2016 International Conference on Circuit, Power and ...rise of supply voltage with amplitude ranges from 110% to 180% of its nominal value. A typical duration of voltage sag and swell

2016 International Conference on Circuit, Power and Computing Technologies [ICCPCT]

Design of Ultra-capacitor based DVR for Power Quality Improvement

S. Preetha R. Bhavani Dr. N. Rathina Prabha

PG scholar, Dept. of EEE Assistant Professor, Dept. of EEE Associate Professor, Dept. of EEEMepco Schlenk Engineering College Mepco Schlenk Engineering College Mepco Schlenk Engineering College

Sivakasi, Tamilnadu, India. Sivakasi, Tamilnadu, India. Sivakasi, Tamilnadu, India. [email protected] [email protected] [email protected]

Abstract–Power Quality (PQ) is the most significant perspectives at transmission and distribution levels. The supply of high grade electrical services required to the customers illustrates this concept. The voltage sag and swell are the most frequent PQ problems that mainly occur in the distribution systems since it may cause equipment tripping, failure of drive systems, shutdown for domestic and industrial equipment. The Dynamic Voltage Restorer (DVR)connected in series has magnificent dynamic capabilities and is a flexible solution for PQ problems.Ultra-capacitors (UCAP)have ideal characteristics such as high power and low energy density essential for the mitigation of voltage sag and swell.This paper presents an enhanced DVR topology capable of delivering deep, extended mitigation for power quality problems. In the proposed DVR, UCAP is used as energy storage as it provides excessive power in a short interval of time. The DVR is integrated into Ultra-capacitor via bidirectional DC-DC converterwhich supports in presenting a rigid dc-link voltage, and also helps in compensating temporary voltage sag and voltage swell. PI Controller is used in DVR for power quality enhancement. The simulation model for the proposed system has been developed in MATLAB and the performance over conventional DVR is validated with the results obtained.

Index terms-Dynamic Voltage Restorer (DVR), Ultra-capacitor (UCAP), DC-DC converter, sag/swell, PI controller.

I. INTRODUCTION

The concept of PQ in utility side has found to be

acknowledged in the recent years. Due to the continuous growth of electric load and transfer of high regional power via a large interconnected network, the security of power system may reduce and leads to a complex operation. It deals with a broad range of disturbances such as harmonics, voltage sags, voltage swells, flicker, interruptions and other distortions [1], [2].Among these power quality problems voltage sag and swell are the most frequent problems in the distribution system. Voltage sag occurs when the supply voltage drops with amplitude varies from 10% to 90% and lasts for a time duration of half a cycle to one minute. Alternatively, Voltage swell may occur when the sudden

rise of supply voltage with amplitude ranges from 110% to 180% of its nominal value. A typical duration of voltage sag and swell is 10 ms to 1 minute according to IEEE 1159-1195 and IEEE 519-1992 standards. The mitigation can be done with a number of available methods using custom power devices such as DSTATCOM, DVR and UPFC [3].

Among the custom power devices, Dynamic Voltage Restorer (DVR) is used as the most efficient device to restore the quality of voltage [4]. The system configurations and analysis reveals the operating performance of Dynamic Voltage Restorer [5]. The voltage capability of DVR depends on the ability of maximum voltage injection. An alternative solution proposed in DVR to compensate for the voltage sag which is done by injecting a lagging voltage in quadrature with the line current [6].In the recent past, the cost of the rechargeable energy storage has been drastically decreasing due to various developments in technologies such as solar, wind, hybrid electric vehicles (HEVs). Numerous types of rechargeable energy storage technologies based on flywheels (FESS), batteries (BESS), Superconducting magnets (SMES) and Ultra-capacitors (UCAPs) are designed for integration into advanced power applications like DVR [7], [8], [9]. There has been improved interest to integrate rechargeable energy storage at the dc-terminal of power quality products like STATCOM and DVR is addressed in [10]. Matrix converter based DVR is given in [11], [12] where there is no requirement of energy storage device for emergency purpose of the grid but it suffers from drawbacks such as high cost, high energy requirement and in [13] H-bridge with cascaded connection in DVR with an inductor controlled by thyristor is introduced to minimize the necessity of energy storage. Ultra-capacitors are best suited for many applications among other energy storage technologies which need active power support in the range of milliseconds to seconds [14], [15].

Ultra-capacitors have various potential advantages that make them unbeatable in many applications because they require neither cooling nor heating, no moving parts,itdoes not

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undergo internal chemical changes as part of their function. In addition, no frequent maintenance is required with reduction in lifetime degradation due to deep cycling and they are very efficient and robust. The application of super-capacitor in wind energy is deliberated in [16]. The integration of super-capacitor into the DVR for the distribution grid is proposed in [17].

This paper presents the integration of UCAP based DVR since DVR can supply only limited amount of real power and is not able to compensate for higher values of PQ problems. The UCAP-DVR connected via bidirectional DC-DC converter is implemented to achieve aprecise and fast response of the DVR. In addition, UCAPs have high powerdensity and low energy density ideal characteristics for effective compensation of PQ problems such as voltage sag and voltage swell investigating the high quality of power in the distributed power generation.

II. PROPOSED BLOCK DIAGRAM

The block diagram of the integrated UCAP-DVR

system is shown in Fig.1. It consists of three phase series inverter, a bidirectional DC-DC converter connected with UCAP energy storage device. The three phase supply voltage of 415 V, 50 Hz is connected to the sensitive load of 15 via line impedance.

Fig. 1. Block diagram of integrated UCAP-DVR.

The three phase voltage source inverter which acts as a power stage is connected in series to the grid and is responsible for the voltage sag and swell compensation. The energy storage device UCAP cannot be connected directly to the inverter; it can be connected through abi-directional DC-DC converter to maintain stiff dc-link voltage.

2.1. Dynamic Voltage Restorer (DVR)

The three phase series DVR model is shown in Fig. 2.It consists of the Insulated gate bipolar transistor (IGBT), its gate-driver; filter circuit consists of theinductance of 1.2 mH and

capacitance of 120 μF and an isolation transformer. The dc-link voltage Vdc is maintained at 260 V. The key purpose of a DVR is the protection of sensitive loads from power quality problems

Fig. 2. Model of DVR.

such as voltage sag and swell. If a fault arises on the transmission lines, DVR injects a series voltage and pre-fault value is obtained by compensating the load voltage. The control of momentary amplitudes of the three injected phase voltages takes place to eliminate the adverse effect of bus voltage into the load voltage . This states that equivalent voltage generated by the inverter will compensate any discrepancy of voltages produced by transient disturbances in the ac feeder. The injected voltage of the DVR connected in series can be written as

sloadinject VVV += (1)

Where, is the required voltage magnitude of load and Vs is the source voltage during sag and swell condition. The fault level of the load bus determines the system impedance ( ). The DVR introduces a series voltage , when there is a drop in the system voltage ( ) via injection transformer in order to maintain the required load voltage magnitude .

thLthmDVR VIZVV −+= (2)

In order to provide dynamic voltage restoration, there are various methods available to control the inverter connected in series. The control method requires the use of the proportional and integral controller. The output signal of the PI controller is directly proportional to the sequence of measured actuating error signal and its time. The transfer function is given by,

Transfer Function = PKs

K +1 (3)


Power quality problem includes voltage sag and swell are generated at the load terminals by inducing the fault. The sensing of load voltage takes place and is passed through the sequence analyzer and its amplitude is compared with a reference voltage (Vref). At the load terminals, a three phase 50Hz sinusoidal voltage is produced by applying Pulse Width modulated control technique in inverter switching. In order to maintain the base voltage across the load terminals, the IGBT inverter is controlled by using PI controller. Using DVR, the compensation is achieved by either injecting or absorbing the real and reactive power into the system. The main disadvantage of DVR states that it can supply only limited amount of real power during compensation. For effective compensation to take place, DVR can be connected through the energy storage device.

2.2. Ultra-capacitor (UCAP)

Ultra-capacitor consists of the electrode, electrolyte, collector, exhaust valve, membrane for isolation, sealing materials and connection pole. The performance of Ultracapacitor depends on Electrode materials, composition of electrolyte, quality related to separation membrane and manufacturing technology. According to the energy storage mechanism, UCAP can be divided into three categories namely double-layer capacitor, metal-oxide electrode super-capacitor and organic polymer electrode Ultracapacitor. The frequently used carbon electrode double-layer capacitance is shown in Fig.3.

Fig. 3. Ultra-capacitor model.

While charging, the positive plate attracts electrolyte anion and negative plate attracts cation, a double layer capacitor is formed on the surface of two layers, thus the name double layer capacitor. When discharging, it can release all stored energy instantly. UCAP is mainly suitable for short term high-power application.

.

2.3. Bidirectionalmode DC-DC converter

There arises an increasing need for the systems with the competency of bidirectional transfer of energy between two dc buses, thus bidirectional dc-dc converters have recently received a lot of attention. It acts as an interface circuit in energy storage system for UCAP as awide range of voltage varies during charging and discharging. Moreover in the voltage side, if an inverter is connected, there is a need for the UCAP to provide a stable DC voltage for the inverter circuit. Thus, DC-DC converter plays a major role in this system.The model of bidirectional DC-DC converter is shown in Fig. 4. with UCAP as energy storage.

Fig. 4. Model of Bidirectional DC-DC converter with UCAP energy storage.

During Voltage sag event, the DC-DC converter should be able to withstand the power generated during the discharge mode. Depend on depth and duration of the voltage sag; the grid decides the amount of active power support. Conversely, during voltage swell event the DC-DC converter may able to absorb the additional power from the grid. Thus bidirectional DC-DC converter acts in boost mode while discharging and, on the other hand, it acts as buck mode during charging.

III. SIMULATION RESULTS

The simulation of the integrated UCAP-DVR is carried out in MATLAB/Simulink for a 415 V, 50Hz system.


Fig. 5. A simulation model for three phase Distribution system.

The simulation model for three phase distribution system is shown in Fig. 5. and creation of voltage sag and the swell event is shown in Fig. 6.(a)-(b) for a time interval of 0.15s and 0.35s.The voltage sag and swell events can be compensated efficiently by integrating the distribution system with the DVR connected with UCAP.

Fig. 6. Simulation Waveform for (a) Voltage sag. (b) Voltage swell.

3.1. Ultra-capacitor design

Ultra-capacitor (UCAP) is used as energy storage device as it has high power density. It stores energy through charge separation and thus need for chemicals are reduced. The advantage of using UCAP is that they have long life time and it has large charge as well as discharge cycles. The choice of selecting the number of UCAPs depends on the factors such as a terminal voltage of UCAP, dc-link voltage and voltage of the distribution grid. In this paper, three UCAP each of 48 V are in series connection to obtain 144 V and capacitance of 165F. The design parameters are shown in TABLE-I. The value of capacitance and Equivalent series resistance (ESR) depends on the formula given by,

nC

C cellsy = (4)

nESRESR cellsy *= (5)

TABLE-I DESIGN PARAMETERS

PARAMETERS RATINGS

Rated Voltage 48 V

Leakage Current 5.2 mA

Rated Capacitance 165 F

ESR 7 m

Operating temperature 25o C

The capacitance of the UCAP varies directly with the parallel plate area A and inversely with the distance a between the plates and is given by,

aA

C rεεο= [F] (6)

Where o is the vacuum permittivity constant and r is the relative constant of insulating dielectric between the plates.

3.2. Simulation model of UCAP with DC-DC converter

The simulated model of UCAP with DC-DC converter is shown in Fig. 7. Three UCAPs are needed to be connected in series with the voltage of 144 V which acts as input to the

Seconds (a)

Seconds (b)


bidirectional DC-DC converter. The nominal load of 213.5 acts as the output for preventing the operation at the no-load condition and is connected to the dc-link of the DVR. Fig. 8 shows the input and output simulation result of UCAP with bidirectional DC-DC converter. The model of integrated UCAP-DVR was done for open loop and PI controller. Due to peak overshoot in the voltage waveform, bidirectional DC-DC converter uses fuzzy logic controller circuit with the fine output voltage.

Fig. 7. Model of UCAP with bidirectional DC-DC converter.

Fig. 8. Input and Output Voltages of DC-DC converter.

3.3. Integration of UCAP-DVR

The result for the integration of UCAP-DVR for the voltage sag and swell event is explained as follows. The injected voltage of the series inverter [Vinja, Vinjb, Vinjc] for the voltage sag is shown in Fig. 9(a). It can be observed from Fig. 9(b) that the injected voltage Vinjalags V0ab by 30o, which shows that it is in-phase with the line-neutral source voltage V0ab. Fig. 9(c) represents the compensation voltage for voltage sag event. The injected voltage of the series inverter [Vinja, Vinjb, Vinjc] for the voltage swell is shown in Fig. 10(a).

Fig. 9. (a) Injected voltage [Vinja, Vinjb, Vinjc] during sag. (b) Injected voltageVinja (green) and V0ab(blue) during sag.

(c) Compensated voltage during sag. It can be observed from Fig. 10(b) that the injected voltage Vinjalags V0ab by 150o, which shows that it isout of phase with 180oin line-neutral voltage source V0ab. Fig. 10(c) represents the compensation voltage for voltage swell event. Thus, the voltage sag and swell problems are compensated effectively by the integration of UCAP with Dynamic Voltage Restorer connected

Seconds (a)

Seconds (b)

Seconds (c)

Seconds (b)

Seconds (a)


Fig. 10 (a) Injected voltage [Vinja, Vinjb, Vinjc] during swell. (b) Injected voltage Vinja (green) and V0ab (blue) during swell.

(c) Compensated voltage during aswell.

via bidirectional DC-DC converter. Here the active power may be absorbed by UCAP-DVR from the grid during voltage swell event through the bidirectional converter and the inverter. It can be observed that due to the injected voltage, the magnitude of the source voltage is reduced, but the load voltage remains constant and thus the voltage sag event can be compensated.

Similarly, it can be observed that due to the injected voltage, the magnitude of source voltage has increased, but the load voltage remains constant and thus the voltage swell can be compensated.

This overall modeling and its result can be compared with the uncompensated and conventional through the THD analysis. This can be done for voltage and thus, the total harmonic (THD) in the system can be found out. This analysis can be done to justify that the integrated UCAP-DVR works efficiently than the uncompensated and conventional system, thus compensating the problems of voltage sag and voltage swell without any distortions. The uncompensated system consists of fault creation while the conventional system comprises of DVR connected in series with the three phase distribution system without any energy storage devices.

(a)

(b)

Fig. 11. THD for conventional system (a) Voltage sag. (b) Voltage swell.

(a)

Seconds (a)

Seconds (b)

Seconds (c)


(b)

Fig. 12.THD for integrated UCAP-DVR system (a)Voltage sag. (b) Voltage swell.

Thus, some distortion may be seen in compensated voltage is specified in the Total harmonic distortion analysis. The voltage sag and swell event in terms of THD for the conventional system is shown in Fig. 11(a) and (b). Conversely for the integrated UCAP-DVR, the THD for voltage sag and swell is shown in Fig. 12(a) and (b).The comparison of total harmonic distortion between the uncompensated, conventional and the proposed system is shown in Table-II and the comparison of various systems is shown in Fig. 13. The analysis shows that the integrated UCAP-DVR connected through the bidirectional DC-DC converter performs well with the total harmonic distortion and the uncompensated system and conventional system.

TABLE-II THD COMPARISON

System THD for Voltage sag

THD for Voltage swell

Uncompensated 7.35 % 8.45 %

Conventional 3.22 % 4.02 %

Integrated UCAP-DVR 1.55 % 1.71 %

Fig. 13. The comparison graph for various systems.

IV. CONCLUSION

In this paper, a new approach was proposed to improve the voltage profile of distribution power system. The proposed model is equipped with DVR as a suitable FACTS device and UCAP as rapid energy storage system. The design and modeling of bidirectional DC-DC converter were discussed as UCAP cannot be directly connected to the dc-link of the DVR. The UCAP plays an important role; since they can provide very high power in a short duration of time and to explore the feasibility and stability of the energy storage system for improving the electric power quality and this is a cheap solution of solving PQ problems in the distribution grid. Simulation result shows that the proposed DVR provide compensation in efficient and deep manner. The results that obtained are compared with conventional DVR in terms of THD.UCAP based energy storage can be adopted in the future on various distribution grid in order to prevent sensitive loads from disturbances.

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% T

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2016 International Conference on Circuit, Power and ...rise of supply voltage with amplitude ranges from 110% to 180% of its nominal value. A typical duration of voltage sag and swell