Line protection
MEASURING PRINCIPLESCommunication schemes
The Distance protection relays can communicate in “Permissive”
or “Blocking” schemes.
InPermissive schemes an acceleration signal “CS” is sent to the remote end, when the fault is detected “forward”. Tripping is achieved when the acceleration signal “CS” is received if the local relay has detected a forward fault as well.
Two main types of permissive schemes exist, see figure 12:
1) Underreaching schemes, where the acceleration signal is sent from a zone underreaching the remote end, usually zone 1, “Z1”.
2) Overreaching schemes, where the acceleration signals are sent from one overreaching zone, usually zone 1 “Z1” or zone 2 “Z2“overreaching the remote end A directional start can also be used.
The overreaching schemes are normally used for short lines (<15 km) to improve the resistive coverage.
Receiving the carrier signal will, dependent of the selected scheme mean a time acceleration of Zone 1 (at Z1 overreach), or Zone 2 or Zone 3.
MEASURING PRINCIPLES
Line protection
Figure 12. Communication with remote end, in Permissive schemes or Blocking schemes. CS from Underreaching zone or Overreaching zone.
In Blocking schemes a blocking signal “CS”, is sent to the remote end when the fault is detected to be reverse. Tripping is achieved when the acceleration signal “CS” is not received within a time of T0 and if the local relay has detected a forward fault as well. A time margin T0 of 20-40ms to check if the signal is received is al-ways needed in a blocking scheme.
The accelerated tripping after “T0” are from “Z2” or “Z3”.
If different types, or manufacturer, of Distance protection relays are used at the two line ends blocking schemes should be used only after checking that the relays will operate together. A relay
Z<
Z<
Carrier send CS = Z< Forward, under or overreach Trip = Z< Forward Z1, Z2 or Z3 & Carrier received
Z<
Z<
Permissive system
A B
Carrier send CS = Z< Reverse zone
_____________
Trip = Z< Forward Z1, Z2 or Z3 & Carrier received &T0 Blocking system
MEASURING PRINCIPLES
where a blocking signal is sent for “start” but not “forward” should, as a general principle, never be used together with a relay with a true reverse directional element sending the blocking signal.
The using of Permissive schemes or Blocking schemes in the system depends on the preference for Security or Dependability.
A Blocking scheme will be Dependable, i. e. it will operate for an internal fault also with a failing communication link, but it has a lower security as it can maloperate for an external fault due to a failing communication link. The Permissive scheme has the op-posite behavior. The advantage with the blocking scheme is that communication signals are sent on healthy lines whereas on per-missive scheme the communication signals are sent over a faulty line.
Reasons for incorrect impedance measurement
To enable a correct impedance measurement the measured volt-age must be a function of only the locally measured current “IA” and the impedance at the fault. This is naturally not always the case in double-end infeed and meshed transmission networks.
REMOTE FAULTS If a fault occurs on an outgoing line in the re-mote substation where the own line will feed fault current “If1”, see fig 13, the other lines in the remote station will also contribute with the fault currents “If2”and “If3”. The measured impedance at the local station will then be as in the figure and the measured im-pedance at the fault will seem much higher than the “true” imped-ance to the fault. The relays will thus get an apparent underreach.
This means that, in practice the possibility to get a “remote back-up” in a transmission network is limited. A local back-up must therefor normally be provided.
MEASURING PRINCIPLES
Line protection
Figure 13. The Distance protection underreach at a remote fault.
HIGH RESISTIVE LINE FAULTS At high resistive line faults on a transmission line with double-end infeed a similar situation will occur. In normal service a load current “Ib” flows through the line.
The current level is expressed:
If1
If2
If3 If1+If2+If3 Z<
If1 ZL
Um
Measured voltage ZF
Um=If1*ZL+(If1+If2+If3)*ZF Measured impedance
Zm=Um/Im If
R [ ] X
[ ]
ZL
Load [Ω]
[Ω]
Ib
EA –EB ZL
---=
MEASURING PRINCIPLES
“UA” and “UB” must have a phase difference to allow the current to flow. This, if the voltage amplitude is the same, which is a “nor-mal case” in the transmission network. When a fault occurs, the currents “IA” and “IB”, are lagging “EA” respectively “EB”, and have therefor also a phase difference compared to each other.
The resistance “R” at the fault will be seen as a resistance plus a reactance reduction (at export) and a reactance addition (at im-port).
Figure 14. Measuring error at a high resistive earth fault in a line with double side infeed.
The measuring error will be:
At the two line ends (see fig), “Θ” is the phase angle difference between “A” and “B” stations. The exporting end will thus over-reach i. e. a fault at the line end can be seen as an internal fault,
UB
IA IB
–---RfsinΘ IB
– IA
---RfsinΘ
MEASURING PRINCIPLES
Line protection
and the import end will underreach i. e. a fault will seem further off than it is in reality.
If the power direction always is the same a compensation can be made when setting the relays. The earth fault loop setting must especially be considered as the high resistive faults normally only occur at earth faults only.
PARALLEL POWER LINES When parallel lines at the same tower are used there will be a mutual impedance, normally only of in-terest when an earth fault occurs. Normal values of the mutual impedance at earth faults for transmission line towers are “Zm”=
“0,5-0,6xZ0”.
The mutual impedance will for earth faults mean over- respective-ly underreach for the two line ends. The over- or underreach is dependent on the fault position and the zero sequence sources at the two line ends. The level of overreach resp underreach in the measurement is in the range up to 16%.
Different situations will occur when:
- Parallel lines are in service.
- Parallel lines are out of service and earthed.
- Parallel lines are out of service (floating).
Compensation have to be done for the different cases when set-ting the relay earth fault reach. The “KN” factor is adjusted to achieve a suitable setting. Normally the worst case is selected and the “KN” is set to prevent overreaching, and thus unneces-sary operation. The overreach is caused by the parallel line out of service and earthed at both ends and the factor KN should be set as:
MEASURING PRINCIPLES
When the parallel line is out of service but ungrounded the reach will be reduced with a factor k1. This factor should be checked and the overlapping of zone 1 from both end at underreaching schemes should be verified.
The setting of KN factor for zones 2 and up should always be as the normal setting with an extended factor of 1,25 to cover a worst case which occurs when no fault current is fed in at the re-mote terminal.
Note that the change of reach for parallel lines with compensated KN is only valid for single phase faults. For multi phase faults the reaches are not influenced.
Figure 15 shows the principle of the mutual impedance.
Figure 15. Influence of mutual impedance, at parallel lines, at the same tower.
MEASURING PRINCIPLES
Line protection
Special functions
Some special functions are of interest with Distance protection relays and should be mentioned.
SWITCH ONTO FAULT (SOTF) At energizing a power line onto a for-gotten earthing, portable or fixed, no measuring voltage will be available and the directional measuring can thus for three phase faults not operate correctly. A special SOTF function is thus pro-vided in Distance protection relays. Different principles can be utilized, from an one phase current/one low voltage measure-ment to an undirectional impedance measuring as per figure 16.
The SOTF function is connected for some second/s only when energizing. The criteria that there is a SOTF condition can either be taken from the manual closing signal (called DC SOTF) acti-vating an input in the relay, or can be detected internally by the relay (AC SOTF) where a no voltage-no current condition for a certain time is taken as confirmation that the line is dead.
Figure 16. Switch Onto Fault function ensures fast tripping when energizing a line onto a forgotten earthing.
Z<
U=0 V If
R [ ] X [ ]
ZL
R [ ] X [ ]
ZL [Ω]
[Ω]
MEASURING PRINCIPLES
POWER SWING BLOCKING (PSB) FUNCTION
Power swings can occur at double end in-feed networks. A Power swing can be started by a sudden load change, due to e. g. man-ual switching, or at a fault somewhere in the network.
Close to the centre of the power swing low voltages and thus low impedances will occur. A Distance protection relay can thus op-erate at a power swing and this is in most countries not accepted.
A Power Swing Blocking device is available for all schemes. The most used principle used is to “measure the speed of the imped-ance locus”. This is done by two impedimped-ance circles, or rectan-gles, and a measuring of the time between passing the outer and the inner line. Normally the time used is 35-40ms.
Exceeded time will make an alarm telling that a Power swing has occurred. A blocking of the Distance protection zones will then be made. Normal power swings in networks have a swing cycle of 0.5-10 seconds.
Figure 17. Power Swing Blocking function. Two rectangles can be provided and the time between passing the outer and inner rectangle is measured.
STUB PROTECTION FUNCTION In one and a half and ring busbar arrangements the voltage transformer will be located outside the
R [ ]
MEASURING PRINCIPLES
Line protection
line disconnector but the bays can be in full service with the breaker closed even if the line disconnector is opened e. g. due to make maintenance on the power line.
A fault in the line bay section, will not be possible to detect by the distance protection. It’s also a risk of incorrect directional mea-surements due to induced voltages or back-feed of voltage from the remote end. The Distance protection directional impedance measuring is blocked and a so called Stub protection is intro-duced. The Stub protection is a simple current relay activated only when line disconnector is open.
Figure 18. Stub protection function protects the line exit when the line disconnector is open.
MEASURING PRINCIPLES
3.4 APPLICATION PROBLEMS
A number of interesting application problems occurs in a trans-mission line. Among them should be mentioned: Current reversal In the quite common “parallel line applications” current reversal can occur. The principal problem is shown in figure 17 and an ex-ample of the logic used to ensure correct operation is shown in figure 18
Figure 19. Fault current reversal occurring, with parallel lines, at the same tower.
Figure 20. Fault current reversal logic for overreaching scheme in RELZ 100/REL 511/521./531 “ZM2” is measuring zone 2, “ZM3R” is Zone 3 re-verse and “CR” is carrier receive.
When a fault occurs at a line, on a parallel connection, the fault will always be cleared from one line end first. When the first breaker opens, the fault current in the parallel line will have a change of direction and if nothing is done in the logic for commu-nication, maloperation can occur on the parallel healthy line. It should be noted that the problem occurs for overreaching schemes only.
MEASURING PRINCIPLES
Line protection
SIMULTANEOUS FAULTS In the quite common “parallel line appli-cations” simultaneous faults can occur. A fault can occur between
“L1” and “L2” but the phases are at different lines. A full scheme relay is a must to give correct operation for simultaneous faults as the measuring loops for earth faults must detect one forward- and one reverse fault in different phases and a logic must be provided in the relay to get the correct operation. Many Distance protection relays (even of new types), will risk maloperation at these faults and care must also be taken when the relay zones and the phase selection are set.
Switched schemes can due to the starter function not operate correctly for simultaneous faults. The starter will select both in-volved phases to the measuring element on both inin-volved lines distance protection relay. The measurement will then naturally be incorrect.
Figure 21 shows the principle for simultaneous faults. The new Distance protection relays from ABB Network Partner provides a logic to cover the problem.
Figure 21. Simultaneous faults, in parallel lines, at the same tower.
SERIES CAPACITORS The use of forward impedance measurement using the reactive characteristic of the transmission lines, implies that Distance protection relays can’t be used in series compen-sated lines without special care. Experts must be consulted for such applications. This is also valid for surrounding lines in the same station.