The 2014 International Power Electronics Conference

Modelling, Design and Control of Grid Connected Converter for High Altitude Wind Power

Application

Jeevan Adhikari, Student Member, IEEE, Akshay K. Rathore, Senior Member, IEEE S K Panda, Senior Member, IEEE Department of Electrical and Computer Engineering, National University of Singapore, Singapore

Abstract-High altitude wind based renewable energy generating system can be connected to a distribution level grid. The generated power at high altitude above the ground is transmitted at medium voltage DC to the ground based station. Thus, transmitted power is interfaced with the distribution grid at the ground station. This paper presents the power electronic converter (PEC) rated at 100 kW HAWP application that converts medium voltage DC to three phase distribution level grid voltage. The proposed converter topology consists of a neutral point clamped (NPC) three level DC-DC converter followed by three phase grid connected two level inverter. The designed power electronic converter uses four high voltage (HV) rating power semiconductor switches for buck converter before inversion to three phase AC distribution voltages. The active and passive components selection for two stage conversion is presented in the paper. The grid side current is controlled using quadrature axes current control method and inverter switches are switched using space vector modulation (SVM) technique. Simulations of the proposed PEC and control of the inverter are carried out using software programs PSIM-9 and MATLAB. The designed converter converts the 8 kV DC transmission voltage to 415 V grid side voltage with current total harmonic distortion (THD) of about 1.2%.

Key Words: High altitude wind power (HAWP), three level neutral point clamped (NPC) DC-DC converter, gridconnected inverter, total harmonic distortion (THD)

I. INTRODUCTION

Wind and solar energy are two major renewable energy sources those have potential to reduce the number of fossilfuel based power generating system. Solar power has low energy density and conventional wind power (CWP) harvesting system has low capacity factor and needs huge infrastructure constructions [1], [2]. High altitude wind power (HAWP) harvesting system generates wind power at low cost and high capacity factor [3], [4]. HAWP harvesting system using light gas filled blimp/aerostat is enlightened in [3], [5]. Fig 1 shows a prototype of blimp supported HAWP generation system developed by Altaeros Energy [6]. A light weight airborne wind turbine drives a permanent magnet synchronous generator (PMSG) at high altitude above the ground. Thus, the generated power is transformed into medium voltage DC for efficient transmission and to minimize the weight of the power transmission cable. An electro-mechanical tether is used to transmit power to the ground based station where it is interfaced with a distribution grid. Power electronic converter (PEC) topology for harvesting HAWP using blimp/aerostat is

illustrated in Fig. 2. The PEC topology consists of a rectifier and a DC-DC converter in an air-borne unit and a grid connected converter at the ground based station. Detail study of DC-DC converters for blimp supported HAWP application is explained in [7], [8].

Fig. 1: A prototype of HAWP generating system develope d by Altaeros Energy [6]

Air-borne Turbine Air-borne PECs VDC transmissi n J.r;........;

AC/DC DC/DC Tether

Ground based Station '-------------- ------------- -..."...-

Air-borne Unit

Fig. 2: Electrical architecture for harvesting high altitu de win d power generating system

Various inverter topologies and their design, modelling and limitations are explained in [9]- [10] for industrial use. The multi-level inverters explained in [9]- [10] are used to drive high power motor drive system, compressor pumps and other electrical loads. Grid connected inverters used for wind farms are described in [11]- [14]. In [11]- [14], and the PECs designed for different power level for on-shore/off-shore wind farms. The close loop current control using different modulation techniques for the designed converter are explained as

978-1-4799-2705-0/14/$31.00 2014 IEEE 1775

well. However, grid connected converter for HA WP has not been explored yet. In contrast to conventional inverters, the converters for HA WP should be capable of converting medium voltage DC to three phase distribution level grid voltages. For 100 kW HAWP application, optimal transmission voltage is 8 kV [4] which gives maximum power-to-weight (PIW) ratio for an air-borne unit. Transmission voltage at the ground station acts as DC link voltage for the converter which is interfaced to distribution grid voltage at 415 V (RMS line-line). A HAWP harvesting system considered in the paper is a distributed renewable generation system connected to the distribution grid.

Fig. 3: The converter topology for interfacing HAWP to distribution gri d

The paper introduces the converter that transforms 8 kV DC link voltage to 415 V grid voltage connected to three phase power distribution system. The proposed converter consists of three level zero voltage switching (ZVS) isolated DC-DC converter and two level SVM inverter. The isolation provided by DC-DC converter protects the grid side power system from unwanted power signals from lightening. The converter consists of four high voltage switches due to the use of three level isolated buck converter instead of using isolated full bridge DC-DC buck converter. Two level inverter is used to transform the output of 3-level buck converter into 3-phase AC with controlled grid side current. The inverter is switched using SVM technique where total harmonic distortion (THD) of the grid side current lies within industrial standard. The paper explains the design of the converter, selection of devices depending on the rating of the switches and control of grid connected inverter to get current distortion with in the permissible limits. Fig. 3 shows the proposed converter for HA WP application for grid interface.

The description and design of the converter are given in Sections II and III. Sections IV and V sUlmnarize the modeling and the control of the converter. Switching strategy for the inverters with space vector modulation is explained in Section VI. Simulation results to verify the design of the converter is shown in Section VII.

II. DESCRIPTION OF THE CONV ERTER

The specifications of the ground based converter are:

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Power Rating: 100 kW Input Voltage: 8000 V (MV DC transmission voltage) Intermediate DC link voltage to inverter: 700 V Output Voltage: 415 V (L-L RMS) The proposed converter consists of multilevel isolated buck

converter followed by two level inverter. Transforming 8000 V to 700 V using an isolated full bridge DC-DC converter requires series connection of the switches (with high voltage rating) in the primary side in order to withstand high input voltage. The series connections of switches (high voltage rating) reduce the reliability of the converter and increase switching and conduction losses. In addition, higher rating switches limit the switching frequency of the converter. So, three level ZVS based isolated buck converter is better choice which reduces the switch voltage stress to half of the input voltage and increases the switching frequency due to soft switching characteristics. A three level isolated buck converter as explained in [17] is used for step down operation as shown in the Fig. 4.

Fig. 4: NPC three level DC-DC converter for HAWP

The input voltage to the inverter is reduced to 700 V using multilevel buck converter. This allows the use of two level SVM inverter which gives reduced THD, increased switching frequency and reduced filtering requirements. Different multilevel inverters are explained in [9]- [10] which suffer hardware design difficulty, voltage balancing issues etc. Two level inverter has reduced numbers of switches, capacitors and diodes, so use of multilevel inverter is skipped by reducing DC link voltage to a lower value. Schematic circuit diagram for two level inverter for HAWP application is shown in Fig. 5.

III. DESIGN OF THE CONV ERTER

This section gives ratings of switches, diodes and passive components for three level DC-DC converter and two level inverter. Table I gives the components' ratings for DC-DC converters for buck operation. The devices selected for step down operation are shown in Table II. The use of three level topology reduces the switch stress and capacitor voltage stress to half. The complete buck operations using multilevel

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Fig. 5: 1\vo level inverter for interfacing HAWP to distribution gri d

topology is explained in [17]. The conduction of antiparallel diode just before turning on the switch facilitates ZVS of all HV switches.

The DC-DC converter uses four Infenion IGBT (FZ200R65KF2) and two split input capacitors. The three level neutral point clamped topology eliminates series connection of HV switches. Similar step down operation can be done using full bridge isolated DC-DC converter which requires eight (4*2) FZ200R65KF2 switches with HF transformer.

TABLE I: Device rating for multilevel isolate d DC-DC buck opera-tion

Parameters Rating Parameters Rating

Switch 4000 Rectifier diode 700 voltage (V) voltage (V) Peak switch 85 Rectifier diode 280 current (A) peak current (A) Av. switch 35 Rectifier diode 160 current (A) avo current (A) Clamp diode 4000 Filter capacitor 700 voltage (V) voltage (V) Av. clamp diode 25 Filter capacitance (mF) 13.3 current (A) Inductor peak 85 Peak clamp diode current (V) current (A) HF transformer 105 Maximum 8000 KVA voltage across Input capacitor 4000 inductor (V) voltage (V) Leakage 332 Input capacitance (uF) 625 inductance (uH) Filter 3 Turns ratio 3.42: I inductance (mH)

Two level inverter interfaces 700 V dc link voltage to 415 V AC grid. The inverter consists of 3 legs with 2 switches in each leg. Ratings of the switches are given in Table III along with the selected modules' details. Infenion fast IGBT modules are selected for the inverter application.

IV. MODELING OF GRID SIDE INV ERTER

The schematic diagram of grid side converter is shown in Fig 3. Modelling of the grid side converter is carried out by modelling the DC link side and grid connected inverter.

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TABLE II: Name of switch an d dio de selecte d multilevel isolate d DC-DC buck operation

IGBT Module Voltage Clamp Diode Voltage Rectifier diode Voltage

Primary Side

FZ200R65KF2 6600 V

DD250S65K3 6500 V

Secondary Side

BSM300GA 120DN2 1200 V

TABLE III: 1\vo level inverter devices ratings

Inverter switch

Voltage (V) 700

RMS 135 current (A)

Peak current (A)

184

Selected module FS200R12KT3

DC link capacitor stabilizes the DC link voltage. Current flowing through the DC link capacitor is given by:

dVdc . . CTt=ts - tg (1)

where Vdc is DC link voltage, is is current of DC link after buck converter, ig is current of DC link at the grid side and C is the DC link capacitance.

Considering that the grid side inverter does not consume power, eqn 1 can be expressed as:

C dVdC = Ps _ ig

dt Vdc (2)

where Ps is generated output power by an air-borne electric generator which is equal to electromagnetic power (Pe = Tews), Te is electrical torque of the generator, Ws is angular frequency of rotation of the air-borne turbine.

The voltage equation per phase between the inverter leg and the grid is given by:

. dia ea = Va + tRa + La dt (3)

where ea is grid side phase voltage, Va is output voltage of one of the leg of inverter, La and Ra are equivalent series inductance and resistance between grid and inverter leg per phase.

Similar voltage equations can be written for phase-b and phase-c as expressed in eqn. 3. Three rotating vectors can be transformed into two stationary vectors by 0: - (3 transformation. The stationary vectors are transformed into rotating d - q frame by park transformation. Three phase grid voltages (ea, eb and ea) are represented by single rotating voltage es, which rotates at angle e along the rotating frame. es is aligned along d-axis of rotating frame which results the q-axis component of rotating voltage zero. The d - q transformation of three

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voltage equations results the following equations:

did . . Vd = Lddi + RsZd - wrLqzq + es

diq . . Vq = Lq di + Rszq + WrLdZd

(4)

(5)

where Vd and Vq are quadrature axes voltages of the inverter, Ld and Lq are the inductances in d - q axes, id and iq are the currents in d - q frame, Wr is angular frequency of grid.

Active power flow through the inverter is controlled by daxis current control and reactive power is controlled by q-axis current. The equations for active (P) and reactive (Q) power are given by:

(6)

(7)

Thus, P can be controlled by Id and Q can be controlled by Iq as expressed in eqns. 6 and 7.

V. CONTROL OF GRID SIDE INV ERTER

The converter consists of two stages: buck stage and inversion stage. The buck operation is controlled to maintain DC link voltage to reference DC link voltage determined by the generated power by HAWP system. The grid side inversion is controlled using quadrature current control method and it is carried out for the following purposes:

1) Regulation of DC link voltage which value should be greater than L-L grid voltage. DC link voltage is controlled by controlling d-axis current.

2) Control of reactive power flowing into the grid by controlling the q-axis current.

Grid side phase voltages are sensed and fed into phase lock loop (PLL) to get the orientation of the rotating voltage vector. Three grid side currents are converted into corresponding d - q axis currents using the angle obtained from PLL. Using quadrature axes currents; quadrature axes voltages Vd and Vq are calculated as shown in Fig 6 and 7.

v' d

Fig. 6: Control block diagram of d-axis current generating Vd

The quadrature axis converter voltages are converted into a - b - c frame by inverse park transformation. Three reference voltages Va, Vb and Vc are used to generate six switching signals using space vector techniques.

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Q'

v' q

Fig. 7: Control block diagram of q-axis current generating Vq

V I. SWITCHING OF GRID SIDE TWO LEV EL INV ERTER

Space vector modulation has been extensively used for high power and high voltage application. SVM utilizes the DC link voltage better than sine PWM method of switching and generates low current ripple. Moreover, implementation of SVM in digital signal processor (DSP) is easier than other modulation techniques [15]- [16]. In two level inverter, eight different switching vectors are possible (six active vectors and two zero vectors).

V II. SIMULATION RESULTS

The 3-level NPC DC-DC converter of the proposed grid connected converter allows zero voltage switching of the IGBT switches (51 - 54) due to the conduction of antiparallel diode (Dl - D4) just before turning on IGBTs. The ZVS operations of switches are illustrated in Fig. 8. Softswitching characteristic enables the converter to operate at higher switching frequency that reduces the size of magnetic used. The DC-DC converter transforms 8 kV transmission voltage into 700 V DC link voltage as demonstrated in Fig. 9.

Switch current Switch voltage (scaled down by 50)

80

60

40

20

o

-20 ZVS during turn-o

0.09102 0.09104 0.09106 0.09108 0.0911 0.09112 0.09114

1000

800

600

400

200

Time (s)

Fig. 8: Switch transient of three level DC-DC converter

Scaled down DC transmission voltage (V)

DC-DC converter output voltage (V)

0.01 0.012 0.014 Time (5)

0.016

Fig. 9: Input/output voltages of three level DC-DC converter

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Fig. 10: Complete control diagram of gri d si de inverter in HAWP application

The output of the DC-DC converter is fed to an inverter for interfacing HAWP with distribution grid. The complete block diagram for interfacing the grid and inverter is shown in Fig 10. Two level inverter is controlled using d - q current control strategy and switched using SVM technique. Fig. lla shows the three phase sinusoidal voltage at grid side while slightly distorted grid side current is pointed out in Fig lIb. However, injected grid side current is roughly sinusoidal.

400,----..-----,,_,---_,---_,

> -; 200 " OJ $ " "0 iii "0 -200

_2 cr:'

;; -;:-1 QJ 3 QJ ' > . -1 QJ cr:

-20 0.05 0.1 0.15 0.2 time

Fig. 14: Reactive power flowing to the gri d from HAWP system

current into the grid. The proposed converter controls the injected current harmonics with in the industrial standard limit. Fig 15 shows the harmonic spectrum of grid side current. Fifth, seventh and eleventh harmonics are injected into the grid current, but the value of total harmonic distortion is within the industrial limit. The value of RMS phase current drawn by the grid is approximately 130 A with total harmonic distortion of 1.2%.

rmm c E 0.8 III 0.6 :J u. 04 '0'

Fundamental (50Hz) = 180 I THO= 1.14%

j I O --L-----------------------_I -J 0 1 00 200 300 400 500 600

'--______ ,EJ:equency (1:Iz.) _____ ----' Fig. 15: T HO of the gri d si de current

V III. CONCLUSION

HAWP generating systems can be interfaced into the distribution grid as a source of renewable generation. The converter proposed in the paper is used to connect 8 kV DC transmission line of HAWP system to a 415 V distribution grid. The paper has presented the design of the converter with cascaded isolated three level DC-DC converter followed by two level space vector modulated inverter. Design and description of 3-level DC-DC converter and 2-level inverter has been carried out in Sections II and III. Current and voltage ratings of the active and passive devices for the complete converter are listed. In addition, for the calculated ratings suitable power semiconductor device modules are itemized in the paper. The converter uses four high voltage rating switches during buck operating. The use of buck converter facilitates the use of low voltage rating IGBT switches for two level inversion operation. The inverter is controlled using quadrature current control method and switched using SVM techniques. The proposed converter delivers 100 kW active power to the grid at unity power factor with grid side current THD of 1.2%.

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[I]

[2]

[3]

[4]

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