Principal Research Results

Unnecessary Operation Prevention Method of Islanding Detectionin Photovoltaic Power Generation
Background
When distribution lines are disconnected from the utility system due to distribution line failure or maintenance, dangerous
islanding phenomena of distributed power generation sources (DG) are prevented by detecting voltage change or frequency change
including sudden voltage phase change under such disconnection of distribution lines. On the other hand, sudden voltage phase change
occurs in normal conditions during route changes of distribution lines and so on. As a result, DG power sources may stop their operations simultaneously due to mistaken islanding detection. Simultaneous stoppage of operations may cause either abnormal drop of thedistribution line voltage or instability of the whole system.
CRIEPI is developing a technique for coping with both prevention of the unnecessary detection in normal operation andrapid ＊ 1 reliable detection of islanding, based on past research and development concerning the islanding detection method. For establishment of the method, it is necessary to clarify improved methods by testing and evaluating the unnecessary detection characteristicsof the current islanding detection function.

Objectives
The purpose of this study is to test and evaluate unnecessary detection characteristics regarding sudden voltage phasechanges in the power conditioning subsystem (PCS) for residential photovoltaic (PV) power generation currently being disseminated.
Also, an improved method for balancing prevention of the unnecessary detection and speeding up necesary detection of islanding isclarified through developing a simulation model for analyzing transient characteristics of the PCS.

Principal Results
1. Characteristics of unnecessary detection to sudden phase change and proposal of improved method
A demonstration test of sudden phase change for 10 Japanese representative types of PCSs is carried out. As the result, it isclear that 70% of the PCSs tend to stop the operation by the unnecessary detection of islanding detective function in the event of phasesudden change less than actual 10 degrees. On the other hand, it is also clear that establishment of a detection time counter is usefulfor preventing the unnecessary detection because PCS considered the continuance time of the deviation from threshold level does notreach the unnecessary detection. From the result, an improved method based on the current method of detecting the frequency change
rate is proposed. In this method, frequency change rate is calculated by moving average of 2 cycles and islanding is detected when thefrequency change rate exceeds threshold level continuously more than twice as much as the moving average cycles.
2. Development of simulation model for transient analysis and evaluation of developed method
Based on both the result of past investigation including demonstration test and above experimental result of PCS, a generalized model of PCS control and main circuits is developed for the transient analysis less than 5 seconds (Fig.1). From the results ofEMTP simulation using the developed model comparing with the conventional passive islanding detection method, it is confirmed thatboth prevention of the unnecessary detection in normal operation and rapid reliable detection of islanding are best satisfied by theproposed method shown in Table 1. Besides, proper operation of the proposed method according to design is also confirmed by ademonstration test. From above, the validity of the proposed method is verified.

Future DevelopmentsWe will establish stable operation technique for PV through investigating prevention of unnecessary detection in the eventof voltage sag in addition to the sudden voltage phase change.
Main Researcher: Hiromu Kobayashi, Dr.,
Senior Research Scientist, Customer Systems Sector, System Engineering Research Laboratory
Reference
H. Kobayashi, et.al., 2008, “Development of Transient Analysis Model of Grid Interconnected PCS for Photovoltaic Power
Generation”, CRIEPI Report R07027 (in Japanese)
H. Kobayashi, et.al., 2008, “Development of Islanding Detection Measure for PV Power Generation to Achieve Stable Operation inSudden Phase Change of the Grid Voltage”, CRIEPI Report R07029 (in Japanese)＊1：0.1 second is targeted taking account of required system shut down time in the event of ground fault of 6.6kV high voltage distribution line
due to failure of pole mounted transformer supplying low voltage customers.

66

4. Power Deliverya) Simulation model of PCS for PV power generationb) Comparison between experimental result andcalculation result of islanding time of each PCS.
Fig.1 Developed transient analysis model of PCS for PV and result of validity conformation

Table 1 Simulation results of passive islanding detection methods
Type of islanding
detection

Detection method

Response tosudden phasechange(0 to 90 deg.)Frequency of over
0.1 sec. detection ofislanding〈Note)

Ranking

Detection over6 deg.（×）

Small

3Ｍidle2

Small１<Conventional>
Sudden voltage phasechange detection I

・Monitoring phase change
・Detection time counter ：N.A.

<Conventional>
Sudden voltage phasechange detection II

・Monitoring phase change
・Detection time counter ：More than
4 cycles deviation from thresholdlevel.

No detection<Proposed>
Improved frequencychange rate detection

・Monitoring frequency change rate・Detection time counter ：More than
4 cycles deviation from thresholdlevel.

No detection(Note) Characteristic when frequency shift method is combined.

67

（○）

（○）

4