Bài giảng Trang thiết bị điện trên hệ thống truyền tải điện - Nguyễn Hữu Phúc

uite uncommon. The valve windings are exposed to AC and DC dielectric stress and therefore a special insulation assembly is necessary. Furthermore, special lead systems connecting the turrets and windings have to be installed in order to withstand the DC voltage of rectifier. Additionally, the load current contains harmonic components of considerable energy resulting in higher losses and increased noise. Above all, special bushings are necessary for the valve side to access upper and lower winding terminals of each system from outside. Conclusively, two identical bushings are installed for star or delta system. Phase Shifting Transformers  Phase shifting transformers are used to control the phase angle across the transformer  Since power flow through the transformer depends upon phase angle, this allows the transformer to regulate the power flow through the transformer  Phase shifters can be used to prevent inadvertent "loop flow" and to prevent line overloads. Phase Shifter 345.00 kV 341.87 kV 283.9 MW 283.9 MW 39.0 Mvar 6.2 Mvar 500 MW slack 164 Mvar 500 MW 100 Mvar Phase Shifting Transformer 216.3 MW 216.3 MW 125.0 Mvar 0.0 deg 93.8 Mvar 1.05000 tap ComED Phase Shifter Display Phase shifting transformer Voltage angle adjustment. UU  P=12 sinδ+α X 2 U1 UU  Q=12 cosδ+α XX Phase shifting transformer • Allows for some control over active power flows • Mechanically switched ==> minutes UU  P= 12sinδ+α X+XPST Phase shifting transformer (II) Principles • Injection of a voltage in quadrature of the phase voltage • One active part or two active parts Asymmetric Symmetric D U 25 ° ==> 10 % voltage rise ==> 40 kV @ 400 kV Power Flow in Parallel Circuits Phase Shifting XFMR Fig. A/ Fig.B/ • A Phase Shifting XFMR schematic is shown in Fig. B/ operating in a three-phase power system. • In practice, the configuration in Fig .C/ is preferred because it can keep identical modules while allowing Fig. C/ phase adjustment between input voltages and output voltages. It is the type of construction in use for transformers in France. >> In Fig. is shown a Phase Shifting XFMR of 65 MVA installed in Sainte-Cecile substation (France). It is the first of its type installed in 63 kV network and allows forcing more power flow on the line Sainte-Cecile-Vaison and, at the same time, relieving the other two lines Chateauneuf- du-Rhone– Montmartel and specially, the line Montmartel- Valaurie, which are both near their capacity limits. The numbers shown in Fig. are the percent of the line power flow capacity, without parentheses before XFMR installation and with parentheses after. >> In 2009, in France there were 6 more XFMRs existing in HV lines, 4 of of 225 kV in Pyrenees region, 1 of 400 kV in Savoie and 1 of 90 kV in Pas de Calais. The characteristics of these XFMRs are as follows:- site of La Paz: 400kV - Capacity: 1081 MVA- phase shifting adjustment between 5.5 ° and – 14.5 ° by means of on-load tap changers of 33 positions. Phase shifting transformer (III) One active part • Series voltage injection • In quadrature to the phase voltage • One active part: low power/low voltage (high shortcircuit currents at low angle) 3' 1' 2' 3 3' Voltages over coils on the same transformer leg are in phase 1 2 1 2 3 Phase shifting transformer- Regulating • Changing injected voltage: – Tap changing transformer – Slow changing of tap position: ½ min • Control of the injected voltage: – Centrally controlled calculations – Updates every 15 minutes – Often remote controlled – Can be integrated in WAMS/WACS system Phase shifter influence Base case 1018 MW 500 MW A B G G Slack bus 500 MW 1000 MW 344.3 MW 173.5 MW 170.4 MW Flow of A to B gets C distributed according to G 800 MW the impedances losses: 18 MW 800 MW Phase shifter influence 1 phase shifter placed 1024.6 MW 500 MW A B G G 500 MW 1000 MW 491.8 MW 15 ° 32.8 MW 33 MW C Flow of A to B is taken G 800 MW mostly by line A-B losses: 24.6 MW 800 MW Phase shifter influence 1 phase shifter placed: overcompensation 1034 MW 500 MW A B G G 500 MW 1000 MW 580 MW 30 ° 42.3 MW 41.4 MW C Overcompensation G causes a circulation 800 MW current 800 MW losses: 34 MW Phase shifter influence 2 phase shifters: cancelling 1052.3 MW 500 MW A B G G 500 MW15 ° 1000 MW 313.9 MW 15 ° 221 MW 238.4 MW C The phase shifting G transformers can 800 MW cancel their effects losses: 52.3 MW 800 MW Phase shifter influence 2 phase shifters: cancelling FLOWS relative to base case (no PS) 1052.3 MW 500 MW A B G G 500 MW15 ° -8.8 % 1000 MW 313.9 MW 15 ° 221 MW 238.4 MW +14.6 % +18.8 % When badly C controlled, little G influence on flows, Additional losses: 800 MW more on losses + 34.4 MW 800 MW Phase shifter influence 2 phase shifters: fighting FLOWS relative to base case (no PS) 10541052.3 MW MW 500 MW A B G G 500 MW 30 -°8.8 % 1000 MW 259.7 MW 15 313.9 MW 15 ° ° 221 MW 238.4 MW 259.7 MW 294.3 MW +14.6 % +18.8 % The Whenphase badly shifting C controlled, little G transformersinfluence on flows, can 800 MW more`fight' on losses Additionallosses: 54 losses: 800 MW + 34.4MW MW Phase shifter influence 2 phase shifters: fighting FLOWS relative to base case (no PS) 1054 MW 500 MW A B G G 500 MW15 ° -24.5 % 1000 MW 259.7 MW 30 ° 259.7 MW 294.3 MW +28 % +35 % C The phase shifting G transformers can 800 MW `fight' losses: 54 MW 800 MW Phase shifters in Belgium • Zandvliet – Zandvliet • Meerhout – Maasbracht (NL) • Gramme – Maasbracht (NL) – 400 kV – +/- 25 ° no load – 1400 MVA – 1.5 ° step (34 steps) • Chooz (F) – Monceau B – 220/150 kV – +10/-10 * 1.5% V (21 steps) – +10/-10 * 1,2° (21 steps) – 400 MVA Transformer Mechanical Construction three-leg transformer The magnetic return paths of the three cores can be dropped, which results in the usual type of three-phase transformers. One primary and one secondary winding of a phase is arranged on any leg. a/ shell-type transformer b/ core-type transformers Five-leg transformers are used for high low overall height high magnetic leakage low magnetic leakage power applications (low overall height). windings: low-leakage models •joints: air gaps are to be avoided a/ cylindrical winding b/ double cylindrical winding c/ disc winding/sandwich winding •core cross sections: adaption to circle VA kVA MVA Tank Heating of Three-Legged Core Transformers Zero-sequence flux in three-legged core transformers enters the tank and the air and oil space. ■ Zero-sequence fluxes will ―escape‖ the core on three-legged core designs (the most popular design for utility distribution substation transformers). The 3d, 9th, 15th, etc., harmonics are predominantly zero-sequence. Therefore, if the winding connections are proper to allow zero-sequence current flow, these harmonic fluxes can cause additional heating in the tanks, core clamps, etc., that would not necessarily be found under balanced three-phase or single-phase tests. Tertiary Winding of Transformer 1. What is tertiary winding? What is Three Winding Transformer ? In some high rating transformer, one winding, in addition to its primary and secondary winding, is used. This additional winding, apart from primary and secondary windings, is known as Tertiary Winding of Transformer. Because of this third winding, the transformer is called Three Winding Transformer or 3 Winding Transformer. 2. Advantages of using tertiary winding in transformer Tertiary Winding is provided in Electrical Power Transformer to meet one or more of the following requirements: i. It reduces the unbalancing in the primary due to unbalancing in three phase load ii. It redistributed the flow of fault current iii. Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer. iv. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral. 3. Stabilization by tertiary winding of transformer In star - star transformer comprising three single units or a single unit with 5 limb core offers high impedance to the flow of unbalanced load between the line and neutral. This is because, in both of these transformers, there is very low reluctance return path of unbalanced flux. If any transformer has N - turns in winding and reluctance of the magnetic path is RL, then, mmf = N.I = ΦRL ....... (1) Where I and Φ are current and flux in the transformer. Again, induced voltage V = 4.44ΦfN ⇒ V ∝ Φ ⇒ Φ = K.V (Where K is constant)....... (2) Now, from equation (1) & (2) , it can be rewritten as, N.I = K.V.RL ⇒ V/I = N/(K.RL) ⇒ Z = N/(K.RL) ⇒ Z ∝ 1/RL From this above mathemetical expression it is found that impedance is inversely proportional to reluctance. The impedance offered by the return path of unbalanced load current, is very high where very low reluctance return path is provided for unbalanced flux. Three Winding Transformer In other words, very high impedance to the flow of unbalanced current in 3 phase system between line and neutral. Any unbalanced current in three phase system can be divided in to three sets of components like wise positive sequence, negative sequence and zero sequence components. The zero sequence current actually co-phasal current in three lines. If value of co-phasial current in each line is Io, then total current flows through the neutral of secondary side of transformer is In = 3.Io. This current can not be balanced by primary current as the zero sequence current can not flow through the isolated neutral star connected primary. Hence the said current in the secondary side set up a magnetic flux in the core. Low reluctance path is available for the zero sequence flux in a bank of single phase units and in the 5 limb core, consequently the impedance offered to the zero sequence current is very high. The delta connected tertiary winding of transformer permits the circulation of zero sequence current in it. This circulating current in this delta winding balances the zero sequence component of unbalanced load, hence prevent unnecessary development of unbalanced zero sequence flux in the transformer core. In few words it can be said that, placement of tertiary winding in star - star - neutral transformer considerably reduces the zero sequence impedance of transformer. 4. Rating of tertiary winding of transformer Rating of tertiary winding of transformer depends upon its use. If it has to supply additional load, its winding cross - section and design philosophy is decided as per load and three phase dead short circuit on its terminal with power flow from both sides of HV & MV. In case it is to be provided for stabilizing purpose only, its cross - section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line - to - ground fault being the most onerous Three Winding Transformers 5. What is need of tertiary winding in transformers? Tertiary windings generally serve one of two purposes. 1. One purpose may simply be to provide an additional terminal, often at a different voltage. While the use of ‗three winding transformers' is relatively unusual, it can be a powerful tool is system architecture. 2. The other objective for a tertiary winding is to provide a closed-circuit path for zero-sequence currents/fluxes. That is most typically the case with transformers that, for system reasons, must be connected wye-wye (star-star for those in the IEC world). With a wye-wye transformer, the only path for zero-sequence flux is the tank of a transformer (and if the three-phase bank is comprised of three single-phase units, even that does not exist). The tank is a high-impedance path, so the result is that the zero sequence impedance seen looking into the transformer is very high. Also, the circulation of zero sequence current through the tank will cause the tank to heat and lead to paint failure. A tertiary winding presents a low impedance path to zero sequence currents, thereby reducing the zero sequence impedance presented to the outside world, while avoiding the problem of tank heating. Maintenance of Power Transformer Instruction Manual Weekly Check Please conduct weekly check in operating condition once per 1 or 2 weeks. It is to confirm the operating condition of the transformer visually, find the abnormality in early stages and prevent accidents. Please use binoculars when you check the top of the transformer visually. When the control criteria are not met, please contact TMT&D. Special inspection is necessary in some cases Inspection point Inspection item Inspection procedure Control criterion and remedy Oil leak Check if there is oil leak from No oil leak the gasket or the weld. General Rust, damage Check if there is rust or damage. No marked rust or damage Bolts and nuts Check if they are not missing. No missing Transformer Operating sound, Check if there is abnormal noise No abnormal noise or foreign odor body foreign odor or foreign order. Gauges in Internal moisture Visually check if there is moisture No moisture condensation general condensation condensation inside the glass of gauges. Oil temperature Temperature Check and record temperature The indication should be much indicator indication indication as well as load factor the same as the past measured and outside air temperature of data (in condition of approxi- the transformer. mate load factor and outside air temperature). Winding Temperature Check and record temperature The indication should be much temperature indication indication as well as load factor the same as the past measured indicator and oil temperature (temperature data (in condition of approxi- indication of the oil temperature indicator)of the mate load factor and oil tempe- transformer. rature). Resistance bulb Temperature Check temperature indication of The difference of temperature For oil temperature indication the resistance bulb and the oil temperature indicator. indications between the resistance bulb and the oil temperature indicator should be 5°C or less. Winding Temperature [In case that winding temperature The difference of temperature temperature indication indicator is attached] indications between the winding transmitter Check temperature indication of temperature transmitter and the winding the winding temperature transmitter and the winding temperature temperature indicator. indicator should be 5°C or less. [In case that winding temperature The indication should be much indicator is not attached] the same as the past measured Check and record temperature data (in condition of approxi- indication as well as load factor mate load factor and oil and oil temperature (temperature temperature). indication of the oil temperature indicator)of the transformer. Oil level Oil level Check oil level indication as well The oil level indication should gauge (except for indication as oil temperature (temperature be within the allowable range one for bushing) indication of the oil temperature indicator). of the oil level monitoring curve. Inspection point Inspection item Inspection procedure Control criterion and remedy Degree of Check the color of the desiccant If the desiccant turns from blue dehydration through the inspection window. to pink, replace it. (See “Instructions for Dehydrating Breather” for replacement procedure.) Oil level Visually check the oil level in the If the oil level is below the oil oil pan. level line (red line), refill with oil to the level of the line. (See “Instructions for Dehydrating Breather” for oil refill procedure.) Dehydrating Drift of oil and Check the dirt condition of oil and If the oil pan or the desiccant is breather desiccant desiccant in the oil pan. significantly dirty, renew the oil, and clean the oil pan and the desiccant. (See “Instructions for Dehydrating Breather” for procedures of changing oil and cleaning the oil pan and the desiccant.) Operation of Check if bubbles go in and out Bubbles should go in and out. breathing through the tip of the breathing pipe in the oil pan when the temperature changes. Pressure relief Oil spillage Visually check any trace of oil spillage at There should be no trace of oil device [In case a oil the bottom of the oil discharge pipe. spillage. discharge pipe If there is any oil spillage, a special is attached] inspection of the interior of the pressure-relief device is required. Contact TMT&D. Target condition Visually check the presence/absence of The target should not be erect. [In case a target is attached] the If the target is erect, oil spillage may target operation. be suspected. Check any trace of oil spillage. If there is any oil spillage, a special inspection of the interior of the pressure-relief device is required. Contact TMT&D. Drain valve Penetration of Open the drain valves of the There should be no rainwater rainwater terminal box for low voltage discharge from the drain valve. bushing and the bus duct to check It may be due to leaks on the discharge of rainwater visually. IPB. Close the drain valves after Please contact the IPB inspection. manufacturer. Inspection point Inspection item Inspection procedure Control criterion and remedy Air bushing Dirt, breakage Visually check if there is dirt or If the porcelain tube is signifi- damage on the porcelain insulator. cantly dirty, stop the circuit securely for the occasion and clean the porcelain tube. (See “Instructions for Bushing” for cleaning procedure.) If there is a breakage, please contact TMT&D. Oil leak Visually check if there is oil leak. There should be no oil leak. If you leave oil leak, it may lead to dielectric breakdown due to oil level decrease. Oil level gauge Oil level [In case of dial type indication] The oil level indication should for bushing indication Check the oil indication. be between H and L or 5% and 100%. If it is out of the control criteria, it may be due to oil leak of the bushing. [In case of direct-view indication] The float should float on the Visually check the state of the oil. float through the inspection window. If it is out of the control criteria, it may be due to oil leak of the bushing. Radiator Corrosion of Check if there is corrosion. No marked corrosion. cooling pipe Cooling fan Operating sound Check the operating sound of the No abnormal noise. of the cooling fan cooling fan. Operating Check the operating condition of The cooling fan that should be condition of the cooling fan. running should not be stopped. the cooling fan Oil pump Operating sound Check the operating sound of the No abnormal noise. of the oil pump oil pump. Operating Check the indicated direction of The oil pump that should be condition of the oil flow indicator. (During running should not be stopped. the oil pump operation: FLOW, during halt: STOP) Forced-oil Operating sound Check the operating sound of the No abnormal noise. forced-air of the cooling fan and the oil pump cooling fan and the oil pump. cooling Operating Check the operating condition of The cooling fan and the oil equipment condition of the the cooling fan. pump that should be running cooling fan and Check the indicated direction of should not be stopped. the oil pump the oil flow indicator. (When the oil pump is operating: FLOW, when stopped: STOP) Inspection Control criterion and Inspection item Inspection procedure point remedy Forced-oil Operating sound Check the operating sound of No abnormal noise. forced-water of the oil pump the cooling oil pump. equipment Operating Check the indicated direction The oil pump that should be condition of the of running should not be oil pump the oil flow indicator. (During stopped. operation: FLOW, during halt: STOP) Coolant level Check the coolant level with a The prescribed amount of coolant gauge. water should flow. Coolant leak, Check if there is coolant leak No coolant leak or oil leak. oil leak or oil leak through the inspection window of the leak detector. On-load tap See “Instructions for On-load Tap Changer” for inspection items, inspection procedures, changer control criteria and remedies. Motor-driven See “Inspections for Motor -Driven Operating Mechanism” for inspection items, inspection operating procedures, control criteria and remedies. mechanism Hot line oil See “Inspections for Hot Line Oil Purifier” for inspection items, inspection procedures, purifier control criteria and remedies. Periodic Inspection Please conduct periodic inspection after stopping operation at regular periods. Mainly for the points that cannot be checked during weekly check, external inspection of abnormality, internal diagnosis with measuring devices and performance test are conducted to maintain the performance. When the control criteria are not met, please contact TMT&D. Special inspection is necessary in some cases. 5. High Voltage Circuit Breakers Phu My- Ho Chi Minh City (Phu My - Nha Be - Phu Lam) 500 kV Transmission Line Project Package#1 substation works Circuit-Breaker technical description 3. Circuit Breakers A high-voltage circuit breaker is an indispensable piece of equipment in the power system. The main task of a circuit breaker is to interrupt fault currents and to isolate faulted parts of the system. Besides short-circuit currents, a circuit breaker must also be able to interrupt a wide variety of other currents at system voltage such as capacitive currents, small inductive currents, and load currents. We require the following from a circuit breaker: • In closed position it is a good conductor; • In open position it behaves as a good isolator between system parts; • It changes in a very short period of time from close to open; • It does not cause overvoltages during switching; • It is reliable in its operation. •The electric arc is, except from power semiconductors, the only known element that is able to change from a conducting to a nonconducting state in a short period of time. •In high-voltage circuit breakers, the electric arc is a high-pressure arc burning in oil, air, or sulphur hexafluoride (SF6). •In medium-voltage breakers more often, the low-pressure arc burning in vacuum is applied to interrupt the current. The current interruption is performed by cooling the arc plasma so that the electric arc, which is formed between the breaker contacts after contact separation, disappears. •This cooling process or arc-extinguishing can be done in different ways. Power circuit breakers are categorised according to the extinguishing medium in the interrupting chamber in which the arc is formed. •That is the reason why we speak of oil, air-blast, SF6, and vacuum circuit breakers It becomes clear that current interruption by an electrical arc is a complex physical process when we realise that the interruption process takes place in microseconds, the plasma temperature in the high-current region is more the 10 000 K, and the temperature decay around current zero is about 2000 K/μs per microsecond while the gas movements are supersonic. . 3.1 THE SWITCHING ARC Figure 3.1 The arc channel can be divided into an arc column, a cathode, and an anode region Figure 3.2 Typical potential distribution along an arc channel 3.2 SF6 CIRCUIT BREAKERS Figure 3.3 Operating principle of an SF6 puffer circuit breaker 3.3 VACUUM CIRCUIT BREAKERS Figure 3.4 Vacuum interrupter with slits in the contacts to bring the arc in a spiralling motion 3.4 ARC–CIRCUIT INTERACTION •Current interruption of a circuit breaker occurs normally at current zero within a time frame of microseconds. In the process of current interruption, several processes take place at the same time. •The arc voltage after maintaining a constant value during the high current interval, increases to a peak value, the extinction peak, and then drops to zero with a very steep du/dt. •The current approaches its zero crossing with a more or less constant di/dt but can be slightly distorted under the influence of the arc voltage. •The arc is resistive and therefore the arc voltage and the current reach the zero crossing at the same instant. •Around current zero, the energy input in the arc channel is rather low (at current zero there is even no energy input), and when the breaker design is such that the cooling by the extinguishing medium is at its maximum, the current can be interrupted. •After current interruption, the still-hot gas between the breaker contacts is stressed by a steep rate of rise of the recovery voltage and in the resulting electric field the present charged particles start to drift and cause a hardly measurable so-called post-arc current. •The post-arc current, together with the transient recovery voltage, results in energy input in the still-hot gas channel. •When the energy input is such that the individual gas molecules dissociate into free electrons and heavier positive ions, the plasma state is created again and current interruption has failed. •This is called a thermal breakdown of the circuit breaker. •When the current interruption is successful, the hot gas channel cools down and the post-arc current disappears; still a dielectric failure can occur when the dielectric strength of the gap between the breaker contacts is not sufficient to withstand the transient recovery voltage. Figure 3.5 Simple lumped-element representation of the network connected to the breaker terminals The simplest lumped element representation of the power system, as seen from the terminals from the circuit breaker, consists of a voltage source with the value of the system voltage, an inductance with the value of the total short-circuit inductance, a capacitance resembling the stray capacitance of the bus bars, voltage transformers, current transformers, and power transformers in the substation. In parallel with this capacitance, a resistor simulates the characteristic impedance of the connected overhead lines Zs = √(Ls/C) is the characteristic impedance •For testing purposes, the short-circuit currents are divided in so-called duties, each covering a percentage of the full short-circuit current. •The IEC 60056 high-voltage circuit breaker standard distinguishes for the breaker terminal fault duties the 10%, 30%, 60% and 100% current ratings. •To each duty a specific rate of rise of the recovery voltage is specified and this enables us to calculate the characteristic impedance of the connected network. •For instance, for a 100 percent short-circuit current of a 145 kV–40 kA–50 Hz circuit breaker, the specified du/dt is 2000 V/μs and the di/dt is 0.444 ∗ 40 = 17.8 A/μs. •This results in a characteristic impedance of Rp = du/dt / di/dt = 2000 / 17.8 = 112 ohms •The capacitance Cp in parallel to the characteristic impedance is rather small: in the nF range. •This capacitance Cp causes a delay for the rate of rise of the recovery voltage because immediately after current interruption this capacitance must be charged. •The value of this time delay is also specified in the IEC 60056, and for a rate of rise of 2000 V/μs connected with the 100 percent duty the delay time is 2 μs. •The value of the time delay equals the RC-time of Cp and Rp, and in this way the value of the capacitance Cp can be calculated. 6. Surge Arresters 7. Transmission Capacitor Bank Technologies Series capacitors Shunt capacitor units are connected from phase to neutral or across the load. Series capacitors are connected in series in the circuit and hence carry the full line current. >> the voltage across the shunt capacitor remains constant, and the drop across the series bank changes with load. Shunt compensation. Series compensation. SERIES CAPACITORS ON RADIAL FEEDERS Radial feeder circuit. Radial feeder circuit with series capacitor. The corresponding approximate voltage drop (VD’: If XC = XL, then the voltage drop is IR cos θ. In practical applications, XC is chosen to be smaller than XL in order to avoid overcompensation. Using kVA of the sending and receiving ends, the voltage relation can be calculated as: Qc is the reactive power supplied by the series capacitor bank. The series capacitors are used on long transmission lines to increase the power transfer capability and to improve system stability. Series capacitor scheme with spark gap and varistor protection. for a given phase angle difference between the voltages ,the power transfer is greater with a series capacitor. Thus, by making a greater interchange of power possible, the normal load transfer and the synchronizing power flowingduring transient conditions are increased, thereby improving stability. Power transfer of a line without and with series capacitors. Fig.02 Single line diagram for MOV protected Fig.01 Single line diagram of spark gap series capacitor bank. protected series capacitor bank. The single gap scheme (Fig. 1) can be called the original series capacitor scheme. It is simple and is used mainly where there is only one transmission line. In cases where there are two or more parallel lines, the MOV scheme (Fig. 2) is normally used. In case of faults outside the line section where the series capacitor bank is located, the MOV will protect the capacitors but the bank will not be bypassed. This will increase the stability of the transmission system. Fig. 03 Single line diagram for a thyristor controlled series capacitor The thyristor controlled scheme may be used to dampen (TCSC) bank. oscillations in the line, when the weak network does not dampen the oscillations or these do not dampen sufficiently. Equipment founded on the series capacitor bank installations in service in the utility electrical high voltage network are: • Capacitor units; • Spark gap; • By-pass circuit breaker; • Damping circuit (reactor and resistor); • By-pass isolator; • Line isolator; • Sensing devices (voltage transformers, current transformers, etc.); • Control system; • Protection system; • Platform; • Insulators; and • Steelwork. The capacitor units provide the reactive power component required for achieving the compensation function. The spark gap and by-pass circuit breaker (connected in parallel over the capacitors) are used to protect the complete series capacitor against any over voltage condition that may be experienced as a result of any fault condition. The damping circuits provides damping out of the high frequency inrush currents that will be present during the operation of the spark gap and/or the by-pass circuit breaker produced by the fault condition and the discharge of the capacitor units into the protective primary equipment. The by-pass and line isolators are used for isolating the series capacitor bank, after it has been de-energised, during outage intervals for the purpose of maintenance. The control and protection systems speak for its self as to basically facilitate for the control and protection functions needed for the proper operation of the series capacitor bank. The sensing devices provide the necessary input signals for the control and protection systems. The platform, support insulators and associated steel structures provide for the mechanical construction to house the associated equipment and to provide for the basic insulation level needed in the series capacitor bank. Brief History • 1920s; Applying capacitors becomes general practice with externally fused capacitors • Until early 1930s, capacitor elements made of tin or lead foil, linen paper and mineral oil • Mid 1930s; Wood pulp paper (Kraft paper) • Mid 1940s; Chlorinated aromatic hydrocarbons (PCB) and internally fused capacitors • Late 1960s; Paper-Film dielectric (Polypropylene) • 1971; All-Film dielectric • 1975; Non-PCB aromatic hydrocarbon • 1989; Fuseless capacitor banks Capacitor Unit Design Fuseless or Major Insulation Externally Fused Element Series Section Capacitor Unit Design Internally Fused Capacitor Elements Internal Connections Extended Foil Mechanical Crimp Patented Mechanical Crimp Advantages • The thickness across the pack is smooth, allowing a high stacking factor and reducing the possibility of film or foil damage. • Very low resistance foil-to-foil connection on every turn for even current distribution throughout the element. • The crimp connection is significantly stronger than other connection methods resulting in improved performance during high inrush or discharge transients. Allows for 10 or 15 kA fuse coordination and a unified and definite tank rupture curve. • Each individual joint can be tested in the factory. Design Equations 2  V   O  Q O =Q R    V R  Where: QO = Operating Reactive Power QR = Rated Reactive Power VO = Operating Voltage VR = Rated Voltage Design Equations  f   O  Q O =Q R    f R  Where: QO = Operating Reactive Power QR = Rated Reactive Power fO = Operating Frequency fR = Rated Frequency Capacitance & Temperature Capacitance vs Dielectric Temperature 1.04 1.03 1.02 1.01 1 0.99 0.98 Relative Capacitance 0.97 0.96 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 Dielectric Temperature (C) Tank Rupture Energy Handling Capability t E= Ri 2 dt 0 Where: E = Energy dissipated in capacitor unit R = Series Resistance i = Current t = time Tank Rupture Tank Rupture Curves • Unified: All capacitor sizes share same curve • Definite: Not probabilistic EX-7L Defined through 10 kA, I2t withstand = 2×106 A2 sec EX-D Defined through 15 kA, I2t withstand = 5.6×106 A2 sec EX-7L Tank Rupture Curve Current Time 10 kA, 1.2 Cycle Capacitor Losses Sources of Losses • Discharge resistor • Internal fuses • Dielectric • Other I2R Capacitor Tank • 409 stainless steel stabilized with titanium. • All robotic TIG welding with 308L stainless steel filler wire. • Light-gray or brown, wet-process porcelain bushings soldered to tank. • Tin-plated brass bushing terminal. • Brass ground stud brazed to tank. Robotic Tank Welding Robotic Tank Welding Welded Hanger Leak Testing Helium Mass Spectrometer Leak Testing HMS Leak Detection sensitivity 10-7 cc/sec Grit Blasting • Grit blast surface prep before primer coat • Detergent wash, rinse and forced air dry • Robotic spray application of: • Two-part high solids epoxy primer • Oven cure primer (43°C) • Two-part high solids urethane • Oven Cure (43°C) Grit Blasting Grit Blasted Unit Grit Blasting Before and After Capacitor Coating System • Exceeds ANSI C57.12.31 Polemount Standard • Meets or exceeds ANSI C57.12.29 Coastal Coatings Standard (1500 Hour Salt Fog) • Robot spray process provides consistent application and coverage • Superior adhesion, thickness, and coverage versus hand applied coatings • Reduces HAP emissions 50% Robotic Tank Painting Capacitor Fusing Types of Capacitor Fusing • External (Expulsion or Current Limiting) • Internal • Fuseless External Fusing Eight, 50 Mvar, 154 kV Banks in parallel External Fusing Capacitor Fusing Why do we fuse capacitors? Capacitor Unit Design Fuseless or Major Insulation Externally Fused Element Series Section Capacitor Unit Design Internally Fused Dielectric Failure The Safe Failure Mode of the All-Film Capacitor • Breakdown of the polypropylene dielectric • Arc from electrode to electrode • Stored energy discharges into the fault • Arc punctures several layers of film and foil • Film melts and recedes from the puncture point • Several layers of the aluminum foil touch • The aluminum foil electrodes weld together • A stable electrical joint is formed All-Film Field Experience • Safe failure mode allows use of higher voltage units • Utility measurements indicate that partially failed capacitor units can operate indefinitely without case swelling or fuse operation Fusing Issues • Type of Failure • Parallel Energy • Voltage • Unbalance Detection • Harmonic Filter Tuning • Maintenance • Losses Type of Failure • Dielectric Failure • Insulation Failure Dielectric Failure Externally Fused • As elements in an externally fused capacitor unit fail, the series section is shorted. • As series sections are shorted, the impedance of the unit goes down and thus the current in the capacitor unit goes up. • After enough series section failures, the capacitor current causes the external fuse to operate. • If the capacitor unit fails to a short circuit, the discharge of the parallel connected capacitors help the fuse operate quickly. Dielectric Failure Externally Fused I Dielectric Failure Internally Fused • Each element is individually fused. • When an element fails, it becomes a short circuit and the parallel connected elements discharge into the failed element. • This discharge through the fuse causes a current limiting action and the failed element is isolated. • This discharge is required for proper fuse operation. Insulation Failure Externally Fused • The fault current resulting from an insulation failure will operate the external fuse. • After the operation of the fuse, the bank remains in service. Insulation Failure Internally Fused • Internal fuses provide no protection for insulation failures and the bank must be removed from service rapidly to minimize damage. • Parallel connected energy is not a concern for a typical dielectric failure. However, it is of great concern for insulation failures due to the probability of tank rupture. • Insulation failures in banks with capacitor unit cases grounded may result in fault to ground. Insulation Failure Fuseless • As there are no external fuses, the bank must be removed from service rapidly to minimize damage. • Capacitor units are not connected in parallel so the energy discharged into the fault will be very low thus minimizing the probability of tank rupture. Parallel Energy • Must be within the capability of external expulsion fuses. • Not an issue with external current limiting fuses. • Typically not an issue with internal fusing except for insulation failures. • Not an issue with fuseless banks because capacitor units are not connected in parallel. Voltage • For both external and internal fusing, the voltage across the fuse must be within the rating of the fuse. • After a fuse has operated, the open gap must be able to withstand the continuous voltage and any transients. • For an externally fused bank, the open gap is very large and easily capable of withstanding the largest transients. • For an internally fused bank, the open gap is very small and contaminated with products of the fuse operation. The withstand of the open gap is of great concern. Unbalance Detection Purpose 1. To increase bank availability by warning personnel of problems in the bank. 2. Remove the bank from service before severe damage occurs. Unbalance Protection Externally Fused Primary Safe failure mode of all-film dielectric Secondary External Fuse Operation Final Protective Relaying Unbalance Protection Internally Fused Primary Internal Fuse Operation Final Protective Relaying Unbalance Protection Fuseless Primary Safe failure mode of all-film dielectric Secondary Remaining series sections limit voltage and current Final Protective Relaying Unbalance Detection Sensitivity • Externally Fused: External fuse operations (108 fuses) • Fuseless: Series section short (240 series sections) • Internally Fused: Element fuse operation (2,880 fuses) The above figures show that the sensitivity of the unbalance detection scheme needs to be greatest for the internally fused bank. Ambiguous Indications Occurs when canceling failures occur in the bank which result in an indication that the bank is balanced. Ambiguous Indication C C IU=0 C C Ambiguous Indication C C IU> 0 C C+C Ambiguous Indication C C+C IU= 0 C C+C Unbalance Detection X & Y Banks Fuseless è The unbalance detection scheme will detect the first failure of a series section to a short circuit. è The unbalance detection scheme will not Internally Fused detect the first internal fuse operation.  Due to the danger of ambiguous indications, periodic capacitance measurements of each unit is required for the internally fused banks. Harmonic Filter Tuning Externally Fused • Dielectric failures cause the capacitance of the bank to increase. (Decreases tuning point) • Fuse operations cause the capacitance of the bank to decrease. (Increases tuning point) • Typically large steps of capacitance change necessitating an evaluation of the effects. • Typically alarm on 1st fuse operation and trip on the 2nd. Harmonic Filter Tuning Internally Fused • Fuse operations cause the capacitance of the bank to decrease. (Increases tuning point) • Smaller change in capacitance than for an externally fused bank. • Detection of the capacitance change is very problematic. • Documented problems with undetected capacitance changes resulting in parallel resonance. Harmonic Filter Tuning Fuseless • Dielectric failures cause the capacitance of the bank to increase. (Decreases tuning point) • Smaller change in capacitance than for an externally fused bank. • Only the fuseless capacitor bank results in only a decrease in the tuning point of the filter with dielectric failures typically resulting in a shift away from a parallel resonance with the system Maintenance Annual maintenance to check for: • Bulged / leaking capacitor units • Contaminated / damaged porcelain • Foreign matter in the bank Maintenance Externally Fused • Only the maintenance mentioned in the previous slide is recommended. • Location of failed capacitor units is accomplished very quickly with a visual inspection. Maintenance Internally Fused • Annual maintenance to locate partially failed capacitor units is required due to the issue of ambiguous indications. • Locating failed capacitor units requires the measurement of every capacitor unit and is very time consuming and problematic. Capacitance & Temperature Capacitance vs Dielectric Temperature 1.04 1.03 1.02 1.01 1 0.99 0.98 Relative Capacitance 0.97 0.96 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 Dielectric Temperature (C) Maintenance Fuseless • Ambiguous indications, may result in annual visual inspection. • Location failed capacitor units requires only measurements to locate the string containing the faulty unit followed by measurements of the units within the string. Fuseless Capacitor Banks 145 kV, 100 Mvar • 4 + 4 Strings • 8 Units / String • Unit Rating: 521 kvar 10,464 V 158 kV, 90.7 Mvar 158 kV, 90.7 Mvar • 3 + 3 Strings • 12 Units / String • Unit Rating: 420 kvar 7.6 kV Not a New Technology • Fuseless capacitor banks are an application of the all-film technology introduced by McGraw-Edison in 1971. • Cooper Power Systems first fuseless bank was delivered in 1989. • Fuseless capacitor banks are the ―Standard‖ in the United States, Canada and South Africa. Capacitor Unit Design Fuseless or Major Insulation Externally Fused Element Series Section Fuseless Capacitors How can we apply the all-film capacitor without fuses? Fuseless String String of 4 Series Connected Capacitor Units Capacitor Unit 32 Series Sections from phase-to-ground Fuseless String One Shorted Series Section Capacitor Unit 32 Series Sections from phase-to-ground SS 32 OV = OV = = 1.03 PU SS - F 32 - 1 Design Considerations • Rated voltage  24 kV for best economics • Maximum current per string of 75 A • Capacitor units connected in series Applications • Shunt Capacitor Banks • Series Capacitor Banks • Harmonic Filters Shunt Capacitor Banks • Typically applied at  24 kV • Have been applied as low as 12 kV by de-rating capacitor units Series Capacitor Banks • Saves platform space • Reduces weight on platform • Parallel energy not a concern • Typically applied at Vc  50 kV Harmonic Filters Harmonic Filter Tuning Point 1 f = 2 LC • Small change in tuning point • Shifts tuning away from parallel resonance • Low losses = sharper tuning Protective Relaying • Overcurrent • Overvoltage • Unbalance Protective Relaying Overcurrent and overvoltage protection of fuseless capacitor banks is identical to that of externally and internally fused banks. Maintenance • Inspection • Locating defective capacitor units • Cleaning of bushings / insulators Disadvantages • No external fuses to protect for insulation failures • Locating defective capacitor units • Sensitive unbalance relaying required • Knowledge of capacitor unit internal construction necessary Advantages Advantages of Fuseless over Externally Fused Capacitor Banks • Cost from 10 to 30% less • Approximately 50% less substation space required • No spurious fuse operations • No coordination of fuse TCC with relaying • Parallel energy not a concern • Low resulting over-voltage with element failure • D C for harmonic filters • Animal proofing / Less bus bars • Simplicity Advantages Advantages of Fuseless over Internally Fused Capacitor Banks • Cost from 10 to 30% less • 40% less losses • Easier more reliable unbalance relaying • D C for harmonic filters • Low resulting over-voltage with element failure • Longer life • Easier to locate partially failed units • Simplicity Unbalance Protection Cooper Power Systems McGraw Edison Power Capacitors Greenwood, SC USA Purpose To increase bank availability by warning personnel of problems (Failures) in the bank and removing the bank from service before severe damage occurs. Failure Events • Dielectric Failures • Bushing / Insulator Flashover • Major Insulation Failure Fuseless Bank Capacitor Unit 32 Series Sections from phase-to-ground One Shorted Series Section Capacitor Unit 32 Series Sections from phase-to-ground 110kV Configuration 110kV Configuration 66kV Configuration 66kV Configuration Types of Unbalance • System • Inherent • Internal Failures Unbalance Relaying Criteria Fuseless Banks • Notify personnel (Alarm) in the event of the first series section short (if possible). • Remove the bank from service (Trip) when enough series sections have shorted in a string to result in greater than a 110% overvoltage on the remaining series sections. • Remove the bank from service (Trip) when the number of shorted series sections in a string equal that contained in one capacitor unit. 8. Intro to High Voltage Direct Current (HVDC) Technologies HVDC advantages over AC transmission High Voltage Direct Current HVDC • High voltage DC connection – No reactive losses • No stability distance limitation • No limit to underground cable length • Lower electrical losses P=U I – 2 cables instead of 3 DC DC – Synchronism is not needed • Connecting different frequencies • Asynchronous grids (UCTE – UK) • Black start capability? (New types, HVDC light) – Power flow (injection) can be fully controlled • Renewed attention of the power industry History of HVDC HVDC Configurations: Transmission modes (I) • Back to back • Monopolar (Sea) • Bipolar • Multiterminal + - HVDC Configurations: Transmission modes (II) LCC HVDC • Thyristor or mercury-arc valves • Reactive power source needed • Large harmonic filters needed VSC HVDC • IGBT valves • P and Q (or U) control • Can feed in passive networks • Smaller footprint • Less filters needed HVDC Example- Norned cable HVDC Example- Norned cable: schematic HVDC Example- Norned cable: sea cable HVDC Example- Garabi back to back HVDC Example- Garabi back to back (4x) VSC HVDC Example- Murray link • Commissioning year:2002 • Power rating: 220 MW AC • Voltage:132/220 kV • DC Voltage:+/- 150 kV • DC Current: 739 A • Length of DC cable:2 x 180 km VSC HVDC Example- Troll • Commissioning year: 2005 • Power rating: 2 x 42 MW AC Voltage:132 kV at Kollsnes, 56 kV at Troll • DC Voltage: +/- 60 kV • DC Current: 350 A • Length of DC cable:4 x 70 km HVDC- Current sizes