Fundamentals of Distance Protection

Fundamentals of Distance Protection
GE Multilin

Outline Transmission line introduction What is distance protection?
Non-pilot and pilot schemes Redundancy considerations Security for dual-breaker terminals Out-of-step relaying Single-pole tripping Series-compensated lines

Transmission Lines A Vital Part of the Power System:
Provide path to transfer power between generation and load Operate at voltage levels from 69kV to 765kV Deregulated markets, economic, environmental requirements have pushed utilities to operate transmission lines close to their limits.

Transmission Lines Classification of line length depends on:
Source-to-line Impedance Ratio (SIR), and Nominal voltage Length considerations: Short Lines: SIR > 4 Medium Lines: 0.5 < SIR < 4 Long Lines: SIR < 0.5

Typical Protection Schemes Short Lines
Current differential Phase comparison Permissive Overreach Transfer Trip (POTT) Directional Comparison Blocking (DCB)

Typical Protection Schemes Medium Lines
Phase comparison Directional Comparison Blocking (DCB) Permissive Underreach Transfer Trip (PUTT) Permissive Overreach Transfer Trip (POTT) Unblocking Step Distance Step or coordinated overcurrent Inverse time overcurrent Current Differential

Typical Protection Schemes Long Lines
Phase comparison Directional Comparison Blocking (DCB) Permissive Underreach Transfer Trip (PUTT) Permissive Overreach Transfer Trip (POTT) Unblocking Step Distance Step or coordinated overcurrent Current Differential

What is distance protection?
RELAY (V,I) Intended REACH point Z F1 I*Z V=I*ZF I*Z - V For internal faults: IZ – V and V approximately in phase (mho) IZ – V and IZ approximately in phase (reactance)

F2 RELAY (V,I) Intended REACH point Z I*Z V=I*ZF I*Z - V For external faults: IZ – V and V approximately out of phase (mho) IZ – V and IZ approximately out of phase (reactance)

Intended REACH point Z RELAY

Source Impedance Ratio, Accuracy & Speed
Relay Line System Voltage at the relay: Consider SIR = 0.1 Fault location Voltage (%) Voltage change (%) 75% 88.24 2.76 90% 90.00 0.91 100% 90.91 N/A 110% 91.67 0.76

Source Impedance Ratio, Accuracy & Speed
Relay System Line Voltage at the relay: Consider SIR = 30 Fault location Voltage (%) Voltage change (%) 75% 2.4390 0.7868 90% 2.9126 0.3132 100% 3.2258 N/A 110% 3.5370 0.3112

Challenges in relay design
Transients: High frequency DC offset in currents CVT transients in voltages 1 2 3 4 steady-state output power cycles -30 -20 -10 10 20 30 voltage, V CVT output

Transients: High frequency DC offset in currents CVT transients in voltages 1 2 3 4 steady-state output -60 -40 -20 20 40 power cycles voltage, V 60 CVT output

-0.5 0.5 1 1.5 -100 -50 50 100 Reactance comparator [V] power cycles S POL OP Sorry… Future (unknown) In-phase = internal fault Out-of-phase = external fault

Transient Overreach Fault current generally contains dc offset in addition to ac power frequency component Ratio of dc to ac component of current depends on instant in the cycle at which fault occurred Rate of decay of dc offset depends on system X/R

Zone 1 and CVT Transients
Capacitive Voltage Transformers (CVTs) create certain problems for fast distance relays applied to systems with high Source Impedance Ratios (SIRs): CVT-induced transient voltage components may assume large magnitudes (up to 30-40%) and last for a comparatively long time (up to about 2 cycles) 60Hz voltage for faults at the relay reach point may be as low as 3% for a SIR of 30 the signal may be buried under noise

Zone 1 and CVT Transients
CVT transients can cause distance relays to overreach. Generally, transient overreach may be caused by: overestimation of the current (the magnitude of the current as measured is larger than its actual value, and consequently, the fault appears closer than it is actually located), underestimation of the voltage (the magnitude of the voltage as measured is lower than its actual value) combination of the above

Distance Element Fundamentals
Z1 End Zone XL The total line impedance is normally highly reactive, and looks at follows an a resistance/reactance diagram: Any Distance element measures the current through the line, and the voltage at the beginning of the line. The ratio between the measured voltage and the current gives the impedance of the system at this point. This point is under normal system conditions far from the line impedance, and is close to the R-axis If a fault occurs at the end of the line with close to zero fault resistance, the impedance the distance element would measure is equal to the total line impedance, since the current measured will be the current in the line and the measured voltage will be the voltage drop across the line. We need to set the first zone of distance, typically known as Zone 1, to trip for faults on the line instantaneously. Any distance element gets exposed to transient conditions during faults, which include voltage, or capacitive voltage transformer transients; current transformer transients (typically much less than voltage transformer transients); or mutual impedances due to a parallel line. Due to the transients it is impossible for the distance element to distinguish accurately between faults at 99% or 101% of the line impedance. For this reason, the distance element can exhibit a transient overreach, making it impossible to set Zone 1 to be exactly 100% of the line impedance. System Transients The following transients are generally those that should be considered, but the importance of each type will depend on the specific design and application of the relay in question. A. DC offset in the current Ð This transient has long been a consideration in relay design and will be familiar to everyone. B. DC offset in voltage Ð This transient results from the dc component of current in systems where the source impedance angle is different from the line impedance angle. C. High frequency transients Ð These transients arise as a consequence of the line shunt capacitance, which will have a charge prior to the fault that can be significantly different from the charge during the fault. The transition results in high frequency transient currents and voltages which modify the current and voltage signals to the relays. The effect on the relays' ability to make the measurement is usually a function of the relative magnitude of the high frequency quantities with respect to the fundamental frequency quantities. The frequency of these transients is also a significant factor. D. Capacitor voltage transformer transients Ð CVT's used for protective relaying usually consist of R, L and C circuits which introduce transients when there is a sudden change in the applied voltage. In general, the lower the capacitance of the coupling capacitor, and the more drastic the change in voltage as a result of the fault, the more significant will be the transient in terms of relay response. E. Current transformer transients or errors Ð Current transformer transients or errors arise primarily as a result of CT core saturation. In general, the error between the primary and secondary currents has been more of a consideration of dependability rather than security in relay design. However, it cannot be disregarded in considerations of security. F. Transients due to series capacitors Ð If series capacitors are used on the system, a low frequency transient may be introduced rather than the dc offset that is often introduced in uncompensated systems. This low frequency transient is usually more difficult to analyze than the dc offset in terms of the relay response. Zone 1 is normally set to 80 to 90% of the total line impedance, ensuring no overreaching will occur, and thus covers 80 to 90% of the line’s length. Pilot protection is used to ensure accurate and fast tripping of the last 10 to 20% of the line, also known as the end-zone. For lines with a very high zero sequence impedance, it might be necessary to add ground directional elements to the pilot scheme for enhanced security, ensuring all faults in the end-zone will be cleared. For example, wood pole lines tend to have a much larger zero sequence impedance than a steel tower line will. Since this impedance is much larger, the total impedance to ground is also generally larger, which may cause problems for a ground distance element, even if it has a quadrilateral shape. In a case like this, it might be necessary to change the protection scheme to include an additional ground directional over-current element to find an effective solution. R XC

Impedance locus may pass
below the origin of the Z-plane - this would call for a time delay to obtain stability

CVT Transient Overreach Solutions
apply delay (fixed or adaptable) reduce the reach adaptive techniques and better filtering algorithms

CVT Transients – Adaptive Solution
Optimize signal filtering: currents - max 3% error due to the dc component voltages - max 0.6% error due to CVT transients Adaptive double-reach approach filtering alone ensures maximum transient overreach at the level of 1% (for SIRs up to 5) and 20% (for SIRs up to 30) to reduce the transient overreach even further an adaptive double-reach zone 1 has been implemented

CVT Transients – Adaptive Solution
The outer zone 1: is fixed at the actual reach applies certain security delay to cope with CVT transients The inner zone 1: has its reach dynamically controlled by the voltage magnitude is instantaneous

Desirable Distance Relay Attributes
Filters: Prefiltering of currents to remove dc decaying transients Limit maximum transient overshoot (below 2%) Prefiltering of voltages to remove low frequency transients caused by CVTs Limit transient overreach to less than 5% for an SIR of 30 Accurate and fast frequency tracking algorithm Adaptive reach control for faults at reach points

Distance Relay Operating Times

35ms 25ms 30ms 20ms 15ms

SLG faults LL faults 3P faults

Actual maximum reach curves
Relay 4 Relay 2 Relay 3 Relay 1

Maximum Torque Angle Angle at which mho element has maximum reach
Characteristics with smaller MTA will accommodate larger amount of arc resistance

Mho Characteristics Traditional Directional angle “slammed”
Directional angle lowered and “slammed” Because the zone uses separate mho and directional maximum torque angles and limit angles, a variety of shapes can be programmed by the user. The phase zone uses just three comparators: Memory-polarized mho Reactance Directional Both MHO and directional angles “slammed” (lens)

Load Swings +XL +R Load Trajectory Operate area No Operate area
+ = LOOKING INTO LINE normally considered forward Reach Load Trajectory Operate area No Operate area The load looks more resistive: about a 300 phase angle. At the time of the fault the MTA will be typically 800 to 900: almost totally inductive because no load just the inductive reactance of the line. Typical load characteristic impedance +R Reach set for say 3.2  Line = 4  +XL Reach Operate area Load Almost pure resistance because customer tries for unity P.F. +R

“Lenticular” Characteristic
Load Swings “Lenticular” Characteristic Load swing

Load Encroachment Characteristic
The load encroachment element responds to positive sequence voltage and current and can be used to block phase distance and phase overcurrent elements.

Blinders Blinders limit the operation of distance relays (quad or mho) to a narrow region that parallels and encompasses the protected line Applied to long transmission lines, where mho settings are large enough to pick up on maximum load or minor system swings

Quadrilateral Characteristics
Five comparators of the QUAD zones are identical as compared with the MHO function. They are: Reactance (zero-sequence polarized thus bending for extra security) Zero-sequence directional Negative-sequence directional Phase-selector supervision (avoid operation on double-line-to-ground faults) Zero-sequence directional for Zones 2 through 4 The QUAD zones of the D60 are very flexible. This suits well retrofit applications.

Quadrilateral Characteristics
Ground Resistance (Conductor falls on ground) R Resultant impedance outside of the mho operating region Quadrilateral characteristic Superior for ground faults because they typically have resistance the resultant impedance of which will be beyond the mho characteristic. In North America mho is used to detect phase faults while quadrilateral is used to detect ground faults. XL

Distance Characteristics - Summary
Mho Lenticular Quadrilateral JX R Standard for phase elements Better coverage for ground faults due to resistance added to return path Used for phase elements with long heavily loaded lines heavily loaded

Distance Element Polarization
The following polarization quantities are commonly used in distance relays for determining directionality: Self-polarized Memory voltage Positive sequence voltage Quadrature voltage Leading phase voltage

Memory Polarization Positive-sequence memorized voltage is used for polarizing: Mho comparator (dynamic, expanding Mho) Negative-sequence directional comparator (Ground Distance Mho and Quad) Zero-sequence directional comparator (Ground Distance MHO and QUAD) Directional comparator (Phase Distance MHO and QUAD) Memory duration is a common distance settings (all zones, phase and ground, MHO and QUAD)

Memory Polarization jX ZL R ZS
Static MHO characteristic (memory not established or expired) ZL Dynamic MHO characteristic for a reverse fault Dynamic MHO characteristic for a forward fault Impedance During Close-up Faults ZS

Memory Polarization Memory Polarization…Improved Resistive Coverage jX
Static MHO characteristic (memory not established or expired) ZL Dynamic MHO characteristic for a forward fault RL ZS Memory Polarization…Improved Resistive Coverage

Choice of Polarization
In order to provide flexibility modern distance relays offer a choice with respect to polarization of ground overcurrent direction functions: Voltage polarization Current polarization Dual polarization

Ground Directional Elements
Pilot-aided schemes using ground mho distance relays have inherently limited fault resistance coverage Ground directional over current protection using either negative or zero sequence can be a useful supplement to give more coverage for high resistance faults Directional discrimination based on the ground quantities is fast: Accurate angular relations between the zero and negative sequence quantities establish very quickly because: During faults zero and negative-sequence currents and voltages build up from very low values (practically from zero) The pre-fault values do not bias the developing fault components in any direction

Distance Schemes Pilot Aided Schemes Non-Pilot Aided Schemes
(Step Distance) All protection schemes used in distance relaying fit into one of two categories. Pilot aided schemes and non-pilot aided schemes. The main difference between the two types of schemes is that the distance relays used in pilot aided schemes communicate with each other to determine whether the fault is located on the transmission line or not. The relays used in Non-pilot aided schemes do not communicate with each other and instead use delays and other forms of co-ordination to determine whether the fault is located on the transmission line or not. All co-ordination in these schemes is done within each individual relay itself. This seminar focuses only on Pilot Aided schemes. Communication between Distance relays No Communication between Distance Relays

Step Distance Schemes Zone 1: Trips with no intentional time delay
Underreaches to avoid unnecessary operation for faults beyond remote terminal Typical reach setting range 80-90% of ZL Zone 2: Set to protect remainder of line Overreaches into adjacent line/equipment Minimum reach setting 120% of ZL Typically time delayed by cycles Zone 3: Remote backup for relay/station failures at remote terminal Reaches beyond Z2, load encroachment a consideration

Step Distance Schemes Local Z1 Z1 Remote BUS BUS
On any distance scheme, any fault that occurs in the mid section of a transmission line that fits into zone 1 of both relays, will be cleared instantly by the local and remote relays located at each end of the line. BUS BUS Z1 Remote

Step Distance Schemes Local End Zone Z1 End Zone Z1 Remote BUS BUS
If the fault however is located on the end Zone of the local relay, it will not be cleared instantly by the local relay, but the remote relay will clear it instantaneously since this fault is in zone 1 of the remote relay. Without a pilot scheme, any fault in the end zone will thus not be cleared instantaneously at both ends of the transmission line. BUS BUS End Zone Z1 Remote

Step Distance Schemes Local Z1 Breaker Tripped Breaker Closed Z1
BUS BUS Breaker Closed Z1 Remote

Step Distance Schemes Local Z2 (time delayed) Z1 Z1 Z2 (time delayed)
The fault will not be cleared at the end zone of the local relay until the time delay in Zone 2 of that relay has expired, if a non-pilot stepped distance scheme was implemented. (Zone 2 is set normally to 120% of the total line impedance) The time delay for zone 2 operation is usually in the range of 250 to 400 milli-seconds. BUS BUS Z1 Z2 (time delayed) Remote

Step Distance Schemes … Z3 (remote backup) Z2 (time delayed) Z1 BUS
The fault will not be cleared at the end zone of the local relay until the time delay in Zone 2 of that relay has expired, if a non-pilot stepped distance scheme was implemented. (Zone 2 is set normally to 120% of the total line impedance) The time delay for zone 2 operation is usually in the range of 250 to 400 milli-seconds. BUS BUS

Step Distance Protection

Distance Relay Coordination
Over Lap Local Relay – Z2 Zone 2 PKP Remote Relay – Z4 Zone 4 PKP Local Relay Remote Relay

Need For Pilot Aided Schemes
BUS BUS Local Relay Remote Relay Pilot schemes speed up the clearing of faults that occur on the transmission line and inside the end zone of the local relay by communicating with the relay at the remote end of the line to determine if the fault is actually on the transmission line. Therefore, all pilot aided schemes require a communication channel be provided between the two relays. This communication between the relays can consist of a single channel or multiple channels depending on the particular application. Over this communication channel, the two relays share information regarding the fault allowing the clearing of faults on the transmission line to occur as fast as possible. Communication channels include power line carriers, Microwave radio channels, SONET channels to list a few. It is desirable to have a very secure and reliable communications channel for this purpose. Communication Channel

Pilot Communications Channels
Distance-based pilot schemes traditionally utilize simple on/off communications between relays, but can also utilize peer-to-peer communications and GOOSE messaging over digital channels Typical communications media include: Pilot-wire (50Hz, 60Hz, AT) Power line carrier Microwave Radio Optic fiber (directly connected or multiplexed channels)

Distance-based Pilot Protection

Pilot-Aided Distance-Based Schemes
DUTT – Direct Under-reaching Transfer Trip PUTT – Permissive Under-reaching Transfer Trip POTT – Permissive Over-reaching Transfer Trip Hybrid POTT – Hybrid Permissive Over-reaching Transfer Trip DCB – Directional Comparison Blocking Scheme DCUB – Directional Comparison Unblocking Scheme The most common Pilot aided schemes are : The DUTT scheme, which stands for Direct Under-reaching Transfer Trip The PUTT scheme which stands for Permissive Under-reaching Transfer Trip The POTT scheme, which stands for Permissive Over-reaching Transfer Trip The Hybrid POTT scheme which stands for the Hybrid Permissive Over-reaching Transfer Trip The Directional Comparison Blocking Scheme And the Directional Comparison Unblocking scheme We will discuss the POTT, DCB and DCUB schemes in the following sections.

Direct Underreaching Transfer Trip (DUTT)
Requires only underreaching (RU) functions which overlap in reach (Zone 1). Applied with FSK channel GUARD frequency transmitted during normal conditions TRIP frequency when one RU function operates Scheme does not provide tripping for faults beyond RU reach if remote breaker is open or channel is inoperative. Dual pilot channels improve security

DUTT Scheme Direct Under-reaching Transfer Trip
Zone 1 Bus Bus Line Direct Under-reaching Transfer Trip Under-reaching units at each end. Local Zone 1 PKP causes local trip Under-reaching units transmit on operation Received signal trips remote breaker Zone 1

Permissive Underreaching Transfer Trip (PUTT)
Requires both under (RU) and overreaching (RO) functions Identical to DUTT, with pilot tripping signal supervised by RO (Zone 2)

PUTT Scheme & OR Local Trip Rx PKP Zone 2 Zone 1
Permissive Under-reaching Transfer Trip Under-reaching units at each end Over-reaching units at each end Under-reaching units transmit on operation Received signal trips ONLY IF (permissive) local over-reach element is operated Trip = Local Zone 2 + Remote Zone 1 Rx PKP & Local Trip Zone 2 OR Zone 1

Permissive Overreaching Transfer Trip (POTT)
Requires overreaching (RO) functions (Zone 2). Applied with FSK channel: GUARD frequency sent in stand-by TRIP frequency when one RO function operates No trip for external faults if pilot channel is inoperative Time-delayed tripping can be provided

POTT Scheme Permissive Over-reaching Transfer Trip
Designed for two terminal lines Over-reaching units at each end Over-reaching units transmit on operation Received signal trips ONLY IF (permissive) local and remote over-reach element have operated Strategy: Communications channel established between relays sending fault/no fault status to the other relay. Both relays set up for zone 2. If they both see the fault, the fault is on the line. If only one relay sees the fault, the fault is behind the other relay.

POTT – Permissive Over-reaching Transfer Trip
POTT Scheme POTT – Permissive Over-reaching Transfer Trip End Zone The POTT pilot aided scheme stands for the Permissive Over-reaching transfer trip scheme; and like other Pilot aided schemes, is used to speed up the clearing of faults that occur in the end zone of a transmission line. As for all pilot aided schemes, a communication channel must be provided between the two relays located at each end of the transmission line for the POTT scheme to operate. BUS BUS Communication Channel

Communication Channel
POTT Scheme Local Relay FWD IGND Ground Dir OC Fwd OR Remote Relay FWD IGND Ground Dir OC Fwd OR Local Relay – Z2 ZONE 2 PKP Remote Relay – Z2 POTT TX ZONE 2 PKP TRIP Communication Channel In the POTT scheme, the Remote relay speeds up the tripping of an end zone fault by sending a permission to Trip key from the Remote relay to the local relay under 2 circumstances. ----The first reason that the remote relay will send a Permissive key is when the it detects a fault occurring within it’s over-reaching zone 2. This is where the expression “over-reaching” comes from in the term Permissive Over-reaching Transfer Trip. ---The second reason that the remote relay will send a permissive key is when it detects that ground directional overcurrent is flowing in it’s forward direction if this feature is enabled and configured. -----Therefore either Negative sequence directional overcurrent Forward element or the Neutral directional overcurrent Forward element, if configured, will send a POTT key to the Local relay, if both are configured,----- as well as when the over-reaching zone 2 pickup flag turns ON The Local relay POTT logic will only cause the breaker to trip ---if it gets the POTT key from the remote relay in the form of a receive AND, -----the local relay has detected a fault within it’s zone 2 area of protection OR, -----it detects that ground directional current is flowing in it’s Forward direction if this function is configured ----Therefore either the local relay’s Forward Negative sequence directional overcurrent element or the Forward Neutral directional overcurrent element as well as the picking up of a Zone 2 fault, will cause the POTT scheme to trip the breaker if it receives a permissive key from the remote relay. POTT RX Remote Relay Local Relay

Communications Channel(s)
POTT Scheme Communications Channel(s) POTT RX 1 POTT TX 1 A to G POTT RX 2 POTT TX 2 B to G POTT RX 3 POTT TX 3 C to G POTT RX 4 POTT TX 4 Multi Phase In conjunction with the Phase selector feature which determines which phase is faulted, the D60 distance elements can determine which phases of the transmission line are faulted. Therefore, the Remote D60 has the ability, -----through the communication key signals, to let the Local D60 know which phases are actually faulted. ------If the breakers used on the transmission line have the ability to trip single pole, the D60 can trip only the faulted pole of the breaker based on the received fault type. The POTT scheme in the UR D60 handles this signaling of which phase is faulted by having the ability of sending up to 4 different POTT key or transmit signals that are available in the relay. Therefore, in order to fully utilize this feature the, scheme would need to have a communication channel between the relays that can share more than one bit of information. If the communication channel you are using can only send and receive on piece of information such as a power line carrier can, which phase of the transmission line is faulted can not be sent and single pole tripping is less secure specially on evolving faults. Local Relay Remote Relay

POTT Scheme Current reversal example TRIP Some additional logic has been added to the POTT scheme to add extra security to transmission lines that are connected parallel to other transmission lines. ---For example if a fault had occurred on the paralleled transmission line as shown here, the local and remote relays will operate in the following way. --The remote D60 will detect that ground current is flowing in it’s transmission line in the Forward direction, and send a permissive key to the Local relay. ---The local D60 will detect that ground current is flowing in its reverse direction. This reverse ground over-current detection does not meet the criteria defined by the POTT scheme and thus the Local relay will not Trip. After a certain time period, the breaker on the parallel line correctly tripped to attempt to clear the fault. The ground current would now begin to flow through our transmission line in the opposite direction to feed that fault if the remote breaker on the opposite end if it did not open yet. The local and the remote D60’s would now operate in the following way. ----First the Local D60 would have its Forward Negative Sequence Directional overcurrent identify current the current is flowing in the Forward direction. ----At the same time the the local D60 identified the change in direction of ground current, the remote D60 will identify this change as well. However, due to the delay in the communications channel, the POTT permissive key will not immediately be removed. Since the Negative sequence directional element of the local D60 indicates that the fault is in the forward direction and the local relay is still receiving the permission key, the POTT scheme will cause the breaker to trip,---- shutting down the transmission line when it did not need to be. Local Relay Remote Relay Timer Expire GND DIR OC FWD GND DIR OC REV POTT RX Start Timer Communication Channel GND DIR OC REV POTT TX ZONE 2 OR GND DIR OC FWD

Communication Channel Communication Channel
POTT Scheme Echo example Remote FWD IGND POTT TX Remote – Z2 Open OPEN TRIP Communication Channel The POTT scheme has added one more feature to help to speed up the tripping of a faulted transmission line, if one of the ends is open. If the breaker on the local end of the transmission line is open for any reason, the local D60 will not detect any current flow into the transmission line and therefore will not detect any faults within it’s zones of protection. If a fault did occur on the line, no key will be sent to the remote D60. ----If a fault occurred in the remote D60’s end zone as seen here,--- the remote D60 will detect the fault in it’s zone 2 and send the POTT key to speed up tripping of the local D60. Since the local D60 will not send a key because it does not see the fault, the tripping of the remote breaker will not occur until zone 2 of that relay has timed out. The additional logic in the POTT scheme which is called the Echo function works in the following way. The Local D60 must first detects that it’s own breaker is open. If it’s local breaker is open, and it receives a POTT key from the remote D60, ----it will send the POTT key sequence it received directly back to the remote relay. The local D60 is telling the remote D60 that it is ok to trip because the local end of the line is already open and clearing of the fault will be much faster, ensuring the system will not become unstable. ----The remote relay will then take this echoed POTT key and trip it’s breaker. If the Echo function is going to be used, the Line Pickup protection element must first be configured. The description of the Line pickup function will be covered later in the course. POTT RX POTT TX POTT RX Local Relay Remote Relay Communication Channel

Hybrid POTT Intended for three-terminal lines and weak infeed conditions Echo feature adds security during weak infeed conditions Reverse-looking distance and oc elements used to identify external faults

Hybrid POTT & & Local Trip Remote Trip Rx Remote Zone 2 PKP 27
The scheme is intended for three-terminal applications and for weak-infeed conditions. - Fault occurres - Remote Zone 2 picks up - Local Zone 1 does not pick up because weak system in Zone 4 - Local Zone 4 does no pick up - Local under voltage pick up - Local trip - Transmits trip to remote relay - remote relay trips Local Trip Rx Remote Zone 2 PKP & Zone 4 PKP 27 Zone 2 PKP Remote Trip & Receives Trip from local relay

Directional Comparison Blocking (DCB)
Requires overreaching (RO) tripping and blocking (B) functions ON/OFF pilot channel typically used (i.e., PLC) Transmitter is keyed to ON state when blocking function(s) operate Receipt of signal from remote end blocks tripping relays Tripping function set with Zone 2 reach or greater Blocking functions include Zone 3 reverse and low-set ground overcurrent elements

DCB Scheme Typically applied on power line carrier. The line is sending the signal and is faulted therefor communications isn’t very reliable… we use this scheme even though it is slower then POTT because it will eventually trip line where as POTT requires a reliable comms. link. Operation: If local zone 2 operates and have not received a block from remote relay local relay will trip.

End Zone The Directional Comparison Blocking scheme that is available in the D60 is one of the most popular types of tele-protection schemes used in distance applications today. The purpose of the scheme is to still speed up the tripping of faults that occur in the end zone of a transmission line, just like the POTT scheme. As for all pilot aided 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. BUS BUS Communication Channel

Internal Faults Local Relay – Z2 Zone 2 PKP FWD IGND GND DIR OC Fwd OR TRIP TRIP Timer Start In the Directional Blocking scheme, the local D60 has an additional delay timer that is started by the detecting of a fault inside it’s 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 D60 will trip the local breaker unless the------Local D60 receives a block message or key from the remote D60. Expired Dir Block RX NO Local Relay Remote Relay

Directional Comparison Blocking (DCB) External Faults Local Relay – Z2 Zone 2 PKP Remote Relay – Z4 Zone 4 PKP FWD IGND GND DIR OC Fwd OR TRIP Timer Start No TRIP REV IGND GND DIR OC Rev OR The remote D60 will only send this blocking key if it detects that the fault is located in it’s zone 4 area of protection, or, it detects that ground current is flowing in the reverse direction. Both of which, would indicate an external fault. In the event that the communications channel failed, the local relay will misoperate. This is one of the disadvantages of the DCB scheme. Dir Block RX DIR BLOCK TX Communication Channel Local Relay Remote Relay

Directional Comparison Unblocking (DCUB)
Applied to Permissive Overreaching (POR) schemes to overcome the possibility of carrier signal attenuation or loss as a result of the fault Unblocking provided in the receiver when signal is lost: If signal is lost due to fault, at least one permissive RO functions will be picked up Unblocking logic produces short-duration TRIP signal ( ms). If RO function not picked up, channel lockout occurs until GUARD signal returns

DCUB Scheme

Directional Comparison Unblocking (DCUB)
End Zone The Directional Comparison Unblocking scheme that is currently not available in the D60, but will be implemented in the near future. This is a scheme that was developed to operate ONLY with FSK (Frequency Shift Keying) channels, like Power Line Carriers The purpose of the scheme is to still speed up the tripping of faults that occur in the end zone of a transmission line, just like the POTT and DCB schemes. As for all pilot aided schemes, a communication channel must be provided between the two relays located at each end of the transmission line for the Directional Comparison Unblocking scheme to operate. This channel is normally a single FSK power line carrier. This scheme utilizes principles from the POTT and DCB schemes, making it the most reliable scheme when the communications channel is a power line carrier. BUS BUS Communication Channel

Directional Comparison Unblocking (DCUB) Normal conditions Load Current FSK Carrier FSK Carrier GUARD1 RX GUARD1 TX Local Relay Remote Relay NO Loss of Guard GUARD2 TX GUARD2 RX NO Loss of Guard NO Permission NO Permission Communication Channel

Directional Comparison Unblocking (DCUB) Normal conditions, channel failure Load Current Loss of Channel FSK Carrier FSK Carrier NO RX GUARD1 RX GUARD1 TX Local Relay Remote Relay NO RX Loss of Guard Block Timer Started GUARD2 TX GUARD2 RX Loss of Guard Block Timer Started Block DCUB until Guard OK Expired Block DCUB until Guard OK Expired Communication Channel

Directional Comparison Unblocking (DCUB) Internal fault, healthy channel Local Relay – Z2 Zone 2 PKP Remote Relay – Z2 ZONE 2 PKP TRIP Z1 TRIP FSK Carrier FSK Carrier TRIP1 RX TRIP1 TX Local Relay GUARD1 RX GUARD1 TX Remote Relay TRIP2 TX TRIP2 RX GUARD2 TX GUARD2 RX Loss of Guard Permission Communication Channel

Duration Timer Started
Directional Comparison Unblocking (DCUB) Internal fault, channel failure Local Relay – Z2 Zone 2 PKP Loss of Channel Remote Relay – Z2 ZONE 2 PKP TRIP Z1 TRIP FSK Carrier FSK Carrier NO RX TRIP1 TX Local Relay GUARD1 RX GUARD1 TX Remote Relay TRIP2 TX NO RX GUARD2 TX GUARD2 RX Loss of Guard Block Timer Started Duration Timer Started Loss of Guard Communication Channel Expired

Redundancy Considerations
Redundant protection systems increase dependability of the system: Multiple sets of protection using same protection principle and multiple pilot channels overcome individual element failure, or Multiple sets of protection using different protection principles and multiple channels protects against failure of one of the protection methods. Security can be improved using “voting” schemes (i.e., 2-out-of-3), potentially at expense of dependability. Redundancy of instrument transformers, battery systems, trip coil circuits, etc. also need to be considered.

Redundant Communications
End Zone BUS BUS AND Channels: OR Channels: POTT Less Reliable POTT More Reliable Communication Channel 1 DCB More Secure DCB Less Secure Communication Channel 2 Loss of Channel 2 More Channel Security More Channel Dependability

Redundant Pilot Schemes

Pilot Relay Desirable Attributes
Integrated functions: weak infeed echo line pick-up (SOTF) Basic protection elements used to key the communication: distance elements fast and sensitive ground (zero and negative sequence) directional IOCs with current, voltage, and/or dual polarization

Pilot Relay Desirable Attributes
Pre-programmed distance-based pilot schemes: Direct Under-reaching Transfer Trip (DUTT) Permissive Under-reaching Transfer Trip (PUTT) Permissive Overreaching Transfer Trip (POTT) Hybrid Permissive Overreaching Transfer Trip (HYB POTT) Blocking scheme (DCB) Unblocking scheme (DCUB)

Security for dual-breaker terminals
Breaker-and-a-half and ring bus terminals are common designs for transmission lines. Standard practice has been to: sum currents from each circuit breaker externally by paralleling the CTs use external sum as the line current for protective relays For some close-in external fault events, poor CT performance may lead to improper operation of line relays.

Accurate CTs preserve the reverse current direction under weak remote infeed

Saturation of CT1 may invert the line current as measured from externally summated CTs

Direct measurement of currents from both circuit breakers allows the use of supervisory logic to prevent distance and directional overcurrent elements from operating incorrectly due to CT errors during reverse faults. Additional benefits of direct measurement of currents: independent BF protection for each circuit breaker independent autoreclosing for each breaker

Supervisory logic should: not affect speed or sensitivity of protection elements correctly allow tripping during evolving external-to-internal fault conditions determine direction of current flow through each breaker independently: Both currents in FWD direction  internal fault One current FWD, one current REV  external fault allow tripping during all forward/internal faults block tripping during all reverse/external faults initially block tripping during evolving external-to-internal faults until second fault appears in forward direction. Block is then lifted to permit tripping.

Single-pole Tripping Distance relay must correctly identify a SLG fault and trip only the circuit breaker pole for the faulted phase. Autoreclosing and breaker failure functions must be initiated correctly on the fault event Security must be maintained on the healthy phases during the open pole condition and any reclosing attempt.

Out-of-Step Condition
For certain operating conditions, a severe system disturbance can cause system instability and result in loss of synchronism between different generating units on an interconnected system.

Out-of-Step Relaying Out-of-step blocking relays
Operate in conjunction with mho tripping relays to prevent a terminal from tripping during severe system swings & out-of-step conditions. Prevent system from separating in an indiscriminate manner. Out-of-step tripping relays Operate independently of other devices to detect out-of-step condition during the first pole slip. Initiate tripping of breakers that separate system in order to balance load with available generation on any isolated part of the system.

Out-of-Step Tripping The locus must stay for some time between the outer and middle characteristics When the inner characteristic is entered the element is ready to trip Must move and stay between the middle and inner characteristics

Power Swing Blocking Applications:
Establish a blocking signal for stable power swings (Power Swing Blocking) Establish a tripping signal for unstable power swings (Out-of-Step Tripping) Responds to: Positive-sequence voltage and current

Series-compensated lines
Benefits of series capacitors: Reduction of overall XL of long lines Improvement of stability margins Ability to adjust line load levels Loss reduction Reduction of voltage drop during severe disturbances Normally economical for line lengths > 200 miles

SCs create unfavorable conditions for protective relays and fault locators: Overreaching of distance elements Failure of distance element to pick up on low-current faults Phase selection problems in single-pole tripping applications Large fault location errors

Series-compensated lines Series Capacitor with MOV

Series-compensated lines Dynamic Reach Control

Series-compensated lines Dynamic Reach Control for External Faults

Series-compensated lines Dynamic Reach Control for Internal Faults

Distance Protection Looking Through a Transformer
Phase distance elements can be set to see beyond any 3-phase power transformer CTs & VTs may be located independently on different sides of the transformer Given distance zone is defined by VT location (not CTs) Reach setting is in sec, and must take into account location & ratios of VTs, CTs and voltage ratio of the involved power transformer

Transformer Group Compensation
Depending on location of VTs and CTs, distance relays need to compensate for the phase shift and magnitude change caused by the power transformer

Setting Rules Transformer positive sequence impedance must be included in reach setting only if transformer lies between VTs and intended reach point Currents require compensation only if transformer located between CTs and intended reach point Voltages require compensation only if transformer located between VTs and intended reach point Compensation set based on transformer connection & vector group as seen from CTs/VTs toward reach point

Distance Relay Desirable Attributes
Multiple reversible distance zones Individual per-zone, per-element characteristic: Dynamic voltage memory polarization Various characteristics, including mho, quad, lenticular Individual per-zone, per-element current supervision (FD) Multi-input phase comparator: additional ground directional supervision dynamic reactance supervision Transient overreach filtering/control Phase shift & magnitude compensation for distance applications with power transformers

For improved flexibility, it is desirable to have the following parameters settable on a per zone basis: Zero-sequence compensation Mutual zero-sequence compensation Maximum torque angle Blinders Directional angle Comparator limit angles (for lenticular characteristic) Overcurrent supervision

Additional functions Overcurrent elements (phase, neutral, ground, directional, negative sequence, etc.) Breaker failure Automatic reclosing (single & three-pole) Sync check Under/over voltage elements Special functions Power swing detection Load encroachment Pilot schemes

Questions?

Fundamentals of Distance Protection

Similar presentations

Presentation on theme: "Fundamentals of Distance Protection"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Fundamentals of Distance Protection

Similar presentations

Presentation on theme: "Fundamentals of Distance Protection"— Presentation transcript:

Similar presentations

About project

Feedback