design a fast digital protective relay algorithm for high voltage

DESIGN A FAST DIGITAL PROTECTIVE RELAY ALGORITHMFOR HIGH VOLTAGE TRANSMISSION LINE

DESIGN A FAST DIGITAL PROTECTIVE RELAY ALGORITHM FOR HIGH VOLTAGE TRANSMISSION LINE Auday A.H. Mohamad1 and Essar Gafar Ahmed2 1

Computer Technology Engineering Department, AL-Mansour University College, Baghdad-Iraq 2 Electrical and Electronics Engineering Dept. Omdurman Islamic University, Khartoum-Sudan

Received Feb. 2014, accepted after revision March 2014

‫ســت َْخـلـَص‬ ْ ‫ُمـ‬ .‫ػًهٍت ًَذجت يزحالث انحًاٌت يفٍذة جذا يٍ انُىاحً االقخصادٌت ألجم حقٍٍى أداء انًزحالث وأَظًت انحًاٌت‬ ‫( بىاسطت بزَايج‬MHO) ‫هذِ انىرقت حؼزض يقخزح جذٌذ نخصًٍى خىارسيٍت يزحم حًاٌت يسافٍت َىع‬ ‫) وانخً ًٌكٍ بىاسطخها حقذٌز‬FFT( ‫ انخىارسيٍت انًقخزحت يبٍُت ػهى ححىٌم فىرٌٍز انسزٌغ‬.(MATLAB) ‫ إضافت إنى حُؼٍى‬،‫انقًٍت انفؼهٍت نهًزكبت انصفزٌت انخً ححذد خالل فخزة انؼطم وإسانخها حًايا إٌ وجذث‬ ‫ حى اخخبار أداء انخىارسيٍت انًقخزحت بىاسطت بٍاَاث يحاكاة نخط َقم يزوي‬.‫انخشىٌش انًىجىد فً اإلشارة‬ 3(PSCAD/EMTDC) ‫ كى ػٍ طزٌق بزَايج انًحاكاة‬7.632 ‫ هزحش وطىل‬05 ‫ كٍهىفىنج بخزدد‬055 ‫ػطبزة‬ ‫( يٍ خالل يجًىػت يٍ األػطال‬MATLAB) ‫انبزَايج انًسخخذو نخصًٍى وحقٍٍى انخىارسيٍت انًقخزحت هى‬ ‫انخً ححاكً األػطال انخً ًٌكٍ أٌ ححذد فً أَظًت خطىط َقم انطاقت فً انؼذٌذ يٍ انظزوف يثم األػطال‬ ‫ ػٍُاث يٍ َخائج هذِ انذراساث حظهز‬.‫فً يىاقغ يخخهفت فً خط انُقم وقٍى يخخهفت نًقاويت وسواٌت انؼطم‬ ٍ‫( يٍ حٍذ سي‬PSCAD/EMTDC) ‫انًقارَت بٍٍ انخىارسيٍت انًقخزحت وانخىارسيٍت انًىجىدة فً بزَايج‬ .‫انفصم‬ Abstract Modeling of protective relays is economical and feasible alternative to investigate the performance of relays and protection systems. This paper presents a new approach for MHO Relay Algorithm in MATLAB based on Fast Fourier Transform Algorithm (FFT) which can estimate exact magnitude of DC offset component and completely eliminates it from operating quantities during faults and also makes use of smoothing window to filter out noise if any. The proposed Numerical Algorithm performance is tested on simulated transmission line of Merowe-Atbara 500 KV, 50 Hz, and 236.7 Km using data generated by PSCAD/EMTDC. The proposed MHO algorithm evaluated by using MATLAB to models a power system and simulates many fault conditions on a selected transmission line such as different Fault locations, resistances with various fault angles. Sample results of these studies show the comparison between the presented Algorithm and PSCAD/EMTDC in detection and tripping time. Keywords: relays and protection systems, (FFT) Algorithm, PSCAD/EMTDC, MHO.

1

INTRODUCTION

Relay models helps engineers and consultants to select the relay types suited for a particular application and to analyze the performance. Researchers use relay model to investigate and improve protection design and algorithms. Instead of

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using actual prototypes, manufacturers use relay models to expedite and economize the process of developing new relays. Electric power utilities use relay models to confirm how the relay would perform during systems disturbances and normal operating conditions and to make the necessary

Sudan Engineering Society Journal,

March 2014, Volume 60; No.1

Auday A.H. Mohamad and Essar Gafar Ahmed

corrective adjustment on the relay settings. The Computer models of relays permit investigators to observe in a very detailed way the performance in each internal module of the relay [1].

2 ESSENTIAL QUALITIES OF PROTECTIVE RELAYING A protective relaying scheme should have certain important qualities, such an essential qualities of protective relaying are [2], [3]:  Reliability.  Selectivity and discrimination.  Speed and time.  Sensitivity.  Stability.

3 MODELING OF PROTECTIVE RELAYS [4] The most important advantage of using relay models, however, is that the models allow the user to observe the processing of inputs signals in a very detailed manner during the relay operation. Several techniques for modeling numerical relays have been developed in the past. In most modeling approaches, the interfacing of the models with an electromagnetic transient program is important for making the models more useful for the protection engineers. PSCAD/EMTDC, developed by the Manitoba HVDC Research Center, is an electromagnetic transient analysis program that uses a graphical user interface for constructing input data files. This approach eliminates the chances of either not providing the required data or the data being out of the normal range. More recent developments have interfaced the EMTP with FORTRAN, EMPT with MATLAB, ATP with MATLAB, and PSCAD/EMTDC with MATLAB for enhancing their abilities for processing the generated

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numerical data with signal processing techniques.

3.1 Numerical Relay Modeling Modeling of numerical relays is important for the protection industry because it allows the users to observe the internal performance of relays during normal operating states of the power system as well as during system disturbances. Relay models are used in a variety of processes, such as designing new prototypes and selecting appropriate protection algorithms, setting relay parameters, and training personnel [5].

3.2 Phasor Estimation Algorithms The estimated phasors of voltages and currents are used in the implementation of protection algorithms in numerical relays. The ratio of appropriate voltages and currents then provide the impedance to the fault. The performance of all of these algorithms is dependent on obtaining accurate estimate of the fundamental frequency component of a signal from a few samples [6]. The algorithms are classified according to the approach used to calculate the impedance based on the voltage and current measurements [5]. A phasor is a representation of a sinusoidal voltage or current of the nominal frequency, f 0 and its positive going zero crossing is  radians ahead of the time equal to zero. The mathematical representation of a phasor is as follows [7]. V  V e j  V (cos   j sin  )

(1)

The real and imaginary parts of the phasor are expressed as follows. Re(V )  V . cos 

(2)

Im(V )  V . sin 

(3)

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DESIGN A FAST DIGITAL PROTECTIVE RELAY ALGORITHMFOR HIGH VOLTAGE TRANSMISSION LINE

The magnitude and phase of the phasor can be calculated using the real and the imaginary parts of the phasor as follows [7]. V  Re(V ) 2  Im(V ) 2

 Im(V )    Re(V ) 

(4) (5)

  tan 1 

Discrete Fourier transform (DFT) is generally used to calculate the phasor of the fundamental frequency component in digital protective relays. Fast Fourier Transform FFT The FFT is simply an algorithm to speed up the DFT calculation by reducing the number of multiplications and additions required. The Fast Fourier Transform (FFT) which requires only ( N / 2) log 2 ( N ) complex multiplications. The computational efficiency of the FFT versus the DFT becomes highly significant [8]. The FFT equation can be defined as: X (k ) 

1 N

N 1

 x(n)W n 0

nk N

(6)

The output of the FFT is X (k ) , contains a real and imaginary component that can be converted into amplitude and phase from equation (4) and (5).

4 DISTANCE PROTECTION RELAYING A distance relay responds to input quantities as a function of the electrical circuit distance between the relay location and point of faults. There are many types of distance relays, including impedance, reactance, offset distance, quadrilateral, self-polarize, and MHO [7]. Distance Relaying determines the fault impedance from the measured short circuit voltage VR and current IR at the relay location as shown in Figure 2. The relay measured fault impedance and then compares it with known line impedance, if the measured fault impedance is smaller than the set line impedance, an internal fault is detected and a trip command issued to the circuit breaker [3].

 j 2

Where: WN  e

N

The radix-2 FFT algorithm breaks the entire DFT calculation down into a number of 2point DFTs. Each 2-point DFT consists of a multiply-and-accumulate operation called a butterfly, there are two representations of the butterfly as shown in figure 1 [8].

(a) (b) Figure 1: Butterfly Computation in the Decimation-in-Time FFT Algorithm, (a) Actual Functional Representation; (b) Simplified Diagram [9].

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Figure 2: Distance Protection Principle, Measurement of Fault Impedance [7].

4.1 Comparators for Distance Protection Relay measuring elements whose functionality is based on the comparison of two independent quantities are essentially either amplitude or phase comparators [2]. A phase comparator checks the difference between the phase angles of the two composite signals and operates if the difference is within a specified range. A magnitude comparator compares the amplitude of the two composite signals and operates if the amplitude of one signal is greater than the amplitude of the other signal [9].

Sudan Engineering Society Journal,

March 2014, Volume 60; No.1

Auday A.H. Mohamad and Essar Gafar Ahmed

4.2 Input Signals of Distance Relays

S 2  Vr 0 o

In Table 1 are shown the input signals employed by ground and phase distance relays. In this Table, K0 is a compensating factor. The determination and use of K0 is explained later [1], [5], [8].

Dividing these equations by the line current I r r give the following equations:

Table 1: Input Signals of Ground and Phase Distance Relays [1], [5], [7] and [9] Fault Type Phase to Ground Phase to Phase

Distance Element Phase A Phase B Phase C Phase A - B Phase B - C Phase C - A

Voltage signal Va Vb Vc Va - Vb Vb – Vc Vc – Va

Current signals Ia + K03I0 Ib + K03I0 Ic + K03I0 Ia - Ib Ib – Ic Ic – Ia

The composite signals in a phase comparator are denoted by S1 and S2. An angular displacement is considered positive if S1 leads S2.The output of a phase comparator operates if the following condition is satisfied [7], [9].  90  S1  S 2  90

(7)

(10)

S '1  Z r  r  Z R  z

(11)

S ' 2  Z r  r

(12)

As seen in Figure 3, the impedances S ' 1 and S ' 2 are placed in the extremes of the constant impedance Z R  z .When the system impedance Z r  r is inside the operating characteristic, as shown on Figure 3(a), the angle between S '1 and S ' 2 fulfills equation (7) and the relay operates. In Figure 3(b) is shown the case of Z r  r lying outside the operating characteristic. Now, the angle between S ' 1 and S ' 2 is outside the range specified in Equation (7), and the relay does not operate. The constant parameter Z R  z marks the diameter of the circular characteristic that passes through the origin.

The composite signals in an amplitude comparator are denoted by S0 and S R , operating and restraining signals, respectively. The comparator operates if the following condition is satisfied [7], [9]. SO

 SR

(8)

4.3 Distance Relay Characteristics [9] MHO relay have been widely deployed worldwide. The methods used for obtaining MHO operating characteristics by the phase and magnitude comparators are presented in the following. MHO Characteristic Phase Comparator The phase comparator signals S1 and S2 for producing the MHO characteristic are defined as follows [7]: S1  Vr 0 o  Z R  z .I r    r

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(9)

(a) (b) Figure 3: Definition of the MHO Characteristic Phase Comparator (a) Operating Condition, (b) Non-operating condition [9], [10].

4.4 Computation Impedance

of

the

Apparent

In the case of phase distance relays, phaseto-phase voltages and differences between line currents are used. For example, a relay designed to detect phase-B to phase-C faults computes the impedance as expressed in the following equation (13) [9]. Z seen 

Vb  Vc Vbc  I b  I c I bc

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(13)

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DESIGN A FAST DIGITAL PROTECTIVE RELAY ALGORITHMFOR HIGH VOLTAGE TRANSMISSION LINE

The ground distance relay protecting phaseA computes the apparent impedance using the following equation. Z seen 

Va I a  K 0 3I 0

(14)

Where K 0 compensation factor can be expressed as [2], [7]:  Z  Z L1   K 0   L 0  3Z L1 

(15)

Where Positive-sequence Z L1 impedance from the fault to relay location

The proposed Algorithm in figure 5 is an improvement for the previous modeling of MHO relay in figure 4 which is discussed in previous sections by adding anti-aliasing filters to eliminate the higher order frequency components. The Algorithm also has a changeable data size of FFT for extracting the fundamental frequency component to increase the accuracy. The new Algorithm of MHO Relay designed in MATLAB.

Zero-sequence impedance

Z L0

from the fault to the relay location

4.5 MHO Relay Modeling When a transmission line subjected to a fault, the voltage signals and current signals contain decaying dc components, higher order frequency components and lower order frequency components. The higher order frequency components can be eliminated using low pass anti-aliasing filters with appropriate cut-off frequency, but the anti-aliasing filters cannot remove decaying dc components and rejects lower order frequency components. This affects the performance of digital relay. Figure 4 shows Mho relay modeling [1].

Figure 4: General Mho Relay Modeling Algorithm [1].

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Figure 5: Proposed MHO Relay Algorithm.

5 METHODOLOGY DESCRIPTION The evaluation methodology of the new proposed design of MHO relay requires a modeling power system in PSCAD/EMTDC to generate voltage and current signals. The new proposed MHO relay Algorithm is designed in MATLAB based on Fast Fourier Transform (FFT) to extract the magnitude and phase of current and voltage waveforms to estimate transmission line impedance. The output of the FFT from equation (6) is X(k), contains a real and imaginary component that can be converted into amplitude and phase from equation (4) and (5). The ratio of appropriate voltages and currents provide to computes the apparent

Sudan Engineering Society Journal,

March 2014, Volume 60; No.1

Auday A.H. Mohamad and Essar Gafar Ahmed

impedance as expressed in equation (13) and (14) which used to detect phase to ground and phase to phase faults signals employed to distance relay algorithm from Table (1). The composite signals in a phase comparator are denoted by S1 and S2 are

6 RESULTS Sample results of these studies show the comparison between the presented MATLAB algorithm and PSCAD/EMTDC in detection and tripping time according to different fault incident impedance ZR %, fault resistances with various fault angles on a selected transmission line. The faulted voltage and current signals are extracting and tested in PSCAD firstly and transferred to test in MATLAB.

defined in equations (11) and (12). An angular displacement is considered positive if S1 leads S2.The output of a phase comparator operates if the condition in equation (7) is satisfied.

5.1 Test System

Table 2 and Table 3 show the sample results of comparison tripping time (ms) between MATLAB and PSCAD/EMTDC. In case of tripping time is more than (50 ms) that means the trip signal comes after fault duration complete (fault duration = 50 ms). The sign (-) means there is no fault detection.

The single line diagram (SLD) shown in figures 6 representing an overhead Transmission line of Merowe-Atbara (Transmission line 3) 500 kV, 50 Hz, and 236.7 Km connected with dynamic load. The positive and zero sequence impedance of the Transmission line are:

Fault Incident angle

0 10 -

Figure 6: Three Phase Line of the Simulation.

50 80 100 -

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Fault Type AB

AG

ABCG MATLAB

The positive sequence impedance of source is 66900 Ω, with apparent power 1400 MVA.

Fault incident impedance ZR %

PSCAD

Where: the capacitance of the line is neglected.

MATLAB

Ω /km

PSCAD

Z0= 0.3445 + 0.981j

Table 2: Comparison between MATLAB and PSCAD/EMTDC Tripping Time (ms) with Fault Resistances Equals to . Ω.

MATLAB

Ω /km

PSCAD

Z1= 0.028 + 0.276j

1.6 2.07 1.80 1.86 1.6 2.07 1.59 1.86 2.72 2.07 2.81 2.8 2.7 2.07 2.8 2.8 2.72 3.08 2.81 2.87

0.06 0.3 0.2 0.1 0.066 0.1 0.1 0.3 0.26 0.3 0.1 0.3 0.26 0.1 0.2 0.3 0.166 0.3 0.2 0.3

6.9 7.3 11.1 7.12 7.9 8.35 12.1 13.1 10.0 14.6 14.1 10.1 12 16.6 16.1 12.1 19.3 18.6 17.2 20.6

1.56 2.4 0 1.4 1.76 0.001 1.8 1.7 2.16 2.1 2.1 3 4.17 3.3 3.3 8.6 4.5 4.3 3.9 10.1

13 12.6 13.1 14.4 15.2 16.6 14.1 16.4 39.1 37.5 35 30 39.1 37.5 35 30 39.1 37.5 35 30

2.16 2.2 2.3 10.3 0.86 3 4.1 1.7 0.96 3.7 4.6 1.8 1.9 2.4 3.9 16.2 1.3 2.6 2.7 17.6

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DESIGN A FAST DIGITAL PROTECTIVE RELAY ALGORITHMFOR HIGH VOLTAGE TRANSMISSION LINE

MATLAB

100

PSCAD

80

MATLAB

50

PSCAD

10

MATLAB

1 5-5 10 40 5 1 5-5 10 40 5 1 5-5 10 40 5 1 5-5 10 40 5 1 5-5

PSCAD

Table 3: Comparison between MATLAB and PSCAD/EMTDC REFERENCES Tripping Time (ms) with Fault Resistance Equals to Ω. [1] Yashasvi B, Vidushi K, and Fault Type Ramesh P, “Simulation of Fault AG AB ABCG MHO Characteristics for Fault incident Incident Transmission Line Protection impedance angle Using PSCAD”, International ZR % Journal of Research in 10 1.71 0.26 - 1.36 55.7 0.66 Engineering & Applied 40 2.07 0.3 - 1.4 54.0 2.9 Sciences IJREAS Volume 2, 0 5 1.8 0.2 - 1.4 54.83 1.9 Issue 2, February 2012. 2.87 2.72 2.07 1.80 2.87 2.72 2.07 2.81 2.87 2.72 3.08 2.81 2.87 3.71 3.08 2.81 3.88

0.3 0.06 0.3 0.2 0.1 0.06 0.3 0.2 0.1 0.26 0.3 0.2 0.1 0.266 0.3 0.2 0.4

-

1.3 1.56 1.7 2.1 1.6 7.86 2.2 2.1 7.8 3.4 3.9 -

52.87 55.76 54.09 54.83 53.88 52.72 55.11 55.85 53.88 53.74 55.11 55.85 53.88 56.77 55.11 55.85 53.88

51.2 60.66 1.1 3.1 51.4 1.76 8.2 5 51.9 50.96 51.9 52.8 51.8 53.66 52 52.8 51.9

7 CONCLUSION

[2] ALSTOM Grid, “Network Protection and Automation Guide NPAG”, May 75113 [3] Bakshi U.A., and Bakshi M. V., “Protection and Switchgear”, Technical Publications, Jan 1, 2009. [4] Gerhard Ziegler, “Numerical Distance Protection Principles and Application”, 1999. [5] Dr. Hamid H. Sherwali and Eng. Abdlmnam A. Abdlrahem, “Simulation of numerical distance relays”, Al-Fatah University TripoliLibya, 2010.

 The new algorithm has the ability to detect and classify the Phase-Ground (AG), Phase-Phase (AB) and three Phases to Ground (ABCG) faults.

[6] Emilson Pereira Leite, “Matlab Modelling, Programming and Simulations”, Published by Sciyo, India, 2010.

 The results on table 1 and table 2 show that the new MHO relay algorithm has a fast tripping time.

[7] MathWorks, Simulink®, “Developing SFunctions R751.b”, Reference Manual, Inc. 2013.

 The new algorithm is more Sensitivity and more accuracy than other, especially after adding the filters and used FFT with changeable data size.

[8] Walt Kester, “Mixed-Signal and DSP Design Techniques”, by the technical staff of Analog Devices, Printed in the United States of America, 2003. [9] Sandro Gianny Aquiles Perez, “Modeling Relays for Power System Protection Studies”, Ph3D Research, University of Saskatchewan, Saskatchewan, Canada, July 2006.

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