Wenjuan Du

Investigation on Probabilistic Small-Signal Stability of Power Systems as Affected by Offshore Wind Generation

This paper investigates the impact of the complementarity, connection technology and network control strategy of offshore wind generation on the probabilistic small-signal stability of power systems. The cumulant-based theory is firstly applied to assess the variances of both the complementarity of combined wind power and the probabilistic small-signal stability of power systems. These variances are later employed as two indices to explain how the different complementarity levels, connection topologies and DC network control schemes of offshore wind generation affects the probabilistic distribution of critical eigenvalue and thus the probabilistic small-signal stability. In the paper, an example of 16-machine 5-area power system with three grid-connected offshore wind farms is presented. The index of probabilistic small-signal stability is computed and then compared under two complementarity conditions, two typical HVDC transmission systems through which the offshore wind farms are connected, and two DC grid control schemes, respectively. It is demonstrated that complementarity can effectively enhance probabilistic stability and multi-terminal HVDC may have either beneficial or detrimental impact on small-signal stability depending on the control pattern and setting when compared with point-to-point topology.

Robustness of photovoltaic system based stabilizer to mitigate inter-area oscillation in a multi-machine power system

This paper presents a case of photovoltaic system applying stabilizers in a 16-machine power system to alleviate inter-area power oscillations. As PV power station can exchange both active and reactive power with power grid, stabilizers attached to a PV system is able to function to suppress power system oscillations via regulation of active or/and reactive power. Thus PV system based stabilizers can be classified into two types, active stabilizer and reactive stabilizer. In the paper, firstly mathematical model of PV power station installed in a multi-machine power system is described. The damping function of stabilizers attached to the PV system is analyzed. Then a case of applying PV generation stabilizers in multi-machine power system is presented. Both the active and reactive stabilizer attached to the PV system is designed and their performance to suppress inter-area power oscillations is compared.

Modal Analysis of Small-Signal Angular Stability of a Power System Affected by Grid-Connected DFIG

This chapter introduces modal analysis of small-signal angular stability of a power system affected by a grid-connected DFIG. First, a method of decomposed modal analysis is proposed. The method can separately examine the impact of load flow change and dynamic interactions brought about by the grid-connected DFIG. Second, the impact of dynamic interactions introduced by the grid-connected DFIG on power system small-signal angular stability is examined comprehensively. Two indices to estimate the impact are introduced.

Probabilistic Analysis of Small-Signal Stability of a Power System Affected by Grid-Connected Wind Power Generation

The operation of power system is stochastic in nature, and the uncertainties can be brought about by many aspects of the system, such as the stochastic variation of load conditions and intermittence of renewable energy generations (e.g., wind and solar). Analysis of power system small-signal stability introduced in this book so far (i.e., modal analysis and damping torque analysis) as well as system time-domain simulation are based on the deterministic system operating conditions with specific loading situation and constant network configuration. Hence, they are not able to deal with stochastic fluctuations of random variables in power systems and may bring impractical results to the system stability analysis. This limitation of deterministic analysis motivates the research of probabilistic analysis into small-signal stability issues, in which the uncertainty and randomness of power system behaviors can be fully considered and discussed.

Small-Signal Angular Stability of a Power System Affected by Strong Dynamic Interactions Introduced from a Grid-Connected VSWG

Small-signal angular stability of a power system may be affected considerably by a grid-connected VSWG under the condition of open-loop modal resonance. This chapter introduces the concept of open-loop modal resonance. Why strong dynamic interactions may be induced by the grid-connected VSWG when the open-loop modal resonance happens is examined. Analysis is presented to indicate that the open-loop modal resonance may very likely degrade the small-signal angular stability of the power system. Both the grid-connected PMSG and DFIG are studied.

Linearized Model of a Power System with a Grid-Connected Variable Speed Wind Generator

Linearized model of a power system with a grid-connected variable speed wind generator (VSWG) is derived in this chapter in three steps. First, the linearized model of the VSWG is established. That includes the establishment of linearized model of a PMSG and a DFIG. Second, the linearized model of the power system is derived. Finally, the linearized model of the power system with the VSWG is established by combining the model of the VSWG and the power system.

Damping Torque Analysis of Small-Signal Angular Stability of a Power System Affected by Grid-Connected Wind Power Induction Generators

Induction generator based wind power generation has been dominating the wind market since the rise of wind power industry at the end of last century and will be continuously in a favorable position for large-scale grid connection given its lower cost and more mature technology compared with other wind generation for the foreseeable future [1]. Fixed-speed induction generator (FSIG-Type 1 Wind Gen Model) and doubly-fed induction generator (DFIG-Type 3 Wind Gen Model) are two main types of induction generator adopted for wind power generation especially considering the fact that DFIG is the most frequently-used technology to date.

Small-Signal Stability of a Power System with a VSWG Affected by the PLL

Grid connection of a VSWG to a power system is realized by the converter control, which normally adopts the current vector control algorithm. This has been introduced in Chap. 2. Implementation of current vector control needs to track the relative positions between the d − q coordinate of the converter and the common x − y coordinate of the power system in order to determine the d − q coordinate of the converter. As being illustrated by Fig. 2.8, the direction of the terminal voltage (PCC voltage) of a PMSG in x − y coordinate of the power system is normally taken as that of d axis of d − qcoordinate of the GSC of the PMSG. Figure 2.10 shows that the direction of the terminal voltage (PCC voltage) of a DFIG in x − y coordinate of the power system is often taken as that of d axis of d − q coordinate of the RSC of the DFIG. Hence, by tracking the phase of the terminal voltage of the VSWG, d − q coordinate of the converter can be determined for implementing the current vector control. This task of phase tracking is normally fulfilled by a phase locked loop (PLL).

Small-Signal Stability of a Power System Integrated with an MTDC Network for the Wind Power Transmission

As it is introduced in Chap. 1, small-signal stability of an AC power system integrated with a multi-terminal DC (MTDC) network for the wind power transmission is determined by the dynamic interactions between the VSCs and synchronous generators (SGs). The dynamic interactions are through both the MTDC network and AC grid. Hence, the small-signal stability of an MTDC/AC power system is a complicated issue, which is addressed in this chapter by focusing on the following three particular aspects.

Damping torque analysis of virtual inertia control for DFIG-based wind turbines

The increasing penetration of large-scale wind generation in power systems will challenge the power system inertia due to the reason that the converter based variable speed wind turbines have no contribution to the system inertia. Traditionally, a doubly fed induction generator (DFIG)-based wind power plant naturally does not provide frequency response because of the decoupling between the output power and the frequency. Moreover, DFIGs also lack power reserve margin because of the maxi mum power point tracking (MPPT) operation. In this paper, a virtual inertial control strategy of the DFIG based wind turbines called supplementary control loop for inertial response is investigated. When the system frequency is changing severely, the out put power of DFIG should respond to it rapidly through the virtual inertial controller at the same time. The rotor speeds of wind turbines can also be adjusted into this procedure. The inertial control methods proposed in this paper can supply controllable virtual inertia of DFIGs to the power system so that the system frequency stability can be strengthened through inertial control of wind turbines on the basis of damping torque analysis.