Difference between grid connections of large-scale wind power and conventional synchronous generation

1 Introduction

In China,regions rich in wind energy resources are generally located at the end of power grids.The power grid architecture in these regions is usually not sufficiently strong,and the energy structure is relatively simple.Therefore,connection of large-scale wind power units considerably burdens the peak load regulation and threatens stable operation of the power grids in these regions [1-8].In addition,most wind turbines use power electronic converter technology.The impact of this technology on the safety and stability of power grids is different from that of conventional synchronous generators.Furthermore,fluctuations in wind power cause the output of a wind farm to fluctuate.Thus,devising and implementing suitable power generation plans for wind farms are difficult,unlike for conventional thermal power plants [9-17].The capacity of a single conventional thermal power unit consisting of a synchronous generator is large—currently,the capacity of a single thermal power unit in operation or to be commissioned is typically more than hundreds of thousands of kilowatts.Moreover,the capacity of the most advanced ultra-supercritical thermal power units has reached a million kilowatts.Thermal power units have rated output line voltages varying from several kilovolts to tens of kilovolts.Thus,they can be directly incorporated into transmission networks of 110 kV,220 kV,or higher voltage levels using primary step-up transformers.Load centers and areas having abundant primary resources are high-priority locations for the construction of thermal power plants.Each thermal power unit is equipped with an excitation system and a speedregulating system to control the terminal voltage and active power of the generator,respectively.Furthermore,each thermal power unit in a power plant strictly implements the generation plan and maintenance plan formulated by the power grid dispatcher.The generation plan prescribes the steps to optimize a certain objective function,such as the minimum active power loss of the system and the minimum total active power output of the generator.The generation technology and grid connection scheme differ considerably between wind power and conventional thermal power.Moreover,the active and reactive power control abilities of wind turbines are weaker than those of thermal power units;thus,additional auxiliary equipment are typically required to control a wind turbine [18-24].The capacity of a single wind turbine that drives a squirrel cage asynchronous generator,doubly fed induction generator (DFIG),or permanent magnet synchronous generator is low.Currently,the maximum capacity of a single wind turbine in operation is 10 MW.Wind turbines with capacities of 1-5 MW are the most commonly used;these wind turbines consist of variable-speed fans with partial power inverters or full-scale frequency converters [25,26].Owing to the wind speed fluctuation,wind turbine design/manufacturing technology,and operational characteristics,the power generation efficiency of wind turbines is lower than that of conventional thermal power units.However,wind power generation does not consume fossil fuels or release pollutants;therefore,it can still be promoted to ensure renewable development and environmental conservation [27].The rated output line voltage of a wind turbine is generally several hundred volts.The power output of several or a dozen wind turbines that are closer to the user side can be connected to 10-kV and 35 kV distribution networks through only one step-up transformer.However,large wind farms located far from the load center contain several dozens or even hundreds of wind turbines.In such cases,their power output is first-boosted to 10 kV using a box transformer,subsequently connected to the main transformer through the field collection lines,and finally boosted to a 110 kV,220 kV,or even higher-voltagelevel transmission network.During normal operation,each variable-speed wind turbine in a field controls its active power and reactive power by itself.However,in case of an emergency,instructions are provided by the grid dispatcher to control the power output of the entire wind farm.

The discussion thus far clearly indicates that the power generation technology and the grid connection scheme differ significantly between wind power generation and conventional thermal power generation.The objective of this study is to analyze the differences between the grid connection and collection strategies of wind power and thermal power generation.Based on this analysis,the differences in the power control modes of wind power and thermal power are further investigated.Finally,the stability of different control modes is evaluated via simulation.

2 Analysis of difference between grid connection schemes

2.1 Grid connection schemes

Because of the weak reactive voltage control ability of wind power plants,additional reactive voltage control equipment must be installed in power grids,such as the 750 kV transmission channel in the Hexi Corridor of Gansu Province.Existing reactive voltage control technologies mainly include series compensation,staric var compensator,static synchronous compensation,controllable high impedance,and other flexible AC transmission technologies;these technologies can be used in wind power transmission lines.In general,the power transmission lines of thermal power plants only use a fixed high reactance to offset the charging power of high-voltage lines.

Currently,the equivalent operating hours (EOH) of thermal power plants in China is approximately 5000 h.Furthermore,the EOH of wind farms is ～2000 h,and the maximum wind power output of general wind farms do not reach their installed capacity [28].The larger the scale of a wind farm,the lower its maximum output simultaneity rate.Fig.1 shows the maximum output probability of a wind power base with an installed capacity of 5000 MW.

Fig.1 Probability distribution of actual output of a wind power base

The figure shows that the probability of the actual wind power output being greater than 63% of the installed capacity is less than 5%.This means that the frequency of the actual wind power output less being than 63% of the installed capacity is 95%.Therefore,considering the economy of the wind power transmission line project,the conductor section of the transmission line is generally not selected based on the total installed capacity of the wind power.Because there is a lot of full-load time,the conductor section of the transmission line of a thermal power plant of the same scale is generally selected according to the rated power of the thermal power plant.Moreover,owing to the low installed capacity and the small probability of full power generation,the transmission line of a small wind farm is generally not considered to meet the N-1 requirements in the grid connection scheme design.This means that if the transmission line of wind power fails and is cut off,the wind farms will also be cut off.In contrast,two transmission line circuits are generally available for thermal power units to ensure reliable power supply.

2.2 Comparison and analysis of actual cases

This section compares and analyzes the grid connection schemes of wind power and conventional thermal power based on actual cases.Here,wind farm refers to a millionkilowatt wind power base in North China,and a thermal power plant has the same power generation scale as the wind farm.The total installed capacity of the wind power base is 2500 MW,which is obtained from seven wind farms (Fig.2).

Fig.2 Grid connection scheme of a million-kilowatt wind power base

Fig.3 Grid connection schemes of the equivalent thermal power plant

It is assumed that the annual EOH of the wind turbines (2500 MW) in the wind power base is 2000 h,which is equivalent to 1000 MW thermal power generators with an annual EOH of 5000 h.Therefore,the wind power base is equivalent to a power plant with two 500 MW thermal power units,considering the same power generation scale.Fig.3 shows the transmission line of the thermal power plant.The power collection system and the reactive power control system of the wind power base are different from those of the thermal power plant.Therefore,additional equipment,such as high-reactance,low-reactance,seriescompensation,and SVC equipment,must be installed at the collection station side [29].

The demarcation point of property right between the thermal power plant and the power grid is generally selected as the first tower of the thermal power plant transmission line.Therefore,the property rights of the step-up transformer of the power plant and the line high-reactance equipment on the thermal power plant side generally belong to the power plant.The differences between the wind power base and the assumed thermal power plant are shown in Table1.

According to the statistics of annual power generation,a 2500 MW wind power base is equivalent to a thermal power plant with an installed capacity of 1000 MW.However,the grid connection of the thermal power plant involves considerably less additional control equipment than that of the wind power base does.The total installed capacity of the wind power base is larger;therefore,the conductor section of the wind power base transmission line is also larger than that of the thermal power plant.

Table1 Comparison of wind power base and thermal power plant

Wind power base Thermal power plant Installed capacity 2500 MW 1000 MW Annual equivalent operating hours 2000 h 5000 h Annual power generation 5000 GW·h 5000 GW·h Power collection system LGJ-300*2/200 km LGJ-400*2/100 km None 500 kV wind power Collection Substation 220 kV side operates in sections,10 outgoing lines in total None Transformer capacity of collection station 1200/1500 MVA*2 None 500 kV high-voltage reactor 150 Mvar None 66 kV low-voltage reactor 360 Mvar None 66 kV low-voltage capacitor 240 Mvar None 66 kV LV SVC 180 Mvar None Series compensation 358 Mvar None Reactive power compensation of substation 60 Mvar capacitor,2 None 500 kV intervals of substation 1 1 66 kV intervals of substation 2 None Transmission line model LGJ-300*6 LGJ-300*4 Transmission line length 150 km 150 km

3 Comparison of power control and its influence on stability

3.1 Active and reactive power control

Active power control of the thermal power unit is realized using a governor and a prime mover system,and terminal voltage control is realized using an excitation regulation system.The excitation system can control only the reactive power,because the amplitude of the excitation current is the only adjustable value of this system.The unit controls the output of the active power according to the generation plan.It cuts off the synchronous generator if the grid develops a severe fault or if the synchronous generator is out of step.Each wind turbine in the wind farm has its own independent control system.In addition,the entire wind farm must have an overall control function to control all wind turbines simultaneously,to ensure that the output characteristics of the entire wind farm are in accordance with the instructions of the grid dispatcher.

The operation of a single DFIG involves controlling the rotor AC excitation frequency converter,which has three adjustable quantities,namely excitation current frequency,excitation current amplitude,and excitation current phase.Because the excitation current of a DFIG has two additional adjustable values compared with that of a synchronous generator,it offers more flexible control.Furthermore,a DFIG is capable of variable-speed and constant-frequency operation through variation in the frequency of the rotor excitation current.By changing the phase of the rotor excitation current,the relative positions of the generator potential phasor and grid voltage phasor can be adjusted.Further,the power factor angle can be changed to adjust both the reactive power and active power.When the power unit absorbs reactive power,the stability of the operation is often reduced owing to the increase in the power factor angle.By adjusting the phase of the AC excitation current,the power angle of the generator can be reduced,and the stability of the unit can be improved.Using vector transformation control to change the phase and amplitude of the DFIG rotor excitation current,decoupled control of the active power and reactive power of the DFIG stator output can be achieved.Therefore,a DFIG offers more advantages than a synchronous generator for power regulation.The control strategy of active power is to achieve the maximum wind power tracking control within a certain range of wind speed.When the output power is larger than the rated power,the pitch angle serves to maintain the rated output.On the other hand,the control strategy of reactive power is to achieve constant-terminal-voltage control or constantpower-factor control.

The control function of an entire wind farm includes active power control and reactive power control.The active power control system receives the active power control instructions sent remotely by the dispatching department,and automatically follows these instructions.It realizes continuous and smooth control of its active power output and ensures that the changes in the active power and reactive power of the wind farm do not exceed the values provided by the grid dispatching department.In case of a power grid emergency,the wind farm should control its active power output according to the instructions of the power grid dispatching department.This ensures the rapidity and reliability of the active power control system of the wind farm.If necessary,the active power of the wind farm can be quickly and automatically cut off or reduced using a safety automatic device.The reactive power control system should have reactive power control and voltage control abilities.According to the instructions of the power grid dispatching department,the wind farm automatically adjusts its sent (or absorbing) reactive power to realize voltage control at the grid connection point.Its regulation speed and control accuracy should meet the requirements of the power grid voltage regulation.In case of a grid voltage drop caused by a grid fault or disturbance,the reactive power control system should ensure that the wind turbines in the field can realize low-voltage ride-through operation and should provide dynamic reactive power support if necessary.In general,thermal power units have a better reactive voltage regulation ability than wind power units do.The power factor of large thermal power units can reach 0.85.The stepup transformer of a thermal power plant is not equipped with additional reactive voltage regulation equipment.The high-voltage-side line is only equipped with a fixed high reactance to offset the charging power of the line.However,wind farms,especially large-scale wind power collection bases,must usually be equipped with series-compensation,controllable high-reactance,low-reactance,low-capacitance,and SVC equipment,owing to their lack of reactive voltage regulation ability.Furthermore,the specific capacity must be determined according to the conditions of different wind power bases.

3.2 Influence on power system operation

Generally,the function of a traditional power system is to balance the predictable load and the controllable power supply in real time.The forecasting accuracy for power system load can exceed 99%.It can ensure that the thermal power,hydropower,and other conventional power plants generate electricity according to the predicted second day load.Furthermore,it ensures that the power is balanced in real-time across the entire power system,thereby maintaining stable operation of the power system.Therefore,the traditional operation scheme of a power system can be regarded as the arrangement of the generator starting mode considering the optimal economy and other constraints.A thermal power unit outputs a real-time and controllable power supply.Considering the minimum combustion temperature of the boiler,the active power range of a large thermal power unit is generally between 40-100% of its rated power;however,wind power differs in this regard.The driving force for wind power generation is natural wind,which is uncontrollable.Thus,wind power generation involves significant challenges in terms of the real-time balance of traditional power systems.Currently,three main dispatching management modes of wind farms have been implemented in China:(a) direct management of provincial grid dispatching;(b) management of local grid dispatching,which is entrusted with provincial grid dispatching;and (c) joint management of provincial and local grid dispatching.Generally,the management mode is adopted based on the capacity of the wind farm incorporated into the grid,the level of connection voltage,and the impact on grid security.

As the scale of grid connections of wind power increases,the proportion of wind power in the power grid continues to increase.Thus,the contradiction between the unconditional implementation of the policy of full purchase of wind power and the safe and stable operation of the power grid is exacerbated.Unconditional implementation of the full-purchase policy for wind power does not comply with the objective law of wind power development and safe operation of the power grid.When the conventional power source of the power system reaches its peak regulation limit and the power system is faced with safety and stability problems,the wind power output must be appropriately limited in the corresponding period of time to ensure safe and stable operation of the power grid.

The wind power dispatching mode is based on the principle of priority dispatching of wind power after the wind power is connected to the power grid.Furthermore,it is based on the requirement of satisfying the peak load regulation and standby demands of the system.For wind power generation,the output power of a wind farm can be regarded as a negative load.After the wind farm is connected to the power grid,the capacity,i.e.,the difference value between the available peak load regulation capacity of the grid and the reserve capacity for balancing load fluctuation.It can be used for the peak load regulation of wind power generation.According to the regulations of power grid dispatching management,to ensure safe operation of the power grid,2-3% of the rotating reserve capacity must be reserved for power grid dispatching to balance the random changes in the load.When the peak regulation capacity of the grid for wind power is limited and the fluctuation of the output power of the wind farm cannot be completely balanced,the power injected into the grid by wind power plants must be limited.Owing to the intermittent characteristics of wind power,the generation capacity of wind power plants cannot be dispatched.Therefore,the grid must have a reserve capacity as the generation reserve of wind power to ensure normal power supply from the grid.With the continuous expansion of wind power generation,power grids should include a certain capacity of peak load regulation power supply.

3.3 Influence on power system stability

The differences between the impacts of wind power and thermal power on the stability of a power system were analyzed through a simulation of the wind power base and the thermal power plant,which were considered to be under the same disturbance [30].In the simulation,a fault disturbance was assumed to occur in the power grid to which the transmission system is connected.Two scenarios were analyzed:(a) full power generation from all wind power units (2500 MW),which replaces part of the thermal power unit (1000 MW),and (b) outage of all wind power units.The changes in system voltage and frequency were analyzed,and the variation curve of the system frequency and voltage was obtained (Fig.4).

Fig.4 Variation curve of system frequency and voltage

Fig.4 clearly shows that when the power grid is disturbed,replacing thermal power with wind power reduces the system stability,the system frequency oscillation in the grid becomes apparent,and voltage fluctuation intensifies.

4 Conclusion

Wind power generation is intermittent and involves fluctuations.Currently,it is impossible to control the output level and operation state of a wind farm according to the control requirements of a conventional power supply.As the grid connection capacity and voltage level of wind farms increase,power grids are being increasingly affected by wind power in a wider range and to a greater extent.To alleviate these problems,this study focuses on two aspects:the grid connection scheme and its impact on system stability.From the economic perspective,to receive wind power,except for the grid connection,investments toward matching transmission and transformation equipment to strengthen the grid structure are required.From the technical perspective,after large-scale wind farms are connected to a grid,the operation mode of the grid changes,and the reliability and stability of the power system are affected.Therefore,for large-scale wind power generation,these two aspects must be considered separately to ensure stable operation and effective consumption of wind power.

Acknowledgments

This work was supported by National Key Research and Development Program of China (2018YFB0904000).

Declaration of Competing Interest

We declare that we have no conflict of interest.