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Figure-6: A phase saturation GIC in ?-Y-type transformer in the flow path

Circuit
below give a description for GIC passes through neutral point for Y-side to ?-side in three-phase power transformer.

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Where l
is the length of the magnetic circuit, is permeability of the material and A is the cross-section
area of the circuit. The reluctanceis the function of the excitation, the magnitude of dc flux  depends of the AC
excitation and the level of the saturation, not only the GIC magnitude 19.

 

                                         (4)

 

Where
N is number of turns, GIC is magnitude of dc current, is reluctance it is given by the equation:

                           (3)

 

The
magnitude of the DC flux  depends on three
factors, given by the following equation:

 

 

                         (2)

The GIC calculated by the
following equation:

 

Negative half-cycle means DC
flux is no saturation, and positive half- cycle means the core will go to
saturation due to the DC flux.

                                   (1)

For the
half-cycle saturation the DC flux flow into the core by the following equation:  

Figure-5:
Three-phase transformer model with GIC

 This section give brief description for
three-phase transformer circuit model, with or without GIC, figure-5 shows this
circuit.

IV.               
Three-Phase
Transformer Circuit

 

NO

Name

Symbol

Description

 
 
1

 
 
Disconnect at neutral

 
 
 
 

This is common method
to block the GIC is disconnect the neutral from the ground where the GIC enters.
When the neutral opening that causes unpredictable voltage transients and hinders
the ground fault detection.

2

Inductor at neutral

Inductor reduce the
ground fault current levels.

3

Resistor at neutral

 

Resistor in neutral
just reduce it at the cost of loss in protection sensitivity and does not
eliminate the GIC.

4

Capacitor at neutral

 

Capacitors in neutral
totally eliminate the GIC but may cause ferroresonance and very expensive.

5

Capacitor with by-pass at neutral

 

This
method eliminate the GIC and ferroresonance, also it is more expensive than
method 4.

Table
1: Equipment’s that can be used at the transformer neutral for the mitigation
of GIC

 

 

 

This paper proposes a one
method to mitigation GIC, Inductor disconnect at the neutral has been discussed,
Inductors are generally used in the neutral to reduce the ground fault current
levels, such an inductor has very little effect on reducing the GIC.
GICs are induced in the system in the transmission lines. If
the electric field is assumed to be uniform for a given
transmission circuit, so if put the resistance in the ground of three-phase
transformer the GIC doesn’t eliminate just the loss in transmission has been
reduced.

There are a lot of academic
research papers study the effect of GIC in power transformers and power system
and how to mitigate this phenomena, table1 give brief descriptions for all
methods of reduction
or mitigation the GIC.

Mitigation Methods:

                       Figure-4: chain of
events during a GMD event

3.      
Prevention of power system
components and protection system from the effects of the asymmetric
magnetization current.

2.      
Prevention of transformer
core from saturation.

1.      
Prevention of creation of
quasi-DC GICs or reduction of them.

 Hence, the
strategies regarding mitigation can be classified in three categories as
fllows:

It is obvious that an impact on solar activity or
interaction of solar storms with the earth’s magnetic field is not possible.
Therfore, mitigation methods can be applied after the stage when the DC voltage
has been induced in the closed loop of transformer windings, ground, and
transmission lines.

As is seen in figure-4 a chain of events occur and may
eventually results to an undesired effect on the power system and its
components during a GMD. Therfore, any measure that can cut the chain before
reaching its final step will help to prevent or mitigate the adverse effects of
it.

Figure-4 illustrates the chain of events eventullay to
adverse consequents of a GMD event. The effect of geomagnetic disturbance due
to sloar storms was reported in the past in North America and Europe ( in
september 1859) 116.

III.              
 Mitigation Methods of GIC

DC current that are injected in the transformer
windings cause saturation of the core and create asymmetric high value
magnetization currents. The increase of winding losses is one important results
of this phenomenon. The saturation of the core changes the magnetic circuit of
the transformer and leads to an increase of leakage fluxes. Besides the
magnetization current contains higher harmonics and creates an unbalanced
magnetomotive force, MMF. Hence, in order to understand the phenomena, the
effects of each factor that can play role regarding the increase of the winding
losses have been studied separately. To realize the impact of each factor helps
in proposing a methods for estimation of the winding losses during a GIC event.   

2.2     Winding Losses

Hysteresis losses depend on the waveform of the flux
densities. By crossing the knee point these losses show considerable increase
in comparison with the losses associated with flux densities between knee
points. However, an increase of the flux density to more than maximum
saturation level of the core due to its reversible does not create hysteresis
losses.  

       A
qualitative analysis indicated that the saturation curve is symmetric about the
vertical axis when no DC current is present. However, when the DC component is
present, the curve shifts to the left, which indicates increasing saturation.
However, when the GICs level is higher, the area increased due to the level of
harmonics increase.

Figure-3: Transformer magnetization curves with and without GIC

The operation point of the magnetic core is obtained
from the graphical representation of the magnetic flux density (B) and the
magnetic field intensity (H). The B-H curve shows how the magnetic density
changes as the alignment of the magnetic domains changes within the magnetic
circuit. Once all domains have been aligned, the saturation point is reached,
and further increases in magnetic field intensity have little changes of the
magnetic flux density. The area of the hysteresis loop corresponds to the
energy dissipated as heat by the magnetization and demagnetization process
during each cycle. Figure-3 shows two operation conditions with and without
GIC.

2.1     Core Losses

The design of transformer are one of the important
factors in analysis of GIC effects. Single-phase and three-phase transformers
have different core design so the sensitivity to the GIC 14.

       The GIC
in transformer will destroy current waveforms. This will produce even and odd
harmonics due to this reactive power consumption increases in transformer. The
reactive power consumption cause voltage fluctuations and harmonics creates
relay and protective devices problem in power system 13.

      GIC
current is low frequency current about (mHz). It is considered as the DC bias.
The magnitude of GIC is depends upon the strength of the earth magnetic field.
The highest value of GIC current noted is about 300A. The magnitude of GIC is
sufficient to cause saturation of the transformer 12.

      The risk
of damages of transformer is depends upon the magnitude of GIC current and the
design of transformer. The magnitude of GIC is depends upon the location and
the environment condition of the area 11. So as the design of transformer
changes the effects of GIC varies.

II.                 
GIC effects
on  Power Transformers

           However, what happens during a GIC
events is not as simple as mentioned above. In fact, there are transient
phenomena due to the interaction between the transformers and power network
before reaching the mentioned steady state condition of a GIC event. Also, the
transformer type and network parameters play an important role in the transient
and steady state behavior. A deep understanding of the phenomenon is essential
to predict its adverse effects and develop proper mitigation methods for
protection of transformers and the power system.

Figure.2:
Effect of DC magnetization on the magnetization current (half-cycle saturation)

As explained in many papers
1-14, the DC current saturates the core of transformer within a half cycle
and causes an asymmetric magnetization current. The transformers usually are
designed in a way that the core is magnetized between the positive and negative
knee points of its magnetization curve for the rated symmetric linkage flux. In
fact, the DC current induces a DC offset in the core flux, which forces the
core magnetization over the knee point in one of the half periods of the power
cycle. As a result the required magnetization current dramatically increases
during the period when the core is magnetized over the knee point. A famous
diagram that is referred to in many studies in order to explain this phenomenon
is shown in Figure.2.    

The largest first GIC event
was observed in south of England where due to earth currents all telegraph
lines in the whole Great Britain where stopped in 1847. The largest blackout
happened on September 22, 1957 and on February 11, 1958 in the Toronto area of
Canada 10.

       These geomagnetically induced currents
(GIC) flow from high resistance path to low resistance path. GIC enter from
neutral wire of transformer and exit from power transmission systems through
the neutral wire of transformer. These include reactive power consumption and
system voltage instability creating load-flow problems, in particular at system
interties. Also, erratic operation of voltage regulators and tap changers has
happened, as well as transformer tripping due to differential relay operation.
Other problems that may result from GIC are overloading of filters in HVDC
transmission system and switching problems 8. GIC can cause partial power
system damage or whole system black-out. The largest electrical system blackout
was occurred on March 13, 1989 due to high magnitude of GIC current in the
entire zone of Quebec. This Hydro-Quebec blackout damaged a large step-up
transformer on the generating side in power system at a nuclear plant on the east
coast of the United States 9.

Figure.1:
Symbolic network for description of GIC event

When this DC current flow
through a transformer it will cause the saturation of transformer core. This
saturated core produces harmonics in the transformer. This harmonics damage the
transformer. GIC produces excessive heating of transformer due to which
transformer windings may get burned. It increases the exciting current of
transformer rapidly such as above maximum flux density value the transformer
core gets saturated. GIC is dangerous for the power system as it causes
increase in reactive power consumption, miss-operation of relay 7.

During geomagnetic storms, a
potential difference is induced on the surface of the earth because of the
earth’s geomagnetic field fluctuations. The resulting earth surface potential
(ESP) produces a current, known as geomagnetically induced current (GIC),
through the grounded neutral of transformers and flowing along the transmission
lines. The frequency of GIC is very low, so it can be as a quasi-DC 2.  GIC can cause DC bias (DC current) of the
transformer. This result in a highly distorted exciting current of transformer,
dramatic increase in transformer reactive power consumption, miss-operation of
relay and some other problems of power system 3-6.

               Geomagnetically Induced Current
(GIC) is caused by solar activity, for example, sun flare 1. The solar
activity can emit a lot of charged particles to the earth. The interaction of
the charged particles with the earth’s magnetic field can produce auroral
currents, which follow circular paths around the earth’s geomagnetic poles at
altitudes of 100 km or more. These auroral currents disturb the earth’s
normally dormant magnetic field and when the disturbances are of sufficient
severity they are termed geomagnetic storms.

I.                   
Introduction

               

 

Keyboard: Geomagnetically
Induced Current (GIC), Harmonics, Three-Phase Transformer, Half-cycle
saturation and reactive power consumption.

Development of
mitigation strategies and risk assessment should be carried out. Geomagnetically
Induced Currents (GICs) causes a lot of problems in power transformers and
power system like,   half-cycle
saturation, increase reactive power consumption, heating, also winding burning,
the higher harmonic also happened that leads harm the transformer. MATLAB-SIMULINK
software used to simulate Three-Phase Transformer.

Abstract – This paper
present the effects of Geomagnetically Induced Currents (GIC) on three-phase
power transformer. Geomagnetically Induced Current (GIC) flow can cause
tripping of components with to sensitive safety limits and damaging of
transformer. Cascading failure may be initiated, which in turn can lead to
blackouts and loss of production.

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