Protection Challenges for Transmission Lines with Long...
Transcript of Protection Challenges for Transmission Lines with Long...
Protection Challenges for Transmission Lines with Long TapsPRESENTED BY: JENNY PATTEN, QUANTA TECHNOLOGY
AgendaEffects of infeed on apparent impedance
Real world examplesExample 1 – using communication-aided tripping to speed clearingExample 2 – impact of tap location on apparent impedanceExample 3 – system strength impact on apparent impedance
Fault locating challengesConclusions
Questions
Effects of Infeed on Apparent Impedance
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝐼𝐼𝐼𝐼𝐴𝐴𝐴𝐴𝐼𝐼𝐴𝐴𝐴𝐴𝐼𝐼𝐴𝐴 =𝑉𝑉𝐼𝐼
=0.5 ∗ 1 + (1 ∗ 1)
0.5= 3 𝑜𝑜𝑜𝐼𝐼𝑜𝑜
~ ~
Terminal A
Tap C
I = 0.5Z = 1.0
Terminal B
X
I = 0.5Z = 1.0
I = 1.0Z = 1.0
Effects of Infeed on Apparent Impedance
X
R
X No infeed
X with infeed
Effects of Infeed on Apparent ImpedanceInfeed increases apparent impedance to faultMakes fault appear further away
Must consider scenarios with weakened system
For lines with mutual couplingWhen determining apparent impedance for ground faults, consider scenarios
with mutually coupled line out of service or grounded
Real World Examples
Example 1 – Communication aided tripping
R
Packer
Line 1
18.38 ohms
17.96 ohms
Badger
Line B
R
Impedances in Ohms Primary
Line A
13.10 ohms
Line C
12.91 ohms115/69KV T1
Brewer
Line 219.86 ohms
11.5 mi9.1 ohms
8.9 mi7.0 ohms
15.4 mi16.7 ohms
Example 169 kV line
15 mile tap roughly in the middle of ~20 mile line Impedance from terminal to end of tap > impedance between terminals
without infeed
Path Apparent Impedance
Packer-Badger 16.1 ohms
Packer-Brewer (worst case) 62.5 ohms
Badger- Brewer (worst case) 42.5 ohms
Example 1Set Badger zone 2 reach at 130% of max apparent impedance
1.3 * 42.5 = 55.25 ohms
Fault at Bear
Fault on Buck 12kV
Example 1Since Zone 2 element sees through a distribution transformer, Z2 time delay must be increased to 90 cycles to coordinate with the transformer protection
Having such a slow clearing time for faults on the line increases risk of equipment damage
To speed clearing for faults on line, a POTT scheme over fiber was implementedAllows faster clearing for faults on the line while maintaining
coordination with surrounding protection
Example 2 – Impact of tap location on apparent impedance
Example 269 kV line
2.8 mile tap on ~24.5 mile lineTap is located 5% from the Cheddar end; 95% from the Colby end
Path Apparent Impedance
Cheddar - Colby 18.4 ohms
Cheddar – Muenster (worst case) 8.5 ohms
Colby – Muenster (worst case) 31.5 ohms
Example 2Set Colby zone 2 reach at 130% of max apparent impedance
1.3 * 31.5 = 40.9 ohms
Fault at Manchego
Example 2Colby Zone 2 overreaches remote lines, so Z2 time delay must be increased to coordinate
An additional forward zone (zone 4) was added that does not overreach any remote lines and uses a typical Z2 delay
Example 3 – system strength impacts on apparent impedance
Example 369 kV line
1.6 mile tap around the middle of a 4.3 mile line
Elm is a much stronger source than OakElm SIR = 0.43Oak SIR = 3.84
Path Apparent Impedance
Oak – Elm 3.3 ohms
Oak – Ash (worst case) 14.76 ohms
Elm – Ash (worst case) 2.83 ohms
Example 3Tap is long from Oak end, but not from the Elm endThe stronger source provides more infeed
Like Example 2, the Zone 2 setting at Oak is set to cover faults on the tap to AshZ2 delay coordinated with surrounding relaysZone 4 is implemented for faster clearing on as much of the line as
possible
Fault Locating ChallengesSingle-ended fault location methods used by microprocessor relays use the apparent impedance to calculate the fault locationWhen apparent impedance > line impedance the fault will look further
away If sum of the fault location from the relays at both ends of the line is
greater than the line length, then the fault is probably located on the tap
Two-ended fault location tools can improve fault location estimate
ConclusionInfeed for faults on a tap can make the apparent impedance for faults on the tap larger than the impedance of the main line section
Apparent impedance is affected byLength of tapLocation of tap along lineRelative strength of the line ends.
ConclusionRelays must be set to cover largest apparent impedance seen for a fault anywhere on the line In cases where the relay overreaches remote protection, time delays
should be coordinated
When very slow time delays are required for coordination, consider using communication-aided tripping
An additional “Short Zone 2” element that doesn’t overreach remote protection can provide faster clearing for portions of the line
Thank You!
Questions