Paper id 26201417

International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637

To Improve Power Quality of Wind Farm Grid Using UPQC V.Ramesh PHD Scholar, EEE Department KL University, Green fields Vaddeswaram, Guntur Dist, India Email: rameshvaddi6013@kluniversity.in

P.Anjappa HOD & Associate Professor Department of Electrical Engineering, GVIC Engineering, Madanapalli, A.P, India Email: anji_abhi@yahoo.co.in

N.Devasana, P. Lakshmipathi, U.Haritha, K. Reddy Swathi Email: devasena.nagishetty@gmail.com, plp.826@gmail.com, haritha.uttaradi3@gmail.com reddyswathik777@gmail.com

Abstract - Squirrel Cage Induction Generator (SCIG) directly connected to Wind Farms (WF) to form the grid, represent a large percentage of the wind energy conversion systems around the power sector scenario. A situation commonly found in such scheme is that the power generated is comparable to the transport capacity of the grid. This case is known as Wind Farm to Weak Grid Connection, and its main problem is the poor voltage regulation at the point of common coupling (PCC). Thus, the effect of weak grids, wind power fluctuation and system load changes produce disturbances in the PCC voltage, weakening the Power Quality and WF stability. This can be improved by using Unified Power quality Conditioner (UPQC) at PCC. In this paper UPQC device was developed to regulate the voltage in the WF terminals, and to mitigate voltage fluctuations at grid side. The internal control strategy is based on the management of series and shunt active filters and the exchange of power between the converters through UPQC DC–Link. This approach increases the compensation capability of the UPQC with respect to other custom strategies that use reactive power only. Simulations results show the effectiveness of the proposed compensation strategy for the enhancement of Power Quality and Wind Farm stability.

Index Terms: UPQC, PCC, SCIG, SVC

1. INTRODUCTION The location of generation facilities for wind energy is determined by wind energy resource availability, often far from high voltage (HV) power transmission grids and major consumption centers.[1] In case of facilities with medium power ratings, the WF is connected through medium voltage (MV) distribution headlines. A situation commonly found in such scheme is that the power generated is comparable to the transport power capacity of the power grid to which the WF is connected, also known as weak grid connection. The main feature of this type of connections is increased in voltage regulation sensitivity to changes in load. So, the system’s ability to regulate voltage at the point of common coupling (PCC) to the electrical system is a key factor for the successful operation of the WF. [2]-[3]It is well known that due to the random nature of wind resources, the WF generates fluctuating electric power. These fluctuations have a negative impact on stability and power quality in electric power systems.

Moreover, in exploitation of wind resources, turbines employing squirrel cage induction generators .This power disturbances propagate into the grid, and can produce a phenomenon known as “flicker”, which consists of fluctuations in the illumination level caused by voltage variations. In case of “weak grids”, the impact is even greater. In order to reduce the voltage fluctuations that may cause “flicker”, and improve WF terminal voltage regulation, several solutions have been proposed. In recent years, the technological development of high power electronics devices has led to implementation of controlling the power flow in transmission systems using Flexible AC Transmission System (FACTS) devices enhancing the power quality in distribution systems employing Custom Power System (CUPS) devices.

The use of these active compensators to improve integration of wind energy in weak grids is the approach adopted in this work. In this paper it is proposed to analyze a compensation strategy using an UPQC, for the case ofSCIG–based Connected to a 170

International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 weak distribution power grid. This system is taken from a real case [4]. The UPQC is controlled to regulate the WF terminal voltage, and to mitigate voltage fluctuations at the point of common coupling (PCC), caused by system load changes and pulsating WF generated power, respectively. The voltage regulation at WF terminal is conducted using the UPQC series converter, by voltage injection “in phase� with PCC voltage.

the shunt converter is represented by a current source IC. Note that all the currents and voltages are 3 dimensional vectors with phase coordinates. Unlike in the case of a UPFC, the voltages and currents may contain negative and zero sequence components in addition to harmonics[7]. The operation of a UPQC can be explained from the analysis of the idealized equivalent circuit shown in below.

A. Operation of UPQC

, Figure 1. Study case power system .

On the other hand, the shunt converter is used to filter the WF generated power to prevent voltage fluctuations, requiring active and reactive power handling capability [5]. The sharing of active power between converters is managed through the common DC link. Simulations were carried out to demonstrate the effectiveness of the proposed compensation approach.

2.

UNIFIED POWER QUALITY CONDITIONER

The provision of both DSTATCOM and DVR can control the power quality of the source current and the load bus voltage. In addition, if the DVR and STATCOM are connected on the DC side, the DC bus voltage can be regulated by the shunt connected DSTATCOM while the DVR supplies the required energy to the load in case of the transient disturbances in source voltage[6]. The configuration of such a device (termed as Unified Power Quality Conditioner (UPQC)) is shown in Fig. 2. This is a versatile device similar to a UPFC. However, the control objectives of a UPQC are quite different from that of a UPFC. The operation of a UPQC can be explained from the analysis of the idealized equivalent circuit shown in above. Here, the series converter is represented by a voltage source VC and

Figure 2 - Basic System Configuration of UPQC Here, the series converter is represented by a voltage source VC and the shunt converter is represented by a current source IC. Note that all the currents and voltages are 3 dimensional vectors with phase coordinates. Unlike in the case of a UPFC, the voltages and currents may contain negative and zero sequence components in addition to harmonics.

Neglecting losses in the converters, we get the relation

Where X,Y denote the inner product of two vectors, defined by

Let the load current IL and the source voltage VS be decomposed into two Components given by

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637

Where I1p L contains only positive sequence, fundamental frequency components Similar comments apply to V 1pS. IrL and V rS contain rest of the load current and the source voltage including harmonics. I1pL is not unique and depends on the power factor at the load bus. However, the following relation applies for I1p L.

This implies that hIrL; VLi = 0. Thus, the fundamental frequency, positive sequence component in IrL does not contribute to the active power in the load. To meet the control objectives, the desired load voltages and source currents must contain only positive sequence, fundamental frequency components and

Where V ¤ L and I¤S are the reference quantities for the load bus voltage and the source current respectively. Ál is the power factor angle at the load bus while Ás is the power factor angle at the source bus (input port of UPQC). Note that V * L(t) and I¤S (t) are sinusoidal and balanced. If the reference current (I¤C ) of the shunt converter and the reference voltage (V * C) of the series

converter are chosen as

With the constraint

We have,

Note that the constraint implies that V 1p C is the reactive voltage in quadrature with the desired source current, IS . It is easy to derive that

.

The above equation shows that for the operating conditions assumed, a UPQC can be viewed as a inaction of a DVR and a STATCOM with no active power through the DC link. However, if the magnitude of V ¤ L is to be controlled, it may not be feasible to achieve this by injecting only reactive voltage. The situation gets complicated if V 1p S is not constant, but changes due to system

disturbances or fault. To ensure the regulation of the load bus voltage it may be necessary to inject variable active voltage (in phase with the source current).

If we express

In deriving the above, we assume that

This implies that both ¢VC and ¢IC are perturbations involving positive sequence, fundamental frequency quantities (say, resulting from symmetric voltage sags). the power balance on the DC side of the shunt and series converter. The perturbation in VC is initiated to ensure that

Thus, the objective of the voltage regulation at the load bus may require exchange of power between the shunt and series converters.

B. Voltage Fluctuations Although blackouts seem to get all the attention, major power problems can be attributed to voltage fluctuations such as sags, surges and impulses. Thus, it is not surprising that users tend to overlook this issue and do not pay much attention to protection against invisible voltage fluctuations. The majority of users believe uninterruptible power

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 supply (UPS) systems are a universal remedy for all power-related problems. The end result is a modest market for voltage regulators used to regulate the ac from the outlet. A recent study conducted by research firm Frost & Sullivan reveals this market generated worldwide revenue of only $203.3 million in 2003, and is expected to grow at an average annual growth rate of 4.5% over next few years. At this rate, it is estimated to reach $277.7 million by 2010.By comparison; UPS is expected to grow from $4.7 billion in 2003 to $6.3 billion, according to Venture Development Corp. Although, this market has also suffered because of recent economic conditions and cuts in spending, the global awareness of the benefits of power protection and recovery in the market is expected to improve revenues in the coming years.

C. Basic Problems with Wind Turbines in Weak Grids-Voltage level

often will be the case in sparsely populated areas the voltage profile of the feeder will be much different from the no wind case. Due to the power production at the wind turbine the voltage level can and in most cases will be higher than in the no wind case.

As is seen on the figure the voltage level can exceed the maximum allowed when the consumer load is low and the power output from the wind turbines is high. This is what limits the capacity of the feeder.

The voltage profile of the feeder depends on the line impedance, the point of connection of the wind turbines and on the wind power production and the consumer load. For a simple single load case the voltage rise over the grid impedance can be approximated with ∆U * (R * P + X *Q) /U using generator sign convention. This formula indicates some of the possible solutions to the problem with absorption of wind power in weak grids. The main options are either a reduction of the active power or an increase of the reactive power consumption or a reduction of the line impedance.

D. Voltage fluctuations

Figure 3. Example of voltage profile for feeder with and without wind power The main problem with wind energy in weak grids is the quasi-static voltage level. In a grid without wind turbines connected the main concern by the utility is the minimum voltage level at the far end of the feeder when the consumer load is at its maximum. So the normal voltage profile for a feeder without wind energy is that the highest voltage is at the bus bar at the substation and that it drops to reach the minimum at the far end. The settings of the transformers by the utility are usually so, that the voltage at the consumer closest to the transformer will experience a voltage, that is close to the maximum value especially when the load is low and that the voltage is close to the minimum value at the far end when the load is high.

This operation ensures that the capacity of the feeder is utilised to its maximum. When wind turbines are connected to the same feeder as consumers which

Another possible problem with wind turbines in weak grids are the possible voltage fluctuations as a result of the power fluctuations that comes from the turbulence in the wind and from starts and stops of the wind turbines. As the grids becomes weaker the voltage fluctuations increase given cause to what is termed as flicker. Flicker is visual fluctuations in the light intensity as a result of voltage fluctuations. The human eye is especially sensitive to these fluctuations if they are in the frequency range of 1-10 Hertz.

Flicker and flicker levels are defined in IEC1000-3-7, [1]. During normal operation the wind turbulence causes power fluctuations mainly in the frequency range of 1-2 Hertz due to rotational sampling of the turbulence by the blades. This together with the tower shadow and wind shear are the main contributors to the flicker produced by the wind turbine during normal operation. The other main contribution to the flicker emission is the cut-in of the wind turbine.

During cut-in the generator is connected to the grid via a soft starter. The soft starter limits the current but even with a soft starter the current during cut-in can be very high due to the limited time available for cutin. especially the magnetisation current at cut-in contributes to the flicker emission from a wind turbine.

3. UPQC CONTROL STRATEGY The UPQC serial converter is controlled to maintain the WF terminal voltage at nominal value (see U1

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 bus-bar in Fig.4), thus compensating the PCC voltage variations. In this way, the voltage disturbances coming from the grid cannot spread to the WF facilities. As a side effect, this control action may increase the low voltage ride–through (LVRT) capability in the occurrence of voltage sags in the WF terminals [4], [9]. Fig.5 shows a block diagram of the series converter controller. The injected voltage is obtained subtracting the PCC voltage from the reference voltage, and is phase–aligned with the PCC voltage (see Fig.3). On the other hand, the shunt converter of UPQC is used to filter the active and reactive power pulsations generated by the WF. Thus, the power injected into the grid from the WF compensator set will be free from pulsations, which are the origin of voltage fluctuation that can propagate into the system. This task is achieved by appropriate electrical currents injection in PCC. Also, the regulation of the DC bus voltage has been assigned to this converter. Fig.6 shows a block diagram of the shunt converter controller. This controller generates both voltages commands

flicker. The control action for the DC–bus voltage loop.This control loop will not interact with the fluctuating power compensation, because its components are lower in frequency than the flicker– band. The powers PshuC and QshuC are calculated in the rotating reference frame, as follows: Ignoring PCC voltage variation, these equations can be written as follows. Taking in consideration that the shunt converter is based on a VSI, we need to generate adequate voltages to obtain the currents in (6). This is achieved using the VSI model proposed sin, leading to a linear relationship between the generated power and the controller voltages. The resultant equations are:P and Q control loops comprise proportional controllers, while DC–bus loop, a PI controller. In summary, in the proposed strategy the UPQC can be seen as a “power buffer”, leveling the power

injected into the power.

Figure 5 Power buffer concept System grid. The Fig.7 illustrates a conceptual diagram of this mode of operation. It must be remarked that the absence of an external DC source in the UPQC bus, forces to maintain zero–average power in the storage element installed in that bus.

This is accomplished by a proper design of DC voltage controller. Also, it is necessary to note that the proposed strategy cannot be implemented using other CUPS devices like D–Statcom or DVR. The power buffer concept may be implemented using a DStatcom, but not using a DVR. On the other hand, voltage regulation during relatively large disturbances cannot be easily coped using reactive power only from DStatcom; in this case, a DVR device is more suitable.

Figure 4 Shunt compensator controller 4.

SIMULATION RESULTS

Deviations are calculated subtracting the mean power from the instantaneous power measured in PCC. The mean values of active and reactive power are obtained by low–pass filtering, and the bandwidth of such filters is chosen so that the power fluctuation components selected for compensation, fall into the

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 Matlab design of case study:

Figure 6.3 Power of the capacitor in the dc bus

Figure 6 Simulink Diagram The model of the power system scheme illustrated in Fig, including the controllers with the control strategy detailed in section III, was implemented usingMatlab/Simulinksoftware.Numericalsimulations were performed to determine and then compensate voltage fluctuation due to wind power variation, and voltage regulation problems due to a sudden load connection. The simulation was conducted with the following chronology: • at t = 0.0′′ the simulation starts with the series converter and the DC–bus voltage controllers in operation.

• at t = 0.5′′ the tower shadow effect starts; • at t = 3.0′′ Q and P control loops(see Fig.6) are enabled; • at t = 6.0′′ L3 load is connected.

• at t = 6.0′′ L3 load is disconnected

Figure 6.4 Voltage of the capacitor in the dc bus

Figure 6.5 Voltage at WF

Figure 6.6 Series injected voltage at “a” phase

Figure 6.7 shunt and series converter active power Figure 6.1 Active and reactive power demand at power grid side

Figure 6.8 DC bus voltage

Figure 6.2 PCC voltage voltage wind farm

V. CONCLUSION In this paper, a new compensation strategy implemented using an UPQC type compensator was presented, to connect SCIG based wind farms to weak distribution power grid. The proposed compensation scheme enhances the system power 175

International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 quality, exploiting fully DC–bus energy storage and active power sharing between UPQC converters, features not present in DVR and D–Statcom compensators. The simulation results show a good performance in the rejection of power fluctuation due to “tower shadow effect” and the regulation of voltage due to a sudden load connection. So, the effectiveness of the proposed compensation approach is demonstrated in the study case. In future work, performance comparison between different compensator types will be made.

REFERENCES [1] M.P. P´alsson, K. Uhlen, J.O.G. Tande. “Largescale Wind Power Integration and Voltage Stability Limits in Regional Networks”; IEEE 2002. p.p. 762–769 [2] P. Ledesma, J. Usaola, J.L. Rodriguez “Transient stability of a fixed speed wind farm” Renewable Energy 28, 2003 pp.1341–1355 [3] P. Rosas “Dynamic influences of wind power on the power system”. Technical report RISØR1408. Ørsted Institute. March 2003.

[4] R.C. Dugan, M.F. McGranahan, S. Santoso, H.W. Beaty “Electrical Power Systems Quality” 2nd Edition McGraw–Hill, 2002. ISBN 0-07- 138622X [5] P. Kundur “Power System Stability and Control” McGraw-Hill, 1994. ISBN 0-07-035958-X [6] N. G. Hingorani y L. Gyugyi. “Understanding FACTS”. IEEE Press; 2000.

[7] Z. Saad-Saoud, M.L. Lisboa, J.B. Ekanayake, N. Jenkins and G. Strbac “Application of STATCOM’s to wind farms” IEE Proc. Gen. Trans. Distrib. vol. 145, No. 5; Sept. 1998

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