International Journal of Advanced Electrical and Electronics Engineering (IJAEEE)
ENHANCED UPQC FOR A SINGLE PHASE SUPPLY SYSTEM TO IMPROVE POWER QUALITY USING SUPER CAPACITOR
ENERGY STORAGE SYSTEM
1
V. Prakash,
2T Rama Subba Reddy,
3S. Tara Kalyani
1Talla Padmavathi C. E, 2Wgl Arjun C.E, 3JNTU C.E, Hyd
Email : 1[email protected], 2[email protected], 3[email protected]
Abstract— In recent years, energy storage systems are playing a major role in all areas, because of supplying energy in remote locations. The use of energy storage devices in many applications are increased extensively by the researchers. In this paper, we propose a concept to enhance the UPQC (Unified power Quality conditioner) for improving power quality at the end-user using a super capacitor energy storage system (SCESS). The power quality problem occurred as a non-standard current, frequency, harmonics and voltage and leads to failure of sensitive loads and also causes service interruptions. The UPQC has the capability of improving power quality at the point of installation on power distribution systems. In other words, the function of UPQC is to eliminate the disturbances that affect the performance of the critical load in a power system. A constant revision is happening to solve the PQ related issues at present. For a country like India, Power Quality is a big issue as there are frequent variations in power, outages and frequency.
Hence, it is mandatory to take vital steps towards the development. This research describes the UPQC principles and power restoration (voltage and current) for balanced / unbalanced voltage sags/swells in a distribution system. This paper proposes a typical configuration of UPQC that consists of a DC/DC converter supplied by a super capacitor at the DC link. A suitable series-shunt controller is employed for controlling the UPQC. The operation of the proposed system is simulated with MATLAB / Simulink.
Index Terms— Power Quality (PQ), UPQC, Super Capacitor, DC/DC Converter and MATLAB/Simulink.
NOMENCLATURE RL - Load Resistance LL - Load Inductance Cdc - DC Link Capacitance Vdc - DC Link Voltage Rs - Source Resistance Ls - Source Inductance Lsh - Shunt Inductance Lse - Series Inductance
A - Interfacial area between electrodes and electrolyte (m2)
c - Molar concentration (mol m −3) equal to c = 0.86/(8NAr3)
F - Faraday constant I - Current density (Am−2) if - Leakage current (A)
i0 - Exchange current density i0 = if/A (Am−2) k - Stefan-Boltzmann constant
N - Number of layers of electrodes NA - Avogadro constant
Np - Number of parallel supercapacitors Ns - Number of series supercapacitors Q - Electric charge (C)
R - Ideal gas constant r - Molecular radius equal x2 x2 - Helmholtz layer length (m)
α - Charge transfer coefficient, Tafel equation (0<alpha<1)
ΔV - Over potential
I. INTRODUCTION
With the increasing demand on non linear loads, the power quality problems are increasing. The modern equipments that are used in home and commercial are prone to harmonics and also effected with the poor power factor. The power quality problem is also due to the different faults conditions occurring on the power system network. These conditions cause voltage sag or swell in the system and malfunctioning of devices which damages the sensitive loads. The UPQC is expected to be one of the most powerful solutions to large capacity loads that are sensitive to the changes in supply voltage, flicker or imbalance. The Unified Power Quality Conditioner (UPQC) has a single topology that combines Dynamic Voltage Restorer (DVR) and Distribution Static Compensator (DSTATCOM) with a common DC link. These two are connected in a back to back configuration.
DSTATCOM compensated all current related distortions and hence it is a shunt compensator and DVR compensates all voltage related distortions and hence it is a series compensator. This compensation is done if there is storage medium at the DC link. The operation of both DVR and DSTATCOM are based VSC (Voltage Source Converter) Technique. The shunt compensator also works for reactive power compensation, current harmonic compensation, load unbalance compensation and power factor improvement. Whereas, the series compensator acts for voltage harmonics, voltage sags, voltage swells, flickering etc. and this is done with the harmonic isolation between load and supply. The super capacitor is used as a battery storage device across the DC link.
The conventional UPQC consists of energy storage in the form of batteries or flywheels to supply the real power required for compensation. A large capacitor is used to ensure a stiff voltage to the inverter. However the battery has a high storage capacity but unreliable and flywheels requires a lot of maintenance. The discharge rate is slower in batteries because of slower chemical process. But now the future is turned to higher rate of charging and discharging the energy which is possible with the super capacitors or ultra capacitors. The super capacitors stores less energy because of lower energy density but the power transfer capability is high when compared to the conventional batteries. The rate of discharge while compensation is fast and it takes even a small current for charging. The reason behind employing the super capacitors in UPQC system is due to less weight; faster charge/discharge cycle (106) time, higher power density, higher efficiency (95%) and almost maintenance free.
The scope of this work is to examine how the SCESS technology can be best operated for UPQC. This merged unit is developed to enhance the capability of the UPQC to maintain a high quality voltage, current and power factor at the point of common coupling.
This paper suggests a new form of UPQC, DC/DC converter and energy storage system. The operation of the intended system was verified through MATLAB \ SIMULINK software. The block diagram representation for the proposed system is shown below in fig 1.
II. UNIFIED POWER QUALITY CONDITIONER
Fig. 1 shows a 1-phase, UPQC connected to a power system feeding a combination of linear and non-linear loads. It consists of a two leg voltage controlled VSI used as a series APF and a two leg current controlled VSI used as a shunt APF. The dc link of both of these active filters is connected to a common dc link capacitor. The two-leg VSI based shunt active filter is capable of suppressing the harmonics in the source currents, load balancing and power factor correction.
The series filter is connected between the supply and load terminals using a single phase transformer. The main aim of the series APF is to obtain harmonic isolation between the load and supply. In addition to injecting the voltage, these transformers are used to filter the switching ripple content in the series active filter. A small capacity rated R-C filter is connected in parallel with the secondary of each series transformer to eliminate the high switching ripple content in the series active filter injected voltage. The VSIs for both the series and shunt APFs are implemented with IGBTs (Insulated Gate Bipolar Transistors). The load under consideration is a combination of linear and non-linear type.
Fig1. Block Diagram of
the Proposed System
Fig. 1 shows a basic system configuration of a general UPQC with series and shunt APFs. The DC link capacitance is fed by the DC/DC converter and a super capacitor bank. This bank consists of number of series and parallel capacitors to increase the current and voltage at the DC link and the DC/DC converter is used to maintain constant voltage at the DC link irrespective to the super capacitor bank by boosting the voltage level when sag appears in the line, and when there is a swell in the line then the excess voltage is drawn by the DC link capacitance and fed to the DC/DC converter and Fig 1. Block Diagram of the Proposed System from there the voltage is maintained constant at the super capacitor by reducing the voltage level (i.e; buck operation) of the DC link to the capacitor bank. In almost all of the papers on UPQC, it is shown that the UPQC can be utilized to solve PQ problems simultaneously. The UPQC for harmonic elimination and simultaneous compensation of voltage and current, which improve the PQ, offered for other
harmonic sensitive loads at the point of common coupling (PCC). UPQC has the capability of voltage imbalance compensation as well as voltage regulation and harmonic compensation at the consumer end. The shunt APF is used to absorb current harmonics, to compensate for reactive power, and to regulate the dc- link voltage between both APFs. The UPQC, therefore, is expected to be one of the most powerful solutions to large capacity loads sensitive to supply-voltage- imbalance distortions. In this paper, the proposed synchronous reference- frame (SRF)-based control method for the UPQC system with a DC/DC converter to control voltage at the super capacitor end is used and the system performance is improved. In the proposed control method, load voltage, source voltage, and source current are measured, evaluated, and tested under unbalanced and distorted load conditions using MATLAB/Simulink software. The values of the circuit parameters and the loads under consideration are given in the Appendix.
III. CONTROL SCHEME OF SERIES CONVERTER
The role of the proposed series converter is to eliminate harmonics and to provide reactive power requirement of the load so that ac source feeds only active component of sinusoidal quantity of unity power factor current. Since this series converter is connected in series with load, it improves the system efficiency.
The proposed control strategy is calculating the reference voltage at the load end which cancel out the distortions present in the supply voltage by injecting the voltages from the series APF, thus making the voltage at PCC a pure sinusoidal with a desired amplitude as shown in fig 2.
Therefore, the sum of the supply voltage and the injected series voltage makes the desired load voltage.
Fig. 3 shows the block diagram of an overall control scheme for the series APF system. DC bus voltage, supply voltage and current are sensed to generate pulses for the series converter. AC source supplies fundamental active power of load current and a fundamental current to maintain the dc bus voltage to a constant value. The sensed dc bus voltage of the APF along with its reference value are processed in the P-I voltage controller.
Fig-2 : Circuit Diagram of Enhanced UPQC
Fig 3. Control Scheme for Series Active Power Filter A P-I (proportional-integral) controller is used to regulate the dc bus capacitor voltage of the series APF.
Ve(n) = Vr(n) – V(n)---(1) The output of PI-Controller is
Vo(n)=Vo(n-1)+Kp{Ve(n)-Ve(n-1)}+KiVe(n) ----(2) Where Kp and Ki are proportional and integral constants. The output of the P-I controller is taken as peak of source current. A unit vector in phase with the source voltage is derived using its sensed value. The peak source current is multiplied with the unit vector to generate a reference sinusoidal unity power factor source current. The reference source current and sensed source Fig 3. Control Scheme for Series Active Power Filter Fig 2. Circuit Diagram of Enhanced UPQC current are processed in hysteresis current controller to derive gating signals for the switches of the APF.
Fig 3. Simulink Block Diagram for Series APF In response to these gating pulses, the APF impresses a PWM voltage to flow a current through filter inductor to meet the harmonic and reactive components of the load Current as shown in Fig 3.
IV. CONTROL SCHEME OF SHUNT CONVERTER
The SRF-based control method is one of the most conventional and the most practical methods. The SRF method presents excellent characteristics but it requires decisive PLL techniques. The shunt active filter shown in Fig. 1 is a current controlled voltage source inverter (VSI), which is connected in parallel with the load. It is controlled in such a way to generate the required reactive and harmonic currents of the load. Hence, the utility needs to supply only the active part of the fundamental component of the load current and thus the power pollution problem along the power line could be avoided. The control algorithm computes the reference for the compensation current to be injected by the shunt active filter. The choice of the control algorithm therefore decides the accuracy and response time of the filter. The calculation steps involved in the control technique have to be minimal to make the control circuit compact. The control strategy has an objective to guarantee balanced and sinusoidal source current at unity power factor. This objective can be easily realized if the active part of the fundamental component of the load current is accurately and instantaneously determined.
The hysteresis current control scheme used for the control of shunt active filter is shown in figure above.
The reference for compensation current to be injected by the active filter is referred to as iref and the actual current of the active filter is referred to as iinj. The control scheme decides the switching pattern of active filter in such a way to maintain the actual injected current of the filter to remain within a desired hysteresis band (HB) as indicated in figure above.
The switching logic is formulated as follows:
If iinj < (iref − HB) S1, S2 ON & S3, S4 OFF If iinj > (iref + HB) S1, S2 OFF & S3, S4 ON
The switching frequency of the hysteresis current control method described above depends on how fast the current changes from upper limit to lower limit of the hysteresis band, or vice versa. Therefore the switching frequency does not remain constant throughout the switching operation, but varies along with the current waveform. Furthermore, the filter inductance value of the active filter is the main parameter determining the rate of change of active filter current.
Fig 3. Simulink Block Diagram for Shunt APF
V. SUPER CAPACITOR ENERGY STORAGE SYSTEM
Super capacitor is a double layer capacitor; the energy is stored by charge transfer at the boundary between electrode and electrolyte. The amount of stored energy is function of the available electrode and electrolyte surface, the size of the ions, and the level of the electrolyte decomposition voltage. Super capacitors are constituted of two electrodes, a separator and an electrolyte. The two electrodes are separated by a membrane, which allows the mobility of charged ions and forbids no electronic contact.
The electrolyte supplies and conducts the ions from one electrode to the other. Usually super capacitors are divided into two types: double-layer capacitors and
electrochemical capacitors. The former depends on the mechanism of double layers, which is result of the separation of charges at interface between the electrode surface of active carbon or carbon fiber and electrolytic solution. Its capacitance is proportional to the specific surface areas of electrode material. The latter depends on fast faraday redox reaction. The electrochemical capacitors include metal oxide super capacitors and conductive polymer super capacitors. The working voltage of electrochemical capacitor is usually lower than 3 V. Based on high working voltage of electrolytic capacitor, the hybrid super-capacitor combines the anode of electrolytic capacitor with the cathode of electrochemical capacitor, so it has the best features with the high specific capacitance and high energy density of electrochemical capacitor. The capacitors can work at high voltage without connecting many cells in series. The most important parameters of a super capacitor include the capacitance(C), ESR and EPR (which is also called leakage resistance).
The size of super-capacitors is determined depending on the size of load connected and the duration of voltage interruption. Therefore, total energy to be released during the voltage interruption is 30kJ. The maximum current flows through the super-capacitor bank, when it discharges the maximum power. The minimum voltage across the super-capacitor bank can be determined with the maximum discharge power and the current rating as the following.
Ubank _min = 20kW / 360A=55.5V
c RT 8 A N N rshin Q F
RT 2 NN A N N
Qx V NN
0 2
P S
0 2 P
2 S
It is assumed that the super-capacitor is charged by 2.43V, which is 90% to the maximum charging voltage of 2.7V, for consideration of 10% margin.
Sl.
No.
Manufacturer Specification of Super Capacitors 1 Power Star
China Make (single Unit)
50 F/2.7V, 300F/2.7V,
600F/2.7 V, ESR less than 1m
2 Panasonic Make (Single Unit)
0.022-70F, 2.1-5.5V, ESR 200 m©-350 3 Maxwell Make
(Module)
63F/125V, 150A ESR 18 m, 94F/75 V, 50 A, ESR 15 m
4 4 Vinatech Make
10-600F/2.3V, ESR 400 -20 m, 3- 350F/2.7, ESR 90-8 m 5 Nesscap Make
(module)
15V/33F, ESR 27 m 340V/ 51F, ESR 19 m
The super capacitor block implements the Stern Equation and Tafel Equation
NRT N V V N F V S exp Ai t
i S
Max S 0
c
The State of Charge for a fully charged capacitor is 100% and for an empty super capacitor it is 0%.
The SOC is calculated as
100 Q *
d ) ( i Q SOC
T t
o init
The lowest discharged voltage is determined to be 2.1V using the following.
Uunit _min = 3/ 4´Uunit _max = 2.1V
Therefore, the lowest discharge voltage and the minimum unit voltage determine the number of units to be connected in series as the following.
N =Ubank _min /Uunit _min =55.5/ 2.1= 26.5
However, the bank can be designed using total 28 units of super-capacitors for the purpose of safety margin.
VI. OPERATION OF DC-DC CONVERTER
The DC/DC converter can operate in bi-directional mode. The operation voltage of the super-capacitor bank is in the range between 60-75V, while the dc link voltage is about 700V. The converter should have high current rating at the bank side and high voltage rating at the DC link side. A DC/DC converter with two full bridges is shown in Figure below.
A filter reactor is inserted between the bank and the full bridge to reduce the ripple of charging and discharging current. The full-bridge in bank side is as a current-fed converter and the full-bridge in DC link side works as voltage-fed converter. The DC/DC
converter boosts the super-capacitor voltage up to the nominal DC link voltage in discharge mode. The super-capacitor voltage is controlled between 60-75V, while the DC link voltage increases up to 700V. The switches SC1 and SC2 operate with a duty ratio of higher than 0.5. The current through transformer rises linearly and its peak value becomes larger than the current through the boost inductor. When the auxiliary switch is turned off then the magnetic energy stored in the leakage inductance of the transformer flows through the diode. So, the zero voltage turn-on condition is provided. The DC/DC converter decreases the nominal DC-link voltage down to the level of super-capacitor voltage in charge mode. The power in the primary side is transferred to the secondary side.
The secondary voltage charges the capacitor Ch through the reverse-connected diode of auxiliary switch Sa. If the charging voltage is high enough to make the charging current zero, switch Sb1 turns off.
Switch Sb3 turns on with zero-voltage scheme while the capacitor C1 is charged and the capacitor C3 is discharged. When auxiliary Sa turns on, the voltage across the auxiliary capacitor affects the primary voltage of the coupling transformer. This voltage is applied to the leakage inductance LLk with reverse polarity. This makes the primary current zero and switch Sb2 turns off with zero-current scheme.
VII. RESULTS AND DISCUSSIONS
Nonlinear behavior of load current and mitigation in source current
Application of sag from 0.5 to 0.7 sec.
Improvement at load voltage.
Stabilizing of DC capacitor voltage
Dc current injected out from DC-DC capacitor
Harmonic compensation along with the compensation current at shunt compensator
Voltage injected by the series compensator at sag. (injection component outoff phase to the
source leading to the inphase component to the load and its support.
Reduced burden on the shunt compensator with the sag compensation by series compensator.
Power factor correction due to upqc.
Independent of energy storage system.
Comparison of source voltage load voltage magnitude
Change in source current due to sag. With and without super capacitor energy storage.
Injected voltage by series inverter.VIII. APPENDIX
Vs = 230V, f = 50Hz, Rl = 10, Ll = 25mH, Cdc = 4700uF, Vdc = 700V, Rs = 0.01, Ls = 50uH, Lsh = 8mH, Lse = 2.5mH.
Super Capacitor Ratings: Crated = 1000F, Rse = 2.1mohms, Vrated = 65V, Surge Voltage = 75V, Ns = 28, Np = 1, Initial Voltage = 60V, leakage Current = 5.2mA, Temperature = 25 C
Stern-Tafel Parameters:
N = 6, r = 1.23*10-9, ΔV = 0.3V, α = 0.3, Charge Current=100A
IX.CONCLUSION
The proposed system is compensating the voltage sag and swells with improved power factor. This paper proposes a new configuration of UPQC that consists of the DC/DC converter and the super capacitors for compensating the voltage interruption. The proposed UPQC can compensate the reactive power, harmonic current, voltage sag and swell, voltage unbalance, and the voltage interruption. The control strategy for the proposed UPQC was derived based on the Synchronous reference frame method. The operation of proposed system was verified through simulations with MATLAB/SIMULINK software. The proposed UPQC has the ultimate capability of improving the power quality at the installation point in the distribution system. The proposed system can replace the UPS, which is effective for the long duration of voltage interruption, because the long duration of voltage interruption is very rare in the present power system.
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