2023年7月17日发(作者：)

TA Kahraman Yumak

/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. Why do we use relays in the power systems?

2. What are the ANSI/IEEE codes of overcurrent relays?

3. What type of relay is overcurrent relay?

4. What is pick-up current? How can it be selected regarding to fault current and load current?

5. How many types of overcurrent relays are there?

6. What are the differences between the types of overcurrent relays?

7. What is the purpose of using time delay in the overcurrent relays?

8. What are the types of inverse time overcurrent relay?

9. What is meant by time dial setting?

10. How the directionality of current flow can be found in a power system?

11. Which types of overcurrent relays can be used for ground fault protection?

12. 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 Numbers2 - Time Delay Starting or Closing Relay21 - Distance Relay25 - Synchronizing or Synchronism-Check Device27 - Undervoltage Relay30 - Annunciator Relay32 - Directional Power Relay37 - Undercurrent or Underpower Relay38 - Bearing Protective Device40 –Field (over/under excitation) Relay46 – Rev. phase or Phase-Bal. Current Relay47 - Phase-Seq. or Phase-Bal. Voltage Relay49 - Machine or Transformer Thermal Relay50 - Instantaneous Overcurrent51 - AC Time Overcurrent Relay59 - Overvoltage Relay60 - Voltage or Current Balance Relay63 - Pressure Switch64 - Ground Detector RelayB – BusF - FieldG – Ground or generatorN – NeutralT – Transformer67 - AC Directional Overcurrent Relay68 - Blocking or “out of step” Relay69 - Permissive Control Device74 - Alarm Relay76 - DC Overcurrent Relay78 - Phase-Angle Measuring Relay79 - AC-Reclosing Relay81 - Frequency Relay85 – Pliot Comm., Carrier or Pilot-Wire Relay86 - Lockout Relay87 - Differential Protective Relay94 - Tripping or Trip-Free Relay

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TA Kahraman Yumak

/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:

A

R(a)Bcurrent

IfaultIload on(b)AB

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

Its operation criterion is only current magnitude (without time delay). This type is applied to the

outgoing feeders.

timeno

triptripIpcurrent

Figure 3. Connection diagram of

instantaneous overcurrent relays

Figure 2. Characteristic of instantaneous

overcurrent relays

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TA Kahraman Yumak

/yumakk

ELK412 - Distribution of Electrical Energy Lab. Notes v1.0 2012 Spring

2.1.2. Definite Time Overcurrent Relays

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.

timeT3T2T1I1I2I3current

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.

inversevery inverseextremely inversetimecurrent

Figure 6. Characteristic of inverse time overcurrent relays

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TA Kahraman Yumak

/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.

10Operating

time

in

seconds1108643211/210Multiples of pickup setting (If/Ip)400.1Time

dial

settingFigure 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.

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TA Kahraman Yumak

/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

/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-S2190V AC (L-L)LANIL1F1P1 +F2F3IL2IL3F5F6F7F4IEF8Circuit BreakerAP1

-A1K1A2B01C9-NOC10-NCC11-COM31NC32Circuit BreakerCB offKEKEBI1C3C4BI2C5C651NC52-S424V DC2L+2L-PSC1(+)C2(-)

Figure 13. Connection scheme of the test set-up

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TA Kahraman Yumak

/yumakk

ELK412 - Distribution of Electrical Energy Lab. Notes v1.0 2012 Spring

6. References

1. Ahmed Safie Eldin, Practical Introduction to Power System Protection and Control, 2005

2. Arun G. Phadke, James S. Thorp, Computer Relaying for Power Systems, John Wiley & Sons, 1988

3. IEEE Standard C37.2-2008 : IEEE Standard for Electrical Power System Device Function Numbers,

Acronyms, and Contact Designations

4. /ee/power/EE525/Lectures/L9/

5. Catalog of SIPROTEC Compact 7SJ80 Multifunction Protection Relay

6. 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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