IEEE Power & Energy Magazine - May/June 2017 - 79
during fault conditions, when there could be sub-
stantial loss of conventional rotating machines, thus
affecting the safety of the electric power system.
* As smart inverters have no inertia, a California Inde-
pendent System Operator study-conducted in sup-
port of the state's goal of achieving 30% renewable
penetration by 2020-suggests a significant reduc-
tion in system inertia to the extent that normal sys-
tem contingencies can drive the frequency below the
presently set underfrequency load shedding levels.
* While smart inverters can regulate voltage, the miti-
gating impacts of reducing voltage may increase the
grid's reactive power requirements. Thus, close co-
ordination with volt/var control is required.
* Large utility-scale PV installations using older invert-
ers can impact several different aspects of distribution
systems planning, engineering, and operations, such
as creating temporary overvoltages under fault condi-
tions that can damage customer and utility equipment.
* "Pirate" PV installations without proper intercon-
nection technology can energize downed overhead
conductors, causing safety problems for the public
and maintenance personnel seeking to repair the
damaged circuit.
* Legacy inverter-based DERs may trip off and discon-
nect during transient low voltage or frequency condi-
tions caused by a transmission fault. In a scenario that
includes high penetration of DERs, this may lead to
losing the contribution from a large amount of DG,
with an aggregated effect that might be comparable
to or greater than the outage of a single conventional
power plant. Concerns in this area have prompted the
✔ DER data and cybersecurity. While vendor and de-
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
Voltage Profile Difference:
"PV at 0.99 PF" Minus "No PV"
Voltage Increase/
Violations Mitigated
Via Advanced Inverters
0
2
Voltage (PU)
Voltage (PU)
veloper information systems keep track of new DER
sales and installations, these sources of information
should be better integrated with utility information
systems. At the same time, privacy and cybersecu-
rity issues must remain a high priority, particularly in
tackling consumer confidentiality and data ownership
implications for DERs not owned and operated by
utilities. The Internet of Things (IoT) promises low-
cost, ubiquitous communications with DERs, which
would facilitate incorporating them into advanced
market and operations processes. However, distribu-
tion systems are expected to have a high level of reli-
ability, security, and availability, even in catastrophic
situations, requiring upgrades to improve capacity and
reliability with increased automation.
✔ Smart inverter technology. The sometimes extreme
variability that comes with higher penetration of re-
newable DERs, particularly distributed PVs, will create
problems for the distribution system (in terms of volt-
age and power flow fluctuations) and, eventually, for
the bulk power system (in terms of managing system
frequency and area generation-load balance). Inverters
are helpful technologies to facilitate DER integration,
as shown in Figure 2. However, the use of this tech-
nology introduces additional challenges related to the
monitoring, control, coordination, and interaction be-
tween fleets of smart inverters and existing distribution
voltage control, regulation, and protection equipment.
* Unless addressed by more advanced adaptive pro-
tection schemes, inverter technologies will not con-
tribute enough current to activate protection devices
4
6
8
10
Distance from Substation (mi)
(a)
1
13
2
14
3
15
4
16
12
5
17
6
18
7
19
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
8
20
Voltage Profile Difference:
"PV at Unity PF" Minus "No PV"
Voltage Increase/
Violations Caused
by PV Interconnection
0
9
21
2
10
22
4
6
8
10
Distance from Substation (mi)
(b)
11
23
12
12
24
figure 2. Example benefits of using advanced inverters to mitigate the impacts caused by PV proliferation (over 5 MW).
Plots show the voltage profile difference for operating a PV plant at (a) unity power factor and (b) absorbing reactive
power at 0.99 constant power factor using advanced inverters. Positive voltage values represent increases caused by PV
interconnection. Each dot is the voltage difference (PU) of a feeder node (miles from substation) at a specific time of the
day for the respective cases.
may/june 2017
ieee power & energy magazine
79
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