IEEE Power & Energy Magazine - May/June 2021 - 49
voltage). Every primary feeder has a connection through a
network unit (NU) to every secondary main. A schematic of
an NU is shown in Figure 1(b). The NU is a subsystem that
contains the three-phase network transformer (NT) that steps
the voltage from MV to low voltage (LV), or a secondary
voltage, and a network protector (NP) that includes a directional power relay (defined in IEEE Standard C37.2-2008 as
an " IEEE function 32 " device), a phase angle relay ( " IEEE
function 78 " ), and the associated circuit breaker (CB). The
secondary mains are typically operated at a utilization voltage (usually 120/208 V or, in a few cases, 480/277 V).
Secondary Spot Networks
A secondary spot network is a smaller system designed to
provide multiple redundant paths for the power flow from
a primary service circuit to a single facility or load. Spot
networks do not provide the same extensive redundancy
as grid networks, but they are considerably less expensive.
Secondary spot network systems usually provide 277/480-V
or 120/208-V service. A typical secondary spot network is
described in Figure 2. All the conductors are three phase.
A single main service is shown at the top, but secondary
spot networks can have more than one. The main service
supplies N primary feeders, which are colored differently to
make the diagram easier to read. Each primary feeder, in turn,
serves an NU that connects to a single secondary main. Each
secondary main serves a three-phase load bus, also called a
collector bus or a paralleling bus. Load buses are tied together
via normally closed (NC) switches. Some spot networks use
fast-acting CBs instead of NC switches to provide fault isolation on the load bus.
Protection of Secondary Networks
General Protection Strategy
The following are two key aspects of the secondary network
protection strategy that are important to the discussion of
DERs and microgrids:
P, Q
P, Q
CB
Main Service (MV)
CB P, Q
Primary Feeder 1 (MV)
Primary Feeder N (MV)
NU
P, Q
NU
NU
P, Q
To Primary Feeder
NU
NT
Secondary
Main 1 (LV)
Loads
NU
=
32
Loads
Loads
NP
P, Q
To Secondary Main
Loads
P, Q
78
(b)
Secondary
Main K (LV)
Loads
Loads
Loads
(a)
figure 1. A typical secondary grid network without DERs: (a) the network topology, showing the directions of the active
(P) and reactive (Q) power flows, and (b) a network unit (NU). CB: circuit breaker; LV: low voltage; NP: network protector;
NT: network transformer.
may/june 2021
ieee power & energy magazine
49
IEEE Power & Energy Magazine - May/June 2021
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021
Contents
IEEE Power & Energy Magazine - May/June 2021 - Cover1
IEEE Power & Energy Magazine - May/June 2021 - Cover2
IEEE Power & Energy Magazine - May/June 2021 - Contents
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IEEE Power & Energy Magazine - May/June 2021 - Cover3
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