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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Electrical aspects and grid integration' taking place on Thursday, 13 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Elías Jesús Medina Domínguez Instituto Tecnológico de Canarias, S. A., Spain
Co-authors:
Elías Jesús Medina Domínguez (1) F P José Fernando Medina Padrón (2)
(1) Instituto Tecnológico de Canarias, S. A., Santa Lucía. Gran Canaria., Spain (2) Instituto Universitario SIANI. Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain

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Abstract

The critical clearing time, a factor influencing wind energy integration in small isolated power systems. Lanzarote-Fuerteventura 2020: a case study

Introduction

This paper deals with the relationship between the amount of wind energy that can be integrated in a small power system and the critical clearing time (CCT).
The CCT is an important parameter used for designing a reliable and effective protection scheme in power systems.
This relationship is more significant in the case of small isolated power systems due to their own characteristics.



Approach

Large power systems, such as the European interconnected power system (UCTE), can withstand high penetration levels of wind energy.
However, small isolated power systems, such as those present in small islands, show very important problems related to security issues [1].

In general, the amount of wind energy that can be connected to a power system depends on several factors, such as:
- Power system size
- Transmission system topology
- Conventional generation technology
- Wind generation technology
- Interconnections with other power systems [2].

Another issue that could be considered is the CCT, particularly in small isolated systems.
The CCT is defined as the maximum time that a short circuit can last without becoming a critical disturbance for the overall system.
A critical disturbance may be a unit outage due to a loss of synchronism, a load loss or a violation the steady-estate security criteria [3] [4].

The CCT is a boundary point from which a power system is unstable. In fact, it is used as a stability indicator [5] [6].
Transmission system operators (TSO) use the CCT to establish the general protection criteria.
General protection criteria are designed to achieve a protection system that minimizes the impact of the disturbances.
Therefore, the appropriate protection systems are designed when the CCT is known [7].

Functionality protection system can be affected by variations in the value of the CCT with respect to its initial value.
In this case, it may have consequences on the power system, such as a blackout.

There is an important relationship between the wind energy and the CCT. For this reason, it should be considered in the study of wind energy integration in small isolated power systems.

Part of this work was carried out within the project Transition to a Sustainable Energy Model for Madeira, the Azores and the Canary Islands (TRES) (PCT-MAC 2007-2013) and funded by European Union through FEDER funds.

Main body of abstract

The existence of the relationship between the wind energy introduced and the CCT is verified in the power system model of Lanzarote-Fuerteventura expected by the year 2020 (figure 01), where there will be connected a significant amount of wind power.



This model was performed with the software PSS®E v32. Several transient stability analyses have been performed.
The power system was analyzed during short circuits in Punta Grande, Las Salinas, Haría/Teguise and Jandía substations, with different amounts of wind power connected to determinate the values of the CCT.

The Lanzarote-Fuerteventura power system has two power plants: Punta Grande (244.24 MW) and Las Salinas (187.43 MW).

The wind generation has 100.86 MW of rated power. Asynchronous wind generators are an important part of the wind generation.
Therefore, models of wind generators have not LVRT capability. In future works, LVRT capability will be considered.

CCT´s obtained are shown below.



Looking at the values, there is some decrease in all of them. The more wind power is introduced in the system, the more CCT decreases.

The figure 03 illustrates the evolution of CCT values depending of wind generation.



The progressive decrease in previous CCTs is due to the outage of wind farm after a short circuit.

Because of the voltages dips and over voltages caused by these short circuits, most of wind farms are disconnected.
When voltages dips take place in small systems, as Lanzarote-Fuerteventura power system, it affects most of the wind farms. (Figures 04;05)





The wind farm outage disturbs the power balance between demand and generation. In fact, the more wind power is connected to the system, the more this balance increases.
In this situation, conventional generation has to correct the imbalance caused by the outage of wind farms.

Swing equation [12] shows how wind farms outages influence power system stability. When wind farms outages occur, there is a sudden increase in the real power generated by the conventional generators.
After that, the imbalance between mechanical power and electrical power causes an angular deceleration that causes a frequency deviation.

Therefore, the more wind power is disconnected, the more angular accelerations and frequency deviations are produced.

When wind power increases, the duration of short circuits is shorter in order to reach the CCT. This is due to frequency deviations caused by short circuits seen before.


The decrease of CCTs is also caused by the reduction in the number of conventional generators connected.
While wind power is introduced in the power system, the number of conventional generators decreases in order to follow the Boucherot criteria.

Inertia constant (H) diminishes when the number of conventional generators decreases. Regarding the swing equation [12], the lower the inertia constant is, the higher the angular acceleration is for the same power imbalance.
These higher accelerations cause higher frequency deviations. Therefore, CCTs are reached faster in shorter duration short circuits. See figure 06.



In this case, when there is 50 MW of wind power connected to the system, the frequency decrease is higher than the frequency decrease on other cases with lower wind power.
It happens because to connect 50 MW of wind power, it is necessary to disconnect one of the conventional generators. The same situation occurs for 60 MW of wind power.

In figure 03, the slope of the chart changes significantly between 40 MW and 50 MW. The slope changes again between 50 MW and 60 MW.
Theses variations in slopes are caused by the lack of conventional generators.



Another important question obtained from results is the difference between the values for short circuits in Punta Grande and Las Salinas substations and the values for short circuits in Haría/Teguise and Jandía substations.

A voltage dip appears in the point of the power system where a short circuit occurs. In this point, the value of voltage is very low or even zero.
The voltage dip is noticeable near the short circuit, but it is lower in farther points due to the impedance of the system [13]. This is shown in figures 04 and 05.
The voltage dip is lower in substations that are far from Las Salinas short circuit. Finally, these different voltage dips produce the difference of the CCTs values.



Conclusion

This paper analyses the relationship between the integration of wind power in small isolated power systems and the critical clearing times (CCT).
Examples of these small power systems can be found in small islands, such as the Canary Islands.

In power systems, the CCT is an important factor in the elaboration of generation and transmission protection schemes.

Thanks to the practical case of Lanzarote-Fuerteventura power system for 2020, it can be appreciated the relationship between the CCT and the integration of wind energy. In fact, CCT decreases when the wind power connected to the system is higher.
This is due to two important factors. The first one is the disconnection of the wind generation caused by voltage dips and over voltage produced by short circuits. The second one is the decrease of the inertia constant (H) of the system produced by the substitution of conventional generation for wind generation.
The decrease of the CCT may become a limit to the integration of wind energy in small power systems.

Due to the disconnections caused by voltage dips in wind farms, it is necessary that wind generators keep connected when voltage dips or over voltage take place in small power systems.
In this way, when wind generation keeps connected during disturbances, its influence on CCT is lower.

For this reason, it is not only important that wind generators are equipped with low voltage ride through (LVRT) devices, but also that grid codes for small isolated power system consider the necessity of LVRT devices.



Learning objectives
There is an important relationship between wind generation connected to small power systems and the CCT.
For this reason, wind power should be considered in the analyses of CCTs. When wind power is introduced in different small power systems, it is necessary to study each case separately because each one has different characteristics that influence the CCT.
In this way, more reliable and effective schemes will be designed for small power systems.


References
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[2] F. Fesquet, P. Juston, I. Garzulino. Impact and Limitation of Wind power generation in an Island Power System. IEEE Bologna Power Tech Conference, 2003 June 23 th-26 th, Bolonga, Italy. IEEE Conference Publications.

[3] Red Eléctrica de España, S. A. Criterios Generales de Protección del Sistema Eléctrico Peninsular Español.

[4] Red Eléctrica de España, S. A. Criterios Generales de Protección de Los Sistemas Eléctricos Insulares y Extrapeninsulares.

[5] B. K. Saharoy, A. K. Pradhan, A. K. Sinha. Computation of Critical Clearing Time using an Integrated Approach. Power Systems, 2009. ICPS '09. International Conference on. IEEE Conference Publications.

[6] Prechanon Kumkratug. Investigation of the Critical Clearing Time of Power System with Synchronous Machine Model Including Saliency. American Journal of Applied Sciences 9 (2): 227-230, 2012. Science Publications.

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[9] Gobierno de Canarias. ORDEN de 4 de agosto de 2009, por la que se resuelve, para el sistema eléctrico de Fuerteventura, el concurso público para la asignación de potencia en la modalidad de nuevos parques eólicos destinados a verter toda la energía en los sistemas eléctricos insulares canarios, convocado por Orden de 27 de abril de 2007.

[10] Gobierno de Canarias. PECAN. Revisión del PECAN 2006-2015. Enero 2012.

[11] Gobierno de España. Planificación de los sectores de electricidad y gas 2012-2020. Desarrollo de las Redes de Transporte. Primer borrador, Julio 2012.

[12] Prabha Kundur. Power System Stability and Control. McGraw Hill, New York, 1994.

[13] Heine, P., Lehtonen, M., Voltage Sag Distributions Caused by Power System Faults, IEEE Transactions on Power Systems, Vol. 18, No. 4, November 2003, pp. 1367-1373.