Novel control strategy of grid-connected photovoltaic power supply for frequency regulation

: The maximum power point tracking (MPPT) mode is widely applied in photovoltaic (PV) power generation system. In this article, instead of MPPT, a novel control strategy of grid-connected PV power supply for frequency regulation is proposed. First, the static frequency characteristic curve of PV plant is proposed, and a method of synthesising the PV power output power–voltage characteristic curve and static frequency characteristic curve into frequency regulation curve of PV power supply is introduced. So the PV plant can participate in frequency regulation of the system. Through simulation analysis in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC), PV plant realises the function of frequency regulation when load changes, so the variation of system frequency can be reduced and the frequency stability of power system can be improved, and the effectiveness of the strategy is verified.


Introduction
As a clean, pollution-free, and resource-rich energy, solar energy can effectively relieve the pressure on the power industry caused by energy crisis and environmental crisis. In the long run, with the continuous improvement of new energy technologies and the gradual saturation of other forms of energy utilisation, by 2050, PV generation will become one of the mainstream forms of energy utilisation [1][2][3]. In recent years, large-capacity photovoltaic (PV) power plants (hundreds of MW level) have emerged all over the world. To make full use of energy, MPPT is applied in the current PV power generation system [4,5]. In this operating mode, the output power of PV plant is determined by the current temperature and solar irradiance and remains at the maximum power point. The output power of PV plant does not respond to the change of frequency, and the PV plant does not participate in the frequency regulation of the system. In large power grids, the capacity of PV plant has less impact on the system due to its much smaller capacity than traditional power plants. However, when a largescale PV plant is connected to an isolated power grid, the inertia of the system may be decreased and the frequency regulation capacity may be insufficient [6]. If the frequency is disturbed due to the system emergencies, the stability of the frequency cannot be guaranteed. At this moment, the participation of PV plant in the system frequency regulation will be of great practical significance. When the frequency of the system fluctuates, the speed control system of conventional synchronous generator will automatically adjust to increase and decrease the output power of the generator to restore the frequency to the normal allowable deviation range that is primary frequency regulation [7]. Therefore, the frequency regulation capability of a PV plant can be developed by referring to the primary frequency regulation characteristic of a conventional synchronous generation unit.
At present, many new energy power generation systems have been involved in the study of power system frequency regulation, most of which are related to wind power generation systems [8,9]. Compared with wind power generation systems, PV power generation system has the advantage of fast adjusting the active power. This feature can be utilised by large-scale grid-connected PV plants to develop faster and more efficient frequency regulation characteristics. So the frequency regulation pressure of conventional synchronous generation units can be shared. This paper proposes a novel control strategy of grid-connected PV power supply for frequency regulation. The specific control strategy is introduced in Section 2, and to verify the effectiveness of the control strategy, simulations are carried out in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) in Section 3. Finally, conclusions are presented in Section 4.

Proposal of static frequency characteristic curve of PV plants
For the current operating PV plants, MPPT is widely used to control the active power, so the solar energy can be fully used of. When the MPPT control of PV plant is used, the output power can be reduced at any time, but the output power cannot be increased to participate in the frequency regulation of the system. Therefore, if the PV plant needs to participate in the frequency regulation of the system, spare capacity of active power should be reserved. In order to determine the spare capacity required by frequency regulation, load shedding level of PV plant σ% is defined. Considering the generation efficiency and frequency regulation ability of PV plants, the load shedding level of this paper is set as follows: where P m is the maximum output power of the PV plant. Therefore, the active power the PV plant involved in frequency regulation generated at the rated frequency can be represented as (1 − σ%)P m . The power of load in the power system is provided by the generator. When the active power of the system changes, the active power provided by the generator will change accordingly so that the frequency deviations can be remained within the allowable range. The relationship between the output power of the generator and the frequency is called the static frequency characteristic of the generator. The static frequency characteristic of conventional synchronous generation units is referenced to set the static frequency characteristic of PV plants. The static transfer coefficient of PV plant is defined as follows: J. Eng The negative sign is added because the static transfer coefficient is often regarded as a positive value and the sign of frequency change is opposite to the sign of the power change. The per unit of δ can be expressed as follows: where f N is the rated frequency of the system. The range of δ * is 0.02 − 0.06, and this paper takes δ * = 0.05. The static frequency characteristic curve of PV plants is shown in Fig. 1. The upper and lower limits of output power of PV plant are set. The upper limit of output power of PV plant is set as the maximum power output of the PV plant P m , and considering the generation efficiency of the PV plant, the lower limit output power of PV plant is set as 50%P m . θ in Fig. 1 satisfies the following equation:

Synthesis of frequency regulation curve of photovoltaic power supply
The PV power output power-voltage characteristic curve can be determined at certain temperature and solar irradiance. So the frequency regulation curve of PV power supply can be synthesised by the PV power output power-voltage characteristic curve and the static frequency characteristic curve. The concrete realisation process is shown in Fig. 2.
Firstly, the PV grid-side three-phase AC voltage is measured by the phase lock loop; therefore, the frequency of power grid f can be obtained; according to f, the corresponding active power can be determined by the static frequency characteristic curve of PV plants, and it is regarded as the reference value of active power P ref sent by the PV power supply. By measuring the current temperature and solar irradiance, the PV power output powervoltage characteristic curve can be obtained from the database, and according to P ref , the reference value of PV power outlet voltage U ref can be determined by the right side of PV power output power-voltage characteristic curve in Fig. 2. From the above steps, the relationship line between f and U ref can be determined that is frequency regulation curve of PV power supply.

Process of frequency regulation
The topology and control strategy of PV grid-connected power generation system this paper used are shown in Fig. 3. First, the PV array is boosted by a chopper circuit, and then the DC power is converted into AC power by the inverter to achieve grid connection. PV array, filter capacitor, boost converter, inverter, filter, and AC system are mainly parts included. Frequency regulation control, boost converter control, and inverter doubleloop control are the three control sections contained. The input signals of frequency regulation controller include the frequency of the grid, solar irradiance, temperature, the output voltage, and current of the PV power supply, and the output signal of frequency regulation is the reference voltage of PV power outlet side. By controlling the switching device action of boost converter, output power of PV array can be controlled. The double-loop control is used in inverter control, which is the same control strategy as the control strategy applied in the traditional bipolar PV power generation system. The DC voltage/reactive power control is applied in the outer-ring control, and the current control is applied in the inner-ring control.
The specific implementation process of frequency regulation is as follows: when the frequency of gird-side fluctuates, the frequency can be measured by the phase lock loop, and the reference voltage of PV power outlet side can be obtained from the frequency regulation curve of PV power supply, and then through the control of boost converter and inverter, the regulation of active power-frequency characteristic can be achieved.

Simulation cases
In order to verify the feasibility of the matrix interconnection DC collector system of PV plants and its control strategy, the system shown in Fig. 4 is carried out in PSCAD. The system consists of two thermal power plants G1 and G2, the rated capacity of which is 667 MVA and a PV plant which is running under standard conditions with the maximum output power of 600 MW. Take 0.051 as the per unit of the static transfer coefficient of two thermal power units, and take 0.05 as the per unit of the static transfer coefficient of the PV plant. Load 1, load 2, load 3, and load 4 are active loads, and the rated active power of which is 200, 300, 900, and 300 MW, respectively. The rated frequency of the system is 50 Hz. In the simulation results, the power is expressed in terms of per  unit. The rated capacity of each power generation unit is selected as the reference power. The dynamic response of the PV plant before and after the frequency regulation when the load suddenly decreases and increases will be analysed in the following sections.

Sudden decrease in load
In the initial state, the load shedding level of PV plant is set as 20%P m . The initial output power of thermal power plants is both set as 0.8 p.u., and all loads are put into operation. At 8 s, load 1 is being cut-off and causes the rise of system frequency. The simulation results are shown in Fig. 5.
In Fig. 5, the dynamic response of the system frequency, the output power of PV plant Ppv, and the output power of thermal power plants Pg1 and Pg2 are compared under the conditions that PV plant participates in the system frequency regulation and does not participate in the system frequency regulation. When PV plant does not participate in the system frequency regulation, it does not respond to changes of the system frequency and its active power remains unchanged; the thermal power plants participate in the frequency regulation alone; after the cut-off of load 1, the frequency increased from 50 to 50.32 Hz. When PV plant participates in the system frequency regulation, it will reduce its output active power according to the frequency regulation curve under the condition that the system frequency increases; and after the cut-off of load 1, the frequency increased from 50 to 50.22 Hz. Therefore, after the participation of PV plant in system frequency regulation, the change in frequency decreases and the frequency regulation pressure of thermal power plants is shared. Under the condition of frequency fluctuation, the output power of PV plant changes very quickly, so the frequency will be stable faster.

Sudden increase in load
In the initial state, the load shedding level of PV plant is set as 20%P m . The initial output power of thermal power plants is set as 0.7 p.u., and all loads are put into operation except load 1. At 8 s, load 1 is being put in and causes the decline of system frequency. The simulation results are shown in Fig. 6.
In Fig. 6, the dynamic response of the system frequency, the output power of PV plant Ppv, and the output power of thermal power plants Pg1 and Pg2 are compared under the conditions that PV plant participates in the system frequency regulation and does not participate in the system frequency regulation. Consider the condition that PV plant does not participate in the system frequency regulation, and after the put-in of load 1, the frequency decreased from 50 to 49.68 Hz; when PV plant participates in the system frequency regulation, it will increase its output active power according to the frequency regulation curve under the condition that the system frequency decreases; and after the put-in of load 1, the frequency decreased from 50 to 49.78 Hz. Therefore, after the participation of PV plant in system frequency regulation, the change in frequency decreases and the frequency regulation pressure of thermal power plant is shared.

Conclusion
This paper proposes a novel control strategy of grid-connected PV power supply for frequency regulation. The static frequency characteristic curve of PV plant is proposed, and the load shedding level of PV plant is set. So PV plant has the margin to increase and decrease its active power, and after the frequency of the grid side of the PV power plant has been measured, the reference voltage of PV power outlet side can be determined. The simulation results show that PV plant can respond to the change of system frequency quickly, and when PV plant participates in the frequency regulation, the change of the system frequency can be decreased after the fluctuation and the frequency regulation pressure of the thermal power plants can be shared. New ideas and new directions for the development of PV power generation technology can be provided by the proposed strategy.