IEEE Power & Energy Magazine - May/June 2017 - 56

table 1. The CLASS project fact sheet.
*	 Total	cost:	US$12	million*
*	 Primary	substations:	60
*	 	Measurements:	Phase	V,	I,	P,	and	Q	(up	to	1-s	resolution)
*	 Project	length:	Two	years
*	 Customers	involved:	350,000+
*	 	TSO	 ➞	 DNO	 communications	 infrastructure:	 ICCP	
link,	VF	line
*	 Project	Partners:	
DNO:	ENWL	
TSO: National	Grid	
	Academia:	University	of	Manchester	
	Others:	 Siemens,	 General	 Electric,	 Impact	 Research,	WSP/Parsons	Brinckerhoff	
*	 Customer	interviews:	1,300+
*	 	Areas	 of	 study:	 Engineering,	 social	 sciences,	 environment,	economics
*All dollar values in this article are reported adopting the following conversion rate:
GBP£1➝ US$1.22. ICCP: Inter Control Centre Protocol; VF: voice frequency.

3) The TSO control signal then reaches the selected primary substations to change the OLTC tap position and
consequently trigger the required voltage reduction.
4) This voltage reduction then triggers a demand
reduction in many of the appliances of all downstream customers.

1

Communication
TSO ↔ DNO

I Need
X MW

TSO's
Control
Center

2

5) The demand reduction from the selected primary substations, each feeding hundreds of thousands of appliances, results in an aggregated demand reduction at
the transmission-distribution (T-D) interface, which is
expected to satisfy the request of the TSO.
This process requires a communication infrastructure
to send signals from the TSO to the DNO as well as from
the DNO to the OLTCs at primary substations (illustrated in
Figure 2 with dashed lines). In addition, the OLTCs need to
translate these signals into control actions. The voltage-led
LM scheme proposed in CLASS achieved this in a practical
way by leveraging the existing infrastructure, as described
in the following section.

Existing Network Infrastructure
The voltage-led LM scheme proposed in CLASS requires
communication infrastructure and remote control capabilities (see Figure 3). Fortunately, such capabilities are, to different extents, part of existing distribution networks. The
following assets can be found in the United Kingdom:
✔ Voice-frequency (VF) line. This is the communication
infrastructure that allows the exchange of digital and
analog data between the DNO control center and any
primary substation. This is realized via serial 4 wire
VF lines running at 9.6 kb/s using the Distributed Network Protocol (or DNP3) and Teleconnect II protocols
with an overall communications latency of approximately 9 s. This is part of modern supervisory control and
data acquisition functionalities introduced to facilitate
real-time visibility (active and reactive power measurements) of key infrastructure.

I Accept to Provide
the Service
DNO's
Control
Center

DNO Shared Dashboard

Communication
DNO ↔ Primary Substation

Voltage
Reduction

1 Control Signal from TSO to Reduce Demand
2 DNO Accepts the Provision of the Service
3 Change Tap Position of Primary Substations
4 The Demand Reduces
5 Aggregated Demand Reduction Provided
3

Demand
Reduction

4

OLTC

5

X MW
Provided at the
T-D Interface

A Few
Megawatts

Hundreds
of Kilowatts

Dozens of
Watts

A Few
Watts

figure 2. An illustrative example of the five steps involved in the deployment of the voltage-led LM scheme proposed
in CLASS.
56

ieee power & energy magazine

may/june 2017



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