CHAPTER-4

DESIGN OF POWER SYSTEM

The proposed system is composed of a 100 MVA, 20KV conventional power

plant, composed of 3-phase synchronous machine, connected by 200 km long 154 KV

distributed-parameters transmission line through a step-up transformer of

20/154KV( TR1). At the substation (TR2), the voltage is stepped down to 22.9 kV from

154KV. High power industrial loads of 6MW and 330MVAR loads are connected to the

conventional source. The 10 MVA wind farms is composed of three fixed-speed

induction-type wind turbines each having a rating of 3.33MVA. The wind farm is

operating at 20KV, and is this voltage is stepped up to branch network voltages of

22.9KV through a 20KV/22.9KV transformer (TR3). The wind farm directly connects

with the branch network (B1) through a transformer (TR3) and is providing power to the

domestic loads. The 10MVA wind farm supplied to the customer loads at 400V, through

distribution transformers of 22.9KV/400V. Three domestically loads are separated by

each 5KM transmission line and at each end of 5KM transmission line a domestically

load is connected through a distribution transformer, as shown in Fig.1. The most severe

fault in the power system is three phase to ground fault, it results in very high current in

the system. The designed fault current limiter is in such a way that it should reduce the

fault current due to three phase to ground fault, then it can successfully reduces the fault

current in micro grids for all remaining faults. Artificial faults are marked as Fault 1,

Fault 2 and Fault 3, which represent three-phase-to-ground faults in distribution grid,

customer grid and transmission line respectively as shown in the Fig.1.and for each and

every fault the wind farm fault current observed. To reduce this wind farm fault current

the designed superconducting fault current limiter is placed at three scenarios for three

different faults in the power system. First, we assumed that single SFCL was located at

Location 1 (Substation). Second, single SFCL was located at Location 2 (Branch

Network). Third, single SFCL was located at Location 3 (Wind farm integration point

with the grid). Finally, in order to clarify the usefulness of dual SFCL installed together

for different locations, SFCLs were located at Location 1 (Substation) and Location 4

(Wind Farm) respectively.

Superconducting fault current limiter

Superconducting Fault Current Limiter (SFCL) is innovative electric equipment

which has the capability to reduce the fault current level within the first cycle of fault

current [11]. The first-cycle suppression of fault current by a SFCL results in an

increased transient stability of the power system carrying higher power with greater

stability. The concept of using the superconductors to carry electric power and to limit

peak currents has been around since the discovery of superconductors and the realization

that they possess highly non-linear properties. More specifically, the current limiting

behavior depends on their nonlinear response to temperature, current and magnetic field

variations. Increasing any of these three parameters can cause a transition between the

superconducting and the normal conducting regime. The current increase can cause a

section of superconductor to become so resistive that the heat generated cannot be

removed locally. This excess heat is transferred along the conductor, causing the

temperature of adjacent sections to increase. The combined current and temperature can

cause these regions to become normal and also generate heat. The term “quench” is

commonly used to describe the propagation of the normal zone through a superconductor.

Once initiated, the quench process is often rapid and uncontrolled. Though once initiated

the quench process is uncontrolled, the extent of the normal region and the temperature

rise in the materials can be predicted.

. SFCL performance evaluation graph indicating the relationship between SFCL impedance and reduction in fault current

Simulation Model of SFCL

The resistive type SFCL was modeled considering four fundamental parameters

of a resistive type SFCL. These parameters and their selected values are: 1) Transition or

response time = 2m.sec 2) Minimum impedance = 0.01Ω. Maximum impedance = 20Ω

3) Triggering current and =550 A 4) Recovery time. = 10 msec. The SFCL working

voltage is 22.9kV. The maximum impedance value can be varied from 20 ohms to 27

ohms. The SFCL model developed in Simulink/SimPower System is shown in Fig.3.1.

The SFCL model works as follows. First, SFCL model calculates the RMS value of the

passing current and then compares it with the characteristic table. Second, if a passing

current is larger than the triggering current level, SFCL‟s resistance increases to

maximum impedance level in a pre-defined response time. Finally, when the current level

falls below the triggering current level the system waitsuntil the Recovery time and then

goes into normal state. The SFCL characteristic table shown in Fig.3.1 plays a main role

which consists of standard parameter values of SFCL.

The current limiting resistance value is calculated and this value is implemented

in the simulation model. The important parameter to be given in SFCL is the current

limiting resistance value. It is stored in the SFCL characteristic table. In order to avoid

harmonics caused by transients, filter is used. The SFCL model developed is tested in

both single phase and three phase test systems and the current waveforms are recorded

with the presence and absence of SFCL. The simulation model of a single phase test

system with and without SFCL is shown in Fig.3.2 and Fig.3.3 respectively and the

current waveforms are recorded. The fault current is induced in the source directly in

order to reduce the complexity of the simulation model.

Fault in the Distribution Grid (Fault 1)

Comparison between fault current from the wind farm (measured at output of TR3

in Fig. 1) for different SFCL locations in the case when a three-phase-to-ground fault was

initiated in the distribution grid (Fault 1 in Fig. 1).

In the case of SFCL located at Location 1 (Substation) or Location 2 (Branch Network),

fault current contribution from the wind farm was increased and the magnitude of fault current is

higher than ‘No FCL’ situation. These critical observations imply that the installation of SFCL in

Location 1 and Location 2, instead of reducing, has increased the DG fault current. This sudden

increase of fault current from the wind farm is caused by the abrupt change of power system’s

impedance. The SFCL at these locations (Location 1 or Location 2) entered into cur-rent limiting

mode and reduced fault current coming from the conventional power plant due to rapid increase

in its resistance. Therefore, wind farm which is the other power source and also closer to the

Fault 1 is now forced to supply larger fault current to fault point (Fault 1).

In the case when SFCL is installed at the integration point of wind farm with the grid, marked

as Location 3 in Fig. 1, the

Comparison of the wind farm fault currents for four SFCL locations in case of fault in customer grid (Fault 2).

wind farm fault current has been successfully reduced. SFCL gives 68% reduction

of fault current from wind farm and also reduce the fault current coming from

conventional power plant because SFCL is located in the direct path of any fault current

flowing towards Fault 1.

With dual SFCL installed at Location 1 and Location 4, 45% reduction in fault

current is also observed. However, even though two SFCLs were installed, wind farm

fault current reduction is lower than what was achieved by the single SFCL installed at

Location 3. From the simulation results, it was known that the installation of two SFCLs

(Location 1 and Location 4) is economically and technically not feasible.

B. Fault in Customer Grid (Fault 2)

Fig. 5 shows a comparison between fault current from the wind farm (measured at

output of TR3 in Fig. 1) for different SFCL locations in the case when a three-phase-to-

ground fault was initiated in the customer grid (Fault 2 in Fig. 1). Fault 2 is

comparatively a small fault as it occurred in low voltage customer side distribution

network. The results obtained are similar to what were observed in the case of

distribution grid (Fault 1) as explained in Section III-A.

Once again the best results are obtained when a single SFCL is located at

Location 3, which is the integration point of the wind farm with the distribution grid.

Comparison of the wind farm fault currents for four SFCL locations in case of fault in customer grid (Fault 2).

Comparison of the wind farm fault currents for four SFCL locations in case of fault in transmission line (Fault 3).

The resistive type SFCL was modeled considering four fundamental parameters of a

resistive type SFCL. These parameters and their selected values are: 1) Transition or response

time = 2m.sec 2) Minimum impedance = 0.01Ω. Maximum impedance = 20Ω 3) Triggering

current and =550 A 4) Recovery time. = 10 msec. The SFCL working voltage is 22.9kV. The

maximum impedance value can be varied from 20 ohms to 27 ohms. The SFCL model developed

in Simulink/SimPower System is shown in Fig.3.1. The SFCL model works as follows. First,

SFCL model calculates the RMS value of the passing current and then compares it with the

characteristic table. Second, if a passing current is larger than the triggering current level,

SFCL‟s resistance increases to maximum impedance level in a pre-defined response time.

Finally, when the current level falls below the triggering current level the system waits until the

Recovery time and then goes into normal state. The SFCL characteristic table shown in Fig.3.1

plays a main role which consists of standard parameter values of SFCL.

The current limiting resistance value is calculated and this value is implemented in the

simulation model. The important parameter to be given in SFCL is the current limiting resistance

value. It is stored in the SFCL characteristic table. In order to avoid harmonics caused by

transients, filter is used. The SFCL model developed is tested in both single phase and three

phase test systems and the current waveforms are recorded with the presence and absence of

SFCL. The simulation model of a single phase test system with and without SFCL is shown in

Fig.3.2 and Fig.3.3 respectively and the current waveforms are recorded. The fault current is

induced in the source directly in order to reduce the complexity of the simulation model.

Simulation model of single phase system with SFCL

The type of the fault induced in the model is single phase to ground fault where it is induced

through the AC voltage source. An RMS block is needed in order to calculate the RMS value of

the incoming current signal and to increase the impedance value according to the limited fault

current value specified in the SFCL characteristic table.

The SFCL subsystem can be implemented in various types of single phase test systems and the

operation can be tested. The performance of the SFCL can also be tested in a power system

generation and distribution systems using three phase SFCL for controlling the fault current for

each phase. The subsystem specified in Fig.3.2 is the SFCL model designed for single phase

system which is shown in Fig.3.3. Harmonics filtration is used in order to reduce the harmonics

caused due to the abnormal fault current. A normal first order filter is used for reducing the

harmonics. The type of the filter can be changed depending upon the application of the system. A

controlled voltage source is connected in order to compensate the voltage sag caused due to the

induced fault current which is caused due to both internal and external causes.