International Journal of Electrical and Computer Engineering (IJECE) Vol.
No.
February 2017, pp.
ISSN: 2088-8708.
DOI: 10.
11591/ijece.
Reach and Operating Time Correction of Digital Distance Relay Sarwade1.
Katti2.
Ghodekar3 Departement of Electrical Engineering.
Dr Babasaheb Ambedkar Technological University.
Lonere.
India Retired Principal.
Govt College of Engineering.
Shivaji University.
India
Article Info
ABSTRACT
Article history:
Current and voltage signals recieved from conventional iron core Current Transformer (CT) and Voltage Transformer plays very important role for correct operation of Distance Distance Relay (DDR).
Increase in secondary burden connected to CT causes it to saturate at earlier stage.
The saturated CT produces distorted secondary current, causing DDR to under reach and to operate by certain time delay.
Rogowski Coils (RC.
are attaining increased acceptance and use in electrical power system due to their inherent linearity, greater accuracy and wide operating current range.
This paper presents use of RC as an advanced measurement device suitable for DDR.
Case study for validation of use of RC is carried out on low voltage system.
The simulation results of Distance protection scheme used for protection of part of 220kV AC system shows excellent performance of RC over CT under abnormal Received Oct 14, 2016 Revised Nov 15, 2016 Accepted Dec 8, 2016 Keyword:
CT saturation Distance relay
PSCAD-EMTDC
Rogowski coil Under reach Copyright A 2017 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Sarwade.
Research Scholar.
Departement of Electrical Engineering.
Dr.
Babasaheb Ambedkar Technological University.
Lonere.
Raigarh.
Maharashtra.
India.
Email: asarwade@yahoo.
INTRODUCTION
Power system is a complex interconnected network which consists of generation, transmission and distribution utilities.
Short circuit and other eccentric conditions which occur in the power system called faults .
According to statistical data about 70 to 80% faults on transmission line are single line to ground faults .
The performance distance relays (DR.
used for protection of transmission line when a fault occurs in the system is important for improvements in the stability of the system and reduction of their effect on sensitive loads.
Reducing the fault clearing time for more possible fault conditions is one of the main goals in the development, application and setting of such relays .
Fault occurred on transmission line produce very large and abnormal currents in the power system.
Traditionally normal and abnormal current measurement is accomplished with magnetic core Current Transformer (CT).
CT produces reduced current accurately proportional to current which can be conveniently connected to measuring and recording instruments.
But CT exposes a series of defects such as complex insulating structure, saturation potential and catastrophic failure due to secondary open .
CT saturation cause distance relay to see lower effective current than they would see and causes them to reach a shorter distance than they would, if there were no CT saturation.
This also causes the distance protection scheme to provide its trip decision with certain time delay .
To overcome this issue, a new measuring technique is required for measurement of current.
In order to use microprocessor-based or numerical relays, more advanced instrument current transducers must be introduced for measurements .
Rogowski Coil (RC) has attracted much attention of electric power industry as it can meet the requirements of protective relaying due to its superior performance, inherent linearity, outstanding dynamic response, wide bandwidth and no magnetic saturation.
The position of the primary conductor inside RC, magnetic field created by nearby conductors and harmonics will not create Journal homepage: http://iaesjournal.
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ISSN: 2088-8708
any type of deviation in RC output .
, .
High degree of selectivity and characteristics of protections which require current measurements can be increased significantly in the protection system with the help of RC.
So.
RC can be considered as alternative to conventional current transformers for applications in harsh operation environments .
So far RC is used as current transducer in differential and over current protection.
This paper presents use of RC as a best alternative to conventional CT in 220 kV distance protection scheme.
RESEARCH METHOD
This paper gives the comparision of the performance of 220 kV distance protection scheme when CT and RC are used as current transducers.
The stages involved while developing a distance protection scheme are shown in Figure 1.
The fault created on an AC system produces current and voltage signals with some The voltage signal is collected with the help VT and the current signal is collected with the help of RC, ideal CT and actual CT simultaneously.
In order to get correct value of the line impedance up to fault point, it is very essential to remove the transients and retain signals with fundamental frequency.
So these signals are further processed through signal processing stage which carries FFT module.
FFT module helps to obtain current and voltage signals at fundamental frequency .
By using these current and voltage signals, apparent impedances (Zap.
are calculated.
Finally these Zaps are fed to MDRs which compares these impedances with its setting and issues trip signal instantaneously or with some time delay.
Figure 1.
Distance Protection Scheme Stages Modelling of 220kV AC System The details of the 220 kV AC system (Figure .
is given in Table 1.
Line between bus A and bus B is protected by using MDR.
The line AB is divided in two parts as TLine1 and TLine2 to obtain its Zone 1 setting (Z1se.
The line lengths of these two parts can be varied to create a fault inside and outside of Zone 1 of line AB.
Single line to ground (SLG) fault is created with the help of time fault logic .
Figure 2.
PSCAD model of 220kV AC System Reach and Operating Time Correction of Digital Distance Relay (A.
Sarwad.
A ISSN: 2088-8708 Table 1.
220 kV AC System Details Parameter Source Voltage Source Impedance System MVA Length of AB Positive sequence impedance.
er k.
Zero sequence Impedance .
er k.
Load
compensation factor
Specifications 220 kV, 50Hz
15O850E
100 MVA
200 km 2928O86.
11O74.
MVA Modeling of Actual Current Transformer The actual CT with the following specifications is used (Figure 3 & Table .
Figure 3.
Actual CT Model Table 2.
CT Details Parameter CT ratio (CTR) Secondary winding Resistance (R.
Secondary winding Inductance (L.
Magnetic Core Area Magnetic Path Length CT Burden (Z.
Specifications 5 E 6x10-3 mm2 .
5 j0.
Modelling of Ideal Current Transformer The primary current is divided by number of turns which have been considered in actual current transformer, to get ideal value of secondary current (Figure .
Figure 4.
Ideal CT Model Modelling of Rogowski Coil The RC module & integrator with the following specifications is used (Figure 5 & Table .
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ISSN: 2088-8708
Figure 5.
Rogowski Coil Model Table 3.
Rogowski Coil Details Parameter Mutual Inductance L of Rogowski Coil R of Rogowski Coil C of Rogowski Coil Z of Rogowski Coil Specifications 2 AAH 8 mH 235 pF Parameter R of Integrator(Rin.
C of Integrator(Cin.
No of turns Output RMS
Rated Current
Specifications 100 E
1 AAF
100mV/1 kA
RESULTS AND ANALYSIS
Zone-1 setting (Z1se.
of MDR used for protection of line AB (Figure .
is given by Equation 1 .
Using Equation 1.
Z1set of MDR for 200 km line is done at 160 km .
% of 200 k.
To observe the under reach phenomenon of the MDR, line length of TLine1 is adjusted as 150 km (Figure .
The burden connected to CT secondary is increased to obtain CT saturation condition.
Use of ideal CT, actual CT and RC in distance protection scheme is analyzed with the help of secondary current waveforms.
B-H curves.
Zap trajectories and operating time of MDR.
Impact of CT Secondary Burden To observe the effect of unsaturated and saturated CT, the burden connected to its secondary is varied from 0.
5 E to 10 E.
Transient Response Figure 6a to 6e shows the secondary current waveforms generated by use of Ideal CT .
Actual CT .
and RC .
When the fault is created at maximum value of voltage (Vma.
, with relay burden (R.
5 E, it is observed that ideal CT, actual CT and RC produces symmetrical secondary current waveforms which are overlaying on each other (Figure 6.
With the same burden Rb, when the fault is created at zero voltage, the current waveforms found to be shifted upwards from the reference due to DC offset and some distortions are observed in secondary current waveforms produced by actual CT (Figure 6.
When Rb is increased to 2 E, 5 E and 10 E, it is observed that the actual CT secondary waveform obtains more and more clipped and distorted shape (Figure 6.
Ae 6.
), whereas it is found that RC transforms primary current faithfully on secondary side as its secondary current waveform overlaying on secondary current waveform produced by ideal CT.
Comparison of the secondary current root means square .
values at different burdens are given by Table 4.
It is observed that rms value of the secondary current produced by ideal CT and RC are approximately equal, but in case of actual CT it goes on reducing with increase in burden.
Table 4.
Secondary Currents at different CT Burdens Instant of Fault Relay Burden(R.
Without CT (A) With CT (A) With Rogowski Coil (A) v = Vmax Secondary Burden Reach and Operating Time Correction of Digital Distance Relay (A.
Sarwad.
A ISSN: 2088-8708 Figure 6.
Secondary Current Waveforms .
Rb = 0.
5 E & Fault at v=Vmax, .
Rb = 0.
5 E & Fault at v=0, .
Rb = 2 E & Fault at v=0, .
Rb = 5 E & Fault at v=0, .
Rb=10 E & Fault at v=0 B-H Curves of CT Figure 7.
B-H curves generated by magnetization of actual CT.
CT gives linear B-H curve (Figure 7.
), when the fault is created at Vmax with CT burden as 0.
With same CT burden, when the fault is created at v=0.
CT gets saturated (Figure 7.
CT goes in deep saturation when the burden is increased from 2 E to 10 E and it requires more magnetizing force to produce same amount of flux density (Figure 7.
After CT saturation, it is observed that, increase in CT burden increases magnetizing force required to produce same amount of flux density (Table .
Table 5.
B & H parameters at last saturation point with different burdens Instant of Fault Rb (Relay Burde.
B (Wb/m.
H (AT/.
v =Vmax 5E IJECE Vol.
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Figure 7.
B-H Curves at different Burden, .
Rb = 0.
5 E & fault at v=Vmax, .
Rb = 0.
5 E & fault at v=0, .
Rb = 2 E & fault at v=0, .
Rb =5 E & fault at v=0, .
Rb = 10 E & fault at v=0 Rogowski output voltage V-I Characteristics of Rogowski Coil (Case Stud.
Rogowski coil which was installed in Gujarat state for Electric Furnace purpose is shown in Figure 8 .
The results of the prototype installation for induction Furnace application are given in Table 6.
The input output characteristics of Rogowski coil is shown in Figure 8.
It is observed that the characteristics remain linear throughout the operating range of 0 Amp to 10 kA.
Input Current Figure 8.
Installation of Rogowski Coil for Electric Furnance Application and its V-I Characteristics Table 6.
Parameters observed on Input and Output side of Rogowski Coil Sr.
No.
Input Current Rogowski output voltage Output from Integrator Reach and Operating Time Correction of Digital Distance Relay (A.
Sarwad.
A ISSN: 2088-8708 Apparent Impedance Figure 9a to Figure 9e shows Zap trajectories with ideal CT .
, actual CT .
and RC .
along with Mho circle, when SLG fault is created at 150 km.
Before saturation of CT, it is observed that all the Zap trajectories are overlaying on each other (Figure 9.
Figure 9b-9e shows that the Zap trajectory .
is significantly impacted by the CT saturation.
To have a correct tripping of the relay.
Zap trajectory must fall inside Zone 1.
But when the CT gets saturated.
Zap trajectory lies outside of its Zone 1 boundary.
As the CT comes out from saturation state, the impedance seen by MDR matches the unsaturated plot.
Therefore.
MDR shows to have a tendency to under reach.
Table 7 gives the values of Zap obtained with different fault instants and increased burdens.
The clipping of secondary current due CT saturation increases the magnitude of impedance seen by Mho element.
It is observed that with increase in burden.
Zap increases.
Figure 9.
Impedance Trajectories on Mho Element with different Burdens .
Rb = 0.
5 E & fault at v=Vmax, .
Rb = 0.
5 E & fault at v=0, .
Rb = 2 E & fault at v=0, .
Rb =5 E & fault at v=0, .
Rb = 10 E & fault at v=0 Table 7.
Zap at different Burdens Fault instant Ideal CT With CT With RC v =Vmax 5E 63O79.
64O79.
62O79.
69O78.
06O65.
68O78.
69O78.
14O65.
68O78.
69O78.
24O63.
68O78.
69O78.
32O63.
68O78.
Operating time The operating time of a DR is considerable to make sure of high speed tripping.
Before CT saturation, all Mho relay elements issues their tripping signals at same instant (Figure 10.
When CT burden is increased from 2 E to10E.
CT goes in deep saturation.
This CT saturation process causes the Zap to lie outside of Zone1 for some time and to return back when CT comes out of saturation.
It delays Mho relay element operation connected to actual CT and result in slower than expected tripping times (Figure 10b-10.
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Table 8 gives the time required for the DR to operate, when the burden is increased from 0.
5 to 10E.
It is observed that increase in CT burden, increases the magnitude of the Zap, causing delay in the time of Figure 10.
Tripping Signals with different Burdens .
Rb = 0.
5 E & fault at v=Vmax, .
Rb = 0.
5 E & fault at v=0, .
Rb=2E & fault at v=0, .
Rb =5 E & fault at v=0, .
Rb = 10 E & fault at v=0 Table 8.
Tripping Times at different Burdens Fault Instant Relay Burden Without CT With CT With RC v =Vmax 5E Instantaneous Instantaneous Instantaneous 5 E Instantaneous After 0.
Instantaneous 2E Instantaneous After 0.
Instantaneous 5E Instantaneous After 0.
Instantaneous 10 E Instantaneous After 0.
Instantaneous CONCLUSION Low voltage case study and conducted simulations on 220kV AC system show use and importance of RC in digital DPS.
Influence of secondary burden of CT was investigated and it is proved that saturated CT produces a highly distorted secondary current.
After changing the burden from 0.
5 to 2.
5 a small indication of core saturation was observed for at least 6 cycles after the fault.
After setting burden to 10.
distortions were present during the whole simulation and they caused RMS current to be smaller than in fact it was.
This means that for a highly reactive fault path the current measured by a CT in the first few cycles is significantly smaller than the actual fault current.
This can cause the Distance Relay to under reach and trip after a longer period of time than it was originally anticipated.
Rogowski coil produces exact replication of primary current without distorting it with any load burden and prevent under reach phenomenon.
REFERENCES