DG Integrated Common DC Link Fed Parallel DSTATCOM for Power Quality Improvement and Active Power Injection

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DG Integrated Common DC Link Fed Parallel DSTATCOM for Power Quality Improvement and Active Power Injection

P. V. V Satyanarayana*, P. V. Ramana Rao**

* Research Scholar, ** Professor and Head, Departement of Electrical and Electronics Engineering,

Acharya Nagarjuna University, Guntur, AP, India.

Abstract: Power quality is a thrust area taken up by electrical engineers as power quality issues cause immense disturbances in the power system. Power quality refers to the capability of load component to absorb the supplied power. DSTATCOM is a renowned power electronic active filter for harmonic compensation. Distributed Generation (DG) integration is the key aspect in the present day power system scenario. This paper presents the parallel DSTATCOM configuration (with common DC-Link) in a power system connected at point of common coupling. One of the VSIs in parallel DSTATCOM configuration acts as a power quality enhancer reducing the harmonics and the other as active (Distributed Generation) power injector. This topology avoids the use of extra (separate) VSI for the integration of Distributed Generation. The proposed model is developed and the result analysis is put forward with MATLAB/SIMULINK software.

Keywords: Power quality, harmonics, parallel DSTATCOM, DG integration.

1. INTRODUCTION

Power quality, as referred to source, is consistency and the power quality as referred to load is power delivered for the reasonable operation of load equipment. The quality of power [1-3] refers to the capability of load component to absorb the supplied power. Power quality issues have considerable impact on the efficiency of electrical equipment. With low power quality, the usage of electrical energy increases and also results in equipment failure and instability. The power supply system can only control the quality of the voltage and it has no control over the nature of load current. Harmonics are one issue that diminishes the quality of power [4-5].

DSTATCOM in distribution system is viable option [6-7] optimizing power utilization and reduces power costs. Controlled DSTATCOM estimates the harmonics and ensures the distortion in current shape is well within the prescribed limit. Figure 1 illustrates the DSTATCOM connected power system.

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C^DC

Non-Linear Load

Controller I^L V^S

ZS

ZC

i^C

i^S iL

V^S pcc

Source

DSTATCOM

Figure 1: Block diagram of DSTATCOM in distribution system

Inverter Grid

RES Output

Figure 2: Block flow diagram of DG integration to grid

Integration of renewable energy source in power system is becoming trendy as global countries are concerned much on global warming caused by emissions from fossil fuels. Power generated from renewable sources at distribution level is called distributed generation (DG) [8-9]. The units of distributed generation generate power on the customer side (close to the load) and can feed distribution system to meet the load demands. Figure 2 illustrates the block flow diagram of DG integration to grid. Distributed generation evolution has taken place mainly due to recent innovations in the technology and environment regulatory conditions. Limitations on the erection of new transmission lines and increased load demand also contribute to the development of distributed generation.

This paper presents the parallel DSTATCOM configuration (with common DC-Link) in power system connected at the point of common coupling which serves the purpose of power quality improvement and DG integration to grid. One of the VSIs in parallel DSTATCOM configuration acts as a power quality enhancer reducing the harmonics and the other as active (Distributed Generation) power injector. This topology avoids the use of extra (separate) VSI for the integration of Distributed Generation. IRP [10-12] control algorithm is used to control DSTATCOM.

2. PARALLEL DSTATCOM TOPOLOGY WITHOUT DISTRIBUTED GENERATION INTEGRATION

2.1 Parallel DSTATCOM Configuration

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Source Impedance

CDC

Source

Parallel DSTATCOM with Common DC-Link

DSTATCOM - 2 DSTATCOM - 1

Harmonic Load

ZDSTAT1 ZDSTAT2

Figure 3: Parallel DSTATCOM configuration

Power quality is a major issue in the present day power system owing to extensive use of modern electric equipment. It is a well known fact that 70% of the power quality issues regard to modern power devices at the load centers that produce harmonics. DSTATCOM is an active filter based on power electronic circuit which operates to induce compensation currents to set right the deformed source current wave shape. DSTATCOM is a shunt device connected in parallel to the distribution system.

Figure 3 illustraes the Parallel DSTATCOM configuration without integration of DG. The two VSIs of parallel DSTATCOM configuration are connected with common DC-Link. Parallel DSTATCOM is a topology introduced to share the burden of generating compensating signals besides reducing the burden on power switches and the switching losses.

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2.2 Control of Parallel DSTATCOM Configuration

Vdc(act) Vdc(ref)

PI Controller Kp

Calculation

Vα Vβ V0

Calculation

Iα Iβ I0

Calculation

P0

P Q

HPF

Filter

Calculation Calculation

p0

Phase Voltages

Load

Currents Source

Reference Currents

Actual Currents from DSTATCOM2 Gate Pulses to

DSTATCOM1

Ki/s

DC-Link Voltage Control

P Gain Gain

Actual Currents from DSTATCOM1

Gate Pulses to DSTATCOM2

HCC

Figure 4: Control algorithm of Parallel DSTATCOM configuration

Current harmonics are compensated using instantaneous reactive power (IRP) theory. Clarke’s transformation of three-phase terminal voltage and load current signals transform voltage/current signals to stationary quadrature signals as presented in equations (1) and (2).

[ 𝑉₀ 𝑉_𝛼

𝑉_𝛽] = √2 3⁄ [

1

√2 1

√2

1 ⁻¹

2

−1 2

0 ^√3

2 −^√3

2] [

𝑉_𝑎𝑛 𝑉𝑏𝑛

𝑉_𝑐𝑛

] (1)

[ 𝐼₀ 𝐼_𝛼

𝐼_𝛽] = √2 3⁄ [

1

√2 1

√2

1 ⁻¹

2

−1 2

0 ^√3₂ −^√3₂] [

𝐼_𝑎 𝐼_𝑏 𝐼_𝑐

] (2)

The three-phase instantaneous active power components can be illustrated as in equation (3)

𝑃_3Ø= 𝑉₀𝐼₀+ 𝑉_𝛼𝐼_𝛼+ 𝑉_𝛽𝐼_𝛽 (3)

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Source Impedance

CDC

Source

Parallel DSTATCOM with

Common DC- Link DSTATCOM - 2

DSTATCOM - 1

Harmonic Load

ZDSTAT1 ZDSTAT2

Figure 5: Control and arrangement of common DC-Link fed parallel DSTATCOM

Only one sensor is required in controlling the DC-Link voltage of common DC-Link fed parallel DSTATCOM. The power loss component is calculated by sensing common DC-Link voltage and comparing the same with reference value generating an error signal. PI controller generates power loss component of power taking the error signal as input. The active power component, reactive power along with power loss is processed to obtain source reference signals in stationary components. Clarke’s inverse transformation of reference signals gives out reference signals in rotary frame a-b-c components and illustrated in equation (4).

[ 𝐼_𝑐𝑎^∗ 𝐼_𝑐𝑏^∗ 𝐼_𝑐𝑐^∗

] =^√2

3

[

1

√2 1 0

1

√2

−1 2

√3 2 1

√2

−1 2

−√3 2 ]

[ 𝐼_𝑐0^∗ 𝐼_𝑐𝛼^∗ 𝐼_𝑐𝛽^∗

] (4)

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Reference signals in a-b-c terms are calculated and processed to the hysteresis controller.

Reference current signals with added gain are processed to hysteresis controller along with actual DSTATCOM1 currents to generate control pulses to power switches of DSTATCOM1. Similarly, Reference current signals with added gain are processed to hysteresis controller along with actual DSTATCOM2 currents to generate control pulses to power switches of DSTATCOM2. Control and arrangement of common DC-Link fed parallel DSTATCOM is illustrated in figure 5.

3. PARALLEL DSTATCOM TOPOLOGY WITH DG 3.1 Parallel DSTATCOM Configuration with DG

Source Impedance

CDC

Source

Harmonic Load

ZDSTAT1 ZDSTAT2

DG

Figure 6: Parallel DSTATCOM configuration with DG integration

Distributed energy sources (power from renewable sources) are sporadic and integrating scheme of Distributed generation to power grid is a tough task. Maintaining power quality (limiting harmonics and maintaining power factor) while integrating DG to grid is an exigent task for power engineers. Figure 6 illustrates the Parallel DSTATCOM configuration with integration of DG. The two VSIs of parallel DSTATCOM configuration are connected with common DC-Link. DG is connected across the DC-link capacitor.

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3.2 Control of Parallel DSTATCOM Configuration with DG

Vdc(act) Vdc(ref)

PI Controller Kp

Calculation Vα Vβ V0

Calculation Iα Iβ I0

Calculation

P0 P Q

HPF

Filter

Calculation Calculation

p0

Phase Voltages

Load

Currents Source

Reference Currents

Actual Currents from DSTATCOM2 Gate Pulses to

DSTATCOM1

Ki/s

DC-Link Voltage Control

P Gain Gain

Actual Currents from DSTATCOM1

Gate Pulses to DSTATCOM2

PD(ref) HCC

Figure 7: Control algorithm of Parallel DSTATCOM configuration with DG integration

Source Impedance

C^DC

Source

Harmonic Load

Z^DSTAT1 Z^DSTAT2

DG

Figure 8: Control and arrangement of parallel DSTATCOM with DG integration

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The power components produced as explained in the IRP theory explained in the section 2.2 of this paper is used to control DSTATCOM configuration with DG integration. The computed power loss component and active power component are added to the DG power and the net power components are fed to generate reference source current signals in α-β components as illustrated in figure 7. Clarke’s inverse transformation of reference signals gives out reference signals in rotary frame a-b-c components

Reference current signals in a-b-c terms are calculated and processed to the hysteresis controller.

Reference current signals with added gain are processed to hysteresis controller along with actual DSTATCOM1 currents to generate control pulses to power switches of DSTATCOM1. Similarly, Reference current signals with added gain are processed to hysteresis controller along with actual DSTATCOM2 currents to generate control pulses to power switches of DSTATCOM2. Control and arrangement of common DC-Link fed parallel DSTATCOM with DG integration is illustrated in figure 8.

4. RESULT ANALYSIS Table-1: System Measurements

Attribute Estimate

Voltage 415 V, 50 Hz

Source Impedance 0.1 Ohms, 0.9 mH

Interfacing filter Impedance 0.0001 Ohms, 5 mH DC-Link Capacitance 1500 micro Farads Table-1 illustrates the system parameters used to develop simulation model.

4.1 Parallel DSTATCOM with fixed DG power

Figure 9: Three-phase source voltage

Figure 9 depicts the three-phase balanced source voltage waveform of 340V peak with no harmonic distortion.

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Figure 10: Source Current

Figure 11: Load current

Figure 10 illustrates the source current. Source current is with 25A magnitude till 0.15 seconds. After o.15 seconds, the source current reduces to 15A but the load current shown in figure 11 is with constant magnitude. This phenomenon indicates that the deficit demand power to the load is supplied from DG system thus reducing the burden on the source. Source current is undistorted but load current contains harmonics. One Parallel DSTATCOM VSI compensates the harmonics to keep source current undistorted and the other injects DG power to distribution system.

Figure 12: Power factor angle

Power factor is shown in Figure 12. Voltage and current signals are not displaced and power factor tends to unity.

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Figure 13: Source side Active power

Source side active power is shown in figure 13. The active power fed from the source reduces at time 0.15 seconds and the deficit power is fed from DG to grid to meet the load demand reducing the burden on main source. Main (conventional) source generation is reduced leading to considerable reduction in pollution.

Figure 14: Source current THD

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Figure 15: Load current THD

Figure 14 shows source current THD and figure 15 shows load current THD. Source current has very less and limited harmonics of 0.69% and load current has 29.6% harmonics.

4.2 Parallel DSTATCOM with variable DG power

Figure 16: Three-phase source voltage

Figure 16 depicts the three-phase balanced source voltage waveform of 340V peak with no harmonic distortion.

Figure 17: Source Current

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Figure 18: Load current

Figure 17 illustrates the source current. Source current is with 25A magnitude till 0.15 seconds. After o.15 seconds, the source current reduces to 15A and further at 0.25 seconds source current still reduces to 10A but the load current shown in figure 18 is with constant magnitude over the same time period. This phenomenon indicates that the deficit demand power to the load is supplied from DG system thus reducing the burden on source. Source current is undistorted but load current contains harmonics. One Parallel DSTATCOM VSI compensates the harmonics to keep source current undistorted and the other induce DG power to distribution system.

Figure 19: Power factor angle

Power factor is shown in Figure 19. Voltage and current signals are not displaced and power factor tends to unity.

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Figure 20: Source side Active power

Source side active power is shown in figure 20. The active power fed from the source reduces at time 0.15 seconds and 0.25 seconds and the deficit power to load is fed from DG to grid to meet the load demand reducing the burden on main source. Main (conventional) source generation is reduced and pollution reduces.

Figure 21: Source current THD

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Figure 22: Load current THD

Figure 21 shows source current THD and figure 22 shows load current THD. Source current has very less and limited harmonics of 1.31% and load current has 29.61% harmonics.

Table-2 illustrates the THD analysis with single and dual APF distribution system configuration.

Table-2: THD analysis

THD Source Current Load Current

Fixed DG Power 0.69 % 29.60 %

Variable DG Power 1.31 % 29.61 %

5. CONCLUSION

Power quality and DG integration to grid are the interesting topics for power engineers. Quality power can lead to development of the nation and DG system reduces the transmission losses and pollution aspects. The paper presents the concept of parallel DSTATCOM configuration (with common DC-Link) and one of the VSIs in parallel DSTATCOM configuration acts as a power quality enhancer reducing the harmonics and the other as active (Distributed Generation) power injector. This topology avoids the use of extra (separate) VSI for the integration of Distributed Generation. DG power is fed to grid and load current is maintained constant (with reduced source current) in two cases like fixed DG power and variable DG power. In the illustrated cases, THD in source current is below stipulated limits and DG power fed to grid reduces the stress on source system.

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