Reactive power compensation for linear and non linear loads by using active and passive filter for smart grid applications.

Reactive power compensation for linear and non linear loads by using active and passive filter for smart grid applications. 1 Vikas Kumar Chandra, 2 Mahendra Kumar Pradhan 1,2 ECE Department, School of Engg.& I.T.MATS University, Raipur (C.G.) ABSTRACT: In this work a new control strategy for simulation of three phase power filter for power quality improvement is used.this filter consists of shunt passive LC power filter and series active filter, with nonlinear and linear loads and also with variation in the source impedance. A new control method based on dual formulation of instantaneous reactive power vectorial theory is applied by considering a balanced and resistive load as reference load, so that the voltage waveform injected by the active filter is able to attain the objective of achieving reactive power compensation. This also helps in eliminating load current harmonics and also in balancing asymmetrical loads i.e., for achieving ideal behaviour for the set hybrid filter load, Total Harmonic Distortion (THD) is reduced and power factor is improved. This method improves passive filter compensation characteristics without depending on system impedance, avoids the danger of passive filter behaves as harmonic drain of close loads and avoiding series and / or parallel resonance problems. And compensation is also possible with variable loads without detuning the passive filter. And is applied for creating LG/LL faults at the source side. The results show that the active filter improves the compensation characteristics of the passive filter and reactive power is compensated. KEYWORDS: Power Filter, Total Harmonic Distortion, Active Power Filter, Point of Common Coupling. I. INTRODUCTION Power system harmonics Power system harmonics are integer multiples of the fundamental power system frequency. Power system harmonics are created by nonlinear devices connected to the power system. Harmonics are voltage and current frequencies riding on top of the normal sinusoidal voltage and current waveforms. The presence of harmonics (both current and voltage) is viewed as `pollution' affecting the operation of power systems. The most common source of harmonic distortion is electronic equipment using switch-mode power supplies, such as computers, adjustable-speed drives, and high-efficiency electronic light ballasts. Harmonic waveforms are characterized by their amplitude and harmonic number. When a sinusoidal voltage is applied to a certain type of load, in which the load cause the current to vary disproportionately with the voltage during each cyclic period. These are classified as nonlinear loads, and the current taken by them will be a non sinusoidal waveform. When there is significant impedance in the path from the power source to a nonlinear load, these current distortions will also produce distortions in the voltage waveform at the load. Waveform distortion can be mathematically analyzed to show that it is equivalent to superimposing additional frequency components onto a pure sine wave (figure 2.1). These frequencies are harmonics (integer multiples) of the fundamental frequency, and can sometimes propagate outwards from nonlinear loads, causing problems elsewhere on the power system. The harmonics generated by the most common non-linear loads have the following properties: Lower order harmonics tend to dominate in amplitude If the waveform has half-wave symmetry there are no even harmonics Harmonic emissions from a large number of nonlinear loads of the same type will be added.power factor correction (PFC) is a mandatory functionality of electronic products in the industrial and commercial market in order to mitigate grid harmonics and operate a power system economically. Since the load characteristics of most PFC applications such as home appliances, battery chargers, switched mode power supplies and other digital products support unidirectional power flow, the general ac-dc boost converter with step-up chopper is considered a popular topology. This is because they are low cost, simple, and their performance is well-proven. Its main task inside the system is to maintain dc-link voltage constantly in order to feed loads at different power ratings. In addition, it is necessary to control input current with a pure sinusoidal waveform in phase with input voltage. Active power filters (APF) are another approach capable of improving grid power quality. Many research endeavours have included APFs in their circuit topologies and control strategies. Unlike PFC circuits, the APF is a system in itself which provides compensation of harmonics and reactive power in order to reduce undesirable effects from non-linear loads and uncontrolled passive loads in power systems. In this method capacitor acts as filter. Electronic filters are analog circuits which perform signal processing 40

functions specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both. Electronic filters can be: Passive or active Analog or digital High-pass, low-pass, band-pass, band-stop Discrete-time or continuous-time Linear or non-linear Infinite impulse response (IIR type) or finite impulse response (FIR type) II. BASIC PRINCIPLE ACTIVE AND PASSIVE FILTERING FOR HARMONIC CURRENT COMPENSATION The simplest method of harmonic filtering is with passive filters. They use reactive storage components, namely capacitors and inductors. Among the more commonly used passive filters are the shunt-tuned LC filters and the shunt low-pass LC filters. They have some advantages such as simplicity, reliability, efficiency, and cost. Among the main disadvantages are the resonances introduced into the ac supply; the filter effectiveness, which is Harmonic Compensation in Power System Using Active Power Filters function of the overall system configuration; and the tuning and possible detuning issues. These drawbacks are overcome with the use of active power filters. Most of the active power filter topologies use voltage source converters, which have a voltage source at the dc bus, usually a capacitor, as an energy storage device. This topology, converts a dc voltage into an ac voltage by appropriately gating the power semiconductor switches. Harmonic distortion in power distribution systems can be suppressed mainly by, passive and active filtering. The passive filtering is the simplest conventional solution to mitigate the harmonic distortion. The uses of passive elements do not always respond correctly to the dynamics of the power distribution systems. Passive filters are known to cause resonance, thus affecting the stability of the power distribution systems. Frequency variation of the power distribution system and tolerances in components values affect the passive filtering characteristics. As the regulatory requirements become more stringent, the passive filters might not be able to meet future revisions of a particular Standard. This may required a retrofit of new filters. Remarkable progress in power electronics had spurred interest in Active Power Filters (APF) for harmonic distortion mitigation. Active filtering is a relatively new technology, practically less than four decades old. The basic principle of APF is to utilize power electronics technologies to produce specific current components that cancel the harmonic current components caused by the nonlinear load. APFs have a number of advantages over the passive filters. First of all, they can suppress not only the supply current harmonics, but also the reactive currents. Moreover, unlike passive filters, they do not cause harmful resonances with the power distribution systems. Fig-1 Active filtering of harmonics Consequently, the APFs performances are independent on the power distribution system properties. Active filtering is a relatively new technology, practically less than four decades old. There is still a need for further research and development to make this technology well established. Fig -2 Basic principle of active and passive filtering III. PROPOSED METHEDOLOGY REACTIVE POWER CONTROL WITH HARMONIC CURRENT COMPENSATION Several power electronic devices such as MOSFETS,SCR,IGBT,UJT which are based on switching phenomenon techniques consumes less power due to this now a days these power electronics components are used in power system network for improvement of power factor correction and to increase the efficiency and to reduce the switching losses. A power diode or MOSFET efficiently used in the operation of versatile AC to DC converter to carry a huge amount of power and easily operate in the reverse bias condition. Fig-3 Proposed method for reactive power control with harmonic current compensation 41

reactive power, is shown in Fig. 7 and can be written respectively as (a) (b) Fig -4 Current flow diagram at PCC, (a) without HCC, (b) with HCC The grid current, ig, includes the harmonics, ihn, from a non-linear load as shown in Fig. 5(a). These harmonics are undesirable and should be removed. If the unidirectional ac-dc converter can generate the harmonic current capable of canceling the harmonics of the nonlinear load, the grid current will be comprised of only fundamental components of the converter current and load current as shown in Fig, 2(b). Therefore, the new current reference for the current controller of the converter can be expressed as where Is* is the magnitude reference from the dc -bus voltage controller. The harmonic current of the load can be obtained by subtracting the measured load current from the fundamental load current which can be estimated by employing a band pass filter in real implementations. where Is* is the magnitude reference from the dc -bus voltage controller. The harmonic current of the load can be obtained by subtracting the measured load current from the fundamental load current which can be estimated by employing a band pass filter in real implementations. C. Reactive Power Cmpensation Unlike non-linear loads, the current waveform of a linear load is sinusoidal at the frequency of the power system [15]. The power factor is almost unity when loads behave as a pure resistor. In contrast, the power factor can be significantly exacerbated when the load is capacitive or inductive. Reactive power oscillates between the ac source and the capacitor or reactor, at a frequency equa lto two times the rated value (50 or 60 Hz) [8]. This reactive power increases the total current unnecessarily in power systems, which causes increased conduction loss or deteriorated performance of voltage regulation at the PCC. Therefore, reducing reactive power is required. Fig. 6 shows the current waveform of a typical inductive load such as a single-phase induction motor. The current flow, consisting of the converter current with RPC and the load current consuming IV CONTROL STRATEGY FOR ACTIVE POWER FILTERS The proposed control strategy of the unidirectional ac-dc boost converter including a feedforward controller, HCC, and RPC is shown in Two control blocks for HCC and RPC have been added to the conventional control The new current reference should be limited within its power rating and consider the harmonic distortion from the converter current. Also, it is worthwhile to mention that functionalities of HCC and RPC in unidirectional ac- dc boost converters are available only when the converter supplies power to its dc load. Thus, the current reference able to be used for HCC and RPC is highly dependent on its power rating and its existing loads. As well as the control method for an individual converter under given harmonic current and reactive power references, the supervisory control strategy when multiple unidirectional converters are available as shown in Fig. 11 can be suggested as follows. Step 1: Calculate the compensation amount for harmonic-producing components and reactive power to improve the grid power quality. Step 2: Obtain the available capacities from an individual converter in order to compensate HCC and RPC. Step 3: Determine and distribute commands to individual converters. Step 4: Measure THD of the grid current. If the grid current is below 5%, the commands of RPC need to be reduced. Otherwise, the commands of RPC can be increased up to their maximum to achieve unity power factor. Step 5: Repeat Step 1 through Step 4. Using these steps, the grid power quality can be enhanced as long as the available capacities of converters for HCC and RPC remain. 42

Fig- 5 Control strategy for HCC and RPC V. SIMULATION RESULTS In order to investigate the effectiveness and performance of the proposed control method for a unidirectional ac - dc boost converter, a 2kW bridgeless PFC converter model, a nonlinear load with 80% THD and a linear load with 0.8 PF are implemented in MATLAB/Simulink. For the evaluations of performances, the three converter operation modes are simulated: 1) HCC mode, 2) combined operations of HCC and RPC. Fig 6 Simulation results in HCC mode the simulation results in harmonic current control mode when a single -phase induction motor connected to the unidirectional ac-dc boost converter at the PCC is used as linear load with a poor PF of 0.8. It can be observed that the power factor of the grid is improved from 0.950 to 0.985 when the converter generates 500 Var in RPC mode. However, the THD of the grid current increases from 1.3% to 8% due to inherent distortions of reactive power current in unidirectional ac-dc converters. Thus, as explained in previous sections, the amount of reactive power used for compensation should be limited to maintain low THD of the grid current. A. Combined Compensation Mode. shows the simulation results for combined operations of HCC and RPC when the two emulated loads used in previous simulations are connected at the PCC. When the converter is operating in PFC mode, the grid PF and the THD of the grid current are 0.95 and 10%, respectively. HCC and RPC begin simultaneously, the resulting grid power quality improves to 0.982 PF and 5% THD. This means that Fig-7 Simulation results in HCC and RPC mode The voltage controller allows the dc voltage to have an almost constant value. The output signal of the dc-link voltage controller determines the value of the active current of the mains load and losses of the power unit of the restoring system. The reactive current is calculated by the reactive power and flicker estimation module of the control unit. The control value of the current control loop is the supply current. This current is a result of the sum of the measured load current (see Fig.1) and the ac current of voltage inverter. These two three-phase system currents are added together and then are transformed to a signal of the two-phase quantities. To control harmonic amplitudes in the network, the harmonic calculator is used. The principle of the 43

operation is based on the direct harmonic control method VI. CONCLUSION Hence a control strategy for a power filter constituted by a series active filter and a passive filter connected in parallel with the load is proposed. The control strategy is based on the dual vectorial theory of electric power. The new control approach achieves the following targets. Suitable for variable loads as the reactive power variation is compensated by the active filter. Therefore, with the proposed control strategy, the active filter improves the harmonic compensation features of passive filter and reactive power is compensated. Also the currents harmonics are eliminated. Simulations with the MATLAB-Simulink platform were performed with different loads and with variation in the source impedance. The proposed technique can also be extended by creating LG /LL faults at the source side. The shunt passive and series active filters works effectively to compensate the source currents by injecting compensating currents at the point of common coupling under the application of LG/LL faults at the source side. REFERENCES [1] Akagi, H.: New Trends in Active Filters for Power Conditioning. IEEE Transactions on Industry Applications, Vol.32, No.6, pp. 1313-1322, 1996. [2] Kalachnikov, S. Three-phase rectifier for AC- Drives incorporated with Active Power Filter, Proceedings of the International Power Electronics and Motion Conference, 1998, Prague, Czech Republic., pp. 2125-2130. [3] Rummich,E., Kalaschnikow, S., Oberschwingungs- arme Netzeinspeisung von Windkraftanlagen mit Hilfe von aktiven Filtern, e&i 117. Jg.(2000), H.2, pp 129-133. [4] Sabanovic, N., Sabanovic, A., Jezernik, K., Kaynak, O.M.,: Current Control in three-phase switching Converters and ac electrical Machines. In: Proceedings of IECON 94, Italy, 1994, pp. 581-586. [5] Gao, W., Wang, Y., Homaifa, A.,: Discrete-time variable Structure Control System. In: IEEE Transactions on Industrial Electronics, Vol.42, No.2, pp. 117-122, 1995. [6] Fernando, J, Orlandi, E., Pais, M.: Sliding Mode Control of Unity Power Factor Three Phase Boost Converters, Proceedings of International Power Electronics and Motion Conference,1998, Czech Republic.,pp.1125-1130 [7] Kalachnikov, S.: Control of the Switch-Mode Rectifier without Mains Voltage Sensors, SPEEDAM 98, Italy, pp.125-128 [8] Kalachnikov, S., Berger, H.: A New Control Strategy for DC-Link Voltage of a Three-Phase Bi-directional PWM Rectifier, EPE`95, Spain, 1995, pp 2558-2562. [9] Guillaume de Préville: Flicker mitigation. Application to a STATCOM, Proceedings of the European Conference on Power Electronics and Applications,2001, A 44