Frequency regulation scheme for islanded microgrid

When the main grid of the power grid malfunctions undergoes emergency repairs, the microgrid is in off-grid operation. At this time, the microgrid in an isolated state must have self-frequency regulation capability. This article proposes a frequency control strategy for isolated microgrids, which can improve their ability to resist frequency interference. The proposed control strategy considers the different characteristics and capabilities of various micro-sources for frequency regulation. It classifies the frequency modulation micro-sources in the microgrid. Judgment conditions and parameters are set to determine whether each micro-source participates in frequency modulation and the participating components. The isolated microgrid has an economical and fast frequency modulation strategy. Finally, a microgrid simulation model is established using Matlab/Simulink simulation software to verify the effectiveness of the proposed frequency modulation strategy.


Introduction
With the decrease in fuel and the increase in pollutant emissions, countries have stepped up their research on renewable and clean energy, especially microgrids, including wind energy, solar energy, energy storage, and load [1].Microgrid is connected to the medium and low voltage power grid of the main power grid and can operate in two modes: on-grid and off-grid.When the main grid is operating normally, the load inside the microgrid is mainly provided by the micro-source in the microgrid, and insufficient or excess power is provided or absorbed by the main grid.When the main power grid fails or needs maintenance, the microgrid is disconnected from the power grid to form an isolated microgrid [2][3].In order to ensure the balance between power generation and power consumption in the isolated microgrid, it is necessary to study the frequency regulation control in the isolated operation of the microgrid.In [4][5][6], researchers studied the frequency control methods of isolated microgrids, but most of them focused on the control level of power electronic inverters.Kim et al. [7] proposed a power management strategy for a microgrid with multiple power electronic interfaces.The dispatchable micro-sources respond to changes in system frequency to achieve the purpose of adjusting the frequency of the microgrid, and the adjustment of each micro-source is independent of each other.In this paper, a complete frequency modulation strategy of isolated microgrids at the system level is proposed.Considering the different characteristics and capabilities of various micro-sources for frequency regulation, parameters are set to determine whether each micro-source participates in frequency modulation and the participating components.In this way, the frequency of isolated microgrids can be adjusted economically and quickly.

The structure of a microgrid system
Figure 1 shows a single line diagram of a simple microgrid system connected to a 35 kV distribution network.The microgrid system includes a variety of power generation units, among which a hydropower plant is connected through feeder line 1; a wind farm composed of a doubly-fed wind power system is connected through feeder line 2; a photovoltaic power station is connected through feeder line 3; an energy storage system is connected through feeder line 4; a centralized load is connected through feeder line 5.The microgrid system can operate in two states, on-grid and off-grid, respectively, with the operation of circuit breaker 2. When the main power grid is in normal operation, the circuit breaker 2 is in a closed state.The doubly-fed wind power system runs at the maximum power state point at a constant pitch angle.The photovoltaic power system outputs at the maximum power.The energy storage system does not output power.The hydropower plant runs in a stable state.When the main power grid fails or is overhauled, circuit breaker 2 is disconnected, and the microgrid operates in isolation from the power grid.

Frequency modulation control strategy of wind power system
The rotor of a conventional generator is directly connected to the grid.When the system frequency changes, the kinetic energy of the rotor can be released or absorbed, as shown in Equation (1): where ΔE is the change of kinetic energy; J is the inertial time constant; Δω is the speed change value; ω0 is the initial speed.At unit value ω=f, the derivative of time t is taken on both sides of Equation (1).By setting ω=f, we can obtain: It can be seen that the system frequency change will cause the rotor speed change and then change the output power.
The rotor side of the doubly-fed wind power system is connected to the microgrid through a dual PWM converter.The speed will not automatically respond to the change in the system frequency.In order to achieve auxiliary frequency control, primary frequency modulation needs to be added to the doubly-fed wind power system.From Equation (2), it can be seen that the power change value is related to the frequency deviation and the differential of the frequency deviation.Therefore, the active power change value of the fan in response to the frequency change of the microgrid system can be set as follows: The sum of the auxiliary active power reference value change generated by Equation (3) in response to the system frequency change and the reference value obtained by the maximum power curve is the new power reference value of DFIG, as shown in Figure 2. Douply-fed wind power generation system is usually operated under maximum power tracking control [8].In order to effectively control the frequency, when the load component changes for a long period and large amplitude, the output power of the fan is required to have a certain redundancy and participate in secondary frequency modulation.
The maximum power output of the wind turbine can be expressed in per unit value as: where Cp_pu is the maximum wind energy utilization coefficient; Vpu is wind speed; β is the pitch angle; λopt is the optimal tip velocity ratio.kp is the active power when Cp_pu=1 and Vpu=1.Cp is a function of pitch angle β and tip velocity ratio λ, as shown in Equation ( 5).
( ) in the equation, According to Equation (4), at a certain wind speed, the output power of the fan is determined by the wind energy utilization coefficient.The wind energy utilization coefficient Cp has a great relationship with pitch angle β and tip velocity ratio λ, which can be expressed by the curve in Figure 3.As can be seen from Figure 3, when the pitch angle changes, the optimal tip speed ratio and the maximum wind energy utilization coefficient both change and change with the output power of the fan.Therefore, the wind power generation system can have a certain adjustment capacity by setting the initial pitch angle, and the output power of the fan can be changed by adjusting the pitch angle.It can be seen from Equations ( 4) and ( 5) that the relationship between pitch angle, optimal tip speed ratio, and fan output power is very complicated.It is difficult to get the proper pitch angle adjustment for a certain adjustment power.In this paper, the relationship between optimal tip speed ratio and pitch angle is obtained by curve fitting so that the wind energy utilization coefficient in Equation ( 5) is a function of univariate pitch angle.Then, the polynomial of pitch angle and wind energy utilization coefficient is fitted, as shown in Equations ( 6) and (7), with the fitting degrees of 99.7% and 99.9%, respectively.Finally, the relationship between output power and pitch angle is obtained.The curves of the fitted pitch angle, the optimal blade tip velocity ratio, and the wind energy utilization coefficient are shown in Figure 4 and Figure 5, respectively.Equation ( 4) is substituted into Equation (7) to obtain the relationship between pitch angle and fan output power, as shown in Equation (8).
According to Equations ( 4) to ( 8), the secondary frequency modulation controller of the wind power generation system can be designed, as shown in Figure 6.ΔP is the required adjustment value of power, Ppu is the initial output power of the fan, V is the per unit value of wind speed, Pnom is the rated power of the wind power generation system, and Cpnom is the wind energy utilization factor rating.When the system frequency changes, the power management system of the microgrid can assign a certain regulation power ΔP to the wind farm.It can add the original output power to get the power that the wind farm needs to output and convert it into the pitch angle command of the fan through Equation (8).In this way, it can realize the control of changing the active output of the wind farm by adjusting the pitch angle.

Frequency modulation control of battery system
The battery is a dispatchable micro-source.The active power and reactive power of the battery system controlled by PQ decoupling are independently controlled by the id and iq components of the current on the AC side [9], where id*=P*/us, iq*=Q*/us, and us are the voltage amplitude on the AC side.The frequency modulation controller of the battery is shown in Figure 7. P* is the regulated power value emitted by the microgrid management system, and Q* is the reactive power reference value, which is set to 0 in this paper.

Frequency control of microgrids
The frequency of the microgrid reflects the balance between power generation and power consumption of the microgrid system.The frequency control of the microgrid in isolated operation is completed by adjusting the active power output of each micro-source.Frequency control includes three modules: one is to regulate the power generation module, the second is the power distribution module, and the third is the frequency modulation controller module of each micro-source.The power generation module and power distribution module are collectively referred to as the microgrid power management module.The regulating power generation module receives real-time data in the microgrid system.It converts it into the regulating power of the microgrid.Then it distributes the regulating power to the frequency modulation controller of each micro-source according to the participating component through the power distribution module, as shown in Figure 8.

Adjust the power generation module
The power generation module calculates the current power required to be adjusted according to the real-time data of the system.The regulating power with PI control is shown in Equation ( 9).
( ) ( ) where ΔPP and ΔPI are proportional components and integral components, respectively.kP and kI are proportional gain and integral gain coefficients, respectively.ACE is the regional control deviation.B is the frequency response coefficient, expressed in MW/0.1 Hz.

Power distribution module
The frequency modulation characteristics and capabilities of various micro-sources in the microgrid system are different.Hydropower is a conventional power supply, and the hydro generator installed with a governor and a frequency modulator has relatively slow static frequency characteristics.As can be seen from the analysis in section III A), the primary frequency modulation function of the doublyfed wind power generation system can be completed by changing the rotor speed, and the adjustment speed is fast.In contrast, the secondary frequency modulation function is realized by adjusting the pitch angle, and the adjustment speed is relatively slow.The output power of the battery system can be scheduled, and the response speed is very fast.Considering the actual capacity and life of the battery, the active power rate output of the battery needs to be finally returned to zero [10].When t=t1, the frequency of the isolated microgrid system is lower than the rated value, and the schematic curve of the frequency response characteristics of each micro-source is shown in Figure 9. Considering the different frequency modulation characteristics and economic characteristics of each micro-source, the micro-sources with the ability of frequency modulation are divided into traditional micro-sources and renewable micro-sources during the frequency control of the microgrid.When the system needs frequency modulation, the renewable energy sources will be the first to adjust the frequency.The traditional micro-sources will participate in frequency modulation when the adjustment limit is reached.The regulation components of each class are calculated according to Equation ( 9).The regulation power is allocated according to the frequency modulation speed and frequency modulation capacity of each micro-source.The regulation power calculation method of each micro-source is as follows: where ai and bi are the distribution coefficients, ai is related to the adjustment capacity, and bi is related to the adjustment speed.The regulation power generation and distribution principle of the microgrid power management system is shown in Figure 10.

Frequency modulation strategy simulation verification
Matlab/Simulink simulation software was used to establish the microgrid system, as shown in Figure 1, and the microgrid operated in off-grid mode.The hydropower plant a rated capacity of 100 MW.
The wind farm has a rated capacity of 25 MW.The photovoltaic power plant has a rated capacity of 1.6 MW.The battery capacity is 3 MWh.According to the classification principle in Equation ( 10), the first type of frequency-modulated micro-source in the microgrid established in this paper is battery and wind power generation.The second type is the hydropower plant.The frequency modulation controller proposed in III is installed in the wind farm and battery system.The governor and frequency modulator are installed in the hydropower plant.The load is increased by 3 MW at t=10 s, and another 8 MW at t=40 s.Wind farms achieve changes in active power output by adjusting the pitch angle.Therefore, the judgment in Figure 10 can be based on whether the pitch angle of wind farms is adjusted to 0° or the maximum limit is 45°, to determine whether the hydropower plant participates in secondary frequency adjustment.The parameters of the microgrid power management module are B=5 MW/0.1 Hz, k1I=2, k1P=0.15,a11=0.8,a12=0.2,b11=0, b12=1, k2I=1, k2P=4.1,a21=1, b21=1.ΔP11, ΔP12, and ΔP21 are the regulated power of the battery, wind power, and hydropower, respectively.The simulation results are shown in Figures 11-14.Figure 11 shows the regulation power value of each micro-source generated by the power distribution module when the system has a load surge.Figure 12 shows the curve of rotor speed and pitch angle of the wind farm under frequency modulation control.It can be seen from the curve that when the load increases by 3 MW, the speed basically does not change due to little frequency fluctuation until the speed rises and stabilizes at 1.2 pu after the pitch angle adjustment response.This increases the active power output of the wind farm.After the load is increased by another 8 MW, the frequency disturbance is large.The primary frequency modulation auxiliary amount generated by the wind farm is large.Additionally, the speed decreases rapidly, and the rotor kinetic energy is released.
The wind farm receives the regulating power generated by the power management module.The pitch angle decreases from 1.2° and soon reaches the regulating limit value 0°.The rotor speed increases from the lowest point of 1.17 and finally stabilizes at 1.21 pu. Figure 13 shows the active power output of each micro-source under frequency disturbance.The active power output of each micro-source changes correspondingly under the action of frequency modulation controller.The active power of load increases by 3 MW at t=10 s.Because the wind power does not reach the frequency modulation limit, the hydropower plant does not participate in secondary frequency modulation, only the wind farm and the battery participate in frequency modulation.Finally, the active power output of the battery is adjusted to 0. When t=40 s, the load is increased by another 8 MW.The fan pitch angle is adjusted to 0°.The wind farm reaches the frequency modulation limit.The hydropower plant participates in secondary frequency modulation.It can be seen from Figure 13 that the active power output of the wind farm first increases and then decreases under primary frequency modulation control.After secondary frequency modulation, the active power output rapidly increases and soon reaches the adjustment limit.Finally, with the maximum power output, the active power output of the hydropower plant increases under the action of secondary frequency modulation.The output power of the battery responds to the change of the system frequency and is finally adjusted to 0.
Figure 14 shows the frequency response of the microgrid under two modes of frequency modulation with or without wind power in the case of load variation.The comparison curve of frequency change in the two modes shows that the dynamic frequency deviation of the microgrid in the mode in which the wind farm participates in frequency modulation is within the frequency deviation limit of the normal operation of the power grid of ±0.5 Hz.In contrast, in the case of mode 2, when the impact load is 8 MW, the frequency deviation is as high as 2 Hz.This can seriously affect the normal operation of the system.The frequency modulation speed of mode 2 is significantly slower than that of mode 1.It can be seen that the participation of wind power in frequency modulation improves the antifrequency interference capability of the microgrid and speeds up the response speed.
The simulation results show that the frequency control system of the isolated microgrid can reasonably and economically allocate the regulating power when the system load changes abruptly.The local controller of the microgrid can accurately respond to the regulating power and change its own active power output.The simulation results verify the correctness of the proposed frequency modulation strategy of the isolated microgrid and the necessity of wind power participating in frequency modulation.

Conclusion
Based on the actual model of the micro-source, this paper designs the frequency modulation controller of the wind farm and battery in detail.It proposes an economical and effective frequency modulation control strategy for the isolated operation of the microgrid.It verifies the necessity and effectiveness of the participation of wind power in frequency modulation by comparing the frequency changes in the two cases when the microgrid is operating in isolation without the participation of the wind farm.The proposed frequency control strategy lays a foundation for the economic operation and scheduling of the subsequent microgrid.

Figure 2 .
Figure 2. Control schematic diagram of primary frequency control.

Figure 6 .
Figure 6.Control schematic diagram of second frequency control.

Figure 7 .
Figure 7.Control schematic diagram of battery frequency regulation.

Figure 8 .
Figure 8.The relation graph among modules of frequency control.

Figure 10 .
Figure 10.Production and distribution of regulated power.

Figure 11 .
Figure 11.The calculated value of regulated power of micro-sources.