Lead Acid Battery Commissioning

1. Inspection/check

           The battery installation and charging unit must be inspected for mechanical soundness. All bolted connections within the circuit must be properly tightened for optimum contact as set out in the operating instructions.

           The charging unit must be checked for operational readiness. Ensure that the polarity is correct.

            Before filling the cells ensure that the conditions as set out in DIN VDE 0510 Part 2 regarding installation and ventilation are observed.

             Should a higher charging current be used than permitted for the ventilation layout by start up charging, the ventilation in the battery room must be increased according to the loading current applied for the start-up period and for one hour afterwards, e.g. by additional portable ventilators. The same applies to occasional special battery charging processes.

2. Filling cells

              Acid with the density according to Table 1 must comply with the purity specifications according DIN 43 530 Part 2.

If concentrated sulphuric acid is supplied, the mixing instructions must be observed.

             The acid temperature should be in the range of 15°C to 30°C. Before filling the temperature must be measured and noted in the commissioning report.

              Aher removing the transport plugs or opening the vent plugs the cells must be filled to the lower electrolyte level mark using acid-resistant filling devices.

It is not permitted to use transport plugs when operating the battery. They must be replaced by vent plugs delivered with the batteries.

Higher temperatures reduce the electrolyte density and lower temperatures increase the electrolyte density. The associated correction factor is 0.0007 kg/l per K.

Example: Electrolyte density 1.23 kg/l at 35°C corresponds to a density of 1.24 kg/l at 20°C.

3. Reaction time

              Aher filling the cells a reaction time of 2 hours must be observed. Subsequently, depending on the total number of cells, the temperature and electrolyte density of at least 4 to 8 cells (pilot cells) must be measured and noted in the commissioning

report.

If the temperature rise is less than 5 K and the electrolyte density has not fallen more than 0.02 kg/l below the acid density, a commissioning charge as under 4.1 or 4.2 is adequate.

                Should one of the deviations be higher than an extended commissioning charge as under 4.3 is necessary.

4. Commissioning

                With non-transparent cell containers the vent plugs remain open in order to observe whether gassing is taking place evenly in all the cells towards the end of the charge.

                  It is important that the first charge is carried out to completion. This is only possible with a charging voltage above 2.35 V/cell. Interruptions should be avoided if possible. Commissioning should be recorded in the commissioning report overleaf.

During start-up the cell voltage of the pilot cells must be measured and on start-up completion the cell voltage, electrolyte density and temperature of all cells must be measured and noted with the time and date in the commissioning report.

  

                 The electrolyte temperature must not exceed 55°C, if necessary the charge operation must be interrupted.

4.1 Commissioning charge with constant voltage (IU chararteristic)

                   A charge voltage of 2.35-2.4 V/cell is required. The charge current on commencing the charge shouid be a minimum of 5 A per 100 Ah C10.

                   The electrolyte density only rises slowly during the charge. The charge time can therefore take several days before reaching a minimum electrolyte density of nominal electrolyte density -0.01 kg/l.

                    Subsequently switch to the float charge voltage as set out in the operoting instructions. The electrolyte density rises to the nominal density during operation.

4.2 Commissioning charge with constant (I chararteristic) or decreasing current (W characteristic)

The maximum permitted currents can be obtained from Table shows maximum permitted charging currents in A per 100 Ah C10 for I and W charging

Charging must continue until

1. All cells have reached a minimum of 2.6 V
2. The electrolyte density in all cells has risen to
3. The nominal value of ± 0.01 kg/l and these cease to rise over a further period of 2 hours.

Subsequently switch to the floot charging voltage as set out in the operating instructions.

4.3 Extended commissioning charge

                  Extended storage or climatic influences (humidity, temperature fluctuations) reduce the charge state of the cells. This makes an extended commissioning charge along the following procedure necessary:

1. Charge at 15 A per 100 Ah C10 until 2.4 V/cell is achieved (ca. 3 - 5 hours),
2. Charge for 14 hours with 5 A per 100 Ah C10 (voltage exceeds 2.4 V/cell),
3. Interrupt for one hour,
4. Charge for 4 hours with 5 A per 100 Ah C10.

Repeat items 3 and 4 until

— all cells have reached a minimum 2.6 V

— the electrolyte density in all cells has risen to

the nominol value of ± 0.01 kg/l and these

cease to rise for a further 2 hours.

Subsequently switch to the float charge voltage

1. as set out in the operating instructions.

4.4 Electrolyte level adjustment

                 On completion of commissioning top up with acid to bring the electrolyte level to the upper electrolyte level mark.

4.5 Electrolyte density adjustment

                 If the electrolyte density at the end of commissioning is too high, reploce part of the electrolyte with purified water as specified in DIN 43 530 Part 4. The comparative electrolyte density in individual cells should not deviate more than 0.01 kg/l. With greater deviations adjust the electrolyte density and then carry out an equalizing charge as set out in the operating instructions.

Motor Electrical Signature Analysis

Motor Current Signature Analysis (MCSA)

             MCSA detect broken rotor bars using current signal only & it tells the user what is from the point of test towards the load.                    

Electrical Signal Analysis(ESA)

           ESA detects Motor fault using both Voltage and current & it tells the user about information from the point of test towards the supply

MCSA Vs ESA

 MCSA is primarily used by the vibration industry using special current probes which allow the vibration data collectors to take current input. This current is then converted from analog to digital, filtered and produced as an FFT (Fast Fourier Transform) spectra of amplitude versus frequency. ESA has been primarily used by the dedicated ESA instrument manufacturers and includes the voltage waveform as an input. The primary difference is that current tells the user what is from the point of test towards the load and voltage provides information from the point of test towards the supply. This allows the user to quickly determine where a particular signature exists.

ESA Advantages

1.    ESA provides the capability of detecting power supply issues, severe connection problems, airgap faults, rotor faults, electrical and mechanical faults in the motor and driven load, including some bearing faults

2.    It is important to note that the technology should not be considered a replacement for vibration analysis in mechanical analysis, but provides excellent data on motor condition from incoming power through to the rotor

3.    From the bearings to the mechanical load still remains in the realm of vibration, in most cases.

Fault Detection Using ESA

            One of the original concepts behind the development of ESA was to eliminate the loss of instrumentation to test MOV’s in the dangerous areas within nuclear power plants. The primary failure of these machines is the rotor which would overload and melt when limit switches failed. It was discovered that the rotor bar failure signature was unique enough that not only could the signature be quickly identified, but that condition values could be applied easily.

              When the Pole Pass Frequency sidebands (P1 and P2) of Figure 1 are compared to the values in Table 1, and the condition of the rotor bars can be determined. However, in this case, the motor is 4,160 Vac and the data was taken from the Motor Control Center (MCC) Current Transformers (CT). The result can be a dampening effect on those peaks resulting in the analyst needing to estimate the severity of the fault.

           Where SS is Synchronous Speed, RS is Running Speed, LF is Line Frequency and PPF is the Pole Pass Frequency

            Concerning most other faults detected with ESA, the number of rotor bars and stator slots in the design of the motor is necessary. Many of the ESA instrument manufacturers have built algorithms into their software which can assist the analyst in estimating either number.

            In figure 2, the motor is an 800 horsepower, 1785 RPM, 101 Amp, Louis Allis motor with 58 rotor bars and 72 stator slots. SM1 and SM2 are peaks related to the movement of the coil ends of the motor windings. As measured through the CT’s, the values are about -78 dB which would be more severe if the current was measured directly. With an RPM of 28.793 Hz (1727.6 RPM), the stator mechanical (coil movement) frequencies would be the number of stator slots times the running speed plus and minus the line frequency. In this case, 2013.1 Hz and 2133.1 Hz which relates to the fields passing through the coils ends and interacting with the rotor fields.

           Excessive coil movement will cause fractures in the coils as they leave the stator slot. In the case of the 800 horsepower motor, this movement coupled with oil contamination caused the winding to fail where the windings leave the slot.

Ni-Cd battery working principle, construction and applications

Working Principle

          Any secondary cell is a combination of active materials which can be electrolytic oxidized and reduced repeatedly. The oxidation of the negative electrode occurring simultaneously with the reduction of the positive generates electric power. In a rechargeable battery both electrode reactions are reversible and the input of current in the proper direction from an outside source will drive the primary or discharge reaction backwards and in effect recharge the electrodes.

           In the uncharged condition the positive electrode of a nickel-cadmium cell is nickelous hydroxide, the negative cadmium hydroxide. In the charged condition the positive electrode is nickelic hydroxide, the negative metallic cadmium. The electrolyte is potassium hydroxide. The average operating voltage of the cell under normal discharge conditions is about 1.2 volts. The over-all chemical reaction of the nickel cadmium system can be considered as:

          During the latter part of a recommended charge cycle and during overcharge, nickel-cadmium batteries generate gas. Oxygen is generated at the positive (nickel) electrode after it becomes fully charged and hydrogen is formed at the negative (cadmium) electrode when it reaches full charge. These gases must be vented from the conventional nickel-cadmium system. In order for the system to be over chargeable while sealed, the evolution of hydrogen must be prevented and provisions made for this reaction of oxygen within the cell container. These things are accomplished by the following:

Construction:

         Energizer nickel-cadmium cells are available in cylindrical configuration and range in capacity up to 5 Amp hours in sizes from AAA to D.

Cylindrical Cells

         This cell type incorporates a different electrode arrangement than the button cell. Sintered plates are used in all cylindrical cells for the positive electrode. This electrode consists of thin, highly porous nickel plaques impregnated with active materials. The plaque is made by heating nickel powder in an inert atmosphere until the particles are welded together. The metallic phase serves as a highly conductive supporting structure for the electrode. The structure of the plate is such that a large surface is furnished for reaction of the active materials. With the sintered electrode it is possible to build cells of very low internal resistance.

                 The negative electrode of most Energizer cylindrical cells is a pasted electrode which consists of blended active materials pressed onto a metal carrier. It is this electrode that gives Energizer cylindrical nickel-cadmium cells outstanding cycle life, long term overcharge capability, with essentially no fade and with little or no memory effect.

              Sealed nickel-cadmium cells under certain abuse conditions such as excessive charge or overcharge rate, deep discharge with subsequent polarity reversal, may develop high internal gas pressure. Usually the gas is oxygen, although hydrogen is also evolved in some cases. Either or both hydrogen and oxygen must be vented. All Energizer high rate cylindrical cells have a resealing pressure vent. This vent permits the cell to release excess gas evolved if the cell, for example, is abused. When the internal pressure has dropped to an acceptable level, the vent will reseal, permitting the cell to be recycled in the normal manner with little or no further loss of electrolyte or capacity. Repeated venting will reduce capacity and cycle life.

Contact Material

              External electrical connections can be made with any good conductor having adequate current handling capabilities.

Potting

              Nickel-cadmium cells or batteries of any type should not be totally potted. Energizer cells have resealable vent mechanisms which would be rendered inoperative by the potting compound.

Capacity

              The capacity rating of Energizer nickel-cadmium cells and batteries is based upon output in discharge at the 1 hour rate to an endpoint of 1.0V/cell for all cylindrical cells. If current is withdrawn at faster rates than these standards, capacity is decreased.

Storage

               At elevated storage temperatures self-discharge will be considerably higher than at room temperature. It is recommended that batteries be stored at 21°C (70°F) or lower for this reason.

Application

1. Calculators
2. Cassette players and recorders
3. Dictating machines
4. Digital Cameras
5. Instruments
6. Personal Pagers
7. Photoflash equipment
8. Portable communications equipment
9. Portable hand tools and appliances
10. Portable computers
11. Radios
12. Radio control models
13. Shavers
14. Tape recorders
15. Television sets

Ni-Cd battery overcharging,discharging & Electrical Characteristics

Continuous Overcharge

           The overcharge capability of Energizer cylindrical nickel-cadmium cells is outstanding. The next chart illustrates initial and subsequent discharge curves after 2 years continuous overcharge without periodic discharges. The first discharge after the 2 year charge period yields a slightly reduced voltage curve and 65% capacity. The second cycle after 2 years continuous overcharge provides essentially the same discharge curve as the initial one.

                The chart above illustrates maintenance vs. months of continuous overcharge at the 20 hour rate with periodic discharges every 3 months at the 1 hour rate. The cells maintain 90% of their initial capacity after 2 years of this overcharge regimen. This pattern of use would occur if batteries are left on charge continuously and used one cycle only on an occasional basis.

Memory Effect

                Memory effect is that characteristic attributed to nickel-cadmium cells wherein the cell retains the characteristics of the previous cycling. That is, after repeated shallow depth discharges the cell will fail to provide a satisfactory full depth discharge. Energizer cylindrical nickel-cadmium cells are particularly excellent with regard to lack of memory effect. The chart below depicts initial and subsequent cycles after repeated shallow discharges. The graphs show the initial discharge curve and the first and second discharge curves after 100 cycles @ 40% depth of discharge. You will note that the subsequent full depth discharges yield nearly equal capacity to the initial curve at slightly reduced voltage levels.

Self-Discharge

                    Self-discharge characteristics of Energizer nickel-cadmium cells are shown in the chart below. The characteristics are shown as a decline in percent of rated capacity available. Self-discharge is increased by elevated temperatures. Batteries are not harmed even if not used for long periods of time.

High Current Pulse Discharge

                 High rate nickel-cadmium cells will deliver exceedingly high currents. If they are discharge continuously under short circuit conditions, self-heating may do irreparable damage. The heat problems vary somewhat from one cell type to another, but in most cases internal metal strip tab connectors overheat or the electrolyte boils. In some instances both events occur. General overheating is normally easy to prevent because the outside temperature of the battery can be used to indicate when rest, for cooling, is required. In terms of cutoff temperature during discharge, it is acceptable practice to keep the battery always below 45oC (113oF). The overheated internal connectors are difficult to detect. This form of overheating takes place in a few seconds or less, and overall cell temperature may hardly be affected. It is thus advisable to withdraw no more ampere seconds per pulse, and to withdraw it at no greater average current per complete discharge, than recommended on the data sheet for the "Eveready" cell in question. In special cases, where cooling of the cell or battery is likely to be poor, or unusually good, special tests should be run to check the important temperatures before any duty cycle adjustment is made. Output capacity is any discharge composed of pulses is difficult to predict accurately because there are infinite combinations of current, "on" time, rest time, and end point voltage. Testing on a specific cycle is the simplest way to get a positive answer.

Recommended Charging

                         Constant current charging is recommended for sealed nickel-cadmium cells. The 10 hour rate should not be exceeded unless overcharge is specifically to be prevented. The recharge efficiency of sealed nickelcadmium cell is dependent on a number of things, but it is most important to remember that charging becomes more difficult as temperature increases and charge rate decreases. It is possible, under certain conditions, to charge at rates much higher than the 10 hour rate, but control devices which prevent high rate over-charge are sometimes required. The nickel-cadmium battery can be trickle charged but floating and constant voltage charging are not recommended. For maximum performance in situations of long term trickle charge current required to keep the battery fully charged is approximately the 30-50 hour rate plus whatever is necessary to compensate for any major withdrawals.

Paralleling of Cells

                      Sealed nickel-cadmium cells should not be charged in parallel unless each cell or series string of the parallel circuit has its own current limiting resistor. Minor differences in internal resistance of the cells may result, after cycling, in extreme variation in their states of charge. This may lead to overcharge at excessive currents in some cells and undercharge in other cells.

Polarity Reversal:

                   When cells are connected in series and discharged completely, small cell capacity differences will cause one cell to reach complete discharge sooner than the remainder. The cell which reaches full discharge first will be driven into reverse by the others. When this happens in an ordinary nickel-cadmium sealed cell, oxygen will be evolved at the cadmium electrode and hydrogen at the nickel electrode. Gas pressure will increase as long as current is driven through the cell and eventually it will either vent or burst. This condition is prevented in some sealed nickel-cadmium cells by special construction features. These include the use of a reducible material in the positive in addition to the nickel hydroxide, to suppress hydrogen evolution when the positive expires. If cadmium oxide is used it is possible to prevent hydrogen formation and to react the oxygen formed at the negative by same basic process used to regulate pressure during overcharge. A cell is considered electrochemically protected against reversal of polarity if, after discharge at the 10 hour rate down to 1.1 volts, it may receive an additional 5 hour discharge with the same current without being damaged or otherwise affected. "Eveready" cylindrical cells are protected against cell rupture, caused by gassing generated during polarity reversal, by a pressure relief vent

Electrical Characteristics

                 Energizer sealed nickel-cadmium cells exhibit relatively constant discharge voltages. They can be recharged many times for long lasting economical power. They are small convenient packages of high energy output, hermetically sealed in steel cases, leak resistant and will operate in any position. The cells have very low internal resistance and impedance, are rugged and highly resistant to shock and vibration. The temperature range under which these cells may be operated is wide. Use at high temperatures, however, or charging at higher than recommended rates, or repeated discharge beyond the normal cut-offs may be harmful..

Voltage Characteristics

                  Except in the case of complete discharge, neither cell condition nor state of charge can be determined by open circuit voltage. Within a short while after charging it may be above 1.4 volts. It will fall shortly thereafter to 1.35V and continue to drop as the cell loses charge. During discharge, the average voltage of a sealed nickel-cadmium battery is approximately 1.2 volts per cell. At normal discharge rates the characteristic is very nearly flat until the cell approaches complete discharge. The battery provides most of its energy above 1.0 volt per cell. If the cell is discharged with currents exceeding the rated value, however, the voltage characteristic will have more of a slope, a lower endpoint voltage will be necessary and the ampere hours per cycle will be reduced.

Temperature Characteristics

                Sealed nickel-cadmium cells experience a relatively small change of output capacity over a wide range of operating temperature. Charging, however, must be done in a much narrower range. Temperature limits applicable to operation of the cells are listed in the specification sheets for each battery.

                   The capacity vs. temperature curves which are on some individual specification sheets represent cells discharged at the temperatures shown after charging at room temperature for 14 hours at the 10 hour rate. This characteristic is also generalized on the following curve.

                    Charging nickel cadmium cells below the recommended temperature can cause oxygen pressure build up and activation of the resealable safety vent. Multiple vent activations will reduce cell capacity.

Types of Overhead Conductors

Types of Conductors:

There are four major types of overhead conductors used for electrical transmission and distribution.

·            AAC - All Aluminum Conductor

·            AAAC - All Aluminum Alloy Conductor

·            ACSR - Aluminum Conductor Steel Reinforced

·            ACAR - Aluminum Conductor Aluminum-Alloy Reinforced

All the major cost components of a transmission line depend upon conductor physical, mechanical and electrical parameters. A list of these basic parameters are:

·            conductor diameter

·            weight per unit length

·            conductivity of material(s)

·            crossectional area(s)

·            modulus of elasticity

·            rated breaking strength

·            coefficient(s) of thermal expansion

·            cost of material(s)

·            maximum unloaded design tension

·            resistance to vibration and/or galloping

·            surface shape/drag coefficient

·            fatigue resistance

AAC - All Aluminum Conductor, sometimes referred to as ASC, Aluminum Stranded Conductor, is made up of one or more strands of 1350 Alloy Aluminum in the hard drawn H19 temper. 1350 Aluminum Alloy, previously known as EC grade or electrical conductor grade aluminum, has a minimum conductivity of 61.2% IACS. Because of its relatively poor strength-to-weight ratio, AAC has had limited use in transmission lines and rural distribution because of the long spans utilized. However, AAC has seen extensive use in urban areas where spans are usually short but high conductivity is required. The excellent corrosion resistance of aluminum has made AAC a conductor of choice in coastal areas.

ACSR - Aluminum Conductor Steel Reinforced, a standard of the electrical utility industry since the early 1900's, consists of a solid or stranded steel core surrounded by one or more layers of strands of 1350 aluminum. Historically, the amount of steel used to obtain higher strength soon increased to a substantial portion of the cross-section of the ACSR, but more recently, as conductors have become larger, the trend has been to less steel content. To meet varying requirements, ACSR is available in a wide range of steel content - from 7% by weight for the 36/1 stranding to 40% for the 30/7 stranding. Early designs of ACSR such as 6/1, 30/7, 30/19, 54/19 and 54/7 strandings featured high steel content, 26% to 40%, with emphasis on strength perhaps due to fears of vibration fatigue problems. Today, for larger-than-AWG sizes, the most used strandings are 18/1, 45/7, 72/7, and 84/19, comprising a range of steel content from 11% to 18%. For the moderately higher strength 54/19, 54/7, and 26/7 strandings, the steel content is 26%, 26% and 31%, respectively. The high-strength ACSR 8/1, 12/7 and 16/19 strandings, are used mostly for overhead ground wires, extra long spans, river crossings, etc.

The inner-core wires of ACSR may be of zinc coated (galvanized) steel, available in standard weight Class A coating or heavier coatings of Class B or Class C. Class B coatings are about twice the thickness of Class A, and Class C coatings about three times as thick as Class A. The inner cores may also be of aluminum coated (aluminized) steel or aluminum clad steel. The latter produces a conductor designated as ACSR/AW in which the aluminum cladding comprises 25% of the area of the wire, with a minimum coating thickness of 10% of the overall radius. The reinforcing wires may be in a central core or distributed throughout the cable. Galvanized or aluminized coats are thin, and are applied to reduce corrosion of the steel wires. The conductivity of these thin coated core wires is about 8% IACS. The apparent conductivity of ACSR/AW reinforcement wire is 20.3% IACS.

ACAR - (Aluminum Conductor-Aluminum Alloy Reinforced) - ACAR combines 1350 and 6201 aluminum alloy strands to provide a transmission conductor with an excellent balance of electrical and mechanical properties. This conductor consists of one or more layers of 1350-H19 aluminum strands helically wrapped over one or more 6201-T81 aluminum alloy wires. The core may consist of one or more 6201 strands. The primary advantage of the ACAR conductor lies in the fact that all strands are interchangeable between EC and 6201, thereby permitting the design of a conductor with an optimum balance between mechanical and electrical characteristics. In effect, ACAR is a composite aluminum-aluminum alloy conductor which is designed for each application to optimize properties. Inverse ACAR conductors are also available with the harder 6201 aluminum alloy wires being on the outer surface of the conductor and the 1350 aluminum making up the heart of the conductor.

Bundled Conductors - A bundled conductor arrangement with two or more conductors in parallel, spaced a short distance apart is frequently used for HV and EHV transmission lines. Many electrical reasons can be cited in favor of bundled conductors. From the stand point of current density per unit area, smaller conductors have higher possible current densities, thus greater metal efficiency. The use of multiple conductors per phase having the same total area as a single conductor will operate at lower temperatures yielding lower resistances and losses for equal loads.

Multiple conductors offer significant improvements in reactance over a single conductor of equal area. The inductive reactance of a two conductor bundle is only about 50% of the reactance for a single conductor having the same circular mil area as the bundled pair. Obviously, the greater the spacing between subconductors, the lower the reactance.

Although important, the electrical advantages of bundled conductors may not be the most important factor influencing their use. The concerns of corona and radio noise may dictate the use of bundled conductors since corona loss of a conductor is a function of the voltage gradient at the conductor surface. The subjects of corona and RIV have been well investigated and will not be further discussed here.

The number and size of conductors per phase have not been standardized. It is dependent upon many factors. Today conductor bundles are a standard design practice for transmission lines designed to operate at 345 kV or higher.

Any of the above discussed conductors including VR Cable, can be used as subconductors for bundle conductor designs. This presents the transmission design engineer with limitless design options.

XLPE cable

XLPE(Cross Linked Poly Ethylene) Cables have advantages over conventional paper insulated/ Oil filled cables

í          Lower dielectric losses

í          Lower weight

í          Higher permissible continuous and short circuit current withstand capacity

í          Environment friendly - no risk of oil leakage in to environment

í          Easy installation of accessories

í          Maintenance free

í          No problem with vertical  installation

í          Improved flame retardant  property with PVC sheath, due to absence of any oil etc.

Manufacturing range

í          Cable sizes                                          : 95 sq mm - 2000 sq mm

í          Voltage grade                                    : 66 kV - 220 kV (extendable up to 400 kV)

í          The basic material for XLPE is low density polyethylene(LDPE)- which is thermoplastic. For EHV, the cross linking takes place in an electrically heated tube of extrusion line , in an inert gas atmosphere.The gas pressure in tube is kept around  5-10 bar to avoid formation of voids due to peroxide decomposition.

Purpose of cross linking

î          To improve thermal  rating - continuous , overload & short circuit temperature ratings

î          To improve mechanical properties

î          To improve deformation resistance

Property                                                                     PE                           XLPE

Max  continuous operating temperature ( °C)                            70                           90

Max  short circuit temperature                ( °C)                         150                         250

Dielectric constant                                                                    2.3                          2.3

Dielectric strength(kV/mm)                                                     > 22                        > 22

Tan d                                                                                   0.0005                   0.0005 

Series compensated lines - Distance protection

Why series compensation?

1. Increased power transfer capability
2. Better system stability
3. Reduced transmission system losses

Benefits

1.    Optimization of power capabilities

2.    Power sharing

3.    Technical & environmental benefits

Criteria

1.    Strong transmission links

2.    Maximum power transfer within the steady state stability limits

Possible measures

1.    Increase no of parallel feeders

2.    Increase transmission voltage

3.    Decrease the transfer impedance using series compensation

• Spark gap – to bypass C during high current faults
• Circuit breaker – to close during high current faults and also to discharge TRV (Transient Recovery Voltage)
• To control the voltage across the series capacitor

 Problems:

1. Voltage inversion
2. Current inversion
3. Distance estimation

Current Inversion

·         I_R inductive as impedance to fault has X_L which is inductive

·         I_S capacitive as impedance to fault has X_C which is capacitive

·         For an internal fault, I_R  & I_S are in opposite directions

·         Probability of non- operation of relay for a high impedance fault

·         Current inversion affects directional, distance, phase comparison scheme and differential protection

Voltage Inversion

·         If XC > m XL, voltages V & V’ are out of phase

·         Line side VTs detect a forward fault correctly

·         Bus side VTs detect a reverse fault correctly

Distance Estimation

·         Errors in distance estimates

·         Correct estimation when capacitor out of service – High current fault

·         Impedance reduces because of C ( ZL – j XC )

·         The set distance characteristic over reaches for a high impedance fault

·         Mho element reach can be reduced to 90% when C is in service

·         Reach reduces to 50% when C is out of service

·         For a high impedance fault close to the capacitor, relay of line section AB sees it as a reverse fault

·         Relay of adjacent line section may mal operate depending on the location of the fault

·         Hence, series capacitor can cause nuisance tripping

·         Memory polarization for voltage inversion à polarizing memory should be long enough

·         Blocking of zone-1 element for high impedance faults to prevent over-reach when capacitor is in circuit              

·         Reduce the reach of zone-1 when capacitor is in circuit.

·         Using line differential protection à excellent choice for a series compensated line