IEEE Power & Energy Magazine - May/June 2021 - 50

1)	 Protection against faults on secondary network conductors tends to rely on the high fault current levels
typically available in such systems.
2)	 Protection against faults on the primary feeders relies,
in part, on the unidirectional current flow in the NUs.
Consider first the protection of secondary network mains
and conductors against fault currents. This typically takes
advantage of the available fault currents in secondary networks, which tend to be very high. Fuses and cable limiters are common in the secondary mains of both spot and
grid networks. For certain cable faults, especially in grid
networks, the fault current is so high that the cable itself
acts as a fuse, and the fault " burns clear. " Because there are
many source points within a secondary network, a fault on
any secondary main must be isolated from both sides. Also,
in ac systems with voltages below 150 V, arcs tend to selfextinguish because the voltage is too low to maintain them.
Thus, in grid networks operated at 120 V, many arcing faults
will resolve themselves.

Protection against faults on the conductors between the
primary feeder CBs and the NT also requires that the fault
be isolated from both sides. A fault on a primary feeder can
be directly fed from the main service as well as through the
secondary network, which forms a loop back to the main service on the " downstream " side of the fault. This is illustrated
in Figure 3, which shows the spot network in Figure 2 with
a fault to ground on primary feeder N. In Figure 3, the CBs
and NUs have been numbered. The gold arrows show fault
current flowing to the fault from both sides. In secondary networks, the primary feeder fault is isolated from the main service by the usual overcurrent functions in the primary feeder
CB (CB 3), and from the NU (NU 3) on the " downstream "
side of the fault by the reverse power function (function 32)
in NU 3's NP.
The reverse power function commonly uses a sensitive setting to ensure that the NP just downstream from the primary
side fault (the green one in Figure 3) will trip on the reverse
current before any of the other NPs trips on the forward fault

P
Main Service (MV)
CB

Primary
Feeder 2
(MV)

Primary
Feeder N
(MV)

CB

Primary
Feeder 1
(MV)

CB

To Primary Feeder

NT
NU

NU

P, Q

NU

P, Q

NU

=

32

NP

P, Q

78

To Secondary Main

NC
Tie

NC
Loads

Tie

P, Q

Secondary
Main N (LV)

P, Q

Secondary
Main 2 (LV)

P, Q

Secondary
Main 1 (LV)

(b)

NC
Loads

Tie

P, Q
NC

Loads

Tie

(a)

figure 2. A typical secondary spot network without DERs: (a) the circuit topology, showing the P, Q power flow directions, and (b) an NU. NC: normally closed switch.
50	

ieee power & energy magazine	

may/june 2021



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
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IEEE Power & Energy Magazine - May/June 2021 - Cover3
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