IEEE Power & Energy Magazine - March/April 2015 - 46

integration into the area electric
power system (EPS).
Mismatch in Real Power at Substation (%)
MVar at
Obtaining a clear understanding
Substation
-5% 0% 5% 10% 15% 20% 30% 35% 40% 45% 50% 55%
of system dynamic performance
0 Cap 3.2 0.91 0.91 0.93 0.96 1.01 1.07 1.26 1.38 1.60 1.77 1.29 0.65
and possessing the ability to properly model inverters and power
1 Cap 1.5 1.03 1.04 1.06 1.12 1.22 1.35 1.87 3.01 0.67 0.56 0.42 0.38
electronic equipment have been
2 Cap
0 0.67 2.03 1.99 2.40 0.89 0.64 0.46 0.42 0.38 0.36 0.32 0.29
the key challenges facing utility
3 Cap -1.0 0.13 0.12 0.11 0.11 0.11 0.10 0.10 0.09 0.09 0.09 0.09 0.09
engineers and their consultants. To
figure 3. Sample results for antiislanding scheme response in a high-penetration
underscore the issue, understandcase (7.9 MW DG). Color coding is based on detection time: green signifies a detecing the expected level of current
tion time < 1 s and red signifies a detection time > 1.5 s.
contribution by PV inverters and
modeling their response during
faults
and
typical
grid
disturbances
(e.g., voltage sag initiated
Although communication schemes and methods of deployment
still need to be streamlined among vendors and products, set point through transmission faults) have been key discussion points
commands for power factor, reactive power, and power curtail- among utilities, inverter vendors, and facility developers.
To effectively study a power electronic device or generation
ment (production limiting) can be sent remotely to an inverter to
change active and reactive power output at any time. At present, facility utilizing multiple power converters, information about
Modbus communication protocols (TCP/IP or serial) are com- control and protection capabilities should be available. The
monly used by PV inverter manufacturers. If there is more than information should provide detailed insights into the expected
one inverter in a PV plant or if vendor-specific and proprietary dynamic response and possible interactions with other devices
inverter protocols are used, communication commands can be if impact studies are to be performed accurately. Such studies
exchanged through plant-level and string (cluster) controllers, as should include detailed fault and voltage disturbance analyses.
Knowledge of detailed device models and the ability to charshown in Figure 4.
acterize the dynamic behavior of power electronic interfaces
are expected to become more and more critical and raise many
PV Inverter Study and Testing Practices
To reliably introduce a new power-conditioning device or gen- questions as new smart control functionalities combining generation facility into the system, a series of resource evaluations eration and power-conditioning capabilities are introduced
and detailed planning studies will be needed to determine possi- through emerging smart inverter technologies.
ble impacts on the system and to suggest corrective measures. In
addition, extensive field testing and performance analysis dur- Gaps in Current Testing Practices
ing the commissioning period are required to ensure its proper and Certifications
In current practice, during the testing procedures for PV inverters and power electronic converters provided by certification
agencies (e.g., the UL 1741 method shown in Figure 5), inverters and other devices are tested at the device level rather than
the system level. This is accomplished through applying a load
Utility Control Center
bank and a controlled voltage source that may not accurately
reflect effects of source impedances and complex responses to
Communications
DNP3 or Modbus
nonlinear loads, since the source is an infinite bus and the load
(Commands or
is a passive impedance. Current certification tests are focused
Measurements)
Plant-Level Controller
on the verification of certain aspects of control and protection
(Substation HMI)
design for one device of a specific type. This approach is used
even though most utility-scale PV installations include mulModbus
Modbus
tiple PV inverters connected in parallel or distributed across
an area with connection points on a collector system. Another
String (Cluster)
String (Cluster)
concern with device-level testing is that such procedures
Controller 1
Controller 4
will not be able to characterize the dynamic interactions and
Proprietary
responses typically seen at the primary-system level (12-kV or
Protocol
25-kV feeders).
In general, current certification testing practices at the
Inverter 1
Inverter 16
device level are lacking in the following areas:
✔✔ During testing, there is no consideration given to area
EPS characteristics, such as grid impedance, voltage
figure 4. A sample communication architecture for PV
level, and stiffness.
inverter dispatch.
Trip Times for Generator Protection in Response to Accidental Islanding (s)

46

ieee power & energy magazine

march/april 2015



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

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IEEE Power & Energy Magazine - March/April 2015 - Cover3
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