Frequency based methods developed to detect an islanding condition in modern power grid structures have been discussed. The condition may occur in power grid lines to which additional energy sources with power electronic converters have been connected such as solar panels or wind turbines. It is a hazardous operating state for grid workers and devices connected to the islanded part of the grid. Such a state results from the inability to control amplitude and frequency of basic harmonic of the grid voltage. An islanding problem in a power grid with additional small energy sources has been discussed. A basic passive and active islanding detection method have been presented and compared with known frequency based algorithms, namely active frequency drift and active frequency drift with a positive feedback algorithm. Finally, a laboratory test of the proposed islanding detection methods in a three- -phase grid with an AC-DC converter has been conducted. To demonstrate differences between the tested methods, total harmonic distortion injected into the output converter current and detection time of islanding state have been measured.
 WŁAS M., Distributed generation in power supply system, Zeszyty Naukowe Wydziału Elektrotechniki i Automatyki Politechniki Gdańskiej, 2010, No. 27, 39-42.
 ROPP M., BOWER W., Evaluation of islanding detection methods for utility-interactive inverters in photovoltaic systems, Sandia National Laboratories, US, November 2002.
 SKOCIL T., GOMIS-BELLMUNT O., MONTESINOS-MIRACLE D., GALCERAN-ARELLANO S., RULL-DURAN J., Passive and active methods of islanding for PV systems, 13th European Conference on Power Electronics and Applications, Barcelona, Spain, 2009, 1-10.
 VERHOEVEN B., Probability of islanding in utility network due to grid connected photovoltaic power systems, Report of the International Energy Agency - Phtovoltaic Power Systems Programme T5-07:2002, September 2002.
 ROPP M., BEGOVIC M., ROHATGI A., KERN G.A., BONN R.H. GONZALES S., Determining the reflective effectiveness of islanding detection methods suing phase criteria and nondetection zones, IEEE Trans. Energy Conv., 2000, 15(3), 290-296.
 Polish Standard PN-EN 50160:1998. Voltage characteristics in public distribution systems, Poland.
 SCHWARTFEGER L., SANTOS-MARTIN D., Review of distributed generation interconnection standards, Proc. EEA Conference Exhibition, 18-20.06.2014, Auckland, New Zealand.
 PERSSON D., Islanding detection in power electronic converter based distributed generation, M.Sc. Diss., Lund University, 2007.
 TEOH W.Y., TAN C.W., A comparative study of anty-islanding control techniques for grid-connected photovoltaic systems, The World Congress on Advanced in Nano, Biomechanics, Robotics, and Energy Researches, Seoul, Korea, 2013, 889-917.
 CHEN W., WANG G., ZHU X., ZHAO B., An improved active frequency drift islanding detection method with lower total harmonic distortion, Proc. IEEE Energy Conversion Congress and Exposition, Denver 2013, 5248-5252.
 YAFAOUI A., WU B., KOURO S., Improved active frequency drift anti-islanding detection method for grid connected photovoltaic systems, IEEE Trans. Power Electr., 2012, 27(5), 2367-2375.
 FALKOWSKI P., SIKORSKI A., Predictive control of active power of AC/DC converter with constant average switching frequency, Przegl. Elektr., 2013, 89(12), 53-56.