Continuity of power flow is very important for telecommunication systems, hospitals, computers and critical industrial facilities. Uninterrupted Power Supplies (UPS) have been in use for several years for this purpose. Increased power requirement of these systems has led to operation of several UPS units in parallel. However, this has brought in issues related to the control of these systems. Initially, parallel systems composed of equally rated units were built. The controllers of these systems compared the current drawn by each unit to the average current levels to minimize the error. Equal rated unit selection requirement poses an important limitation for these systems. Later on, a communication line that carries the current and frequency information was used for control purposes. The most important disadvantage of these systems is that the communication lines are open to noise and any noise related problem may affect the whole system. Also, the system becomes more complicated due to the use of communication line. This study investigates the use of power-frequency droop method, which has been used in power systems for a long time, in parallel connected, distributed power systems. Each USP unit is supposed to have its own controller. These controllers operate only on its own output current and voltage data. As there is no communication line in this system, the operation is simpler and thus, more reliable. Controllers compute the active power by using the current and voltage values, and then adjust the frequency based on the power-frequency droop characteristic. Each unit has its own power-frequency characteristic. That means the system may consist of units with different rated power levels. When the load of the system changes each unit responds immediately and contributes in proportion with their rated power levels. This enables each unit to operate at their optimum operating level. It is obvious that this method causes the frequency to deviate from the rated value. The frequency of the system can be restored to the original value by shifting the power-frequency characteristic vertically. In this work, a distributed power system consisting of two parallel UPS units is investigated. The system can be loaded at two different points. Focus of the operation is on the inverter parts. Each inverter uses Sinusoidal PWM (SPWM) method, and has an LC filter at their outputs. Inverter outputs are connected together by a tie-line inductor. This line is required to transfer power from one unit to a load located near the other unit. PWM signals of each inverter are generated by using the frequency that is determined after using the power-frequency droop method. Since each unit has a different power-frequency droop characteristic and different power rating, they also have a different PWM frequency. This leads to a phase difference between the output voltages, and to a power flow through the tie-line. After a transient period following a load change, each unit shares the load power in proportion with their rated power values. First of all a system simulation was done by using Matlab-SIMULINK. Then, the simulation results were confirmed by an experimental set-up including a DS-1103 DSP controller and two ASIPM modules. One of the inverters was chosen to be 50% higher rated to experience transient frequency deviations when the load changes. As expected, different frequency values were observed following load changes. This led to a phase difference between the units, and thus power flow through tie-line. The frequencies are equalized at the steady state, and the phase difference is fixed. Frequency restoration algorithm then takes over, and brings the frequencies back to the rated level. The two load units used in the system are equal, but the inverters supply these loads in proportion with their power ratings, meaning that one supplies 50% more power than the other one. Simulation and experimental results are in agreement. Power-frequency droop algorithm yields the synchronous operation of two units in parallel, at a higher power level. As the individual units operate near their rated power levels the efficiency is increased. Due to the lack of communication line, the control is simpler and the operation is more reliable. Parallel operation also adds redundancy to the system. Keywords: Parallel operated UPS systems, distributed power systems, load sharing, mircogrid.Bu çalışmada, iki adet üç fazlı kesintisiz güç kaynağından (KGK) oluşan bir dağınık güç sisteminde aktif güç paylaşımı incelenmektedir. Çalışmada KGK sistemlerinin, çıkış katındaki eviricileri üzerinde durulduğundan, sistemin iki adet paralel eviriciden oluştuğu varsayılmıştır. Bu çalışmada enterkonnekte güç sistemlerinde yıllardır kullanılmakta olan yük-frekans düşüm yönteminin paralel çalışan KGK birimlerine uyarlanabildiği gösterilmiştir. Bu yöntemde her bir KGK birimi kendi denetleyicisine sahiptir ve sadece denetlediği birimin çıkış akım ve gerilim bilgilerini kullanmaktadır. Bu sayede herhangi bir haberleşme hattının kullanılmasına gerek kalmamaktadır. Denetleyiciler, birimlerden aldığı akım ve gerilim bilgilerinden aktif gücü hesaplayarak, yük-frekans düşüm karakteristiğine göre belirlenen yeni frekansı, ayar değeri olarak kullanır. Her bir birimin kendine ait bir yük-frekans düşüm karakteristiği olduğundan, sistemde farklı nominal güç değerlerine sahip KGK birimleri kullanılabilir. Sistemdeki toplam yük değişimine her bir birim anında tepki vererek, değişim miktarını kendi güçleri oranında karşılar. Böylece farklı güçlerdeki birimlerin her biri optimal bir güç seviyesinde çalışır. Her bir eviriciye ait olan kontrol algoritması, düşüm karakteristiğini kullanarak, eviricilerin güçleri oranına ve sistemde bulunan yüklerin değerine göre eviricilerin frekanslarını, tespit eder. Bu frekanslar, çıkış gerilimleri arasında faz farkına yola açar ve eviricilerin sisteme verecekleri güç ve ara bağlantı hattından akacak güç akışı sağlanmış olur. Sonuçta paralel çalışma ve sistemin toplam yükünün eviricilerin güçleri oranında paylaşılması gerçekleşmiş olur. Sistemin benzetimi Matlab-Simulink paket programında yapılmıştır. Deneysel çalışmada ise ASIPM evirici modülleri ve denetleyici kart olarak da DS-1103 kullanılmıştır. Anahtar Kelimeler: Paralel çalışan KGK sistemleri, dağınık güç sistemleri, yük paylaşımı, mikro şebeke. 
