Hybrid power system with electric generator and auxiliary power source
A hybrid power-generator system includes an engine, an electric generator, first and second rectifiers, first and second DC-DC voltage converters, a DC bus, an inverter, and one or more controllers. The system provides a unique method of joining two power sources such that the relative proportion utilized can be changed to any value seamlessly, such as to avoid daily and/or seasonal variations in utility charges. Since the AC output portion of the circuit is independent of the utility grid, power can be supplied at variable frequencies to motor loads with significant positive impacts in load efficiency. Power increases required by the load(s) that occur rapidly can utilize the electrical grid to assist for the brief transient, allowing the engine, which is maintained at a fixed and wide-open-throttle position, to continue operation and in a more gradual process to resume its blend target for power generation.
1. A hybrid power-generator system comprising:
an engine that runs with a wide-open throttle;
an electric generator mechanically coupled to the engine to produce a generator alternating current (AC) having a frequency;
a first rectifier having an input that is electrically coupled to an output of the electric generator, the first rectifier configured to rectify the generator AC to a generator direct current (DC);
a second rectifier having an input that is electrically coupled to an electric grid to receive a grid AC, the second rectifier configured to rectify the grid AC to a grid DC;
a first DC-DC voltage converter having an input that is electrically coupled to an output of the first rectifier to receive the generator DC;
a second DC-DC voltage converter having an input that is electrically coupled to an output of the second rectifier to receive the grid DC;
a DC bus electrically coupled to an output of the first DC-DC voltage converter and to an output of the second DC-DC voltage converter;
an inverter electrically coupled to the DC bus, the inverter supplying an inverter AC to a load motor;
a first controller that receives as inputs a grid power target signal and a grid power feedback signal and produces an engine speed target signal based, at least in part, on the grid power target signal and the grid power feedback signal; and
a second controller that receives as inputs the engine speed target signal, an engine speed feedback signal, a DC bus voltage feedback signal, and a DC bus target voltage signal, and produces first and second duty-cycle output signals based, at least in part, on the engine speed target signal, the engine speed feedback signal, the DC bus voltage feedback signal, and the DC bus target voltage signal,
wherein:
the first duty-cycle output signal sets a duty cycle of the first DC-DC voltage converter,
the second duty-cycle output signal sets a duty cycle of the second DC-DC voltage converter, and
the duty cycle of the first DC-DC voltage converter causes the engine to operate at a corresponding speed.
2. The system of claim 1 , wherein:
the grid power target signal indicates a target grid power from the electric grid,
the grid power feedback signal indicates an actual grid power from the electric grid, and
the first controller produces the engine speed target signal based, at least in part, on a difference between the target grid power and the actual grid power.
3. The system of claim 2 , wherein the first controller includes a proportional-integral-derivative controller.
4. The system of claim 1 , wherein:
the engine speed target signal corresponds to a target speed of the engine,
the engine speed feedback signal corresponds to an actual speed of the engine,
the DC bus voltage feedback signal corresponds to an actual voltage of the DC bus,
the DC bus target voltage signal corresponds to a target voltage of the DC bus, and
the second controller produces the first and second duty-cycle output signals based, at least in part, on a difference between the target speed and the actual speed of the engine and a difference between the target voltage and the actual voltage of the DC bus.
5. The system of claim 4 , wherein the second controller includes:
a first proportional-integral-derivative (PID) controller that receives as an input an engine speed difference signal that corresponds to the difference between the target speed and the actual speed of the engine, the first PID controller producing a first output signal;
a second PID controller that receives as an input a DC bus voltage difference signal that corresponds to the difference between the target voltage and the actual voltage of the DC bus, the second PID controller producing a second output signal;
a current controller that receives as inputs the first and second output signals, the current controller producing first and second reference currents;
a third PID controller that receives as an input a first current difference signal that corresponds to a difference between the first reference current and the generator DC, the third PID controller producing the first duty-cycle output signal; and
a fourth PID controller that receives as an input a second current difference signal that corresponds to a difference between the second reference current and the grid DC, the fourth PID controller producing the second duty-cycle output signal.
6. The system of claim 5 , wherein the second controller includes:
a first pulse-width modulator (PWM) that receives as an input the first duty-cycle output signal, the first PWM producing a first pulse-width modulated signal based on the first duty-cycle output signal; and
a second PWM that receives as an input the second duty-cycle output signal, the second PWM producing a second pulse-width modulated signal based on the second duty-cycle output signal,
wherein:
the first DC-DC voltage converter is configured to convert a voltage of the generator DC using the first pulse-width modulated signal, the first pulse-width modulated signal having the first duty cycle, and
the second DC-DC voltage converter is configured to convert a voltage of the grid DC using the second pulse-width modulated signal, the second pulse-width modulated signal having the second duty cycle.
7. The system of claim 6 , wherein the second controller includes a first current-limit circuit that receives as an input the first current difference signal and that produces as an output:
the first current difference signal when a target generator DC is less than or equal to a maximum generator DC, and
a first maximized current difference signal when the target generator DC is greater than the maximum generator DC.
8. The system of claim 7 , wherein the second controller includes a second current-limit circuit that receives as an input the second current difference signal and that produces as an output:
the second current difference signal when a target grid DC is less than or equal to a maximum grid DC, and
a second maximized current difference signal when the target grid DC is greater than the maximum grid DC.
9. The system of claim 1 , wherein:
the inverter is a first inverter,
the load motor is a first load motor,
the inverter AC is a first inverter AC, and
the system further comprises:
a second inverter electrically coupled to the DC bus, the second inverter supplying a second inverter AC to a second load motor.
10. The system of claim 1 , wherein:
the load motor is a first load motor, and
the system further comprises:
a third DC-DC converter electrically coupled to the DC bus, the third DC-DC converter supplying a DC power to a second load motor.
11. The system of claim 1 , wherein the engine and the electric generator are components of a combined heat and power system.
12. The system of claim 1 , further comprising a combined controller that includes the first and second controllers.
13. A hybrid power-generator system comprising:
an engine that runs with a wide-open throttle;
an electric generator mechanically coupled to the engine to produce a generator direct current (DC) having a generator voltage;
a rectifier having an input that is electrically coupled to an electric grid to receive a grid AC, the rectifier configured to rectify the grid AC to a grid DC;
a first DC-DC voltage converter having an input that is electrically coupled to an output of electric generator to receive the generator DC;
a second DC-DC voltage converter having an input that is electrically coupled to an output of the rectifier to receive the grid DC;
a DC bus electrically coupled to an output of the first DC-DC voltage converter and to an output of the second DC-DC voltage converter;
an inverter electrically coupled to the DC bus, the inverter supplying an inverter AC to a load motor;
a first controller that receives as inputs a grid power target signal and a grid power feedback signal and produces an engine speed target signal based, at least in part, on the grid power target signal and the grid power feedback signal; and
a second controller that receives as inputs the engine speed target signal, an engine speed feedback signal, a DC bus voltage feedback signal, and a DC bus target voltage signal, and produces first and second duty-cycle output signals based, at least in part, on the engine speed target signal, the engine speed feedback signal, the DC bus voltage feedback signal, and the DC bus target voltage signal,
wherein:
the first duty-cycle output signal sets a duty cycle of the first DC-DC voltage converter,
the second duty-cycle output signal sets a duty cycle of the second DC-DC voltage converter, and
the duty cycle of the first DC-DC voltage converter causes the engine to operate at a corresponding speed.
14. A hybrid power-generator system comprising:
an engine that runs with a wide-open throttle;
an electric generator mechanically coupled to the engine to produce a generator alternating current (AC) having a frequency;
a first rectifier having an input that is electrically coupled to an output of the electric generator, the first rectifier configured to rectify the generator AC to a generator direct current (DC);
a first DC-DC voltage converter having an input that is electrically coupled to an output of the first rectifier to receive the generator DC;
a second DC-DC voltage converter having an input that is electrically coupled to an output of an auxiliary DC power source;
a DC bus electrically coupled to an output of the first DC-DC voltage converter and to an output of the second DC-DC voltage converter;
an inverter electrically coupled to the DC bus, the inverter supplying an inverter AC to a load motor;
a first controller that receives as inputs a grid power target signal and a grid power feedback signal and produces an engine speed target signal based, at least in part, on the grid power target signal and the grid power feedback signal; and
a second controller that receives as inputs the engine speed target signal, an engine speed feedback signal, a DC bus voltage feedback signal, and a DC bus target voltage signal, and produces first and second duty-cycle output signals based, at least in part, on the engine speed target signal, the engine speed feedback signal, the DC bus voltage feedback signal, and the DC bus target voltage signal,
wherein:
the first duty-cycle output signal sets a duty cycle of the first DC-DC voltage converter,
the second duty-cycle output signal sets a duty cycle of the second DC-DC voltage converter, and
the duty cycle of the first DC-DC voltage converter causes the engine to operate at a corresponding speed.
15. The system of claim 14 , wherein the auxiliary DC power source comprises one or more batteries, one or more supercapacitors, a photovoltaic solar array, or other DC power source.