MYSTIQUES IN POWER SYSTEM PROTECTION
By Doods A. Amora, PEE
[PART 2 OF A SERIES OF 3]
[PART 2 OF A SERIES OF 3]
OVERVIEW OF FAULTS
Before attempting to set-up any system protection scheme, faults as complex phenomena must first be understood & their magnitudes calculated.
As the name implies, a ‘fault current’ is one which flows outside the normal conducting paths. Fault modes refer to: Three-Phase Faults, Single Phase (Line-to-Line) Faults, Double Line-to-Ground Faults and Single Line-to-Ground Faults. And these currents always come in large magnitude!
Contributing sources of fault currents into a system in focus include: Grid Generation, Local Generation, In-Plant Synchronous Motors and Induction Motors. Therefore, Fault Calculation is needed to establish new levels of fault duties brought about by any changes in the grid or in the industrial plant system itself.
Three-Phase Bolted Short Circuits: This describes the condition where the three conductors are physically held as if they were bolted together. In this condition, the impedance between these conductors or terminals is zero and the short circuit current flowing into the fault point at the time is influenced by the sub-transient impedance of the system at the inception (1/2 cycle) of the fault. This establishes a “worst case” condition, which results in maximum thermal and mechanical stress in the system. While ‘bolted short circuit condition’ seldom occurs, it generally results in maximum short-circuit values and for this reason that the “basic short circuit calculation” in power systems is employed.
Subsequently, it is from these ½ cycle maximum values that the selection of fault duty ratings of circuit breakers, power fuses, other protective devices and switchgear withstand ratings shall be based, busway bracings included.
Line-to-Line Bolted Short Circuit: From the three-phase fault calculation, other types of fault conditions can be obtained. The levels of line-to-line (single phase) bolted short circuit currents in most three-phase systems are approximately 87% of three-phase bolted short circuit currents, but this calculation is seldom required because it is not the maximum value, especially for establishing circuit breaker ratings. But then these values are needed as bases in relay settings or other purposes.
Line-to-Ground Bolted Fault Circuits: In solidly grounded systems, single line-to-ground bolted short circuit current in general terms, can be almost equal to the three-phase bolted short circuit current. Most of the time, actual SLG fault currents are lower than the 3Φ short circuit current due to the impedance of the ground return circuit and due to the non-zero-sequence current contribution from the motors which are usually ungrounded.
In resistance-grounded medium voltage systems common in generators, the Neutral Grounding Resistor (NGR) is generally selected to limit ground fault current to a value ranging between say, a few tens or hundreds of amperes allowed to pierce into the neutral of the generator. Magnitudes of Line-to-Ground fault currents on these systems are limited primarily by the grounding resistor itself and a line-to-ground fault calculation using symmetrical components is generally required to size up the resistor.
Arcing Faulted Circuits: In actuality, faults in many power systems tend to be arcing in nature. Statistics say that Single-Line-to-Ground Arcing Faults are the most frequent faults experienced in any power system.
Arcing faults are much lower level short circuit currents than the bolted ones at the same fault point. These lower levels of currents are due to the impedance of the arc ‘inserted’ into the circuit and the impedance of the ground return path. Normally, arcing fault currents fall in the range from 40% to 50% of the bolted values or could be much lower especially in limestone earthing environments.
In the real world, statistics showed the frequency of occurrence of these faults are as follows:
Single Line-to-Ground Faults: 70% – 80%
Double Line-to-Ground Faults: 10% - 17%
Phase-to-Phase Faults: 8% - 10%
Three-Phase Faults: 2% - 3%
FAULT DUTIES
For new installations, Fault Calculations must precede any effort to procure system protection devices purposively to arrive at the appropriate ratings & capabilities that fits various system conditions to include future considerations.
For existing plants where Power System Study is conducted, this activity is likewise imperative to establish that the protective devices such as circuit breakers in switchgears are still within operating limits. Updating the awareness in system fault duties is always true to situations where the Grid has changed significantly where fault duties have also changed.
While overloads do occur at somewhat modest levels, the ‘short-circuit’ or ‘fault current’ can be hundred times (or more) larger than the normal operating current. A high level fault in the medium voltage systems may be 40,000 amperes (or even larger). If not interrupted within a matter of a few thousandths of a second (depending on the magnitude of fault current), damage & destruction can become very serious. There can be severe insulation damage, melting of conductors, vaporization of metals, ionization of gases, explosion, arcing & eventually fires. Moreover, high level short-circuit currents can develop huge magnetic-field stresses between switchgear buses that can reach destructive forces beyond their short-time ratings that even heavy bracings may not be able to keep them from being distorted beyond repair.
Protective devices such as circuit breakers in switchgears must be rated to withstand the destructive energies of fault currents. If a fault current exceeds a level beyond the capability of the protective device, the device may rupture and disintegrate in its attempt to interrupt a fault. This is the first step.
It is therefore important to de-mystify the stigma of faults and its counter-measures. Again, the system designer is responsible for the selection of the right equipment; and would generally have the task of calculating system short circuits.
PROTECTIVE RELAYS
Today, Fault Control by protective relays is just one part of a protective umbrella covering such conditions as equipment & component deterioration, natural hazards, reliability requirements and similar considerations. As one article on the web puts it: "But if system protection is the “heart” then, electronic devices now being integrated into electrical systems have become the “nerves” of today’s systems. In just one generation, the introduction of new, “smarter” devices has significantly changed equipment and design practices."
Before attempting to set-up any system protection scheme, faults as complex phenomena must first be understood & their magnitudes calculated.
As the name implies, a ‘fault current’ is one which flows outside the normal conducting paths. Fault modes refer to: Three-Phase Faults, Single Phase (Line-to-Line) Faults, Double Line-to-Ground Faults and Single Line-to-Ground Faults. And these currents always come in large magnitude!
Contributing sources of fault currents into a system in focus include: Grid Generation, Local Generation, In-Plant Synchronous Motors and Induction Motors. Therefore, Fault Calculation is needed to establish new levels of fault duties brought about by any changes in the grid or in the industrial plant system itself.
Three-Phase Bolted Short Circuits: This describes the condition where the three conductors are physically held as if they were bolted together. In this condition, the impedance between these conductors or terminals is zero and the short circuit current flowing into the fault point at the time is influenced by the sub-transient impedance of the system at the inception (1/2 cycle) of the fault. This establishes a “worst case” condition, which results in maximum thermal and mechanical stress in the system. While ‘bolted short circuit condition’ seldom occurs, it generally results in maximum short-circuit values and for this reason that the “basic short circuit calculation” in power systems is employed.
Subsequently, it is from these ½ cycle maximum values that the selection of fault duty ratings of circuit breakers, power fuses, other protective devices and switchgear withstand ratings shall be based, busway bracings included.
Line-to-Line Bolted Short Circuit: From the three-phase fault calculation, other types of fault conditions can be obtained. The levels of line-to-line (single phase) bolted short circuit currents in most three-phase systems are approximately 87% of three-phase bolted short circuit currents, but this calculation is seldom required because it is not the maximum value, especially for establishing circuit breaker ratings. But then these values are needed as bases in relay settings or other purposes.
Line-to-Ground Bolted Fault Circuits: In solidly grounded systems, single line-to-ground bolted short circuit current in general terms, can be almost equal to the three-phase bolted short circuit current. Most of the time, actual SLG fault currents are lower than the 3Φ short circuit current due to the impedance of the ground return circuit and due to the non-zero-sequence current contribution from the motors which are usually ungrounded.
In resistance-grounded medium voltage systems common in generators, the Neutral Grounding Resistor (NGR) is generally selected to limit ground fault current to a value ranging between say, a few tens or hundreds of amperes allowed to pierce into the neutral of the generator. Magnitudes of Line-to-Ground fault currents on these systems are limited primarily by the grounding resistor itself and a line-to-ground fault calculation using symmetrical components is generally required to size up the resistor.
Arcing Faulted Circuits: In actuality, faults in many power systems tend to be arcing in nature. Statistics say that Single-Line-to-Ground Arcing Faults are the most frequent faults experienced in any power system.
Arcing faults are much lower level short circuit currents than the bolted ones at the same fault point. These lower levels of currents are due to the impedance of the arc ‘inserted’ into the circuit and the impedance of the ground return path. Normally, arcing fault currents fall in the range from 40% to 50% of the bolted values or could be much lower especially in limestone earthing environments.
In the real world, statistics showed the frequency of occurrence of these faults are as follows:
Single Line-to-Ground Faults: 70% – 80%
Double Line-to-Ground Faults: 10% - 17%
Phase-to-Phase Faults: 8% - 10%
Three-Phase Faults: 2% - 3%
FAULT DUTIES
For new installations, Fault Calculations must precede any effort to procure system protection devices purposively to arrive at the appropriate ratings & capabilities that fits various system conditions to include future considerations.
For existing plants where Power System Study is conducted, this activity is likewise imperative to establish that the protective devices such as circuit breakers in switchgears are still within operating limits. Updating the awareness in system fault duties is always true to situations where the Grid has changed significantly where fault duties have also changed.
While overloads do occur at somewhat modest levels, the ‘short-circuit’ or ‘fault current’ can be hundred times (or more) larger than the normal operating current. A high level fault in the medium voltage systems may be 40,000 amperes (or even larger). If not interrupted within a matter of a few thousandths of a second (depending on the magnitude of fault current), damage & destruction can become very serious. There can be severe insulation damage, melting of conductors, vaporization of metals, ionization of gases, explosion, arcing & eventually fires. Moreover, high level short-circuit currents can develop huge magnetic-field stresses between switchgear buses that can reach destructive forces beyond their short-time ratings that even heavy bracings may not be able to keep them from being distorted beyond repair.
Protective devices such as circuit breakers in switchgears must be rated to withstand the destructive energies of fault currents. If a fault current exceeds a level beyond the capability of the protective device, the device may rupture and disintegrate in its attempt to interrupt a fault. This is the first step.
It is therefore important to de-mystify the stigma of faults and its counter-measures. Again, the system designer is responsible for the selection of the right equipment; and would generally have the task of calculating system short circuits.
PROTECTIVE RELAYS
Today, Fault Control by protective relays is just one part of a protective umbrella covering such conditions as equipment & component deterioration, natural hazards, reliability requirements and similar considerations. As one article on the web puts it: "But if system protection is the “heart” then, electronic devices now being integrated into electrical systems have become the “nerves” of today’s systems. In just one generation, the introduction of new, “smarter” devices has significantly changed equipment and design practices."
Protective relays have been described as the watchdogs or silent sentinels in the power system. They come in anticipation for faults. Again, to minimize the effects of faults on the system, these devices should be selective in operation so that the one nearest the fault downstream will operate first and, if any device should fail to function, the next closest device on the upstream side should open the circuit. This is what is globally referred to as “failing well”.
It may be convenient to think of the circuit breaker as the muscle providing brute force that does the work of isolating the component, while the relay is the brain which decides that isolation is required and the command for the circuit breaker to trip.
Most switchgear-type relays are enclosed in a semi-flush-mounting draw-out case. Relays usually are installed on the door of the switchgear cubicle. Again, protective relays are arguably the least understood component of medium voltage circuit protection. And, coordination doesn't need to be complicated too, that is, if we know some basic relay and sensor information. Let's then try to unravel the mystery.
Protective Relays come on the form of electro-mechanical, solid state and more recently the digital relays.
Electro-mechanical Relays: These relays are now extinct and therefore need no further discussion as far as this article is concerned.
Solid State Relays: These relays can perform all the functions that can be performed by electro-mechanical relays and, since the precision of solid state electronic relays is greater than that of electro-mechanical relays, they allow closer system coordination. In addition, because there is no mechanical motion and the electronic circuitry is very stable, they retain their target accuracy for a long time. Incidentally however, Solid State Relays had a short-lived popularity. Sooner than expected, these types of relays are now out of manufacture.
Digital Relays: Solid State relays were replaced by the development of more modern & superior Digital Relays that are now used in newer installations. Compared to its predecessors, digital relays carry superior functions than the electro-mechanical and solid state units. Because of the versatility of digital circuitry and micro-processors, these relays provide many functions not previously available in electro-mechanical & solid state counterparts.
Today’s Digital Relays are built immune to severe electrical environment of industrial or utility applications. They are built to withstand failure, especially from high transient voltages caused by lightning, on-site switching and other rugged application conditions. Digital Relays have gained a strong and rapidly growing position in power systems in terms of accuracy, dependability, versatility, and reliability, and most of all; they come in, much cheaper.
To be continued...
DAA 3/8/2009
It may be convenient to think of the circuit breaker as the muscle providing brute force that does the work of isolating the component, while the relay is the brain which decides that isolation is required and the command for the circuit breaker to trip.
Most switchgear-type relays are enclosed in a semi-flush-mounting draw-out case. Relays usually are installed on the door of the switchgear cubicle. Again, protective relays are arguably the least understood component of medium voltage circuit protection. And, coordination doesn't need to be complicated too, that is, if we know some basic relay and sensor information. Let's then try to unravel the mystery.
Protective Relays come on the form of electro-mechanical, solid state and more recently the digital relays.
Electro-mechanical Relays: These relays are now extinct and therefore need no further discussion as far as this article is concerned.
Solid State Relays: These relays can perform all the functions that can be performed by electro-mechanical relays and, since the precision of solid state electronic relays is greater than that of electro-mechanical relays, they allow closer system coordination. In addition, because there is no mechanical motion and the electronic circuitry is very stable, they retain their target accuracy for a long time. Incidentally however, Solid State Relays had a short-lived popularity. Sooner than expected, these types of relays are now out of manufacture.
Digital Relays: Solid State relays were replaced by the development of more modern & superior Digital Relays that are now used in newer installations. Compared to its predecessors, digital relays carry superior functions than the electro-mechanical and solid state units. Because of the versatility of digital circuitry and micro-processors, these relays provide many functions not previously available in electro-mechanical & solid state counterparts.
Today’s Digital Relays are built immune to severe electrical environment of industrial or utility applications. They are built to withstand failure, especially from high transient voltages caused by lightning, on-site switching and other rugged application conditions. Digital Relays have gained a strong and rapidly growing position in power systems in terms of accuracy, dependability, versatility, and reliability, and most of all; they come in, much cheaper.
To be continued...
DAA 3/8/2009
2 comments:
Integrated UPS (Uninterruptible Power Systems) and standby generators make sure the system is regularly running that is imperative with the announcement of a possible energy crisis that's expected by power consultants.
Power transformers in India | Transformer manufacturer in India
I like your blog, because you shared a valuable information about Safety anaysis and Relays. These relays must be able to differentiate between the normal operating currents that including short time over currents that may appear due to certain equipment normal operation such as motor starting currents, Transformer inrush currents and sustained overcurrent due to fault conditions. If you have any queries, Please visit Relay Coordination
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