Modelling and transient stability of large wind farms

doi:10.1016/S0142-0615(02)00017-0

International Journal of Electrical Power & Energy Systems

Volume 25, Issue 2, February 2003, Pages 123-144

https://doi.org/10.1016/S0142-0615(02)00017-0 Get rights and content

Abstract

The paper is dealing with modelling and short-term voltage stability considerations of large wind farms. A physical model of a large offshore wind farm consisting of a large number of windmills is implemented in the dynamic simulation tool PSS/E. Each windmill in the wind farm is represented by a physical model of grid-connected windmills. The windmill generators are conventional induction generators and the wind farm is ac-connected to the power system. Improvements of short-term voltage stability in case of failure events in the external power system are treated with use of conventional generator technology. This subject is treated as a parameter study with respect to the windmill electrical and mechanical parameters and with use of control strategies within the conventional generator technology. Stability improvements on the wind farm side of the connection point lead to significant reduction of dynamic reactive compensation demands. In case of blade angle control applied at failure events, dynamic reactive compensation is not necessary for maintaining the voltage stability.

Introduction

Denmark has currently about 2300 MW wind power capacity in on-land and few offshore settings [1], which corresponds to more than 20% of power consumption (in average). Further, construction of two large-scale offshore wind farms of 150 MW power capacity each has been announced. The first large offshore wind farm in Denmark will be constructed at Horns Rev by the year 2002 in the area of the system operator ELTRA [2]. This will be followed by the first in the area of the Eastern Danish system operator, ELKRAFT System, large offshore wind farm at Rødsand by the year 2003 [3].

The installed capacity in on-land settings and in combined heat-power units (CHP) will increase as well, whilst the power production and control ability of the conventional power plants with respect to voltage and frequency are reduced. In the years to come, the power production pattern in the Danish power system will change from the power supply from conventional power plants—as it is known today—to a power supply mix, where about 30–40% of power consumption (in average) is covered by wind power. In other words, the power technology will undergo changes from a well-known technology built-up about conventional power plants to a partly unknown technology—wind power.

In the years to come, it will be focusing on maintaining power system stability and voltage stability, for example at a short circuit fault, ensuring power supply safety and other important tasks [4] as amount of wind power is drastically increasing. This situation makes it necessary to find solutions with respect to maintaining dynamic stability of the power system with large amount of wind power and its reliable operation. These solutions are based on a number of requirements that are formulated with respect to operation of the large offshore wind farms and the external power system in case of failure events in the external system.

The paper contains separate subjects dealing with design of windmills for large offshore applications and their control that shall be taken into account with respect to improving the short-term voltage stability.

Section snippets

System stability requirements

In terms of short-term voltage stability, the major goal is the voltage re-establishing after failure events in the power system with large amount of wind power. The transmission system operator is responsible for maintaining power system stability and reliable power supply.

As the situation is today, the majority of the Danish windmills on-land are stall wind turbines equipped with conventional induction generators and ac-connected to the power system. In case of a short circuit fault in the

Wind farm model

The windmill technology in offshore settings has to be robust, developed and known from practical applications. The wind turbine concept with conventional induction generators has been in operation in on-land settings in Denmark during many years, which is why it may be considered that this technology will be used offshore as well. The wind turbines are equipped with blade angle control systems—pitch or active stall that make it possible to adjust the set-points of the wind turbines by the

Dynamic reactive compensation

In this work, the dynamic reactive compensation of the large offshore wind farm is a SVC of the capacity that will be necessary for maintaining the short-term voltage stability. The model of the SVC is as in Ref. [15].

When operating as stall windmills

Blade angle control is primarily used for optimisation of the wind turbine mechanical power with respect to incoming wind [11], [12] and hence, this control ability is not necessarily available at failure events in the external power system with respect to maintaining the short-term voltage stability. This implies that the pitch or active stall wind turbines may operate as conventional (passive) stall wind turbines, by the same way as windmills on-land, with the exception that they may not be

Dynamic stability improvements within conventional technology

The movement equation of a windmill in terms of the lumped-mass system is [9], [16] $d d t (ω_{L})= T_{M} −T_{E} 2(H_{M} +H_{G}),$ where T_M and T_E are the mechanical torque of the rotating mill and the electric torque, respectively, and ω_L is the lumped-mass system speed [9] $ω_{L} = H_{M} ω_{M} +H_{G} ω_{G} H_{M} +H_{G},$ where ω_M and ω_G are the mill mechanical speed and the electric speed of the generator, respectively, and T_M=P_M(w)/ω_M at the given wind, w.

The dynamic stability limit of the windmill is found [16] from the movement , as the speed ω_L

Dynamic rotor resistance

The ideas behind the design of the dynamic rotor resistance are:

1.
In normal operation, the rotor resistance shall be low with respect to minimise of the power losses.
2.
When a failure event in the power system is registered, the rotor resistance is increasing. By this way, the critical speed of the windmill, ω_C, is expanded and the dynamic stability is improved.
3.
During failure events, however, some higher, but still fair, losses in the rotor circuit may be accepted.
4.
When the voltage is re-established,

Blade angle control

In terms of the dynamic stability limit considerations, the short-term stability of the wind turbines may be improved by a temporary reduction of the wind turbine mechanical power [17]. This principle is illustrated in Fig. 12.

This principle can be realised by use of the blade angle control acting at failure events in the external power system for maintaining the short-term voltage stability. This control principle—the blade angle control—is already realised in modern wind turbines, where

1.
active

Mixed wind farm

The commonly asked question is what happens if a large wind farm will be equipped with the wind turbines having different mechanical parameters. This question is related if there is any risk of oscillations between the wind turbines with different mechanical parameters in case of disturbances. When the windmill mechanical parameters are different, the windmill natural frequencies—the shaft torsional modes—will be also different.

Since the wind turbines are equipped with induction generators,

Robustness of control principles

In practical operation, control equipment failures may happen contemporarily with or be even caused by disturbances in the power system. Such failures can be experienced, for instance, in the dynamic reactive compensation units or in the blade angle control systems of the individual windmills. Such failures of the control systems may not lead to fatal outage with respect to maintaining the voltage stability. Robustness of the control principle will, in this term, relate to how large part of the

Conclusions

A physical model of the large offshore wind farm consisting of 80 2 MW wind turbines is set up. The farm is ac-connected to the transmission network.

Transient stability of the wind farm is investigated when the external network is subjected to a short circuit fault and the faulted line is tripped. In accordance with the Danish Specifications for the large offshore wind farm [5], the farm may not be tripped from the power system at the failure events, but will maintain its operation. The goal has

Acknowledgements

This work is granted by the electric power distribution company NESA A/S, Copenhagen, Denmark, and The Danish Academy of Technical Sciences, Lyngby, Denmark.

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