Duk Hong Kang, “개선된 DFT 을 이용한 무효전력변동 단독운전 검출기법의 성능 개선,” 아주대학교 공학석사 학위 논문, 2010. > Paper

These days, interests in connected grid operation of power variation utility has been increased as the research in renewable energy

is actively in progress because of the domestic and foreign problems such as oil rises. PV generation was attracted the attention as

the key answer for both environmental way and potential possibilities. But this system could bring out anti-islanding problem which

is the situation that part or whole electric power distribution could not receive the electric power but it is maintained its electric

power supplies by self generating system. If anti-islanding state is being continued, It could be occurred a close distance accident

such as engineer’s electric shock during line working. Because of this problem, connected grid type of PV generation system should

recognize it and halt its operating immediately if connected grid is disconnected. The antiislanding methods which have been

researched until now are passive and active detection techniques. The passive method is mostly the way to recognize the anti-

islanding by observing electric voltage, frequency, and change of phase when cut off the grid connection. There are phase jump

detection, over/under voltage and frequency detection and voltage harmonic monitoring. There is possibility that great load variation

in electric grid could be occurred spontaneously though it is in normal state that the connection with the grid is maintained. Passive 

method misjudges this situation to islanding state. Therefore, to leave the room for not to take it for islanding state, it should set

an islanding Non Detection Zone (NDZ) range of possibility in passive method such as electric voltage and frequency method.

However, if there are no changes of electric voltage and frequency in passive method, when islanding occurs, it is presented NDZ

like Fig. 1.1, and it is impossible to recognize the islanding as it is under the range of NDZ. Recently, to complete this drawback,

various researches about active method have been conducted to recognize the islanding by changing the output of PV system at

discretion and examining the changes of load voltage and frequency accordingly. Active method is affecting to inverter output

current in system and is the method to actively deal with the islanding by breaking the comparably maintained equilibrium between

generation of PV system and power. The performance has been improved by adopting the Goertzel algorithm as a method to detect

the frequency which is necessary to RPV, one of the active methods. RPV method has been improving the NDZ. When using RPV

method, if anti- islanding occurs, the excellence of anti-islanding detection is determined by the method of detecting the frequency

as it should halt its voltage from PV system by recognizing the change of frequency. In this paper, as a method to improve the

performance of islanding detection, the performance of detecting the islanding is improved by adopting the Discrete Fourier

Transform (DFT) using Goertzel algorithm and estimating the frequency. The simulation results are presented to demonstrate the

excellence of the Goertzel algorithm by comparing the zero crossing detection method which is frequently used in the past and PLL.