IEEE Power & Energy Magazine - March/April 2015 - 45

ratio of inverter-based generation units to conventional rotating
machine-based generation plants in the area. Figure 2 illustrates
the frequency response for a feeder with a generation mix consisting of rotating-machine DG and multiple PV systems. Under high
power mismatches, e.g., 45% and above as shown in Figure 2(a),
the power from conventional generation units is dominantly supplying the load. Due to high mismatch between load and generation, power frequency decreases when there is an accidental
islanding of the feeder. In Figure 2(b), the power mismatch is further reduced by adding more generation from PV units, yet there
is still more load than generation. The power frequency shows an
increasing rate, however, with the much faster trip time of inverter
units in response to unintentional islanding. Based on these observations, it can be concluded the following:
✔ In a rotating machine-dominated case, a shortage of
generation versus load causes frequency to decrease,
while excess generation causes frequency to increase.
✔ In an inverter-dominated power mismatch scenario,
simulations and field testing (in a controlled environment) have shown that the frequency variations will
be dictated by reactive power imbalance on a feeder.
For instance, there will be situations in which the system frequency will increase under a shortage of active
power but a surplus of reactive power.
Further laboratory investigations with testing of multiple
commercial PV inverters have shown similar behavior. This
is mainly due to the fact that the frequency response of PV
inverters is very much influenced by the design of the phaselocked loop (PLL) circuit in grid-tie inverters.
Simulation studies of high-PV-penetration scenarios have
also shown that conventional antiislanding protection schemes

Control and Communications
for PV Inverters
To manage the voltage profile and power flow on a feeder under
high PV penetration, reactive power control of PV inverters
and the ability to curtail output power are two essential control
schemes. The objective is to move away from fixed power factor
set points and constant reactive power and be able to dynamically adjust the reactive power absorption or injection by PV
inverters according to changing circuit conditions throughout the
day. Utilities are looking into standard communication and control methods to utilize these types of controls and dispatch PV
plants remotely as part of day-to-day system operation. Remote
controls will provide the flexibility and controllability needed
to properly operate distribution systems while increasing the
use of existing assets and mitigating the adverse impacts often
experienced under conditions of extreme PV power production.
Most commercially available PV inverters offer reactive power
adjustment and power curtailment through communications.

System Frequency (Generator Dominant)

75
70
F (Hz)

will trip faster when the frequency response is dominated to a
greater extent by inverters and there is surplus reactive power.
As shown in Figure 3, islanding scenarios with negative reactive power mismatch (surplus) have all had fast detection time
irrespective of the penetration level. Hence, one potential way
to ensure effective antiislanding protection for all inverter-based
generation is to cause a surplus of reactive power on a feeder.
To achieve this natural effect, the reactive power resources on a
feeder (switched capacitors, dynamic var units, and so on) can
be controlled so as to maintain a net negative reactive power
flow (or net outflow) in an area of a feeder where the minimum
load-to-generation ratio is less than 2.

55%

50%
45%

65
60
55
0

0.5

1.5
Time (s)
(a)

2.5

System Frequency (Inverter Dominant)
75
F (Hz)

35%

20%
10%
-5%

65
60
55
0

0.5

1.5
Time (s)
(b)

2.5

figure 2. The frequency response for the multiple-DG technology case: (a) synchronous generator dominated and
(b) inverter dominated.
march/april 2015

ieee power & energy magazine

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