International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 1
Enhancement of Power Quality in
Distribution System Using D-STATCOM
KOSURI AMALA RAJANI Assistant Professor Department of EEE D.M.S.S.V.H. College of Engg,.
Machilipatnam
KUNAPAREDDY HARI HARA AYYAPPA M.Tech Scholar
Department of EEE D.M.S.S.V.H. College of Engg,.
Machilipatnam
Abstract: This paper presents the enhancement of voltage sags, harmonic distortion and low power factor using Distribution Static Compensator (D- STATCOM) with LCL Passive Filter in distribution system. The model is based on the Voltage Source Converter (VSC) principle. The D-STATCOM injects a current into the system to mitigate the voltage sags.LCL Passive Filter was then added to D-STATCOM to improve harmonic distortion and low power factor.
Keywords: DSTATCOM, LG, LL, LLG.
I.
INTRODUCTIONPower quality is certainly a major concern in the present era; it becomes of especially important with the introduction sophisticated devices, whose performance is very sensitive to the quality of power supply. Modern industrial processes are based on a large amount of electronic devices such as programmable logic controllers and adjustable speed drives. Electronic devices are very sensitive to disturbances and thus industrial loads become less tolerant to power quality problems.
Power Quality (PQ) has become an important issue since many loads at various distribution ends like adjustable speed drives, process industries, printers, domestic utilities; computers, microprocessor based equipments etc. have become intolerant to voltage fluctuations, harmonic content and interruptions.Power Quality (PQ) mainly deals with issues like maintaining a fixed voltage at the Point of Common Coupling (PCC) for various distribution voltage levels irrespective of voltage fluctuations, maintaining near unity power factor power drawn from the supply, blocking of voltage and current unbalance from passing upwards from various distribution levels, reduction of voltage and current harmonics in the system and suppression of excessive supply neutral current. Recently, the importance of power quality issues has increased due to various reasons. First of all, there have been changes in the nature of electrical loads. On one hand, the characteristics of load have become more
complex due to the increased use of power electronic equipment, which results in a deviation of voltage and current from its sinusoidal waveform. On another hand, equipments have become more sensitive to power quality due to its electronic nature.
Figure 1: Block diagram of a power electronic system.
Deregulation of the electrical power market is a second factor that has increased the importance of power quality. Deregulation has divided what was a single utility into three: supplier, transmitter and distributor. It is important to evaluate power quality level and identify the source of faults that origin electrical disturbances in electrical power systems, which determines the responsibility of a bad quality of power. In order to evaluate and identify the disturbances and its origin, power quality monitoring is the tool that utilities and customers use.
II.
LETARATURE REVIEW The common Power Quality issues are Interrupts, Sags (dips), and Swells. In order to overcome the problems such mentioned above, the concept of custom power devices is introduced recently; custom power is a strategy, which is designed primarily to meet the requirements of industrial and commercial customer. The concept of custom power is to use power electronic or static controllers in the medium voltage distribution system aiming to supply reliable and high quality power toInternational Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 2 sensitive users. Power electronic valves are the basis
of those custom power devices such as the static transfer switch, active filters and converter-based devices. Converter based power electronics devices can be divided in to two groups: shunt-connected and series-connected devices.SOLID-STATE control of ac power using thyristors and other semiconductor switches is widely employed to feed controlled electric power to electrical loads, such as adjustable speed drives (ASD's), furnaces, computer power supplies, etc. Such controllers are also used in HV dc systems and renewable electrical power generation.
As nonlinear loads, these solid-state converters draw harmonic and reactive power components of current from ac mains. In three-phase systems, they could also cause unbalance and draw excessive neutral currents. The injected harmonics, reactive power burden, unbalance, and excessive neutral currents cause low system efficiency and poor power factor.
They also cause disturbance to other consumers and interference in nearby communication networks.
Passive LC filters and fixed compensating devices with some degree of variation like thyristor switched capacitors, thyristor switched reactors were employed to improve the power factor of ac loads. Such devices have the demerits of fixed compensation, large size, ageing and resonance.Conventionally passive L-C filters were used to reduce harmonics and capacitors were employed to improve the power factor of the ac loads. However, passive filters have the demerits of fixed compensation, large size, and resonance. The increased severity of harmonic pollution in power networks has attracted the attention of power electronics and power system engineers to develop dynamic and adjustable solutions to the power quality problems. Such equipment, generally known as active filters (AF's)is also called active power line conditioners (APLC's), instantaneous
Table 1: Classification of electromagnetic phenomena in the field of power quality
Broad Categories Specific Categories Characterization Methods Typical Causes
Transients
Impulsive
Peak magnitude, rise time and duration
Lighting strike, Transformer energisation and capacitor
switching
Oscillatory
Peak magnitude, Frequency components
Line or Capacitor or Load Switching
Short duration voltage variation
Sag
Magnitude duration
Ferro resonant transformers Single line to ground fault
Swell
Magnitude duration
Ferro resonant transformers Single line to ground fault
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 3 Interruption
Duration
Temporary (self-Clearing faults)
Long duration voltage variation
Under voltage
Magnitude duration
Switching on loads, Capacitor de-energisation
Over voltage
Magnitude duration
Switching off loads, Capacitor energisation
Voltage Unbalance Symmetrical components
Single-phase load, Single phasing condition
Waveform distortion
Harmonics
THD, Harmonic spectrum
Adjustable speed drives and nonlinear loads
Notching
THD, Harmonic spectrum Power Electronics converter
Voltage Flicker
Frequency of occurrences, modulating frequency
Arc furnace, Arc Lamps
reactive power compensators (IRPC's), active power filters (APF's), and active power quality conditioners(APQC's). Active Power Filters are classified as Series Active power Filter, Shunt Active Filter and Hybrid Active Power Filters (Combination of Series and Shunt Active Power Filters).
Classification of Power Quality Problems:
These are undesirable but decay with time and hence not a steady state problem. A broad definition is that a transient is “that part of the change in a variable
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 4 that disappears during transition from one steady
state operating condition to the other”.
Voltage Sags:
Sag is a decrease of rms voltage to a value between 0.1 and 0.9 p.u. and lasting for duration between 0.5 cycles to 1 minute. Voltage sags are mainly due to system faults and last for durations ranging from 3 cycles to 30 cycles depending on the fault clearing time. Starting of large induction motors can result in voltage sags as the motor draws a current up to 10 times the full load current during the starting. Also, the power factor of the starting current is generally poor.
Voltage swells:
A voltage swell is defined as an increase to between 1.1 and 1.8 p.u. in rms voltage at the powerfrequency for duration between 0.5 cycles to 1 minute. A voltage swell is characterized by its magnitude and duration. As with sags, swells are associated with system faults. A SLG fault can result in a voltage swell in the un-faulted phases. Swells can also result from energizing a large capacitor bank.
III.
TEST SYSTEMThe below figure shows the test system used to carry out the various D-STATCOM simulations presented in this section. Single line diagram of the
test system for STATCOM is shown in Figure 2 and the test system employed to carry out the simulations for DVR is shown in Figure 2 Such system is composed by a 13 kV, 50 Hz generation system, feeding two transmission lines through a 3-winding transformer connected in Y/Δ/Δ, 13/115/115 kV.
Such transmission lines feed two distribution networks through two transformers connected in Δ/Y, 15/11 kV.
Figure 2: Single line diagram of the test system for D-STATCOM
IV.
SIMULATION RESULTS a) Simulation of System without DSTATCOM Figure 3 gives the Simulation model of system without DSTATCOM. In a distribution system with a non linear load connected to the system will generates Harmonics. A nonlinear load in a power system is characterized by the introduction of a switching action and consequently current interruptions.Figure 3 Simulation model of system without DSTATCOM This behavior provides current with different
components that are multiples of the fundamental frequency of the system. These components are called harmonics. The amplitude and phase angle of a harmonic is dependent on the circuit and on the load it drives. Power systems that are conceived to operate at the fundamental frequency are susceptible to erroneous behavior as more and more nonlinear loads
are connected to the network. Harmonics increase the resistances of the conductors due to skin effect and cause an abnormal neutral-ground voltage difference.
The first simulation contains no D-STATCOM and three phase fault is applied at point A in Figure 2, via a fault resistance of 0.4 Ω, during the period 0.1–0.2 s. The voltage sag at the load point is 40% with
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 5 respect to the reference voltage. Figure 3, shows the
load voltage without D-STATCOM.
The text model is developed on MATLAB for the cases of Three Phase Symmetrical Fault, Single Line to Ground fault and Double Line to
ground fault. In each case fault is connected at 0.1Sec to 0.2 Sec. For each case performance of the test system is studied and results are as follows. Figure 5, 6 and 7 gives rms Load Voltages.
Figure 4: Load voltage without D-STATCOM.
Figure 5: Voltage Vrms at the load point: without DSTATCOM with Three Phase Fault.
Figure 6: Voltage Vrms at the load point: without DSTATCOM with out Single Line to Ground Fault.
Figure 7 (a): Voltage Vrms at the load point: without DSTATCOM with Double Line to Ground Fault
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 6 Figure 7(b): Voltage Vrms at the load point: without DSTATCOM with
LL Fault
b) Simulation of System with DSTATCOM Now simulation carried out with D- STATCOM and three phase fault is applied at point A in Figure 8, via a fault resistance of 0.86 Ω, during the period 0.1-0.2 s. The voltage sag at the load point is compensated. Where the very effective voltage regulation provided by the D-STATCOM can be
clearly appreciated. The D-STATCOM supplies reactive power to the system. In spite of sudden load variations, the regulated rms voltage shows a reasonably smooth profile, where the transient over shoots is almost nonexistent. Here in the Figure 8, DSTATCOM is controlled by closed loop control.
The aim of the control scheme is to maintain constant voltage magnitude at the point where a sensitive load is connected, under system disturbances. The control system only measures the r.m.s voltage at the load point, i.e., no reactive power measurements are required.
Figure 8: Simulation model of system with DSTATCOM The VSC switching strategy is based on a
sinusoidal PWM technique which offers simplicity and good response. Since custom power is a relatively low-power application, PWM methods offer a more flexible option than the Fundamental Frequency Switching (FFS) methods favored in FACTS applications. Besides, high switching frequencies can be used to improve on the efficiency of the converter, without incurring significant switching losses. The proposed simulation controller is shown in Figure 8. The controller input is an error signal obtained from the reference voltage and the
value rms of the terminal voltage measured. Such error is processed by a PI controller the output is the angle δ, which is provided to the PWM signal generator. It is important to note that in this case, indirectly controlled converter, there is active and reactive power exchange with the network simultaneously: an error signal is obtained by comparing the reference voltage with the rms voltage measured at the load point. The PI controller process the error signal generates the required angle to drive the error to zero, i.e., the load rms voltage is brought back to the reference voltage.
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 7 Figure 9: Simulink model of DSTATCOM Controller.
Figure 10: Voltage Vrms at the load point: with DSTATCOM with Three Phase Fault.
Figure 11: Voltage Vrms at the load point: with DSTATCOM with Single Line to Ground Fault
Figure 12: Voltage Vrms at the load point: without DSTATCOM with Double Line to Ground Fault
Figure 10, 11 and 12 gives the Voltage Vrms at the load point: without DSTATCOM with Three Phase Fault , Single Line to Ground Fault and Double Line to Ground Fault. It is observed that sag is compensated.
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 8 Figure 13: waveform of distortion output current without LCL Passive Filter
Figure.14: Harmonic spectrum of distortion output current without LCL Passive Filter
Figure.15: waveform of distortion output current with LCL Passive Filter
International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 9 Figure.16: Harmonic spectrum of distortion output current with LCL
Passive Filter
V.
CONCLUSIONThere are many solutions in mitigating the power quality problems at a distribution system such as using surge arresters, active power filters, isolation transformer, uninterruptible power supply and static VAR compensator. It proposed a new D- STATCOM control algorithm to compensate Sags.LCL filter is introduced to reduce THD value.
The simulation results show that the voltage sags can be mitigate by inserting D-STATCOM to the distribution system. By adding LCL Passive filter to D-STATCOM, the THD reduced very effectively.
Thus, it can be concluded that by adding D- STATCOM with LCL filter the power quality is improved. It is observed from these studies that the proposed DSTATCOM with LCL filter gives better improvement in Power Quality.
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International Journal for Modern Trends in Science and Technology (ISSN: 2455-3778), Volume 4, Special Issue 4, July 2018 10 Authors
K Amala Rajani received the B.Tech degree in electrical and electronics engineering from D.M.S.S.V.H College of engineering, Machilipatnam, A.P, India in 2004, M.Tech from JNTU Hyderabad, India. Her Research interests are power system analysis, power system optimization, soft computing applications, and power quality
K Hari Hara Ayyappa received the B.Tech degree in electrical and electronics engineering from D.M.S.S.V.H College of engineering, Machilipatnam, A.P, India in 2017, Pursuing M.Tech from JNTU Kakinada AP, India. His Research interests include power system analysis, soft computing applications, and power quality.
