Modelling, Simulation, and Performance of Active Series ... · PDF fileMATLAB model for Active Series Compensator . SSSC is located at bus 3. It is in series with the transmission

International Journal of Electronics Engineering Research.

ISSN 0975-6450 Volume 9, Number 3 (2017) pp. 429-439

© Research India Publications

http://www.ripublication.com

Modelling, Simulation, and Performance of Active

Series Compensators

Dipesh Shirishbhai Doshi

School of Electronics and Electrical Engineering, Lovely Professional University, Phagwara, India.

Shashank Shukla

School of Electronics and Electrical Engineering, Lovely Professional University, Phagwara, India.

Abstract

This paper represents the basic and schematic controls of the FACTS devices

which contains static synchronous series compensator (SSSC). A controllable

compensating voltage can be provided by the SSSC over an identical inductive

and capacitive range. In this type of compensator, external DC power supply

can be used, which will compensate the resistive voltage drop across the

impedance of the line. Phase Locked Loop is used for the proper

interconnection of SSSC with the grid. PLL has a beneficial effects on the

dynamic functioning of the SSSC and it will improve the dynamic

performance of the SSSC, a new auxiliary regulatory proposed. A model of

SSSC with the two area system is developed and simulated with the MATLAB

7.14 with different types of faults and SSSC will make the system stable after

fault.

Keywords: SSSC, FACTS, Voltage stability, compensation.

I. INTRODUCTION

It is very hard to work all the electrical devices without electricity. It is necessary to have quality and reliable supply with the increase of the power transmission and distribution [1]. With the time, cost and time consumption for construction of new buildings, new transmission lines and distribution lines increasing. Optimum power transfer and distribution add an advantage to decrease the transfer losses and to have

430 Dipesh Shirishbhai Doshi and Shashank Shukla

proper amount of energy at the receiving end. The main aim to establishment of transmission network is to pool the load centers power plant generation to minimize the fuel cost and generation capacity of power [2]. With the increase in power transfer, complexity of power system increases and it will become hard to operate it within the security limits. So to operate system in the security limits, proper controls is needed. The full potential of the power system cannot be utilized when there is excess reactive power in different part of the system and if there are large dynamic swings in the various parts of the system. Restructuring of grid enhance the system performance with the increase in the capabilities of transmission lines alone [15].

A. Principle of the series controller When the line voltage is in the quadrature with the current flowing through line then

the series controller produces or absorbs the reactive power from the system and when

it not in phase quadrature at the same time controller produces and absorbs the

reactive power as well as the real power from the system [2]. Static Synchronous

Series Compensators (SSSCs), TCSRs (Thyristor controlled series reactor), TSSC

(Thyristor switched series capacitors) are the good examples of the such types of

controllers. To damp out the system oscillation and to control the power flow and

current into the system, these types of controller work efficiently and effectively,

SSSC is the most crucial series FACTS devices which is solid state voltage source

inverters, injects voltage which is sinusoidal in nature in variable magnitude in series

with the system or can say with the transmission line. As told earlier series

compensators can inject voltage in almost in phased quadrature with the line current

[4].

Application of the SSSC are

1. For the power flow control

2. To improve the limits of power transfer

3. Transient stability improvement

4. To damp out the oscillation of power system

5. To damp out the Sub-Synchronous Resonance (SSR)

II. STATIC SYNCHRONOUS SERIES COMPENSATOR

A. Operating priciple The basic operating principle of SSSC cane dictate with the help of the conventional series compensation with capacitors as shown in figure below [5].

Figure 1: Basic two machine model of the system with the series compensator with the capacitors with the relative phasor diagram.[17]

Modelling, Simulation, and Performance of Active Series Compensators 431

The phasor diagram itself explains that voltage over the series capacitors force the line current opposite polarity voltage over the impedance line [16]. By increasing the voltage across the impedance of the named line is done by the series compensation [7]. To get same steady state power for transmission which can be established if the synchronous ac voltage source is provided with the series compensation as shown in figure 1, whose output voltage can be matched with the voltage of the voltage across series capacitor,

Vq = Vc = -jXcI = -jkXI (1)

Where

Vc = Injected compensation phasor voltage

I = Line current

Xc = Line reactance of the transmission line

k = Xc / X, it is degree of series compensation

j = sqrt (-1)

Figure 2: Operation block diagram of the synchronous voltage source based on a voltage sourced converter [17]

SSSC injects almost pure sinusoidal voltage with the variable magnitude voltage in series with the line. The injected voltage in the transmission line almost quadrature with the current in transmission line [6]. A voltage in phase with the transmission line which has small magnitude of injected voltage which causes losses in inverter circuit. The SSSC may include the energy storage devices which feed or absorb the real power which improve dynamic behavior of the system by adjusting for the real power, which also helps to diminish the total resistive voltage drop across the transmission line [5].

Above figure shows the relation between transmitted powers Pq versus the transmission angle δ. It can be expressed for two machine model as shown below [8]

P = (V2 / X) sin δ + (V / X) Vq cos (δ/2) (2)


The above figure shows the SSSC enhances or diminish the power transferred by a not variable fraction of the maximum power which is transferrable by the line which are uncompensated which is independent with the load angle δ with the load angle rang δ between 0 ≤ δ ≤ 90˚ [11].

Figure 3: Transferred power vs transmission angle furnished by the Static Series Synchronous Compensator as a parametric function of the series compensating voltage.

Moreover this interposed voltage is made bigger than the voltage which is across uncompensated voltage by the receiving and sending end system then the power flow will be reversed in the system.

B. Conventional PI controller for SSSC The possible controlling scheme for the controlled SSSC converter as depicted in

figure 3 [9]. This Controlling scheme is used to reduce the unnecessary output voltage

components due to the transition of DC capacitors voltage by using sub synchronous

and other line currents. If DC power supply which is also called as sink is used with

the system then it will provide both real and reactive power for the line compensation

[10].


Figure 4: Operation internal control of the Static Series Synchronous Compensator

applying a directly controlled converter. [17]

Here Phase Locked Loop plays main role to synchronize the line current components.

The control signals are operated by getting signals from three reference signals:

VqQRef defining the desired magnitude of the series reactive compensating voltage

[12]. The reference reactive voltage VqQRef which can be used to determine with the

help of the line current the reactive power exchange for series compensation and the

overall reference active voltage VqRRef + VdcRef. This real reference voltage with the

line current determines the active power exchange for the compensation of optinal

active power of the transmission line[14]. From the figure, it is visible that magnitude

Vq and the θ + Φ are used to generate the gate signal for the convertor valves. Here

operating voltage VdcRef is used to compensate the real voltage.

The power flow can be controlled by using the slow closed loop system, by operating

it from the selectable references which are XqRef or VqRef or IRef or PRef. other network

variables which are Xq or Vq or I or P is found from the voltage and current processors

and it will be compared to another reference. The error which will be further

amplified at the output terminal of the proportional integral controller which is

provided by the reference signals derived from the internal control [15].

To damp out subsynchronous oscillation and also improves transient as well as steady

state stability by the auxiliary control signals, which are derived from the frequency

variation, relevant system variables, power floe variation or the subysnchronous

oscillation variation, or synthesization of measured voltage and currents and also from

the subsynchronous oscillation which will be damp out from the auxiliary control

circuit and which is fed directly to the internal control block[13]. These auxiliary

control circuit also controls the main power flow control operation to avoid the

opposite reference requirements.


Figure 5: External control method for SSSC [17]

III. MODELLING AND SIMULATION

SSSC, consists a voltage source converter and a static device which is transformer connected in series with the line. A layout which is used in this paper is shown in figure below. SSSC is used to damp out the oscillation caused by three phase fault in to the system.

Figure 6: Test system

As shown into figure 6, figure consists two generating stations and a load at bus number 3. The generator 1 has ratings of 2100 MVA and the generator number two has the rating of 1400 MVA. Generating voltage for the both the generators are 13.8 KV. Almost approximately 2200 MW load is connected to the system. This load is the main cause of absorber of active and reactive power and is a function of the system voltage.


Generator 1 is connected to long transmission line L1 and L2 where L1 is 250 km long and transmission line L2 is 150 km long. Here transmission line split into two lines to simulate a three phase fault. The generator 2 is also connected to load with 50 km long transmission line.

Figure 6: MATLAB model for Active Series Compensator

SSSC is located at bus 3. It is in series with the transmission line L1. SSSC is rated with 100 MVA and it is possible to inject about 10% of normal system voltage. Here SSSC is made of typical three level PWM SSSC. In SSSC, a DC power source is provided with the ratings of 40KV which has capacitors which has capacitance of 375 micro faraday. POD (Power Oscillation Controller) is used to control the injected voltage reference by SSSC, whose output is connected to the Vqref input of the SSSC. The inputs of the POD controller is the voltage from the bus 2 and current flowing through the transmission line L1.

Figure 7: Output of the Voltage injected and voltage reference graph without any fault into the system


Figure 8: power output graph for bus 2 without any fault into the system.

When a fault occur into system which is single phase fault and occurred in the Phase A to ground fault which has transition time of 1/60 to 2/60. Then at that time voltage injected and voltage reference graph is going to be checked. Power output for bus 2 is also going to be plotted for the same transition time period.

Now here three phase fault is going to be considered and output graph for the voltage injected by SSSC and voltage reference is plotted and also power at bus 2 will be taken into consideration. Here fault for three phase to ground fault is taken into consideration which has transition period of [1/60 2/60].

Figure 8: output of the Voltage injected and voltage reference graph with fault in phase A to ground into the system


Figure 9: power output graph for bus 2 with fault in phase A to ground into the system.

Figure 10: output of the Voltage injected and voltage reference graph with three phase to ground fault into the system

Figure 11: power output graph for bus 2 with fault in all three phase into the system.


IV. CONCLUSION

In this paper, simulation is done with the various conditions, which are, improving

transient stability and power oscillation damping with the use of MATLAB. Here in

this paper, by modelling of the power system with the SSSC is shown and transient

stability of the system has been improved, has been explained. When LLLG fault is

introduced into the system, results are shown in the above section. From output plots,

it can be conclude that when LLLG fault has been created system will not go out of

synchronism and it will recover from it and again it will become stable. But without

the installation of SSSC system will undergo to loss of synchronism and system will

become unstable. Same thing happened when LG fault has been introduced to system.

System recovers from transient period immediately and became stable. So, we can

say that SSSC has the capability to improve the transient stability of the system. From

the results it is observed that controlling of power flow in to the system is preety fast

by introducing of SSSC in to the system.

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Modelling, Simulation, and Performance of Active Series ... · PDF fileMATLAB model for Active Series Compensator . SSSC is located at bus 3. It is in series with the transmission