Control of small distributed energy resources
A method of controlling the output inverter of a microsource in a distributed energy resource system is disclosed. Embodiments of the invention include using unit or zone power controllers that reduce the operating frequency of the inverter to increase its unit output power. Preferred embodiments includes methods wherein the inverter reaches maximum output power and minimum operating frequency at the same time, and further comprising using a voltage controller implementing a voltage vs. reactive current droop. Other aspects of this embodiment relate to an inverter that implements such methods, and a microsource containing such an inverter. These methods can be extended to control inverters in a plurality of microsources, organized in a single zone or in a plurality of zones.
1. A method of controlling distributed energy resources, the method comprising:
providing a microsource, wherein the microsource is located in a microgrid, and further wherein the micro source is configured to deliver a power P 1 at a frequency ω1;
operating the microsource in a grid mode in which the microsource is connected to a utility grid, wherein during operation in the grid mode the frequency ω1 is approximately equal to a frequency ω o and the power P 1 is equal to a power P 1 o , wherein the frequency ω o is an operating frequency of the utility grid; and
transferring the microsource from the grid mode to an island mode such that the microsource is disconnected from the utility grid, wherein ω1 is equal to ω island and P 1 is equal to P 1 island during the island mode;
wherein ω island >ω 0 , and P 1 island <P 1 0 if the microgrid was exporting power prior to the transfer from the grid mode to the island mode;
wherein ω island <ω 0 , and P 1 island >P 1 0 if the microgrid was importing power prior to the transfer from the grid mode to the island mode; and
wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , wherein the microsource has a slope switch frequency ω switch , and
ω
island
≈
ω
0
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
.
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
max
.
2. The method of claim 1 , wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , and wherein
ω
island
≈
ω
0
(
ω
0
-
ω
min
P
2
0
-
P
2
max
)
(
P
2
0
-
P
2
island
)
.
3. The method of claim 1 ,
wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
≥
ω
switch
.
4. The method of claim 1 , wherein the microsource delivers a maximum output level P 1 max at a frequency ω min wherein the microsource has a maximum operating frequency ω max ,
wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
max
;
and
wherein
ω
island
≈
ω
max
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
≥
ω
max
.
5. The method of claim 1 , wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , wherein the microsource has a slope switch frequency ω switch , wherein the microsource has a maximum operating frequency ω max ,
wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
switch
;
wherein
ω
island
<
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
ω
switch
≤
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
max
;
and
wherein
ω
island
≈
ω
max
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
≥
ω
max
.
6. The method of claim 1 , wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , wherein the microsource has a minimum power output level P 1 min ,
wherein
P
1
island
≈
P
1
0
-
(
P
1
0
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
0
-
(
P
1
0
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
>
P
1
min
;
and
wherein
P
1
island
≈
P
1
min
if
P
1
0
-
(
P
1
0
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≤
P
1
min
.
7. The method of claim 6 , wherein the microsource includes power storage such that P 1 min <0.
8. The method of claim 1 , further comprising:
providing a second microsource, wherein the second microsource is located in the microgrid, and further wherein the second microsource is configured to deliver a power P 2 at a frequency ω2;
operating the second microsource in the grid mode in which the second microsource is connected to the utility grid, wherein during operation in the grid mode, the frequency ω2 is approximately equal to the frequency ω o and the power P 2 is equal to a power P 2 o ; and
transferring the second microsource from the grid mode to the island mode such that the second microsource is disconnected from the utility grid, wherein ω2 is approximately equal to ω island and P 2 is equal to P 2 island during operation in the island mode;
wherein ω island >ω o and P 2 island <P 2 o if the microgrid was exporting power prior to the transfer from the grid mode to the island mode; and
wherein ω island <ω o and P 2 island >P 2 o if the microgrid was importing power prior to the transfer from the grid mode to the island mode.
9. The method of claim 8 , wherein (P 1 island −P 1 0 )≈(P 2 island −P 2 0 ) such that an amount of change of P 1 when the microsource is transferred from the grid mode to the island mode is approximately equal to an amount of change of P 2 when the second microsource is transferred from the grid mode to the island mode.
10. The method of claim 8 , further comprising providing a power slope m, and wherein ω island ≈ω 0 −m(P 1 0 −P 1 island )≈ω 0 −m(P 2 0 −P 2 island ).
11. The method of claim 8 , further comprising providing a power slope m, wherein the microsource has a minimum operating frequency ωmin, wherein ω island ≈ω 0 −m(P 1 0 −P 1 island )>ω min ; and
wherein ω island ≈ω min if ω 0 −m (P 1 0 −P 1 island )≦ω min .
12. The method of claim 8 , further comprising providing a power slope m, wherein the microsource has a maximum operating frequency ω max , wherein ω island ≈ω 0 −m(P 1 0 −P 1 island )>ω max ; and
wherein ω island ≈ω max if ω 0 −m(P 1 0 −P 1 island )≦ω max .
13. The method of claim 8 , further comprising providing a power slope m, wherein the microsource has a minimum operating frequency ω min and a maximum operating frequency ω max , wherein ω island ≈ min if ω−m(P 1 0 −P 1 island )≦ω min wherein ω island ≈ω max −m(P 1 0 −P 1 island )>ω min ; and
wherein ω island ≈ω max if ω 0 −m (P 1 0 −P 1 island )≦ω max .
14. The method of claim 1 , wherein the microsource has a maximum power output level P 1 max , a minimum power output level P 1 min , and a minimum operating frequency ω min , and wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
min
-
P
1
max
)
(
P
1
0
-
P
1
island
)
.
15. The method of claim 1 , wherein the microsource has a maximum power output level P 1 max , a minimum power output level P 1 min , and a minimum operating frequency ω min ,
Wherein
P
1
island
≈
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
>
P
1
min
and
wherein
P
1
island
≈
P
1
min
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≤
P
1
min
.
16. The method of claim 1 , wherein the microsource has a maximum power output level P 1 max , a minimum power output level P 1 min , and a minimum operating frequency ω min ,
wherein
P
1
island
≈
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
<
P
1
max
and
wherein
P
1
island
≈
P
1
max
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≤
P
1
max
.
17. The method of claim 1 wherein the microsource has a maximum power output level P 1 max , a minimum power output level P 1 min , and a minimum operating frequency ω min ,
wherein
P
1
island
≈
P
1
min
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≤
P
1
min
;
wherein
P
1
island
≈
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
min
<
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
<
P
1
max
;
and
wherein
P
1
island
≈
P
1
max
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≥
P
1
max
.
18. A microgrid comprising:
a microsource comprising a power controller configured to
control a frequency ω1 and a power P 1 of the microsource;
operate the microsource in a grid mode in which the microsource is connected to a utility grid, and wherein the frequency ω 1 is approximately equal to an operating frequency ω o of the utility grid and the power P 1 is equal to P 1 o ; and
transfer the microsource to operate in an island mode in which the microsource is disconnected from the utility grid, wherein the frequency ω1 is equal to ω island and the power P 1 is equal to P 1 island in the island mode;
wherein ω island >ω 0 , and P 1 island <P 1 0 if the microgrid was exporting power prior to the transfer from the grid mode to the island mode;
wherein ω island <ω 0 , and P 1 island >P 1 0 if the microgrid was importing power prior to the transfer from the grid mode to the island mode; and
wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , wherein the microsource has a slope switch frequency ω switch , wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
switch
;
19. The microgrid of claim 18 , wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , and wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
2
0
-
P
2
max
)
(
P
2
0
-
P
2
island
)
.
20. The microgrid of claim 18 ,
wherein
ω
island
<
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
≥
ω
switch
.
21. The microgrid of claim 18 , wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , wherein the microsource has a slope switch frequency ω switch , wherein the microsource has a maximum operating frequency ω max ,
wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
switch
;
wherein
ω
island
<
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
if
ω
switch
≤
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
<
ω
max
;
wherein
ω
island
≈
ω
max
if
[
ω
0
-
(
ω
0
-
ω
min
P
1
0
-
P
1
max
)
(
P
1
0
-
P
1
island
)
]
≥
ω
max
.
22. The microgrid of claim 18 , wherein the microsource delivers a maximum power output level P 1 max at a frequency ω min , wherein the microsource has a minimum power output level P 1 min ,
wherein
P
1
island
≈
P
1
0
-
(
P
1
0
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
0
-
(
P
1
0
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
>
P
1
min
;
and
wherein
P
1
island
≈
P
1
min
if
P
1
0
-
(
P
1
0
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≤
P
1
min
.
23. The microgrid of claim 22 , wherein the microsource further comprises a power storage unit such that P 1 min <0.
24. The microgrid of claim 18 , wherein the microsource has a maximum power output level P 1 max , a minim urn power output level P 1 min , and a minimum operating frequency ω min , and wherein
ω
island
≈
ω
0
-
(
ω
0
-
ω
min
P
1
min
-
P
1
max
)
(
P
1
0
-
P
1
island
)
.
25. The microgrid of claim 18 , wherein the microsource has a maximum power output level P 1 max , a minimum power output level P 1 min , and a minimum operating frequency ω min ,
P
1
island
≈
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
>
P
1
min
;
and
wherein
P
1
island
≈
P
1
min
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≤
P
1
min
.
26. The microgrid of claim 25 , wherein the microsource further comprises a power storage unit such that P 1 min <0.
27. The microgrid of claim 18 , wherein the microsource has a maximum power output level P 1 max , a minimum power output level P 1 min , and a minimum operating frequency ω min ,
P
1
island
≈
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
<
P
1
max
;
and
wherein
P
1
island
≈
P
1
max
if
P
1
0
-
(
P
1
min
-
P
1
max
ω
0
-
ω
min
)
(
ω
0
-
ω
island
)
≥
P
1
max
.