Frequency Control of Power System with Solar and Wind Power Stations by Using Frequency Band Control and Deadband Control of HVDC Interconnection Line

In recent years, environmental problems are becoming serious and renewable energy has attracted attention as their solutions. However, the electricity generation using the renewable energy has a demerit that the output becomes unstable because of intermittent characteristics, such as variations of wind speed or solar radiation intensity. Frequency fluctuations due to the installation of large scale wind farm (WF) and photovoltaics (PV) into the power system is a major concern. In order to solve the problem, this paper proposes two control methods using High Voltage Direct Current (HVDC) interconnection line to suppress the frequency fluctuations due to large scale of WF and PV. Comparative analysis between these two control methods is presented in this paper. One proposed method is a frequency control using a notch filter, and the other is using a deadband. Validity of the proposed methods is verified through simulation analyses, which is performed on a multi-machine power system model.


Introduction
since the Great East Japan Earthquake that occurred in 2011. Therefore, from the viewpoint of securing energy security and improving environmental problems, renewable energy has been attracting. It is a theme that can greatly contribute to the revitalizations of industry and economic activities, and the improvement of people's lives. Among renewable energy power sources, wind power generation and solar power generation have been attracting. Wind power generation has the advantages of relatively low power generation cost and high energy conversion efficiency compared with other types of power generation using renewable energy. In solar power generation, there is an advantage that the power generation efficiency is almost constant irrespective of the size of the installation system, and noise is not generated during power generation. In fact, wind power generation and solar power generation have been increasing year by year, and the amount of wind power generation in the world of 2016 is about 486.7 GW [1] and the amount of solar power generation is about 303 GW [2]. The renewable energy is likely to continue increasing in the future. However, the power generation using such renewable energy system may cause frequency and voltage fluctuations in the power system because of their fluctuating output due to intermittent characteristics, such as variations of wind speed or solar radiation intensity. Based on this fact, the frequency fluctuations in the power system with renewable power sources have been suppressed by the frequency controls in the thermal power plants and the hydraulic power plants so far. However, if the renewable power sources will increase, it is expected that the frequency fluctuations could not be suppressed by those controls sufficiently. If the frequency fluctuations cannot be suppressed within the permissible range of power system frequency, it is possible that some power plants may be stopped and some electrical equipment cannot operate normally. In order to solve this problem, various methods have been investigated and reported. Among them, there have been many researches about application of battery system, however, the battery systems are, in general, very expensive due to installation cost etc. Therefore, in recent years, frequency control using HVDC transmission line instead of battery has been studied [3] [4] [5] [6] [7]. Although, in these references [3] [4] [5] [6] [7], various studies have been presented for frequency control by HVDC transmission line connecting an offshore wind farm and an onshore power system, there are few papers reporting frequency control using HVDC transmission line connecting two power systems [8] [9]. Reference [8] presents a control method to suppress the frequency fluctuations occurring due to a large amount of PV power generation and wind power generation, which is based on power flow control of HVDC interconnection line using a different type of deadband from those in this paper in its frequency control system. In this paper, two methods are proposed to suppress the frequency fluctuations in one power system installed with large PV power station and wind power station, which is based on power flow control of HVDC interconnection line. And, these two proposed methods are examined and compared through simulation analyses.

K. Tada et al. Journal of Power and Energy Engineering
One of the two methods is a frequency band control using a notch filter. Generally, frequency fluctuations in the power system are suppressed by frequency control of the synchronous generators. However, there is a frequency band in which the control performance is not sufficient. Therefore, by determining the frequency band in which the frequency adjustment ability decreases in the power system with renewable power sources installed, a notch filter capable of effectively extracting only this band can be designed. Then, using the designed notch filter, fluctuating power in that frequency band, which cannot be suppressed sufficiently in the power system with renewable power sources, is transmitted to another power system through the HVDC interconnection line. As a result, frequency fluctuations in the power system with renewable power sources are suppressed.
The other method is a frequency control using deadband. This control is very simple. Only when the frequency deviation in the power system with renewable power sources exceeds a threshold value, the frequency control of HVDC interconnection line activates and the fluctuating power is transmitted to another system. In the case of the frequency control using notch filter, the frequency control of HVDC transmission line is always in operation. However in the case of the frequency control using deadband, the frequency control of HVDC transmission line is in operation only when the frequency deviation exceeds the threshold value. Therefore fluctuations in the HVDC transmission line power in the latter case become smaller than those in the former case.
The effectiveness of the above two proposed methods is verified by simulation analysis executed on PSCAD/EMTDC software (4.2.1).

Power System Model
The power system model used in this study and its parameters are shown in  [11]. 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 (infinite bus, System B) and the positive direction of its power flow is from System A to System B.

Governor Model
Governor is a device for controlling the rotational speed of the synchronous generator and turbine. When the balance between the turbine output and generator output does not hold, the rotational speed changes. In order to return the rotation speed to the synchronous speed, the turbine output is controlled by the K. Tada        shown in Figure 3 and The frequency deviation is input to the Low Pass Filter (LPF) to remove components of short period as shown in Figure 4, and then LFC signal is generated through PI controller. This is because the LFC is used to control frequency fluctuations with a long period of time. The angular frequency ω c (=2πf) of LPF is set to 0.005 [Hz]. The governor models used in this paper are based on the standard models of the Institute of Electrical Engineers of Japan.

Wind Turbine Model
Wind turbine model used in this paper is shown in Equations (1)-(5) [13].

PV System
PV model used in this study is shown in Figure 5 [8]. 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 V PV [kV], and the obtained current (I PV ) is entered to the grid from the current sources. PV voltage (V PV ) is fixed at 6.6 kV.

HVDC System
The HVDC system model used in this study is shown in Figure 6 [7] [9]. In this study, to shorten the calculation time of the simulation, the HVDC model is expressed by the simple model [11] using controlled voltage sources instead of IGBT based inverter and rectifier.

Control Model of System Aa Side Converter
The control block of the converter that converts from three-phase AC to DC voltage is designed as shown in Figure 7. First, the phase angle is obtained by detecting the phase voltage V r from the three-phase terminal voltage at the converter. Next, the d-axis and q-axis components (I rd , I rq ) of the current are obtained from the phase angle and the three-phase current. Active power P r and reactive power Q r of the converter are controlled independently by the d-axis and the q-axis components. For this purpose, the d-axis and q-axis components ( )  Table 3.

Control Model of System B Side Inverter
The control block of the inverter that converts the DC voltage to the three-phase AC is designed as shown in Figure 8. As shown in Figure 8, the phase angle is obtained by detecting the phase voltage V q from the three-phase terminal voltage at the inverter. Next, the d-axis and q-axis components (I qd , I qq ) of the current are obtained from the phase angle and the three-phase current. DC voltage V dc   The parameters of PI controllers (PI3, PI4) are shown in Table 3.

DC-Link Model
The DC-Link model is shown in Figure 9, in which the DC-link voltage is expressed by Equation (6) [7] [9].
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.
Rated voltage of the HVDC line is 250 kV.

Frequency Band Control Using Notch Filter
Firstly frequency characteristics of System A are analyzed in order to clarify the frequency band in which the synchronous generators and their frequency control systems in System A cannot suppress frequency fluctuations sufficiently. In order to perform the frequency characteristic analysis, System A of Figure 1 is expressed by using the simple frequency block model shown in Figure 10 [14] [15]. Figure 11 shows the result of frequency characteristics of System A, in which X-axis denotes frequency of input power variation and Y-axis denotes the maximum frequency deviation in System A. "No frequency control" in Figure  11 shows the result calculated from transfer function from output variations of WF and PV to frequency variations of System A, which is obtained from the frequency block model. Next, in order to construct a control system for HVDC line, a notch filter has been designed. The role of the filter is to decrease the frequency components that the synchronous generators and their frequency control systems in System Journal of Power and Energy Engineering   A cannot suppress sufficiently. Since the permissible range of power system frequency deviation in Japan is, in general, within ± 0.2 Hz, the filter is designed so that the maximum frequency deviation is equivalent to 0.2 Hz. "with Notch filter" in Figure 11 are the results. In this paper, the transfer functions of the designed notch filter is shown in Equation (7), where ζ 1 is damping coefficient, ω c1 is center angular frequency, and their values are set to ζ 1 = 4.9, ω c1 = 0.387 respectively.   Figure 14, the output image of the deadband is shown.
The threshold value "a" of the deadband is set to 0.05 [Hz] in this paper, and P gain is set to %K G = 15.0, which is the system constant of System A. The equation about the system constant %K G is shown in Equation (8).

Simulation Results
To confirm the effectiveness of the proposed methods, simulation analyses for the model system of Figure Table 4. The wind speed data shown in Figure 15 is input into the wind turbine model in the simulation analyses. This is real wind speed data measured in Hokkaido Island, Japan. The calculated output of wind turbine generator is shown in Figure 16. The PV output data shown in Figure 17 is used in the simulation analyses. This is also real PV data measured in Hokkaido Island, Japan. The total of this output of wind turbine generator and PV station is shown in Figure 18.
Simulation results of 3 cases are shown in Figure 19 and Figure 20. Figure 19 shows responses of System A frequency and it is seen the frequency variations      Table 5 shows the maximum Journal of Power and Energy Engineering     Frequency control with Notch filter (HVDC transmission power is controlled within 0.5 ± 0.1 p.u.) 3 Frequency control with Deadband (HVDC transmission power is controlled within 0.5 ± 0.1 p.u.) Table 5. Maximum deviation and standard deviation of system frequency. deviation and standard deviation of the system frequency fluctuations, from which it is also seen that the system frequency fluctuations can be suppressed effectively in Case 2 and Case 3. Table 6 shows the maximum deviation and Journal of Power and Energy Engineering

Discussion
In the simulation analysis of Section 3, two proposed methods for stabilizing the frequency fluctuations of system A have been analyzed. From Table 5, it is seen that the frequency control using the deadband (Case 3) can suppress the frequency fluctuations slightly more than the frequency control using the notch filter (Case 2). This is because, the frequency control based on the notch filter suppresses the frequency fluctuations just within ±0.2 Hz while the control using deadband suppresses all components of the frequency fluctuations whenever the fluctuations become over the threshold value. On the other hand, it is seen from Table 6

Conclusion
In order to suppress the frequency fluctuations in the power system with large amount of PV and WF installed, two methods of frequency control based on the power flow control of HVDC interconnection line are proposed in this paper, and their validity is examined through simulation analyses. As a result, it is shown that both methods can suppress the frequency fluctuations effectively.
Considering the ability to suppress the system frequency fluctuations with fewer fluctuations in HVDC line power flow, however, it can be concluded that the