Overcurrent and Ground Fault Protection

TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
Overcurrent and Ground Fault Protection
Announcement: You are not supposed to prepare a pre-report. But there will be an oral examination, so you are
strongly advised to study this note regarding to the pre-study questions below. After the lab, you will need to
deliver a post-report which contains what you have done in the lab, data, related graphs and answers of the
questions.
Pre-Study Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Why do we use relays in the power systems?
What are the ANSI/IEEE codes of overcurrent relays?
What type of relay is overcurrent relay?
What is pick-up current? How can it be selected regarding to fault current and load current?
How many types of overcurrent relays are there?
What are the differences between the types of overcurrent relays?
What is the purpose of using time delay in the overcurrent relays?
What are the types of inverse time overcurrent relay?
What is meant by time dial setting?
How the directionality of current flow can be found in a power system?
Which types of overcurrent relays can be used for ground fault protection?
What are the meanings of DMT and IDMT?
1. Objective
To study the principles of overcurrent and ground fault protection.
2. Theory [1, 2]
Relays are used to detect abnormal conditions in the power systems. After detection of a fault, relays
close circuit breakers and disconnect faulty circuits from the general supply system in order to minimize
the damage. There is a list of ANSI/IEEE codes of different types of protection relays as follows [3, 4]:
ANSI /IEEE Standard Device Numbers
2 - Time Delay Starting or Closing Relay
67 - AC Directional Overcurrent Relay
21 - Distance Relay
68 - Blocking or “out of step” Relay
25 - Synchronizing or Synchronism-Check Device
69 - Permissive Control Device
27 - Undervoltage Relay
74 - Alarm Relay
30 - Annunciator Relay
76 - DC Overcurrent Relay
32 - Directional Power Relay
78 - Phase-Angle Measuring Relay
37 - Undercurrent or Underpower Relay
79 - AC-Reclosing Relay
38 - Bearing Protective Device
81 - Frequency Relay
40 –Field (over/under excitation) Relay
85 – Pliot Comm., Carrier or Pilot-Wire Relay
46 – Rev. phase or Phase-Bal. Current Relay
86 - Lockout Relay
47 - Phase-Seq. or Phase-Bal. Voltage Relay
87 - Differential Protective Relay
49 - Machine or Transformer Thermal Relay
94 - Tripping or Trip-Free Relay
50 - Instantaneous Overcurrent
51 - AC Time Overcurrent Relay
B – Bus
59 - Overvoltage Relay
F - Field
60 - Voltage or Current Balance Relay
G – Ground or generator
63 - Pressure Switch
N – Neutral
64 - Ground Detector Relay
T – Transformer
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TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
2.1. Overcurrent Protection
Overcurrent protection is practical application of magnitude relays since it picks up when the magnitude
of current exceeds some value (setting value). Overcurrent relays can be used to protect practically any
power system elements, i.e. transmission lines, transformers, generators, or motors.
As an example, a radial transmission line can be used. For a fault within the zone of protection, the fault
current is smallest at the end of the line and greatest at the relay end. If the minimum fault current
possible within the zone of protection is greater than the maximum possible load current, it would be
possible to define the operating principle as follows:
| |
fault zone, trip
| |
no fault in zone, do not trip.
where is the current in the relay and
is the pickup setting of the relay.
should be selected as:
B
current
A
A
R
(a)
Ifault
Iload max.
location
B
(b)
Figure 1. Overcurrent protection of transmission lines.
(a) Radial system protection. (b) Fault current magnitude as a function of fault location.
There are four types of overcurrent relays; instantaneous, definite time, inverse time and directional
overcurrent relays.
2.1.1. Instantaneous Overcurrent Relays
time
Its operation criterion is only current magnitude (without time delay). This type is applied to the
outgoing feeders.
trip
no
trip
Ip
current
Figure 2. Characteristic of instantaneous
overcurrent relays
Figure 3. Connection diagram of
instantaneous overcurrent relays
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TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
2.1.2. Definite Time Overcurrent Relays
time
In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting
value and the fault must be continuous at least a time equal to time setting of the relay. Modern relays
may contain more than one stage of protection each stage includes each own current and time setting.
T3
T2
T1
I1 I2
I3
current
Figure 4. Characteristic of definite time overcurrent relays
Definite time overcurrent relay is the most applied type of over current. It is used as:
1. Back up protection of distance relay of transmission line with time delay.
2. Back up protection to differential relay of power transformer with time delay.
3. Main protection to outgoing feeders and bus couplers with adjustable time delay setting.
Figure 5. Connection diagram of definite time overcurrent relay with internal timer
2.1.3. Inverse Time Overcurrent Relays
In this type of relays, operating time is inversely changed with current. So, high current will operate
overcurrent relay faster than lower ones. There are standard inverse, very inverse and extremely inverse
types.
time
inverse
very inverse
extremely inverse
current
Figure 6. Characteristic of inverse time overcurrent relays
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TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
The operating time of an overcurrent relay can be moved up (made slower) by adjusting the ‘time dial
setting’. The lowest time dial setting (fastest operating time) is generally 1/2, and the slowest is 10.
10
8
6
4
3
1
Time dial setting
Operating time in seconds
10
2
1
0.1
1/2
10
Multiples of pickup setting (If/Ip)
40
Figure 7. A typical commercial time overcurrent relay characteristic
2.1.4. Directional Overcurrent Relays
When the power system is not radial (source on one side of the line), an overcurrent relay may not be
able to provide adequate protection. This type of relay operates in on direction of current flow and
blocks in the opposite direction. Three conditions must be satisfied for its operation: current magnitude,
time delay and directionality. The directionality of current flow can be identified using voltage as a
reference of direction.
Figure 8. Connection diagram of directional overcurrent relay
2.2. Ground Fault Protection
Ground fault currents are dependent upon system grounding and they produce zero sequence currents
whereas there is very little zero-sequence current during normal operation. Thus the pick-up settings of
the ground fault relays can be made more sensitive than those of phase fault relays. A separate relay
responding to the zero sequence current
is provided for ground fault protection. Types
of ground fault relays are similar to overcurrent relays but with only one coil for current in the case of
instantaneous, definite time or inverse time ground fault types. One voltage coil is added in the case of
directional ground fault relays.
4
TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
There are different connection types of current circuits:



Current coil may be connected to return path (neutral) of a current transformer
Current coil may be connected to the secondary side of current transformer which is installed at
the star point of power transformer
Current coil may be connected to secondary side of ring type current transformer installed at
power cables
Figure 9. Ground fault relay connected at
the neutral point of current transformer
Figure 11. Ground fault relay connected to
ring type current transformer
Figure 10. Ground fault relay connected
star point of power transformer
Figure 12. Connection diagram of
directional ground fault relay
3. Practical Information
In this lab, the overcurrent functions (50, 51) of SIPROTEC Compact 7SJ80 Multifunction Protection Relay
will be used. Order number is as follows: 7SJ8031-1EB90-1FC1
Order number includes many information about the relay, such as; housing, binary inputs and outputs,
measuring inputs, language settings and its functions. It can be seen in the catalog of the relay[5].
In the relay, three definite time over current (DMT) protection elements are available both for the phase
and ground elements. The current threshold and delay set time can be set. Inverse time overcurrent
protection characteristics (IDMT) can also be selected and activated.
5
TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
4. Procedure
You don’t need to set up any connection. Only study the connection scheme. In the lab, you will
measure several current values and related time delays in the context of overcurrent protection.
5. Connection Scheme
SIPROTEC Compact 7SJ80
IL1
-S2
190V AC (L-L)
LA
F1
P1 +
N
A
IL3
IL2
F2
F3
F4
F5
IE
F6
F7
F8
P1 -
Circuit Breaker
K1
A1
B01
C9-NO
A2
NC
C10-NC C11-COM
31
32
Circuit Breaker
CB off
KE
-S4
24V DC
2L+
2L-
BI1
KE
C3
BI2
C4
C5
NC
C6
51
52
PS
C1(+)
C2(-)
Figure 13. Connection scheme of the test set-up
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TA Kahraman Yumak
web.itu.edu.tr/yumakk
ELK412 - Distribution of Electrical Energy Lab. Notes v1.0
2012 Spring
6. References
1.
2.
3.
4.
5.
6.
Ahmed Safie Eldin, Practical Introduction to Power System Protection and Control, 2005
Arun G. Phadke, James S. Thorp, Computer Relaying for Power Systems, John Wiley & Sons, 1988
IEEE Standard C37.2-2008 : IEEE Standard for Electrical Power System Device Function Numbers,
Acronyms, and Contact Designations
http://www.ee.uidaho.edu/ee/power/EE525/Lectures/L9/RelayDeviceNumbers.pdf
Catalog of SIPROTEC Compact 7SJ80 Multifunction Protection Relay
IEC 60255-151:2009 : Functional requirements for over/under protection
7. Questions for the Post-Report
1. What do you understand by
Primary relay?
Secondary relay?
Auxiliary relay?
2. What is meant by
Pick-up current?
Drop out current?
Dropout ratio?
3. What are the equations of IEC normal inverse, very inverse and extremely inverse overcurrent
relays?
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