HVDC

AN INTRODUCTION TO HVDC TRANSMISSION One of the most exciting new technical development in electric power system in the last three decades has been “High Voltage Direct Current transmission”. From the first of HVDC links to the recent, the voltage has increased from 100 KV to 800 KV, the rated power from 20 MW to 6300 MW and the distance from 96 km to 1370 km. Preceding and accompanying this rapid growth of Direct Current Transmission were developments in High Voltage, High power valves, in control and protection system, in DC cables and in insulation for overhead DC lines. In India three HVDC projects are in operation.

(i) The Rihand-Delhi HVDC transmission project having 1500 MW capacity and 500 KV DC voltage is the first commercial long distance DC transmission project in India.

(ii) Vindhyachal 2x250 MW Back to back DC converter station which

asychronously connect the Northern and Western regions for exchange of power.

(iii) The Nation HVDC experimental line project, which links Lower Sileru in

A.P. to Barsoor in M.P. Phase 1 of this project is capable of transmitting 100 MW at 100 KV DC.

The main advantages of High Voltage Direct Current transmission are – (1) Asynchronous operation (2) Controllability

(3) Stability

(4) Reliability

(5) Low right of way requirement

(6) Economy on overall basis

(7) Greater power per conductor

(8) Simple line construction

(9) No skin effect, charging current and less corona loss and interference

(10) Ground return can be used

(11) Cables can be worked at a higher voltage gradient

(12) May inter connect AC systems of different frequencies

(13) Low short circuit current on DC line.

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Principal Applications of DC Transmission

1. For cables crossing bodies of water wider than 32 km. [Ex Sweden-Got land link, a 20 MW, 100 KV DC single conductor submarine link to supply power to the island of Got land.]

2. For inter connecting AC systems having different frequencies or where

asynchronous operation is desired. 3. For transmitting large amounts of power over long distances by over head lines. 4. In congested urban areas where it is difficult to acquire right of way for overhead

lines and where lengths involved make AC cables impracticable. Economic Factors The cost per unit length of a DC is lower than that of an AC line of the same power capability with comparable reliability, but the cost of the terminal equipment of a DC line is much more than that in an AC. A graph is plotted between the cost of transmitting an amount of power by onemethod and the distance over which it is transmitted, below:

The vertical intercept of each curve is the cost of the terminal equipment alone. The slope of each curve is the cost per unit length of the line and of that accessory equipment which varies with length. The curve for AC transmission intersects that for DC transmission at an X axis which is the break even distance, Transmission by DC is cheaper than AC for distance above 500 km. Type of DC Links HVDC back to back link: This link is used to connect two AC grids, each AC grid can have its own load frequency control. Direction of power flow can be controlled by adjusting the characteristics of convertor valves. There is no increase in fault level and cascade trippings in the network are avoided. [Ex. Vindhyachal Back to Back HVDC link].

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Monopolar link: This links has one conductor, usually of negative polarity, and ground or sea return. Bipolar link: This link has two conductors one positive, the other negative. Each terminal has two convertors of equal rated voltages in series on the DC side. The neutral points (junction between convertors) are grounded at one or both ends. If both neutrals are grounded, the two poles can operate independently. Normally the operate at equal currents: then there is no ground current. In the event of fault on one conductor, the other conductor with ground return can carry upto half the rated load. Homopolar link: This links has two or more conductors all having the same polarity, usually negative, and always operate with ground return. In the event of a fault on one conductor, the entire convertor is available for connection to the remaining conductor or conductors, which, having some over load capability, can carry more than half of the rated power and perhaps whole rated power, at the expenses of increased line loss. In a Bipolar scheme reconnection of the whole convertor to one pole of the line is more complicated and is usually not feasible because of graded insulation. In this respect a Homopolar line is preferable to a Bipolar line in cases where continuous ground current is not objectionable. An additional advantage, through minor is less corona loss and negative polarity is preferable to have less radio interference.

HVDC back to back link

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Figure shows HVDC Bipolar system in which there are two poles one is negative and the other is positive. Each pole consists of one 12 pulse covnertor at both ends in which sending end will act as rectifier and receiving end will act as invertor. The 12 pulse convertor consists of two series connected 6 pulse bridges which are connected to two convertor transformers. The transformers are of Y/Y and Y/D to provide 30° phase shift for 12 pulse operation. HVDC Bipolar System Layout HVDC Rectifier and invertor station in HVDC Bipolar systems consists of following parts

1. AC switchyard 2. AC filter area 3. Convertor transformers 4. Valve hall and control room 5. DC switchyard and smoothing reactor 6. Electrical and mechanical ausiliaries

AC Switchyard The AC switchyard is generally at 400 KV or 760 KV voltage level corresponding to the standard of EHV/UHV transmission voltage. The AC yard is of one half breaker bus system. The advantage of one and half a breaker system is it permits use of only three breakers for two circuits. In one and half a breaker system the circuits one and two can take supply either from Bus I or Bus II, thus in the event of fault on any bus the supply is maintained in the circuits by unfaulty bus. Hence, high security against loss of supply.

One and Half Breaker Scheme

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The insulation coordination of the AC yard is correlated with that of DC yard and over voltages approaching from DC side. Metal oxide arrestors are used in AC yard and DC yard. The AC yard is designed in similar principles like usually EHV AC switchyards with following additional considerations:

- A large area on AC yard is covered by AC harmonic filter bank. - More space is provided for movement of large convertor transformers and

cranes. - No. of surge arrestors are provided at strategic locations in AC yard. - Protection and control of an eneterprise with valves and DC yard. The circuit breakers used in HVDC substation have preinsertion resistors to

reduce switching over voltage and to reduce large magnetic inrush current during switching of convertor transformers. AC Harmonic Filter Area A large portion of the area in AC yard is covered by AC filter bank. The filters are required to filter out harmonics generated due to the operation of 3 phase AC/DC conversion which generates kp± 1th harmonic on AC side, p is the integer and K is the

number of pulses of convertor valve. This is derived using fourier analysis. Harmonics in AC for 12 pulse system for which K = 12 are 1, 11, 13, 23, 25

th, 5

th

and 7th

harmonics are of 10% to 25% which are generated due to the formation of 12 pulse by series connection of sixpulse connection. Each filter bank has the following components. AC filter capacitor bank Reactor Resistor bank Current transformers Circuit breakers These AC harmonic filters are essential to reduce the harmonic contenet in the AC voltage within the limits. AC filter capacitor also provide the leading reactive power consumed by the convertor (shunt compensation). AC harmonic filters comprise RLC series circuit connected in shunt with the AC busbard. Separate branches are provided for predominant 5

th, 7

th, 11

th and 13

th Harmonic and a high phase filter for higher than

23rd

harmonic and above.

A C Harmonic Filter Circuit

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Reactive power demand and compensation :- The operation of the convertor requires a certain amount of reactive power. This is due to – -- The manner of controlling HVDC convertors introduces a phase shift between the

fundamentals of AC current and voltage. The magnitude of this phase shift is strongly dependant on the firing angle and in rectifier and extinction angle y in invertor.

-- The commutation process, in which the DC current is connected from one valve

to another, also introduces further displacement of the AC current. Covertor consumes reactive power both when it operates as rectifier as well as invertor. Besides the reactive power demand is also due to magnetizing current of convertor transformer. Considering normal valves of a (rectifier) firing angle and extinction angel y (Invertor) the reactive power demand usually in the range of 50% - 60% of the transmitted active power.

Convertor Reactive Power Demand

This reactive power in the range of 50% to 60% of the transmitted active power (each convertor station) is compensated by several ways depending on the quality of the connecting AC network. The different possibilities for suitable reactive power production are mentioned here. -- Ac filters -- Shunt capacitors -- Excessive reactive power from the AC network -- Static compensation -- Synchronous condensers When choosing reacting power generation equipment one must consider both economic and technical aspect. The least costly solution is shunt capacitor. However, if AC network is weak a synchronous condenser is required a static compensator.

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A C Voltage

Converter Transformer The convertor transformers are connected between the AC busbars and the converter valves. Converter transformers have AC supply on primary side and DC load of the valves on the secondary side. The secondary side has high voltage DC component in addition to the transformed AC voltage component. Thus the insulation of windings in specially designed. The converter transformer is fitted with on load tap changers. A twelve pulse conversion requires one star-star and star-deltatransformer to create a phase different of 30° in feeding AC power. The pulsation of direct voltage in one bridge are staggered with respect to the other giving a 12 pulse convertor. The following are four alternatives available for convertor transformer considering phase winding arrangement:

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The choice of convertor transformer is made after studying the complete transportation weight and project. The best choice of convertor transformer is considered as 3 winding single phase transformer. Valve Hall and Control Room The valve hall and control room are located between AC yard and DC yard. The valve hall houses quadruple thyristor valves, air core reactors, terminal bushings associated bus bars and surge arrestor. The control room building houses control panels for AC yard, DC yard and valves etc. in Bipolar HVDC substation there are two valve halls, each valve hall houses three quadruple valves. The control room is in between two valve halls. The valve hall is provided with uniform earthing mat in the flooring and uniform earthed screen in the wall and the roof. The screen protects the control circuits from the electromagnetic interference produced by the operation of thyristor valves. The valve hall is provided with air conditioning system. The temperature inside the valve hall is high due to valve losses and the lowest temperature of valve hall maintained is 10°C and the highest temperature of valve hall maintained is 55°C. The control room houses the following control panels:

-- protection, metering and control panels for AC yard, DC yard and convertor transformers.

-- control panels for valves -- PLC communication and tele control panels etc. -- monitoring panels. The auxiliary switchgear, low voltage switchgear, DC supply systems is generally

installed in a separate floor of the control room building. The convertor valves are either supported on the valve hall floor on insulator columns or are under hung from the roof by means of insulators. Thyristor Valves Since the individual thyristors has a limited voltage ratings nearly 7 KV, several thyristors are connected in series to achieve desired rated voltage. The assembly formed is called a thyristor valve. A thyristor valve for an HVDC convertor comprises of the following:

-- Several thyristors connected in series to achieve the required insulation level. Each thyristor has its associated thyristor control unit.

-- Snubber (voltage grading) circuit for equal distribution of voltage across

thyristors and protection of thyristors in the string. -- Cooling system to removal heat from the cathode silicon wager. In HVDC

system pure deionised water is circulated in a closed cycle to remove heat from heat sinks.

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A valve is made up of stacking four valve modules in a vertical formation is called a quadruple valve. The valve is supported on procelain insulator parts or is suspended vertically from the roof by means of glass fibre reinforced plastic rods.

A typical Bipolar twelve pulse convertor substation has two valve halls, one for each pole. Each valve hall houses three quadruple valves. The active part of thyristor is a semi conductor mono crystalline silicon wafer with a thickness of half a milli meter and an area in the range 8 to 60 cm². The wafer has been treated to obtain P-N-P-N with desired current and voltage properties. The junction temp. with stand capability is 100° to 125°C. the water cooled wafer has 45 cm² area and a threshold voltage drop of 0.8 to 1.0 V. the thyristors are mounted on heat sinks. The modules are cooled in parallel with two cooling circuits in each module giving equal coolings. As the water should be insulating a special water processing unit is installed to deionise the water to limit the amount of oxygen in the water. The valve losses are about 0.5 percent of DC power transfer.

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Cooling System for Water Cooled Thyristor Valve

Triggering of Thyristors A thyristor needs positive pulse at the gate to start conducting. Such triggering pulses are necessary for each thyristor and for each cycle of the AC wage throughout the service life. In normal control applications, the triggering pulses from control panel to the thyristor gate are via insulated copper control cables. In case of HVDC valve this method is not suitable because individual thyristors are at higher potential and galvanic connection between high potential and control devices at nearly ground potential is not practicable. This problem is solved by using optical firing for triggering the thyristors.

Optical Firing System

In Optical firing system the electrical pulses at the source are converted to light signal and transmitted through fibre optical cables and then converted to electrical signal units in the thyristor control unit for triggering the thyristors. DC Yard The DC yard has the following essential equipment:-

- DC smoothing reactor - DC filters - DC busbars and isolators, earthing switches, current transducers, voltage

dividers, surge arrestors. - Switchgear for switching from ground return to the metallic return.

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Smoothing Reactor: HVDC smoothing reactors of a 0.4 henrys to 1 henry are

generally used. There are oil filled reactors. Smoothing reactor is connected in series with the convertor bridges in order to reduce the current harmonics in the direct current and to reduce valve stresses due transients such as DC line faults and commutation failures by limiting the fault current and the rate of rise current. A DC smoothing reactor is located on the low voltage side and air core reactors on the line side of the convertors. The later to limit any steep front surger entering the station from the DC side. Additional air core reactors are installed in each phase on the AC side to reduce the rate of rise of current during thyristor turn on. DC Harmonic Filters Using Fourier analysis we can evaluate the harmonicas on DC side for 12 pulse connector which is “Kp”, p is the integer and K is the pulse number. For 12 pulse system the harmonics generated on DC side are 0, 12, 24, etc. a high pulse DC filter turned to 12

th harmonic is usually provided on DC side.

Single Line Dia of Single Pole Giving Details of DC Yard 1. Surge arrestor 6. Direct voltage measuring device 2. Converter trfr. 7. DC filter 3. Air core reactor 8. Current measuring transducer 4. Thyristor valve 9. DC line 5. Smoothing reactor 10. Electrode line

The measuring equipment i.e. a voltage divider, current measuring transducer and current transformer, provide the necessary input signals for the control and protection circuits. Earth Return In monopolar configuration, the return path is usually through earth or sea. Earth return or sea return reduces the cost of transmission system.

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In Bipolar system the normal power flow is through pole conductors and only negligible out of balance current flow through earth. The mid points of convertors at both ends are earthed. In monopolar the return is through earth. The earth electrode station is usually built 10 to 25 km from main HVDC substation to avoid galvanic corrosion of pipes, foundation structures, cable theatres, earthing material due to cathodic corrosion. The connection between mid point of convertor valve and a distant earth electrode is an electrode line. Electrode line is insulated from earth and connected to the earth electrode.

1. Neutral Bus Switch 2. Switch for Metallic to Ground Transfer 3. HVDC Breaker of Ground to Metallic Return Transfer DC circuit breaking is difficult due to non-availability of current zero in the DC.

Hence links do not have any provision of DC circuit breakers. HVDC links do not have parallel lines and T off lines due to lack of HVDC circuit breaker. HVDC circuit breaker using artificial current zero is produced by discharging a precharged capacitor bank through the breaker contacts has been developed but it is complex and not economical.

Metallic Return: In the case of fault on a pole the power transfer taken place in

Monopolar mode using ground return in addition to this the line of the pole which is out of order can be used for return path. This type of current return is called Metallic return. Electrical and Mechanical Auxiliaries

1. Cooling water system for convertor valves 2. Ventilation and air conditioning system for valve hall, control room etc. 3. Auxiliary low voltage DC supply for protection and control. 4. Auxiliary low voltage and medium high voltage AC supply for auxiliaries. 5. Station lighting system 6. Fire fighting system 7. Internal telephone system 8. Power line carrier communication system (PLCC)

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Control of A HVDC Link In a direct current control of the HVDC link, the transmitted current is given by Id = Vd1-Vd2 R Id is changed by changing the difference between Vd1 and Vd2. The methods available for changing the terminal voltage difference:

1. Change the tap position of convertor transformer. 2. Change the delay angle of thyristors.

The tap changers are slow. The tap changing issued for slower variation of DC

voltage. For rapid variation, the delay angle is controlled. Angle a must be kept near zero to reduce KVAV demand of the convertor. In practice the valve between 15° and 18° is chosen to ensure that valves of the bridges are ignited at the same instant of time and secondary to allow small voltage margin for an immediate small increase in power. Both tap changer control as well as delay angle control is used at both stations. But delay angle control is used initially for rapid variation of voltages. This is followed by tap changer control. The fig. Gives basic control system of control in HVDC.

Each convertor has a closed loop current control system . the input controlling parameter to this system is called current order. Thus, the convertor tries to adjust the DC voltage until there is no difference between current order and current response (DC current) or the maximum voltage is reached when fixing at minimum delay angle.

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On the other hand if the current response is larger than the current order the direct voltage is correspondingly decreased. The decreasing action is limited when fixing at the least permitted commutation margin invertor operation.

Ud / ID Characteristics of the Converter

By giving the two convertors different current orders, the convertor with highest current order will output a positive voltage and the other a negative voltage. Thus the convertor with highest current order will operate as rectifier and the other as inverter.

Further, the converter which can create the highest voltage will also control the

current. Normally the control angle may be smaller in rectifier (a = a min) than in Inverter (y = y min) and the rectifier will control the current. The other convertor, consequently normally the inverter, controls the DC voltage with constant control angle y = y min. In simple words the terminal with the highest voltage limit will control the current, and the terminal with the lowest voltage limit will determine the voltage.

Normal Operation Rectifier Controls the Current

Normal Operation In practice the controls are adjusted so that under normal conditions the rectifier controls the current and the inverter controls the voltage. This is arranged in the following way. Both terminals are basically given the same current order. the power director is then established by applying a small additional negative current order to the converter which is intended to be run as an inverter. The effective current order will thus lower for this converter, which according to the foregoing will operate as inverter. The negative current order contribution is called the “current margin” and is an important feature for the operation of an HVDC link.

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In fact that the current order of the inverter is lower than that of the rectifier

means that the inverter is forced to fire at the highest permitted firing angle. For this mode of operation the control system keeps the margin of commutation’ constant, i.e. at y=yo and accordingly the inverter defines the voltage on the DC line. The direct voltage is kept at the desired reference in the inverter by adjustments of tap changers of the converter transformers. In the rectifier the firing angle is measured and maintained at approximately 15° by making adjustment to the rectifier transformer tap changers. The current is kept constant by the current control loop. Sudden changes in transmission parameters will of course lead to immediate changes in firing angle in an effort to keep the transmitted power constant when steady state condition prevail again the tap change control will bring the firing angle back to within the region of a =15° Operation during disturbance: If suddenly the AC voltage in the rectifier drops, then also the DC voltage drops to such a value that the horizontal part of rectifier Vd/Id characteristics fall below that os the inverter. In the inverter the AC voltage still is normal and the current will decrease. When the current has dropped to a value equal to the current order to the inverter, the inverter will start to reduce its voltage by increasing its y so as to maintain this current level. The inverter takes over control of the current and the inverter voltage will be reduced to a value equal to the voltage which is defined by the rectifier with reduced Vd and working at (a = x min). The current will now be less than the original current transmitted between the terminals by an amount equal to the current margin. The firing angle of the rectifier will be at its extreme minimum since this terminal is trying to increase the DC voltage so as to increase DC current, whereas in the inverter the firing angle has now been decreased in order to control current. This new point B is shown in figure below for the case of inverter controlling the current.

Inverter controls the current

HVDC back to back station is similar to HVDC Bipole station. The only differences are in back to back station. Both rectifier and inverter stations are at one place and there is no DC line between these two and earth electrode station is not required as the distance between rectifier and inverter is nothing.

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