A New Bidirectional Synchronous-VSC for PV based Smart Grid

IJIRST 窶的nternational Journal for Innovative Research in Science & Technology| Volume 1 | Issue 6 | November 2014 ISSN (online): 2349-6010

A New Bidirectional Synchronous-VSC for PV based Smart Grid B. Bhargava Reddy M.Tech Student Department of Electrical & Electronics Engineering Srinivasa Institute of Technology & Science, Kadapa, A.P, India

R.Venkata Subbaiah Assistant professor Department of Electrical & Electronics Engineering Srinivasa Institute of Technology & Science, Kadapa, A.P, India

G. Venkata Suresh Babu Associate Professor& HOD Department of Electrical & Electronics Engineering Srinivasa Institute of Technology & Science, Kadapa, A.P, India

Abstract Smart grid is a concept of matching electricity production with demand using multiple identical generators which can start, stop and operate efficiently at chosen load, independently of the others, making them suitable for base load and peaking power generation. Matching supply and demand, called load balancing, is essential for a stable and reliable supply of electricity. Shortterm deviations in the balance lead to frequency variations and a prolonged mismatch results in blackouts. Operators of power transmission systems are charged with the balancing task, matching the power output of all the generators to the load of their electrical grid. The load balancing task has become much more challenging as increasingly intermittent and variable generators such as wind turbines and solar cells are added to the grid, forcing other producers to adapt their output much more frequently than has been required in the past. This concept presents a new control strategy for voltage source converters (VSCs) in the frequency angle domain which enables dc-link voltage regulation via frequency and load angle adjustment. This controller presents high stability margin and fast dc-link voltage regulation, where as it can provide frequency support in the ac side during contingencies. Frequency and voltage amplitude are adjusted by two separate loops. Two different variants are proposed for dc link voltage control; namely direct dc-link voltage control and indirect dc-link voltage controller. Simulation results provided to validate the controller effectiveness.

Keywords: Control topology, dc-link voltage regulation, power control, voltage-source converters (VSCs). _______________________________________________________________________________________________________

I. INTRODUCTION Recent innovations in small scale distributed power generation systems combined with technological advancements in power electronic systems led to concepts of future network technologies such as micro grids. Nowadays, because of high penetration levels of renewable energy resources, the paradigms of micro-grids (MGs) and distribution generation (DG) are gaining vital role in power and distribution systems. MGs are categorized as ac MGs, dc MGs, and hybrid ac窶電c MGs. Since a considerable portion of renewable energy resources, such as wind turbines, photovoltaic (PV), fuel cells and energy storage systems, and many modern loads such as communication technology facilities, data centers, and motor drives is dc-type, dynamics and controls of rectifiers and dc MGs are gaining high interest [1]. However, in dc grids, many generation units such as wind turbines must be interfaced to the utility grid via electronically interfaced (EI) rectifiers. Based on predictions given in [2], the resistive load share will be significantly reduced whereas the EI loads share will increase to 60-80% of the total load by 2015.

One of the major challenges facing future power systems is significant reduction in grid equivalent rotational inertia due to the expected high penetration level of EI units, which in turn may lead to frequency-stability degradation. To overcome this difficulty, controlling VSCs as virtual synchronous machines is proposed for power system frequency stabilization [6] by embedding a short-term energy storage to the VSC facilitating power flow to and from to the energy storage device proportional to the variation in grid frequency. Therefore, the development of VSCs with well-defined angle, frequency, and dc-link voltage characteristics are of high interest for future smart power systems with a high penetration of VSCs. Moreover, a general control scheme which is suitable for both rectification and inversion modes without reconfiguration is very attractive in power system applications since bidirectional VSCs can work in generative and motoring modes similar to SMs. Novel control strategies using nonlinear synchronizing power are addressed in [17] to provide self-synchronization ability and large-signal stability for VSCs in MGs and very weak grid applications.

Instabilities due to dc-link dynamics are one of major sources of instabilities in VSCs. Most of previous works on virtual SGs and/or self-synchronization of VSCs consider the dc link as an ideal battery with infinite energy. However, it is obvious that this is not the case and, in most transient scenarios, dc-link voltage varies; also, its energy and power are limited. Moreover, if dc-link All rights reserved by www.ijirst.org

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voltage dynamics are slow and the voltage passes some thresholds for a relatively long time, under or over-modulation and, consequently, voltage instability is expected. To improve dc-link voltage stability, fast response short-term energy storage can be installed in DG units.

To overcome the aforementioned difficulties, in this paper, a comprehensive control strategy is proposed for VSCs. It augments all of the aforementioned requirements in one compact topology which has the following salient features.

(1) It emulates the behavior of SMs with proper adjustment of the dc-link voltage so the power grid views the VSC dc-link as a virtual rotor. Thus, it can be easily integrated to grids with numerous SMs. (2) It introduces some inertia for the frequency; therefore the stored energy in the dc-link can be used for frequency regulation during contingencies which is the same role of SM’s rotor.

(3) It is feasible in both rectifying and inverting modes without reconfiguration, i.e., similar to an SM, it can be operate either in motoring or generative mode. Furthermore, since it provides bidirectional power transfer, even active loads can be used as an asset for frequency stabilization.

(4) Since the controller has cascaded frequency, angle and dc-link voltage loops, it offers extra damping and synchronizing powers, therefore, it can automatically synchronize itself with the main grid; offer self-synchronization capability; and eliminate the need for a PLL. This feature is a continuation where the concepts of cooperative droop and nonlinear selfsynchronization are proposed.

(5) Since dc-link voltage dynamics is inherently taken into account during design and analysis, it presents more practical control topology and design process. In a simple control strategy for dc-link voltage regulation in frequency domain and polar coordinates is proposed; however, it lacks a comprehensive and general framework for emulating and integrating SM characteristics in VSCs. (6) It yields well-defined dynamics for frequency, angle and dc-link voltage, which in turn makes power system analysis easier.

(7) Similar to SMs, it has a fault-ride-through capability.

Two different topologies, namely, virtual torque and direct dc-link voltage control strategies, are developed for the frequency control loop. In these topologies, the output power and the dc-link voltage are used as control variables, respectively.

II. POLAR CONTROL OF VSCS IN FREQUENCY-ANGLE DOMAIN In this paper, a control topology is proposed in which the frequency, load angle, and dc-link voltage are control variables rather than conventional vector controls which employ current components for dc-link voltage regulation and reactive power control.

Thus, direct control of frequency, angle, and dc-link power and voltage is available which in turn provides easier controllability and system analysis. In the following, the control topology is described in detail. One of the main goals in VSC control is to maintain the dc-link voltage constant. Assuming a lossless VSC, the input ac power is equal to the dc power PDC = VDC IDC; and the reactive power does not correspond to any real power exchange with the converter dc-side. The power-circuit part involves the VSC and the connecting impedance between VSC and the grid and is similar to conventional grid-connected VSC systems.

Usually in high power VSCs used in power systems, which are the main scope of this paper, a pure inductor without capacitor is used as output filter. If the VSC is employed to regulate frequency variations during contingencies, an energy storage system can be installed at the dc-link to transfer power from the energy storage device to the grid to damp frequency and angle oscillations.

However, for rectifier applications in which no energy storage is used, the active load can be used for power exchange with the grid.

Fig. 1: SM principal operation and control concept.

Synchronous Machine Model Generally, an SM involves excitation and damper windings mounted on the rotor and three-phase windings in the stator.

Although the damper windings significantly improve transient performance in terms of synchronism and damping capabilities, and the reluctance torque increases power density of an SM, in this paper, the model of a round rotor synchronous motor without damping windings is considered. In fact, the effect of damping and synchronizing powers can be emulated by the control functions in the VSC without physical damping components as in conventional SMs. The mechanical dynamics is expressed by A.

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Fig. 2: Proposed control topologies for frequency and dc-link voltage regulation. (a) Virtual torque control. (b) Direct dc-link voltage control.

The virtual electrical torque and reactive power in this case are governed by the following equations:

B. Frequency Control Loop Here, the proposed topologies for frequency control are presented. For the frequency control, two variants are proposed— namely, virtual torque control and direct dc-link voltage control as depicted in Fig. 2. To generate a three-phase sinusoidal voltage in the polar system, its amplitude, frequency, and angle are required. The voltage generation principle in a synchronousVSC is similar to back-EMF generation in SGs shown in figure.

III. SMALL-SIGNAL ANALYSIS To investigate the dynamic performance and transient response of the system, a small-signal model is developed. The smallsignal analysis gives insights on how controller variables variation affects the stability and convergence characteristics of the controlled variables. It also helps in optimum tuning of controller parameters to reach the best tradeoff among design objectives.

In the following, the small-signal analysis of the frequency controller shown in Fig. 2is presented. The voltage control loop used for reactive power regulation, shown in Fig. 3 is analyzed. The small-signal model of other topologies can be obtained in a similar way.

IV. CONTROLLER DESIGN The transfer function of the controller is obtained as

The controller natural frequency (ωn) should be chosen much smaller than the switching frequency (ωn). Another concern related to is the system rise time. Usually, the most inner controller bandwidth is selected to be less than 20% of the switching

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frequency. Unlike real SMs, the rotational momentum and friction factor can be selected equal to values that are not possible for physical electrical machines.

Fig. 3: Schematic view of the simulated system

Equation (23) is the main tool for calculating the virtual momentum of inertia to introduce desirable virtual inertia to the grid.

The lower the gain, the lower the frequency time-constant is. Nevertheless, it may lead to a steady-state error in the output. If the extra phase angle shift is removed, the transfer function is reduced to

which gives more straight-forward approach to determine the system time-constant and calculate control variables

V. SIMULATION RESULTS This section presents detailed simulation results of the proposed control system. The simulated system is shown in Fig. 3.

Simulation studies are carried out in the MATLAB/SIMULINK environment. Different conditions in both generative and rectification modes are considered to show effectiveness and generality of the controller in all cases.

A. Rectification Mode: In this section, the transient behavior of the proposed controller topology in rectification mode is presented. The corresponding waveforms are shown in Fig. 4. The first event occurs at t=0.9 s due to a sudden load power increment from 9.0 to 12.0 kW. The dc voltage encounters 60-V sag due to the load power change; however, the recovery time is less than 0.05 s, presenting a very fast response. At t=2.5 s, the input reactive power reference is varied from 0 to 3.0 kVAR. In this case, the rectifier stars to absorb reactive power from the grid. The controller successfully tracks the new set value within 1 s. The effect of reactive power set variation on the dc-link voltage is almost negligible which proves the decoupling between real and reactive power regulation.

Fig a: D.C. Voltage

Fig c: Frequency

Fig b: Real Power

Fig d: Reactive Power Fig. 4: Simulation results for rectifying mode.

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Fig a: Current Fig b: Phase voltage Fig. 5: (a) Current and (b) output filter voltage waveforms for the case of load power increment.

At t=4 s, the dc-link voltage reference is reduced to 270 V. The controller tracks the voltage reference accurately within 0.2 s whereas the reactive power slightly varies.

Inverting Mode: The behavior of the synchronous VSC in the inverting or equivalently the generative mode is studied. The initial voltage of the dc-link voltage in this case is 300 V. At t=2 s, the output dc current of the DG unit is increased from 20 to 30 A. The waveforms of the dc-link voltage, real power, frequency for the inverting mode are shown in Fig.5. When a grid voltage sag occurs at t=3.0 s, the controller successfully damps this disturbance with a negligible effect on the real power and dc-link voltage. After the DG output power is increased, the dc-link voltage catches its reference value within 0.08 s and the output power is increased with some minor spikes. At 4 s, the dc-link voltage reference is reduced to 250 V and the VSC tracks the reference value with highly damped transient response.

B.

Fig. 6 a: D C Voltage

Fig. 6 b: Real Power

Fig. 6 c: Frequency Fig. 6: Simulation results for inverting mode.

Fig. 7 a: DC-link voltage

Fig. 7 b: Instantaneous currents

Fig. 7 c: Instantaneous output filter voltages.

Fig. 7: Fault-ride-through capability of the synchronous-VSC.

Fault-Ride-Through Capability: To investigate this scenario, an unsymmetrical single-phase to ground bolted fault occurs after the filter at 1.0 s and is cleared at 1.1 s, as shown in Fig. 7. The initial conditions are similar to case A, i.e., dc-link voltage 300 V, reference reactive power t=0 VAr, and load current of 30 A to investigate whether the VSC can override the fault and synchronize itself with the grid during and subsequent to an unsymmetrical fault without PLL. The VSC works in the rectification mode when a fault occurs in phase ―b.‖ The corresponding waveforms of the dc-link voltage, three-phase instantaneous currents, and output filter voltage are shown in Fig. 6. During the single phase fault, is reduced to approximately 180 V and, subsequent to the fault, a voltage overshoot of 75 V is observed to charge the capacitor; the controller needs 0.2 s to recover the reference voltage. Also, as expected, a secondC.

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harmonic ripple with frequency of 120 Hz appears in the dc-link voltage during the fault due to the unbalanced power exchange conditions between the ac- and dc-sides.

Comparison with the Conventional Synchronous DQ-Frame Vector Control: Fig. 11 shows the diagram of a VSC with conventional vector control system with outer dc-link voltage controller and inner current control loops. The controller shown in Fig. 8 is implemented to compare its features with the proposed synchronousVSC. The controller parameters are given in Table II. D.

(a)DC-link voltage. (b) VSC’s frequency Fig. 8: System responses of the vector control for the case of the load power increment

The parameters are tuned optimally for a 10-kHz switching frequency to achieve adequate performance. A PLL is necessary to observe grid frequency and angle and transform voltage and current variables from abc to dq frame and vice versa. Two scenarios similar to synchronous-VSC case studies are taken into account in the rectifying mode. In the first scenario, the load current is increased from 30 A to 40 A at 1.5 s and a single phase-to-ground fault occurs after the filter at t=2.8 s. However, the voltage undershoots in synchronous-VSC and vector control are limited to 52 and 32 V, respectively, representing lower voltage drops for the vector control.

(a).DC-link voltage. (b) Instantaneous currents Fig. 9: System response during the fault using the vector control.

Fig. 10: DC-link voltage variation during the fault using the vector controller with fault resistance Rfault = 1â„Ś .

E. Virtual Torque Control Strategy: The virtual torque control loop adjusts the dc-link voltage in a way such that damping and synchronizing torques are generated within the controller during transients similar to SMs.

(a).DC-link voltage. (b) VSC frequency Fig. 11: System response with the virtual torque controller.

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The simulation results have been provided to validate controller performance under a wide range of typical operating conditions.

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