Study Of The Micro-grid Islanding Detection Technology Based On The Controlled Short Circuit Technique

With the development of the economy, the problems of energy and environment are more and more prominent. In order to accommodate the national policy of sustainable development, energy reservation and consumption reduction, some measures should be carried out to achieve the harmonious development between the economy, environment and the society. While the main electricity grid is vigorously developed, distributed generating technologies are considered more and more important all around the world. The most important features of the distributed generating technologies are of no pollution to the environment, widespread and abundant reservation, low cost of the initial construction, and flexible power generating patterns, all of these strongpoint makes it to be the most effective approach to solve the energy shortage and environment problems.The islanding detection of the distributed generating system is the key to ensure the safety and the reliability of the distributed generating system. When the breaker which connects the main grid and the distributed generating system tripped out, the distributed generator and its local load is isolated to be an island. When the distributed generators runs into the islanding state, it may cause damages to the user's power devices and the distributed generators, threaten the safety of the operators, and may affect the power quality of power supply in the island. The island must be detected out whenever the island occurred in time, so the distributed generators in the island have enough time to be adjusted to maintain the reliability of the system.To achieve such a goal, many kinds of islanding detection schemes have been proposed at home and abroad. Nevertheless, every scheme has its limitations and disadvantages, that is to say, every scheme has its applicable scope. Especially, when more than one distributed generators carry out islanding detection simultaneously, the detecting signals will interfere each others, and the detecting signal is very difficult to pick up. In order to overcome the above problem, this paper proposes a new and effective active islanding detection technology, which can unfailingly detect out the islanding running state of the distributed generating system. At the same time, it is an advanced technique which can solve the problem of disturbances among detection signals in active islanding detection. Besides, the detecting signals can be artificially controlled and easy to be detected and picked up.In this active islanding detection technique proposed by this paper, a thyristor is placed between any two phases just by the port of the distributed generator. A artificial short time phase-to-phase short circuit is formed by triggering the thyristror to conduct, so a short circuit current pulse can be extract out. The change of system impedance can be reflected by the amplitude change of the phase-to-phase short circuit current pulse, so that whether island has occurred or not can be determined by the amplitude of the current pulse. This islanding detection technique is called the Micro-grid Islanding Detection Technology Based on the Controlled Short-circuit Technique. The main advantages of this active islanding detection technique can be summarized as follows:1. The active islanding detection scheme which is proposed by this paper, is essentially different from the former proposed active islanding detection schemes, because it makes up a phase-to-phase short circuit artificially and drives out the short-circuit current pulse rather than injects a detection signal to the distributed generating system in the former active islanding detection techniques. Because the thyristor can be put between different phases, the triggering angle and frequency of the thyristor can be controlled, so this islanding detection scheme has several different detecting signal channels which have no interference to each others. As a result, this active islanding detection scheme can effectively solve the disturbance between the different distributed generators'detecting signals which lies in the former active islanding detection schemes.2. In the active islanding detection scheme proposed by this paper, the triggering angle of the thyristor can be artificially controlled, so that the short circuit current pulse can be adjusted according to the need of the islanding detection.3. In the active islanding detection scheme which is proposed by this paper, the triggering time of the thyristor can be artificially controlled, so that the emergence of the short circuit current pulse can be forecasted. It is convenient to detect and capture this signal.The active islanding detection scheme proposed by this paper can unfailingly detect out the islanding running state of the distributed generators. And this active islanding detection scheme can also effectively solve the signal interference question which lies in the former active islanding detection techniques when the distributed system has not just one distributed generators. At the same time, the amplitude and beginning time of the detecting signal, in this occasion, the phase-to-phase short-circuit current pulse, can be adjusted by controlling the triggering angle of the thyristor, so that the need of signal detection can be easily satisfied. So, this active islanding detection scheme provides a novel and effective approach for the islanding detection, and has great contributes to the improvement of power quality and power supply reliability of the distributed generating system.