Power Quality Improvement in Non-Linear Loads using Dynamic Voltage Restorer with Special Reference to Induction Furnace

Power Quality Improvement in Non-Linear Loads using Dynamic Voltage Restorer with Special Reference to Induction Furnace

Tejinder Singh Saggu

Asst Prof., Electrical Engg. PEC Uni of Tech Chandigarh, India &

Research Scholar, Punjab Tech Uni, Jalandhar, India

Lakhwinder Singh

Professor, Electrical Engg, BBSBEC, Fatehgarh Sahib, India

Abstract It is a well-known fact that power is the basic and urgent need of every country due to the industrial revolution. Every sector whether it is industrial, commercial or domestic depends profoundly on power because all require power for their existence in this world. As the number of industries employing non-linear loads is increasing day by day and is expected to increase dramatically in the years ahead, it has become essential to set up criteria for limiting problems associated with these loads from system voltage degradation. Induction furnace is one such type of nonlinear load mainly used in the steel industry which results in the production of harmonics due to which load current waveform gets distorted. It leads to production of current harmonics from the load side which tend to flow towards the source side through Point of Common Coupling (PCC). These current harmonics further gives rise to voltage harmonics on the source side due to which nearby consumers can be affected. Thus, it is recommended to control the current harmonics on the load side itself at PCC so that they cannot interfere with the neighbouring loads. In this paper, a typical study of induction furnace is presented and the solution methodology using custom power device technology based Dynamic Voltage Restorer is adopted to improve the power quality in induction furnace.

KeywordsInduction furnace, DVR, Harmonics, Power quality, Simulink

1. INTRODUCTION

   The power quality concept is exceedingly imperious to industrial and commercial designs of the power system. During the last two decades, non-linear loads in industries have been significantly increased. The supply voltage at PCC may contain harmonics due to the extensive use of power converters in the manufacturing process thereby results in the power quality degradation [1] [2]. The equipment may face abnormal operating conditions and may lead to breakdown [3]. As the same distribution network feeds many other users as well, hence it is important to recognize the various reasons for power quality disturbance at PCC [4]. In the industries as well as in the offices, Uninterruptable Power Supply (UPS) plays an important role. However, UPS systems are quite expensive, and while they do a good job in protecting their own load, they are also the major power system polluters and often cause problems for neighbouring loads. Thus, there exists the need to modify or design new and innovative circuits that can be placed at the user end to reduce distortion

   level or to cancel the effect of transient phenomena, and provide back up for end users which we get now through UPS systems. Thus, harmonics in the supply varies arbitrarily and hence various compensating techniques are being used such as passive filter, active filter or hybrid filter to eliminate harmonics at a particular frequency [5], [6] Also various configurations of these filters have been adopted depending upon the system configuration and its control strategy [7],[8]. After filters, advanced power electronics technology came into action known as custom power device technology. So, power quality improvement is possible either by harmonic cancellation or reactive power compensation by injecting current to improve the load current at PCC using these devices [9].

   The power quality problems can be addressed by two approaches. The first approach is to be possible through consumer side and the second approach is through utility side. The consumer side approach involves load conditioning, which ensures that the connected equipment should not be sensitive enough to power quality disturbances, i.e. if any voltage dip or voltage swell occurs, then it may not affect the operation of the equipment. The other solution is to install line conditioning filters that can counteract such types of disturbances. They may involve PWM techniques and can be connected in series or parallel to the system. In order to compensate current harmonics on the load side, series active filters acting as a controllable voltage source should be connected in concurrence with shunt passive filters which are acting as a controllable current source. This is possible using PWM inverters, a DC bus and any reactive element. This whole system configuration can play a major role in improving power quality problems.

2. INDUCTION FURNACE- A GLANCE

   In Indian industry, induction furnace plays crucial role in steel production. The mild steel is usually produced from scrap material using induction furnace in the industries. Earlier, arc furnaces were typically used in the steel industries and rolling plants but with the evolution of induction furnace, the use of arc furnace is limited. The Table I shows the comparison of an arc furnace and induction furnace.

   TABLE 1: COMPARISON OF INDUCTION FURNACE AND ELECTRIC ARC FURNACE

   S. No	Parameters	Approximate Range
   		IF	EAF
   1.	Net Output (kWh/t)	550	500
   2.	Electrodes (kg/t)	Nil	2.5
   3.	Dust Production (kg/t)	2	8-12
   4.	Flux (kg/t)	Nil	26-30
   5.	Refractory (kg/t)	3.5	4.2
   6.	Slag Production (kg/t)	12-15	65-70
   7.	Noise (dB)	84-86	100-120

   For the production of one ton of steel in a steel industry, an arc furnace would require around 440 kWh of energy at a temperature of around 1500°C. The oxidation elements like iron, carbon and silicon along with the burning of natural gases results in chemical energy formation which when mixed with electrical energy results in the melting of steel along with the various losses as shown in the fig. 1 below. This production efficiency has been significantly improved since last 25-30 years and also the consumption of electrical energy has got reduced up-to 50% by using induction furnace.

   Fig. 1: Steel production process using EAF

   The Table II shows the history of induction furnace and its linkage with the Indian steel industry:

   TABLE II: EVOLUTION OF INDUCTION FURNACE

   tr>

   S. No.	Period (in years)	Induction furnace history
   1	1870	Experiments on induction furnace performed
   2.	1900	First induction furnace patented
   3.	1907	Steel production begins using induction furnace in the USA
   4.	1965	Entry of induction furnace in Indian industry
   5.	1975	Medium Frequency induction furnace imported
   6.	1982-1995	Sudden growth of induction furnace started in the industry
   7.	2000-2015	Huge production of stainless steel from scrap and mild steel production

3. OPERATION OF INDUCTION FURNACE

   During the production of mild steel through induction furnace, certain precautionary measures in terms of chemical analysis of the input materials are required in order to furnish

   the end product as per our requirement. First, the input materials are being charged up to 50% and then analysis of bath sample is done for its further chemical composition by addition of certain metallics. The iron content of the charged material is enhanced if there is a high percentage of Sulphur, Carbon and Phosphorus in the bath sample. When about 80% of the melting process is completed, bath sample is considered to be in the final stage of further analyzing the sample. The pig iron is added to the sample, if there is low carbon content in the charged material. The oxidation of Silicon and Manganese is done using sponge iron, which also helps in removing Sulfur and Phosphorous in the charged material and hence the traceable elements for the production of steel can be controlled using sponge iron.

   The block diagram of a typical induction furnace used in this industrial unit is shown in fig. 2 below. This system is quite different from other systems because of the frequency selector unit which raises the normal system frequency of 50 Hz to several kHz. This is being done in order to ease in the melting process. The greater the frequency, lesser will be the melting time. The metal loading describes the type of metal scrap to be used for manufacturing process. In this paper, the whole induction furnace unit is being simulated using the data obtained from industry and then corrective action is applied using DVR.

   Frequency Selection Unit

   AC INPUT

   Metal Loading

   Frequency Selection Unit

   AC INPUT

   Metal Loading

   Induction Furnace Coil

   Induction Furnace Coil

   Power Unit

   Power Selection Unit

   Impedance Matching

   Power Selection Unit

   Impedance Matching

   Fig 2: Layout of Induction Furnace

   For production of Ingots (Cast Iron), the power consumption required is 550 kWh/ton. This value varies for different metals and the quality of scrap used for the production of Ingots. The melting time matters a lot as during this period, induction furnace draws heavy current from the supply mains and inject harmonics in the system. The melting time is calculated as follows:

   Input Power available = 600 kW

   Power consumption for Ingots = 550 kWh/ton Thus, Melting Time = x 1 hour

   = 0.91 hour

   = 54.6 min

   Thus, for melting 1 ton of Ingots, it requires 54.6 min. The melting time varies from metal to metal. The Table III shows the power consumption of different metals and alloys:

   S. No.	Scrap Type	Power Consumption (kWh/ton)
   1.	Steel	620-640
   2.	Solid Aluminium	500-550
   3.	Light Aluminium	600-630
   4.	Cast Iron	500-570
   5.	Spheroidal Graphite Iron	550-620
   6.	Mild/ Stainless Steel	600-660

   S. No.	Scrap Type	Power Consumption (kWh/ton)
   1.	Steel	620-640
   2.	Solid Aluminium	500-550
   3.	Light Aluminium	600-630
   4.	Cast Iron	500-570
   5.	Spheroidal Graphite Iron	550-620
   6.	Mild/ Stainless Steel	600-660

   TABLE III: POWER CONSUMPTION OF DIFFERENT METALS

   The complex power injected by DVR is given by:

   (2)

   (3)

4. WORKING PRINCIPLE OF DVR

   A DVR is mainly used in both low and medium voltage levels to solve power quality problems. It is generally connected in series with the network through an interfacing transformer. It is mainly used to detect voltage sag and swell in the system, thereby regulating the load voltage. When the system voltage is normal, DVR will be acting as silent but as it varies from actual value, it will inject the necessary compensating voltage thereby balancing the load voltage [10]. A DVR unit is generally installed at the PCC and a capacitor bank is also provided for energy storage. DVR can also be used to improve the voltage harmonics in the supply using series injection transformer. The DVR can also be used to compensate voltage harmonics, fault current limitation and suppressing voltage transients. During the event of voltage sag, the compensation and phase angle shift is provided by DVR. Its main components are energy storage unit, filter unit, inverter circuit and series injecting transformer as shown in fig

   3. It can operate in three modes, namely standby mode, protected mode and injection mode. Hence, any disturbance in the system will be compensated by a comparable voltage using a converter and injected by means of a transformer.

   Fig.3: Dynamic Voltage Restorer

   The impedance of system depends upon the fault level at load terminal. As the system voltage (Vth) decreases, series voltage (VD) is provided by injection transformer in order to achieve the desired load voltage (VL). The equation given by:

   (1)

   Where VL is the load voltage, IL is the load current, Zth is the load impedance and Vth is the system voltage during fault conditions.

   The load current IL is given by

   The reactive power will be provided by the DVR using voltage injection techniques restricted by various load conditions, type of fault, its voltage magnitude, phase shift and power rating of the equipment. Hence, various control strategies are dependent upon the type of load characteristics.

5. TESTING OF INDUCTION FURNACE USING DVR IN SIMULINK

   The whole system was modelled in Matlab, Simulink software. The block diagram of the complete test system is as shown in fig 4.

   Three Phase Supply

   11 kV, 50 Hz

   Step down Transformer 11/0.415 kV

   Twelve Pulse Rectifier

   DC Choke Coil

   Inverter 50-500

   Hz

   Induction Furnace

   Three Phase Supply

   11 kV, 50 Hz

   Step down Transformer 11/0.415 kV

   Twelve Pulse Rectifier

   DC Choke Coil

   Inverter 50-500

   Hz

   Induction Furnace

   Firing Circuit

   Coupling Transformer

   Firing Circuit

   Coupling Transformer

   DVR

   DVR

   Fig.4: Block Diagram of Test System in Simulink

   The specifications of various components involved in the above process are shown in Table IV.

   TABLE IV: SPECIFICATIONS OF TEST SYSTEM

   S. No	System Parameters	Specifications
   1.	Input	3 Ø, 11/0.415 kV, 50 Hz
   2.	Inverter	IGBT Based, 50-500 Hz
   3.	Induction furnace	8 Ton, 12 pulse, 500 Hz,
   4.	Injection transformer	1:10, R (pu)= 0.002, X (pu)= 0.05
   5.	DC link capacitor	7500 µF

   The DVR test system mainly consists of a nonlinear load, controller and injection transformer. The performance and testing of the DVR is done by fault occurrence in the system at a particular time. It involves the basic process followed by detection of voltage sag/swell, correcting voltage magnitude; generating trigger pulses to the sinusoidal PWM based inverter, correcting deviations in series voltage injection & the termination of trigger pulses when the event has passed. The controller can make the inverter to act as a rectifier and to charge capacitors through the DC energy link in case there is no voltage sag/swell in the circuit. The abc- dq0 transformation technique has been proposed to control DVR. This method gives information about voltage sag and swells in the system along with their event of occurring. The voltage is first converted from a-b-c frame to d-q-0 reference frame with zero phase sequence components ignored. This detection process is carried out by comparison of reference voltage & measured terminal voltage. Voltage sags can be detected by this system when measured voltage drops below 90% of reference level & swell is detected when measured voltage increases above 25% of reference level.

   In this model, the inverter switching in DVR is done through hysteresis voltage controller based upon synchronous

   reference frame theory. The per unit values of induction furnace are converted from abc to dq0 coordinates using Park transformation technique. The phase locked loop (PLL) is used to derive unit vectors sin and cos given by the following expression:

   =

   (4)

   If Vdref and Vqref are the reference values corresponding to direct and quadrature axis in rotating reference frame respectively, then voltage of DVR can be obtained as,

   = – (5)

   = – (6)

   Using inverse Parks transformation, the voltages of DVR in abc frame can be find out using the values of VDd and VDq from the above expressions as:

   instant of voltage sag, swell or interruption occurs, DVR will compensate such instant in order to improve the voltage profile of the system and improvement in THD content.

   =

   (7)

   The voltages obtained from above expression are used to generate gating pulses for power switches i.e. VSC containing IGBTs or MOSFETs. The PWM technique is used to generate commutation pattern and hence correction in voltage is done using this modulation. The block diagram representation of this transformation technique is shown in fig 5.

   Fig.6: Voltage sag mitigation using DVR

   1. Voltage injection by DVR

   2. Voltage sag occurrence at time interval from 0.2 to 0.3

   3. Correction in voltage waveform

   4. Correction in current waveform

Then, the Total harmonic distortion (THD) of the load voltage and load current has been measured using FFT analysis in the Simulink. The improvement in the THDs of both voltage and current is shown in Table V which is strictly as per IEEE-519 standards.

Three Phase Input Voltage

abc-dq0 transform-

ation

Compare

TABLE V: TOTAL HARMONIC DISTORTION IMPROVEMENT

Reference Voltage

PLL

Conversion to dq0 transforma- tion

Conversion to abc coordinate system

Output signal for PWM

CONCLUSION

In this paper, a special reference of induction furnaces has been presented for power quality improvement in steel production process. The whole system has been modelled in Matlab-Simulink platform and degradation in power quality using induction furnace load has been shown. Then, the

Fig 5: Transformation Technique

The overall improvement in voltage profile of the system is shown in figure 6. Due to the induction furnace load, the voltage sag was produced from time interval 0.2 to 0.3 sec, as shown in part b. The DVR compensated this voltage sag during the same interval as shown in part a. The improvement in voltage and current due to this compensation is shown in part c and d respectively. It may be noted that whenever such

solution methodology using custom power devices based DVR has been presented for the improvement of power quality. This methodology revealed the effectiveness of DVR for voltage profile improvement and harmonic improvement in induction furnace as per IEEE recommendations.

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