Enhancement of Power Transfer Capability by using IGBT based Unified Power Flow Controller

Enhancement of Power Transfer Capability by using IGBT based Unified Power Flow Controller

Sujata Tandon

M.Tech Scholar Electrical Engg. Department BBSBEC, Fatehgarh Sahib

Lakhwinder Singh

Professor Electrical Engg. Department BBSBEC, Fatehgarh Sahib

Manpreet Singh

Assistant Professor Electrical Engg. Department BBSBEC, Fatehgarh Sahib

Abstract: Global demand for electrical power consumer has been drastically increased but the generation has not been increased proportionally. Due to this huge gap between generation capacity and consumer load demand, transmission lines are forced to operate at their maximum limits. To satisfy this condition, Flexible AC Transmission System (FACTS) controllers are widely used. In this paper, Unified Power Flow Controller (UPFC) is applied to enhance the power transfer capability of transmission line. UPFC is a combination of series and shunt Voltage Source Converter (VSC) and both the converters are coupled to each other through a DC link. UPFC controls the power flow in the transmission line by controlling line impedance, magnitude of voltage and phase angle of injected voltage or current in line. Methodology to improve power transfer capability of system has been tested in MATLAB simulation software.

Keywords: Unified Power Flow Controller (UPFC), Flexible AC Transmission System (FACTS), Series Converter, Shunt Converter, Voltage Source Converter (VSC).

1. INTRODUCTION:

   Since last two decades load has been increased at higher rate than generation capacity. As a result, transmission lines are overloaded and are more prone to severe outages. To cope this situation several methods are implemented to improve power transfer capability of transmission lines and one of them is FACTS controllers. FACTS are most acceptable devices in the world to solve the problem of power stability and voltage stability. Flexible AC transmission system controller (FACTS) is a power electronic static device which provides control over several lines simultaneously. FACTS controllers are divided as, First Generation FACTS Controllers; It is the earliest generation of FACTS controllers in which manual switching technology is used. In first generation SVC and TCSC are included. Second generation of FACTS Controllers; It is Thyristorised FACTS controllers generation, which includes STATCOM, SSSC, UPFC and IPFC. Third generation of FACTS controllers, it includes generalized unified power flow controller (GUPFC), Hybrid power flow controller(HPFC)

   Fourth generation of FACTS controllers, includes APFC. In power system, FACTS controllers are divided in four types.

   1. Series Controllers.

   2. Shunt Controllers.

   3. Combined series -series controllers

   4. Combined series-shunt controllers

   Series Controllers: Series controllers are connected in series with the transmission line. It could be variable impedance type. For example TCSC, TCVL, SSSC and IPC.

   Shunt Controllers: These controllers are connected in parallel with transmission line. Shunts controllers includes variable impedance, variable source or a combination of variable impedance and source. For example SVC, TSC, TCR, TCBR, STATCOM, BESS, SMES.

   Combined Series-Series Controllers: It is a unified series- series controller which provides independent control of reactive power flow in each line. For example IPFC. Combined Series-Shunt Controllers: In this type of controller both series and shunt controllers are connected to each other in coordinated manner. For example UPFC.

2. UNIFIED POWER FLOW CONTROLLER:

   Unified power flow controller is second generation family of FACTS controller which is used to control power flow over transmission line. It is a combination of two, back to back series shunt converters which is coupled through a DC link. Series converter injects voltage of controlled magnitude and phase angle in series to transmission line through series coupling transformer, similarly shunt converter injects current of controlled magnitude and phase angle to parallel with line through shunt transformer. Reactive power is a function of voltage, therefore by controlling voltage of transmission line, reactive power can be controlled and if reactive power in transmission line is limited, then active power flow in line will rise. Typical schematic diagram of UPFC controlled power system has been shown in Fig. 1.

3. METHODOLOGY:

   Figure. 1: UPFC controlled transmission line

4. MODELING OF FACTS CONTROLLER:

   There are many techniques available to solve the problem of power transfer capability and voltage sag like STATCOM, SSSC etc but in this paper UPFC scheme is used to solve the problem of power transfer capability. UPFC are more reliable and can control active and reactive power over transmission line, as shown in Fig. 2.

   The model of power flow study of UPFC is known as power injection model. Series converter is a main component which controls, phase angle, voltage magnitude and power injection in series with line. Therefore, only series voltage converter is described in Fig. 3.

   Figure. 2: Internal structure of UPFC

   For injection modeling following points are considered.

   • Consider transmission line as line series reactance jx. It is reactance taken from terminal voltage and is equal to

     max

     max

     .xs = xkr2 (SB/Ss) 3.1

     Where, xk resembles transformer series reactance, rmax denotes maximum per unit injected voltage magnitude, SB

     denotes base power of system and Ss denotes nominal rating power of series converter.

   • Replace the voltage source se by a current source inj= -jbs se in parallel with xs rmaxand 0 2 3.2

   se = rke-jwhere 0 r

   Figure. 3 Injection model of series converter

   The current source inj corresponds to injection powers iand j which are defined by

   2 2

   2 2

   i = i (-inj) = -rbsVi Sin() jrbsVi cos() 3.3

   j= j (inj) = rbsViVjsin(ij ) + jrbsViVjcos(ij ) 3.4

   Where ij=i j, and bs = 1/ xs.

   Fig 3 shows injection model of the series converter part of upfc, in this

   Pi = -real(i ), Qi = -imag(i )

   Pj = -real(j), Qj = -imag(j)

   If we neglect UPFC loss then,

   Pconv 1 = Pconv 2 3.5

   Apparent power generated by the series voltage source converter is:

   CONV 2 =

   sese

   = rej i( ) 3.6

   Equations of active and reactive power supplied by series converter are:

   i

   i

   Pconv2 =rbsViVjsin(i – j+ ) rbsV 2sin 3 .7

   Qconv2 = – rbsViVjcos(i – j+ ) rbsV 2cos + r2bsV 2 3.8

   i i

   At the UPFC shunt side, the active and reactive power flows are given as

   2

   2

   Pi1 =- rbsViVjsin(ij + ) bsVi sinij 3.9

   2 2

   2 2

   Qi1 = – rbsViVjcos + Qconv1 + bsVi cosij – bsVi 3.10

   Where at series side they are

   Pj2 =rbsViVjsin(ij + ) +bsViVjsinij 3.11

   2 2

   2 2

   Qj2 = rbsViVjcos + Qconv1 + bsVi cosij – bsVi 3.12

   The UPFC injection model is thereby defined by the constant series branch susceptance bs which is included in the system bus admittance matrix, and the bus power injections Psi, Qsi, Psi, and Qsj

5. MATLAB SIMULATIONS:

   Simulation model of both compensated and uncompensated transmission line has been shown in Fig. 4, Fig. 5 and Fig. 6, respectively. In compensated model UPFC controller is used to compensate line. Load 2 (4KW) is connected with transmission line through circuit breake at 0.3 sec. After the connection in transmission line voltage sag is present in uncompensated line which can be observed from Fig. 8 and also active and reactive power transfer is low, which is shown in Fig. 7. But in case of compensated line voltage profile is improved as well as power transfer capability is also increased as compared to uncompensated line shown in Fig. 9 and Fig. 10.

   Figure.4: Model of Uncompensated Line

   Figure. 5: Model of Compensated line

   Figure. 6: Open loop model of UPFC

   Figure. 7: Power flow graph of uncompensated line

   Figure.8: Voltage sag across load in uncompensated line

   Figure.9: Power flow graph in compensated line

   Figure.10: Voltage graph in Compensated line

6. RESULTS:

To enhance the power transfer capability of transmission line, UPFC controller has been used. Simulation results of both the uncompensated and compensated line have been obtained and are summarized in Table 1 given below.

Table 1: Comparison of uncompensated and compensated line response.

CONCLUSIONS:

In this paper, Unified Power Flow Controller (UPFC) has been successfully implemented on transmission line having non linear load. By the output response of simulation, it can be observed that the UPFC improves voltage profile as well as power transfer capability of transmission line.

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