IEEE Power & Energy Magazine - May/June 2017 - 57

✔ Remote terminal unit (RTU). The RTU provides the in-

✔ Remote customer monitors. Voltage monitors, installed at

terface between the primary substation and the DNO
control center.
✔ Automatic voltage control (AVC) relay. This relay operates the OLTC based on local measurements and a predefined logic so as to maintain voltages of a specific point
in the network (e.g., the busbar) within predefined limits.
✔ OLTC. These voltage-control devices, as pictured in
Figure 4(a), are installed in primary substation transformers and are responsible for changing the transformer tap position during load conditions.
Although this network infrastructure was used in CLASS,
some enhancements were required to add extra capabilities.
These are discussed in the following sections.

the remote ends of low-voltage distribution feeders, verified whether the voltages of the furthest customers were
below statutory limits during the CLASS deployment.
The implemented updates of the existing infrastructure
were found to be 57 times faster and approximately 12 times
cheaper than the traditional reinforcements that would otherwise be required to incorporate the same capabilities.

Enhancing Existing Network Infrastructure
For CLASS to be deployed successfully, the communication
infrastructure should be able to carry control signals from
the TSO to each OLTC. In addition, the OLTC should be able
to translate these control signals by changing its tap position
accordingly (see Figure 3). For this purpose, the capabilities
of the existing infrastructure were enhanced as follows.
✔ ICCP link. ICCP is a protocol that provides a mechanism for real-time data exchange (typically used
only to exchange data between TSO and DNO control
rooms/centers). For the first time in the United Kingdom, an ICCP was designed and built to provide the
TSO with control functionalities over the distribution
network assets (OLTCs), which are essential for the
eventual wide-scale implementation of a voltage-led
LM scheme.
✔ Autonomous substation controller (ASC). This device, shown in Figure 4(d), translates the control signals produced by the TSO (delivered via the ICCP
link and VF lines) or DNO into signals the AVC can
use to trigger the required tap changes in the OLTC.
The ASC is the key device required in any of the primary substations participating in the voltage-led LM
scheme proposed in CLASS.
✔ Updating or replacement of the AVC relay. Old AVCs
are not capable of receiving signals from the ASC
and therefore need to be replaced with modern units
[Figure 4(d)]. In some cases, it was possible to adopt a
less expensive solution in which a third component, an
Argus 8 relay, was used as the interface between the
ASC and an old AVC.
✔ Transducers and data transmitter (DT). New transducers [Figure 4(b)] were deployed at all 60 primary
substations in the trial area to allow the corresponding monitors to provide higher resolution measurements. Then, a DT provided by the Nortech Envoy storage and a 3-G converter unit [shown in Figure 4(c)],
channeled these measurements (adopting an RS485
output) to a cloud-based storage to be used for further analysis.
may/june 2017

Unlocking the Potential:
Assessing the Benefits of CLASS
To quantify the potential volumes of demand reduction that
could be provided by the CLASS voltage-led LM scheme, it
is first necessary to assess the time-varying load models and
voltage capabilities (defined as follows) for each primary
substation involved in the trial (also illustrated in Figure 5):
✔ load model: This is the time-varying voltage-demand
relationship that, in the case of primary substations, is
determined by the aggregation of hundreds of thousands of appliances.
✔ voltage capability: This is the time-varying extent
to which voltages can be reduced. The interactions
across the whole distribution network (upstream and
downstream from the primary substation) must be
taken into account to produce an adequate demandreduction quantification.
Two different approaches were employed to determine
realistic load models: a measurement-based and a component-based model. The former uses measurements from
the CLASS trial in which both parallel primary transformers

TSO's
Control
Center

Transmission
Network
ICCP
Link

DNO's
Control
Center
VF
Line

Primary Substation
OLTC

AVC
ASC
RTU

OLTC

DT
Secondary
Key
Substation
Existing
New Infrastructure
for CLASS
Updated/
New for CLASS
Monitor

Remote
Customer

figure 3. An overview of the existing communication,
measurement, and control infrastructure and the new
infrastructure required by CLASS.
ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2017

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