gy using microprocessors have led to many improvements in distribution
protection: lower installation and maintenance costs, better reliability, improved protection and control,
and faster restoration of outages.
Microprocessor-based distribution relays provide technical improvements and cost savings in several
ways. One improvement is the use of programmable logic to reduce and simplify wiring. The relays also
provide protection for bus faults, breaker failure, and high-side transformer blown fuse detection at no or
minimal additional cost. The relays have metering functions that reduce or eliminate the need for panel
meters and transducers and provide remote targeting and fault location information to assist operators in
the restoration of electrical service. Finally, microprocessor-based relays reduce maintenance costs by
providing self-test features and high reliability.
I
NTRODUCTION

Microprocessor-based distribution relays contribute to improved reliability and reduced costs on electric
power systems. Microprocessor-based relays, also called digital relays, have a proven track record of
reliability, with over 100,000 relay-years of field experience. Microprocessor-based relays provide
technical improvements and cost savings in several ways:

The relays use programmable logic to reduce and simplify wiring.

The relays provide protection for bus faults, breaker failure, and high-side transformer blown
fuse detection at no or minimal additional cost.

The relays have metering functions to reduce or eliminate the need for panel meters and
transducers.

The relays reduce maintenance costs by providing self-test functions and high reliability.

The relays provide remote targets and fault location information to assist operators in restoration
of electrical service.
In this paper, we show many examples of how these technical improvements and cost savings are
manifest.
U
SING
M
ICROPROCESSOR
-B
ASED
R
ELAYS
R
EDUCES AND
S
IMPLIFIES
W
IRING

Many microprocessor-based relays have features that, when implemented, reduce and simplify the wiring
and connections of an installation. We show three examples of this:

How to use programmable logic to implement a fuse-saving scheme on a distribution feeder. 2

How to use programmable logic and control inputs to provide fast bus protection to replace a
current differential protection scheme.

How to simplify CT connections for transformer differential protection.
Using Programmable Logic to Implement a Fuse-Saving Scheme
In a typical fuse-saving scheme, we apply time-overcurrent (51) and instantaneous overcurrent (50)
relays with automatic reclosing on a breaker (F) to coordinate with a downstream fuse (F1). For a fault
beyond the fuse F1, the intention is for the instantaneous overcurrent relay to trip the breaker so the fault
clears before the fuse begins to melt. Then, we automatically reclose breaker F. If the fault is temporary,
we avoid a prolonged outage to customers served from the F1 tap. However, all of the customers served
by the feeder will have a momentary outage. If the fault is permanent, we block the instantaneous
overcurrent at F, and allow the fuse to clear the fault. Figures 1a and 1b show a one-line diagram and
time-overcurrent coordination for a fuse-saving scheme.
50/51
F
100T
FUSE
(F1)
R

100
10
1.0
0.1
100
1000
10000
I
t
50 @ Substation
(+Bkr Oper Time)
Minimum Melt Time, F1
(100T)
51 @ Substation

Figure 1a: One-Line of Distribution Feeder
Figure 1b: Time-Overcurrent Curves for
Fuse-Saving
Let us suppose we apply four single-phase overcurrent relays with time (51) and instantaneous (50)
elements. In the dc control circuit connections, we must parallel all of the 51 elements and parallel all of
the 50 elements. Then, we use a contact from a separate reclosing relay (INST BLOCK) to block the
instantaneous elements after the first trip.
Using a microprocessor-based relay, we can program these functions internally. Suppose we want the
phase and ground time-overcurrent elements (51, 51N) to trip directly and the phase and ground
instantaneous (50, 50N) to trip only for the first shot. If we use the Boolean symbols AND (*) and OR
(+), we can program the TRIP conditions as follows:
TRIP = 51 + 51N + (50 + 50N) * (1st shot only)
Therefore, the relay programming allows a trip for phase OR ground time-overcurrent. Also, the relay
programming allows trip for (phase OR ground instantaneous) AND (1st shot only). If a fault occurs on
the feeder, the relay trips the breaker instantaneously on the first shot to save the fuse from operating.
The relay then blocks the instantaneous overcurrents (50, 50N) on subsequent trips to allow the fuse to
trip. Figures 2a and 2b show the control circuit connections using traditional relaying and
microprocessor-based relaying with programmable logic. 3
51
50
51
50
51
50
51
50
Inst Block
52A
Trip Coil
+
_

Trip Output
52A
Trip Coil
+
_

Figure 2a: Trip Circuit Using Conventional
Relaying
Figure 2b: Using Microprocessor-Based
Relay
By using programmable logic, we require only one output contact, significantly reducing wiring and
simplifying the control circuit.
Using Programmable Logic and Control Inputs to Provide Fast Bus Protection
Many utilities are applying microprocessor-based overcurrent relays in place of current differential relays
to provide fast bus protection. In many cases, utilities do not apply bus differential protection because of
the high installation cost of the breaker CTs and the profusion of CT wiring.
F
F
B
F
Control

50
F
50
F
Feeder Breaker
Trip Circuit
I
Feeder
Bus
T1
Control
Pickup
Timer
T1
(50T
B
)
50
B
50
B
Bus Breaker
Trip Circuit

Figure 3a: Fast Bus Trip Scheme
Figure 3b: DC Control Implementation
Figures 3a and 3b show fast bus protection. Instantaneous overcurrent relaying on the feeder breakers
(50F) provides a control input to an instantaneous overcurrent relay with a short definite-time delay on a
low-side transformer breaker or switch (50B, T1). If any of the feeder relays (F) assert, they trip their
respective breaker and block the backup relay (B). However, if the fault is on the bus, none of the feeder
relays operate, and the backup relay trips the bus nearly instantaneously. One utility has successfully
applied this scheme with a 2-cycle time delay (pickup timer T1) for bus faults [1].
Simplifying Transformer Differential CT Connections
On power transformers greater than 10 MVA, most utilities apply transformer differential relays. With
conventional relays, when any transformer winding is connected delta, you must connect the CTs wye,
and vice versa. For example, if a transformer is connected delta-wye, you must connect the CTs wye on
the delta side of the bank, and delta on the wye side of the bank. Figure 4 shows a typical connection. 4

Figure 4: Delta-Wye Transformer With Wye-Delta Connected CTs
Microprocessor-based transformer differential relays can "make" the delta internally. Therefore, you can
connect the CTs in wye on both sides of the bank, regardless of the transformer bank connection. Also,
microprocessor-based relays can provide easy current magnitude and angle checks to ensure proper
connections.

Figure 5: Delta-Wye Transformer With Wye-Wye Connected CTs 5
This provides two advantages:

Wye-delta transformer applications no longer require dedicated CTs. We can use the transformer
differential CTs for other overcurrent protection or metering functions.

Using wye-connected CTs eliminates common wiring errors that often occur when making up a
delta connection.
I
MPROVED
P
ROTECTION AND
C
ONTROL FOR
C
OMMON
D
ISTRIBUTION
P
ROBLEMS

Backup Protection
One common concern when using microprocessor-based relays is backup protection. What if a relay
fails? Do you have all eggs in one basket?
Here is an example of how numerous utilities address this concern. An alarm contact from each of the
feeder relays is connected to permit the backup relay to directly trip the breaker for the alarmed feeder.
At the same time, the settings on the backup relay can be changed to provide additional sensitivity to
permit the backup relay to adequately protect the alarmed feeder, without sacrificing coordination with
other feeder relays.

Figure 6: Improve Backup Relaying
If a feeder relay fails, its alarm contact closes. We connect the alarm contact in series with a trip output
from a backup relay, which, when asserted, produces a feeder breaker trip. If the utility applies a low-
side transformer overcurrent relay, there is no additional cost except for the control wiring of the trip
circuit.
Breaker Failure Protection
Many microprocessor-based distribution relays are equipped with internal timers that, along with a relay
trip condition, can be used to provide breaker failure protection. 6


Figure 7: Breaker Failure Relaying
Change Protection Based on Day/Date/Hour
Utilities may wish to provide fuse-saving or other sensitive protection, but they may also wish to avoid or
reduce nuisance operations during hours when critical customers are in operation. Microprocessor-based
relays allow the utility to change protection settings and logic based on time-of-day and day-of-week.

Figure 8: Change Protection Based on Time-of-Day and Day-of-Week
Change Protection Based on System Conditions
Protection requirements can change with system load and configuration. Conventional protection
schemes must accommodate the worst-case o