Power Quality Improvement Using PI and Fuzzy Controller Based Dynamic Voltage Restorer
Parveen Saini
1and Vijay Kumar Garg
21
PG Scholar (EE), U.I.E.T Kurukshetra University, Haryana, India
1
parveen.npti@gmail.com
2
Assistant Professor (EED), U.I.E.T Kurukshetra University, Haryana, India
2
vijaygarg_kuk@yahoo.com
ABSTRACT
The Power quality problem is one of the dominant issue at consumer side. Due to this problem there is a massive loss in terms of time and money. The short circuit faults and sudden injection of heavy loads, creates power quality disturbances. To bring out the improved power quality using custom power devices which leads to mitigate the problem. The Dynamic Voltage Restorer gives the controlled power supply by switching the inverters. The performance of PI based DVR when compared with Fuzzy based DVR controller, the fuzzy based controller provide better transient response and reduced total harmonic distortion during switching the dynamic voltage restorer. The results from comparison of both techniques fuzzy controller showed improved power quality and stable power supply at the load end. The validity of the proposed method and achievement of the desired compensation are confirmed by the results of the simulation in Matlab/ Simulink.
Keywords— DVR, PI-controller, Fuzzy Logic Controller, Power Quality.
1. INTRODUCTION
The advancements in technology have proven a path to the modern industries to extract and develop the innovative technologies within the limits of their industries for the fulfillment of their industrial goals. Their ultimate objective is to optimize the production while minimizing the production cost and thereby achieving maximized profits while ensuring continuous production throughout the period.
The demand of high quality power requires for the brakeless operations of manufacturing and process equipments in industry. It has been found that the blame of degraded power quality cannot be solely put on to the hands of utility itself.
The most of the conditions are generated by non-linear loads with in the industries caused disrupted power supply. Some of them are listed here:
1. Voltage sags 2. Phase outages 3. Voltage interruptions
4. Transients due to Lighting loads, capacitor switching, non-linear loads
5. Harmonics, etc.
As a result of above abnormalities the industries may undergo burned-out motors, lost data on volatile memories, erroneous motion of robotics, unnecessary downtime, increased maintenance costs and burning core materials. Among those power quality abnormalities voltage sags and surges or simply the fluctuating voltage situations are considered to be one of the most frequent type of abnormality occur in a fraction of a cycle to few cycles.
The solutions should come from both and are named as utility [1] based solutions and customer based solutions respectively.
There are two types of solutions are FACTS devices (Flexible AC Transmission Systems) and Custom power devices. The utility can directly controlled the FACTS devices, whereas the Custom power devices are operated, maintained and controlled by the customer itself and installed at the customer premises.
Both the FACTS devices and Custom power devices are based on solid state power electronic components. As the new technologies come into view, the manufacturing cost and the reliability of those solid state devices are enhanced, hence the
protection devices which assimilate such solid state devices can be purchased at a reasonable price with better performance than the other electrical or pneumatic devices available in the market. Uninterruptible Power Supplies (UPS), Dynamic Voltage Restorers (DVR) and Active Power Filters (APF) are examples for commonly used custom power devices. Among those DVR is used to compensate for voltage sags.
In this paper, a systematic approach for the comparative analysis of dynamic voltage restorer along with the implementation of proportional integral controller and fuzzy logic controller. The control of a Dynamic voltage restorer for three phase voltage sags has been studied. The VSI in DVR must be controlled correctly to inject the required current (in shunt connection) or voltage (in series connection) into the system in order to compensate for a voltage sag. It is also worth noting that the control circuit suggested in this study is easy to accomplish, hence the development cost and implementation investment can be largely reduced.
2. CONVENTIONAL SYSTEM
A conventional system configuration of a dynamic voltage restorer installed in radial distribution lines, where the feeder is represented by an inductor connected in series with a resistor. In the figure, a DVR is seen composed of an inverter, a dc power source, and a series insertion transformer. They are installed at a feeder connected to critical loads. When voltage sag takes place, the inverter is activated to inject a compensation voltage into the transformer such that the profile of input voltage is expected to preserve. The load impedance ZL depends on the fault level of the load bus. When the system voltage VS drops, shown in fig. 1 the DVR injects a series voltage VINJ through the injection transformer so that the desired load voltage magnitude VL can be maintained.
Fig. 1 Equivalent Circuit Diagram of DVR.[1]
The series injected voltage of the DVR can be written as:
VINJ = VL +ZL IL -VS (1) where
VL : Desired load voltage magnitude ZL : Desired load impedance
IL : Desired load current VS : The system voltage during fault condition
The load current IL is given by:
IL = (PL +iQL )/VL (2)
When VL is considered as a reference, the equation can be rewritten as:
VINJ ∟α = VL ∟0 + ZL ∟(β – θ)IL –VS∟δL (3)
Symbols α, β, δ are angles of VINJ, ZL , VS respectively and is Load power angle. The complex power (SDVR) injection of the DVR can be written as,
SDVR = VINJ * IL (4)
It requires the injection of only reactive power and the DVR itself is capable of generating the reactive power.
3. PROPOSED METHOD
3.1 Main Circuit
The configuration of the proposed DVR shown in Fig. 3, where the outputs of a three-phase full-bridge inverter are connected to the utility supply via a Y-connected series transformer. In case of three line to ground fault the connectivity switch detects the fault current/voltage in order to connect the DVR to the disturbed distributed line with the addition of PWM-based control scheme, the inverter output can be steered in phase with the incoming ac source while the load voltage is maintained constant. With such a design, the quality of demand side can be better ensured, while that of the supplying power can be less affected.
3.2 Control Model
In the proposed method, the control system is performed to offer the required switching signals for the semiconductor switches (IGBT). As the current-controlled inverter technology comes with a fast response, it was adopted in the proposed method. Since the method performs the direct
control of the load voltage, there is no need to calculate the series compensation voltage in advance, relieving the computation burden and shortening the circuit design procedures in a significant manner.
Fig. 3 Proposed DVR configuration [3]
3.2.1 Pi Controller
The endeavor of the control method is to maintain constant voltage magnitude at the point where a sensitive load is connected, under system disturbances. The control system only evaluates the root mean square (RMS) voltages at the load terminals, i.e., no reactive power measurements are required. The VSC switching approach is based on a sinusoidal PWM technique which proposes simplicity and good response. Since custom power is a comparatively low- power application, PWM methods offer a more flexible alternative than the Fundamental Frequency Switching (FFS) methods. Besides, high switching frequencies can be used to get better on the efficiency of the converter, without incurring considerable switching losses. The controller input is an error signal acquired from the reference voltage and the RMS value at the terminal voltage measured. Such error is processed by a PI Controller, the output angle is δ, which is provided to the PWM signal generator. 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, then produces the required angle to drive the error to zero, i.e. the load RMS voltage is brought back to the reference voltage
Fig. 4 Proportional Integral controller
.
Fig. 5 control circuit using PI controller
Fig.5 shows the control circuit designed in matlab/simulink.
The error output voltage V2 measured at the load is fed as the input to the PI controller. The voltage sag is detected by measuring the error between the output voltage and the reference values. The error signal is then fed to PI controller.
Set Kp to 0.1 and Ki to 1. The tuning of PI gives stable and well responses to system disturbances.
3.2.2 Fuzzy Logic Controller
The fuzzy logic controller for the proposed DVR has two real time inputs measured at every sampling time i.e. error and error rate and one output as actuating-signal for each of the phases. The input signals are fuzzified and represented in fuzzy set notations by membership functions. The defined ‘if
… then …’ rules produce the linguistic variables and these variables are defuzzified into control signals for comparison with a carrier signal to generate PWM inverter gating pulses.
Err = Vset –Vmeas (5) δErr = Err(n) –Err(n-1) (6)
Fuzzy logic control involves three steps:
1. Fuzzification, 2. Decision-making and
3.
De-fuzzification.
Fuzzification Knowledge base
Fig. 6 Fuzzy Logic controller
Fuzzification transforms the non-fuzzy (numeric) input variable measurements into the fuzzy set (linguistic) variable that is a clearly defined boundary. Three state conversation between output signal and input signal which performs logical control with rule based programming
.
Fig. 7 Membership function Input variable “Error”
Fig. 8 Membership function Input variable “Error rate”
Fig. 9 Membership function for output
In the proposed controller, the error and error rate are defined by linguistic variables such as:
LN = large negative MN = medium negative SN = small negative S = small
SP = small positive MP = medium positive LP = large positive
The memberships are curves that define how each point in the input space is mapped to a membership value between 0 and 1.
The membership functions belonging to the other phases are identical Membership functions for the inputs are shown in Fig.4 and Fig.5. The membership function of output variable is shown in Fig.6.
Error Rate
LP MP SP S SN MN LN
Error
LP PB PB PB PM PM PS Z MP PB PB PM PM PS Z NS
SP PB PM PM PS Z NS NM S PM PM PS Z NS NM NM SN PM PS Z NS NM NM NB MN PS Z NS NM NM NB NB LN Z NS NM NM NB NB NB
Table 1 Decision table for fuzzy logic control
The decision table for fuzzy logic control rules is shown in Table1. There are 49 rules to carry out optimum control action and each rule expresses an operating condition in the system.
The correct combinations of these rules improve the system performance
4. SIMULINK IMPLEMENTATION
In this section the modelling of DVR using PI and FUZZY controller have purposed shown in fig. 10 and the output response of each controller with dynamic voltage restorer are shown in fig. 12 and fig. 13.
Output Input
Inference (Rule /Data base)
Defuzzification Scaling Factor
Plant
Output scaling factor,
Normalization Sensor
Fig. 10 Simulink diagram of DVR
5. SIMULATION RESULTS
Three phase load end voltage with fault occur.
Fig. 11 Three phase fault at load point.
Injection of DVR voltage using different controllers after prediction of fault.
Fig. 12 Three phase compensated voltage with DVR using PI controller
Fig. 13 Three phase compensated voltage at load end with DVR using Fuzzy controller.
6. CONCLUSION
On the bases of MATLAB/SIMULINK model of three phase distribution system having dynamic voltage restorer with different controllers, the simulation graphs obtained for the mitigation of sag voltage with different controllers. The output response of given system obtained by using different controllers are providing by output voltage as feedback signal to controller as input. The output response of DVR using PI controller with setup of Kp and Ki improves the voltage quality. In another case, the performance of DVR using FLC with 49 rule bases also gives output voltage in improved manner to the load. Thus, the power quality of proposed system is improved as their voltage quality improved by using DVR with different controllers.
REFERENCES
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