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Available online at www.ijiere.com
International Journal of Innovative and Emerging
Research in Engineering
e-ISSN: 2394 - 3343 p-ISSN: 2394 - 5494
SRF BASED CONTROL FOR POWER QUALITY
IMPROVEMENT USING D-STATCOM
Riya B. vasava
ME ELECTRICAL SCET, SURAT
Abstract:
Three phase three wire distribution system are facing severe power quality problems such as poor voltage regulation , high reactive power and harmonics current burden , load unbalancing , etc. For the improvement of power quality FACTS devices are used. FACTS devices are SSSC, UPFC, SVC , D-STATCOM etc. In this paper D-STATCOM is used to solve power quality problems. There are many different control strategies are used to control the D-STATCOM. Different control strategies are Instantaneous power theory (IRP) , Synchronous Reference frame theory (SRF) , Symmetrical component theory (SC), modified p-q theory etc. In this SRF control strategy is used to control the D-STATCOM. MATLAB simulation is presented with D-STATCOM using SRF control strategy.
Keywords: power quality, D-STATCOM, voltage source converter , control strategy (SRF), Hysteresis control
INTRODUCTION
Initially for the improvement of power quality or reliability of the system FACTS devices like static synchronous compensator (STATCOM), static synchronous series compensator (SSSC), inter line power flow controller (IPFC), and unified power flow controller (UPFC) etc are introduced. These FACTS devices are designed for the transmission system. But now a day as more attention is on the distribution system for the improvement of power quality, these devices are modified and known as custom power devices. The term“ custom power” describes the value-added power that electric utilities will offer to their customers. The value addition involves the application of high power electronic controllers to distribution systems, at the supply end of industrial, commercial consumers. The main custom power devices which are used in distribution system for power quality improvement are distribution static synchronous compensate or (DSTATCOM). A DSTATCOM is utilized to eliminate the harmonics from the source currents and also balance them in addition to providing reactive power compensation to improve power factor or regulate the load bus voltage. The compensating type custom power devices can be classified on the basis of different topologies and the number of phases. For power quality improvement the voltage source inverter (VSC) bridge structure is generally used for the development of custom power devices, while the use of current source inverter (CSI) is less reported. The topology can be shunt (DSTATCOM).A DSTATCOM is a custom power device which is utilized to eliminate the harmonics from the source currents and also balance them in addition to providing reactive power compensation to improve power factor or regulate the load bus voltage. A Distribution Static Compensator is in short known as D-STATCOM. It is a power electronic converter based device used to protect the distribution bus from voltage unbalances. It is connected in shunt to the distribution bus generally at the PCC. The schematic diagram of a D-STATCOM is as shown in Fig.1.1 [1]
Fig.1.1 Schematic Diagram of D-STATCOM [2]
Operating principle:A D-STATCOM is capable of compensating either bus voltage or line current. It can operate in two modes based on the parameter which it regulates .They are,[2]
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Current Mode Operation: In this mode of operation, the D-STATCOM forces the source current to be a balanced sinusoid irrespective of the load current harmonics.
The basic operating principle of a D-STATCOM in voltage sag mitigation is to regulate the bus voltage by generating or absorbing the reactive power. Therefore, the DSTATCOM operates either as an inductor or as a capacitor based on the magnitude of the bus voltage.[2]
Inductive Operation: If the bus voltage magnitude (VB) is more than the rated voltage then the D-STATCOM acts as an inductor absorbing the reactive power from the system. The circuit and phasor diagram are shown in Fig.1.2. [2]
Fig.1.2 inductive mode operation [2]
Capacitive Operation: If the bus voltage magnitude (VB) is less than the rated voltage then the D-STATCOM acts as a capacitor generating the reactive power to the system. The circuit and phasor diagram of this mode of operation are shown in Fig 1.3. [2]
Fig.1.3capacitive mode [2]
I. CONTROL STRATEGY
DSTATCOM has been used extensively for reactive power compensation, load balancing and harmonic mitigation in the distribution system. The objective of the compensating scheme is to supply the oscillating component of power such that the dc component can be supplied by the source. The performance of DSTATCOM depends on the control algorithm used for extraction of reference current components. For this purpose, many control algorithms have been reported in literature, and some of these are, Instantaneous Reactive Power theory (IRP) , interpretations and modifications on IRP, Synchronous Reference Frame theory (SRF), Symmetrical Component theory (SC), current compensation using dc bus voltage regulation ,computation based on per phase basis and scheme based on neural network techniques.[3,4,5]
The aim of control scheme is to generate the reference current waveforms which are to be injected to serve the required objective. The compensator would produce desired results as long as its bandwidth is sufficient to follow the fluctuations in the load. There are many control approaches available for the generation of reference source currents for the control of VSC of three-phase, four-wire DSTATCOM system. [3,4,5]
(A) SRF THEORY:
128 AC AC AC Linear/non linear load Zs Zs A B C i_sa i_sb i_sc Lf i_La i_Lb i_Lc
3 phase AC source
C_dc V_dc
control
VSa VSb VSc iLa iLb iLc Vdc isa isb isc
Fig.2.1 Basic circuit diagram of the DSTATCOM system.[3]
[ iLd iLq iL0
] =2 3
[
cos 𝜃 − sin 𝜃 1 2
cos (𝜃 −2𝜋
3) − sin (𝜃 − 2𝜋
3) 1 2
cos (𝜃 +2𝜋
3) sin (𝜃 + 2𝜋 3) 1 2 ] [ 𝑖𝑙𝑎 𝑖𝑙𝑏 𝑖𝑙𝑐
] (2.1)
[ 𝑖𝑠𝑎∗ 𝑖𝑠𝑏∗ 𝑖𝑠𝑐∗ ] = [
cos 𝜃 sin 𝜃 1 2
cos (𝜃 −2𝜋
3) sin (𝜃 − 2𝜋
3) 1 2
cos (𝜃 +2𝜋
3) sin (𝜃 + 2𝜋 3) 1 2 ] [ 𝑖𝑑∗ 𝑖𝑞∗ 𝑖0∗
] (2.2)
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Clarke’s transformation
Park’s
transformation filter
Reverse park’s transformation
Reverse clarke’s transformation Hysteresis
current controller Switching signals
to VSC PLL
Vs(abc)
iL(abc)
Is(abc) theta
Fig.2.2 Basic block diagram of SRF method.[4]
A three-phase PLL (phase locked loop) is used to synchronise these signals with the PCC voltage. These d-q current components are then passed through low pass filters to extract the dc components of iLd and iLq. A SRF controller extracts dc quantities by a low pass filter and hence the non dc quantities (harmonics) are separated from the reference signal. The d-axis and q-axis currents consist of fundamental and harmonic components as, [3]
ild= id dc+ id ac (2.3) ilq= iq dc+ iqac (2.4)
The compensation strategy for DSTATCOM considers that the source must deliver the dc component of the direct-axis component of the load current (𝑖𝑑𝑑𝑐) along with the active power current component for maintaining the dc bus and meeting the losses (𝑖𝑙𝑜𝑠𝑠) in DSTATCOM. Moreover, the source must deliver the dc component of the quadrature axis current (𝑖𝑞𝑑𝑐) and the component obtained from the PI controller (𝑖𝑞𝑟) used for regulating the voltage at PCC. The output of PI (proportional-integral) controller at the dc bus voltage of DSTATCOM is considered as the current (𝑖𝑙𝑜𝑠𝑠)for meeting its losses. [3]
iloss(n)= iloss(n−1)+ kpd(vde(n)− vde(n−1)) + kidvde(n) (2.5)
Where, vde(n)= vdc∗ − vdc(n) is the error between the reference (vdc*) and sensed (𝑉𝑑𝑐) dc voltage at the nth sampling instant. 𝑘𝑝𝑑 and 𝑘𝑖𝑑 are the proportional and the integral gains of the dc bus voltage PI controller. The reference direct axis source current, [3]
id∗ = id dc+ iloss (2.6)
The amplitude of ac terminal voltage (𝑉𝑠) at the PCC is controlled to its reference voltage (𝑉𝑑𝑐∗) using a PI controller. The output of PI controller is considered as the reactive component of current (𝑖𝑞𝑟) for zero voltage regulation of ac voltage at PCC. The amplitude of AC voltage (VS) at PCC is calculated from the ac voltages (vsa, vsb, vsc), [3]
vs= ( 2 3)
1 2 ⁄
(vsa2 + vsb2 + vsc2) (2.7)
Then, a PI controller is used to regulate this voltage to a reference value as,
iqr(n)= iqr(n−1)+ kpq(vte(n)− vte(n−1)) + kiqvte(n) (2.8)
130 Three-phase reference source currents are obtained by reverse Park’s transformation using eqn. In an indirect current controller, the sensed (isa, isb, isc) and reference source currents (isa*, isb*, isc*) are compared and a proportional controller is used for amplifying these current errors in each phase before comparing with a triangular carrier signal to generate the gating signals for six IGBT switches of VSC of DSTATCOM. The generated gating signals control the IGBT switches to inject a current such that the sensed source currents exactly follow the reference source currents. [3]
II. SIMULATION RESULTS
Fig.3.1 Matlab simulation model of SRF control
Fig.3.2 Voltage Vrms(p.u) at the load point (a) without STATCOM 22.4% voltage sag (b) with D-STATCOM 5% voltage sag
powergui Discrete, Ts = 1e-005 s
PI Subsystem4 I_d I_q theta I_0 Is_a Is_b Is_c Subsystem2 IL_b IL_a IL_c theta I_d I_q I_0 Subsystem1 IS_abc* IS_abc Out1 Out2 Out3 Out4 Out5 Out6 Scope 2 Scope 1 Scope PLL VS_a VS_b VS_c theta Out3 Fo=45 Hz
131 Fig.3.3 Voltage Vrms(p.u) at the load point
(a) without D-STATCOM 29% voltage swell (b) with D-STATCOM 5.6% Voltage Swell
Fig.3.4 Voltage Vrms(p.u) at the load point (a) without D-STATCOM 33% voltage sag
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Fig.3.5 Voltage Vrms(p.u) at the load point (a)without D-STATCOM 66.5% voltage sag (b)with D-STATCOM 20% voltage sag for Two phase to ground fault
133 Fig.3.7 Voltage Vrms(p.u) at the load point (a) without D-STATCOM 98.75% voltage sag
(b) with D-STATCOM 30% voltage sag for Three phase fault
Voltage improvement table
Before
compensation(p.u)
After
compensation(p.u) SRF
Voltage sag 0.776 0.95
Voltage Swell 1.29 1.056
Single phase to ground fault
0.67 0.91
Two phase to ground fault
0.335 0.8
Three phase to ground fault
0.013 0.72
Three phase fault 0.0125 0.7
Table.3.1 Voltage improvement table
Appendix
Parameters Values
Supply voltage 230 kv
Frequency 50 Hz
Coupling transformer 230/11/11 kv
Dc bus voltage of DSTATCOM 700 v Dc bus capacitance of DSTATCOM 750 µF
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III. CONCLUSION
From above simulation results conclude that D-STATCOM is promising device which is used for voltage sag, swell and phase to ground fault mitigation at distribution side.
SRF theories have demonstrated the satisfactory behaviour of DSTATCOM.
REFERENCES
