Keywords

1 Introduction

Unlike conventional generators, the wind energy produces stochastic power output due to inherent wind characteristics. In recent years, the power production from wind energy resources has increased exponentially. Therefore, the stability of power system becomes an important issue in modern power systems. The transient stability is among one of the electrical power system stability. According to IEEE/CIGRE joint task report [1] “Transient stability is concerned with the ability of the power system to maintain synchronism when subjected to a severe disturbance, such as a short circuit on a transmission line.” In Denmark, the power output from wind energy supplies more than 20% of the local electricity consumption and the aim of the Danish government to increase this share to 50% by 2025 [2, 3]. The voltage and transient stability of wind farms integrated into the power grid was studied in [4]. The ability of fast response under large disturbances, the power converter of doubly-fed induction generators (DFIGs) enables wind farm to reach steady-state conditions much faster than conventional generators. In [5], the sensitivity-based method was investigated in order to determine the impact of DFIG based wind turbine generators (WTGs) on small-signal and transient stability of power systems. The intermittent generation characteristics of wind farms lead to fluctuating power output which may further push the stability margin to its limit. In [6], a method was proposed for fast assessment of the transient stability margin (TSM) considering the uncertainty of wind generators. In [7], a wide area control (WAC) was proposed to enhance the transient stability of the DFIG integrated power grid. A real-time method for the assessment of transient stability of a power system comprising wind turbine was proposed in [8]. The proposed real-time method utilizes the corrected kinetic energy method for determining the critical clearing time. In [9], a coordinated control scheme for superconducting magnetic energy storage (SMES) devices in the wind power integrated system were investigated in order to enhance the overall transient stability of the system. To enhance the transient stability of large power system the researchers had applied the STATCOM and BESS [10]. It is observed that power system integrated with DFIG based wind farms was less sensitive to transient disturbances such as fault clearing time, voltage sags and wind penetration below a certain threshold. Above threshold the wind farm has adverse effect on transient stability of system [11]. Authors in [12] had utilized energy capacitor system (ECS) comprised of electric double layer capacitor (EDLC) and power electronics device to improve the system transient stability. In [13], the performance of the DFIG based wind farm on different transient disturbances had investigated.

In this paper, the transient disturbances such as sudden load change, three-phase faults, sudden change in wind speed, and wind gust are investigated in a power system comprising wind farms. To enhance the system transient stability, the devices, i.e., BESS and STATCOM are also considered. The paper is structured as follows: Sect. 2 explains the model of DFIG based wind farm and its control. Section 3 discusses the BESS and its control. Section 4 describes the STATCOM and its control. Finally, Sect. 4 concludes the results. The standard IEEE 14 bus test system is used to obtain the results. All simulation studies have performed in DlgSILENT PowerFactory software.

2 Modeling of DFIG Based Wind Power

In literature [14,15,16], different models of DFIG based wind power for the transient stability study have been proposed. The schematic diagram of the grid integrated DFIG is represented in Fig. 1. The DFIG based wind turbine has the ability to independently control the active and reactive power during transient disturbances. In this paper, the DFIG based wind turbine model available in PowerFactory library have utilized. The block diagram of DFIG with its controllers is shown in Fig. 2.

Fig. 1
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Grid integrated DFIG based wind power

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Fig. 2
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Block diagram of DFIG with controllers (PowerFactory library model)

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3 Modeling of BESS and Its Control

The BESS technology provides fast active power compensation in power systems during transient disturbances. The BESS technology comprises of two parts: battery and rectifier/inverter. A voltage source converter (VSC) functions as a rectifier and inverter during charging and discharging, respectively. The schematic diagram of a typical battery storage is represented in Fig. 3.

Fig. 3
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Block diagram representation of battery

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The state of charge (SOC) defines the current status of battery. If SOC is one, then the battery is fully charged while SOC is zero, then the battery is fully discharged. There are two main constraints on the system: first is the rated power/current of the converter and the second is the capacity of the battery that is the amount of stored energy. In this paper, the BESS of 30 MVA capacity has utilized. The composite model of battery with its controller is shown in Fig. 4.

Fig. 4
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Block diagram representation of composite model of BESS

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The voltage output from a typical BESS can formulate using Eq. (1).

$$U_{\text{DC}} = U_{{\max} } \times SOC + U_{{\min} } \times (1 - SOC) - IZ_{i}$$
(1)

where SOC: State of charging; Umax: Maximum voltage output; Umin: Minimum voltage output; I: current; Zi: equivalent internal impedance.

4 Modeling of STATCOM and Its Control

The static compensator (STATCOM) is a FACTS device connected in shunt position and used for reactive power compensation in the transmission network. The schematic diagram and V-I characteristics of STATCOM is shown in Fig. 5. The basic STATCOM device consists of VSC, coupling transformer, and capacitor bank. The STATCOM has the ability to supply/absorb reactive power independent with system voltage at the point of common coupling (PCC) during a transient disturbance. The control scheme for typical STATCOM is shown in Fig. 6.

Fig. 5
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STATCOM and V-I characteristics

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Fig. 6
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Control scheme of STATCOM

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5 Simulation and Results

In this paper, the IEEE 14 bus test system is utilized to investigate the transient stability of the system comprising of the DFIG based wind turbine which is shown below in Fig. 7. The test system consists of five generators (Synchronous generator -02 Nos, Synchronous Compensator -03 Nos), 17 transmission lines, 11 constant load demand, 19 buses, and 8 transformers. The total system active and reactive load demand is 259 MW and 73.5 MVAr, respectively. The 30% load scaling has set for the test system. In order to compensate for the increased load demand, the 13 Nos. DFIG based wind turbine having a 6 MW capacity each has integrated into the test system. The transient stability of test system comprising DFIG based wind farms in the presence of STATCOM and BESS has been investigated during following transient disturbances.

  1. 1.

    Sudden change in load demand

  2. 2.

    Three-phase fault in transmission line

  3. 3.

    Sudden change in wind speed

  4. 4.

    Effect of Wind gust.

The following cases are taken into consideration. Case 1: Test system with DFIG based wind farm Case 2: Test system with DFIG based wind farm, STATCOM, and BESS.

Fig. 7
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IEEE 14 bus test system

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5.1 Sudden Change in Load Demand

The sudden change in load demand is considered as a transient event. The sudden increase in load demand at selected bus can introduce system transients. In this paper, the power system performance is investigated for 20% increase in load demand at bus 3, 4, and 9 at after 1.0 s. The temporary difference in the power balance between the mechanical and electrical power of each generator can lead to acceleration of rotor angle for the whole system. The variation in various parameters of different elements of the IEEE 14 bus test system is represented in Fig. 8.

Fig. 8
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Variation of various parameters under sudden load change

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It is observed from Fig. 8, the transient response of various parameters in case 2 has better performance compared to case 1.

5.2 Three-Phase Fault in Transmission Line

The most severe disturbance in the power network is three-phase fault on the transmission network. In this paper, a symmetrical three-phase fault is created at 1.0 s on transmission line connected between bus 2 and 3, which is cleared at 1.5 s. When three-phase fault occurs in the system, the dynamics during the post fault can become unstable because of inadequate damping supplied from the generator. The combination of BESS and STATCOM offers an additional degree to add damping in the system and assist with mitigating the instability problem. The results obtained through simulations are represented in Fig. 9.

Fig. 9
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Variation in different parameters under three-phase fault

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5.3 Sudden Change in Wind Speed

The simulation is started normally and sudden increase in wind speed from 11.019 m/s to 14 m/s. When the wind speed is increased, the negative slip is also increased, therefore the power delivered from the stator side is decreased and power delivered from rotor side is increased. The system gets back in stable state after some seconds. When generator speed is increased then the pitch angle of the system settles to a new value. The power delivered by the generators also settles to a new value so that the maximum power can be achieved from the new speed. The simulation results are shown in Fig. 10.

Fig. 10
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Variation in different parameters under sudden change in wind speed

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5.4 Effect of Wind Gust

A wind gust starting at 2 s and ending at 5 s is simulated. All parameters are settling back to its original position. Hence a wind gust can reach its original position of the event, but this event takes a longer time to settle down to the steady-state. The results obtained from the simulation are represented in Fig. 11.

Fig. 11
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Variation in different parameters under wind gust transient condition

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6 Conclusion

In this paper, the transient stability of a power system comprises of the DFIG based wind farm is investigated. To enhance the transient stability of the system, STATCOM and BESS are applied. The transient disturbances, i.e., sudden change in load demand, three-phase fault on the transmission line, sudden change in wind speed, and the effect of wind gust has investigated on wind energy integrated power system with/without STATCOM and BESS. It is observed that STATCOM and BESS are the most effective solution to improve the transient stability of a power system with wind farms. They can absorb or produce active and reactive power, according to the requirement under transient events.