Identification of Fault Condition and Switching Event for … · magnitude of this current depends...

International Journal of Electrical Electronics & Computer Science Engineering

Volume 1, Issue 6 (December 2014), ISSN : 2348 2273

Available Online at www.ijeecse.com

31

Identification of Fault Condition and Switching Event for Improvement of

Overcurrent using Symmetrical Components

Dhanashree Kotkar1, Nandkumar Wagh

2

1M.TechResearch Scholar, SDCOE, Selukate, Wardha 2Associate Prof. Electrical Engg. Deptt. DESCOET, Dhamangaon

Abstract: In power system, the transient current, inrush

current and overcurrent is quite common and sometimes it

can be represented as fault. As the overcurrent is for very

short period of time, it can be cleared automatically within

1minute and does not damage the system. But the transient

current during switching is treated as a fault current and

system gets disturbed, due to tripping of the relays. To

analyze such conditions, the method of criterion function ‘R’

is proposed in this paper based on symmetrical components.

Which is further implemented in MATLAB as a simulation

for 13 bus bar distribution system and output waveform for

particular value of criterion function ‘R’ is shown and

explained for different fault conditions on power system

equipments such as transformer and Induction Motor.

Keywords: identification, fault condition, overcurrent

Protection, switching event, symmetrical components.

I. INTRODUCTION

A fault is abnormal system condition, which is in most cases is short circuit, and occurs as a random event. The

purpose of protection system is to prevent harm to people,

to limit further damage to power system by removing

faulted equipment [2]. A power is not static but its

parameters changes during operation (switching on or off

of generators, three phase transformers, capacitor banks,

circuit breakers, shunt capacitors and transmission lines)

and during planning (addition of generators and

transmission lines). Faults usually occur in a power

system due to insulation failure, flashover, physical damage or human error. These faults may either be three

phases in nature involving all three phases in a

symmetrical manner, or may be asymmetrical where

usually only one or two phases may be involved. Power

system switching, such as motor starting and transformer

energizing, is the most important source of undesirable

operation of the relay protection.

Traditional methods of over current protection have been

based mainly on phase currents. Inrush currents

associated with motor starting and transformer energizing

can cause interaction problems with other loads in a facility or on the power system. Protection devices can

misinterpret these events as fault currents and may cause

protection devices to trip. Energizing a transformer has

the additional issue of harmonics in the inrush current,

which can excite system resonances and cause dynamic

overvoltage. Here, the high currents occur to energize the transformer core. The steady-state magnetizing current for

a transformer is very low, but the momentary current

when it is first energized can be quite high.

II. OBJECTIVE

The development of deregulation in power systems leads

to a higher requirement on power quality. In the area of relay protection this means that a faster protection is

needed, while undesirable operation of the protection

system is almost unacceptable. A faster protection can

guarantee that an abnormal operation mode somewhere in

a system, such as voltage sag caused by faults, can be

quarantined quickly, so as not to propagate to the rest of

the system and cause instability. In the area of relay

protection employed in distribution system network,

transients caused due to non fault switching events such

as motor starting and transformer energizing are source of

undesirable operation of the protection system. Therefore identification of those factors that produces this

undesirable operation of the relay and introducing

procedures for their discrimination from the real fault

cases are very important. To do this, a relay protection

should be sensitive. Unfortunately, high sensitivity

sometimes causes undesirable operation of relay

protection when there is no fault in the system. In a

deregulated power market this directly leads to penalty

compensation to the users that suffer from the blackout.

Therefore, identification of those factors that produce this

undesirable operation of the relay and introducing

procedures for their discrimination from the real fault cases are very important.

III. METHODOLOGY

An algorithm based on symmetrical components is

proposed to detect and identify the balance condition of

the power system during the fault. An expert system is

presented here that is able to classify different types of

power system events to the underlying causes (i.e., events) and offer useful information in terms of power

quality using MATLAB/SIMULINK software that are

used to analyze real time distribution system. A

MATLAB/GUI based simulation tool has been developed

to analyze power system operation [1]. The criterion

function „R‟ based on concept of symmetrical




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components presents method for preventing the

undesirable relay operation due to over currents following

switching. It will give a method for improving over current relay operation. It formulates an algorithm based

on different behavior of current components during fault

and non-fault conditions. Based on these differences, a

criterion function „R‟ is introduced, considering

undesirable operations of over current relays due to

switching is presented in [4].

𝑉𝑎𝑏𝑐 = 𝑉𝑎𝑉𝐵

𝑉𝐶

1)

Therefore, the three symmetrical components phasors

arranged into a vector are as follows:

𝑉012 = 𝑉0

𝑉1

𝑉2

2)

Where the subscripts 0, 1, and 2 respectively refer to the

zero, positive, and negative sequence components. A phase rotation operator 'a' is defined to rotate a phasor

vector forward by 120 degrees. Matrix A can be defined

using this operator to transform the phase vector into

symmetrical components:

𝐴 = 1 1 11 𝑎2 𝑎1 𝑎 𝑎2

3)

The phase voltages are generated by the sequence equation.

𝑉𝑎𝑏𝑐 = 𝐴𝑉012 4)

Conversely, the sequence components are generated from

the analysis equations.

𝑉012 = 𝐴−1𝑉𝑎𝑏𝑐 5)

Where 𝐴−1 =1

2 1 1 11 𝑎 𝑎2

1 𝑎2 𝑎

6)

When transformer is switched, inrush current will happen.

This current has some features, which it is enough to

identify itself. In this paper, to extract these features, a

new criterion is proposed to discriminate inrush currents

from internal faults in power transformers. The point is the value of negative sequence is different from positive

sequence in faulty conditions. Helping this rule, the

criterion is introduced:

𝐼0

𝐼1

𝐼2

=1

3 1 1 11 ∝ ∝2

1 ∝2 ∝ 7)

∝=∝⦟120⟹I_a=1⦟0,I_b=1⦟+120,I_c=1⦟-120 8)

In faulty condition, it is obvious that the value of I2 (negative current) is larger than I1 (positive current) in

normal condition. Using this feature, define new below

criterion:

R = 𝐼1 − 𝐼2

𝐼1 + 𝐼2 9)

IV. EFFECT OF SWITCHING ON RELAY

a. Transformer Energizing: When the primary winding

of an unloaded transformer is switched on to normal voltage supply, it acts as a nonlinear inductor. In this

situation there is a transient inrush current that is required

to establish the magnetic field of the transformer. The

magnitude of this current depends on the applied voltage

magnitude at the instant of switching, supply impedance,

transformer size and design. Residual flux in the core can

aggravate the condition. The initial inrush current could

reach values several times full load current and will decay

with time until a normal exciting current value is reached

[3]. The decay of the inrush current may vary from as

short as 20 cycles to as long as minutes for highly

inductive circuits. The inrush current contains both odd and even order harmonics. Although digital relay‟s filter

is used to extract the fundamental component of the

current, the magnitude of the signal may lead to

undesirable operation of the relay. Another concern about

transformer energizing is transient propagation. This

causes considerable amount of even harmonics and dc

component in the voltage. These disturbances may

propagate through transformers to the rest of the system,

and be magnified due to resonance effect.

b. Motor switching: Starting the medium voltage (MV)

and low voltage (LV) induction motors is another subject to be considered. The starting current of a large induction

motor is typically five to six times the rated current. In

fact, the starting current has a very high initial peak. That

value is damped out after a few cycles, normally no more

than two cycles depending on the circuit time-constant

[3]. Then, it drops rapidly to a multiple value of its

nominal level, and is maintained during most of the

acceleration process. The current is then smoothly

reduced to the nominal value that depends on the

mechanical load of the motor. This trend that corresponds

to the direct starting of a three-phase motor connected to

the supply at the worst switching angle.

V. CONCEPT OF CRITERION FUNCTION

The criterion function „R‟ for discriminating fault

from non fault switching is defined as follows:

R = (I1 - I2) / (I1 + I2)




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Since there is a considerable negative component in

the asymmetrical fault case, according to criterion

function the value of „R‟ is close to zero. In the switching case, the negative component is very small

and „R‟ is close to 1.

The suggested criterion is independent of the amplitude of the current which is advantageous. The

reason is that it operates based on the relative

difference between the negative and positive

component of the current [5].

Another advantage of the suggested criterion function

is that its proper operation is independent of the

power system balancing it also operates properly.

The reason is that during the asymmetrical fault, the negative component of current increases and the

value is much smaller than that before fault event.

R<0.35 indicates the fault; otherwise, overcurrent is the result of switching. It can be mathematically proved as

follows:

a. Analytical model:

Example 1: The single line diagram of power system

considered is shown in figure 1, where negative and

positive sequence reactances are also given. The neutral

of generator and -Y transformers are solidly grounded.

The motor neutral is grounded through reactance Xn = 0.005 per unit on motor base. Pre fault voltage is VF =

1.0500 per unit. Pre fault load current and -Y transformers phase shifts are neglected [6].

Fig. 1: Single Line Diagram for Example 1

Solution: The sequence network of zero, negative and positive for above power system is below

Fig. 2: Zero Sequence Network for Example 1

Fig. 3: Positive Sequence Network for Example 1

Fig. 4: Negative Sequence Network for Example 1

Fig. 5: Thevenin Equivalent of Zero Sequence Network for Example 1

Fig. 6: Thevenin Equivalent of Positive Sequence Network for Example 1




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Fig. 7: Thevenin Equivalent of Negative Sequence Network for Example 1

CASE 1: When single phase line to ground fault

occurs, the zero, negative and positive sequence

networks are connected in series.

Fig. 8: Interconnected Sequence Network for Single Line to Ground Fault

I0 = I1 = I2 = 1.0500

j (0.25+0.13893+0.14562)

= 1.0500

j (0.5345)

= -j 1.96427 per unit

Criterion function „R‟

R = (I1 - I2)

(I1 + I2)

R = 0.

CASE 2: When line to line fault occurs, the negative

and positive sequence networks are connected in parallel at fault terminals as follows.

Fig. 9: Interconnected Sequence Network for Line to Line Fault

I2 = - I1 = 1.0500

j (0.13893+0.14562)

= 1.0500

j (0.28455)

= 3.690-900 per unit


R = (I1 - I2)

(I1 + I2)

R = 0.

CASE 3: When double line to ground fault occurs, the

zero, negative and positive sequence networks are connected in parallel at fault terminals as follows.

Fig. 10: Interconnected Sequence Network for Double

Line to Ground Fault

Equivalent Impedance = (Z0 // Z2) + Z1

= (j0.25 // j0.14562) + j0.13893

= j0.23095

I1 = 1.0500

j0.23095

= j 4.5464 per unit

By current division rule

I2 = - (-j 4.5464) x j0.25

j0.25+ j0.14562

= j 2.873 per unit


R = (I1 - I2)

(I1 + I2)

R = 4.5464 - 2.873

4.5464 + 2.873

= 0.23

By solving above power system problem for criterion

function „R‟, the value of „R‟ is below 0.35. Hence

this value can be used for overcurrent protection.




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Example 2: A star connected generator has sequence

reactor X0 = 0.09 per unit, X1 = 0.22 per unit, and X2 =

0.36 per unit. The neutral point of machine is grounded through a reactance of 0.09 per unit. The

machine is running on load with rated terminal voltage

when it suffers an unbalance fault. The fault currents

out of machine armature are Ia = 0, Ib = 3.751500 per

unit, and Ic = 3.75300 with respect to phase „a‟ line to neutral voltage.

Solution:

Where a=11200

I1 = 1 (Ia+aIb+a2Ic)

3

=1(0+(11200)x(3.751500)+(11200)2x(3.75300))

3

= - j 2.5 per unit

I2 = 1 (Ia+a2Ib+aIc)

3

=1(0+(11200)2x(3.751500)+(11200)

x(3.75300))

3

= j 1.25 per unit


R = (I1 - I2)

(I1 + I2)

R = 2.5 – 1.25

2.5 + 1.25

= 0.33

By solving above power system problem for criterion

function „R‟, the value of „R‟ is less than 0.35. Hence

this value can be used for overcurrent protection.

b. Analytical Model: To show the advantage of the criterion function „R‟ for overcurrent protection, a part

of a distribution system 34.5 kV 13 bus is shown in

Fig. 11, is modeled using the MATLAB/Simulink,

also the network parameter of the 13-bus distribution

system is shown. Several non-fault events are applied

to this system along with unsymmetrical faults i.e.,

single line to ground, line to line and double line to ground events at different times. The simulation

results show, how the proposed algorithm could help

the overcurrent relay to discriminate fault from non-

fault events [1].

Fig. 11: 13 Bus (34.5 kV Simulated Distribution System)

VI. SIMULATION RESULT ON 13 BUS SYSTEM

Fig. 12: MATLAB Model of 13 Bus bar system without fault




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Fig. 13: Voltage & Current Waveform13 Bus bar system without fault

Fig. 14: MATLAB Model of 13 Bus bar system with fault

Fig. 15: Voltage & Current Waveform13 Bus bar system with fault

Fig. 16: Subsystem for Criterion Function

Fig. 17: Subsystem for subsystem 4

Fig. 18: Subsystem for subsystem 4 (gate 2)

Fig. 19: Transformer Energizing on bus 9

Fig. 20: Value of Criterion Function with transformer switching on Bus 9




37

Case-I: By considering asymmetrical different faults:

Fig. 21: LG Fault on bus 9

Fig. 22: Value of „R‟ versus Time due to LG on Bus 9

Fig. 23: LLG Fault on bus 9

Fig. 24: Value of R by LLG Fault on bus 9

Fig. 25: LL Fault on bus 9

Fig. 26: Value of R by LL Fault on bus 9

Fig. 27: Motor switching on bus 9

Fig. 28: Value of Criterion Function R on bus 9




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Case-II: Fault through low resistance:

Fig. 29: LG Fault without Overcurrent Relay on Bus 9

Fig. 30: Value of „R‟ versus Time due to LG Fault without Overcurrent Relay on Bus 9

Fig. 31: LG Fault with Overcurrent Relay on Bus 9

Fig. 32: Value of „R‟ versus Time due to LG Fault with Overcurrent Relay on Bus 9

VII. DISCUSSION

The nature of plots for value of „R‟ versus time and three phase currents due to three phase short circuit

without overcurrent relay are shown in fig. 29 and fig.

30 and with relay in fig. 31 and fig. 32respectively.

The three phase current waveforms of LG, LL, LLG,

and simultaneous LL and transformer energizing are

same for ground fault and fault through low resistance

as due to overcurrent relay the faulty phases are disconnected on occurrence of fault.

The criterion function „R‟ effectively works with the use of negative sequence component which arises due

to unbalance of unsymmetrical fault conditions. It

provides accurate discrimination between fault and

non fault cases. It operates effectively in power

distribution network when power system is

unbalanced. The mathematical value of criterion

function „R‟ is used as 0.35 and it is proved with

different power system examples. The value of

criterion function „R‟ varies for fault and non fault

cases. The value of criterion function „R‟ for motor switching is near about same for transformer switching

non fault event.

VIII. CONCLUSION

The criterion function „R‟ as a decision making box can

be implemented in relay and gives the signal to the circuit

breaker for exact operation of the system, while switching and fault conditions. Maloperation can be avoided and

continuous power supply will be easily available without

any disturbance.

The focus of this study is on the application of power system disturbance data for the testing of power system

protection equipment. Towards such a goal, study is

carried out not on particular case study, but on seeking a

general procedure for processing any possible disturbance

before it is applied in protection testing. In other words,

the main concern in this work is how to implement such

an application of disturbance data on testing.

IX. APPLICATION

The criterion function „R‟ with overcurrent protection can be used for overhead distribution lines. The above

work is useful to the researchers working on this topic

and it is also useful to one who wants to check the

results with distribution, and transmission lines. The

work is useful to get reliable results with other power system software like EMTDC / PSCAD software [1].

This work is useful to improve distribution system

overcurrent protection and will provide development

in deregulation with reliable power quality.




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