Simulation of HSAPF for Power Quality Enhancement using PR Controller

Electrical energy is the most efficient and popular form of energy and the modern society is heavily dependent on the electric supply. The life cannot be imagined without the supply of electricity. At the same time the quality of the electric power supplied is also very important for the efficient functioning of the end user equipment. The term power quality became most prominent in the power sector and both the electric power supply company and the end users are concerned about it. Now-a-days with the advancement of technology, the demand for electric power is increasing at an exponential rate. Many consumer appliances demand quality power continuously for their operation. The quality of power delivered to the consumers depends on the voltage and frequency ranges of the power. If there is any deviation in the voltage and frequency of the electric power delivered from that of the standard values then the quality of power delivered is affected. The performance of the end user equipment is heavily dependent on the quality of power supplied to it. But the quality of power delivered to the end user is affected by various external and internal factors. They are like voltage and frequency variations, faults, outages etc. These power quality problems reduce the life time and efficiency of the equipment. Thus, to enhance the performance of the consumer equipment and also the overall performance of the system these problems should be mitigated. Now-a- days with the advancement in technology there is a drastic improvement in the semi-conductor devices. With this development and advantages, the semi- conductor devices got a permanent place in the power sector helping to ease the control of overall system. Moreover, most of the loads are also semi- conductor based equipment. But the semi-conductor devices are non-linear in nature and draws non- linear current from the source. And also the semi- conductor devices are involved in power conversion, which is either AC to DC or from DC to AC. This power conversion contains lot of switching operations which may introduce discontinuity in the current. Due to this discontinuity and non-linearity, harmonics are present which affect the quality of power delivered to the end user. In order to maintain the quality of power delivered, the harmonics should be filtered out. Thus, a device named Filter is used

hybrid. In this paper the use of hybrid power filters for the improvement of electric power quality is studied and analyzed. The rising interest in the use of electronic devices levies nonlinear loads to the source that draw active current, reactive current and harmonic current. Due to the reactive current and harmonic current electromagnetic interference with nearby equipment and heating of transformers occur. Power system can sop up harmonic currents with no problem. Resonant condition mainly affects the power problem.

The forecast of a smart grid associated with the continuous increase of switch-mode power converters, drives, as well as domestic and industrial nonlinear loads has created a serious concern on the power quality of the future distribution power systems as shown in Fig.1. Along with integration of renewables into the smart micro-grids, and expansion of electronic polluting devices as well as electric vehicle charging stations would have harmful impacts on the power quality. Electric vehicles market shows a considerable growth, therefore it became crucial to monitor and evaluates their behaviour and associated current waveforms when charging. Moreover, in some existing North American EV charging stations, the cars are connected between two phases of the 3P4W system, creating heavy unbalances. On the other hand, pushed by social efforts, governments start to investigate more on implementation of renewable energy sources, creating research and developments in this field. This work proposes an efficient three-phase Transformer less Hybrid Series Active Filter (THSeAF) capable to rectify current related issues in the smart micro-grids which also provides sustainable and reliable voltage supply at the PCC where residential and commercial buildings are connected with nonlinear time varying load side demand. The use of this device will facilitate the integration of energy storage systems and renewables for modern systems. The compensator is connected at distribution level of the power system to prevent harmonic currents of a load to flow into the grid while protecting the downstream loads from voltage perturbations coming from the utility and protecting the later from sags and swells. toward power quality improvement of Smart grid‘s power quality development. This compensator cleans the current harmonics from the utility and similarly to a DVR, the Point of Common Coupling (PCC) and utility‘s smart meters will be protected from voltage distortions and wrong computation of power and energy balance. This compensator could inject or absorb active power during grid voltage variations to ensure high quality and reliable supply along with complete decoupling the grid from polluted loads.

The series APF used for the power quality improvement is realized as a Voltage Source Inverter (VSI). It can be a three-phase VSI or three single-phase VSI‟s can also be used. The VSI is connected in series with the source impedance through a matching transformer. The circuit diagram is shown in Fig.2. A capacitor is used at the input if the VSI to provide constant input voltage to VSI.

A passive filter is also connected at the PCC. This filter is tuned to eliminate higher order harmonics. In certain cases there may be two or more LC branches tuned to eliminate specific order harmonics (especially 5th and 7th). A ripple filter is used in series with VSI. The filter parameters are selected such that they do not exceed the transformer burden. The design criteria is-

• XCrf<

• XCrf<< ZS + ZF, to make the voltage divide between Lrf and Crf

Thus, with an efficient control strategy, the APF compensates the voltage unbalances and distortion. The control strategy is designed such that the series APF together with the passive filter act as a balanced resistive load on the overall system. In a four-wire system, the harmonic currents circulated in the neutral wire are also reduced due to series APF.

The series APF should be controlled such that the voltage injected by it should compensate the harmonics present in the system and should help in improving the quality of power. To achieve the above purpose, the output voltage of the APF should be controlled. For this to happen, at first a reference voltage is generated which when injected by APF will serve the desired purpose. Then the actual output voltage of the series connected APF is controlled using a PI controller such that the actual output voltage generated is equal to the reference value. The overall control strategy is shown by the flow chart given in Fig.3.

The compensation strategy to compensate the harmonics is designed based on ―Dual Instantaneous Reactive Power Theory‖. In general, the power company tries to generate electric power at sinusoidal and balanced voltage. To achieve this condition, the load current at the Point of Common Coupling (PCC) should be co-linear with the supply voltage. This condition is satisfied if the load is a linear, balanced and resistive. This condition is expressed in equation form as-

Thus, the average power supplied by the source is given as- In case of unbalanced loads, where harmonics exist, only the fundamental component of the current helps in supplying the active power to the load. So the current in the equation (2) is only the fundamental component and is represented as I1. The load power is the summation of the source power and the compensator power. But the power

Therefore, the equivalent resistance is obtained as- Thus, the voltage at the PCC in α-β co-ordinates is obtained as- By substituting equation (3) in (4) the voltage at PCC is obtained as- From equation (6) the load voltage can be written as- Thus, the compensating voltage of the series APF is obtained as- From equations (6) and (7) the equation (8) is modified as- This is the reference value of the voltage that is to be supplied by the APF in order to make the set load and compensating equipment to act as resistive load.

actual voltage vector. The reference voltage vector given by equation (5.9) is generated by the following control block shown in Fig. 5.3. The fundamental current component calculation is shown in Fig.4. The fundamental component calculation needs the grid voltage angle to calculate the value. The grid voltage angle necessary for this calculation is extracted by using a Phased Lock Loop (PLL).

A Low pass filter (LPF) is used in the fundamental calculation block to filter out the harmonics and extract the fundamental component. A comparison is made between the actual and reference values of the output voltage of APF. The error is passed through a PI controller. The gain values of the controller are tuned in such a way that the error is zero and the actual value matches almost with the reference value. If this condition is achieved harmonics and thus improving the performance of the system. System configuration Conventional series Hybrid active filters have a common configuration as shown in Fig. 6 with a three- phase converter and three isolating transformers, while the proposed configuration do not contain series transformers. The transformer less power hybrid filter shown in Fig. 7 is composed of a two-level H-bridge converter connected in series between the grid and the PCC, where the presented configuration allows an independent control on each individual phase regardless of the others. A bank of tuned passive filters ensures a low impedance path for current harmonics. A DC bidirectional source is connected to inject power

The THSeAF is connected in series in order to inject the compensating voltage. On the DC side of the compensator, auxiliary dc-link energy storage components are installed. The shunt passive filter constituted of several tuned filters is composed of capacitor in series with an inductance with values tuned at desired frequency. Active filters are often keen to compensate current distortions, which is one of the priorities of the compensator. The current harmonics of the load produces voltage distortions when passing through the line and system impedances. Thus, to address such power quality issues, the proposed topology will be connected to the grid without the use of the bulky series transformer. This configuration will compensates source current

The Series active filter represents a controlled voltage source Inverter (VSI). In order to prevent current harmonics to drift into the power system, this series source should have a high impedance characteristic for all harmonic components. The principle of such modelling is well documented in this paper. The use of a passive filter becomes mandatory to compensate current issues while maintaining a constant voltage free of harmonic distortions at the load terminals. The behaviour of the series active filter (SeAF) for a current control approach is evaluated from the phasor‘s equivalent circuit shown in Fig. 6..

always clean of any harmonic component. An additional component is then added to regulate the load voltage terminals which will be described in the next section. This extra component assists the THSeAF to operate similar to a Dynamic voltage restorer (DVR) and maintain the load voltage regulated despite variation in the utility‘s voltage supply.

In the simulation results shown in Fig. 21-25, the compensator starts operating at 0.05s. It can be clearly observed that the major part of the source current whereas the load‘s current is highly polluted with a THD of 21%. The P+R regulator is forcing within three cycles the converter to follow the reference to produce the exact compensating voltage. The voltage generated by the compensator will force the nonlinear loads to draw a sinusoidal current in phase with the corresponding source voltage to achieve a unity power factor. The shunt passive filter in the configuration is essential to prevent current harmonics to flow into the grid. The compensator do not interfere the active power flow, meanwhile it has the ability to control the power flow by lagging or leading the current.

During voltage perturbations, the compensator maintains a harmonic-free and regulated voltage at the load terminals. The simulation results fig 6.35 to 6.40 demonstrates the behaviour of the system during utility‘s voltage highly pollution with fifth and seventh order harmonics with 20 and 15 percent of the fundamental respectively resulting in a three- phase supply (Vs) containing a THD of 25%. The proposed compensator continues to clean the grid‘s current from harmonics and corrects the power factor while the load‘s voltage THD is kept under the 5% limitation imposed by standards and regulations.

CONCLUSION

In this paper a three-phase THSeAF based on VSC type of converters was introduced to improve power the p-q theory was developed to extract voltage and current harmonics as well as unbalance, sags, and swells to be compensates. The key novelty of the proposed topology includes power quality improvement for small residential building and some low voltage distribution system that may result to the enhancement of the global power system. Furthermore, the proposed configuration can regulate and improve the load voltage PCC and when connected to a renewable auxiliary DC source, the topology is able to counteract actively to the power flow in the system similar to a UPS function. It was denoted that the active compensator responds seamlessly to voltage variations by ensuring a constant and distortion-free supply at load PCC. Furthermore, this compensator eliminates source current harmonics and improves grid power quality with no need to use the typical bulky series transformer. The Matlab Simulation of Proposed THSeAF system has been successfully carried out and THD analysis also done in this paper.

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Ph.D Scholar, Electrical Department, UOT, Jaipur, Rajasthan, India