Load shed system
A load shed module configured to be connected in series between a power supply and a load is disclosed. A separate load shed module is connected in series between each load and the power supply. The load shed module determines the frequency of the voltage supplied from the power supply. Based on the frequency, the load shed module determines if utility power is connected or if a generator is connected. If the generator is connected and the frequency of the voltage goes outside of a desired operating range for a preset time, the load shed module disconnects the load from the power supply. Each load shed module includes a priority setting and reconnects its corresponding load after a predetermined time corresponding to the priority setting.
1. A method of reconnecting a load to a power source in a power distribution system after shedding the load, comprising the steps of:
reading a feedback signal corresponding to an alternating current (AC) voltage received at an input of a load shed module from the power source;
determining a frequency of the AC voltage from the feedback signal with a controller in the load shed module;
determining whether the AC voltage at the input of the load shed module is from a utility power source or from a backup power source based on the frequency of the AC voltage;
closing a switch connected in series between the input and an output of the load shed module when the frequency of the AC voltage at the input is within a frequency range, wherein the output of the load shed module is connected to the load, and wherein the switch is connected between a transfer switch and the load; and
keeping the switch open when the frequency of the AC voltage at the input is outside the frequency range.
2. The method of claim 1 , wherein the closing the switch connected in series between the input and an output of the load shed module includes:
determining that a delay period has passed; and
closing the switch connected in series between the input and an output of the load shed module.
3. The method of claim 1 , wherein the determining whether the AC voltage at the input of the load shed module is from the utility power source or from the backup power source based on the frequency of the AC voltage includes:
determining whether the frequency of the AC voltage at the input of the load shed module is within the frequency range; and
determining that the AC voltage at the input of the load shed module is from the utility power source when the frequency is within the frequency range, while determining that the AC voltage at the input of the load shed module is from the backup power source when the frequency is outside the first frequency range.
4. The method of claim 1 , wherein the determining whether the AC voltage at the input of the load shed module is from the utility power source or from the backup power source based on the frequency of the AC voltage includes:
determining a minimum frequency of the AC voltage at the input of the load shed module over a measurement period;
determining a maximum frequency of the AC voltage at the input of the load shed module over the measurement period;
determining a difference between the minimum frequency and the maximum frequency; and
determining that the AC voltage at the input of the load shed module is from the utility power source when the difference is equal to or smaller than a threshold difference, while determining that the AC voltage at the input of the load shed module is from the backup power source when the difference is greater than the threshold difference.
5. The method of claim 1 , wherein the determining the frequency of the AC voltage includes:
detecting a first peak of the AC voltage;
detecting a second peak of the AC voltage, the second peak being next to the first peak; and
determining the frequency of the AC voltage based on the first peak of the AC voltage and the second peak of the AC voltage.
6. The method of claim 1 , wherein the backup power source is a backup generator.
7. A load shed module configured to be connected in series between a transfer switch and a load, the load shed module comprising:
an input configured to receive a first electrical connection from the transfer switch;
an output configured to provide a second electrical connection for the load;
a switch operatively connected in series between the input and the output, wherein the switch is configured to be connected between the transfer switch and the load, and wherein the switch receives a control signal to selectively control the switch;
a sensor configured to generate a feedback signal corresponding to an alternating current (AC) voltage at the input;
a controller connected to the sensor to receive the feedback signal, and connected to the switch to supply the control signal, wherein the controller is operable to:
determine whether the AC voltage at the input of the load shed module is from a utility power source or from a backup power source based on a frequency of the AC voltage;
close the switch when the frequency of the AC voltage at the input is within an expected frequency range; and
keep the switch open when the frequency of the AC voltage at the input is outside the expected frequency range.
8. The load shed module of claim 7 , wherein the operation to close the switch includes:
determine that a delay period has passed; and
close the switch.
9. The load shed module of claim 7 , wherein the operation to determine whether the AC voltage at the input of the load shed module is from the utility power source or from the backup power source based on the frequency of the AC voltage includes:
determine whether the frequency of the AC voltage at the input of the load shed module is within the expected frequency range; and
determine that the AC voltage at the input of the load shed module is from the utility power source when the frequency is within the expected frequency range, while determining that the AC voltage at the input of the load shed module is from the backup power source when the frequency is outside the expected frequency range.
10. The load shed module of claim 7 , wherein the operation to determine whether the AC voltage at the input of the load shed module is from the utility power source or from the backup power source based on the frequency of the AC voltage includes:
determine a minimum frequency of the AC voltage at the input of the load shed module over a measurement period;
determine a maximum frequency of the AC voltage at the input of the load shed module over the measurement period;
determine a difference between the minimum frequency and the maximum frequency; and
determine that the AC voltage at the input of the load shed module is from the utility power source when the difference is equal to or smaller than a threshold difference, while determining that the AC voltage at the input of the load shed module is from the backup power source when the difference is greater than the threshold difference.
11. The load shed module of claim 7 , wherein the frequency of the AC voltage at the input is obtained by detecting a first peak of the AC voltage and a second peak of the AC voltage, the second peak being next to the first peak.
12. The load shed module of claim 7 , wherein the backup power source is a backup generator.
13. A load shed module configured to be connected in series between a transfer switch and a load, the load shed module comprising:
an input configured to receive a first electrical connection from the transfer switch;
an output configured to provide a second electrical connection for the load;
a switch operatively connected in series between the input and the output, wherein the switch is configured to be connected between a transfer switch and the load, and wherein the switch receives a control signal to selectively control the switch;
a sensor configured to generate a feedback signal corresponding to an alternating current (AC) voltage at the input;
a controller connected to the sensor to receive the feedback signal, and connected to the switch to supply the control signal, wherein the controller is operable to:
determine whether the AC voltage is from a utility power source or a backup power source based on a frequency of the AC voltage at the input; and
when the AC voltage is from the utility power source, generate the control signal to close the switch.
14. The load shed module of claim 13 , wherein the operation to generate the control signal to close the switch when the AC voltage is from the utility power source includes:
when the AC voltage is from the utility power source, determine that the frequency of the AC voltage is within an expected frequency range for a first period of time; and
generate the control signal to close the switch.
15. The load shed module of claim 13 , wherein the operation to determine whether the AC voltage is from the utility power source or the backup power source based on the frequency of the AC voltage at the input includes:
determine whether the frequency of the AC voltage is within an expected frequency range; and
determine that the AC voltage is from the utility power source when the frequency is within the expected frequency range, while determine that the AC voltage is from the backup power source when the frequency is outside the expected frequency range.
16. The load shed module of claim 13 , the operation to determine whether the AC voltage is from the utility power source or the backup power source based on the frequency of the AC voltage present at the input includes:
determine a minimum frequency of the AC voltage over a measurement period;
determine a maximum frequency of the AC voltage over the measurement period;
determine a difference between the minimum frequency and the maximum frequency; and
determine that the AC voltage is from the utility power source when the difference is equal to or smaller than a threshold difference, while determine that the AC voltage is from the backup power source when the difference is greater than the threshold difference.
17. The load shed module of claim 13 , wherein the frequency of the AC voltage at the input is obtained by detecting a first peak of the AC voltage and a second peak of the AC voltage, the second peak being next to the first peak.
18. The load shed module of claim 13 , wherein the backup power source is a backup generator.