Protection principles

Analog to Digital ( A / D ) Conversion of Measuring Signals

Example of an A/D Conversion of a Sinusoidal Signal.

A voltage signal 10 sin ω t is sampled at a rate of 1 kHz (Sampling time Δ T = 1 ms) . ω = 2πf, with f being the power frequency = 50 Hz..

How does the output of a 12 bit (11 bits + sign) ADC look like ? Note : 1 ms for a 50 Hz system corresponds to 18 electrical degrees

Sampling Rates used in Siemens Numerical Protection

S.No
Relay Designation
Sampling Rate
1
7UT612
12 Samples / Cycle
600 Hz for 50 Hz system
2
7UT613
16 Samples / Cycle
800 Hz for 50 Hz system
3
7UT63
16 Samples / Cycle
800 Hz for 50 Hz system
4
7SJ61-64
16 Samples / Cycle
800 Hz for 50 Hz system
5
7SA
20 Samples / Cycle
1000 Hz for 50 Hz system
6
7SD
20 Samples / Cycle
1000 Hz for 50 Hz system
7
7SD
20 Samples / Cycle
1000 Hz for 50 Hz system
8
7UM
Depends on network frequency


Further Readings


The Art & Science of Protective Relaying 
By : C Russel Mason 

General Structure of a Numerical Protection Device

Equipment : Lines, cables, transformers, machines Processing : Digital Filters, Numerical Methods,Measuring Algorithms

Signal Conversion : CTs and VTs Signal Analysis : Comparison with Settings, grading 

Signal Tailoring : Signal matching, Anti-Aliasing Filters, A/ D Conversion

Analog to Digital ( A / D ) Conversion of Measuring Signals

Typical equipment data Transformer

Typical equipment data Transformer

UN1/UN2
SN
uK

ZTransf.




380 kV
110 kV
20 kV
380/110 kV
300 MVA
15 %
72 W
6 W
0.2 W
110/20 kV
40 MVA
15 %

45W
1.5 W
20/0.4 kV
630 kVA
6 %


37.0 W
Typical equipment data Line



R’1
X’1
Z’1
C’1
380 kV






Overhead line
0.03
+ j 0.25 W/km
0.25 W/km
14 nF/km
110 kV






Overhead line
Cable
0.07
0.04
+ j 0.38 W/km
+ j 0.11
W/km
0.39 W/km
0.12
W/km
10 nF/km
400 nF/km
20 kV






Overhead line
Cable
0.31
0.20
+ j 0.36 W/km
+ j 0.13
W/km
0.48 W/km
0.24
W/km
10 nF/km
300 nF/km
Planning of power system protection systems



Basic Protection Requirements

 Reliability
 dependability (availability) high dependability = low risk of failure to trip
 Security 
 stable for all operating conditions , high security = low risk of over-trip
 Speed
 high speed minimizes damage high speed reduces stability problems 
 Selectivity
 trip the minimum number of circuit breakers
 Sensitivity
 notice smallest fault value
Protected zone

  • To limit the extent of the power system that is disconnected when a fault occurs, protection is arranged in zones 
  • Zones of protection should overlap, so that no part of the power system is left unprotected 
  • Location of the CT connection to the protection usually defines the zone 
  • Unit type protections have clear zones reach e.g Diff. Relay, REF relay 
  • Zone reach depends on measurement of the system quantities e.g OC , EF, distance relays . The start will be defined but the extent (or ‘reach’) is subject to variation, owing to changes in system conditions and measurement errors
Criteria indicating fault condition

Reasons of Primary Protection Failure

Primary protections failure could be due to any of the following reasons :

1. Current or Potential Transformer failure
2. Loss of Auxiliary Control Voltage
3. Defective Primary Relays
4. Open Circuits in Control & Trip Coil
5. Failure of Breaker

It is therefore logical that back-up relays should not utilise any of the above items as common with primary relays.

Protection Concept

The system is only as strong as the weakest link!