Integrated control of AC-DC converter with harmonics reduction and reactive power compensation for grid application
1Vikas Kumar Chandra, 2Mahendra Kumar Pradhan
1,2ECE Department, School of Engg. & I.T.MATS University, Raipur(C.G.)
Abstract— Ac to DC converter is used for operating DC electrical machine and also supplies DC power to battery which is used for many purposes. The main purpose of this project is to compensate the harmonics by using harmonic current compensation and reactive power control. If reactive power is compensate prior to system then non linearity or harmonics present in the system does not much affect the existing system. In industrial low and medium voltage mains, passive filters and PFC capacitors have traditionally been used to improve the supply quality.
However, they cannot be rated only for the loads being compensated. They are affected by harmonic currents from other non-linear loads or by harmonics from the power system. Compared with passive element compensators, an active harmonic compensator (AHC) can be used to improve the supply quality without worrying about all the problems associated with applying passive elements. The proposed active harmonic compensation AHC for industrial networks can be successfully used with nonlinear loads and consumers with rapid fluctuations of reactive and active power consumption to improve the supply quality of other loads supplied from the same mains. Clear reduction of the voltage wave form distortion and the voltage changes (flicker effects) as well as the stabilisation of the mains voltage are the main advantages of the proposed AHC. These all make the application of the power electronics to improve the supply voltage quality in industrial networks more effective in comparison to passive filters and PFC capacitors.
Keywords –Shunt active power filter (SAPF), Generalized integrator controller, Active power filter (APF), harmonic suppression, voltage stability.
I. INTRODUCTION
Active filtering of electric power has now become a mature technology for harmonic and reactive power compensation in two-wire (single phase), three-wire (three phase without neutral), and four-wire (three phase with neutral) ac power networks with nonlinear loads.
This paper presents a comprehensive review of active filter (AF) configurations, control strategies, selection of components, other related economic and technical considerations, and their selection for specific applications. It is aimed at providing a broad perspective on the status of AF technology to researchers and application engineers dealing with power quality issues.
Fig -1 Active Harmonic Compensator connected to industrial plant.
The AF technology is now mature for providing compensation for harmonics, reactive power, and/or neutral current in ac networks. It has evolved in the past quarter century of development with varying configurations, control strategies, and solid-state devices. AF’s are also used to eliminate voltage harmonics, to regulate terminal voltage, to suppress voltage flicker, and to improve voltage balance in three- phase systems. This wide range of objectives is achieved either individually or in combination, depending upon the requirements and control strategy and configuration which have to be selected appropriately. This project describes the history of development and present status of the AF technology.
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.
II. PROPOSED MATHEDOLOGY
This project introduces a versatile unidirectional ac-dc converter with harmonic current and reactive power compensation. Since numerous unidirectional ac-dc converters can be connected with ac power systems, existing commercial converters possess the ability to improve substantially the stability of ac power systems by compensating harmonic current and reactive power.
In this project, the feasibility and limitations of the unidirectional ac-dc converter are explained when it is employed for harmonic current and reactive power compensation, and a control strategy for such functionalities is proposed. The proposed control method can ameliorate harmonic current and reactive power for improved grid power quality as well as regulation of dc-bus voltage. Even though the amount of HCC and RPC is limited compared to APFs, this control strategy can contribute to a more stable power system as more converters capable of HCC and RPC are available at the point of common coupling (PCC) without extra cost. The proposed unidirectional ac-dc converter has three operation modes i.e., PFC, HCC and RPC. Also, both HCC and RPC can be simultaneously used to improve the distortion and the displacement factors of the grid current.
Fig-2 Block diagram of proposed method A DC-to-DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. It is a class of power converter DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply.
Electronic Linear Mode
Linear regulators can only output at lower voltages from the input. They are very inefficient when the voltage drop is large and the current is high as they dissipate heat equal to the product of the output current and the
voltage drop; consequently they are not normally used for large-drop high-current applications. The inefficiency wastes energy and requires higher-rated and consequently more expensive and larger components.
The heat dissipated by high-power supplies is a problem in itself and it must be removed from the circuitry to prevent unacceptable temperature rises.
Switched-mode conversion
Electronic switch-mode DC to DC converters convert one DC voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors). This conversion method is more power efficient than linear voltage regulation. This efficiency is beneficial to increasing the running time of battery operated devices. The efficiency has increased since the late 1980s due to the use of power FETs, which are able to switch at high frequency more efficiently than power bipolar transistors, which incur more switching losses and require a more complicated drive circuit. Another important innovation in DC-DC converters is the use of synchronous rectification replacing the flywheel diode with a power FET with low "on resistance", thereby reducing switching losses. Before the wide availability of power semiconductors, low power DC to DC converters of this family consisted of an electro- mechanical vibrator followed by a voltage step-up transformer and a vacuum tube or semiconductor rectifier or synchronous rectifier contacts on the vibrator.
Motor-generators can convert between any combination of DC and AC voltage and phase standards. Large motor-generator sets were widely used to convert industrial amounts of power while smaller motor- generators were used to convert battery power to a high DC voltage, which was required to operate vacuum tube (thermionic valve) equipment.
III. CONTROL SYSTEM OF PROPOSED METHODOLOGY
The control scheme is based on a cascade control with a current control in the inner loop without mains voltage sensors. The current controller sets the output voltage of the voltage source inverters for each sampling period of the control system so that the line current has a reference value. 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.
Fig 3 Control strategy for proposed method To reduce the high-frequency switching-ripple of the AHC line current, a high frequency LCL filter is connected in between the mains and the AHC.
Current control
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 i Sa ,b . In Fig. this current is represented as i Sa and i Sb .
The reference value for the current controller i ref d ,q (d and q components) is transformed to the stationary reference frame a −b . The transformation of the vector i refd q, to the vector i1a b, is executed by ejw1t, derived from a phase locked loop PLL (see Fig.2). The selection of the switching sequence for every switching operation of the both voltage source inverters is achieved through the use of a sliding mode controller. The selection of the switching sequence for every switching operation through the use of the sliding mode control is discussed in detail in [4-6]. This makes it possible to control the active filter without mains voltage sensors [7]. It significantly simplifies the hardware configuration of the active mains compensator, especially for medium and high voltage applications. The output signals of two P-controllers with saturation represent two components of the mains voltage vector u Wa , u Wb which are used to detect the position of the voltage vector by PLL.
To control harmonic amplitudes in the network, the harmonic calculator is used. The principle of the operation is based on the direct harmonic control method. T
DC-link Control
With non-sinusoidal mains current of the voltage inverter, the dc-link voltage contains not only a ripple from transistor switching operations, but also a low frequency voltage ripple like the dc voltage at the dc link of the diode rectifier with capacitor. This low frequency voltage ripple must be filtered in the control loop by feeding back the dc voltage otherwise this voltage ripple would be increased by the proportional part of the voltage controller and it would be passed on to the line current control loop. Therefore the line currents would be distorted . To decrease the influence of the dc-link voltage ripple on the current control loop,
the cut-off frequency of the feedback low-pass filter must be f0=50÷75Hz. The low cut-off frequency of the feedback filter causes the large delay time in the dc- voltage measurement and therefore the dc-link voltage control has a low dynamic performance. To improve the time response of the dc-link voltage control, an adaptive control system is used, whose parameter values of the feedback filter and PI controller are changed in accordance with the value of the dc voltage error .
Fig-4 Control system of proposed method
IV. SIMULATION RESULTS
The harmonic current is generated by a typical three phase diode rectifier Adjustable Speed Drive. Since the displacement power factor is close to unity there is no requirement of reactive power compensation in this case, but only harmonic current mitigation. At time 0.16 the AHC is connected to the power system and starts mitigating the harmonic currents from the ASD. The transient takes almost one fundamental period, until the source current resembles sinusoid waveform. Due to its high dynamics the AHC is able to compensate the harmonic currents within one fundamental period. The current distortion of the non-linear load has a THD of 34
% while the source current reaches a THD of 3.11%.
Fig.5 Simulation results of AHC start-up.
The simulation results are compared with the control method of previous methodology of shunt active power filtering and proposed Active Power Filter .
Figure 4 shows the waveform of supply current after compensation. It consist of fundamental current only.
The harmonic current present in the supply current is eliminated by using the Shunt Active Power Filter. The distortion present in the supply current is reduced when compared to PPF compensation.
Fig- 6 Supply current waveform after compensation by using previous method
Fig-6 shows the spectrum analysis of supply current after compensation. The Total Harmonic Distortion of the supply current is reduced to 4.96% .
Fig-7 Spectrum analysis of supply current- after compensation using SAPF
Fig- 8 Supply current waveform after compensation by using proposed method
Fig-9 Spectrum analysis of supply current- after compensation using proposed method
V. CONCLUSION
Integrated control of ac to dc converter is very useful for compensation of harmonics due to nonlinear loads those are connected through the smart grid and provide appropriate harmonic current compensation and reactive power compensation due to which stability of smart grid increases and also increases the efficiency and reliability of the power system network.
VI. FUTURE ASPECTS
AC TO DC converter introduces various types of applications, after the conversion of AC to DC power is fed to the DC bus through which many alternating machines are connected which are used for many purposes this technique reduces power loss in the power system network.
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