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