FMPR-105 Pt2 l Transmission Line Protection v1

Estimated read time: 1:20

Summary

In this video, the advanced features of distance protection relays, specifically GE Multilin D60, are explored. The relays detect faults on transmission lines by measuring current and voltage to calculate impedance, with protection zones set to cover specific line areas. Different schemes, including pilot and non-pilot AED, step distance, and the zone one extension scheme, are discussed for fault detection and handling. These methods ensure the safety and efficiency of power systems, highlighting elements like ground directional overcurrent and power swing blocking.

Highlights

Advanced distance relays like the GE Multilin D60 provide both phase and ground distance protection elements. ⚙️
Relays calculate impedance to detect faults on a transmission line effectively. 🚦
Protection zones vary in reach and direction, offering flexible fault detection.🔄
Pilot and non-pilot AED schemes differ in relay communication for fault management.📞
The zone one extension scheme aids in handling transient faults swiftly.✈️
Pilot AED schemes require a communication channel between relays for enhanced fault detection. 💡
The hybrid PT scheme optimizes security for transmission lines with weak infed sources. 🛡️
Line current differential relaying is noted for its immunity to power swings and high sensitivity. 📈

Key Takeaways

Advanced distance relays, like GE Multilin D60, enhance protection capabilities. 🚀
Protection zones can be strategically set for both phase and ground elements. 🛡️
Distance relay schemes, such as AED and non-AED, tailor fault management. ⚙️
Step distance and zone one extension schemes offer varying protection scopes.📡
Pilot AED schemes employ communication between relays for quicker fault clearing. 💬
Ground directional elements improve sensitivity in detecting faults. 🌐
Power swing blocking in relays prevents unnecessary tripping. 🚫
Line current differential relaying is becoming popular for its speed and reliability.⚡

Overview

In the realm of transmission line protection, advanced distance relays like the GE Multilin D60 are game-changers. These relays incorporate both ground and phase protection elements, enhancing their ability to detect and clear faults by measuring current and voltage to calculate the line impedance. Notably, various zones are employed, each customizable for different protection strategies and directions.

A deeper dive reveals distinct schematics used in distance relaying: pilot AED versus non-pilot AED schemes. These differ primarily in communication requirements, impacting the speed and strategy of fault detection and clearing. The step distance and zone one extension schemes add flexibility, accommodating both partial and extensive line protection needs efficiently.

Furthermore, the video delves into new-age protection such as line current differential relaying. This system stands out for its ability to maintain stability during power swings, high sensitivity, and robustness against faults, offering an advance over traditional methods. This comprehensive approach ensures that modern electrical grids remain reliable, safe, and efficient.

Chapters

00:00 - 00:30: Introduction to Distance Protection Relays This chapter introduces distance protection relays, with a focus on advanced models like the GE Multilin D60. These relays come equipped with features for both ground and phase protection.
00:30 - 01:00: Distance Protection Elements and Zones The chapter discusses distance protection elements in electrical systems, focusing on ground distance elements and phase elements that detect faults between phases. There are four zones of protection available for these elements. Each zone can be set to operate in either the forward or reverse direction along the transmission line and can adopt mo or quadrilateral shaped protection characteristics. Additionally, the operating characteristics of each zone can be configured individually.
01:00 - 01:30: Impedance Measurement and Inaccuracies This chapter discusses the role of distance protection relays in detecting and addressing faults on transmission lines. These relays work by measuring current and voltage to calculate the impedance of the line. If the measured impedance is less than the known impedance, it indicates a fault, as it suggests the line is shorter than expected. However, the accuracy of these impedance calculations can be compromised due to the inherent inaccuracies of Current Transformers (CTs) and Voltage Transformers (VTs).
01:30 - 02:00: Zones of Protection Setup and Strategy The chapter discusses the challenges associated with using distance relays for protection in power systems. It emphasizes how inaccuracies in current transformers (CTs) and voltage transformers (VTs), as well as transient effects, can lead to misjudgment of fault location. For example, if a transmission line's actual impedance is 20 Ohms and a fault occurs slightly beyond the line, the resultant impedance might be measured inaccurately due to these factors, leading the relay to incorrectly identify the fault as being on its protected line.
02:00 - 03:00: Pilot and Non-Pilot AED Schemes The chapter discusses the concept of pilot and non-pilot Automatic External Defibrillator (AED) schemes. It explains how inaccuracies in relay readings can cause circuit breakers to trip unnecessarily, leading to the shutting down of transmission lines. To address this issue, distance zones of protection are not set strictly at the transmission line boundary. Instead, distance relays are programmed with multiple zones of protection. Some zones are set short to cover only a portion of the transmission line, while others are set longer to cover the entire line and some portion of the next adjacent line. This setup ensures better coverage and minimizes unnecessary shutdowns.
03:00 - 05:00: Stepped Distance Protection Scheme The chapter titled 'Stepped Distance Protection Scheme' discusses the strategy used in transmission line protection using relays. The protection zones, either under-reaching or over-reaching, can be programmed to protect in forward or reverse directions. Zones set for forward direction protection look into the line, while those for reverse direction look out of the line. The chapter elaborates on configuring these zones to ensure effective relay transmission line protection.
05:00 - 08:00: Zone One Extension Scheme This chapter discusses the Zone One Extension Scheme which is a type of protection scheme used in electrical distance relaying. There are two categories of protection schemes: pilot AED and non-pilot AED. The primary distinction is that pilot AED schemes involve communication between distance relays to ascertain the location of faults on the transmission line, while non-pilot AED schemes do not involve such communication and rely on delays and coordination.
08:00 - 10:00: Pilot AED Schemes and Communication The chapter focuses on Pilot AED Schemes and Communication, exploring various distance protection schemes within electrical grids. It introduces two main types: the Stepped Distance Protection Scheme and the Zone One Extension Scheme, with an emphasis on the former as it forms the foundational structure for most other distance schemes. The Stepped Distance Scheme is detailed with its use of four different zones of protection, explaining how coordination occurs internally within individual relays.
10:00 - 12:00: Permissive Overreaching Transfer Trip Scheme (PT) This chapter discusses the use of a Permissive Overreaching Transfer Trip Scheme (PT) to protect transmission lines and serve as a backup for distance relays. It illustrates the setup using a power system example, focusing on transmission line number one. The relay is positioned at the far left end of the line. The first zone of protection is defined to 'under reach', covering 80-90% of the line's length. This concept and its application are explained in the context of protecting adjacent transmission lines.
12:00 - 15:00: Hybrid PT Scheme This chapter discusses the 'Hybrid PT Scheme' with a focus on the functionality of protective relay zones in electrical transmission lines. Zone one is immediate with no time delay and covers most of the transmission line but not the last 10 to 20%, known as 'The End Zone'. Faults in 'The End Zone' are handled by other protection zones. Zone two overreaches beyond the transmission line beginning.
15:00 - 17:00: Directional Blocking Scheme This chapter discusses the concept of a Directional Blocking Scheme within transmission lines, focusing on protective zones known as Zone 2. Zone 2 is set to cover 120% of the transmission line's impedance and is designed to detect faults that occur towards the end of the line, beyond the reach of Zone 1. It incorporates a time delay between 0.25 and 0.4 seconds, utilizing the protective characteristics outlined in the previous sections.
17:00 - 18:30: Power Swing Blocking The chapter 'Power Swing Blocking' discusses the ability of protecting the entire length of a transmission line using Zone 1 and Zone 2 protection mechanisms. Zone 1, which has no intentional time delay, immediately activates its operate flag in response to faults. Zone 2, which includes a time delay, does not initially activate. When Zone 1's operate flag triggers, it initiates the relay to trip and clear the fault on the transmission line.
18:30 - 23:30: Line Current Differential Relaying This chapter covers the operational behavior of line current differential relaying, specifically focusing on how different zones react to faults on a transmission line. When a fault occurs outside the primary protection zone (Zone 1), Zone 1 remains inactive as it does not detect the fault. However, if the fault occurs within the boundary of the secondary protection zone (Zone 2), Zone 2 will detect and respond to the fault. The chapter highlights the importance of each zone's role in ensuring the reliability and safety of power transmission systems.
23:30 - 33:30: Breaker Failure Protective Element The chapter discusses the behavior of the breaker failure protective element, particularly when a fault occurs within the zone 2 area of protection. After the zone 2 time delay has expired, the zone 2 operate flag changes to the 'on' state, triggering the relay to trip and clear the fault. Once the fault is cleared, both the pickup and operate flags for zone 2 deactivate. The chapter also addresses scenarios where a fault occurs on an adjacent transmission line but within the zone 2 protection, noting that under normal circumstances, the relay would trip the unfaulted transmission upon expiration of the zone 2 time delay.

FMPR-105 Pt2 l Transmission Line Protection v1 Transcription

00:00 - 00:30 [Music] we will now highlight some of the distance protection features that are available in more advanced distance relays such as the GE multilin D60 these relays will have both ground and phase
00:30 - 01:00 distance protection elements available the ground distance elements will detect faults between a phase and ground while the phase elements will detect faults between phases for both of these types of elements there are four zones of protection available every zone of protection can be programmed to be protecting in the forward or the reverse direction of the transmission line these zones can also be programmed to have a mo or quadrilateral shaped protection characteristic the operating characteristics of each Zone can also be set individually
01:00 - 01:30 a distance protection relay is used to detect and clear faults that occur on a transmission line it accomplishes this type of protection for both the phase and ground distance Elements by measuring the current and voltage and using this information calculates the impedance of the line if the measured impedance is less than the known impedance of the line this indicates the Line is now shorter than expected indicating it is faulted however due to the inherent inaccuracies of CTS and VTS the impedance calculations made by the
01:30 - 02:00 distance relays cannot be guaranteed to be extremely accurate for example let's assume the actual impedance of our transmission line is 20 Ohms a fault occurs just past this transmission line on the next adjacent circuit the actual impedance from the measurement point of the relay to this fault is 20.5 ohms due to the inaccuracies in the CTS and VTS along with some transient effects the relay May measure the impedance to The Fault to be less than it actually is and see the fault as being on the transmission line it is protecting this
02:00 - 02:30 would cause the relay to trip the circuit breaker and shut down this transmission line when it did not need to because of these inaccuracies distance zones of protection are not set to stop right at the boundary of a transmission line distance relays use multiple zones of protection with some of them set short to only protect part of a transmission line and others set longer to protect the entire length of a transmission line plus some of the next adjacent line the zones of protection that are set to cover only part of a
02:30 - 03:00 transmission line are said to be under reaching the zones of protection that are set to cover more than just that relays transmission line are set to be overreaching zones of protection can also be programmed to protect in either the forward direction or reverse Direction zones that are set to protect in the forward direction are said to be looking into the line while zones that are set to protect in the reverse direction are set to be looking out of the line the Strate strategy used to protect
03:00 - 03:30 a line is commonly referred to as a scheme all protection schemes used in distance relaying fit into one of two categories pilot AED and non-pilot AED schemes the main difference between the two types of schemes is that the distance relays used in Pilot AED schemes communicate with each other to determine whether the fault is located on the transmission line or not the relays used in non-pilot AED schemes do not communicate with each other and instead use delays and other forms of coordination to determine whether the fault is located on the transmission
03:30 - 04:00 line or not all coordination in these schemes is done within each individual relay itself in the category of non-pilot AED distance protection schemes there are two main types that we will discuss these are the stepped distance protection scheme and the Zone one extension scheme we will first describe the step distance scheme this scheme is the basic structure for all other distance schemes so it is important to cover it in great detail the stepped distance scheme uses four four different zones of protection
04:00 - 04:30 to protect its own transmission line and to act as a backup distance relay to protect transmission lines that are located next to it we will use the power system shown here to describe how the four zones of protection are set and used the main transmission line we will be protecting will be transmission line number one and the relay will be located at the far left of that line the first zone of protection is set to under reach and extends from the beginning of the transmission line to 80 or 90% of the length of the entire l Line This zone is
04:30 - 05:00 also said to have no time delay therefore if a fault occurs in this Zone the Relay can be sure that the fault is located on its transmission line and can trip immediately Zone number one will not protect the last 10 to 20% of the line which is known as The End Zone any faults that occur in this area will not be seen by Zone number one and will have to be cleared by one of the other zones of protection the second zone of protection is set to overreach and extends from the beginning of the transmission line past
05:00 - 05:30 the end of the line and into the next adjoining transmission line This is called Zone 2 Zone 2 is usually set to 120% of the impedance of the transmission line Zone 2 is set in this manner so that it can detect faults that occur in the end zone area that zone number one cannot detect Zone 2 is also always programmed to have a time delay set between 0.25 and 0.4 seconds using all of the characteristics we have just discussed Zone 1 and z Zone 2 have the
05:30 - 06:00 ability of protecting the entire length of the transmission line in the following way any faults that occur at the beginning of the transmission line will be picked up by both Zone 1 and zone two as seen by the status of their pickup Flags since Zone one has no intentional time delay its operate flag will turn on immediately The Zone 2 operate flag will not yet turn on because it has a time delay programmed within it the Zone one operate flag will then trigger the relay to trip to clear the the fault at that point the fault
06:00 - 06:30 will be cleared the Zone one pickup and operate Flags will turn off the zone 2 pickup flag will turn off and the zone 2 operate flag will stay in the off state with never having turned on on another occasion if a fault occurred in the end zone region of the transmission line the two zones would act differently the fault does not lie within the zone of protection of Zone number one therefore Zone one will not pick up the fault does lie within the zone of protection of Zone number two therefore Zone 2 will
06:30 - 07:00 pick up after the zone 2 time delay has expired The Zone 2 operate flag will change to the on State and Trigger the relay to trip and clear the fault once the fault is cleared both the pickup and operate flags of zone two will turn off you may be wondering what would happen if a fault occurred on the adjacent transmission line but still within the zone 2 area of protection normally after the zone 2 time delay expired the relay would trip the UN faulted transmission
07:00 - 07:30 line however you have to remember that every adjacent transmission line is protected by its own distance relay this fault would fall into the second relay's first zone of protection and the fault would be cleared before the timer of zone two in the first relay expired the third zone of protection is set to overreach past the end of the second adjacent transmission line the third zone is used to act as a backup for that next adjacent transmission line in case the protection on that transmission line fails for for this reason Zone 3 is usually set to extend
07:30 - 08:00 to the same point as Zone number two of the adjacent transmission line so a setting of 220% of the impedance of its own transmission line is often selected if line one and line two's impedances are the same Zone 3 must have a time delay that is longer than the operate time of Zone number two of the adjacent transmission line a time delay of 1 second is commonly used Zone 4 is used as backup protection for a small portion of the adjacent transmission line in the reverse
08:00 - 08:30 Direction the operation of this zone is usually time delayed to allow the first Zone and second zone of that adjacent transmission line to operate first a time delay of 750 milliseconds to 1 second is common once each of the four zones have been set the Ste distance scheme to control the breaker on the left side of the transmission line is complete however the breaker on both ends of the transmission line must be opened to completely clear a fault therefore another distance relay must be located on the right end of the the transmission line to control that
08:30 - 09:00 breaker the relay on the right needs to be programmed with the same characteristics as the relay on the left however this one will be set up to be protecting in the opposite direction now the zones of protection of each relay will detect any faults that occur on our protected transmission line these relays will then proceed to open the breaker and clear the fault from their own respective end of the line each relay will also have ground stepped distance zones of protection which are identical to that of the phase stepped distance zones the difference being that the ground distance elements are used to
09:00 - 09:30 detect single phase to ground faults the other type of non-pilot AED scheme we will discuss is the Zone one extension scheme The Zone one extension scheme is an enhancement to the stepped distance scheme covered in the previous section this protection scheme operates on the principle that most transmission line Faults Are transient in nature which means the fault is not permanent for example the most common cause of faults on transmission lines is lightning if lightning strikes a transmission line it ionizes the air
09:30 - 10:00 which reduces the resistance between the different phases of the line This reduced resistance allows for a path for current to flow and hence creates a fault once the transmission line is tripped and the fault is cleared the ionized air is removed and there no longer exists a path for current to flow closing the transmission line circuit breaker at this point would reenergize the line and transmission of power could resume as mentioned before the Zone one extension scheme is like the stepped distance scheme where it has four zones
10:00 - 10:30 of protection the difference between the two schemes is that in the zone one extension scheme an additional Zone one impedance setting is set higher to overreach into the next adjacent transmission line the impedance of this extended Zone one which is labed z1x is usually set to the same impedance as the zone 2 setting of 120% of the impedance of the entire transmission line This extended Zone one is used by the distance relay until it detects a fault within this zone of protection
10:30 - 11:00 once a fault is detected the relay will immediately trip the transmission line since there is no delay in the operation in the extended Zone one element as you can see the relay will trip the line even though the fault may exist in the area located in the extended Zone one but be in the adjacent transmission line once the breaker is opened and the fault is cleared the relay will then automatically reclose the breaker and reenergize the transmission line once the line has been reclosed the relay will then use the standard Zone one characteristic and now under reached to
11:00 - 11:30 only protect an area within its own transmission line if the fault that occurred was a permanent fault and the fault is located within the transmission Line's newly shortened Zone one the relay will trip and again open the circuit breaker to clear the fault if the fault was transient in nature the first occurrence of tripping the line would have removed the fault and transmission of power would resume if the fault that occurred was a permanent fault and reappeared after the reclosing but was not located in the new shortened Zone one protected area the
11:30 - 12:00 relay would not pick this up for a second time in zone one and not trip to try to clear the fault the permanent fault would then be cleared by the zone 2 protection after the set time delay if the fault fell inside the transmission Lin's End Zone area if the permanent fault was picked up by the relay Zone 2 because it was located at the beginning of the next adjoining transmission line the relay protecting that transmission line would pick it up in its Zone one of protection and clear the fault the disadvantage of the Zone one extension scheme is that external faults within
12:00 - 12:30 the extended Zone One reach of the relay trip the breaker instantaneously even if that breaker did not need to be tripped this needless tripping increases the amount of breaker maintenance needed and causes a needless loss of power supply to some customers all transmission line distance protection schemes that fit into the category of pilot AED schemes are based on the basic concept of stepped distance protection schemes on a stepped distance scheme any that occurs in the midpoint
12:30 - 13:00 of a transmission line that fits into both of the relays Zone one will be cleared instantly by the relay located at each end of the line however if the fault is located on one of the end zones of the transmission line which is also known as a close in fault it will not be cleared instantly at both ends of the transmission line the fault will be cleared instantly by Zone one of the relay that is located near the fault but will not be cleared instantly by the relay at the far end of the line because the fault did not fall in its Zone one the fault will not be cleared at the far
13:00 - 13:30 end of the transmission line until the time delay in zone 2 of that relay has expired you'll remember that the time delay for Zone 2 operation is usually in the range of 250 to 400 milliseconds the main reason to modify and improve upon the stepped distance scheme using pilot AED schemes is to speed up the performance of clearing faults that occur in this end zone of the transmission line pilot schemes speed up the clearing of faults that occur on the transmission line and inside Zone 2 of the local relay by communicating with the relay at the
13:30 - 14:00 remote end of the line to determine if the fault is actually on the transmission line therefore all pilot AED schemes require a communication Channel be provided between the two relays over this communication channel the two relays share information regarding the general location of the fault allowing the clearing of faults on the transmission line to occur as fast as possible the most common pilot AED schemes are the dut scheme which stands for direct under reaching transfer trip the P scheme which stands for permissive
14:00 - 14:30 under reaching transfer trip the PT scheme which stands for permissive overreaching transfer trip the hybrid PT scheme which stands for the hybrid permissive overreaching transfer trip and the directional blocking scheme we'll discuss all of these schemes in the following sections a fault that occurs in the end zone of a transmission line normally will not be fully cleared until the time delay of the Zone to protection element
14:30 - 15:00 has expired the dut scheme which stands for the direct under reaching transfer trip scheme is used to reduce the amount of time needed to wait before clearing a fault that occurs in this end zone of a transmission line as for all pilot schemes there must be a communication Channel set up between the distance relays at each end of the line This Communication channel can be one of many different configurations the communication Channel may have the ability to only send one bit of data back and forth between the relays such as a powerline carrier communication
15:00 - 15:30 link or may be able to send multiple pieces of data back and forth between the two relays such as a fiber optic or microwave link first each of the relays must also have all of its four zones of protection set up to operate as a standard stepped distance application for the examples we will be using in this section we will label the two distance relays that are protecting the transmission line in the following way the distance relay that is located on the left side of the transmission line will be called the local relay the distance relay that is located on
15:30 - 16:00 the right side of the transmission line will be called the remote relay when a fault occurs in the middle of the transmission line using the dut scheme or both Zone ones of the local and the remote relays the relays will trip and clear the fault instantly at both ends of the line This is similar to the stepped distance scheme faults that occur in the local relays end zone of the transmission line will be cleared at the remote end of the line instantly by The Zone one element of the remote relay the breaker at the local end of the line
16:00 - 16:30 would normally not trip until the timer of Zone 2 had expired using the dut scheme if a fault occurred in the transmission line End Zone the remote relay that identified the fault in its Zone one will immediately trip the breaker on its side of the line upon detecting this fault in its Zone one the remote D60 will also immediately send a signal which is known as a key over the communication channel to the local relay located at the other end of the line This is where the term under reaching came from because the keying signal is
16:30 - 17:00 initiated by identifying a fault within the relay's under reaching Zone one as soon as the local relay receives the dut key from the remote relay it will immediately trip its breaker thus clearing the fault on the transmission line the important thing to note with this scheme is that the local relay will trip as soon as it gets a transfer trip signal from the remote relay without requiring any indication of a fault within its own zones of protection hence the term direct transfer trip
17:00 - 17:30 the P pilot edit scheme stands for the permissive under reaching transfer trip scheme just like the dut scheme the P scheme is used to speed up the clearing of faults that occur in the end zone of a transmission line for this scheme as for all pilot AED schemes a communication Channel must be provided between the two relays located at each end of the transmission line in the P scheme the remote distance relay sends a P key signal to the local relay whenever
17:30 - 18:00 it detects that a fault exists within its under reaching Zone one area of protection this is where the expression undering comes from in the term permissive under reaching transfer trip the difference between the dut scheme and the P scheme is what the local relay does when it receives the key signal from the remote relay in the dut scheme the local relay trips as soon as it received the key signal in the P scheme the local distance relay P logic will will now only trip the breaker if it
18:00 - 18:30 receives the P key and the local relay has detected a fault within its Zone 2 area of protection if the local distance relay received the P key and it does not detect default within its Zone 2 area of protection the P scheme will not cause the local distance relay to trip therefore the distance relays P scheme uses the key from the remote relay as a permissive signal that allows it to trip if the local relay detects a possible fault on the transmission line hence the
18:30 - 19:00 term permissive under reaching transfer trip when a fault occurs in a transmission line somewhere on the line currents are flowing to ground measuring the direction of the flow of the ground current is used to assist some of the pilot AED distance protection schemes in determining if the fault and the source of the ground current are located on their transmission lines the ground directional overcurrent elements let the pilot AED schemes know if the ground current rises above a minimum set level
19:00 - 19:30 and if it does what direction the ground current is Flowing the ground directional overcurrent elements are used by the pilot AED schemes because they are sometimes more sensitive to detecting faults than the distance elements are when the source has a weak infeed or high system impedance ratio the benefits of using ground directional functions in Pilot schemes are that the zero sequence and negative sequence currents that are used to detect the direction of ground currents do not contain very many load components therefore the pickup levels for the
19:30 - 20:00 ground directional elements can be set very low and thus are very sensitive the ground directional elements are also very fast operating because the zero sequence and negative sequence currents build up from practically a zero prefault value also since there was no ground current before the fault the prefault zero sequence and negative sequence currents do not bias the direction of the developing fault components the PT pilot AED scheme
20:00 - 20:30 stands for the permissive overreaching transfer trip scheme like the other pilot AED schemes it is used to speed up the clearing of faults that occur in the end zone of a transmission line again a communication Channel must be provided between the two relays for the PT scheme to operate in the PT scheme the remote distance relay speeds up the tripping of an Endzone Fault by sending a permission to trip key from the remote relay to the local relay under two conditions the first condition is when the remote relay detects a fault occurring within its
20:30 - 21:00 overreaching zone two this is where the expression overreaching comes from in the term permissive overreaching transfer trip the second condition under which the remote relay will send a permissive key is when it detects that ground current is flowing in its forward Direction therefore the remote relay's forward negative sequence directional over current element or its forward neutral directional over current elements operation will also cause the remote relay to send a PT key to the local relay in in addition to the overreaching zone 2 pickup flag in the
21:00 - 21:30 PT scheme the local relay PT logic will only cause the breaker to trip if it gets the PT key and the local relay has detected a fault within its Zone 2 area of protection or it detects that ground current is flowing in its forward direction if this function is enabled as a result either the local relay's forward negative sequence directional overcurrent element or the forward neutral directional overcurrent element as well as the pick up of his own two fault will cause the PT scheme to trip
21:30 - 22:00 the breaker once it has received a permissive key from the remote relay any combination of the two reasons the remote D60 sends transmit keys and the two reasons the local relay identifies a fault will isolate the transmission line and verify the fault is located on the transmission line we are protecting as shown in diagram 1 if zone two of both relays identifies a fault it is a clear indication that the fault is on the transmission line so both relays will trip their respective Breakers in
22:00 - 22:30 diagram 2 the remote Zone 2 indicates that the fault is located to the left of the remote relay and the forward flow of ground current as seen by the local relay indicates that the fault is to the right of it therefore the fault is on the transmission line and both relays will trip their Breakers in diagram 3 the forward flowing ground current of the remote relay and the zone two picking up on the local relay is a clear indication that the fault is on the transmission line and so both Breakers will be tripped finally in diagram 4 the
22:30 - 23:00 direction of ground current detected by both relays is flowing into the transmission line which is again a clear indication that the transmission line is faulted resulting in the tripping of both Breakers the hybrid PT or hybrid permissive overreaching transfer trip scheme is a modification of the PT scheme which adds an extra degree of security and additional protection for transmission lines that have a weak infeed Source the hybrid PT scheme works in the very same way as the PT scheme to
23:00 - 23:30 speed up the clearing of Endzone faults in this scheme both a Zone 2 fault and forward flowing ground current being detected by the remote distance relay will send a permissive key to the local distance relay thus allowing it to trip the local distance relay also tries to detect reverse ground current or a Zone 4 fault if either reverse ground current or a fault is picked up in zone 4 the hybrid PT scheme is blocked from tripping resulting in additional security the other advantage of the hybrid PT
23:30 - 24:00 scheme over the PT scheme is its ability to trip on faults that are fed by a source that has a high source impedance this is also referred to as a weak infeed when a fault occurs on the transmission line with a weak infeed that is located close to the local distance relay the characteristics of the transmission line will operate in the following way the voltage measured by the local distance relay will be very close to zero the current flowing through the local distance relay CTS will not be very high therefore the distance zones of
24:00 - 24:30 protection may not pick up because the fault current is below the current supervision level as was discussed in the zones of protection section also the zero sequence and negative sequence current will be almost zero therefore the forward and reverse ground directional over current elements will not operate even though none of the distance relays protection elements have picked up the fault the faulted line still needs to be cleared the weak infeed feature of advanced distance relays can detect this condition and correctly trip the
24:30 - 25:00 relay the directional blocking scheme is one of the most popular types of teleprotection schemes used in distance applications today again we should state that the purpose of the scheme is to speed up the tripping of faults that occur in the end zone of a transmission line as with all pilot AED schemes a communication Channel must be provided between the two relays located at each end of the transmission line for the directional blocking scheme to operate in the directional blocking scheme the local distance relay has an
25:00 - 25:30 additional delay timer that is started by the detecting of either a fault inside its Zone 2 area of protection or the detection of ground current flowing in the forward Direction This timer is set considerably shorter than the normal Zone 2 delay when this additional timer expires the local distance relay will trip the local breaker unless it receives a block message or key from the remote distance relay the remote relay will only send this blocking key if it detects that the fault is located in its Zone 4 area of protection or it detects
25:30 - 26:00 that ground current is flowing in the reverse Direction both of which would indicate an external fault the last subject we will deal with in distance relaying is power swing blocking a stable power swing can be seen as a condition where our system loses synchronism for a short period of time with our neighboring system and then later regain synchronism two reasons that a power swing such as this can occur are one of the systems loses a significant amount of generation
26:00 - 26:30 therefore instantly becomes much weaker and the load of the system remains constant or the system is already weak and the load suddenly increases substantially during a power swing the voltages and currents will fluctuate significantly drastically affecting the value of the measured impedance of the line the plotted impedance as measured by the distance relay will slowly swing from our load Point into the distance operating zones and then back out to a load point if system regains synchronism the power swing blocking feature on
26:30 - 27:00 Advanced distance relays can detect that the impedance swing is a recoverable power swing and will block the distance zones from operating this is detected by creating an additional impedance Zone located around the operating impedance zones as shown a stable power swing is detected if the impedance of the system moves into this new impedance zone for an extended period of time under this condition the distance protection will be blocked if this swing were a real fault that entered the power swing blocking area it would move very fast through this additional area and into
27:00 - 27:30 the operating impedance Zone the power swing blocking characteristics must be configured to be much larger than the distance Zone one characteristic to ensure a stable power swing is detected long before Zone one's operating characteristic is reached the outer boundaries of the two larger zones in this diagram form a tomato- shaped Zone while the area of overlap of these two zones form an inner lens-shaped Zone the resulting tomato- shaped outer zone is what is used to detect the presence of a power swing and sets the block of the
27:30 - 28:00 distance element if the impedance enters the lens shape the relay no longer sees this as a PowerWing but maintains the block signal for a short time to allow the system impedance to leave this area as will happen in a stable swing with the Advent of high-speed lowcost land technology a new form of line protection known as line current differential reling is becoming more popular due to its speed sensitivity immunity to power swings and being a form of unit protection unit protection means the
28:00 - 28:30 zone of protection covers the complete power system component without overreaching line current differential relaying measures and compares the current at both ends of the line using two separate relays and a Communications link the zone of protection is the segment of the line between the two sets of CTS theoretically the sum of the two currents should be zero if there is a difference Beyond a reasonable amount there is a fault on the line and so both relays Triad their respective Breakers isolating the line the communication
28:30 - 29:00 link requirements for differential relaying can be quite modest by today's standards for example the GE multilin L90 current differential relay requires only a 64 Koby per second communication link to enhance reliability modern utility class line current differential relays will provide built-in distance backup in the event of a communication link failure for the following examples we will be using the GE multilin L90 relays let's first take a look at a two-
29:00 - 29:30 terminal application to better understand the operation of line current differential relaying upon PowerUp the two relays first synchronize their internal clocks via the communication link each relay then measures the current at its CTS and sends this information along with a time stamp to the other relay using a form of the percent differential element each relay compares the current measured at its location with a value of current having the same time stamp that was received from the remote location if there is more than an
29:30 - 30:00 acceptable error there is a fault on the line and the element in both relays will operate to isolate that section of line from the rest of the power system the L90 relays support both two and three terminal configurations the relays in a three- terminal configuration will behave in the same fashion as the two terminal configuration with each relay making its own decision as to whether it will trip in this way all L90 relays are said to act as Masters if there is a failure of one Communication channel path the
30:00 - 30:30 relays will automatically switch to a backup Master Slave mode the relay that is still in communication to the other two relays will be the master and the other two will switch to slave mode the master relay will perform the differential element calculations for its location and both of the slave locations if a fault occurs the master will either trip its own breaker and or send the appropriate signal to one or both of the slavery lays to clear the fault with line current differential relaying there are two common types of
30:30 - 31:00 transfer trips that can be sent the 87l direct transfer trip and the key direct transfer trip the 87l direct transfer trip is sent from the master's line differential element and causes the slave that receives the transfer trip to trip the key direct transfer trip is sent by another function such as breaker failure the basic premise for the operation of differential protection schemes is that the sum of the currents and entering the protected zone is zero
31:00 - 31:30 in the case of a power system transmission line This is not entirely true because of the capacitive charging current of the line for short transmission lines the charging current is a small factor and can be treated as an unknown error in this circumstance the relays can be applied without voltage sensors the line charging current will be included as a constant term in the total variance increasing the differential restraint current for long transmission lines the charging current is a significant factor and should be calculated by the relay to
31:30 - 32:00 provide increased sensitivity to fault current compensation for charging current requires that the voltage at the terminals is supplied to the relays the algorithm calculates the capacitance times the change in voltage over the change in time for each phase which is then subtracted from the measured currents at both ends of the line This is a simple approach that provides adequate compensation of the capacitive current at the fundamental power system frequency traveling waves the transmission line are not compensated for and contribute to restraint by
32:00 - 32:30 increasing allowable error in the setting of the differential error if the VTS are connected in y the compensation is accurate for both balanced conditions for example all positive negative and zero sequence components of the charging current are compensated if the VTS are connected in Delta the compensation is accurate for positive and negative sequence components of the charging current since the zero sequence voltage is not available the relay cannot compensate for the zero sequence
32:30 - 33:00 current let's look at an example of a breaker failure protective element in this example the protective relays operate correctly but the associated circuit breaker fails to clear the fault because of some malfunction within the breaker or its control circuits the fault will now remain on the system until some other means is used to clear it in our example the primary or backup relays start a timer once a fault has been detected the timer will then time out and send a
33:00 - 33:30 trip signal to all Breakers that can feed the failed breaker with power [Music]