Backup power system and method

A backup power system connectable to a main AC power system at input terminals and to a load at output terminals, comprising:

• (a) a battery;
• (b) power supply lines forming a main power supply path from the input terminals to the output terminals to normally supply power from an AC power system connected to the input terminals to a load connected to the output terminals;
• (c) means responsive to control signals for interrupting the supply of power through the main power supply path from the AC power system to the load and for selectively providing AC power from the battery to the power supply path and thence to the load;
• (d) control means for providing control signals, upon occurrence of a line fault on the AC power system power provided to the input terminals, to the means responsive to control signals to cause the supply of power to the load to be transferred from the AC power system to the battery, the control means including:
• means for sampling the voltage provided by the AC power system to the input terminals at periodic times and providing digital output data corresponding to the sampled input data;
• means receiving the digital data for producing a reference waveform at selected sample times during the period of the AC line signal, wherein each reference value at each sample time comprises a selectively weighted average of the value at that sample time in the prior cycle of the AC power system voltage waveform and the previous reference waveform value at that sample time;
• and
• means for determining the absolute value of the difference between the present sampled AC power system voltage waveform and the reference waveform at corresponding times in the AC waveforms for each sample, for adding the difference to a sum at each sample and for subtracting from the sum at each sample a selected decrement value to produce a new sum for comparing the new sum with a trigger threshold level, and for determining that line fault has occurred and providing a line fault signal when the sum exceeds the trigger threshold.

A backup power system connectable to a main AC power system at input terminals and to a load at output terminals, comprising:

• (a) a battery;
• (b) power supply lines forming a main power supply path from the input terminals to the output terminals to normally supply power from an AC power system connected to the input terminals to a load connected to the output terminals;
• (c) a controllable switch in at least one of the power supply lines in the main power supply path responsive to a control signal to switch from providing conduction continuity in the power supply line to breaking conduction continuity in the power supply line;
• (d) an inverter operatively connected to the power supply lines between the controllable switch and the output terminals, the inverter having gate controlled switching devices connected to receive DC power from the battery and to provide AC power to the power supply lines in the output terminals when the inverter switching devices are switched;
• and
• (e) control means providing control signals to the controllable switch in the main power supply path when a fault occurs in the AC power system power provided to the input terminals and to provide control signals to the gates of the gate controlled switching devices of the inverter to generate an AC output voltage waveform at the output of the inverter and thereby at the output terminals, and further including means for sensing the current drawn from the battery through the inverter to the load and for turning off the control signals to the gates of the switching devices when a selected current limit is reached for a selected short period of time less than a cycle of the AC waveform, for thereafter turning on the control signals to the gates of the switching devices to provide the AC voltage waveform from the inverter and for again turning off the signals to the gates if the current drawn from the battery through the inverter exceeds the selected current limit level.

A method of controlling the switching of switching devices in an inverter of a backup power system which is coupled to output terminals connectable to a load, the backup power system normally providing power on a main power supply path from input terminals connectable to an AC power system to the output terminals when the AC power system power is available and disconnecting the main power path from the AC power system when the AC power system power fails and providing power from a battery through the inverter to the load during the fault on the AC power system, the inverter having gate controlled switching devices responsive to control signals to switch to provide AC power at an output of the inverter, comprising the steps of:

• (a) sensing the current drawn from the battery through the inverter to the load;
• (b) turning off the control signals to the gates of the switching devices when a selected limit level of current drawn from the battery is reached for a selected short period of time less than a cycle of the AC waveform;
• (c) thereafter turning on the control signals to the gates of the switching devices to provide the AC voltage waveform from the inverter;
• and
• (d), again turning off the control signals to the gates if the current drawn from the battery through the inverter again exceeds the selected current limit level.