IEEE Power & Energy Magazine - March/April 2015 - 33

march/april 2015

Grid Voltage V/Vn

set its reactive power output according to defined control
modes, while active power can be controlled only subject to
the availability of the primary power source.
Inverter functions range from immediate control functions, such as remote connect and disconnect, limiting active
power output, and setting a fixed PF and var output to complex control modes such as volt/var, watt/PF, frequency/watt,
or volt/watt response. In conjunction with storage, additional
charge and discharge control depending on local load or
overall grid conditions can be used.
As an example, the characteristics of the volt/var control determining the inverter's response are preestablished
during installation or remotely configured and enabled
through communication signals or time schedules (see
Figure 2). This command is one of the autonomous functions, so if the local voltage measurements exceed the
deadband between points V2 and V3, reactive power will
be injected or absorbed from the grid to help move the
grid voltage back to the nominal value in the deadband.
Appropriate timing plays an important role to avoid abrupt
changes in DER output. For this purpose, advanced DER
inverters can provide ramp rate control and randomization
time windows to ensure the smooth response of a single
DER installation as well as for the aggregate DER capacity
installed in a distribution system.
Besides complex active and reactive control capabilities,
state-of-the-art DER inverters also provide extensive ridethrough capabilities to support the local distribution grid as
well as the overall power system during abnormal conditions
such as faults and frequency fluctuations. In power systems
with high shares of DERs, the appropriate implementation of
ride-through functions will be crucial to avoiding the simultaneous loss of large DER capacities during remote faults
and during frequency variations, which could potentially
lead to an overall undersupply of power.
In DER converters, the associated capabilities for highand low-voltage ride-through (H/LVRT) as well as frequency
ride-through are implemented by designing the internal current control structures and converter hardware to be capable
of operating at abnormally high or low voltage and frequency
levels. For this purpose, zones are defined by the latest interconnection specifications where the DERs must remain connected and provide grid support. A sample H/LVRT curve is
shown in Figure 3.
To support system voltage during a fault, FRT-capable
inverters can also provide a fast response (fast reactive current injection) according to a predefined volt/var curve, supplying or consuming reactive current during short periods of
low or high voltage.
Going beyond the advanced DER capabilities required
by today's interconnection specification, the full integration
of DERs into future smart grid control concepts will further
enable their ability to provide coordinated grid support services. For this purpose, new strategies for effectively managing the large numbers of dispersed generation systems

1

0
Initiation of Fault

Time (s)

Normal Voltage Range for Permanent Operation
Must Remain Connected and Provide Reactive
Current to Support the Grid
May Disconnect from the Grid
Must Disconnect from the Grid

figure 3. A sample H/LVRT requirement for DER converters (source: AIT).

and utilizing their "smart" capabilities are currently being
developed by the International Electrotechnical Commission (IEC). Concepts and approaches as well as standards
from the information and communication technology (ICT)
domain will play a major role in realizing a coordinated systems approach.
Such a "smart" approach is not fully addressed by today's
grid codes and standards. Moreover, current national interconnection specifications define advanced DER capabilities
in different ways, requiring manufacturers to adapt their
products individually for single national markets. This situation not only leads to increased costs and a longer time-tomarket, but missing or inappropriate testing and validation
specifications can even cause issues with respect to interoperability and reliability.
To remedy this unsatisfactory situation, the IEC made a
first step toward standardizing advanced DER functions in
a 2013 technical report (reference number IEC/TRĀ  6185090-7) that enlarges the scope of the IEC 61850 standard,
which was originally developed for substation automation.
There are still large gaps that need to be filled, however,
with respect to the test procedures and certification standards
for verifying the interoperability and functionality of the
advanced DER functions.
To validate smart inverters, exercise nascent and draft
conformance standards, and work toward a consensus on
certification procedures for the new IEC functions, testing
laboratories around the world are building research platforms for studying advanced DERs. The rapid development
of these research facilities and the collaborations among
ieee power & energy magazine

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

IEEE Power & Energy Magazine - March/April 2015 - Cover1
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IEEE Power & Energy Magazine - March/April 2015 - Cover3
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