Frequency Control of Power System with Renewable Power Sources by HVDC Interconnection Line and Battery Considering Energy Balancing

Recently, introduction of renewable energy sources like wind power generation and photovoltaic power generation has been increasing from the viewpoint of environmental problems. However, renewable energy power supplies have unstable output due to the influence of weather conditions such as wind speed variations, which may cause fluctuations of voltage and frequency in the power system. This paper proposes fuzzy PD based virtual inertia control system to decrease frequency fluctuations in power system caused by fluctuating output of renewable energy sources. The proposed new method is based on the coordinated control of HVDC interconnection line and battery, and energy balancing control is also incorporated in it. Finally, it is concluded that the proposed system is very effective for suppressing the frequency fluctuations of the power system due to the large-scale wind power generation and solar power generation and also for keeping the energy balancing in the HVDC transmission line.


Introduction
Many studies have been reported so far about renewable energy systems like How to cite this paper: Onuka, S., Umemura, A., Takahashi, R., Tamura, J., Sakahara, A., Tosaka, F. and Nakamoto, R. (2020) Frequency Control of Power System with Renewable Power Sources by HVDC Interconnection Line and Battery Considering Energy Balancing. Journal of Power and Energy Engineering, 8, 11-24. wind power generation and photovoltaic power generation [1] [2]. The advantage of wind power generation is that the power generation cost is comparatively lower and the energy conversion efficiency is higher than other renewable energy power generations. In addition, there is the advantage in photovoltaic power generation that no noise is appeared in power generation process and the power generation efficiency is about constant regardless of the installed system size. However, the output of these renewable energy power generations may fluctuate due to intermittent characteristics like fluctuations of wind speed and solar radiation intensity. This can cause power system frequency and voltage fluctuations. Therefore, various methods for controlling frequency fluctuations have been investigated [3] [4] [5] [6]. Among them, control method using HVDC interconnection line can be thought to be strong and effective method [7]. This is because the power ratings of actual HVDC interconnection lines are relatively large in most cases and active power flow on the HVDC transmission line can be controlled, in general, easily and quickly based on the AC/DC converter control. This paper proposes fuzzy PD based virtual inertia control system to restrain system frequency fluctuation caused by fluctuating output of renewable energy power supply, which is based on the coordinated control of HVDC interconnection line and battery. In addition, we also propose energy balancing control which is incorporated in the coordinated control system to keep the transmitted energy (MWh) of HVDC transmission line constant at the end of every specified period. Though there have been some reports so far in which power system frequency fluctuations caused by fluctuating output of renewable power sources are controlled by using HVDC interconnection line and battery as stated above, there is almost no one which achieves also the energy balancing control as far as the authors know. Simulation analyses are performed by using PSCAD/EMTDC software to verify the effectiveness of the proposed method.

Power System Model
The power system model used in this study and its parameters are shown in Figure 1. It is a modified version of the IEEE standard model with 9 buses [8] which is composed of 3 synchronous generators (SG1, SG2, and SG3). SG1 and SG2 are thermal power plants (SG1: 200 MVA, SG2: 200 MVA), and SG3 is a hydraulic power plant (200 MVA). Moreover, a fixed speed wind turbine squirrel cage induction generator (SCIG) based wind farm (WF, 60 MVA), a PV station (60 MVA), HVDC interconnection line (60 MVA), battery (10 MVA), and three loads (Loads A, B, and C) are connected to the main system [9]. Their conditions are shown in Table 1. The HVDC transmission line is connecting the main modified 9-bus power system (System A) and another large power system (System B, expressed by an infinite bus) and the positive direction of the power flow is from System A to System B. Positive direction of battery power corresponds to charging. Journal of Power and Energy Engineering

Governor Model [10]
In this study, SG1 and SG3 are operating under load frequency control (LFC) and SG2 is under governor free (GF) operation. Figure 2 and Figure

Wind Turbine Model [11]
Wind turbine model used in this paper is shown in Equations (1)-(5).

PV Model [3] [7]
Figure 5 shows PV model used in this paper. In this study, the PV model is expressed by a simple model using current sources, in which kilowatts data, P PV [kW], is used. Therefore, PV current (I PV ) is calculated from P PV [kW] and PV voltage, V PV [kV], and the obtained current (I PV ) is entered to the current sources. Figure 6 shows the Voltage Source Converter (VSC) based HVDC system model used in this study. In this paper, in order to shorten the calculation time of the simulation, VSC-HVDC simple model [9] is used which is expressed by con-  The parameters of PI controllers used in Figure 7 and Figure 8 are shown in Table 3.

DC Link Model [4] [7] [12]
DC-link voltage in the DC-Link circuit of the HVDC model shown in Figure 6 is expressed by Equation (6).
where, V dc : DC voltage, C dc : capacitance of smoothing capacitor in the DC link (50,000 μF), P r : active power of the converter, P q : active power of the inverter, V dcn : rated voltage of the HVDC line (250 kV). Figure 9 shows battery model used in this paper. The battery model is expressed by a simple model using voltage sources. Q ref_Battery is fixed at zero. Figure 10 shows the control model of battery converter that converts from three-phase AC to DC voltage. The parameters of PI controllers used in Figure 10 are shown in Table 3. Figure 11 and Figure 12 show the fuzzy PD based virtual inertia control systems for the HVDC interconnection line and the battery proposed in this paper. The controller for the HVDC line shown in Figure 11 is composed of PD controller in which D controller is based on FLC (Fuzzy Logic Controller). As differential value of the system frequency is used in the FLC, it can generate virtual inertia output component, and thus, it makes it possible to respond quickly to sudden frequency fluctuations. Figure 13 and Table 4 show the membership functions and fuzzy rule base designed for the FLC.

Coordinated Control
There is a permissible control range in the transmission power on HVDC interconnection line. This is ±10% of the rated capacity around the steady state transmission power in Japan under normal situation. Therefore, if the system frequency fluctuation is large so that the reference power of the HVDC line is over the permissible range, the HVDC line cannot control the frequency fluctuation. This paper proposes the coordinated control of HVDC interconnection line and battery. Figure 12 shows the control systems for the battery, where the amount of power exceeding the permissible control range of the HVDC line is calculated, and then, sent to the battery as an output reference value.

Energy Balancing Control [13]
Figure 14 and Figure 15 show the control blocks used for energy balancing control of the HVDC system. The HVDC transmission MWh deviation, ΔMWs, from System A to System B is calculated by Equation (7).

( )
where, P DC0 is the reference value for HVDC transmission power, and P DC is the actual transmission power of the HVDC interconnection line. An auxiliary power signal P EB for the balancing control is generated by multiplying ΔMWs by a gain as shown in Figure 14, and subtracted from the sum of PD control references as shown in Figure 11 and Figure 12. In the control block shown in Figure 15, ΔMWs is passed through a Variable Dead Band (VDB), and then the auxiliary power signal P EB is obtained. As shown in Figure 11 and Figure 12, energy balancing control is performed by subtracting the auxiliary power signal P EB from the frequency control compensation power signals in the HVDC interconnection line and the storage battery. As described later, two types of gains that increase with time from a certain time t to t + τ in Equation (7) are used in the control block shown in Figure 14.

Simulation Conditions
Simulation analyses have been performed in the 6 cases shown in Table 5 to confirm the effectiveness of the proposed method. Figure 16 shows the wind speed data and Figure 17 shows the resulting output of WF used in the simula-tion analyses. Figure 18 shows the PV data used in the simulation analyses. Figure 19 shows the total output of WF and PV station. The data used in Figure  16 and Figure 18 were real data measured in Hokkaido, Japan. The simulation time was 1200 seconds and PSCAD/EMTDC software was used for the simulation analyses.  Figure 14. Energy balancing control block using gain.     In order to confirm the effectiveness of the proposed method, the energy balancing control for 10 minutes is performed twice in the simulation for 1200 seconds, i.e., 0 to 600 seconds and 600 to 1200 seconds. As shown in Figure 20, two types of gains are used in the control block shown in Figure 14, where gain I increases linearly from 0 at t = 300 seconds to 1.0 at t = 600 seconds and gain II increases linearly from 0 at t = 550 seconds to 1.0 at t = 600 seconds. On the other hand, Variable Dead Band is used in the control block shown in Figure 15, of which threshold value becomes gradually narrower from ±6 [MWs] at 0 seconds to 0 [MWs] at 600 seconds as shown in Figure 21. In Figure 21, the white area is dead zone, and if ΔMWs exceeds the pink line, which is the threshold line, the energy balancing control is activated. Interval of the integration of Equation (7) for ΔMWs is set to [0, 600].

Simulation Results
The simulation results are shown in Figures 22-25 and Table 6. Figure 22 shows the frequency responses of System A in the 6 cases. Figure 23 shows the HVDC transmission line power. Figure 24 shows the battery power (positive value corresponds to charging). Figure 25 shows the transmitted energy deviation, ΔMWs, obtained from Equation (7). Table 6 shows the maximum deviation and the standard deviation of the frequency fluctuation of System A.
Comparing Cases 1 to 6 in Figure 22 and Table 6, it is seen that the maximum frequency deviation in Case 1 exceeds the permissible range, ±0.2 Hz, which is the standard permissible range of power system frequency fluctuations in Japan. This is because the frequency control is not performed in Case 1. The frequency deviation is largest in this case. Next, the dead band control is performed in Case 2. However, since only the control by the HVDC interconnection line is performed in this case, the frequency fluctuation is also large and the      However, it is seen from Figure 25 that the transmitted energy deviation, ΔMWs, does not become 0 at 600 and 1200 seconds, because the energy balancing control for 10 minutes is not performed in this case. On the other hand, ΔMWs becomes 0 at 600 and 1200 seconds in Cases 4, 5, and 6 as can be seen from Figure 25, because the energy balancing control is performed in these cases. This confirms that the proposed energy balancing control is very effective for keeping the transmitted energy balance. In addition, it is also seen from Table 6 that the performances about the system frequency deviations in these 3 cases are worse than that in Case 3. This is because the energy balancing control is also performed in Cases 4, 5 and 6. However the performances in Cases 5 and 6 are almost the same level as that in Case 3, and the best results are obtained in Case 5 where gain II shown in Figure 20 is used in the energy balancing control block shown in Figure 14.
As a result, it can be concluded that the coordinated control of HVDC interconnection line and battery in Case 5 is the most effective method for achieving both the system frequency control and the energy balancing control in the HVDC transmission line. As the proposed coordinated system (Case 5) can con-trol well power system frequency fluctuations caused by fluctuating output of renewable power sources as well as can achieve the energy balancing in the HVDC interconnection line, it has original and significant values from a point of view of stabilization of power systems with renewable power sources installed.

Conclusions
In this paper, a coordinated control method of HVDC interconnection line and battery is proposed in order to suppress the frequency fluctuations of the power system where wind farm and solar power station are installed, in which the control system is designed based on the fuzzy PD based virtual inertia controller and the energy balancing control in the HVDC transmission line is also considered.
Simulation analyses show the effectiveness of the proposed method. As a result, it is concluded that the system proposed in this paper is very effective for suppressing the frequency fluctuations of the power system caused by the large-scale introduction of wind power generation and solar power generation and also for keeping the energy balancing in the HVDC transmission line.