A New Approach to Current Differential Protection for Transmission Lines

Transcription

1 A New Approach to Current Differential Protection for Transmission Lines

2 CURRENT DIFFERENTIAL MODEL Normal Condition: I 1 + I 2 = I C - the line charging current Fault Condition: I 1 + I 2 = I C Can be done on a per phase basis I c 2 2 I 1 I 2 I c 2

3 PERCENT RESTRAINT CHARACTERISTIC OPERATE I1 + I2 K1 I MIN RESTRAINT k*[ I1 + I2 ] 3

4 Discrete Fourier Transform vt () = V cos( ωt + φ) peak Vreal N 1 2 k = ( ) [ Vk cos( 2 π )] N N k = 0 Vimag 2 N 1 k = ( ) [ Vk sin( 2 π )] N N k = 0 4

5 TRADITIONAL ONE CYCLE FOURIER W1 W2 W1 W2 One cycle sliding Fourier window on fault waveform Magnitude Response of One Cycle Sliding Fourier Window 5

6 VARIABLE FOURIER WINDOW Fault Detection W1 W2 W2 W1 W2 W3 One cycle sliding Fourier window on fault waveform Magnitude Response of Dynamic Fourier Window 6

7 Phaselet Real = C X Phaselet Imaginary = S X Where : p pp k k= + pp P 1 p pp k k= + pp P 1 C k, Sk = Sine and Cosine Coefficients p = phaselet index: there are N / P phaselets per cycle P = the number of samples per phaselet X = kth sample of the input signal k PHASELET CALCULATION k k 7

8 Phasor Re aln Phasor Im aginary PHASOR CALCULATION Phaselets are converted to phasors by the following: n TRR( nw, ) TRI( n, W) PhaseletSumRealn = TIR( n, W) TII( n, W) PhaseletSumImaginary n Where: PhaseletSumReal = Phaselet Real n n p= n W + 1 P PhaseletSum Im aginary = Phaselet Im aginary n n p= n W +1 P n = Phasor index; W = Window size in samples p p 8

9 MIMIC ALGORITHM Time Domain: IZ() t = I() t R + d (()) It dt L Sampled Data: δ δ δ (( Ik+ ) + Ik ( )) (( IZ( k ) R Ik + ) Ik ( )) + = + L 2 2 δ 9

10 DUAL SLOPE PERCENT RESTRAINT OPERATE I1 + I2 Dynamic Restraint K2 K1 RESTRAINT k*[ I1 + I2 ] 10

11 GOODNESS OF FIT CALCULATION N 2 i= 1 i R R I I σ = x ( X PL + X PL ) 11

12 PERCENT RESTRAINT CHARACTERISTIC ON THE COMPLEX PLANE IMAG OPERATE REGION K1 Ir Iop REAL 12

13 ADAPTIVE RESTRAINT CHARACTERISTIC ON THE COMPLEX PLANE IMAG OPERATE REGION Iop REAL STEADY STATE RESTRAINT DYNAMIC RESTRAINT 13

14 ELLIPTICAL RESTRAINT CHARACTERISTIC IMAG OPERATE REGION DYNAMIC ELLIPTICAL RESTRAINT STEADY STATE CIRCULAR RESTRAINT Iop REAL TRADITIONAL CIRCULAR RESTRAINT 14

15 Time synchronization Internet Ping-Pong Relay 1 Relay 2 Forward travel time Return travel time t 0 t f t r t 3 time t f = t r = t 3 - t 0 - (t 2 - t 1 ) 2 t 1 t 2 Relay turn-around time 15

16 DIGITAL FLYWHEEL ω Virtual Shaft ω clock 1 clock 2 Sample clocks set via a 32 bit equivalent counter Counter is set as a result of thousands of Ping-Pong messages If communications is lost, sample clocks continue to free wheel Superior accuracy is obtained with the 32 bit counter. Long term accuracy is only a function of the base crystal stability. 16

17 Differential Path Delay Correction If t r = t f, this differential time delay results in a phase angle error in the measured phasor θ θ 2 θ 2 Communication path time differential is corrected via current measurement 17

18 Data from Relay #1 Data Packet Comparison Address Control Time Tag Data CRC Note: Only data with identical time tags are compared Data from Relay #2 Address Control Time Tag Data CRC 18

19 Communications Architecture Redundant communications with Hot Standby on 2 terminal applications: Communication link #1 Relay 1 Communication link #2 Relay 2 19

20 Communications Architecture Peer-to-Peer operation Each relay has sufficient information to make an independent decision Communication Redundancy Relay 1 Current Phasors communicated from each relay to every other relay Relay 2 Relay 3 20

21 Communications Architecture Master-Slave operation At least one relay has sufficient information to make an independent decision The deciding relay(s) sends a transfer-trip command to all other relays Relay 2 Data Relay 1 Transfer Trip message Relay 3 Data Relay 2 Relay 3 21

22 Communications Architecture Can support up to a 5 terminal line configuration with either Peer-to-Peer or Master/Slave or any combination thereof Relay 1 Relay 2 Relay 5 Relay 3 Relay 4 22

23 CHANNEL MEDUIM OPTIONS Channel Bandwidth Tripping Data Packets Op. Time kbps per cycle (cycle) Dedicated Fiber 64 Single phase 2 3/4-1 (0 to 40kM) RS-422 Sync. 64 Single phase 2 3/4-1 G.703 Sync. 64 Single phase 2 3/4-1 Note: The RS-422 and G.703 interfaces are designed to operate over digital multiplexed communication channels 23

24 Capacitive Error Current Compensation I C = C dv/dt Corrects sample data before a phasor is computed Compensates all frequency components - not just the fundamental x I1 I c 2 I c 2 I2 24

25 SUMMARY Enhanced Performance Time Synchronization - Communication Path Delay Adjustment - Differential Communication Path Delay Redundancy for Loss of Communications Flexible Communication Configurations Communications Media Options 25