An Overview Of The HVDC System Biology Essay

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1.1 Objective of Dissertation

The main objective of this dissertation is to study the behavior of HVDC link system of Transmission line with various configurations. Case study of Transmission line is also performed. Different pulse converter, of 6 pulse, 12 pulse, 24 pulse and 48 pulse converter has also been studied. In this dissertation Total harmonic distortion, Input Power Factor and Distortion factor for 6 pulse, 12 pulse,24 pulse and 48 pulse converter has been calculated.

1.2 Organization of the Dissertation

This dissertation work is organized as follows:

Chapter 1 deals with the general introduction of HVDC system,

Chapter 2 deals HVDC system with FACT controller, literature review, objective and organization of dissertation work.

Chapter 3 presents introduction about the HVDC link system.

Chapter 4 deals the performance analysis of AC/DC converter

Chapter 5 Simulation and result

Chapter 6 Conclusions and future scope

2.1 Role of FACTS Devices in Transmission System

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FACTS has the principal role to enhance controllability and power transfer capability in A system. FACTS involve conversion and /or switching power electronics in the range of a few tens to a few hundred megawatts. Most of the world's electric supply systems are widely interconnected. This is done for economic reasons, to reduce the cost of electricity and to improve its reliability, it must however be kept in mind that these inter connections are very complex and they emerged gradually however be kept in mind that these inter connections are very complex and they emerge gradually based upon the requirements of various power utilities. These interconnections apart from delivering the power pool power plants and load centers in order to pool power generation and reduce fuel cost. Thus they reduce the overall number of generating sources, but as the saying goes a coin has two sides, like wise as the power transfer grows. The power system becomes increasingly complex to operate and system can become less secure for riding through major outages. It may lead to large power flows with inadequate control, excessive reactive power, and large dynamic swings between different parts of the system. Thus the full potential of a transmission connection cannot be utilized. It is very difficult to control such transmission of power in such systems. Most of the controllers designed in the past were mechanical in nature. But mechanical controllers have numerous intrinsic problems. Many power electronics controllers have been designed to supplement the potentially faulty mechanical controllers. These power electronic controllers are all grouped in a category called flexible AC transmission controller or FACTS controllers.[2]

FACTS and FACTS controllers are defined in IEEE Terms and Definitions as:

•Flexible AC Transmission System (FACTS): Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability.

•FACTS Controller: A power electronic-based system and other static equipment that provide control of one or more AC transmission system parameters.

2.2 ADVANTAGE OF FACTS

The following are the benefits that are principally derived by using the FACTS controllers.

(1)The flow of power- is ordered. It may be as per the contract or as per the requirements of utilities.

(2)It increases the loading capability of the lines to their thermal capability. Overcoming their limitations and sharing of power among lines can accomplish this.

(3) It improves the stability of the system and thus makes the system secure.

(4)Provides secure tie line connections to neighboring utilities and regions, thereby decreasing over all generation reserve requirements on both sides.

(5)Provides greater flexibility in sitting new generation.

(6)Upgrade of lines

(7)Reduce loop flows

(8)Minimizes the cost of transmission and hence the overall cost of generation.

(9) FACTS devices improve the speed of operation of the overall system.

2.3 APPLICATION OF FACTS

FACTS are a family of devices which can be inserted into power grids in series, in shunt, and in some cases, both in shunt and series. Important applications in power transmission and distribution involve devices such as SVC (Static VAR Compensators), Fixed Series Capacitors (SC) as well as Thyristor-Controlled Series Capacitors (TCSC) and STATCOM.

SVC and SC have been utilized for a long time. The first SC installations came on line in the early 1950s.Among the pioneering countries are USA and Sweden. SVCs have been available for commercial purposes since the 1970s. Over the years, more than a thousand SVCs and SCs have been installed all over the world.

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FACTS mainly find applications in the following areas:

• Power transmission

• Power quality

• Railway grid connection

• Wind power grid connection

• Cable systems

2.4 COMPARISON OF FACTS WITH HVDC TRANSMISSION

In the transmission area ,application of power electronics consists of High-Voltage Direct

Current (HVDC) power transmission and FACTS. HVDC, a well-established technology, is often an economical way to interconnect certain power systems, which are situated in different regions separated by long distances(over 50 km submarine or 1000km overhead line), or those which have different frequencies or incompatible frequency control .HVDC involves conversion of AC to DC at one end and conversion of DC to AC at the other end. In general, HVDC represents conversion equipment sizes in the range of a hundred megawatts to a few thousands megawatts.

In general, FACTS - the subject matter of this Dissertation are relatively new technology - has the principle role to enhance controllability and power transfer capability in ac systems. FACTS involves conversion and /or switching power electronics in the range of a few ten to a few hundred megawatts.

2.5 CONCLUSION OF FACTS

• Improved power transmission capability

• Improved system stability and availability

• Improved power quality

• Minimized environmental impact

• Minimized transmission losses

CHAPTER-3

HVDC LINK SYSTEM

3.1 RIHAND -DADRI BIPOLAR SYSTEM

The Rihand-Dadri 1500 MW HVDC transmission system is the first commercial HVDC link in India. This 810 km long link operates at 500 kV, and is used for bulk power transmission from coal-based thermal power station at Rihand to load centre at Dadri (near Delhi). This link was commissioned in 1991-92 and has been operating satisfactorily. HVDC option was chosen in preference to two double-circuit 400 kV AC lines because of better overall economy, reduced right-of-way, lower transmission losses, and increased stability on parallel AC lines and better controllability.

In order to enhance the reliability of integrated AC/DC bulk power transmission system, special control features were incorporated in the scheme. These include Power Modulation, Frequency Control, Sub-synchronous Resonance Damping, Reactive Power Control and Runback control. BHEL is the supplier of these two converter stations, at Rihand and Dadri, under a technical collaboration agreement with M/s ABB. ABB and BHEL have both shared supplies of major items for this project. It was during this period (mid-eighties) that BHEL established specific HVDC manufacturing and testing facilities in its premises under the overall engineering guidance of ABB. This enabled BHEL to supply all major HVDC products like converter transformers, smoothing reactor, AC filter capacitors, thyristor valve assembly and select control panels assembly for second pole of the bipole.

An HVDC Transmission system has the following essential parts:

-AC substation and HVDC substation at each terminal

-Interconnecting HVDC lines(s)

-Electrode lines and earth electrodes

The HVDC terminal substation at each end of the HVDC transmission line has following essential part:

AC Switchyard

AC Harmonic filters(F) and shunt compensation(SC)

Converter Transformer(T)

Converter valves (V) housed in valve halls.

Control, protection and monitoring panels along with control cables installed in control room located in between two valve halls.

HVDC yard with switching arrangement for Bipolar to Monopolar change over.MRTB in one terminal.

Smoothing reactors(R), one for each poles

Electrical and Mechanical Auxilaries

HVDC line poles

Electrode lines (EL) and Earth Electrode (E).

Station earthling system.

TABLE-1

MAIN DATA OF RIHAND-DADRI PROJECT

Interconnection Between

Rihand (Northern Region)

Dadri (Northern region)

AC System Frequency

50 Hz

50 Hz

AC System Voltage

400 kV

400 kV

Power Co.

POWERGRID Corp Of India Ltd.

Contractor

ABB Limited

Commissioned

Dec 1990, June 1991

Executed cost

Main Purpose

Bulk Power transmission from Rihand super thermal power plant (Northern region in India) to Dadri (Delhi) in Northern region.

Main Data

Rated Power : 2X 750 MW

Direct Voltage: +/- 500 kV

Direct Current : 1786 A

DC overhead line : 810 km

A.C. Networks

Both stations are connected to the AC network through 400 kV buses. There are six single phase 3 winding converter transformers at each terminal.

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Both terminals have specified Short Circuit Ratio between 2.5 to 3.

Transformer leakage reactance : 18%

Rihand

AC Side

Valve side Y

Valve Side

D

Dadri

AC side

Valve side

Y

Valve side

D

Rated power

MVA

315

157.5

157.5

305

152.5

152.5

Rated Voltage kV

400/√3

213/√3

213

400

206/√3

206

Tapping

range

-10% to

+14%

-10% to +14%

HVDC System

Total length = 810 km

AC Filters

At Rihand 3X118 MVAR, 3/36th, 11/13th, HP24 Harmonic filters, 3X39 MVAR, 11/13th Harmonic filters and 3X70.5 MVAR 5/27th harmonic filters are installed.

At Dadri 3X79.1 MVAR, 3/36th and 11/13th Harmonic filters, 3X77.8 MVAR, 11/13th, HP24 Harmonic filters and 3X70.5 MVAR 5/27th harmonic filters are installed.

DC Filters

2X (12, HP24)

HVDC Valves

Each Valve in Quadruple valve structure consists of 96 No. 6.5 kV thyristor. The Quadruple valve structure is suspended from top.

Valve Cooling

The valves are cooled by deionised water circulated in closed loop system.

Smoothing reactor

Two smoothing reactors are installed per pole. One oil insulated of 360 mH and one air insulated of 180 mH.

Ground Electrode

Each terminal has ground electrode stations.

TABLE-2

HVDC TERMINAL DATA

RATINGS

2X750 MW, ±500 KV

AC VOLTAGE

For Performance: 380 ~ 420 KV

For Rating: 360 ~ 440 KV

AC SIDE FREQUENCY

For Performance: 48.5 ~ 50.5 Hz

For Rating: 47.5 ~ 51.5 Hz

OVER LOAD RATING

1650 MW (Low Ambient OR Two Hour O/L)

SHORT TIME VERLOADS

1000 MW per Pole (5 Sec)

THYRISTOR VALVES

6.5 KV, 1568 Amp, and Water Cooled

CONVERTOR TRANSFORMER

1Φ, 3Wdg, 305 MVA, +14/-10 Taps@1.25 %

AC FILTERS

3 X 230 MVAR (11th/13th, 3rd/36th, 5th/27th,HP 24th)

DC FILTERS

2 X (12th, HP 24th)

3.1.1 Converter Transformers

Of the converter transformers supplied for the project, eight are manufactured by ABB and six by BHEL. They are of the single-phase, three-winding type. The ungrounded-Y valve winding bushings protrude inside the valve hall, while the delta valve-side bushings are outside. The salient features of the converter transformers are Type: Single-phase, 3 winding, Quantity: 12 + 2 spare, Total weight: 358 tonnes.

The star winding bushings protrude directly into the valve hall. The delta connections, which have a lower DC voltage, are made outside the valve hall. The converter transfom1er transforms the ac voltage to a suitable value for feeding the converter. In addition it serves the following functions:

Reactive power is supplied to the converter through tap changing.

Short circuit currents are controlled by suitable impedance values of these transformers.

The reactance of the converter transformer helps in harmonic suppression.

By suitable star-star and star-delta connections, the required 30-degree phase shift for the 12­-pulse operation is achieved.

A converter transformer has a somewhat different design than that of a normal power transformer because in the converter transformer, the insulation system has to be designed to withstand direct voltage stresses. Moreover, in a converter transfom1er the currents have high harmonic content so that special care has to be taken with regard to eddy current losses.

3.1.2 Smoothing Reactors

Two smoothing reactors are installed per pole, one oil-insulated of 360 mH and one air-insulated of 180 mH. The valve-side bushing of the oil-insulated smoothing reactor protrudes directly into the valve hall. It serves the following purposes:

(i) It prevents consequent commutation failures in the inverter by limiting the rate of increase

of direct current during commutation in one bridge when the direct voltage of another

bridge collapses.

(ii) It decreases the incidence of commutation failures in the inverter during dips in the

alternating voltage.

(iii) It decreases the harmonic voltages and currents in the dc line.

(iv) It smoothen the ripple in the direct current sufficiently to prevent the current from becoming discontinuous at light loads.

(v) It limits the current on the rectifier when a short circuit occurs on thee line

3.1.3 Two D.C Filters

Two DC filters are installed in each pole, one double-tuned to the 12th and 24th harmonics, the other single-tuned to the 12th harmonic. The HVDC transmission link can be operated without DC filters. However, the drawback is the higher telephone interference associated with it. Characteristic D.C. side voltage harmonics generated by a 6 pulse converter are of the order 6n and when generated by a 12 pulse converter, are of the order 12n. D.C. side filters reduce harmonic current flow on D.C. transmission lines to minimize coupling and interference to adjacent voice frequency communication circuits. Where there is no D.C. line such as in the back­-to-back configuration, D.C. side filters may not be required.

Single Tuned and Double Tuned identical filters are installed at Rihand-Delhi.

TABLE-3

  DC FILTER PASSIVE ELEMENT SPECIFICATION

Harmonic

N° of

C = F

L = mH

R =

Tuning

Filters

 

 

 

12th

2

1.2

58.6

 

Single

 

 

 

 

Tuned

 

 

 

 

12/24th

2

CH= 1.2

LH=42

RL= 522

Double

 

CL= 1.8

LL=15.38

 

Tuned

 

 

 

 

3.1.4 The AC Filter Banks

To meet the filtering requirements and to control the interchange of reactive power with the 400 kV network three AC filter banks, each rated at 230 MVAR are in-stalled in each station Each AC filter bank consists of two branches double-tuned to the 11th and 13th harmonics, one double tuned to the 3rd and 36th harmonics and one double-tuned to the 5th and 27th harmonics. The last branch is a pure high-pass filter.

The characteristic ac side current harmonics generated by 6 pulse converters are 6n ±1 and 12n ±1 for 12 pulse converters where n equals all positive integers. A.C filters are typically tuned to 11th, 13th, 23rd and 25th harmonics for 12 pulse converters Tuning to the 5th and 7t harmonics is required if the converters can be configured into 6-pulse operation. A.C side harmonic filters may be switched with circuit breakers or circuit switches to accommodate reactive power requirement strategies since these filters generate reactive power at fundamental frequency. A parallel resonance is naturally created between the capacitance of the A.C filters and the inductive impedance of the A.C system. For the special case where such a resonance is lightly damped and tuned to a frequency between the 2nd and 4th harmonic, then a low order harmonic filter at the 2nd or 3rd harmonic may be required, even for 12 pulse converter operation.

Since the 3rd harmonic is the most important low order harmonic. In order to achieve good filtering of the 3rd harmonic even at the lowest power levels the filter is designed double - tuned

with the ordinary high pass filter (36th harmonic),and it is therefore always connected.

The most natural choice would have been to choose a filter tuned to the 24th and 36th harmonic frequencies, which would then take care of the 23rd ,25th and 35th ,37th harmonics. However, the performance requirements were not possible to fulfil with this solution due to high calculated harmonic currents between these frequencies. Different filter solutions were tested and the chosen one turned out to be the best.

Three types DT (Double Tuned), DTHP (Double Tuned High Pass) and HighPass Filters are installed

TABLE-4

AC FILTER PASSIVE ELEMENT SPECIFICATION

Harm.

Mvar

C = F

L = mH

R =

Type

3/36

40.1

CH=0.77

LH=14.13

RH= 480

DT

C1=1.99

LL=416.6

RL=13700

11/13

39.0

CH=0.77

LH=94.43

R = 6420

DT

CL=31.69

LL= 2.29

5/27

70.5

CH=1.39

LH=12.38

RH= 370

DT

CL=6.11

LL=54.78

RL= 2500

HP24

38.8

C= 0.77

L= 23.83

R= 723

DTHP

Legend :

XH --> High voltage component

XL --> Low voltage component

FIG. 3.1 Single line dig. Of RIHAND-DADRI HVDC System

TABLE-5

DC LINE DATA

LINE LENGTH

810 KMS

NUMBERS OF TOWERS

2140

CONDUCTOR PER POLE

4

TYPE OF CONDUCTOR

BERSIMIS

CONDUCTOR SIZE

4 x 725.2 MM2

CLEARANCE PHASE TO PHASE

12.7 METRES

PHASE TO GND

12.5 METRES

AVERAGE SPAN

400 METRES

SUBCONDUCTOR SPACING

457 MM

3.2 CASE STUDY OF RIHAND-DADRI HVDC PROJECT

By using AC filter and DC filter 3rd, 5th, 11th, 13th, &24th characteristic and non characteristic harmonic can be mitigated.[10]

In 6-Pulse bipolar system equation of input current waveform can be written as

ia= sin-) - sin-) - sin-) + sin-) + sin-) - sin-) - sin-1) ………..} ……… …(3.1)

So primary current will contain only fundamental and some higher order harmonic. Generally higher order harmonic has magnitude less than 3%. In Rihand-Dadri HVDC system filters are designed for harmonic higher than 3%.For this purpose three AC filter has been installed at the Rihand side and Dadri side of 230 MVAR which are double tuned & high pass filter.After removing the higher order harmonic we get the Total harmonic distrtion of current wave get reduced to 5.13%.

For reducing the current THD upto 5.13% following arrangement have been made:

(i) A capacitor has been inserted across the DC link of the converter; but no marked improvement in THD of secondary side is observed with this solution; but dv/dt and voltage spikes are getting reduced. Such an application could be similar to capacitor commutated converter which has not been considered under present scope of our discussion.

(ii) Since two numbers 6 pulse bridges are connected in series with a phase shift of 30°, the predominant harmonics in 6 Pulse Bridge (i.e., on the secondary side) will be 5th and 7th. Therefore passive filters of 5th and 7th harmonic order are incorporated in the secondary side of converter transformer. In this case, there is a substantial improvement in current harmonics (this gets reduced to 8% from the original 20%) and voltage harmonics reduced marginally. Since major harmonics source is 6 pulse converter and 5th,7th harmonics are getting generated, once these are filtered at generating source itself it is not likely to get such harmonic transfer to primary side as 11th ,13th.

(iii) When these two solutions (i) and (ii) are combined together, the voltage and current THD are getting reduced to 14.34% and 5.13% respectively. Commutation overlap and dv/dt are also getting reduced.

(iv) When Four Types of AC Filters & Two Types of DC Filters are used the current THD get reduced to 3.4%

THD(ia) = = (Ia112 +Ia132 + Ia232 + Ia252+ ……..)1/2

Idealized Current waveform equation for 12- pulse converter

ia = sin) + sin)+ sin) + sin) + sin) + sin) + sin) + sin) + sin) …………….. }……….(3.2)

After using the AC Harmonic Filter 11th , 13th, 23th , 25th, 35th,& 37th, harmonic component will be disappear

So THD (ia) = [(1/47)2 + (1/49)2 + (1/59) 2]1/2

THD (ia) = .034

=3.4 %

(v) When the Filters are not used the current THD for Idealized current waveform comes to:

THD (ia) = [(1/11)2 + (1/13)2 + (1/23) 2 + (1/25) 2 + (1/35) 2 + (1/37) 2 + (1/47) 2 + (1/49) 2 ]1/2

= 14.17%

CHAPTER-4

ANALYSIS OF AC/DC CONVERTER

4.1 ANALYSIS OF 6 PULSE CONVERTER:

A 6-pulse bridge has 6 valves arranged on 3 limbs for the vertical structure.AC supply is given from the three secondary loads of a converter transformer. The 6 valves are fired in a definite sequence (1,2,3…..6).DC output is obtained at the terminals of the 6-pulse bridge operated in rectifier mode. The DC voltage waveform of a 6 pulse converter has 6 pulses per cycle of AC wave.

The natural firing instant of thyristor occurs at =30° and the firing angle α is reckoned from this instant. At any instant of time, two thyristor of the converters conduct - one in the positive half and other in the negative half.

For α=0°;T1, T2, T3,…T6 behave like diode. This is shown in figure. For α=0°, T1 is triggered at =/6, T2 at 90°, T3 at 150° and so on. The load voltage has, therefore ,the waveform as shown in figure.

The Back emf present in load has a tendency to make current discontinuous. This connection is prone to discontinuous conduction. However choosing sufficiently large inductance ,continuous conduction may be possible .During discontinuous conduction load current falls to zero and is at zero value till the other Thyristor is fired. The load time constant is very small making the current zero before the other Thyristor is fired. The load current is in pulse. Further the presence of backemf accelerates the current to reach zero. Normally even though the load current is continuous with some R-L load, it may become discontinuous if a back emf is connected in series with it.

While during continuous conduction load current does not fall to zero and a finite value of current flows. Load inductance here is not a very large value to cause perfect smoothing but large enough to maintain a non zero current. The load current has ripple

In six-pulse converter when α ≤ /3 conduction will be continuous and if α> /3 conduction will be discontinuous

For α=60°;. The conduction sequence of thyristor T1 to T6 is shown in figure. Here T1 is triggered at 30°+60°=90°, T2 at 90°+60°=150° and so on. If the conduction interval of various Thyristor T1, T2, T3…T6 is shown first, then it becomes easier to draw the voltage and current waveform. Each SCR conduct for 120°, when T1 is triggered, reverse biased thyristor T5 is turned off and T1 turned on . T6 is already conducting. As T1 is connected to A and T6 to B, voltage Vab appears across the load. It varies from 1.5Vm to zero. Here Vmp is the maximum value of phase voltage.

FIG. 4.1 Six Pulse Converter

At α/3

For three phase full converter if the line to line Neutral voltage are defined as

Van= Vm Sin

Vbn=Vm Sin( - /3)

Vcn=Vm Sin( +/3)

Vab=Van-Vbn