Simple droop voltage control system for parallel operation of UPS

Uninterruptible power supply systems are very valuable nowadays for many reasons as they prevent critical systems from shutting down when the mains fails. Safety systems in airports, hospitals, banks etc cannot be turned off at any moment. As the power needs can grow when systems are upgraded, the UPS rated power should also grow. Then, there are two options: buying a more powerful UPS or upgrading the system adding a new small UPS in parallel. This option is very interesting for manufacturers because it is very versatile and large systems can be powered with small UPS. However, AC paralleling is always controversial because phase and voltage amplitude match should be guaranteed. This paper presents a prototype using a very simple system to allow UPS to be paralleled. When the AC output is obtained from a battery, there is usually a step-up converter to boost the input voltage and a dc-ac stage connected in cascade. Therefore, the output voltage amplitude can be controlled changing the dc bus voltage instead of changing the modulation ratio of the inverter PWM signal, making possible to operate the inverter stage without amplitude regulation. As a consequence, parallel connection of the UPSs becomes similar to paralleling dc converters, and well known methods used in dc-dc converters can also be used in this case. Two 3 kW UPSs prototypes using the proposed system have been built and tested to compare theoretical and experimental results.