Electrical arc furnaces in an industrial distribution system results in random dynamic voltage drops in all the three phases of the power supply. Thus the operation of arc furnace is said to be injecting harmonic current into the power distribution network which produces voltage distortion including voltage drops of short duration which not only reduces power supplied to the furnace but also adversely affects the quality of the source voltage. In order to overcome the harmonics problem in EAFs, fast dynamic reactive compensation device called STATCOM is provided at a suitable location of the power supply network.
Dynamic performance of a typical arc furnace in an industrial distribution system is computed with respect to voltage harmonics by using RSCAD in Real Time Digital Simulator (RTDS®) environment. Through simulation study, sizing and control strategy of PWM-based STATCOM have been arrived so as to improve the power quality as per IEEE Standard 519-1992.
Keywords-Arc Furnaces; Power Quality; Harmonics; Pulse Width Modulation; Real-Time Digital Simulation (RTDS)
Introduction
Electric Arc Furnaces (EAFs) are defined as AC or DC powered furnaces, which uses graphite electrodes to melt scrap iron and steel. Heat is supplied from electricity that produces arcs from the electrodes to the metal bath. The EAF comprises a refractory lined shell which holds the charge. The three electrodes, one each connected to the three phases of the AC electric power supply, are held in special clamps on a swing support structure which can be swung aside for charging, and which can allow each electrode to be raised or lowered with hydraulic actuators.
In an Electric Arc Furnace, scrap iron or steel is melted with huge current surge from the electrodes. The arc is produced when the electrodes move above the slag. Then current begins to jump from the electrode to the slag, creating electric arcs. These arcs behave as time varying and mostly resistive impedances which may not always be equal amongst three phases, produces random dynamic voltage drops in all the three phases of the power supply. Thus the operation of arc furnace is said to be injecting harmonic current into the power distribution network. The harmonic currents produce voltage distortion including voltage drops of short duration which not only reduces power supplied to the furnace but also adversely affects the quality of the source voltage influencing the performance of the other loads supplied at PCC (Point of Common Coupling). In order to overcome the harmonics problem in EAFs, fast dynamic reactive compensation device called STATCOM is provided at a suitable location of the power supply network.
A STATCOM, based on voltage-sourced converter is a Static synchronous generator operated as a shunt-connected Static VAR Compensator whose capacitive or inductive output current can be controlled independent of the AC system voltage. If the voltage source converter in STATCOM is appropriately designed with high-band width control capability, it can be used to force three-phase currents of arbitrary wave shape through the tie line inductance into the power line. This unique capability makes the STATCOM an ideal candidate for Electric Arc Furnace compensation. The STATCOM provides a controlled source of reactive power by drawing balanced positive sequence currents from the line at fundamental frequency.
This paper proposes to consider a typical arc furnace in an industrial environment. The system will be simulated using RSCAD in Real Time Digital Simulator environment and its dynamic performance is computed with respect to voltage harmonics. Through simulation study sizing and control strategy of STATCOM have been arrived at so as to improve the power quality as seen from the primary of the arc furnace transformer in the industrial power distribution system.
Power Distribution System of AC EAF:
Fig.1 illustrates the layout of the Industrial power system connected with a typical arc furnace to perform real time analysis under steady state and dynamic conditions.
11.5 kV /650V
4 MVA
Arc Furnace
Transformer
Source Bus
132/11.5 kV
90 MVA
T1
PCC Bus
Local Load
Distribution
MV cable-1
Length, L=500m
XLPE
Fault Level
Scc = 2200 MVA
Fault Level
Scc = 330 MVA
Fault Level
Scc = 180 MVA
Distribution
MV cable-2
Length, L=500m
XLPE
63.5 MVA
Furnace Bus
Arc Furnace
P = 2.5 MW
Figure.1. System Configuration
Vs
The complete system represented in the form of single-line diagram is divided with two voltage grades. The power transformer T1 with Y(grnd)-Y(grnd) is step-downed from 132kV to 11.5kV, while the arc furnace transformer T2 steps 11.5kV down to 650V, which in turn is connected to the arc furnace load of 2.5MW at 0.85pf(lag). The PCC is the point between the nonlinear arc furnace load and other customer loads at which power quality is constrained. Two XLPE (Cross-linked Poly- Ethylene) cables of 11 kV voltage ratings of 25 sq.mm and length 500m each of type Aluminium Round Armoured Cable are also connected with the EAF supply system.
The main parameters of the EAF industrial distribution system are as follows:
T1:90MVA, 132kV/11.5kV, 9.43%;
T2: 4MVA, 11.5kV/650V, 6%;
132 kV H.V. bus bar fault level short circuit capacity: 2200 MVA;
11.5kV PCC bus bar fault level short circuit capacity: 330MVA;
Furnace bus fault level short circuit capacity: 180MVA;
The proposed aggregate system is modelled using RSCAD software under Real Time Digital Simulation (RTDS®) environment as shown in Fig.1.
Voltage Source Converter Based STATCOM:
The Voltage Source Converter (VSC) forms the basis for several power electronic devices including the Static Compensator (STATCOM). The VSC and its implementation in a STATCOM are described in detail in this paper. The STATCOM is functionally an electronic version of the synchronous condenser, with several important advantages. Primarily, it provides very fast voltage control compared to the synchronous condenser, and has no moving parts. Like the synchronous condenser and unlike the SVC, it can supply its full rated reactive power into a very low voltage.
Voltage Source Converter:
The arrangement of a shunt connected, two-level VSC is shown Fig.2. The usual method of firing a two level VSC is with Pulse Width Modulation (PWM), where the GTO's are switched at high frequency.
The magnitude and phase of the ac waveform as well as harmonics at switching frequency are controlled by the switching pattern.
Figure.2. Basic Arrangement of Shunt Connected two-level VSC
An example of PWM firing along with some of the internal signals used is shown in Fig.3.
A very low switching frequency, 6 times fundamental, is illustrated for simplicity.
Figure.3. Example of PWM firing
VSC Firing Circuits:
The PLL:
Firing controls are synchronized to the ac system phase with a second order closed loop system called Phase Locked Loop (PLL). The instantaneous value of ac system phase is tracked, and appears as a saw-tooth waveform varying between 0 and 2 pi (Fig.3).
In the VSC case, a high degree of accuracy is required to control the capacitor voltage. Therefore relatively high gains are used. In this case the PLL with a very high loop gain is introduced with which it follows ac system phase with small transient error and responds to noise introduced into its input from converter switching.
PLL Generated Control Signal:
The PLL phase signal forms the basis for PWM firing pulse generation. Depending on the function of the VSC, various controllers are used to regulate one or more external variables. These control blocks create a pair of signals, VD and VQ of Fig.4, representing the desired phase and magnitude of the ac waveform to be synthesized on the internal nodes of the VSC. In the STATCOM, the phase control signal DELTA is very small and determines if real power will be added to or removed from the capacitor. The magnitude signal VT is called the modulation index and usually doesn't exceed 0.8.
Figure.4: The Phase Locked Loop
A value higher than 21 times 50 or 1.05 kHz is possible, but only if the modulation index is kept well below one.
Triangular Wave Generation:
Triangle waves are produced at some multiple N of the ac frequency. In the illustration shown in Fig.3, the multiple is 6, but actual values are usually a multiple of 3 and an odd number, like 15 or 21, for harmonic minimization. In the example case, 21 is used. With a system frequency of 50 Hz, this gives a carrier frequency of 1050 Hz.
Figure.5. Non-Linear gain Block
The non-linear gain block, shown in Fig.5, is mainly responsible for generation of the triangle waves.
Firing Pulse Generation:
Firing Pulses are produced for a single phase, from the components shown in Fig.6. The Comparator-complete-with-Fraction compares the triangle and reference sine waves. Its' outputs are a firing pulse integer and a floating point fraction to indicate the switching point between time steps necessary to implement firing pulse interpolation in the VSC component.
This aspect of firing is essential in the RTDS, as time steps are typically about 50 microseconds in duration. Without improved firing, the time step would be much too long to generate smooth sine waves of current required at the output of the VSC.
Figure.6. Firing pulse generator
The firing pulse data is transferred to the VSC model with the aid of SENDT0 icons. These are necessary as the VSC model only looks for firing pulse information passed in the T0 communication interval. It is important in the timing of the control that the portion generating firing pulses finish in about 15 microseconds or it delays the normal timing of transfers on the back plane. For this reason, the control has been split into several processors.
The Current Regulator:
In the STATCOM, phase and magnitude of the synthesized ac internal node voltage are produced by the current regulator. AC current is first per unitized and then converted in to two dc quantities Id and Iq. (Id - variable proportional to current in phase with the ac system voltage and Iq - variable in-quadrature with Id).
Current in-quadrature with the voltage is responsible for creation or absorption of reactive power.
The quadrature current regulator obtains its reference input from the ac voltage regulator. Changing the regulated Iq results in transient changes in Eq and the control angle, and large changes in Ed and the modulation index.
Current in-phase with the voltage is nominally zero and is modulated to regulate the voltage on the VSI capacitor and to account for losses. The reference for in-phase current is derived by measuring capacitor voltage, scaling and filtering it.
Figure (7): The Current Regulator
Equivalent Circuit Diagram of VSC based STATCOM:
Figure (8): Equivalent Circuit of Voltage Source Converter
The ac side circuit equations can be written as follows:
p
ia
ib
ic
=
-Rs/Ls 0 0
0 -Rs/Ls 0
0 0 -Rs/Ls
ia
ib
ic
+
1/Ls
ea - va
eb - vb
ec - vc
Using the Transformation of variables, the above equation can be transformed to the synchronously-rotating reference frame as follows:
The current control unit is always in operation, independent of the STATCOM operating mode. The state equations representing the dynamic equations are
* id = -R s/Ls * id + ω I q + (ed - v)/Ls --- (1)
* iq = -R s/Ls * iq - ωid + eq /Ls ---- (2)
Where ed and eq are the direct and quadrature axis components of the supply voltage, and ω is system frequency in radians per second.
From equation (1)
* id + Rs/Ls * id = ωiq + (ed - v)/Ls
Using Laplace Transformation
S * id(s) + Rs/Ls * id(s) = ωiq + (ed - v)/Ls
(SLs + Rs) * id(s) = Ls * ωiq + (ed - v)
id(s) = (1/ ( SLs + Rs)) * [ Ls * ωiq + (ed - v) ]- (3)
From equation (2)
* iq + Rs/Ls * iq = -ωid + eq/Ls
Using Laplace Transformation
S * iq(s) + Rs/Ls * iq(s) = -ωid + eq/Ls
(SLs + Rs) * iq(s) = -Ls * ωid + eq
iq(s) = (1/ ( SLs + Rs)) *[ (- Ls * ωiq ) + eq ] - (4)
From 3 and 4, the two components are decoupled to obtain two independent first order plant models with control inputs ud and uq as shown in following figure.
ud
id
uq
iq
1/ (SLs +Rs)
1/ (SLs +Rs)
Figure (8): First Order plant models
Where,
ud = ed - v + ωLs * iq
uq = eq - ωLs * id
Based on the decoupled models a pair of identical PI controllers may be easily designed.
Block Diagram for complete control scheme:
idref
PI controller
+
-
id
ω*Lf*Iq
Error
ud
V
ed
-
+
+
Figure.9 (A) Control Scheme with Direct current error fed to PI Controller
IDR
iqref
PI controller
+
-
iq
ω*Lf*Id
Error
uq
eq
-
-
Figure.9 (B) Control Scheme with Quadrature current error fed to PI Controller
It has two separate control loops that are capable of independently controlling ID and IQ. In each control loops, the corresponding current feed back signal is subtracted from the current reference and the resulting error signal is fed in to a PI controller. These PI controllers have identical gains.
RTDS Development for EAF Simulation:
In order to control the industrial distribution system with typical electrical arc furnace, incorporation of real time digital simulation system is required to evaluate the performance under actual steady state and dynamic operating conditions. This paper introduces design and implementation of EAF based on recorded data modeled in real time digital simulation. Through simulation, results are observed with sizing and control strategy of STATCOM with respect to voltage harmonics according to distribution supply system feeding EAF.
Until recently the real time digital simulation of FACTS devices was limited to 2-level systems with a Pulse Width Modulation (PWM) frequency of less than 1 kHz. However, developments in simulation technology have made it possible to execute the demanding real time simulation of multi-level VSC converters with PWM switching frequencies in the order of 1.5 - 2 kHz.
This paper presents a flexible and comprehensive model of a two-level Pulse width Modulation (PWM) based Voltage Source Converter (VSC) and tests the substantial firing pulse controls using Real Time Digital Simulation (RTDS®). For its excellent real-time simulation performance, Real Time Digital Simulator (RTDS) is a world class, state of the art, real time power system simulator capable of continuous real time operation. It is specialized power system simulator that uses parallel processing architecture to solve three-phase electromagnetic and electro mechanical power system transients in real time. It solves power system equations fast enough to continuously produce conditions that realistically represent conditions in real time network. The RTDS calculates the power system state with a typical time step of 50 micro seconds (over 300 times per power frequency cycle) utilizing customized hardware and software. The overall network solution technique employed in RTDS is based on nodal analysis and the underlying solution algorithms are those based on Dommel's solution algorithm. This provides a more exhaustive representation of industrial power supply network for the tests.
Modeling of Industrial Power System Arc Furnace under Dynamic conditions:
Utilizing the RTDS to test the controls abridges the recording of results since all signals can now be gathered by Real Time Digital Simulator (RTDS®).
Incorporation of Recorded Field Data of EAF Modelled in RTDS:
The instantaneous value signals of corresponding bus voltage and the phase currents of arc furnace load incorporated with 6 channels are recorded throughout an entire melt cycle lasting approximately 10 minutes with a time step of 84µseconds per power frequency cycle. A new device called GTNET, which provides direct link between an Ethernet LAN and the RTDS simulation is incorporated to accommodate the EAF data recordings in the order of several gigabytes. Using the Playback firmware as shown in Fig.10, the GTNET can read EAF data recording files from a PC hard drive and make the signals available in the real time simulation.
Coordination of Current injection model with respect to source Voltage:
In order to test and validate the VSC controls, the arc furnace load had to be authentically represented in RTDS simulation. The realistic and erratic behavior of electrical arc furnace is quite challenging since the prediction and simulation are very hard. The available arc furnace models in RTDS behave in such a way that its parameters are difficult to choose for its compensation.
Figure.10. Playback Firmware with GTNET Card
Figure.11 (A) Coordination of embedded arc furnace current injection model with PLL control voltage source, (B) FFT Component for Computation of Voltage and Current Harmonics at Furnace Bus and Source Bus respectively.
By injecting currents recorded from a typical arc furnace through the secondary of Arc furnace transformer as shown in Fig.11, the physical model can be validated to study on adaptable power supply system for EAF.
In order to establish the signal synchronization control strategies under simulation of EAF, the embedded current injection model at the arc furnace transformer secondary is coordinated with the recorded transformer secondary voltage waveform whose signals VA, VB and VC are synchronized with phase-locked loop. These signals results an output phase angle PHIREC, which is corrected with 30° (0.523599 radians) difference in phase shift (since the arc furnace transformer is Y-∆ connected) to lock the three phase voltages under fundamental frequency.
Computation of Voltage Harmonics under Dynamic Conditions:
The electrical arc furnace, as described earlier exhibits different arcing characteristics depending on the stage of its operation. In order to determine the most severe effect of the arcing on the power supply system, a few power frequency cycles of the more severe of the different modes of operation is considered to simulate the dynamic behaviour of the arc furnace. Though the non-linear load of the arc furnace affects the entire network, it is of outmost importance that the FFT analysis of voltages and currents at the furnace bus and at the source bus are studied for their individual harmonic contents and Total Harmonic Distortion (THD) which is illustrated in Fig.11 (B).
The information obtained for the arc furnace bus is useful in determining the requirement of corrective action for the excessive harmonics if any and the information at source bus is useful in estimating the extent of the arc furnace load corrupting the utility supply.
Case Studies and results:
The study of harmonic suppression using STATCOM has been divided into three parts; they are:
Consolidation of the steady state network:
The arc furnace of 2.5MW is modelled as steady state impedance to its rated MVA and power factor of 0.85. The Arc furnace Load with Resistance Rarc = 0.121Ohms in each phase and Larc = 2.4 * 10^-4 H in each phase with Star-Grounded is Connected at the Secondary of the Transformer. The magnitudes of voltages and currents in terms of phase-to-ground peak values from the real time digital simulated are given in table.1 and table.2 respectively, which shows the comparison between measured values of voltages and currents with theoretical values.
Table 1: Magnitudes of Steady State Voltages measured from industrial distribution system:
NODE
Voltage Phase-to-ground Peak Value
Measured (kV)
Actual (kV)
Primary Of Arc Furnace Transformer
8.52791 kV
9.389 kV
Furnace Bus
8.87085 kV
9.389 kV
PCC Bus
9.20032 kV
9.389 kV
Source Bus
105.98497 kV
107.77 kV
Table 2: Magnitudes of Steady State Currents measured from industrial distribution system:
NODE
Current Phase-to-ground Peak Value
Measured (kA)
Actual (kA)
Primary Of Arc Furnace Transformer
0.19491 kA
0.2008517 kA
Secondary of Furnace Transformer
3.35855 kA
3.3853 kA
PCC Bus
4.24803 kA
4.508 kA
Source Bus
0.3895 kA
0.39183 kA
Studying the effect of the arc furnace as dynamic load on the voltage profile of the network:
As mentioned in section 5.2.1 in this paper, line-to-ground voltages and line currents at furnace bus and source bus were computed in RTDS and their corresponding waveforms are as shown in fig. 12 and fig.13
respectively.
Fig.14 (A) and Fig.14 (B) show the Voltage THDs at furnace bus and source bus respectively. Similarly, Fig.15 (A) and Fig.15 (B) show the Current THDs at furnace bus and source bus respectively. It is evident from the voltage harmonics at the arc furnace bus that the THD exceeds the 5% limit as per the IEEE standard.
Figure.12 (A) Voltage waveform of Furnace bus under dynamic conditions Figure.12 (B) Current Waveforms of Furnace bus under dynamic conditions
Figure.13 (A) Voltage waveform of Source bus under dynamic conditions
Figure.13 (B) Current Waveform of Source Bus under dynamic conditions
Figure.14 (A) Voltage THD of Furnace bus under dynamic conditions
Figure.14 (B) Voltage THD of Source bus under dynamic conditions
Figure.15 (A) Current THD of Furnace bus under dynamic conditions
Figure.15 (B) Current THD of Source bus under dynamic conditions
Hence there is a need for filtering the current harmonics so that the THD of voltage are controlled to acceptable levels.
Modelling STATCOM to suppress harmonics produced by arc furnace:
The single line diagram of the industrial distribution system with STATCOM is as shown in fig.16 (A). The STATCOM (synchronous static compensator) based on voltage source converter (VSC) is used for voltage regulation in industrial distribution systems. In this paper, the integration and control of static capacitance incase of STATCOM (STATic COMpensator) is developed to mitigate such problems like voltage harmonics, enhance the power quality and improve industrial distribution system reliability.
Source Bus
132/11.5 kV
90 MVA
T1
PCC Bus
Local Load
Distribution
MV cable-1
Length, L=500m
XLPE
Fault Level
Scc = 2200 MVA
Fault Level
Scc = 330 MVA
Distribution
MV cable-2
Length, L=500m
XLPE
63.5 MVA
Furnace Bus
Arc Furnace
P = 2.5 MW
Vs
Figure.16 (A): Industrial power supply with arc furnace and STATCOM
GTOs:
The Gate-Turn-off Thyristors are used for construction of three phase VSC Bridge for which the following parameters are specified as:
Snubber Circuit Resistance = 1000.0 Ohms
Snubber Circuit Capacitance = 0.1 µF
Base Frequency = 50 Hz
Switching Frequency = 1050 Hz
Coupling Transformer:
A VSC Transformer of Y (grnd)-Y (grnd), 2.4 MVA, and 11.5 KV/300V is used to validate the VAr requirement of the arc furnace load for fast dynamic reactive power compensation. In this case the VSC transformer parameters are specified as shown in table.3:
VSC T/F MVA Sbase = 2.4 MVA
VSC T/F Primary voltage Vpri = 11.5 kV
VSC T/F Secondary voltage Vsec = 0.300 kV
VSC T/F Primary current Ipri = 0.12049 kA
VSC T/F leakage reactance Xs = 0.18 pu
VSC T/F leakage resistance Rs = 0.01 pu
Settings of the controller Variables:
The setting of control variables of a Voltage Source Converter based STATCOM; the PI controller constants of current regulator as shown in fig.7 are estimated by tuning it, whose values are:
Proportional Constant Kp1 = 0.94.
Rail-to-Rail Capacitance Voltage = 1.91 kV
Time constant of first PI-Controller = 0.867 sec
From the base current the gain of current regulator is calculated from the Scaling factor, which is formulated as follows:
Scaling of Current regulator = 1/ (Ibase*√2) = 1/ (0.12490* √2) = 5.868 (constant).
Therefore, Gain of the current Regulator = 5.868 (constant).
The parameters of PI-Controller-2 are specified as follows:
Proportional Constant Kp2 = 0.12.
Time constant of first PI-Controller = 0.115 seconds
Results of study:
It is observed from the figures of THDs of voltages and currents corresponding to furnace bus and source bus that there is a substantial reduction in both of them. Hence, A Voltage Source Converter based STATCOM is proved to be an efficient tool for reduction of harmonics in the arc furnace load. The voltage harmonics at the Furnace Bus are drastically reduced and the corresponding Total Harmonic Distortion is reduced from 10.61% to 4.4% which must be less than 5% according to IEEE standards mentioned in [2].
Figure.17 (A) Voltage waveform of Furnace bus with STATCOM under dynamic conditions
Figure.17 (B) Voltage waveform of Source bus with STATCOM under dynamic conditions
Figure.18 (A) Current waveform of Furnace bus with STATCOM under dynamic conditions
Figure.18 (B) Current waveform of Source bus with STATCOM under dynamic conditions
Figure.19 (A) Voltage THD of Furnace bus with STATCOM under dynamic conditions
Figure.19 (B) Voltage THD of Source bus with STATCOM under dynamic conditions
Figure.20 (A) Current THD of Furnace bus with STATCOM under dynamic conditions
Figure.15 (B) Current THD of Source bus with STATCOM under dynamic conditions
CONCLUSIONS:
Based on the simulation study the following are concluded:
Arc furnace loads produce high harmonic distortions in the voltage of an industrial power supply network. The Voltage THD would be of the order of 10.61%.
Need for active filtering is established through simulation studies using typical arc furnace current and voltage data.
The dynamic arcing phenomena are successfully interfaced with the RTDS model of the power supply system using GTNET Card.
STATCOM of two-level, 3-legged, Pulse-Width Modulation type with generic controls is found to be effective in reducing the Voltage THD at the arc furnace transformer primary.
A Two-Level three-legged, Voltage Source Converter based STATCOM model with Pulse-Width Modulation technique, was successfully interfaced with the industrial power supply network in RSCAD under RTDS environment.
The Total Harmonic Distortion at the Furnace Bus at which the STATCOM is placed is reduced to 4.4% from which the effectiveness of STACOM in mitigation of voltage harmonics is predicted.
