•
Fig 36 shows the diagram in which the
DG power generation is calculated
using its current and adjusted in order
to maintain a constant
E
.
•
Here, the DG current
I
and the utility
feeder voltage
E
are measured and
fed back to the LC
control loop.

90
•
The steady state P
ω
characteristics for two DGs are shown in Fig 37, which
allow power to change between P=0 and P=Pmax as ω changes.
•
If power is imported from the grid before islanding, the frequency in island
mode,
ω
imp
will drop to be smaller than
ω
n
. Accordingly, both DG1 and DG2
operating points will shift downward on
droops
.
•
In
this
case,
DG2
reaches
its
maximum
power
and
the
steady
state
characteristic slope switches to vertical, and the operating point moves
downward vertically (right side of the figure).
Regulating the DG Power in Island
Fig 37.
P
Z
characteristics for two DG units

91
•
Fig 38 shows the revised version, if Pmax was not enforced, in which the two
DGs settle in ω1 (shown with circles). Here,
P
1
(at
ω
1
) = P1_1
P
2
(at
ω
1
) = P
max
+
Δ
P
max
•
With Pmax enforced, the frequency is shifted to ω2 as P1 takes on the
additional load so that the final operating point (shown in squares) would
satisfy the limits:
P
1
(at
ω
2
) = P1_1 +
Δ
P
max
P
2
(at
ω
2
) = P
max
Regulating the DG Power in Island
Fig 38. Island mode operation with a limiting Pmax for output power

92
•
To prevent a power injection that exceeds the maximum value, we generate a
power error
errP
max
with the reference set to
P
max
, as shown in Fig 39.
•
This error is passed on to an integral block that has a dynamic limiter which
prevents its output to ever become positive.
•
As long as power is less than
P
max
,
errP
max
is positive and the output of the
integral is dynamically limited to zero.
•
If power is larger than
P
max
,
errP
max
is negative and the integral would start
generating a negative value for the offset. It will keep on translating down until
the offset exactly matches the value for which
errP
max
= 0
Regulating the DG Power in Island
Fig 39. Offset generation to limit max power with integral block

93
•
The opposite consideration in Fig 37 takes place with power initially exported
to the grid
•
Now the attention in this case is to find the changes that need to be done
when zero power limit is exceeded
•
The operation points are compared in Fig.40.
Regulating the DG Power in Island
Fig 40. Limiting
P
min
=0 on output power control
Without P
min
considered, the DGs
operate at
ω
1
(the dots):
P
1
(at ω
1
) = P
min
– ΔP
min
P
2
(at ω
1
) = P2_1
With P
min
considered, the DGs operate
at
ω
2
(the squares):
P1(at ω
2
) = P
min
(=0)
P2(at ω
2
) = P2_1 ΔP
min

94
•
Fig
41 shows that the first step is to calculate
errP
min
, using the minimum
power setpoint as zero.

#### You've reached the end of your free preview.

Want to read all 106 pages?

- Spring '14
- MohammadShahidehpour
- Trigraph, Electric power, Electricity distribution