1. Research & production enterprise «EKRA», Ltd,
Peculiarities of longitudinal line differential protection with digital link
A.N. DONI, N.A. DONI,
Research & production enterprise «EKRA», Ltd,
Russia
2009

2. Operating principle
The operating principle of LDP is based on comparison of current phasors at ends of protected power line in normal and fault states. The current phasor and restraint measurements are sent from one line end to the opposite via digital communication links.

3. Connecting current circuits of LDP
LDP connects to current transformers at substations with following configurations:
one breaker per connection,
two busbar systems with bypass,
two breakers per connection (two current groups),
two breakers and reactor on power line (three current groups)
Digital communication links between protections
two synchronous communication links (CL) 64…512 Kbps,
automatic switching of link in case of link failure,
connecting to protection via multimode fiber (ST connector),
admissible data deference for CL – 30 ms (in one direction),
compensation of fixed CL asymmetry – up to 5 ms

4. Using dedicated optical fiber

5. Using multiplexers

6. Direction of currents for a two-end line
а – normal load current or external fault condition;
b – fault on protected line;
c – fault on power line in case of single-way feed. Direction of currents same as for external fault and normal load

7. Influence of communication link asymmetry and power flow on position of characteristic points in the complex current ratio plane
a=Re(I1/I2),
b=Im(I1/I2)
d =9000 * td, 
 - phase difference
of equivalent EMF at line ends

8. Dynamic errors of real CTа) b) c)
a – secondary current, А
b – secondary current fundament harmonic RMS error (per unit)
c – degree error (degrees)

9. A1=-(1+k)/(1-k), A2=-(1-k)/(1+k), k – restraint coefficient
LDP blocking and operating characteristics
|I1+I2| > I0d ;
1- |I1+I2| > k|I1-I2| – circle zone;
2- |I1+I2| > k(|I1|+|I2|) – cardioids;
3- A2<|I1 / I2|<A1,
( - BL ) < arg (I1 / I2)< ( + BL ) – sector or “rainbow”
A1=-(1+k)/(1-k), A2=-(1-k)/(1+k),
k – restraint coefficient

10. Angle characteristics of LDP blocking at different k values
1 - circle
2 - cardioids
3 - sector

11. Analysis of LDP operation for internal and external faults

12. Capacitive current compensation
When installed on cables and long power lines, LDP performs equalization of currents applied to protections at both line ends during external faults. The equalization is carried out by electric “transposition” of CTs to mid-line using capacitive current compensation scheme.

13. T-line application
The additional impedance functions and zero sequence directional element immune to inrush provide for correct operation of LDP for T-line arrangement schemes.

14. Additional functions
receives and sends up to 20 binary signals,
fault locator two-way measurement without use of zero sequence components,
set of time-graded backup protections:
-5 distance zones, including distance function for ground faults,
-5 steps of current directional zero sequence function,
-power swing blocking:
current rate of change measurement,
DZ / Dt measurement,
-voltage circuits failure monitoring,
-speedup logic,
-current cutoff function,
directional current load shedding,
breaker failure protection;

15. Operating characteristics of distance function
Characteristic for DZ/Dt power swing blocking

16. Field experience data for LDP БЭ2704v091
Line 110 kV Tolyatti Heat Station – Levoberezhnaya (Stavropolskaya-3). Length 10 km. September 2007.
Line 110 kV Tsentralnaya – Magistralnaya (Kazan.) Length 9 km. December 2007.
The monitoring of protections is remotely performed over GSM channels.
In the course of monitoring some 50 external faults with currents measured at A were recorded. One of the faults was three-phase. One of the faults was on secondary side of tapped transformer.
No failure in optical communication links was recorded.
The experience results are positive.
The LDP with trip are installed on 11 power lines at 110 kV voltage.

17. Real case external three-phase fault on 110 kV power line

18. Thanks for your attention!