certain quantity, the heat change in the boiler exceeds the regenerative capacity of the furnace body. At this time, the redundant variable quantity can rapidly transform into the change of the amount of steam and accelerate the steam pressure response speed. However, in order to reduce the impact of the great change of the fuel amount on the air and flue gas system, the rate limit should be set after this instruction exceeds the former amount. Further study findings indicate that the size of this rapid variation is linearly correlated with the loading rate (valve movement speed) and functional relation with the load variation, which makes the practical application of the plan in the changeable working conditions possible. Due to the accelerated steam pressure response, in the initial stage of the load instruction change the valve can move quickly to change the load (see Figure 6.4 ④). If the fuel in the boiler can continue to maintain a certain surplus amount, the steam pressure will not produce a great negative deviation. The rapid load response in the initial stage of the instruction changes can be achieved by reasonably constructing the instruction feedforward model of the boiler main control room.
Figure 6.3 Regulation process of main parameters in the conventional coordination and control mode. ① load; ② main steam pressure; ③ boiler instructions; ④ turbine instructions; ⑤ load instructions; ⑥ total steam pressure value.
Due to the continuous change of the load, the valve needs to be changed constantly. As a result, it is difficult for the steam pressure to produce positive changes. If the steam pressure set value curve in the conventional control is still adopted, then the large deviation will still have an uncertain impact on the fuel amount control. Therefore, it is suggested to conduct a decoupling process on the two functional components of supporting the load and regulating the steam pressure in the boiler main control room. Take increasing load as an example. As shown in Figure 6.4, on the one hand, use the feedforward model of the boiler main control room instruction ③ to rapidly and accurately provide heat support based on load instruction demand; on the other hand, fit its approximate curve based on the response characteristics of the main steam pressure in the coordinating working conditions as the main steam pressure set value model ⑥ so that the PID functions of the controller can finish the accurate control of the main steam pressure. The application of this control idea solves the conflict between the load ① and the demand of the main steam pressure response for heat ② in time. In addition, it can effectively control the overshoot and improve the controllability of the steam pressure so that the turbine and the boiler can formulate a kind of coordination and complementary relationship in the real sense.
6.3. Energy Storage Technologies
In recent decades the energy, transportation, electric power, and telecommunications departments of all countries have attached great importance to the research and development of energy storage technologies. Energy storage technologies have been regarded as an important component in the power grid operation. The introduction of energy storage into the power system can effectively achieve demand-side management. In this way we can not only effectively make use of power equipment and reduce power supply cost but also promote the application of the renewable resources and use it as a means to improve the stability of the system operation, regulate frequency, and compensate load fluctuation.
6.3.1. Introduction of Various Energy Storage Technologies
6.3.1.1. Pumped storage
Pumped storage technology can pump water from the lower pool reservoir to the upper pool reservoir in the power load trough so that electrical energy can be transformed into gravitational potential energy and stored, and the water in the upper pool reservoir can be released to generate power in the power load peak period. The release time of pumped storage ranges from several hours to a few days, and its comprehensive efficiency is 70–80%. It is mainly used to regulate peak load, fill valley load, control frequency, modulate phases, and serve as emergency power supply. The construction of pumped storage power stations is restricted by the terrain. When the power station is far away from the electricity consumption area, the power transmission loss is great. Currently the pumped storage power stations worldwide have a total installed capacity of 90,000 MW, accounting for about 3% of the global installed power generating capacity. It is predicted that by 2020 within the business area of the State Grid Corporation of China the pumped storage capacity will reach 26,920 MW.
6.3.1.2. Compressed air energy storage
Compressed air energy storage technology can use electrical power to compress air in the power load trough so that it can be stored in abandoned mines, sunk in undersea gas tanks, caves, expired oil and gas wells or new gas storage wells, and released in the power load peak period to promote turbines to generate power. The earliest commercially operated compressed air energy storage system was constructed in Germany in 1978 and had an installed capacity of 290 MW. Another successful compressed air storage system was built in Alabama, USA, in 1991; it can store compressed air in the abandoned salt mines 450 m underground and provide compressed air for a 110-MW turbine for 26 consecutive hours. The construction of compressed air energy storage power stations is restricted by terrain and has special requirements for the geological structure. At present with the development of the distributed power system, an 8–12 MW miniature compressed air energy storage system has attracted increased attention.
6.3.1.3. Flywheel energy storage
Flywheel energy storage uses electric motors to drive the flywheel to rotate at a high speed so that the electrical power is transformed into mechanical power and stored, and when necessary, flywheels drive generators to generate power. The flywheel system operates in the high vacuum environment. Characterized by no friction loss, small wind resistance, long life, no impact on the environment, and needing no maintenance, this flywheel system is applicable to power grid frequency modulation and power quality guarantee. However, it also has some shortcomings such as low energy density and the high cost of ensuring the system's security. Its advantages cannot be manifested on a small scale. At present it is mainly used to supplement the battery system.
6.3.1.4. Sodium-sulfur cell
When a sodium-sulfur cell works under the environment of a high temperature of 300 °C, its positive electrode active material is liquid metal sulfur (S) while its negative electrode active material is liquid metal sodium (Na). In between is the perforated ceramic diaphragm. The sodium-sulfur cell is mainly characterized by high energy density (three times as high as that of a lead storage battery), high charge efficiency (reaching 80%), and longer cycle life than a lead storage battery. However, while a sodium-sulfur cell is working, a high temperature needs to be maintained, which presents a certain hidden danger. Tokyo Electric Power Company is an international leader in the development of sodium-sulfur cell systems. In 2004, it installed the then-largest sodium-sulfur cell system in the world with a capacity of 9.6 MW/57.6 MWh in the automation system plant of the Hitachi company.
6.3.1.5. Liquid flow battery
The active substance of a liquid flow battery can be dissolved and put in two large storage tanks. When the solution flows through the liquid flow battery, reduction and oxidation reaction occur, respectively, on the electrode on both sides of the ion exchange membrane. This chemical reaction is reversible, so the battery has the capacity to charge and discharge repeatedly. The storage capacity of this system is decided by the electrolyte volume of the storage tank while its output power is determined by the reaction area of the battery. Since the electrolyte volume of the storage tank and the reaction area of the battery can be designed independently, the design of this system is very flexible and less restricted by the set field. Liquid flow batteries are composed of several systems including all vanadium, vanadium bromide, and sodium polysulfide/bromide. This battery has small electrochemical polarization. The all-vanadium redox flow battery has advantages including high energy efficiency, large storage capacity, 100% depth of discharge, quick charge and discharge, and long life. It is already in commercial operation and can effectively smooth wind power generation power. The 4-MW all-vanadium redox flow battery operated in Japan provides local 32-MW wind farms with energy storage and has run 270,000 cycles. No other energy storage technology in the world can reach this requirement.
6.3.1.6. Lithium-ion battery
The cathode material of the lithium-ion battery is lithium metallic oxide characterized by high energy density and boasting advantages such as stable discharge voltage, wide range of working temperature, low self-discharge rate, long storage life, no memory effect, and no pollution. However, at present there are some problems with the production of large-sized lithium-ion batteries such as high requirements for special package of excessive charging control and high price. As a result, currently lithium-ion batteries cannot be universally used. At this time, the largest lithium-ion battery system in the world has an installed capacity of 2 MW, constructed by A123 Systems.
6.3.1.7. Lead-acid cell
The lead-acid cell is a kind of acid accumulator using dilute sulfuric acid as electrolyte and lead dioxide and fluffy lead as the anode and cathode of the battery, respectively. Characterized by low cost, mature technology, and large energy storage capacity, it is mainly applied in power system standby capacity, frequency control, and constant power system. Its disadvantages include low energy density, fewer charge and discharge times, and posting pollution in the manufacturing process. DEDO in Japan once funded a demonstration project combining lead acid battery and photovoltaic power generation. The total energy storage capacity of the lead-acid cell's energy storage system is 4.95 MW.
6.3.1.8. Nickel-cadmium cell
Nickel-cadmium cells are characterized by the capacity for more than 500 times of charge and discharge, being economical and durable, longer life than lead-acid cells, and small internal resistance. Since the nickel-cadmium cell can realize quick charge and provide a large current for load and has small voltage change in discharging, it is an ideal DC power supply battery. Its shortcomings lie in its memory effect, a shortage of cadmium materials, and high price. The configuration of the nickel-cadmium cell system currently operated in Golden Valley, Alaska, is 4 MW × 15 min.
6.3.1.9. Supercapacitor
The supercapacitors are developed and manufactured based on the electric double-layer theory. It can provide powerful pulse power. When charging, in the surface of the electrode of the ideal polarization state the charge will attract the opposite ions in the surrounding electrolyte solution and enable them to be attached to the electrode surface, forming the double layer that constitutes the electric double-layer capacitor. Through three generations of upgrading and decades of development, there have emerged a series of supercapacitor products with the capacity ranging from 0.5 to 1000 F, the working voltage from 12 to 400 V, and the maximum discharge current from 400 to 2000 A. The maximum stored energy of the storage system can reach 30 MJ. However, supercapacity is quite expensive, and in the power system it is mainly used to smooth load of short time and high power and for the peak power of the electric energy quality such as the start-up support for high-power DC motors and transient recovery voltage. It can enhance the level of electricity supply when the voltage drops and during the transient interference.
6.3.1.10. Superconducting energy storage
Superconducting energy storage uses superconductors to make coils for magnetic energy storage, and in power delivery there is no need to convert the energy form. It has advantages such as fast response (millisecond), high conversion efficiency (≥96%), and high specific capacity (1 ∼ 10 Wh/kg)/specific power (104 ∼ 105 kW/kg), and can realize real-time energy exchange and power compensation with the large capacity in the power system. At present, 1 ∼ 5-MJ/MW low-temperature superconducting energy storage devices have formed products. 100-MJ of superconducting energy storage is already operating in the high voltage transmission grid while 5-GWh superconducting energy storage has already passed feasibility analysis and technical demonstration. Superconducting energy storage can fully meet the requirements of the transmission and distribution power grids for voltage support, power compensation, frequency control, and improvement of system stability and power transmission capacity. Compared with other energy storage technologies, superconducting energy storage is still very expensive. In addition to the cost of superconductors, the cost arising from the improved repair frequency due to the maintenance of the low temperature of the system is quite considerable. Worldwide there are a significant number of superconducting magnetic energy storage projects under construction or in the development phase.
The capacity of the superconducting storage system ranges from tens of kilowatts to several hundred megawatts; its discharge time has a long span ranging from milliseconds to hours; it has a wide range of applications covering power generation, transmission, transformation, distribution, and consumption (Table 6.2, Figure 6.5).
Currently the development of energy storage technologies must meet the following requirements: high energy storage density, low conversion cost, low operating cost, easy to maintain, does not pollute the environment. Large-scale energy storage technologies are the revolutionary breakthroughs in the traditional power mode of “instant consumption of generated power.” They can reduce the investment in the generating equipment and improve the utilization rate of power equipment. When installed near electric equipment, they can reduce line loss; when installed near large cities, they can improve the reliability of power supply. Studies of energy storage technologies have been the research focus both at home and abroad, and the range of study covers energy storage materials and components, engineering equipment, power electronic devices, and energy storage devices. Breakthroughs will be constantly achieved in this field.
Table 6.2
Comprehensive Comparison of Various Energy Storage Technologies
| Energy Storage Technologies | Advantages | Disadvantages | Power Application | Energy Application |
| Pumped storage | Large capacity and low cost | Special site requirements | ∗ | ∗∗∗∗ |
| Compressed air energy storage | Large capacity and low cost | Special site requirements and gas required | ∗ | ∗∗∗∗ |
| Liquid-flow battery | Large capacity | Low energy density | ∗∗∗ | ∗∗∗∗ |
| Metal-air battery | High energy density | Having difficulty in charging | ∗ | ∗∗∗∗ |
| Sodium-sulfur cell | Large capacity, high energy density, and high efficiency | High manufacturing cost | ∗∗∗∗ | ∗∗∗∗ |
| Security concerns | Large capacity, high energy density, and high efficiency | High manufacturing cost and special charging circuit required | ∗∗∗∗ | ∗∗ |
| Nickel-cadmium cell | Large capacity and high efficiency | Low energy density | ∗∗∗ | ∗∗∗ |
| Other advanced batteries | Large capacity, high energy density, and high efficiency | High manufacturing cost | ∗∗∗∗ | ∗∗ |
| Lead-acid cell | Low investment | Short life | ∗∗∗∗ | ∗∗ |
| Flywheel energy storage | Large capacity | Low energy density | ∗∗∗∗ | ∗ |
| Superconducting magnetic energy storage | Large capacity | High manufacturing cost and low energy density | ∗∗∗∗ | ∗ |
| Supercapacitor | Long life and high efficiency | Low energy density | ∗∗∗∗ | ∗∗∗ |
Energy Storage Association.
6.3.2. The Role of Energy Storage Technologies in Improving the Power Grid Peak-Valley Regulation and Power Quality
6.3.2.1. The role of energy storage technologies in improving power grid peak-valley regulation
Energy storage power stations in various forms can charge by obtaining power from the power grid as load in the power grid load valley and then transmit power to the power grid by changing to operate in the generator mode in the power grid load peak. This mode is conducive to reducing the loss of the power system's power transmission network and peak load shifting, thus obtaining economic benefits. Compared with conventional generators and gas turbines, this mode has great cost advantages because it can use power in the power grid load valley and reduce the power consumption cost.
Some renewable energy distributed generation systems are greatly affected by environmental factors. As a result, it is impossible to make specific generation scheduling. If energy storage equipment devices are equipped, then the power grid can supply needed power in the specific time without considering the power capacity of the power generation units at this time. It only needs to generate power according to the previous generation scheduling.
6.3.2.2. The role of energy storage technologies in improving power grid power quality
Power quality is not only of great significance to the safe and economic operation of the power grid but also directly decides whether the user-side equipment can work normally. The application of the energy storage system in improving the power quality mainly focuses on reducing voltage fluctuations and voltage sag (Figure 6.6).
The energy storage system using DSTATCOM/BESS to improve the power quality can realize rapid active and reactive power exchange with the system and effectively improve voltage fluctuations, voltage sag, and voltage and current waveform distortion and flicker. It is applicable to solving the power quality problem brought by wind power integration. The series-parallel hybrid compensation scheme of supercapacitors realizes power exchange between supercapacitors and the system through parallel system in order to smooth the wind power output and effectively improves the reliability of the power supply voltage through the series system in order to inhibit voltage sag.
Energy storage technologies combined with advanced power electric technologies can help reduce the power grid harmonic distortion rate and eliminate the influence of voltage sag and surge current. Since improving power quality is the dynamic compensation of the short-time power, the energy storage system needs to have the capability of regulating millisecond dynamic power. As a result, it is suitable for selecting supercapacitor energy storage, superconductive magnetic energy storage, and battery energy storage systems.
6.4. Demand Response
6.4.1. Concept and Main Content of Demand Response
Demand response (DS) has developed based on the demand-side management. It refers to consumers changing their consumption behavior to enable the wholesale electricity market to have the price flexibility so as to reduce the peak load, improve the reliability of the system, reduce the overall cost of the system, improve the market efficiency, prevent market members from manipulating the market, and enable market participant to avoid the risk of system security and price fluctuation.
Demand response should include load response and price response. Load response refers to consumers reducing their demand in the system peak load or in case of emergency of the system including direct control of the load to reduce load or completely interrupt load. Price response refers to the price change in the wholesale electricity market transmitting to consumers who adjust their demand for electricity based on the electricity price change including real-time (dynamic) pricing, time-of-use electricity price, and request for quotation or repurchase. Request for quotation refers to the market participant giving a lowest quotation for reducing their demand and the quantity of demand to be reduced.
6.4.1.1. Demand response operation
Load response is mostly regulated through contracts. The participant signs contracts with the power system operator beforehand. In case of a system emergency, power is cut or completely interrupted based on the requirements of the power system operator, and ensured compensation is obtained according to the standard of the electricity settlement price in the market. In order to encourage participation in the response plan, a lower limit is usually set for the compensation standard. If the demand for electricity is not reduced, then the participant will be punished. As a typical load response, interruptible load is according to the contract signed by the power supplier and customers, and during the peak load of the power grid customers respond to the power grid dispatching instructions and carry out short-time interruption of designated load equipment in the specified time in order to shift the power grid load peak and ensure the supply and demand balance of the power grid; according to the economic loss generated during the active blackout, customers obtain appropriate compensation electricity load with the compensated rate being referred to the interruptible price. As the comprehensive application of demand response technical means, economic means and policy means, interruptible load is particularly applicable to the “plastic load” in the industry, commerce and service industry in which the power supply reliability requirements can be relaxed. For example, operational procedures can be adjusted through the process adjustment, or the peak consumer load can be avoided through the utilization of stored energy. Since interruption compensation is made through electricity price concession, consumers are willing to use reduced electricity cost to lower the limited power consumption reliability. Some power system operators also regard interruptible load as a part of ancillary services offered to maintain system security and reliable operation, which is called emergency service. For example, it is stipulated in the North American Electric Reliability Council's operation policy that interruptible load is one kind of emergency service.
6.4.1.2. Demand response operators
The demand response operators include independent system operator (ISO), load service enterprise, and utility distribution company. Currently there appears a new demand response operator, curtailment service provider, which makes the minimum requirement for participating in demand response plan based on ISO demand response plan (for example, interrupting 1 MW) and reduces the load of aggregated small consumers to meet the access requirements of the demand response plan.
6.4.1.3. Demand response implementation principles
The demand response plan has attracted increased attention from the industry. Generally, in making demand response policies the following important principles should be followed:
1. Consumer participation, which means the design of the demand response market should encourage consumers of various types and scales to participate in it;
2. Equal treatment, which means demand response resources with the same scale should enjoy equal status;
3. Sound market, which encourages participants of the demand response plan to establish diverse relations;
4. Timely coordination, which means the operator of the demand response market should provide feedback on the demand response performance and make financial compensation in time;
5. Reasonable price, which means the demand response participants should obtain compensation with fair value;
6. Information security, which means the consumer agreement should be kept secret and only accessible to the supervision department in review;
7. Policy promotion, which means the regulatory institutions should fully cooperate with each other to eliminate the obstacles to the implementation of the demand response plan.
Intelligent demand response refers to conducting optimal management of consumers' power use through advanced communication systems, advanced control decision means, and suitable price and incentive means so as to achieve demand response automation, higher electricity consumer efficiency, more flexible load curve, better power quality, and bidirectional interaction with the supply side.
6.4.2. Present Development Situation of Demand-side Response at Home and Abroad
The demand response plans implemented by Independent System Operator-New England (ISO-NE), the operator in the US New England region power system (the six states of Maine, New Hampshire, Vermont, Massachusetts, Connecticut, and Rhode Island), are divided into two categories: one is the load response plan and the other is the price response plan. Consumers can only choose one of them. Participants must reduce load by no less than 100 kW but no more than 5 MW. Consumers with distributed generation and emergency generation facilities can also participate in the load response plan. Consumers participating in the load response plan can reduce their power demand based on ISO-NE instructions to improve the reliability of ISO-NE. Meanwhile, they can also get compensation. ISO-NE control center determines the start and end date and time of interruption and sends the notice to consumers participating in the demand response plan. Consumers will interrupt load within 30 min after receiving the instructions issued by the control center, and the interruption usually lasts no more than 2 h. However, in case of undercapacity or a system emergency, the interruption might last longer. On two occasions consumers participating in the demand response plan will be informed to compulsorily interrupt load: one is when the 30 min operating reserve cannot support the voltage decline; the other is in an emergency when the 10 min operating reserve cannot recover within 30 min. If the load is interrupted, consumers will get compensation. From the first occasion consumers get a compensation sum that is equal to the adjusted electricity settlement price (with the lower limit being $100/MW) × interrupted electricity quantity (MWh). Consumers will also get the management cost calculated according to the 30 min operating reserve settlement price. ISO-NE will pay compensation to market participants. If there is any dispute over the reduced load of the consumers, ISO-NE can be required to look into load reduction according to market rules and procedures. ISO-NE or a unit designated by it shall be responsible for researching the load reduction information and any dispute over the compensation shall be solved through the existing dispute settlement procedures of ISO-NE. If during the investigation participants or consumers are suspected of fraud in order to obtain compensation from ISO-NE, ISO-NE shall have the right to prohibit participants or consumers from participating in the load response plan. For consumers who cannot reduce the lower limit of the load response plan, but still want to participate in the plan, they can be aggregated, and the total load after aggregation must exceed 100 kW. The aggregated load must be in the same region. Every aggregated group is regarded as a single entity. ISO-NE also implements “day-ahead demand response programs.” In the day-ahead market (one day ahead) consumers reduce the quotation of load with 1 MW as the range. The quotation has upper and lower limits: the upper limit is $50/MWh; the lower limit is $500/MWh. In the day-ahead market the consumer who agrees to the accepted quotation will be required to interrupt load and be compensated for it (Table 6.3).
The demand response uses technical and economic means to enable electricity consumers to change their power consumption behavior in order to reduce the peak load, improve the system reliability, and reduce the overall cost of the system. Currently, in power demand response practice in foreign countries load response accounts for a larger part. It is the major measure to cut peak load or a part of ancillary services such as system emergency reserve. However, presently load response is changing into price response in order to better reflect the participation of the demand side in the electricity market and its response to the market price change.
Due to the great peak-valley difference in some regions in China, there is a shortage of peak power supply capacity, and short-term power supply varies from period to period. Therefore, we can borrow successful practices from some foreign countries and promote demand response in a planned and step-by-step manner. Our study focuses on the analysis of the basic connotations, functions, economic efficiencies, pricing and implementation suggestions of the demand response.
Table 6.3
Comparison of Interruptible Load Arrangements in Some Countries and Regions
| Country/Region | Category | Contract Type | Advance Notice (Ahead Time) | Minimum Interrupted Load and Time | Compensation Scheme |
| Alberta, Canada | First type | One-month contract | One hour ahead | 1 MW, at most 4 h | There is a fixed price for MW per month, which is irrelevant to the times of interruption. |
| Second type | Two-week contract | One hour ahead | The price per MWh and load is paid only in interruption. | ||
| California, USA | Contract | 30 min ahead | 1 MW, at most 4 h | Monthly reserve capacity is paid; actual transmitted electric energy is paid. | |
| New York, USA | Contract | 10 or 30 min ahead | 1 or 2 MW, at most 1 h | 10 min spinning reserve market price is compensated for the interrupted 1 MW load; the day-ahead market price is compensated for the interrupted 2 MW load. | |
| Taiwan | First type | Contract | One day, one week ahead | 5 MW, 6 h per day | 50% off the contract demand |
| Second type | One day, 4 h or 1 h ahead | For all industrial consumers, each interruption shall not last more than 6 h. | Depending on the time informed in advance. |
6.4.3. Problems Needed to be Solved in Implementing Demand Side Response Mechanism
Using demand response experience of foreign countries as reference and taking into consideration the specific circumstances in the Gansu electricity market, we have concluded that we are going to face the following problems in effectively promoting and implementing the demand response mechanism:
6.4.3.1. Consumer idea as an obstacle
It still takes time for consumers to change the charging method from average electricity price to fluctuating electricity price and formulate the idea of demand bidding. This is a problem that urgently needs to be solved in introducing the response mechanism into the demand side.
6.4.3.2. Handling of benefits obtained by consumers who do not participate in demand response
Demand response is implemented in the form of consumers' voluntary participation. Both the reduction of average electricity price and the improvement of system stability resulting from the demand response are beneficial to the market. Even those consumers who do not participate in the demand response can enjoy the benefits brought by the increase of demand elasticity. According to the principle of fairness, the apportionment of benefits among consumers who do not participate in the demand response must be handled properly.
6.4.3.3. Evaluation of the effect of demand response measures
The effect of the demand response is obtained by contrasting the power consumption modes before and after the implementation of the demand response. Since the load is affected by various factors such as temperature and energy prices, it is quite difficult to find a baseline demand on which to contrast these two modes and obtain an economic benefit evaluation effect.
6.4.3.4. Demand response measurement and information support system
To implement the demand response mechanism, a large initial investment is needed to install corresponding systems. Purely relying on consumers for investment will reduce their enthusiasm about participating in the demand response. As a result, we need to increase the consumer participation rate and reduce the initial investment cost through stimulation in the consumer market and the incentives of the management authorities.
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