Terminology in relay protection
It’s not unusual to see graduates and engineers from other disciplines experience difficulties in properly interpreting the terminology used in applying relays, analyzing their performance, and designing protection systems. Actually, this is normal, but during the project, this lack makes it difficult for relay engineers to communicate effectively with their colleagues and convey their interpretations of relaying issues and questions effectively.
This technical article is dedicated to graduates and engineers coming from other disciplines as well to experienced power system and protection engineers. It will shed some light on terms concerning the quality of measurements, philosophy of protection and circuit breakers often used by protection engineers.
- Accuracy class,
- Relay stability,
- Primary protection,
- Backup protection,
- Dual protection,
- Device number,
- Breaker failure,
- Phase disagreement,
- Pole flashover, and
- Single-phase tripping
This term is used for at least two different purposes, one to describe the accuracy of a device and the other to specify the accuracy of a measurement. In the first context, accuracy is the degree to which a device (relay, instrument or meter) conforms to an accepted standard.
In the second case, the accuracy of a measurement specifies the difference between the measured and true values of a quantity. The deviation from the true value is the indication of how accurately a reading has been taken or a setting has been made.
If a relay is specified to have ±5% accuracy, it means that the relay should operate when its exciting quantity (current or voltage) is between -5% and +5% of its setting. Let us consider the case of Figure 1 and assume that the CT provides secondary current which is an accurate representation of the primary current.
When the fault current is 12,000 A, the current in the relay will be 100 A. If the relay accuracy is ±5%, it could interpret the current to be of any level from 95 A to 105 A. In case the relay is set to operate at 100 A, it mayor may not operate depending on its interpretation of the level of current in the circuit.
This term is used to define the quality of the steady state performance of a current transformer. The accuracy class of a current transformer (CT) used for protection functions is described by a letter which indicates whether the accuracy can be calculated (class C) or it must be obtained from physical tests (class T).
Examples of accuracy classes for 10% error class C CTs are C1OO, C200, C400 and C800. At this time, there is no accuracy class higher than C800. Examples of accuracy classes for 10% error class T CTs are T105, 1250, T375 and T750.
|IEEE C57.13||IEC 60044-1|
|C100||25 VA 5P 20|
|C200||50 VA 5P 20|
|C400||100 VA 5P 20|
|C800||200 VA 5P 20|
The IEC accuracy designation gives the burden VA at rated input, the accuracy rating (5P), and the limit of 20 times rating.
Reliability is an index that expresses the attribute of a protective relay or a system to operate correctly for situations in which it is designed to operate. This also includes the attribute of not operating (incorrectly) for all other situations.
Reliability is expressed in terms of two competing fundamental attributes, dependability and security.
Dependability is the aspect of reliability that expresses the degree of certainty that a relay will operate correctly. For relay systems, dependability is assured by using redundant protection systems and backup relays.
The primary protection for a transmission line may be provided by using a phase comparison protection scheme. The degree of certainty that this scheme will operate for all faults on the transmission line is the dependability index of the scheme.
To increase this index for the transmission line protection system, distance relays can be included to act as backup relays.
Security is the aspect of reliability that expresses the degree of certainty that a relay will not operate incorrectly irrespective of the nature of the operating state of the power system. Pretty simple.
If a differential relay is designed to operate for faults in a transformer it is protecting, the degree of certainty that the relay will not operate for faults outside the transformer zone is the security index of the relay.
This term is used to express different attributes of devices. One definition expresses it as a ratio of the response of the device to the change of the input. In the power system protection field, sensitivity is the minimum value of an input (or change of an input) that would cause a relay to operate.
An instantaneous ground fault directional relay designed to operate at a minimum current of 0.5 A would be classified as having a sensitivity of 0.5 A.
A relay is considered to be stable if: starting from a steady state, it returns to the same steady state following the introduction and removal of inputs representing a disturbance in the system to which it is connected.
A solid-state timing relay, whose timing accuracy is not affected by the changes in the DC voltage supply used to operate it, is considered to be stable.
Consider that a relay system experiences a momentary loss of de supply used for performing logic and/or tripping functions. If the relay system returns to a normal steady state mode on restoration of the DC supply, the relay is considered to be stable.
A real-life case study of relay coordination (step by step tutorial with analysis)
Take a deep analysis of how four main characteristics of a good protection system: selectivity, stability, speed and sensitivity are implemented. More information here.
The protection system that is designed to operate before other devices respond to a disturbance due to its sensitivity and speed, is said to provide primary protection.
A differential relay protecting a transformer is expected to operate when a fault is experienced in its protection zone. Other devices used to protect the transformer, such as overcurrent relays, are expected to operate if the differential relay fails to detect the fault.
In this case, the differential relay provides primary protection for faults in its zone of protection.
Relays used to provide second line of defense are said to provide backup protection. The operating time of these relays is longer than the operating time of primary relays, and, therefore, they operate but trip appropriate circuit breakers only if the primary relays fail to detect the presence of the disturbance or fail to open the circuit breakers.
These relays could be physically in the substation in which the primary relays are located or may be located in a remote substation.
A phase comparison system can be used to provide primary protection of a transmission line. Distance relays may be used, without permissive overreach or transfer trip, to provide backup protection of the line.
Power system equipment of bulk transmission systems is often protected with dual primary relays. Both primary protection systems are kept independent of each other as far as possible. Depending on the protection philosophy adopted, each protection system may be connected to its own CTs, VTs, relays, trip coils of circuit breakers and batteries.
These systems are sometimes referred to as “Protection system A” and “Protection system B“.
A transmission line may be protected by a differential protection system, which is expected to operate in 10 to 15 ms, and a distance protection system with transfer trip, which is also expected to operate in comparable time. The differential protection in this case could be classified as “Protection system A” or “Protection system 1” and the distance protection system could be classified as “Protection system B” or “Protection system 2“.
The circuit diagrams used in power systems use nomenclatures and device numbers as specified in the ANSI/IEEE Standard C37.2. A device number is assigned for each type of relay and instrument. The phases are identified as A, B, C or a, b, c. The numerals 1, 2 and 3 are not used because I is used to identify positive sequence quantities and 2 is used to identify negative sequence quantities.
To see the complete list of ANSI/IEEE device numbers, read this technical article.
Some of the device numbers specified in the Standard are listed in the following table.
|Instantaneous overcurrent relay||50|
|AC time overcurrent relay||51|
|AC directional overcurrent relay||67|
The failure of a circuit breaker to interrupt fault current following the attempt to energize its trip coil by a protective relay is described as breaker failure. The reason for such failures include:
- Inadequate or damaged interrupter,
- Mechanically damaged mechanism, and
- Lack of electrical continuity of the trip circuit.
The CB may fail to trip due to various reasons, such as trip coil failure, interrupting component failure, dielectric gas pressure low, etc. Faults must be cleared under CB failure conditions. In doing so all the adjacent CBs shall be tripped, which can be accomplished by the backup protection or by installing dedicated CB failure protection (BFP) for each CB.
The following three examples show the circuit breakers that are tripped by a breaker failure relay.
Circuit breaker A of a single bus switching station, shown in Figure 7, has failed to interrupt current flowing to a fault on the line it controls. The condition is identified by the breaker failure relay which issues commands to trip circuit breakers B, C and D. The relay also issues a trip command to trip circuit breaker A.
Circuit breaker A of the switching station, shown in Figure 8, has failed to interrupt current flowing to a fault on line to circuit breaker J at the remote station. Circuit breakers B and J have successfully interrupted the flow of current through them. On detecting circuit breaker failure, the breaker failure relays issues trip commands to circuit breakers D and G, as well as A and B.
If communication facilities are available, the trip command is also sent to circuit breaker J.
Circuit breaker A of the switching station, shown in Figure 9, has failed to interrupt current flowing to a fault on the line to circuit breaker H at the remote station Y. Circuit breakers D and H have successfully interrupted the flow of current through them.
On detecting circuit breaker failure, the breaker failure relay issues trip commands to circuit breakers B and J, as well as A, D and H.
The three examples, one for a single bus switching station, one for a breaker-and-a-half switching arrangement and the third for a ring bus switching station show the local, as well as, remote circuit breakers that could supply fault current through the failed circuit breaker.
The breaker failure relay issues trip commands to these circuit breakers as well as the circuit breakers that have successfully interrupted the flow of fault current.
This is the condition in which one pole of a three-phase circuit breaker is open while the remaining poles are closed. It also includes the condition in which two poles of a three-phase circuit breaker are open while the remaining pole is closed. Such conditions cause negative-sequence currents to flow in the equipment controlled by the circuit breaker.
This condition is sometimes called “pole disagreement“.
Figure 10 shows the contact arrangement of a three-phase breaker which has two breaks per pole. The contacts of interrupters “A” and “B” are closed whereas the interrupters “C“, “D“, “E” and “F” are open.
The condition in which:
- the contacts of an interrupter, or interrupters in one phase are open, and
- the contacts of interrupters in the other two phases are closed
is identified as pole disagreement. If the contacts of interrupter A, or A and B, are open and the contacts of interrupters C, D, E and F are closed, pole disagreement has occurred.
Figure XX Contact arrangement of a three phase circuit breaker which has two interrupters per phase. The contacts of interrupters “A” and “B” are closed whereas the interrupters “C“, “D“, “E” and “F” are open.
A flashover across an open or partially open pole of a three-phase circuit beaker can occur due to lightning, switching surges or loss of dielectric in a pressurized interrupter. This phenomenon is called pole flashover.
Flashover can occur on circuit breakers which have one operating mechanism for all three poles and also on circuit breakers which have independent operating mechanisms for each pole.
When a single-phase fault is experienced on a system, fault current flows in one phase only. In many situations, only one pole of the circuit breaker controlling a line is opened during these faults. Most power system protection engineers call this practice “single-pole tripping” but sometimes it is called “single phase tripping“.
Faults other than single phase to ground faults are usually isolated by tripping all three poles.
- Terms Used by Power System Protection Engineers by IEEE Power System Relaying Committee
- Current Transformer Accuracy Ratings by SEL
- Review of The Breaker Failure Protection Practices in Utilities by Yiyan Xue and Manish Thakhar (American Electric Power Company), Jacob C. Theron (Hydro One Networks Inc.) and Davis P. Erwin (Pacific Gas and Electric Company)