IEEE Power & Energy Magazine - March/April 2015 - 40

ib /ln

Commanded P
Measured P

600
500

20.2

400
300
200
100
0

50

100

150
Time (s)

200

250

PF (cosφ)

figure 13. Measured active power output during the INV2
(adjust maximum generation level up or down) function
test with four different active power set point values: 100%,
66%, 33%, and 0% of the rated power (source: AIT).

1
0.9
0.8
Overexcited
0.7 Overexcited
Underexcited
Underexcited
0.6
250
300
350
400
450
500
Time (s)
Measured PF (cosφ)

Commanded PF (cosφ)

Reactive
Power Q (kVAr)
Active
Power P (kW)

(a)
400
200
0
-200
-400

600

Measured Q
Overexcited Operation
Underexcited Operation
250

300

350
400
Time (s)
(b)

450

500

400
200 Limitation of Active Power Output
as Priority Is Given to Reactive Power
0
250
300
350
400
450
Time (s)

500

Commanded P

Measured P
(c)

figure 14. Measured power factor, reactive power output, and active power output during the test of the INV3
(request power factor) function (source: AIT).

system (DMS) and SCADA from the power grid operator to
the inverter. For the validation of the correct implementation
and operation of the inverter, different set points for the INV2
and INV3 functions have been provided. The corresponding
test results are shown in Figures 13 and 14.
40

Positive Sequence
Reactive Current

-0.5
-1

U/Un

Active Power Output P (kW)

0

700

ieee power & energy magazine

1.2
1
0.8
0.6
0.4

20.4

20.6 20.8
Time (s)
(a)

21

21.2

Positive Sequence Voltage

20.2

20.4

20.6 20.8
Time (s)
(b)

21

21.2

figure 15. (a) Reactive current and (b) transient voltage
curves during a symmetrical grid fault (50% voltage remaining at the terminals of the DER with a fault duration of
985 ms), with k = 3 (source: DNV GL).

For both functions, the correct operation of the PV
inverter together with the IEC 61850 Modbus gateway was
able to be successfully validated, confirming the interoperability of the DER setup.

FPGLab: LVRT Validation
The ability of DER units to ride through fault conditions that
occur in the grid is vital for both voltage and frequency recovery. As the penetration of DERs increases, so will requirements
that the units be able to ride through faults and support the grid's
recovery. At the FPGLab, the option to subject large-scale DER
equipment to the effects of grid faults, such as temporary low
voltage at its terminals, is often used as an inexpensive, repeatable, quick method to verify H/LVRT capabilities.
Figure 15 shows a typical LVRT measurement of a largescale (1-MW) PV inverter providing full grid support during a
symmetrical grid fault, taken at the FPGLab. The inverter being
tested provides reactive power at a given gradient according to
the relative change in voltage from its nominal value, commonly
referred to as the k-factor. In this particular case k was set to 3 and
measured as 3.26. The figure depicts the reactive power contribution of the DER unit during the event to return the grid voltage
to nominal as well as the resumption of normal operation after
the fault is cleared, all while the unit remains in continuously
stable operation and never disconnects from the grid. The actual
terminal voltage therefore only drops by 25% instead of the 50%
it would have if no grid support had been provided.
Figure 16 shows another LVRT measurement of the same
system providing full grid support but this time during an
unsymmetrical grid fault. Again, the equipment under test
provides stable operation and voltage support during the
fault duration but with different contributions from each
phase, as dictated by the grid situation.
These two LVRT tests represent only a small portion of the
voltage scenarios needed to fully support the grid and comply
with the major grid codes, such as the IEC 61400-21 standard.
march/april 2015



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2015

IEEE Power & Energy Magazine - March/April 2015 - Cover1
IEEE Power & Energy Magazine - March/April 2015 - Cover2
IEEE Power & Energy Magazine - March/April 2015 - 1
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IEEE Power & Energy Magazine - March/April 2015 - 96
IEEE Power & Energy Magazine - March/April 2015 - Cover3
IEEE Power & Energy Magazine - March/April 2015 - Cover4
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