Evaluation of the loss profiles of VSC - based hvdc converters

Therefore, according to IEC 61803 (Determination of power losses in HVDC converter stations), the loss calculation models of converter station equipment under operation and standby mode are established in detail alongside the datasheet parameters of the devices.

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EVALUATION OF THE LOSS PROFILES OF

VSC-BASED HVDC CONVERTERS

Gbadega Peter A

Department of Electrical Electronic and Computer Engineering, University of KwaZulu-Natal, King George V Avenue, Durban, 4041, South Africa

Inambao Freddie L

Department of Mechanical Engineering, University of KwaZulu-Natal, King George V Avenue, Durban, 4041, South Africa

https://orcid.org/0000-0001-9922-5434

perosman4real1987@yahoo.com, Inambaof@ukzn.ac.za

ABSTRACT

Voltage source converter based high-voltage direct current systems (VSC-HVDCs) are often used in transmission and distribution regions and accomplish good operating results They are popular due to the recent innovation of controllable semiconductor devices and bulk power transmission for long distances of about 500

km and above However, the loss contributions of VSC-HVDC systems are relatively high compared to the traditional LCC-based HVDCs, which tends to be the main hurdle to the application of VSC-HVDC to high power transmission Thus, the loss characteristics of VSC warrants further investigation In this paper, the loss calculations of two level and three level VSC-based HVDC technologies are studied and corresponding loss reducing measures are obtained from the results An essential step in the thermal management design of the power electronic devices, in this case,

an insulated-gate bipolar transistor (IGBT) and diode, is required for accurate calculation of both conduction and switching losses of these devices In order to achieve optimized designs, tools are needed for accurate prediction of device junction temperature and power dissipation Therefore, according to IEC 61803 (Determination of power losses in HVDC converter stations), the loss calculation models of converter station equipment under operation and standby mode are established in detail alongside the datasheet parameters of the devices

Keywords: HVDC, power losses, VSC, datasheet parameters, IGBT, diode,

conduction losses, switching losses

Cite this Article: Gbadega Peter A and Inambao Freddie L, Evaluation of the Loss

Profiles of VSC-Based HVDC Converters International Journal of Mechanical Engineering and Technology 10(12), 2020, pp 512-526

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=12

1 INTRODUCTION

The evaluation of power losses involves analysis and computation of losses that occur during the operation of a system in order to assist designers and operators in evaluating the technical and economic advantages of the project It is necessary for the supplier to gain knowledge of how and where the losses are generated in order to determine the equipment ratings and subsequently increase the operating efficiency of the transmission system by optimizing the design of the equipment that generates the losses [1, 2] Power loss evaluation during the design stage of VSC-based HVDC technology is imperative because this permits the designers to enhance the overall system performance through a compromise of various design indices The losses of the overall VSC-based HVDC link can generally be computed as the sum of the individual equipment losses on the network The loss contributions of various components of the converter stations are calculated separately The overall losses are mainly the separate losses of each piece of equipment [1, 3] Loss determination of various equipment is based on available standards However, these standards are mainly used for LCC-based HVDC links, therefore, in order to determine the losses occurring in a VSC-based HVDC installation more research has to be conducted to establish similar standards to perfectly determine the losses in a VSC HVDC scheme However, the existing standards can

be used to calculate the power losses in VSC-based HVDC technology The losses related to voltage source converters account for a large proportion of the total losses of the converter stations due to the low voltage level and high switching frequency features of voltage source converters [4] A method was proposed in [5] which presents an analytical method to calculate the efficiency of VSC-HVDC links based on two-level and three-level VSCs The method uses the average and root mean square of the VSC converter current to estimate the conversion losses in the converter (conduction and switching losses) Additionally, the studies

in [6] verified the feasibility, technical as well as economic, of VSC HVDC on loss minimization in mesh networks The losses of the different components of a VSC HVDC link were studied and the impact of the installation on overall system losses in mesh networks was verified through simulations This paper is organized in the following way: Section 2 describes the overview of losses of VSC-based HVDC technologies; Section 3 describes the results and analyses of power loss calculations of VSC-based HVDC technologies (two level and three level configurations); Section 4 concludes the study

2 VSC-BASED VALVE LOSSES

Figure 1 shows the loss hierarchy of VSC-based converter valves which are mainly from insulated-gate bipolar transistors (IGBTs) and freewheeling diodes In order to implement loss evaluation, the losses of converter valves are segmented into various parts, as depicted in Figure 1 Output characteristics curves are used to calculate the IGBT and Freewheeling Diode (FWD) steady state losses and are calculated based on the collector voltage-current characteristics in a transient state The conduction and switching losses of an IGBT device generates about half of the converter valve, and the switching losses of the FWD generates about one-third of the entire energy dissipation

An infinite proportion of losses results from the other parts of the module [7] The proportion of the IGBT device and converter losses tend to reduce as the converter operates in the rectifier state because the current flows through the FWD most of the time, therefore, its losses are usually less than that of the IGBTs’ Changing of the operating states and structure

of the valves will usually have an effect on the converter valve losses [8] The essential aspects to consider are topology of converter, types of the converter valve, scheme of valve

driver, valve unit accessories, and temperature of the device

Total losses of IGBT model

Total losses of accessories

Total losses

of converter valve unit

IGBT

FWD

Voltage divider Heat sink Driver unit

Switching losses steady state losses (conduction losses)

Transient losses (reverse recovery) Steady state Losses(conduction losses)

Other facilities

Turn-on Losses

Turn-off Losses

Figure 1 Classification of losses in a VSC-based converter valve unit [3]

The main contributor to the overall VSC-based HVDC losses are the valve losses The valves composed of IGBTs and diodes are collectively known as an IGBT module In an IGBT module there are various IGBTs and diodes based on the module and the application requirements When conducting or switching from one state to another, all chips dissipate power However, it is expedient to consider each of the devices that make up the valve [8, 9]

2.1 Power Losses Evaluation of Two-Level VSC-Based HVDC Technology

2.1.1 Estimating Power Losses of IGBTs

In thermal design, the first step is the computation of total power losses An IGBT is a voltage-controlled device, which merges the merits of a Bipolar Junction Transistor (BJT) and

a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) IGBT is a three-terminal device, collector, emitter and the gate terminal For high power IGBT module operation it is mandatory to provide a suitable heatsink otherwise it may go into thermal runaway In power electronics circuits which adopt the use of IGBTs it is imperative to consider the two most important sources of power dissipation, namely, conduction losses and switching losses, since IGBTs operate in two states and produce losses in both states, [10, 11]

2.1.1.1 Conduction Losses of IGBTs

The losses in an IGBT module that occur during the On-state of the IGBT or freewheeling diode when current is being conducted are known as conduction losses The dissipated total power is estimated by multiplying the On-state saturation voltage and On-state current (current flowing through the collector or anode) In order to obtain the average power dissipation, the conduction losses, in PWM applications, must be multiplied by the duty factor The first order approximation of conduction losses can be obtained by multiplying the IGBT saturated voltage drop (Vce sat) by the expected average device current [12] Conduction loss is the On-state loss or steady state loss The average power dissipated by the IGBT is expressed as:

∫ [ ( ) ( )] (1)

Where:

Average conduction losses, in W

Vce = the device forward voltage or On-state saturation voltage (IGBT), in Volts

= On-state current, in Ampere

T = Conduction period, in seconds

The forward resistance can easily be computed from the IGBT characteristic curve from the datasheet provided by the manufacturer

(2)

Description as above Forward resistance is the ratio between the collector-emitter voltage difference and the collector current difference The value of ( ) is computed as follows [13, 14]:

( ) (3)

Substituting the expression for the forward voltage in Eq (3) into Eq (1) yields:

(4)

Where:

= Average current through the device during conduction period, in Ampere

= rms current through the device during conduction period, in Ampere

The average and rms current through the device during conduction period are estimated as follows:

∫ ( ) (5) ∫ ( ) (6)

Note that, the value obtained from Eq (3) should be approximately equal with the value in the datasheet to justify the correct computation from the graph However, the average power losses in PWM sine wave switching is expressed as [15]:

( ) (

) (7)

Where

= modulation index

= power factor

= Peak value of sinusoidal output current, in Ampere

Other descriptions as above

2.1.1.2 Switching Losses of IGBTs

Switching losses in power electronic devices usually contribute a substantial amount to the overall losses of the system Thus, ignoring losses due to switching of the electronic devices

in the computation of or weighting of the losses due to conduction of the devices with an estimated factor to actually consider switching losses, might lead to large errors Regarding the total losses, the power dissipated during the turn On and turn Off switching transitions are known as switching losses Switching losses are calculated as the total sum of on-state switching losses and turn-off switching losses, so depend on the switching frequency, device

current and device characteristics Switching losses happen as a result of energy loss during the transition and switching frequency These losses are also related to the dc link voltage, load current and junction temperature If the switching frequency is higher, losses will likewise be higher [3, 16] It is essential to estimate the switching accurately in order to compute the junction-temperature time behavior so that the reliability of the design can be improved To evaluate the average switching power losses it is necessary to obtain the and values from the curve at the expected average operating current Therefore, the average power dissipated is expressed as:

( )

(8)

Based on the conditions provided for any application with respect to the datasheet nominal values, the switching power loss is required to be normalized

( )

(9)

Total losses is expressed as:

(10)

Where:

= IGBT turn on switching energy at and T = 125

= IGBT turn off energy loss at and T = 125

PWM switching frequency

IGBT saturation voltage drop at and T = 125

Peak value of sinusoidal output current

2.1.2 Estimating Power Losses of Freewheeling Diodes

In power electronics, a diode is a two-terminal pn-junction device whose two terminals are anode and cathode It is an electronic device which allows current to pass when it is forward biased in one direction (conduction state), and blocks current in the opposite direction (reverse direction)

2.1.2.1 Conduction Losses of Diodes

The average total power losses in a diode is expressed as:

(11)

The forward resistance can easily be computed from the diode characteristic curve from the datasheet provided by the manufacturer

(12)

From Eq (12) the value of diode resistance can be obtained from change of forward voltage and forward current The forward voltage of a diode is expressed as:

(13)

Similarly, diode threshold voltage can be computed from the characteristic diode graph, which is then used for estimating the forward voltage as shown in Eq (13) This forward voltage can be checked against the value on the datasheet to confirm the validity of the calculation The average power losses in a diode when operating under PWM sinewave switching is expressed as:

( ) (

) (14)

Parameters description as above

2.1.2.2 Reverse Recovery Losses of Diodes

Basically, when switching a diode from the conduction mode to the blocking state, a diode or rectifier tends to have stored charge which must be discharged first, prior to when the diode blocks the reverse voltage This discharge takes a limited amount of time known as Reverse Recovery Time, or During this time, diode current may drift in the reverse direction When the device (diode) turns off as a result of the discharging operation, which still permits the drifting action of the diode current in the reverse direction, losses are generated which is called recovery loss The time needed to recover is called reverse recovery time The recovery losses of a diode is expressed as:

(15)

Where:

= Diode reverse recovery energy (The turn-off energy of the diode due to reverse recovery current)

= Switching frequency

= Diode reverse recovery current

= Diode reverse recovery charge

= Diode reverse recovery time

As previously stated, based on the conditions provided for any application with respect to the datasheet nominal values, the switching power loss is required to be normalized

(16)

Finally, the summation of Eq (14) and Eq (16) results in Eq (11) Therefore, the sum of conduction loss and reverse recovery loss in a diode results in the total average power loss for the diode

Similarly, the average (total) conduction losses of a per valve IGBT module is expressed as:

[ ] (17) [ ] (18)

Note that in a two-level converter, the total conduction and switching losses for the 6-valves becomes:

[ ] (19) [ ] (20)

Where:

= Number of IGBTs connected in series in each valve

The number of IGBTs connected in series in each, valve is expressed as:

( ) ( )

(21)

Where:

= Voltage factor

= Switching safe operating area voltage factor

= Voltage miss distribution

= Switching safe operating area voltage factor for IGBTs

2.2 Power Losses Evaluation of Three-Level VSC-Based HVDC Technology

2.2.1 Conduction Losses

The conduction losses consist of losses in the IGBTs and the freewheeling diodes However, the conduction losses for each switch can be estimated by the following expression:

(22)

Consequently, in a three-level converter configuration, there are four IGBTs and six diodes in each phase The load current and the phase leg voltage are the same as the two-level configuration However, the duty cycle across the switching devices is expressed as follows [4, 17]: ,

,

(23)

Therefore, the average and rms currents that flow in the IGBT and are expressed as follows:

∫

[( ) ] (24)

∫

[ ] (25)

Where: = The peak of the output voltage, in Ampere = The phase difference between the output voltage and current = Modulation index Similarly, the currents for and are expressed as [4]:

*∫ ∫ +

[ ] (26)

*∫ ∫ +

[ ( ) ]

(27) Likewise, the average and rms currents that flow in the freewheeling diodes are expressed as: ∫ [ ] (28)

∫ [ ] (29)

Similarly, the average and rms currents that flow in the clamped diodes are expressed as:

*∫ ∫ +

[( ) ]

(30)

*∫ ∫ + [ ] (31)

Subsequently, the above listed formulas can be used in Eq (22) to compute the total conduction per valve of IGBT module

2.2.2 Switching Losses

Practically, there are only two commutations per output in IGBTs and , so, therefore, the switching losses of these IGBTs are usually excluded However, an ideal case is being considered in this research work, so the switching losses are computed for a qualitative analysis

Furthermore, the switching losses in the IGBTs to for a three-level neutral point clamped converter are expressed as:

∫ ( )

[ ] (32) ∫ ( ) [ ] (33)

The switching losses of the diodes (considering the fact that there is only switch off energy i.e recovery energy) can be obtained using Eq (16)

2.3 Power Losses Estimation in the DC-Link Capacitor

The ESR (equivalent series resistance) value and the current through the capacitor can be used

to determine the DC-link capacitor losses Similarly, the ESR values can be obtained from the capacitor datasheet, and the current can be estimated with the assistance of charging and discharging times of the capacitor using the SKN 130 capacitor datasheet When the charge and discharge current are estimated, the current that drifts through the capacitor can be evaluated from the expression of charge and discharge currents [18, 19] as follows:

√ (34)

Where

= rms value of charging current

= rms value of discharging current

Most of the power losses associated with the converter valve occurs in the switches, but there are also some power losses in the DC-link capacitor The total power loss of the valve is expressed as [20]:

[ ]

(35)

2.4 Evaluation of the Average Junction Temperature

Conduction and switching power losses are expected to surface when operating the power device contained in IGBT and intelligent power modules Therefore, the heat generated due to these losses has to be dissipated away from the power chips and the environment using a heat sink; the power device will over-heat if an adequate thermal system is not adopted, which

could lead to module failure However, the system thermal design in many applications is limited by the maximum useable power output of the module Thus, in order to gain maximum output from the device, it is essential to accurately design the thermal system [21], [22]

The junction temperature is expressed as:

[ ( ) ( ) ( )] (36)

Where

= Junction temperature

= Module power loss

( ) = Thermal Resistance junction to case

( ) = Thermal Resistance case to heat sink

( ) = Thermal Resistance heat sink to ambient

3 RESULTS AND ANALYSES OF LOSSES CALCULATION OF VSC-BASED HVDC CONVERTERS

The purpose of this study is to calculate the loss mechanisms of the popular VSC-based HVDC technologies, with two-level and three-level converter configuration The total loss figure of the these VSC-based technologies are calculated based on the summation of individual loss components, and the individual component losses are determined by applying standardized calculation methods

3.1 Power Losses Estimations in Two-Level VSC-Based HVDC Converters

Table 1 System Parameters

Rectifier side Inverter side

Transformer rating 500/440 kV, 1000

MVA

350/230 kV Transformer

resistance

Transformer reactance

55.5 mH 55.5 mH Coupling reactor 41.8 mH 41.8 mH VSC converter rating 1000 MVA 1000 MVA

DC cable resistance 0.05 Ω/km (500

km long) = 25 Ω

0.05 Ω/km (500

km long) = 25 Ω

DC link voltage 561.64 kV 561.51 kV (number

of IGBTs connected

in series)

Number of valves at each station

Table 2 Datasheet parameters for Dynex IGBT module DIM1200ASM45-TS000

Input

MVA

Rectifier side = 1312.16 A

Inverter side = 1649.57 A

440 kV

√

√ Rectifier side =

1855.7 A

Inverter side = 2332.84 A

DATASHEET

DIM1200ASM45-TS000

Case temperature= = 125 unless stated otherwise

( ) ( /W) 0.008 ( )( /W) 0.016

( ) ( /W) 0.006 ( )( /W) 0.006

( ) ( /W) 0.014 ( )( /W) 0.022

Tables 1 and 2 show the system parameters and the conduction and switching data for Dynex IGBT module DIM1200ASM45-TS000 used for power losses calculation for two level converter topology and PWM sinewave switching

Table 3 Losses Calculations of two-level converter HVDC system

Power conduction losses in IGBT (W) Rectifier terminal Inverter terminal

1836.45 W 2686.11 W Power switching losses in IGBT (W) 5463.86 W 4906.24 W

Total Power losses in IGBT (W)

7300.31 W 7592.35 W Power conduction losses in Diode (W) 418.32 W 587.92 W

Power switching losses in Diode (W)

(Reverse recovery energy loss)

1845.9 W 1657.51 W Total Power losses in Diode (W)

2264.22 W 2245.43 W Total Power losses per IGBT module

(W)

[

9564.54 W 9837.78 W

Table 3 shows the results of the calculation of conduction and switching losses of an IGBT and diode for two-levels using the standardized equations stipulated above