POWER SYSTEM PROTECTION

steamgloomyElectronics - Devices

Nov 15, 2013 (3 years and 7 months ago)

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POWER SYSTEM PROTECTION


1.
Introduction

o
The
purpose of power system protection is to
continuously monitor the system to ensure maximum
continuity of electrical supply with minimum damage to
equipment
and property
.


o

Main idea is to remove faults as quickly as possible while
leaving as much of the system intact as possible




1.
Introduction


Fault sequence of events

1.
Fault occurs somewhere on the system, changing the system
currents and voltages


2.
Current transformers (CTs) and potential transformers (PTs)
sensors detect the change in
currents/voltages


3.
Relays use sensor input to determine whether a fault has
occurred


4.
If fault occurs relays open circuit breakers to isolate fault




1.
Introduction


Protection systems must be designed with both primary
protection and backup protection in case primary
protection devices fail


In designing power system protection systems there
are two main types of systems that need to be
considered:

1.
Radial: there is a single source of power, so power
always flows in a single direction; this is the easiest
from a protection point of view

2.
Network: power can flow in either direction: protection
is much more involved



1. Radial Power System Protection

Radial systems are primarily used in the lower voltage
distribution systems. Protection actions usually result in
loss of customer load, but the outages are usually quite
local.

The
figure
shows



potential protection

schemes for a

radial system. The

bottom scheme is

preferred since it

results in less lost load




1. Radial Power System Protection


In radial power systems the amount of fault current is
limited by the fault distance from the power source: faults
further done the feeder have less fault current since the
current is limited by feeder impedance


Radial power system protection systems usually use
inverse
-
time overcurrent relays.



Coordination of relay current settings is needed to

open the correct breakers




1. Inverse Time Overcurrent Relays


Inverse time overcurrent relays respond
instan
-
taneously

to a current above their maximum setting


They respond slower to currents below this value but
above the pickup current value




1. Inverse Time Overcurrent Relays (Cont’d)


The inverse time characteristic provides backup
protection since relays further upstream (closer to power
source) should eventually trip if relays closer to the fault
fail


Challenge is to make sure the minimum pickup current is
set low enough to pick up all likely faults, but high enough
not to trip on load current


When
outaged

feeders are returned to service there can
be a large in
-
rush current as all the motors try to
simultaneously start; this in
-
rush current may re
-
trip the
feeder




1. Inverse Time Overcurrent Relays (Cont’d)




Current and time

settings are ad
-

justed

using dials

on the relay

Relays have

traditionally been

electromechanical

devices, but are

gradually being

replaced by

digital relays

Classification of Power system Relays

1.
E
lectromechanical (moving parts)

2.
S
tatic
(no moving parts and data processing is analog)

3.
N
umerical
(no moving parts and data processing is
digital).


2. Zones
of
Protection

S
imple
P
ower
S
ystem

CT
connections

Figure
2:
CT connections overlapping

around
a circuit breaker.

Figure
1:
Zones of protection for a simple
power system.

3. Primary
and Backup Protection


Illustration
of back
-
up protection of transmission line section EF.

4. Types
of Protective Relays

1.
Current Relays

2.
Voltage Relays

3.
Power Relays

4.
Frequency Relays

5.
Directional Relays

6.
Distance Relays

7.
Differential Relays

5. Current
Transformers (CTs)

Figure
4:
Schematic Representation of a CT.

Current ratio

Current ratio

Current ratio

50:5

300:5

800:5

100:5

400:5

900:5

150:5

450:5

1000:5

200:5

500:5

1200:5

250:5

600:5



Table I: Standard CT ratios

6.
Capacitor
-
Coupled Voltage Transformer
(CCVT)

Figure
5:
Circuit diagram of a
capacitor
-
coupled voltage
transformer (CCVT).

Thevenin

Impedance

7.1. Magnitude
Relays

Figure
6:
Operating & blocking regions of a time over
-
current relay. Time
T2 is
greater than
T1.

7.1. Magnitude Relays

Figure
7:
Characteristic curves of type IFC
-
53 (General Electric) time
overcurrent relays

7.2. Directional
Relays

Figure 8: Operating principle of a directional relay.

7.3. Ratio
Relays

Figure 9: Impedance relay characteristics (a) Zone of protection for R12 (b)
Nondirectional

impedance relay (c) Mho relay. In (b) & (c) the Z is indicated for
a fault to the left of R12

7.4. Differential
Relays

Figure 10: Differential protection for a generator phase winding.

7.5. Protection of subtransmission lines and
radial distribution networks

Figure 11: Protection of a radial system. Line and transformer
reactance values are marked in ohms

Fault Current
C
alculations

1.
Example 1


A portion of a 13.8 kV radial system is shown in Figure 11.
The system may be operated with one rather than two
source transformers under certain operating conditions.
Assume high voltage bus of transformer is an infinite bus.
Protection system for line
-
to
-
line and three
-
phase faults
has to be designed. Transmission line
reactances

in ohms
referred to the 13.8 kV side are shown in the figure.
Neglect resistance and first calculate the minimum and
maximum fault currents at bus 5.


Fault Current Calculations

Solution Hints


1.
Maximum fault current will occur for a three
-
phase with
both transformers in service
.


2.
Minimum fault in this case is assumed for a line
-
to
-
line
fault.
A line
-
to
-
line fault produces a fault current equal to
times the three
-
phase fault. Also the minimum fault
current happens for line
-
to
-
line faults with both
transformers in service.


Fault Current Calculations

Solution
Hints


Fault at bus

1

2

3

4

5

Max fault

current, A

203

Min fault

current, A

1380

473

329

238

165

Table II: Maximum and Minimum Fault Currents


Designing an Overcurrent Relay

Design Example 2

Consider again the portion of the 13.8 kV radial
system.


For the above system, an over
-
current protection system has to be
designed for line
-
to
-
line and three
-
phase faults (CT ratios, relay tap
(pickup) settings, and the relay time dial settings).


The standard CT ratios
would be used.
Use at all locations the IFC
-
53
relay whose characteristic curves are given in
the notes
and whose
mechanical tap settings are as follows: 1.0, 1.2, 1.5, 2.0, 3.0, 4.0, 5.0,
6.0, 7.0, 8.0, 12.0 A. The relays at each bus 1, 2, 3, and 4 are
designated by R1, R2, R3, and R4, respectively. The breaker at each
bus will open all three phases when tripped by any of the three
associated relays.


Determine the relay settings (CT ratios, relay tap (pickup) settings, and
the relay time dial settings) for R1
-
R4.




Design Principles

1.
Start
you solution first with the calculations for R4. R4
must operate for all currents above 165. A, but for
reliability purpose choose a relay which will operate with
a current in the line which is 30% of the
minimum.

2.
Backup
protection: Over
-
current relay for X, backing up
the next downstream relay Y, is that X must pick
up

1.
For
30% of the minimum current seen by Y
and

2.
For
the maximum current seen by Y but no sooner than 0.3 s
after Y should have picked up for that current.






7.6. Protection
of Loop Systems

Figure 12: One
-
line diagram of a loop system. Arrows beside each circuit
breaker show the direction to the fault for which relay will respond. Relays
having all arrows pointing in the same direction around the loop coordinate
with each other.

7.7. Protection
of HV and EHV Transmission Lines

Figure 13: Distance (or impedance) relays. (a) Zone of protection of protection
shown by solid line is replaced by zones 1 and 2 identified by dashed lines.
Zone 3 provides backup protection for neighboring protection systems. (b)
Time delay and operating time for R12, R23 and R24

7.7. Protection of HV and EHV Transmission Lines

Figure 14: Characteristics of (a) Directional impedance relay (b) Mho relay
for the power system configuration shown in figure 13.