A method of transmitting a signal by a wireless power transmitter in a wireless charging system, wireless power transmitter, and a wireless power receiver are provided. The method includes receiving, from a wireless power transmitter, a first beacon power for a first duration time in every first period; receiving, from the wireless power transmitter, a second beacon power for a second duration time in every second period; if the wireless power receiver determines that an extension of the second beacon power is required, generating load changes while receiving the second beacon power for the second duration time; in response to the generated load changes, continually receiving, from the wireless power transmitter, the second beacon power until a predetermined third duration time, wherein the predetermined third duration time is determined before the second beacon power is transmitted by the wireless power transmitter; and transmitting an advertisement signal to the wireless power transmitter while receiving the second beacon power for the predetermined third duration time.