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Power Quality Improvements in Wind Based DG Systems using Solid State Transformer

Subhadeep Paladhi1 and Ashok S.2

Abstract— Along with the random increment of power demand throughout the world, the amount of renewable energy integration into the conventional grid is also increasing day by day. W ind power is being found as one of the most rapidly increasing renewable energy resources in the present scenario. As a result of the wind power being an uncontrollable resource, various problems regarding power quality, power system stability, reactive power consumption and protection issues arise. Though the Solid State Transformer (SST) has been found to be useful in integration of different distributed energy resources in the distribution grid with multiple functionalities, research gaps are still found in SST application incase of wind power integration. In this paper, the SST based mitigation of some power quality issues like voltage sag and swell at point of common coupling (PCC), reactive power consumption, power factor improvement etc. which exist in the wind based DG systems are presented through PSCAD/EMTDC simulation using suitable case study.

Index Terms— Solid State Transformer, Power Quality, Voltage Sag and Voltage swell, Reactive Power Compensation, Power factor

Improvement.

1 INTRODUCTION

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ITH the increment of power demand throughout the world, the penetration of renewable energy is increas- ing day by day, where the wind energy has risen as a
perfect solution thus far. For being an uncontrollable resource, it becomes a great challenge to integrate large wind farms into the distribution grid as per the power quality, power system stability and protection point of view. Additionally, the step up power transformers are used to boost the voltage level needed for interconnecting wind power to the distribution grid which creates transportation problem because of its bulky size. Placement optimization is another great problem for the reactive power compensators (like STATCOM, capacitor banks etc.) used for improving the power quality issues like voltage level improvement and power factor improvement [1], [2].
Solid State Transformer (SST) is being considered as one the most current research interests for integration of distributed energy resources (DERs). For the unavailability of high volt- age and power level power electronic devices, the research regarding SST was stopped at its early stage of research. With the development of high voltage and high power level power electronic devices, researches have been started again for the application of SST in high power level applications. The re- search approaches regarding SST are divided mainly into two directions. One is the topology and architecture development in order to reduce the power level of the SST component and the other direction is to find the suitability of SST in different applications [4]. SST is already found useful in the several ap- plications such as traction/locomotives, distributed source integration (like solar farm, wind power), charge station, and smart grid application in order to reduce the volume and

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• 1Subhadeep Paladhi is currently working towards his M.Tech. degree in the Department of Electrical Engineering in National Institute of Technology, Cal- icut, Kerala, India-673601. Email- paladhisubha91@gmail.com

• 2Ashok S. is currently working as a Professor in the Electrical Engineering

Department, National Institute of Technology Calicut, Kerala, India-673601

weight of the system and improve the power quality and pro- tection issues where conventional low frequency transformers are already dominating [1], [6].
The reactive power compensation, active power control and voltage conversion properties are already verified in wind power integrated grid using SST [1]. Some fault isolation algo- rithms are proposed and verified for distribution grid using SST with PV and fuel cell as the distributed energy resources (DER) [9], [10], [11], [12]. Though the suitability of SST appli- cation in wind based DG systems are already verified, some important approaches like voltage level and power factor im- provement at the Point of Common Coupling (PCC) in case of dynamic load condition without using any Static Compensas- tors (like STATCOM) have not been verified still now.
In this paper the voltage sag and swell mitigation, power factor improvement at PCC without using any static compen- sators in a SST interfaced wind based DG system are verified along with the active and reactive power control capabilities through PSCAD/EMTDC simulation using suitable case study. Section 2 describes the overview of the SST architec- tures and topologies, conventional and SST interfaced wind power integration techniques and power quality issues pre- sent in a wind based DG system. The modeling of SST with controllers designing approach is presented in Section 3. In Section 4, a system case study based on the data of Kanjikode wind farm is described. The simulation results are shown in Section 5 to show the advantages of SST based technique over conventional.

2 OVERVIEW OF SOLID STATE TRANSFORMER (SST), WIND POWER INTEGRATION TECHNIQUES AND POWER QUALITY ISSUES

2.1 Solid State Transformer (SST)

The SST is typically composed of a high-voltage ac/dc con- verter regulating a high-voltage dc bus (ac voltage during re- active power compensation), an isolated high-frequency oper- ated dual active bridge dc/dc converter regulating the sec-

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ondary dc bus, and a dc/ac converter regulating the output of the terminal ac voltage. The functional diagram of SST has been shown in Figure 1.

Fig.1. Structural Overview of SST
SST was first mentioned in the research in 1980 by James Brooks [8]. But due to the limitation of higher voltage and power level power semiconductor devices, SST was not found as a practical solution at that time. With the advancement of semiconductor technology many organizations are trying to make the high power SST. The researchers at ETH Zurich are working with matrix converter based SST topology with the code name as MAGACube [7]. The FREEDM project is inves- tigating a SST based on a single phase system with different modularity [1], [9], [11], [12]. Four important topologies by ABB, GE, UNIFLEX and EPRI are described in [6].
15 kV SiC MOSFET has been found to be useful than IGBT devices for making high power SST [6]. Different amorphous alloys, nanocrystalline core, METAGLASS core has been found to be useful for high frequency transformer core with maximum flux density [6]. By increasing the number of H bridges in converter stage (in proper combination of series and parallel connection) the problem regarding high voltage and power level can be solved [6], [7].
Conventional low frequency power transformers have been greatly challenged by solid-state transformer technologies. Simply, the main function of conventional transformer is in- put-output voltage and current transformation. So the dis- turbances on one side are fully reflected on the opposite side. SST has been found to be a promising device to overcome this issue [1], [4], [6]. Potential advantages of SST over convention- al low frequency transformers are low volume and weight (due to its high-frequency operation compared with 50-Hz transformer), fault isolation, voltage regulation, insusceptible to harmonics, easy integration of renewable energy resources and energy storage, etc. [1], [4], [5], [6], [7], [8], [9], [10], [11], [12].

2.2 Conventional Wind Power Integrated Grid

Among the various techniques used for the conversion of wind energy into electric energy, induction generator based technology is the most popular and widely used technique. Three mainly used Wind Farm architectures are SCIG based wind energy system, doubly fed induction generator (DFIG)- based wind energy system, and directly driven synchronous generator (DDSG)-based wind energy system, shown in Fig- ure 2. SCIG is considered as the most economical solution be- cause of the direct coupling with the grid as shown in Figure
2(a). Thyristor controlled capacitor bank is generally used for power factor correction compensating local reactive power to

cope up with the wind power generation variation. The wind generation using DFIG and DDSG employs back-to-back con- verters using the power partially and fully to decouple the mechanical and electrical rotor frequencies as described in Figure 2(b) and Figure 2(c).

(a) Conventional Grid Connected SCIG based W F

(b) Conventional Grid Connected DFIG based W F

2.3 SST interfaced Wind Farms

Wind energy systems connected to distribution grid through SST are shown in Figure 3. It eliminates capacitor bank, two trans-formers and one STATCOM for SCIG based wind farm (WF), two transformers and one STATCOM for DFIG based WF and back to back converter, two transformers and one STATCOM for DDSG based WF [1]. This describes the struc- tural advantages of SSTs in interfacing wind power with the grid.

2.4 Power Quality and Stability Issues Present in Wind

Based DG Systems

As for being uncontrollable resource, wind power integrated distributed grid faces different problems like voltage stability problems, poor power factor issues etc. One main problem with induction generator based wind farm is the reactive power consumption issue. So thyristor controlled capacitor banks are needed to compensate local reactive power con- sumption. With dynamic load condition at PCC creates volt- age fluctuations (like sag and swell), which creates rotor angle stability problem in wind generators. Sometimes because of low voltage condition at PCC, wind farms are needed to dis- connect from the grid. This all problems come under power quality and stability issues.

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(a) SST interfaced Grid Connected SCIG based W F

Where,
(1) (2)

(b) SST interfaced Grid Connected DFIG based W F

iga ; igb ; igc are the grid side current, vpcca ; vpccb ; vpccc are the grid side voltage, Rg is the grid side line resistance, Lg is the grid side inductor, EH the high voltage DC bus voltage, CH is the high voltage DC capacitor, dha ; dhb ; dhc are the rectifier PWM duty cycle for the three phases respectively.
The equation 1 and 2 are transformed into d-q coordinate
using the following equation

Fig.3. Overview of SST based Grid connected W Fs

3 MODELING OF SST AND CONTROLLER DESIGNING

LOGIC

Among the different SST topologies, the simple cascaded three phase switching model of SST is used in this paper for wind

[Adq] = [τ].[Aabc]

Where,
(3)

fN is the rated frequency.

power interfacing. AC voltages are controlled for reactive power compensation and the dc voltages are controlled for active power control [1]. Fig. 4 shows a cascaded-type three- phase SST. Physical limitations for power devices and magnet- ic materials are neglected during modeling and simulation. First stage of SST is a three-phase bidirectional ac/dc pulse width modulation (PWM) converter. The dc/dc stage of SST is consisted of a dual active bridge (DAB) or dual half bridge (DHB) converter representing the most attractive candidate for high-power applications requiring isolation, as it can perform zero-voltage switching in a wide operation range [1].

In the aforementioned SST configuration, VPCC is the PCC voltage, ig is the PCC current flowing through the SST, Vhdc is the high dc bus voltage, Vldc is the low dc bus voltage, Vwind is the wind farm side SST terminal voltage, and iwind is the cur- rent through SST terminal at wind farm side.

Fig.4. Switching model of Three Phase SST

3.1 Modeling of Rectifier Stage

The basic differential equations of the rectifier stage are:
The derived differential equations in d-q coordinates are as follows:
(4) (5) Using these equations the controllers are modeled. The de-
tailed controller logic for rectifier stage using d-q vector con-
trol is shown in Fig. 5(a).
As the SST is used for bidirectional power flow, this stage is
used as rectifier for transferring power from grid to secondary
side. But in this paper the power is transferring from wind
farm to distribution grid. So the described approach has been
used to design the wind farm side converter stage.

3.2 Modeling of Dual Active Bridge (DAB) Converter

Stage

The dual active bridge consists of a high voltage inverter, a high frequency transformer and a low voltage rectifier. The DAB converter controls the low voltage dc link voltage. Zero voltage switching for all the switches, relatively low voltage stress for the switches, low passive component ratings and complete symmetry of configuration are the advantages of DAB topology that allows seamless control for bidirectional power flow. Real power flows through the DAB converter is

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given by the equation 6.
(6)
explained in rectifier stage.

4 CASE STUDY

A system case study is carried out with the layout and practi- cal data from Kanjikode 2MW SCIG based wind farm, situated in Kerala, India. The wind farm has nine units of Vestas V27
225kW wind turbine and SCIG is used as wind generator with a rated output of 225kW, 400V, 50Hz. The nine units are con- nected to the three 1MVA, 415V/22kV, 50 Hz step up trans- former divided into three units for each. This is connected to
110kV and the 220kV grid using 25MVA, 22kV/110kV, 50Hz distribution transformer and 160MVA, 110kV/220kV, 50Hz autotransformer respectively. Thyristor controlled 100kVAR capacitor bank is connected with each unit for power factor improvement.
For the simplicity of simulation for the SST interfaced wind farm the all nine units are connected to one 3MVA SST i.e. same as three 1MVA step up transformer unit. The simulation parameters of 3MVA SST are pre-sented in Table I.

TABLE 1

SIMULATION PARAMETERS FOR SST USED IN CASE STUDY

Fig. 5: SST Controller logic for (a) High Voltage Rectifier Stage, (b) Dual Active Bridge Stage, (c) Low voltage inverter Stage

where, EH is input side high voltage DC voltage, fs is switching frequency, L is leakage inductance, EL is output side low voltage DC link voltage referred to input side and is ratio of time delay between the two bridges to one half of switching period.
A phase regulation scheme is used in the DAB controller adjusting the phase shift between high and low-side convert- ers using simple PI controller to control the power flow as shown in Fig. 5(b).
In this paper thsimilar control technique has been used for DAB to control the HVDC bus voltage as the power flows from wind farm to grid.

3.3 Modeling of Low Voltage Inverter Stage

A dual loop strategy in dq coordinate is used for the inverter stage. Cascaded inductor current loop is used as the inner loop for fast dynamic responding. This controller is modified ac- cording to the variation of different wind generators. The con- troller logic for low voltage inverter stage is shown in Fig. 5(c).

In this paper this control logic has been used for first stage i.e. grid side converter stage to flow the power from wind farm to grid and this stage is designed with the control logic
Fig. 6(a) shows the Kanjikode wind farm layout i.e. the SCIG based 2.025 MW wind farm is first connected to 22kV PCC and then 110kV bus and 220kV through step up trans- formers of above mentioned rating and capacitor bank is con- nected to each wind farm unit of the rating mentioned before. Fig. 6(b) shows the same wind farm connected to 22kV PCC through 3MVA (same as step up transformer rating) Solid State Transformer (SST) and then to 110kV bus and 220kV grid in the same way as conventional. But here no capacitor bank is used.
To verify the voltage sag and swell condition under dynam- ic load condition, three different loads are been applied at dif- ferent timing for a definite short period. L1 (1MW+0.2MVAR/ph) for 0 to 3.5s. L2 (3MW+1MVAR/ph) for
3.5s to 4.2s and L3 (0.01MW-1MVAR/ph) for 4,2s to 5s.
IJSER © 2 http://www.ijs

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(a) Conventional Grid Connected Layout of Kanjikode W ind Farm

(b) SST based Grid Connected Layout of Kanjikode W ind Farm

Fig. 6. Grid Connected Layout of Kanjikode W ind Farm

5 RESULTS AND DISCUSSION

The voltage at point of common coupling (PCC) under dy- namic load condition is shown in Fig. 7(a). The active power transfer capability from wind farm to grid and reactive power consumption by the SCIG based wind farm under steady state
SST eliminates the bulky step up transformer and capacitor bank and also able to prevent the power quality issues like voltage sag and swell, power factor etc. controlling the active and reactive power without using any static compensators.

TABLE 2

QUATITATIVE ANALYSIS OF CASE STUDY

condition (i.e. load L1 connected at PCC) are shown in Fig.
7(b) and 7(c) respectively. Fig. 7(d) and 7(e) show the voltage
and current waveform at PCC to show the power factor im-

Parameters Without

SST

PCC Voltage

With SST

provement using SST. The overall comparison between con- ventional and SST based interfacing technique are represented in Table II.

• With L1

• With L2

• With L3

21.50 V

20.93 V

22.23 V

21.98 V

21.97 V

21.99 V

From the results it is observed that the PCC voltage has been maintained approximately at rated value which is bene- ficial for the utilities connected to the PCC.

Active Power Transfer 1.96MW 1.9613MW

Reactive Power Consumption 0.2301pu 7.56X10-30pu

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(a) PCC Voltage at Dynamic Load Condition

been presented in this paper. The constructional advantages and active and reactive power control capability are also veri- fied through case study using PSCAD/EMTDC simulation tool.
Among the three types of wind generators, the SCIG-driven wind farm consumes more reactive power compare to the oth- ers. For that reason more reactive power compensation is re- quired. So the conclusion drawn from this case study can also be applied to the WFs driven by DFIG and DDSG since the control logics are same for all these systems.

ACKNOWLEDGMENT

The authors wish to thank Prof. A. K. Pradhan, EED, IIT KGP, India for his valuable guidance regarding Solid State Trans- former and Kerala State Electricity Board (KSEB) for offering the data needed for case study.

(b) Active Power Transfer by SST from W ind Farm with L1

at PCC

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