COOLING SYSTEM OF HAENAM-JEJU HVDC SYSTEM

Joo-Sik Kwak, Chan-ki Kim, Bong-Eon Koh

Korea Electric Power Research Institute

ABSTRACT . . . . .

This.'paper deals with ,the water cooling system of Haenam-Jeju HVDC in Korea. It is generally accepted that water' is a very effective medium to remove heat losses from any.type of equipment. Because.of this benefits the water cooling method is used in HVDC. The water cooling system consists of a heat exchanger, circulation pump and a connecting pipe. According to temperature level, thyristor junction temperature is maintained within preset range by controlling the fan of exchanger. In this paper, the water cooling system of HVDC system is analyzed and estimated.

1. INTRODUCTION

Electric power conversion is based on the flow and control of electric currents. Whenever electric current flows through electronic component parts, the heat is generated by the flow of electric currents in electronic component parts such as resistors, diodes, transistors, and transformers.

.................... .................

. . L ...................... ................................ i :.... ........... i Source Cnnver ter Load

Fig.1. Diagram of High Power Converter

Heat always flows from the hot area to the cool area. Since the electronic components will usually be the hottest spots in an electronic equipment is, therefore, the removal of intemally generated heat by providing a good heat flow path form the heat sources to an ultimate sink, which is often the surrounding ambient air. There are three basic methods by which heat can be transferred: radiation, conduction, and convection. Among this methods convection is the transfer of heat by the mixing action of fluids. When the mixing is due entirely to temperature differences within the fluids, resulting in different densities,

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. 1.

the 'action is known' as natural 'convection. When the mixing is produced by mechanical means, such as a fan, the action is known as forced convection. Natural convection is not very great because it depends on the density change in the fluid. Therefore any small obstacle or resistance in the flow path. Most electronic systems make use of all three basic methods of heat transfer to some extent, even though one method may dominate the design, because the greatest amount of heat is picked up by forced convection as the cooling air passes over the individual electronic components. The basic principle of heatsink design is to transfer the heat from the thyristor to the cooling area of the fins as efficiently as possible. Although other materials can be used, the majority are made of flat plate aluminum or extruded sections of the same material. In general, a design compromise is reached between the required heatsink cross-sectional shape and the its dies. This paper deals with the practical methods of cooling which have been tried, from air by fan-blown. in high power converter system. The construction of thyristor equipment has always been very dependent on the cooling of the thyristors, and so this subject also is include here. Construction covers a wide area from a single thyristorkooler unit assembly, to groups thyristors with their cooling facilities and protection, to complete cubicle- enclosed units containing from 3 to 300 thyristor at a time with all the necessary cooling firing control, and protection components to make them fully operational. A thyristor will only operate correctly, ie. block forward and reverse voltage and be controllable by gate current, if its junction temperature is kept below a critical level. Above this temperature it will not be possible to control it by gate firing and it will not accept forward voltage without breaking over. This critical junction temperature usually lies between 120 "C and 150 "C

. .

2. NORMAL HEATSINK DESIGN OF HIGH POWER CONVERTER

The heat dissipated in the vicinity of a junction flows to the case and then to the ambient through the externally mounted heat sink, causing a rise in the junction temperature. The maximum junction temperature of a device should be limited because of its adverse effect on leakage current, breakover, tum-off time, thermal stability,

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and long-term reliability of the device. Fig.2 shows the thermal equivalent circuit of a thyristor mounted on a heat sink.

I : I IV : (_..-_..__.___...__._~_..-__.__.-___.___..~_-___I ~ _____..-__-__...-_._______._____ ,...........__._..... i ._.. i.& .....__...__......__, . i . . . . . I

: : i ; ; @ U

! ! . .

Fig.2. Thermal equivalent circuit of a thyristor mounted on a heat sink.

Fig.2 shows the thermal equivalent circuit of a thyristor mounted on a heat sink. For sustained power dissipation Q at the junction, the junction temperature can be calculated as:

R,=h,A, L, (3)

In equation (I) , (2), (3) and (4), and is the ambient temperature and junction temperature and , and represent a contact surface from junction to case, case to heatsink and sink to ambient, and sink to ambient, respectively., represent a thickness from junction to case, case to heatsink., and is the eficiency of, and. the is the coefficient of convection heat transfer. is the thermal conductivity from junction to case, is the thermal conductivity from case to heatsink. From equation (1) it is evident that for a limited, Q can be increased by reducing. This means that a more effrcient cooling system will increase the power dissipation capability of a device. An infinite heat sink will result if are reduced to zero and the case temperature is locked to the fixed ambient temperature. Resistance is a complex summation of surface and inter stitial(gap) resistance known as the mounting or interface thermal resistance. This gap at the interface is caused by the uneven surfaces being joined together. The resultant air gap causes thermal resistance that can be reduced through the use of special compounds

designed for just this purpose. Resistance may also include the overall resistance of insulators placed between case and heatsink; may be minimized by following strict assembly procedures. Mounting force, which is relation to shown in Fig.2 and Equation (3), is necessary to establish a good electrical and thermal contact. It is very important that mounting force stay within the min. and max. range even under worst case condition, ie. over the whole operating temperature range. Too low a mounting force results in an increase in thermal resistance and, Articulately with high currents, damage of the dry interfaces within the housing and possible degradation of the device. Exceeding mounting force leads to an increased mechanical stress on the silicon wafer, particularly in applications with frequent thermal cycles. This reduced the life-expectancy of the device and can lead to premature wear-out. Resistance is very high relative to the total resistance and, because it is a parallel resistance path, is usually ignored. The variable for the designer is the thermal resistance from heat sink to ambient. Controlling with Q fixed determines the sink temperature rise above ambient in the heat path. It can shift all the other temperature up or down. Changing also vanes the overall thermal resistance, which, in tum determines to what extent the input power Q could be increased while not exceeding a specified junction temperature. From Fig.2, the performance of an air-cooled heatsink will depend on follow factors:

1. The surface exposed to the cooling air 2. The design of the heatsink, ie. It's efficiency in transferring the heat from the thynstor to the dissipating surface. 3. The heatsink material. 4. The size of the thyristor surface in contact with the heatsink. 5 . The position of the thyristor on the heatsink. 6 . The ambient air temperature. 7. The volumetric flow of air over the fin surface. 8. The nature of the air flow, ie. laminar or turbulent flow.

In steady state a thermal capacity may be ignored but in transient state or in heatsink test not ignored.

3. WATER COOLING SYSTEM

3.1 Liquid Cooling Heatsink Cooling by liquid can be much more efficient than more air cooling but it brings with it the necessity for inter connecting pipes and pumps and the potential problems of electrolytic corrosion, blockage, and possibly freezing. More compact design can be produced using liquid cooling and the heat can be taken well away from thyristor assembly. Examples of the methods employed are :

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1. Mounting the thysristor on a individual water-cooled block which is then interconnected with others by flexible pipes.

2. Mounting many thyristors on one common water- cooled structure

3. Direct immersion of thyristors into the cooling water

Among the above three methods, the third one is not applicable in case of water. Because the second method, except that is simple to implement, is difficult to maintain and to operate efficiently, the system like as the first thing is used widely. As the double-sided type device can be more effective in cooling, power devices are configured individually or stacked such as Fig.3. Each devices can be connected in series or parallel.

n n n n n n n

Thy* a) Stack Type of Cooling System

Thy. b) Individual Type of Cooling System

Fig.3. Water-cooling System (B.C:Bus- Bar Connection, H:Heatsink, Thy.:Thyristor).

In Fig.5, because in series connected type whenever the coolant pass through the heatsink its temperature goes higher and higher, the cooling efficiency tend to be poor at end of the series. At malhnction it is easy to find out the blockage while in the parallel type it is difficult for repair. With hundreds of devices, high power converter has a mixed configuration that wider coolant pipes are connected in parallel and narrow pipes are series connected at near device.

B.C -

rtn -. Flow output ~ . r iny. n ~ n n f

c .P Flow input

a) Coolant Pipes of Stack Type Connected in series. Flow input -

Flow output 7 Manifold b)

b) Coolant Pipes of Stack Type- Connected in Parallel

Fig.4. Pipe connection of stack type - cooling system (B.C: Bus-Bar Connection, H : Heatsink, Thy. : Thyristor).

3.2 Peripheral Equipment of Water Cooling System Bulk water cooling system is consisted of something like that of Fig.6.

3.2.1 Expansion Tank This provides the main reservoir of coolant to make up

loss due to minor leaks in the system It is also used to allow for the expansion and contraction of the coolant as its temperature changes and to provide the system static pressure which is maintained by a nitrogen bottle.

Fig.5. system

MaIn Pump

Schematic diagram

FUter

of single-circuit water cooling

3.2.2 Main Pump

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The main pump circulates the coolant. Generally there are two main circulating pumps for each cooling system. The pumps are hydraulically in parallel and only one is in operation at any time while the second one is in standby. An automatic changeover of the pumps is performed each time the flow drops under a preset rate as monitored by the flow meters on the return pipe from the thyristor valve.

3.2.3 Heat ExchanPer

and the principle is same as the forced air cooling system. This cool down the coolant flowing through the pipe

3.2.4 De-Ionizer

heatsinks it must removed by de-ionizer. Because the ion in the coolant can corrode the pipe and

3.2.5 Main Filter- A full flow filter is included in the main cooling circuit

before the thyristor valves to prevent debris collected in the system from entering the thyristor valves. The thyristir valve heatsinks must have clear flow ways to maintain their efficiency. Cooling system is a closed system which does not generate debris during normal operation. Extreme care should be taken not to introduce debris or contamination as part of normal maintenance.

3.3 Control of Water Cooling System Water cooling system have to control not to approach the dew point. As air cooling system use the air alone as the coolant ance the surface of cooler hardly be most. But in case water cooling temperater difference between the air and the coolant can make dew in the system. The wetted surface can be hamrful to the power converter by degrading the cooling effieciency and I or lowering electric isolation level.

K Td = - (5 ) (1 I x - 1)

Formal for dew point control is

log( RH / 100) Tu (6) X = +-

17.27 Ta+K

3.4 Comparison of Liquid Coolants Water can have a high speed of flow and can be very effective in cooling but it can make and does, cause electrolytic corrosion and it can freeze. The electrolyk action is caused by the fact that thyristors in most circuits will be at different potentials and current flow through the water can occur. Distilled and de-ionized water may be used and water circuits will be restricted to use of compatible metal, copper, brass, gunmetal, etc. The possibility of freezing results in the use of glycol, plus various inhibitors to prevent further corrosion. Oil is a

more satisfactory medium but it more viscous and its cooling ability is less than water. It does not allow electrolytic currents to flow and it does not freeze. It is normally, however, inflammable and this restricts its use. The liquid will increase in temperature to take the heat away and the following formulae will give a guide to the temperature rise which must be followed for :

Watts 70 x Iiteres I min .flow

Water ( K C) =

(7)

watts 28 . 6x liters I min .flow

Oil ( K C ) = (8)

4. WATER COOLING SYSTEM OF HVDC TRANSMISSION

4.1 Thermal Loss of Power Converter For loss calculation of bulk power converter, loss of snubber circuit should be added to the loss of the cooling system and others. Fig.6 shows the valve arrangement of HVDC converter. Loss of the as follows:

Conduction Loss - Wth

w,, = $[ v, + RI( 31 Turn on Loss -Won

W O N = E , , x f

valve can be calculated by

(9)

EGVI mean coefficient for forward gate current. Generally turn-on loss less than 10 % of that of conduction loss and more detail value are related to system frequency(f), overlap angle( mu), and firing angle(a1pha).

Fig.6. Valve arrangement of HVDC system

1003 ISIE 2001, Pusan, KOREA

one-sixth heat generated by each pole. The design of Haenam-Jeju cooling system link is based on the concept Loss by DC Voltage

} { 2 p + s i n 2 a -sin( 2 a + 2 p ) ) I (11) 7 3 m ( 2 - m ) -(r+

Loss of Damping Resistor, WLF 2 2R 4~ Js+f im'

w w = J 7 ; 2 n f c - $ [ 7 j - - 2 - 8

+ (6 mz - 12m - 7 ) 4 + (1- sin 2a

7 3 m 3312 ')sin( 2 a + 2 p ) +(:+-+- 4 32

4 8 4 32

(12) ~5;;;*,' ) cos2a+-cos (2a+2p) ] G -(-+-

16 8 16

Loss of Damping Capacitor

w, = ~ ~ ~ ~ I , i n ' a + s i n ' c a + p ~ ~ (13)

Loss of turn-off

Hysteresis Loss of saturation reactor

The total loss of thyristor stack can be represented as follows:

WT= W T H + W O N + W D G + w O P + W S R

(16) Here, w m = w K + w, + w, 4.2 Configuration of Thyristor Stack

Fig. 7 shows the cooling circuit of HVDC thyristor tier. H means the heat sink, F means the outlet pipe and R means the inlet pipe. Denoted as H, heat, accompanied by currents flows through damping reactor, snubber resistor, is cooled down with thyristor contacted to heatsink. Inside the heatsink is shaped of spiral to maximize the cooling ability. Fig.8 shows the water-cooled quadrivalve of thyristor valve assemble of Haean-Jeju HVDC converter. Fig.9 shows the heat exchanger where water in the pipes is cooled down. Each pole of the HVDC system have six heat exchanger. One heat exchanger is designed to take

of N +I (50% X 2 + 50%).

-+ -b -b

I

R

Fig.7. Layout of cooling circuitry within tier assembly.

Fig.8. Water -cooling system of thyristor valve assemble.

Heat Exchanger

Fig.9. Layout of heat exchanger.

5. ESTIMATION OF HAENAM-JEJU HVDC COOLING SYSTEM

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5.1. Thermal Losses of Thyristor Valve The specification of thyristor valve is summarized in

table 1. The calculated thermal output is listed in table 2 with respected to formulae mentioned above.

W-Sq l.l[kW] Pole Loss

Table 1. Specification of HVDC Valve

W-T I 82.8[kW 993.756[kW]

~1 35.l[kW

f 6O[Hz] Vt 1[V] Qr 3000[mQ]

Table 2. Thermal Loss of Valve

di/dt I.O[A/ms]

Idc I840[A] I Vdc I 180[kV] I VL I 154[kV]

M W g l

Thyristor Average On-Statecurrent 3 600 [A]

5.2. Cooling Capability of Exchanger In Fig. 10, water-cooling system can be modeled as thermal resistance on the basis of the equations described in chapter 2. The model in Fig.10 describes thermal model that the generated heat, Q out of thyristor junction is spread to outside air. The model is used to control the junction temperature, Tj, not to exceed the allowed maximum value, where RHZA is controlled.

RJC

TJ Tc THI Tco0i T m Q

Fig. 10. Thermal resistance model of water-cooling system.

kNU h A = 7

Cooling system have to be designed to ensure the normal operation at worst situation. In this paper, the cooling ability of Haenam-Jeju is estimated under the worst condition of hot weather in summer. According to estimation, the HVDC water cooling can cool down to the temperature of coolant 48.5 C at 37.5 C of atmosphere temperature when the converter is operating at fill load. But at this condition if the flow speed of coolant is slown down to 2000 [Wmin] or thermal conductivity is less than 1.2 [microkm], HVDC converter can not afford to transfer full load and thyristor junction temperature can go up to 55 c.

R , = v , UmD

kw NU wv hw= D

6. CONCULSUION

Water cooling system is adopted for high power converter such HVDC transmission because of compactness and superior efficiency. The cooling system of Haenam-Jeju HVDC link in Korea is modeled mathematically and its ability is estimated under the operating conditions.

7. REFERENCE

1) H.P.Lips,"Water Cooling of HVDC Thyristor valves", IEEE Tran. on Power Delivery, Vo1.9, No.4, Oct. 1994, pp.1830-1837. 2) P.O. Jackson, etc.3, "Corrosion in HVDC valve cooling systems", IEEE Tran. on Power Delivery, Vol. 12, No. 2, April, 1997, pp. 1049-1052. [3] Cooper, M.G., Mikic, B.B. and Yovanovich, M.M.,"Thermal Contact Conductances",International Joumal of Heat and Mass Transfer, Vol. 12, 1969, pp. 279-300.

1005 ISIE 2001, Pusan, KOREA