Overcurrent trip coordination between inverter and circuit breakers
An electrical installation having a first source of power, such as the utility grid, and a second source of power from an electronic DC-to-AC converter, comprises a smart load center for selecting the source of power independently for each of a number of branch circuits protected by circuit breakers to be the first source or the second source. The smart load center comprises a software-controlled processor for operating the transfer relays that select the first or second source of power for each branch circuit. In the event that a current overload on a branch circuit connected to the DC-to-AC converter causes it to trip before the branch circuit breaker clears the overload, all circuits are transferred to the first power source to trip the fault breaker and then the microprocessor enters a restart sequence to verify that the fault is cleared or else to take other action.
1. A method of operating a DC-to-AC converter operative to supply power to a plurality of branch circuits in an electrical installation, wherein each branch circuit is protected with an overcurrent protection device, comprising:
detecting an overcurrent condition and halting power output in response to the overcurrent condition; and
restarting the DC-to-AC converter by outputting less than a predetermined full output power in a manner coordinated with the time/current trip curve of an overcurrent protection device feeding a faulty branch circuit, so as to cause the overcurrent protection device of the faulty branch circuit to break on overcurrent.
2. The method of claim 1 wherein if the overcurrent protection device of the faulty branch circuit breaks, outputting the predetermined full output power.
3. The method of claim 2 wherein outputting the predetermined full output power comprises ramping up the output power to the predetermined full value in stages, while performing fault detection at one or more such stages.
4. The method of claim 1 wherein if the overcurrent protection device of the faulty branch circuit does not break on overcurrent within a predetermined duration, outputting a fault indication.
5. The method of claim 1 wherein restarting the DC-to-AC converter by outputting less than a predetermined full output power in a manner coordinated with the time/current trip curve of an overcurrent protection device feeding a faulty branch circuit, so as to cause the overcurrent protection device of the faulty branch circuit to break on overcurrent comprises
initially outputting power at less than a predetermined full voltage;
monitoring a current drawn; and
increasing the voltage of output power to
a voltage below the predetermined full voltage in the case that a corresponding current reaches a predetermined maximum allowed current, and
the predetermined full voltage in the case that the corresponding current does not reach the predetermined maximum allowed current; and
in the case that the current is the predetermined maximum allowed current, halting power output after a predetermined duration.
6. The method of claim 5 wherein increasing the voltage of output power comprises increasing the voltage according to a predetermined schedule.
7. The method of claim 5 wherein increasing the voltage of output power comprises:
based on the initial voltage, the corresponding monitored current, and the known voltage waveform, determining a voltage corresponding to the predetermined maximum allowed current; and
outputting power at the determined voltage.
8. The method of claim 5 wherein increasing the voltage of output power comprises, iteratively:
based on a current voltage and the corresponding monitored current, calculating the instantaneous load impedance;
based on the calculated load impedance, calculating a voltage resulting in the predetermined maximum allowed current; and
outputting power at the calculated voltage.
9. A DC-to-AC converter comprising:
a plurality of switching transistors operative to selectively switch the positive or negative value of one or more DC voltage sources, or zero volts, into a series connection operative to sum the switched DC voltages so as to approximate a desired instantaneous AC output voltage;
an AC output current sensor and current overload detector operative to produce digitized current indications;
memory operative to store a plurality of transistor switching sequences, each operative to generate a predetermined AC output waveform; and
a processor operatively connected to the switching transistors, the current sensor, and memory, and operative to
retrieve a transistor switching sequence from memory;
control the switching transistors according to the retrieved transistor switching sequence to produce the associated output waveform;
monitor the digitized AC output current indications; and
shut down the AC output and restart the AC-to-DC controller when the AC output current indications indicate a short circuit load;
wherein upon restarting the DC-to-AC converter, the processor is further operative to
select a transistor switching sequence corresponding to a low voltage and a current waveform of defined peak or rms value; and
proceed to select one or more transistor switching sequences corresponding to waveforms of progressively higher voltage if a current less than a predetermined peak or rms current is sensed by said AC output current sensor.
10. The DC-to-AC converter of claim 9 wherein, if the current sensed by the AC output sensor is greater than the predefined peak or rms value, the processor is operative to:
not select a transistor switching sequence corresponding to a higher voltage waveform; and
continue to generate the last selected AC waveform for a predetermined time-out period; and
after the predetermined time-out period, shut down the DC-to-AC converter.