Now a days one of the main challenges of power industries is facing efficiency and stable delivery of power to their customers. Hence, efficient transmission of power and voltage regulation has become an essential criteria. This subject has been under research for a while and treated as one of the main issue. Many research scholars and scientists had addressed this problem in many ways.
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SVC been one of those solutions in stabilizing the voltage of the power distribution system. SVC is one of the FACTS(flexible alternating current transmission) controller which act as a shunt capacitor in the power system to compensate the reactive power there by increasing the power factor. In this work we have used MATLAB platform to investigate the performance of SCV in regulating system voltage.
I express my deepest gratitude to my project guide Dr. NIGEL SHEPSTONE, for his inestimable conveyance, acutance and encouragement throughout the project work. I would like to thank him for enlightening me on the latest trends in power factor correction techniques, which has helped me to complete this work very efficiently in time.
I am grateful to all the faculty members of the department, for helping me in understanding the various aspects of the Power System during the course work.
I owe my thanks all my classmates and lab mates for their companionship throughout this project as well as the whole course.
I love to dedicate this work to my parents for their sacrifice, co-operation, support and affection shown to me during the course work which made it possible for me to complete this work in time.
Power system engineering forms a vast and major portion of electrical engineering studies. It is mainly concerned with the production of electrical power and its transmission from the sending end to the receiving end as per consumer requirements, incurring minimum amount of losses. The power at the consumer end is often subjected to changes due to the variation of load or due to disturbances induced within the length of transmission line. For this reason the term power system stability is of utmost importance in this field, and is used to define the ability of the of the system to bring back its operation to steady state condition within minimum possible time after having undergone some sort of transience or disturbance in the line.
Today’s changing electric power systems create a growing need for flexibility, reliability, fast response and accuracy in the fields of electric power generation, transmission, distribution and consumption. Flexible Alternating Current Transmission Systems (FACTS) are new devices emanating from recent innovative technologies that are capable of altering voltage, phase angle and/or impedance at particular points in power systems.Their fast response offers a high potential for power system stability enhancement apart from steady- state flow control.
To provide stable, secure, controlled, high quality electric power on today’s environment and to do better utilization of available power system capacities Flexible AC transmission systems (FACTS) controllers are employed to enhance power system stability in addition to their main function of power flow control. The Power electronic based FACTS devices are added to power transmission and distribution systems at strategic locations to improve system performance. FACTS are a family of devices which can be inserted into power grids in series, in shunt, and in some cases, both in shunt and series. During the last decade, a number of control devices under the term FACTS technology have been proposed and implemented. Application of FACTS devices in power systems, leads to better performance of system in many aspects. Voltage stability, voltage regulation and power system stability, damping can be improved by using these devices and their proper control.
In New Zealand first transmission line was designed and constructed in the South Island by the government as a part of Coleridge hydro station department and was commissioned in 1914. Two 66kv transmission line in between Coleridge to Addington in Christchurch was created over a distance of 104km.
After world war- 1, 110kv transmission was begun to develop and connect towns and cities. However, after some time 110kv network capacity was decided to expand due to increasing demand in the region. Because of the low voltage supply while transformer is out for maintenance.
2.1. Power Factor
Power Factor (PF): The Power Factor is an indicator of electric power usage efficiency, denoted by PF. PF is defined as real power dissipated by an AC electric power system to the apparent power (VrmsIrms) in the electric circuit.
Therefore we have,
- Increase in the heating losses in the transformer and distributed system.
- Reduction in plant/equipment life.
- Unstable voltage levels.
- Increased power losses in power systems.
- Upgradation to costly equipment.
- There is decrease in energy efficiency.
- Increase electricity costs by paying power factor surcharges.
Power factor correction is very important (bringing it close to 1) because it minimizes the wastage of electric energy and hence decreases the unnecessary billing in the electric bill of a plant/customer. This can be achieved by installing required number of switched capacitors in the circuit.
- Reduction in demand charges:
Basically, if the power factor is low, then in comparison to KW, KVA will be significantly greater in demand and the electricity providing companies will charge for demand in KW (active power) or a percentage of the highest registered demand in KVA (apparent power), whichever is greater. But there will a reduction in electric bills if we can increase the power factor by any power factor correction means, because there will less demand charges implied.
- Increased load capacity with the existing system
Loads demanding reactive power is in-turn demanding reactive current. If we can install a power factor correction capacitor at the right location before the inductive load/machine, there will be a huge reduction in current carried through each circuit. This reduction in current flow can increase the load capacity which in-turn eliminates the requirement for the upgradation to new power system to handle new loads.
- Improved line voltage
As discussed above loads demanding reactive power is in-turn demanding reactive current. So as the demand for reactive power increases, more reactive current flows through the line and hence increasing the voltage drop in the conductor, this may result in low voltage at the load/equipment end. But we can reduce this voltage drop in the conductor and increasing load voltage by proper power factor correction.
- Reduced power system loss:
We know that the transmission line losses are directly proportional to square of the current moving through the conductor, reduction in line current is directly proportional to the power factor, from this it’s clearly that transmission line losses are inversely proportional to the square of the PF (power factor).
- Reduced carbon footprint:
If there is a reduction in our power system’s demand charge through power factor correction, the company/industry is putting less strain on the electricity grid, therefore reducing its carbon footprint.
- Static Capacitor:
Adding a shunt capacitor to the load which demands for a lagging reactive power can improve the power factor. This happens simply by connecting a capacitor in parallel to the load because the capacitor generates reactive power, and this lagging reactive power is supplied to the load/equipment which is in demand of a lagging reactive power, there by maintain good PF value.
- Synchronous Condenser:
Basically a synchronous condenser is an over-excited synchronous motor. Whenever a synchronous motor is over-excited, it takes current that is leading by an angle of $90^o$ from the supply voltage. This indicates that, it behaves like a capacitor.If we connect a synchronous condenser in parallel with the inductive load (like synchronous motor), Power factor correction is obtained in-turn improving the PF (power factor).
If we follow the definition of IEEE PES Task Force of FACTS: Static VAR Compensator (SVC): A shunt (parallel to the load) connected static VAR generator or absorber whose action is to adjust the reactive (capacitive or inductive) current so as to maintain or control some of the parameters of the electrical power transmission system (basically its bus voltage).
This is a general term used for a Thyristor Controlled Reactor (TCR) or Thyristor Switched Reactor (TSR) and/or Thyristor Switched Capacitor (TSC) Fig 1. The term, “SVC” has been used for shunt connected compensators, which are based on thyristors without gate turn-off capability.
It consists of separate devices for leading and lagging VARs; the thyristor-controlled or thyristor-switched reactor for absorbing reactive power and thyristor-switched capacitor for supplying the reactive power.
In this model we show the operation of a +300 Mvar/-100 Mvar Static Var Compensator (SVC).
In this model Static Var Compensator (SVC) system that regulates voltage on a 6000-MVA, 110-kV system (frequency 50Hz). The Static Var Compensator (SVC) consists of a 110kV/11-kV, 333-MVA coupling transformer, one 109-Mvar Thyristor-controlled Reactor bank and three 94-Mvar Thyristor- switched Capacitor banks (TSC1, TSC2, TSC3) connected on the secondary side of the transformer (frequency 50Hz). The simulation model of Static Var Compensator system is shown in the below figure:
The Static Var Compensator (SVC) is in voltage control mode and its reference voltage is set to Vref =1.0 p.u.. Initially the source voltage is set at 1.004 p.u., so it result in a 1.0 p.u. voltage at Static Var Compensator terminals when the static Var Compensator is out of service. As the Static Var Compensator is in a suspension state, the port current is initially floating (zero current). In this operating point, the Thyristor Switched Capacitor 1 (TSC1) is conducting (Qc = -94 Mvar), and the Thyristor Controlled Reactor bank is at full conduction (alpha=96 degrees). And the voltage drop of the regulator is 0.01 p.u./100VA (0.03p.u./300MVA). Therefore when the Static Var Compensator operating point changes from fully capacitive (+300 Mvar) to fully inductive (- 100 Mvar), the Static Var Compensator voltage varies between 1-0.03=0.97 p.u. and 1+0.01=1.01 p.u.
The following output obtained from MATLAB Simulation:
Power Factor correction using SVC (Static Var Compensator)
At t=0.4s, the source voltage is suddenly lowered to 0.934 pu.(as shown in the below diagram)
The Static Var Compensator (SVC) interacts by generating 256Mvar of reactive power, which causes an increasing in the voltage to 0.974 pu. We can see in the plot, the three Thyristor-switched Capacitors are on and providing their services. (As shown in the below diagram). This plot shows how the Thyristor-switched Capacitors are one after one sequentially switched on and off in order to generate the required reactive power.
The Thyristor-controlled Reactor (TCR) absorbs ~35\% of its nominal Q (reactive power) [ α =120o].
Each time a Thyristor-switched Capacitor (TSC) is switched on the Thyristor-controlled Reactor (TCR)’s α angle changes suddenly from 180o (no conduction) to 90o (full conduction), as we can see in below plot. Because the Thyristor-switched Capacitors (TSC) is put into action, in this stage the voltage and current are also not synchronized with each other, the current is almost leading the voltage by 90o.
At last, at t=0.7s the source voltage is increased to 1.0 p.u. and the Static Var Compensator reactive power is reduced to zero.
In this work we have used MATLAB simulink using static var compensator (SVC) to improve transient stability. The basic structure of (SVC) is operating under typical bus voltage control and its model was described. The model represents the controller as variable impedance that changes with the firing angle of the thyristor reactor (TCR), simulations carried out confirmed that SVC could provide the fast acting voltage support necessary to prevent the possibility of voltage instability at the bus to which the SVC is proposed. Matlab/simulink environment was used to carry out the simulation work and detailed results are shown to assess the dynamic performance of the SVC on the bus voltage stability.
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