4.4.4.2.2. WTGs in constant voltage control
When the coefficient of the proportion and integration links in the PI control on DC side of the controllable HV reactor varies, it will exert some influence over the system stability in the conditions that the Hexi transmission channel is provided with series compensation capacitors and controllable HV reactors, and the Jiuquan-Hexi 750 kV double-circuit transmission line can transmit about 4900 MW power. See Table 4.23.
Table 4.23
Impact of Variation of Proportion and Integration Links Coefficient in the PI Control of the Controllable HV Reactor DC Side on System Stability
| Operating Mode | Proportional Coefficient | Integral Coefficient | Limitation Fault | Stability |
| Summer maximum mode, with serial compensating capacitor and controllable HV reactor | 1.0 (calculated initial value) | 0.1 (calculated initial value) | “Three-permanent” fault on Jiuquan side of the Jiuquan-Hexi 750 kV transmission line | Stable |
| 0.0 | 0.1 (calculated initial value) | |||
| 10.0 | 0.1 (calculated initial value) | |||
| 1.0 (calculated initial value) | 0.01 | |||
| 1.0 (calculated initial value) | 1.0 | |||
| 0.0 | 0.01 | |||
| 10.0 | 1.0 |
Table 4.23 shows that in the PI control on the DC side of the controllable HV reactor, when the coefficient of the integration link stays unchanged and the coefficient of the proportion link increases or decreases by 10X, it will not have obvious negative impact on system stability; when the coefficient of the proportion link stays unchanged and the coefficient of the integration link increases or decreases by 10X, it will not have obvious negative impact on system stability; and when the coefficient of both the proportion link simultaneously increases or decreases by 10X, it will not have obvious negative impact on system stability. As a result, the variation of proportion and integration links coefficient in the PI control of the controllable HV reactor DC side has little impact on system stability.
4.4.4.2.3. WTGs in constant power factor control
When the coefficient of the proportion and integration links in the PI control on DC side of the controllable HV reactor varies, it will exert the same influence as that in case of WTGs in constant voltage control mode in the conditions that the Hexi transmission channel is provided with series compensation capacitors and controllable HV reactors, and the Jiuquan-Hexi 750 kV double-circuit transmission line can transmit about 3710 MW power. See Table 4.23.
4.4.5. Selection of FACTS Control Strategies
4.4.5.1. Voltage-reactive power optimization algorithm
The secondary control objective of the Hexi transmission channel is to realize voltage-reactive power coordination control. In the voltage control of this level, the voltage of the pilot node in the region can be set based on offline experience or by the expert system. As dispatch automation and reactive power optimization algorithm develop, the real-time reactive power optimization has become the development direction of voltage-reactive power control. EMS/SCADA system can offer the data and software platform for real-time reactive power optimization, and the promotion of VOC systems on the plant or substation side has offered hardware foundation for real-time reactive power optimization.
Voltage-reactive power optimization control is mathematically large-scale optimization with nonlinear constraints. At present, the classic reactive power optimization methods include reduced gradient method, sequential quadratic programming, Newton method, linear programming method, mixed integer programming, prime-dual interior point method, etc.
In recent years, to make the reactive power optimization closer to the global optimal value, the following intelligent algorithms have gradually developed: neural network, genetic algorithm, simulated annealing, Tabu search method, expert system, etc. Among them, the genetic algorithm can be easily combined with other numerical optimization methods or artificial intelligence methods to form mixed genetic algorithm, which has been applied to the existing AVC system.
4.4.5.2. Realization of real-time reactive power optimization control strategy
4.4.5.2.1. Neural network control system
Figure 4.52 shows the structure of the control system at the reactive power and voltage network level. Its core is the reactive power and voltage optimization software in real-time operation. The software is mainly designed to work out the settings of the controllable reactive power regulation equipment in the system with system active grid losses as the objective based on the estimated status result in EMS and the network restraint conditions. For the power plant, the setting of the HV bus of the power plant shall be given; the number of capacitor/reactor sets enabled or disabled shall be given for the substation; for the transformer, the control position of the tap shall be given; for the dynamic reactive power compensator, it will indicate the target voltage control value or the reactive capacity.
In the above reactive power regulation methods, the dynamic reactive power compensator and the generator can be continuously regulated. The capacitor/reactor (enable/disable) and the position of the transformer tap cannot be continuously regulated, and the number of actions in a certain time is also limited. As a result, in the reactive power voltage control, the control equipment in the power plant plays a different role from that in the substation, and their control strategies shall be differentiated.
Since continuous regulation is available for the dynamic reactive power compensator and the generator, the associated control strategy is simple, and the associated setting in the result of each reactive power optimization module operation can be directly delivered to each unit. As for the shunt capacitor, the reactor set and the transformer tap in the substation, it must monitor the variation direction of the grid system and the substation loads. If the action direction of some equipment is inconsistent with the variation direction of the grid system and the substation loads in the reactive power optimization results, the equipment shall temporarily not participate in reactive power and voltage optimization. At the same time, the voltage of the main bus of the substation shall be monitored and regulated to a reasonable range with the voltage restraint value as the criterion.
4.4.5.2.2. Control system at the plant/substation level
The control system at the plant/substation level consists of the generator voltage-reactive power controls in the power plant and the voltage-reactive power controls in the substation.
1. The voltage-reactive power control system in the power plant selects the HV bus as the control target. It takes full consideration to the various limit indices of the generator and can ensure that the generator runs in a safe and smooth manner in the qualified parameters. The voltage control target can come from the locally set voltage curve or the voltage target value delivered by the control center. The upstream PC is designed to receive commands and optimize the distribution, and the downstream PC is designed for data acquisition and real-time control. When the voltage target value will reduce the generator-end voltage down to make the auxiliary power voltage excessive, the control system will automatically limit and regulate to ensure the auxiliary power in normal status; and when the voltage target value will result in leading of the power factor and further in excessively large power angle, which may affect the stable operation of the unit, the system will also automatically limit and regulate the power angle in the permissive range. The other indices such as the stator current and the rotor current and the like will not be off-limits.
As a result, no matter if the locally set voltage curve is used or the voltage target value delivered remotely is executed, the system can still execute the regulation command with full consideration to safe and smooth operation of the generator.
2. The reactive power regulation system in the substation shall control based on the control information delivered by the control center. The control center will transmit the settings to the substation reactive power regulation system, including the tap position of the transformer and the number of capacitor banks and reactor sets. Based on the information and the status quantities of the local equipment acquired, the substation reactive power regulation system will control the condition of equipment security and ensure that the shunt equipment and the transformer tap are free from frequent participations in regulation.
For the substation provided with dynamic reactive power compensator, the operation management can refer to the control characteristics of the power plant. For the controllable HV reactor in sets, the control strategy is similar to that of the traditional LV reactor except that the number of operations as the constraint condition can be extended.
4.5. Power Dispatch Technology after Large-Scale WTG Integration
4.5.1. Dispatch Mechanism and Control Strategy of Wind Power
The scientific and reasonable dispatch mode and technology can help improve the grid stability after large-scale integration of WTGs and increase the wind power integration capacity of the grid. Take the actual operation of Gansu Grid as an example. Analyze the limitations of wind power transmission, improve the wind power quality integrated to the grid by means of the implementation rules on dispatch management, set up the daily generation scheduling in the maximum load mode, and carry out excessively short rolling plan and real-time regulation to make sure of safe, energy-saving, and “open, just, fair” dispatch of the grid.
When the wind power transmission is limited, the dispatch department will prepare the dispatch plan of the wind farm in the consistent principle and operation platform. The basic principles for dispatch are to make sure of smooth and reliable grid operation, and dispatch the wind power output in an open, just, and fair manner. In case of output limiting, the output of the wind farm shall be determined on the basis of equal proportion of installed capacity and be regulated in real time according to the wind power resources.
4.5.1.1. Wind power dispatch plan modes
When no wind power prediction system is available or the prediction system has poor accuracy, the wind power dispatch plan shall be prepared in the following modes:
1. Maximum output mode. The maximum output curve of the farm shall be delivered to the wind farm, and the wind farm shall control its output not to exceed the curve.
2. Constant output mode. The wind farm output curve delivered by the dispatcher is a constant value. Because the wind power sees frequent variation with big fluctuation, it is difficult to execute in actual application.
3. Constraint-free mode. The dispatcher has no limits for the real-time output of the wind farm and the wind farm can autonomously regulate its output based on the wind force. The mode is mainly used for the wind farm that has no output limits and its wind power variation has little influence over the grid.
4. Tie-line regulation mode. The dispatcher shall, based on the constraint of transmitting load flow of the wind farm, deliver the wind power output curve.
5. Spinning reserve mode. The dispatcher shall, based on the requirements of grid security operation, consider the spinning reserve capacity of 20% the adjustable installed capacity before delivering the wind farm output curve.
4.5.1.2. Wind power maximum active power output mode
The wind power intelligent control system, developed by the power dispatch center of Gansu Power Corporation, combines the maximum output mode with real-time regulation, which can maximally improve the wind power integration capacity of the Gansu Grid. The actual measures are as follows:
1. Wind power scheduling preparation. The Gansu Grid determines the generation scheduling curve of the thermal power plant according to the principle of “power based on heat demand,” and the purely condensing thermal power unit shall be started in the minimum mode. The output shall be based on the implementation of generation scheduling. The 96-point daily plan of the wind farm and the hydropower plant without regulation capacity for the next day shall be delivered with the generation scheduling curve upper limit based on the stable control requirements of the grid sections and the equal proportion of installed capacity in the same region. The scheduled maintenance of the transmission and substation equipment of the wind farm shall be in principle once a year, which shall fall in the summer, with weakest wind force as much as possible.
2. Real-time dispatch. The wind power intelligent control system shall deliver the output upper limit planned value to the wind farm every 5 min, and the wind farm can report to the control system for additional output plan application in the next period every 5 min with maximum additional capacity of 10 MW. The system will arrange it in chronological order. Having received the request for additional output from the wind farm, the control system shall first detect the output margin of the other wind farms at the tap, and modify the plan by capacity, and it shall detect the output margin of other taps if insufficient, and then adjust the plan with large margin until allocation is done. If the requested capacity cannot be allocated, it shall detect whether an overgenerated tap is present, and if yes, it shall detect the output allocation in the overgenerated tap by capacity to reduce those with more capacity; and if the output is allocated by equal capacity, it shall reduce in equal proportion the capacity until the allocated quota is reached. The control shall detect the load flow margin of each control section every 5 min, and the output plan of the wind farm shall be increased/reduced based on the minimal value and then delivered for execution.
When a wind farm has its output larger than the planned value, the control system will send out alarm signals to the wind farm and the dispatcher, and it shall disintegrate the wind farm first in case the control targets exceed stability boundary. The control system can rapidly modify the setting of the control sections in case of maintenance or emergency. In addition, it can also allocate the output plan based on the control strategies, including constant curve, constant tie-line control, unit classification regulation, manual interference by the dispatcher, and so on.
The above dispatch mechanism and control strategy has been applied to Jiajiu Grid, Gansu, which proves that the mode has good feasibility. In the conditions that the structure of Gansu Grid has no obvious improvement, that the user loads of Jiajiu Grid, Gansu, are reduced by 15% in the period January–August on year-on-year basis, 2009, and that the generation of the regional small-sized hydropower plants rises by 24% on year-on-year basis, the wind power generation in Jiajiu Region grows by 63% on year-on-year basis while the installed capacity of the regional WTGs rises by only 21% on year-on-year basis. As for the difference between the utilization hour of the regional wind farm and the average level, Yumen is below 4.7%, and Guazhou is below 6.5%.
4.5.1.3. Global dispatch mechanism and control strategy of wind power
The dispatch mechanism and control strategy of wind power shall be considered from a global view for the Northwest Grid. After the large-scale wind power is put into operation, it will exert great influence over the existing dispatch mode of the Northwest Grid. Great changes will take place in the grid dispatch system, including generation scheduling, load flow operation control, tie-line assessment and power consumption, integration operation management, etc.
1. Impact on generation scheduling. The traditional generation scheduling is based on reliable power sources and predictable loads, and it shall consider equipment maintenance and mutual power supply. When the power system consists of a lot of wind farms, because the prediction level of wind power cannot reach the accuracy demand of the project application, the predictability is poor if the wind farm is viewed as a negative load; and it cannot guarantee the reliability if the wind farm is viewed as a power source. Consequently, it is very difficult to schedule a generation plan.
The equivalent load fluctuation curve, superimposed by the wind power and the user load, is generally used for study. Compared with the traditional electrical load curve, the equivalent load curve increases the system peak-valley difference, which has increased the grid peak-valley regulation difficulty and will have great impact on the generator set start mode and the grid operation mode. As a result, it shall consider the grid as a whole during generation scheduling. On one hand, it shall ensure the system can keep the associated positive/negative spinning reserve capacity with the rated output of the wind farm in any operation status; on the other hand, the system peak-valley regulation rate shall be available in case of sudden changes of wind farm output.
2. Impact on dispatch operation control. Large uncertainty exists in the operation of wind farms, which increases the difficulty in controlling system frequency regulation, section load flow monitoring, tie-line power regulation, regional voltage stability, and the like. As a result, higher requirements must be proposed for the operation monitoring level and coordination response capacity of the dispatcher and operation as well as the technical means for dispatch automation.
3. Tie-line assessment and power consumption mechanism to be improved. At present, the provinces (regions) of the Northwest Grid have a low proportion of wind power installed, and the existing technical means can still meet the control demand on provincial balance of wind power. The existing dispatch mode can realize provincial balance of wind power where the impact of wind power on the system can be mostly absorbed in the province (region). In addition, the energy settlement method is simple, ensuring to some extent the stability of interprovincial tie-line assessment and the order of grid power energy transaction. As the installed capacity of wind power rapidly grows, it is increasingly difficult for the wind power dispatch mode based on provincial balance to meet the demand on tie-line control. It is urgent to improve the existing wind power dispatch mode. As a result, it's necessary to study the new tie-line assessment and wind power consumption mechanism suited to large-scale wind power operation in advance, explore the new dispatch mode based on the wind power balance in the whole region (even cross regions), and settle the provincial tie-line power preparation and assessment in the existing power market transaction framework as well as a series of problems concerning the existing real-time transactions such as organization, implementation, settlement effective transition, etc.
4.5.1.4. The integration management and operation management of WTGs to be strengthened
Since the WTG involves many types of turbines and wide assess range, integration management and operation management cover many points and a wide scope. As for coordination between the wind farm and the grid, the WTG will exert great impact on the safe and smooth operation of the integration turbine selection, protection configuration, control strategy, and operation characteristics. As a result, it's necessary to develop deep study in advance on the associated technical problems concerning integration of WTGs to the power system, carry out active technical supervision and guide to the wind power from the grid security management, prepare consistent codes on integration of WTGs and the relevant operation management rules, and improve the normalization and coordination of WTG operation.
4.5.2. Coordination between the Wind Farm and the Grid
The coordination between the wind farm and the grid refers to the coordinated dispatch of active/reactive power between the wind farm and the grid. Here the discussion will focus on frequency regulation after large-scale integration of wind farms.
In the range of primary frequency modulation, the power fluctuation in the distributed wind farms and the internal WTGs is random and uncertain, which will not result in obvious rise of grid frequency modulation capacity and performance. For the primary frequency modulation, the short-time fluctuation of wind power in the region as a whole can be ignored compared with the impact of unit trip in the traditional power plant.
The secondary frequency modulation is automatically controlled by AGC in each control zone with action time generally in a range of 3–5 min. Take Jiuquan Wind Power Base as an example. The wind power output variation in 5 min can reach 5–10% installed capacity. When the wind power generation scale is large, the power fluctuation is very large compared with the system AGC control capacity. Accordingly, the large-scale WTG integration will propose higher demand on secondary frequency modulation.
The WTGs have different power characteristics from the traditional units. The former has rapid and transient power characteristics, and the latter has delayed and continuous. Both of them can carry out coordinated control and play its own virtue. Figure 4.53 shows a typical control strategy with wind power participated in WTG-grid coordination.
In Figure 4.53, Rwt is generator rotor winding resistance of WTGs, k is a constant, Φ is generator gap flux, H is inertia constant, J is rotary inertia, S is differential operator, Δf is frequency deviation, dΔf/dt is frequency deviation variation rate, βpatch is blade pitch angle, Tem is EM torque, Tmech is mechanical torque, and ω is the rotary angular speed of the impeller.
The control system is based on leveled architecture. The operation management level shall supervise the behavior of the whole wind farm and decide the control level mode status and power parameter of the wind farm, and coordinate the frequency control of the wind farm and the traditional power plant. The control level of the whole wind farm shall control the power generation of the whole wind farm and transmit the reference value to the local controller of each WTG. In addition, the local control level can control any single WTG and make sure of the reference power level transmitted. The associated information such as generation, estimated available power, and wind velocity of each WTG shall be returned to the control level of the wind farm. The control level of the wind farm shall measure the PCC frequency and increase or decrease the active power output by the whole wind farm in case of system frequency variation based on the sag characteristic curve of the wind farm, making the wind farm participate in system frequency control like traditional power plants.
The control system, by means of server control, can offer rapid active power support in several seconds and additional active power support in a long time. The rapid growth of wind power, however, may to some extent reduce the response of the traditional units. To avoid the adverse impact, the coordinated control strategy between WTGs and traditional units is proposed. See Figure 4.54.
The coordinated generation control is integrated to automatic gain control, power regulation management, power balance control, and imbalance among the given companies in an economical way. Based on the operation time constant of various generator sets, and the capacity and functions of the load frequency controller, as well as the contribution to the primary/secondary frequency modulation control, set the participating coefficient of each traditional generator and WTG, making it compliant with ∑KCi = 1. Suppose nP is the total additionally injected power, the additionally injected power of each generator: PCi = nPKCi.
The wind power generator can easily offer additional 0.1X active power support, rapidly respond to frequency variation, and provide effective power supply in 10 s. By means of coordinated control of WTGs and traditional units, the traditional generator can participate in system frequency regulation in a more effective manner, which is good for system frequency support. As a result, to some extent the wind farm can participate in system frequency control and realize the coordinated control between the wind farm and the grid.
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