# US 11,451,065 B1 · Voltage control and grid synchronization of microgrids in real time

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- Number: US 11,451,065 B1
- Document ID: US11451065B1
- Status: Granted
- Kind: B1
- Application number: 17/742,066
- Filed: 2022-05-11
- Granted: 2022-09-20
- Earliest claimed: 2021-12-21
- Assignees: King Fahd University of Petroleum and Minerals
- Applicants: King Fahd University of Petroleum and Minerals
- Inventors: Muhammed Yibre Worku; Mohamed A. Hassan; Mohamed A. Abido
- Web page: https://patentgrep.com/patent/US11451065B1

## CPC

- H02J 3/46: Controlling the sharing of generated power between the generators, sources or networks
- G05B 19/042: using digital processors (G05B19/05 takes precedence)
- H02J 3/08: Synchronisation of networks
- H02J 3/381: {Dispersed generators}
- H02J 3/388: Arrangements for the handling of islanding, e.g. for disconnection or for avoiding the disconnection of power
- H02J 3/42: with automatic parallel connection when synchronisation is achieved
- G05B 2219/2639: Energy management, use maximum of cheap power, keep peak load low
- H02J 2101/10: Dispersed power generation using fossil fuels, e.g. diesel generators
- H02J 2101/25: {involving maximum power point tracking control for photovoltaic sources}
- H02J 2101/28: Wind energy
- H02J: ELECTRIC POWER NETWORKS; CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- G05B: CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS (systems for controlling or regulating non-electric variables G05D; systems for regulating electric or magnetic variables G05F; control devices or systems insofar as characterised by mechanical features only G05G)

## Abstract

A system and method for synchronizing a utility grid and a microgrid including a diesel generator, a photovoltaic array and a wind turbine. In a grid connected mode, a microgrid voltage, a microgrid frequency and a microgrid phase angle are controlled by a grid side converter which generates a preset reference power at the utility grid frequency; and provides a real power and a reactive power to each of a plurality of loads. In an island mode, the diesel generator operates at a constant speed to generate power at the reference frequency and communicates the reference frequency to a wind turbine, and the microgrid supplies the real power and reactive power to each of the plurality of loads. In a resynchronization mode, a switched capacitor bank is charged to provide the voltage needed to reconnect the microgrid to the utility grid.

## Claims

### Claim 1

A method for synchronizing a utility grid and a microgrid, comprising:
when operating the utility grid and the microgrid in a grid connected mode:
controlling, by a grid side converter located at a point of common coupling (PCC) between the utility grid and the microgrid, a microgrid voltage, a microgrid frequency and a microgrid phase angle to match a utility grid voltage, a utility grid frequency and a utility grid phase angle;
generating, by a diesel generator of the microgrid, a preset reference power at the utility grid frequency by controlling a speed of a diesel generator in a speed droop mode;
providing, by the utility grid and the microgrid, at least one of a first real power and a first reactive power to each of a plurality of loads;
then, when operating the microgrid in an island mode:
generating, by the diesel generator, a second real power and a second reactive power at an output frequency which matches the utility grid frequency of the grid connected mode, by operating the diesel generator at a constant speed;
providing, by a diesel generator controller, the diesel generator output frequency as a reference frequency to a wind turbine;
generating, with the wind turbine, a third real power;
generating, by a photovoltaic array, a fourth real power;
providing, by the microgrid, the second reactive power, the second real power, the third real power and the fourth real power to the plurality of loads;
then, when operating the utility grid and the microgrid in a resynchronization mode:
charging a switched capacitor bank connected to the point of common coupling with the second real power, the third real power and the fourth real power;
controlling, by the diesel generator controller, a plurality of switches of the switched capacitor bank to adjust the second reactive power such that the microgrid voltage, the microgrid frequency and the microgrid phase angle match the utility grid voltage, the utility grid frequency and the utility grid phase angle; and
actuating, by a synchro check relay connected to the point of common coupling, a switch to connect the microgrid to the utility grid.

### Claim 2 (depends on 1)

The method of claim 1, further comprising:
connecting the diesel generator to an AC-AC voltage source converter;
connecting the wind turbine to a first DC-AC voltage source converter;
connecting the photovoltaic array to a second DC-AC voltage source converter; and
connecting each of the AC-AC voltage source converter, the first DC-AC voltage source converter and the second DC-AC voltage source converter to the point of common coupling.

### Claim 3 (depends on 2)

The method of claim 2, wherein operating in the grid connected mode comprises:
receiving, by a proportional integral (SG-PI) speed governor controller connected to the diesel generator, a utility grid frequency reference signal, a utility grid voltage reference signal, a droop reference signal, a power reference signal and a shaft frequency;
storing, by the SG-PI speed governor controller, the utility grid frequency reference signal, the utility grid voltage reference signal, the droop reference signal, the power reference signal and the shaft frequency;
subtracting the shaft frequency from the reference shaft frequency;
generating, by the SG-PI speed governor controller, a power value;
generating a power signal by dividing the power value by a speed droop reference value;
subtracting the power signal from the power reference signal to generate a power error signal;
amplifying, by an amplifier circuit, the power error signal to generate an amplified frequency signal;
adding the amplified frequency signal to the power error signal;
generating a shaft frequency signal;
delaying, by a delay circuit, the shaft frequency signal;
dividing the delayed shaft frequency signal by the utility grid frequency;
generating a torque signal;
transmitting the torque signal to the diesel generator; and
generating, by the diesel generator, the real and reactive power based on the torque signal.

### Claim 4 (depends on 2)

The method of claim 2, wherein operating in island mode comprises:
adding, by an adder, a power reference signal and an amplifier feedback signal to generate a power error signal;
amplifying, by an amplifier circuit, the power error signal to generate an amplified frequency signal;
adding the amplified frequency signal to the power error signal;
generating a shaft frequency signal;
delaying, by a delay circuit, the shaft frequency signal;
dividing the delayed shaft frequency signal by the utility grid frequency;
generating a torque signal;
transmitting the torque signal to the diesel generator; and
generating, by the diesel generator, the real and reactive power based on the torque signal.

### Claim 5 (depends on 4)

The method of claim 4, further comprising:
transmitting, by the diesel generator controller, the utility grid voltage reference signal and the diesel generator output frequency to the wind turbine; and
transmitting, by the diesel generator controller, the utility grid voltage reference, Vref, signal to an MPPT circuit of the photovoltaic array.

### Claim 6 (depends on 2)

The method of claim 2, further comprising:
connecting the wind turbine to a doubly fed induction generator, DFIG;
connecting a stator of the wind turbine to a transformer;
connecting a rotor of the wind turbine to a rotor side converter;
connecting the rotor side converter in parallel with a capacitor;
connecting the capacitor in parallel to a grid side converter;
connecting the grid side converter in series with a resistor and an inductor;
connecting the inductor to the transformer; and
connecting the transformer to the first DC-AC voltage source converter.

### Claim 7 (depends on 6)

The method of claim 6, further comprising:
in the grid connected mode, controlling the grid side converter in a decoupled D-Q current control mode at the utility grid frequency with a unity power factor to provide a portion of the first real power to the plurality of loads; and
in the island mode, operating the doubly fed induction generator in a constant P-Q control mode at the reference frequency with a unity power factor to provide the third real power to the plurality of loads.

### Claim 8 (depends on 7)

The method of claim 7, further comprising:
transmitting, by the diesel generator controller, a set of DFIG voltage reference control signals, mDR, and mQR, to the rotor side converter, wherein mDR regulates a rotor side DC voltage and a rotor side active power of the rotor side converter, and mQR regulates a rotor side AC voltage of a reactive power of the rotor side converter.

### Claim 9 (depends on 8)

The method of claim 8, further comprising:
transmitting, by the diesel generator controller, a set of grid voltage reference control signals, mDG, and mQG, to the grid side converter, wherein mDG regulates a grid side DC voltage and a grid side active power of the grid side converter, and mQG regulates a grid side AC voltage of a reactive power of the grid side converter.

### Claim 10 (depends on 2)

The method of claim 2, further comprising:
generating a voltage, VPV, and a current, IPV, by the photovoltaic array, PV;
applying the voltage, VPV, and the current, IPV, to a maximum power point tracking, MPPT, circuit;
receiving, from the controller of the diesel generator, a reference voltage, Vref;
subtracting the voltage, VPV, from the voltage reference, Vref, to generate an error voltage value;
applying the error voltage value to a photovoltaic array proportional integral, PV-PI, controller;
generating, by the PV-PI controller, a duty cycle control signal based on the error voltage value;
applying the voltage, VPV, the current, IPV, and the duty cycle signal to a DC-DC buck converter connected in parallel with the photovoltaic array;
generating a DC voltage, VDC, by the DC-DC buck converter; and
connecting the DC-DC buck converter to the second DC-AC voltage source converter; and
converting the DC voltage, VDC, by the second DC-AC voltage source converter, to a three phase voltage, wherein each phase has a magnitude of the DC voltage at a frequency of the duty cycle signal.

### Claim 11 (depends on 10)

The method of claim 10, further comprising:
calculating, by the PV-PI controller, the duty cycle signal based on
Duty
=
(
V
r
⁢
e
⁢
f
-
V
P
⁢
V
)
·
(
k
P
+
k
I
s
)
,
where kp is a proportional constant of the PV-PI controller, kI is an integral constant of the PI controller, and s is an output variable of a Laplace transform of the PV-PI controller;
when operating in the grid connected mode:
setting a frequency of the duty cycle signal to match the utility grid frequency;
controlling the second DC-AC voltage source converter in a decoupled D-Q current control mode at the utility grid frequency with a unity power factor to provide a portion of the first real power to the plurality of loads;
then, when operating in the island mode;
setting the frequency of the duty cycle signal to match the reference frequency; and
controlling the second DC-AC voltage source converter in a decoupled d-q current control mode at the reference frequency with a unity power factor to provide the fourth real power to the plurality of loads.

### Claim 12 (depends on 11)

The method of claim 11, further comprising:
transmitting, by the diesel generator controller, a set of photovoltaic array voltage reference control signals, mD, and mQ, to the second DC-AC voltage source converter, wherein mD regulates the DC voltage and the active power, and mQ regulates the AC voltage.

### Claim 13

A system for synchronization of a utility grid and a microgrid, comprising:
a utility grid including a grid side controller, wherein the utility grid is configured to operate at a utility grid voltage, a utility grid frequency and a utility grid phase angle;
a grid side converter configured to transmit the utility grid voltage, the utility grid frequency and the utility grid phase angle to the microgrid;
a diesel generator located in the microgrid in parallel with the utility grid;
a diesel generator controller connected to the diesel generator, wherein the diesel generator controller is configured to calculate a power reference from the utility grid voltage and store the utility grid voltage, the utility grid frequency, and the power reference;
a wind turbine located in the microgrid in parallel with the utility grid;
a photovoltaic array located in the microgrid in parallel with the utility grid;
a point of common coupling switchably connected between the utility grid and the microgrid;
a synchro check relay connected to the point of common coupling;
a switched capacitor bank connected between the synchro check relay and the microgrid;
a plurality of loads connected to the microgrid;
wherein, when operating in a grid connected mode:
the microgrid is configured to match a microgrid voltage, a microgrid frequency and a microgrid phase angle to the utility grid voltage, the utility grid frequency and the utility grid phase angle respectfully, generate at least one of a first real power and a first reactive power, and transmit the at least one of a first real power and a reactive power to each of the plurality of loads;
wherein the diesel generator is configured to operate in a speed droop mode;
wherein, when operating in an island mode:
the diesel generator controller is configured to provide the stored utility grid frequency and power reference to the diesel generator;
the diesel generator is configured to operate at a constant angular speed to generate a second real and reactive power at a diesel generator frequency, which matches the stored power reference and stored utility grid frequency respectively;
the diesel generator controller is configured to transmit the stored utility grid voltage and the diesel generator frequency as a wind turbine reference voltage and a wind turbine reference frequency respectively to the wind turbine and to transmit the stored utility grid voltage and diesel generator frequency to the photovoltaic array as a photovoltaic array reference voltage and photovoltaic array reference frequency respectively;
wherein the diesel generator, the wind turbine and the photovoltaic array are configured to generate at least one of a first real power and a reactive power and transmit at least one of the first real power and the reactive power to each of the plurality of loads;
wherein, when operating in a resynchronization mode:
the switched capacitor bank is configured to be charged by the first real power produced by the microgrid;
the diesel generator controller is configured to actuate a plurality of switches of the switched capacitor bank to adjust the reactive power such that the microgrid voltage, the microgrid frequency and the microgrid phase angle match the utility grid voltage, the utility grid frequency and the utility grid phase angle; and
the synchro check relay is configured to actuate the switch to connect the microgrid to the utility grid.

### Claim 14 (depends on 13)

The system of claim 13, further comprising:
an AC-AC voltage source converter connected to the diesel generator;
a first DC-AC voltage source converter connected to the wind turbine; and
a second DC-AC voltage source converter connected to the photovoltaic array,
wherein each of the AC-AC voltage source converter, the first DC-AC voltage source converter and the second DC-AC voltage source converter are connected to the point of common coupling.

### Claim 15 (depends on 14)

The system of claim 14, further comprising in the grid connected mode:
a proportional integral (SG-PI) speed governor controller electrically connected to the diesel generator, wherein the proportional integral (SG-PI) speed governor controller is configured to:
subtract a shaft frequency of the diesel generator from a reference shaft frequency,
generate a power value,
divide the power value by a droop reference value, and
generate a power signal by dividing the power value by a speed droop reference value;
wherein the diesel generator controller comprises:
a subtractor configured to subtract the power signal from the power reference signal to generate a power error signal;
an amplifier circuit configured to receive the power reference, compare the power reference to the real and reactive power to calculate a power error signal, and generate a shaft frequency signal based on the power error signal; and
a delay circuit configured to delay the shaft frequency signal.

### Claim 16 (depends on 14)

The system of claim 14, wherein in the island mode, the diesel generator controller comprises:
an amplifier circuit configured to receive the power reference, compare the power reference to the real and reactive power to calculate a power error signal, and generate a shaft frequency signal based on the power error signal;
a delay circuit configured to delay the shaft frequency signal;
a divider configured to divide the delayed shaft frequency signal by the utility grid frequency and generate a torque signal and transmit the torque signal to the diesel generator; and
wherein the diesel generator is configured to use the torque signal to generate a second real power and the reactive power.

### Claim 17 (depends on 14)

The system of claim 14, further comprising:
a doubly fed induction generator, DFIG, connected to the wind turbine;
a transformer connected to a stator of the wind turbine;
a rotor side converter connected to a rotor of the wind turbine;
a capacitor connected in parallel with the rotor side converter;
a grid side converter connected in parallel with the capacitor;
a resistor and an inductor connected in series between the grid side converter and the transformer; wherein the transformer is connected to the first DC-AC voltage source converter;
wherein, when operating in the grid connected mode, the grid side converter is configured to operate in a decoupled D-Q current control mode at the utility grid frequency with a unity power factor to provide a third real power to the plurality of loads; and
wherein, when operating in the island mode, the doubly fed induction generator is configured to operate in a constant P-Q control mode at the wind turbine reference frequency with a unity power factor to provide the third real power to the plurality of loads.

### Claim 18 (depends on 14)

The system of claim 14, further comprising:
wherein the photovoltaic, PV, array, is configured to generate a voltage, VPV, and a current, IPV;
an MPPT circuit connected to the PV array, wherein the MPPT circuit is configured to receive the voltage, VPV, the current, IPV, and the photovoltaic array reference voltage, and compare the photovoltaic array reference voltage to the voltage, VPV to generate a voltage error signal;
a photographic array proportional integral, PV-PI, controller connected to the MPPT circuit, wherein the PV-PI controller is configured to generate a duty cycle signal;
a DC-DC buck converter connected to the PV array and the PV-PI controller, wherein the buck converter is configured to receive the voltage, VPV, the current, IPV from the PV array and the duty cycle signal from the PV-PI controller and generate a DC voltage;
wherein the DC-DC buck converter is connected to the second DC-AC voltage source converter, wherein the second DC-AC voltage source converter is configured to convert the DC voltage to a three phase voltage, wherein each phase has a magnitude of the DC voltage at the duty cycle frequency.

### Claim 19 (depends on 14)

The system of claim 14, further comprising:
wherein the PV-PI controller, is configured to calculate the duty cycle signal based on
Duty
=
(
V
r
⁢
e
⁢
f
-
V
P
⁢
V
)
·
(
k
P
+
k
I
s
)
,
where kp is a proportional constant of the PV-PI controller, kI is an integral constant of the PI controller, and s is an output variable of a Laplace transform of an integral controller;
wherein, when operating in the grid connected mode:
the PV-PI controller is configured to set a frequency of the duty cycle signal to match the utility grid frequency; and
second DC-AC voltage source converter is configured to use a decoupled d-q current control mode at the frequency of the duty cycle with a unity power factor to provide a fourth real power to the plurality of loads;
wherein, when operating in the island mode;
the PV-PI controller is configured to set a frequency of the duty cycle signal to match the photovoltaic array reference frequency; and
the second DC-AC voltage source converter is configured to use a decoupled d-q current control mode at the photovoltaic array reference frequency with a unity power factor to provide the fourth real power to the plurality of loads.

### Claim 20

A method for synchronizing a utility grid and a microgrid, comprising:
when operating the utility grid and the microgrid in a grid connected mode:
controlling the microgrid at a utility grid voltage, a utility grid frequency and a utility grid phase angle;
generating, by a diesel generator of the microgrid, a preset reference power at the utility grid frequency by controlling a speed of a diesel generator in a speed droop mode;
providing, by the utility grid and the microgrid, at least one of a first real power and a reactive power to each of a plurality of loads;
then, when operating the microgrid in an island mode:
operating the diesel generator at a constant speed based on the utility grid frequency to output a second real power and the reactive power at a reference frequency;
operating the wind turbine in a decoupled d-q mode at the reference frequency with unity power factor to generate a third real power;
generating, by a photovoltaic array, a fourth real power;
providing the reactive power, the second real power, the third real power and the fourth real power to the plurality of loads;
then, when operating the utility grid and the microgrid in a resynchronization mode: charging a switched capacitor bank connected to the point of common coupling with the second real power, the third real power and the fourth real power; and
actuating, by a synchro check relay connected to the point of common coupling, a switch to connect the microgrid to the utility grid.