Power Conditioning Of Microgrid By Using A Fast-Acting Dc-Link Voltage Controller For 3-Phase D-Statcom

Power Conditioning Of Microgrid By Using A Fast-Acting Dc-Link Voltage Controller For 3-Phase D-Statcom

ABSTRACT:-

Microgrids are systems with clusters of micro generators, which are installed for distributed power generation. When interfaced to the utility grid, microgrids are exposed to common utility power-quality disturbances. The paper focuses on the combination of wind, FC and for sustained power generation. We propose herein a dynamic model, design and simulation of a wind/FC/ hybrid power generation system with power flow controllers.

This paper presents a novel D-Statcom configuration for voltage quality improvement in MICRO GRID. The transient response of the distribution static compensator (DSTATCOM) is very important while compensating rapidly varying unbalanced and nonlinear loads. Any change in the load affects the dc-link voltage directly. The sudden removal of load would result in an increase in the dc-link voltage above the reference value, whereas a sudden increase in load would reduce the dc-link voltage below its reference value. The proper operation of DSTATCOM requires variation of the dc-link voltage within the prescribed limits .Conventionally, a proportional-integral (PI) controller is used to maintain the dc-link voltage to the reference value. It uses deviation of the capacitor voltage from its reference value as its input. However, the transient response of the conventional PI dc-link voltage controller is slow. In this paper, a fast-acting dc-link voltage controller based on the energy of a dc-link capacitor is proposed

I. INTRODUCTION

This white paper proposes that the significant potential of smaller DER (< 100 kW/unit) to meet customers and

current at the point of common coupling (PCC) so that harmonic filtering, power factor correction, and load balancing can be achieved. The DSTATCOM consists of a current-controlled voltage-source inverter (VSI) which injects current at the PCC through the interface inductor. The operation of VSI is supported by a dc storage capacitor with proper dc voltage across it.

One important aspect of the compensation is the extraction of reference currents. Various control algorithms are available in literature [7][11] to compute the reference compensator currents.

However, due to the simplicity in formulation and no confusion regarding the definition of powers, the control algorithm based on instantaneous symmetrical component theory [11] is preferred. Based on this algorithm, the compensator reference currents ( , , ) are given as follows:

)

) (1)

utilities needs can be best captured by organizing these

resources into MicroGrids1. MicroGrids are envisioned as )

clusters of generators (including heat recovery), storage,

and loads that are operated as single controllable systems. MicroGrids can operate both connected to and synchronized with the utility distribution grid and in isolation from the utility distribution grid (as an island).

Power quality problems like voltage sag, voltage swell and harmonic are major concern of the industrial and commercial electrical consumers due to enormous loss in terms of time and money. Due to the increasing complexity in the power system, voltage sags are now becoming one of the most significant power quality problems and deserves attention. Voltage sag is defined as a momentary decrease in voltage rms value between 10% to 90% of the nominal voltage level, for duration of half a cycle to one minute. A power quality study done recently indicates that 92 % of all disturbances in a power system are caused by voltage sags [1].

Therefore, these adverse effects of voltage changes necessitate the existence of effective mitigating devices. There are various solutions to these problems. The shunt-connected custom power device, called the distribution static compensator (DSTATCOM), injects

where = tan/3 is the desired phase angle between the supply voltages and compensated source currents in the respective phases. For unity power factor operation, = 0, thus = 0. The term Plavg is the dc or average value of the load power. The term Pdc in (1) accounts for the losses in the VSI without any dc loads in its dc link. To generate, a suitable closed-loop dc-link voltage controller should be used, which will regulate the dc voltage to the reference value.

For the DSTATCOM compensating unbalanced and nonlinear loads, the transient performance of the compensator is decided by the computation time of average load power and losses in the compensator. In most DSTATCOM applications, losses in the VSI are a fraction of the average load power.

Therefore, the transient performance of the compensator mostly depends on the computation of Plavg. In this paper, Plavg is computed by using a moving average filter (MAF) to ensure fast dynamic response. The settling time of the MAF is a half-cycle period in case of odd harmonics and one cycle period in case of even harmonics presence in voltages and currents. Although the computation of is Pdc

generally slow and updated once or twice in a cycle, being a small value compared to Plavg, it does not play a significant role in transient performance of the compensator.

In some of the electric power consumers, such as the telecommunications industry, power-electronics drive applications, etc., there is a requirement for ac as well as dc loads [12][15]. The telecommunication industry uses several parallel-connected switch-mode rectifiers to support dc bus voltage. Such an arrangement draws nonlinear load currents from the utility. This causes poor power factor and, hence, more losses and less efficiency. Clearly, there are PQ issues, such as unbalance, poor power factor, and harmonics produced by telecom equipment in power distribution networks. Therefore, the functionalities of the conventional DSTATCOM should be increased to mitigate the aforementioned PQ problems and to supply the dc loads from its dc link as well. The load sharing by the ac and dc bus depends upon the design and the rating of the VSI. This DSTATCOM differs from conventional one in the sense that its dc link not only supports instantaneous compensation but also supplies dc loads.

However, when the dc link of the DSTATCOM supplies the dc load as well, the corresponding dc power is comparable to the average load power and, hence, plays a major role in the transient response of the compensator. Hence, there are two important issues. The first one is the regulation of the dc-link voltage within prescribed limits under transient load conditions. The second one is the settling time of the dclink voltage controller. Conventionally, a PI controller is used to maintain the dc- link voltage. It uses the deviation of the capacitor voltage from its reference value as its input. However, the transient response of the conventional dc-link voltage controllers is slow, especially in applications where the load changes rapidly. Some work related to dc-link voltage controllers and their stability is reported in [16][20]. However, the work is limited to rectifier units where switchingpatterns are well defined and analysis can be easily carried out. In this paper, a fast-acting dc-link voltage controller based on the dc-link capacitor energy is proposed. The detailed modeling, simulation, is given to prove the efficacy of this fast-acting dc-link voltage controller. There is no systematic procedure to design the gains of the conventional PI controller used to regulate the dc-link voltage of the DSTATCOM. Herewith, mathematical equations are given

to design the gains of the conventional controller based on the fast-acting dc-link voltage controllers to achieve similar

II Designe of Microgrid

A.Wind Turbine: In this paper the modelling of wind turbine is described. The three bladed rotor is the most important and most visible part of the wind turbine. It is through the rotor that the energy of the wind is transformed into mechanical energy that turns the main shaft of the wind turbine.

1. uel Cell:Fuel cells convert the chemical energy of a fuel and an oxidant directly into electrical energy and heat using electrochemical processesnot combustion. Sir William Grove invented the first fuel cell in 1839. Grove knew that water could be split into hydrogen and oxygen by sending an electric current through it (a process called electrolysis). Perhaps the simplest system, a Proton Exchange Membrane Fuel Cell (PEMFC), combines hydrogen fuel with oxygen from the air to produce electricity, water, and heat. A Basic Proton Exchange Membrane Fuel Cell consists of 3 components: an anode (a negative electrode that repels electrons), an electrolyte in the center, and a cathode (a positive electrode that attracts electrons).As hydrogen flows into the fuel cell anode, a catalyst, often a platinum coating on the anode helps to separate the gas into protons (hydrogen ions) and electrons.

   Figure2: Block Diagram of formation of Anode(H2) of PEMFC

   Figure 3: Block diagram of formation of Cathode(O2) of PEMFC

   The PEMFC has a high power density and a relatively low operating temperature (ranging from 60 to 80 degrees Celsius, or 140 to 176 degrees Fahrenheit). The low operating temperature means that it doesn't take very long for the fuel cell to warm up and begin generating electricity. Fuel Cell Performance Parameters:-Fuel cell performance is affected by design parameters such as cell size, power level, cost and by operating variables such as temperature, pressure, fuel composition andcurrent density.

   1

   fast transient response.

   o

   RT ln{ 2 . P 1}…(2)

   2F 2

   III MODELLING OF FUEL CELL & WIND TURBINE

   Figure 1: Block diagram of the µgrid with D-Statcom to Load

   1. Fuel Cell Steady State Behaviour:-

      The experimental voltage-current data of the 500W Polymer exchange membrane fuel cell (PEMFC)

      built by Fuel Cell Technologies, Ltd,.Fig. 1shows the data points that are curve-fitted using quadratic, cubic, and fourth order polynomials.In addition, the relation between output power and current (P-I curve) is also shown.

      …(3)

      where V is the fuel cell terminal voltage and I is the current.

      The minimum least squares error method is used to obtain the closest approximation.

      V * I = I2 * Z

      V=I * Z…(4)

      where Z is the load impedance. Equations (3) and (4) are solved simultaneously to obtain an equation in terms of the current and the load impedance as follows

      …(5)

      Figure 4: Simulink diagram of Fuel Cell Reaction

      The Newton-Raphson iterative method is employed to solve the non-linear equation (5) and determine the fuel cell current. The method for solving an equation f(x) = 0 is outlined as follows [5]

      … (6)

      wherexn is the estimate at the nth iteration and xn+1 is the updated estimate. The iteration process is terminated when f(xn+1) <= 0, where is the tolerance limit and xn+1 is the required solution. The method is terminated if f1(xn) =0, load impedance is not negative for a fuel cell power system being simulated.

      i.e.E=W= Fs

      According to Newtons law we have,F= ma Hence E = mas …. (7)

      Using Newtons third equation of motion v2 = u2 + 2as

      We can get a = )

      Since the initial velocity of the object is zero, i.e. u=0, we

      get a =

      Substituting above equation in equation (7), we get that the kinetic energy of a mass in motions is

      E = ….(8)

      The power in the wind is given by the rate of change of energy

      P = = ….(9)

      As mass flow rate is given by And the rate of change of distance is given by

      We get

      Hence from equation (3.3) the power can be defined as

      P = …(10)

      Figure 7: Simulink diag of the Wind Turbine connected to µgird

      Figure 8: Internal simulink diagram of the Wind Turbine

      The swept area of the turbine can be calculated from the length of the turbine blades using the equation for the area of a circle:

      Figure 5: Simulink diag of the Fuel Cell connected to µgird

      Figure 6: Internal simulink diagram of above Fuel Cell Stack

   2. Mathematical Model of Wind Turbine:-

Under constant acceleration, the kinetic energy of an object having mass m and velocity v is equal to the work done W in displacing that object from rest to a distance s under a force F,

….(11)

Where the radius is equal to the blade length.

Figure 9: Subsystem of Microgrid

IV Conventional DC-Link Voltage Controller

The conventional PI controller used for maintaining the dc- link voltage is shown in Fig. 2. To maintain the dc-link voltage at the reference value, the dc-link capacitor needs a certain amount of real power, which is proportional to the difference between the actual and reference voltages. The power required by the capacitor can be expressed as follows:

(12)

where Tc is the ripple period of the dc-link capacitor voltage. Some control schemes have been reported in [33] and [34].

However, due to the lack of integral term, there is a steady- state error while compensating the combined ac and dc loads. This is eliminated by including an integral term. The input to this controller is the error between the squares of reference and the actual capacitor voltages. This controller is shown in Fig. 3 and the total dc power required by the dc- link capacitor is computed as follows:

(15)

Figure 10.Conventional DC link voltage controller

Figure 11.Fast acting DC-link voltage controller

The dc-link capacitor has slow dynamics compared to the compensator, since the capacitor voltage is sampled at every zero crossing of phase supply voltage. The sampling can also be performed at a quarter cycle depending upon the symmetry of the dc-link voltage waveform. The drawback of this conventional controller is that its transient response is slow, especially for fast-changing loads. Also, the design of PI controller parameters is quite difficult for a complex system and, hence, these parameters are chosen by trial and error. Moreover, the dynamic response during the transients is totally dependent on the values of Kp, Ki and Pdc when is comparable to Plavg .

V Fast-Acting DC Link Voltage Controller

To overcome the disadvantages of the aforementioned controller, an energy-based dc-link voltage controller is proposed. The energy required by the dc-link capacitor (Wdc) to charge from actual voltage (Vdc) to the reference value (Vdcref) can be computed as

(13)

In general, the dc-link capacitor voltage has ripples with double frequency, that of the supply frequency. The dc power (Pdc) required by the dc-link capacitor is given as

(14)

The coefficients Kpe and Kie are the proportional and integral gains of the proposed energy-based dc-link voltage controller. As an energy-based controller, it gives fast response compared to the conventional PI controller. Thus, it can be called a fast acting dc-link voltage controller. The ease in the calculation of the proportional and integral gains is an additonal advantage. The value of the proportional controller gain Kpe can be given as

(16)

For example, if the value of dc-link capacitor is 2200 F and the capacitor voltage ripple period as 0.01 s, then Kpe is computed as 0.11 by using (12). The selection of Kie depends upon the tradeoff between the transient response and overshoot in the compensated source current. Once this proportional gain is selected, integral gain is tuned around and chosen to be 0.5. It is found that if Kie is greater than Kpe , the response tends to be oscillatory and if is less than Kpe , then response tends to be sluggish. Hence, Kie is chosen to be Kpe/2 .

VI Selection of the DC-Link Capacitor

The value of the dc-link capacitor can be selected based on its ability to regulate the voltage under transient conditions. Let us assume that the compensator in Fig. 1 is connected to a system with the rating of X kilovolt amperes. The energy of the system is given by X X 1000 J/s. Let us further assume that the compensator deals with half (i.e.,X/2 ) and twice (i.e.,2X) capacity under the transient conditions for n cycles with the system voltage period of T s. Then, the change in energy to be dealt with by the dc capacitor is given as

E (2X X/2)nT (17)

Now this change in energy (21) should be supported by the energy stored in the dc capacitor. Let us allow the dc capacitor to change its total dc-link voltage from 1.4 Vm to

1.8 Vm during the transient conditions where Vm is the peak value of phase voltage. Hence, we can write

which implies that

T (18)

(19)

the conventional dc-link voltage controller are not properly chosen, the dc-link voltage would have undesirable overshoot and considerably large settling time. Consequently, the performance of the load connected to the dc link also gets affected due to the above factors.

For example, consider a 10-kVA system (i.e., X = 10 kVA), system peak voltage Vm = 325.2 V, n = 0.5, and T =

0.02 s. The value of Cdc computed using (23) is 2216 F. Practically, 2000 F is readily available and the same value has been taken for simulation and experimental studies.

SIMULATION & RESULTS

Figure 12: Simulink Diagram of the MicroGrid with MLCI DVR connected to Load

Results of Wind Turbine:-Here we considered Wind turbine as rotate with constant speed i.e 19.4 rpm.

Figure 13: Voltage and Current shared by Wind Turbine

Figure 14: Voltage & Power shared by single Fuel Cell

In the case of conventional, at the instant t = 0.4 s. Due to a sudden reduction in the load, the dc-link capacitor absorbs surplus power from the source. Therefore, there is an increase in dc-link capacitor voltage above the reference value. Based on the values of PI controller gains, the dc-link capacitor voltage controller will be brought back to the reference value after a few cycles. Similarly, when the load is switched back to the full load at instant t = 0.8 s, the dc capacitor supplies power to the load momentarily and, hence, the dc-link voltage falls below the reference value. Due to the PI controller action, the capacitor voltage will gradually build up and reach its reference value. If gains of

Figure 15: Transient response of the conventional controller.(a).compensated source current in phase a. (b).DC-link voltage

Fast- Acting Dc-Link, at the instant ts = 0.4 s, the capacitor voltage increases due to the sudden removal of the load. The fast-acting dc-link voltage controller takes action at the instant ts = 0.41 s. This is because the controller output is updated at every half cycle. It computes the dc load power Pdc needed to bring the capacitor voltage to the reference value in a half cycle. Therefore, the dc-link voltage reaches its reference voltage at the instant ts = 0.42 s. When the dc- link voltage is more than the reference value, Pdc is less. Therefore, the source currents are less in magnitude.

At the instant ts = 0.8 s, the dc-link voltage falls below the reference voltage due to a sudden increase in load. As explained earlier, the fast-acting controller brings the dc- link voltage to its reference value at ts = 0.82 s with almost the same rise in voltage as that of the conventional dc-link voltage controller. A close observation of the figure would reveal that the fast-acting dc-link voltage controller can regulate the capacitor voltage within a half cycle period which is indicated by ts. Owing to its good transient performance, it is preferred over the conventional dc-link voltage controller.

Figure 16:Transient response of the fast acting controller.(a).compensated source current in phase a.(b).DC link voltage

Figure 1: fast acting controller

(a) Compensator currents (b) Compensated load currents

CONCLUSION

In this paper, Matlab&Simulink dynamic models of Wind Turbine and PEMFC are developed. The two developed models of the Wind Turbine and PEMFC are inserted in a complete MG system to deal with the system fast response. The obtained results prove that a Wind Turbine and fuel cell can deal with load changes in the micro grid.

An energy-based fast-acting dc-link voltage controller is suggested to ensure the fast transient response of the compensator. Mathematical equations are developed to compute the gains of this controller. The efficacy of the proposed controller over the conventional dc-link voltage controller is established through the Matlab/simulation . It is observed from these studies that the proposed dc-link voltage controller gives fast transient response under load transients.

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Authors biography

Shravani K Asst.Professor in KL University, M.Tech from K L University- Vijayawada, and B.Tech VCEW from JNT University. She has Published 2 International conference papers and 1

International Journal paper. Her area of interest is on Power Quality Improvements and MicroGrids. E-mail: shravani773@gmail.com

I Raghavendra Assoc.Prof & HOD, in TKR College of Engineering, M.Tech from JNTU Hyderabad and B.Tech Vignan Institute of Technology & Sciences. He has Published 2 International conference papers

and 3 International Journal paper. His area of interest is on Power Quality Improvements and Power system restructuring.

C.Ragasudha pursuing M.Tech from Teegala Krishna Reddy Engineering College and B.Tech from Nishitha college of engineering & Technology. Her area of

interest is on Power Quality Improvements with different Facts devices.