Pir

SOURCE : SANVEDA

Main Parts Of The Circuit Breaker 1. INTERRUPTING UNIT , PIR,GRADING CAPACITOR 2. POLE COLUMN 3. OPERATING MECHANISM. MECHANISM

The two methods used to achieve substantial reduction in transmission line energizing overvoltage's are •Resistor Resistor Insertion and •Controlled Controlled closing of the Energizing circuitcircuitbreaker close to voltage zeros

Methods of controlling switching surges On systems of 400 kV and above, the energisation of long transmission lines (200 km and greater) is commonly required. Voltages above 4 pu of the normal system phase voltage peak have been shown to occur by TNA and computer studies. Methods therefore have been employed to reduce these overvoltage's to 2.5 pu or less to achieve economic design of the transmission line and substation. At 420 kV the overvoltage's can be well controlled using metal oxide surge arresters at the send and receive end of the line. At 550 kV, circuit-breaker preinsertion resistors (PIR) have been used with great effect but result in complicated contact arrangements on the circuit-breaker with appropriate increase in maintenance. For 550 kV systems with line lengths below 300 km, metal oxide surge arresters (MOA) can give an acceptable voltage profile along the line length, with the maximum voltage occurring at the line mid-point. With lengths near 300 km and above, additional surge arresters can be placed at the line mid-points. Recent developments in controlled point-on-wave(POW) switching have introduced microprocessor based technology for circuit-breaker operation control. This can be very effective, particularly when used in conjunction with metal oxide surge arresters for transmission line overvoltage control. However, in all cases the substation voltages can be adequately controlled with the surge arresters at the line ends only. Dependent on the line design and the acceptable risk of failure for the 550 kV line, the mid-point surge arrester may not be required even with 300 km lines.

The Transient overvoltage factor can be controlled by: (a) Removing the trapped charges from the line (b) Synchronizing closing which can be accomplished either by closing at a voltage zero of the supply side or by matching the polarity of the line and the supply side (c) Synchronizing opening which optimizes the contact gap at current zero (d) Using pre-insertion resistors From all the listed alternatives only resistors can be considered to be an integral part of a circuit breaker. The practice of including opening and / or closing resistors as part of a circuit breaker is relatively common for circuit breakers intended for applications at voltages above 123 kV

1. SINGLE STAGE RESISTOR INSERTION Energizing a transmission line through single stage resistors results in the waves transmitted along the line being reduced in magnitude and hence the overvoltage's at the receiving end being less severe. Resistors used in this way must also be removed from circuit and the removal of the resistors also initiates travelling waves which create overvoltage's. Figure illustrates the severity of the overvoltage's produced in the initial energizing operations through single stage resistors and the subsequent resistor shorting operations indicating that there is an optimum PIR value where the overvoltage's produced by the initial energisation and the subsequent resistor removal are equal. The optimum resistor value varies with different system conditions but is typically in the range 300-500 ohms.

2 Insertion time and Pole Scatter The overvoltage's generated when energizing transmission lines are relatively insensitive to the resistor insertion time. However, if the insertion time is less than the circuit-breaker pole scatter plus twice the transmission Line transit time an increase in the overvoltage results, especially in the cases when remanant charge exists. Figure 2.8 illustrates the results from a series of studies to investigate the relationship between pole scatter and insertion time. They show that there is significant increase in the overvoltage if one phase is energized through its resistor and the resistor shorted out before another phase has been energized for the first time as the damping effect of the resistor on the mutually induced voltage is ineffective. The effect is most pronounced in the regions of small insertion resistor values where the mutually coupled transient components are greater.

Overvoltage variation With Resistor Insertion Time

Circuit breaker components 1.Breaking 1.Breaking unit 2. Support insulator 3. Support structure 4. Operating mechanism 5. Trip mechanism 6. Gas supervision (on opposite side) 7. Pullrod with protective tube 8. Position indicator 11. Primary terminals

Circuit breaker components 1.Breaking 1.Breaking unit 2. Support insulator 3. Support structure 4. Operating mechanism 5. Trip mechanism 6. Gas supervision (on opposite side) 7. Pullrod with protective tube 8. Position indicator. 9. Grading capacitor (if required) 10. Preinsertion resistor (PIR) (if required) 11. Primary terminals

Pre Insertion Resistor Used in Circuit Breaker and comes into operation at the time of charging the line/ transformer to reduce switching Over voltage

PrePre-insertion resistors (closing resistors) resistors) are used to limit over-voltages / surges in the network during switching operations.
The pre-insertion resistors are only used during closing and consist of resistor blocks that are connected in parallel with the breaking chamber.
The resistor blocks will close the circuit approximately 8-12 ms before the arcing contacts.
Pre-insertion resistors are mainly used at higher system voltages (≥362 kV).
Pre-insertion resistors should not be mixed up with opening resistors, which are used for reducing (damping) the TRV during opening. Opening resistors are mainly used on older types of circuit breakers, e.g. air-blast circuit breakers.

Preinsertion Resistors (PIR) HPL circuit breakers with more than one breaking element per pole can be provided with Preinsertion resistors for switching of no-load lines.

ABB GUIDE :Preinsertion resistors on line circuit breakers are used occasionally at rated voltages 362 – 420 kV and more often at 550 – 800 kV. Their purpose is to reduce the voltage transients generated when a nono-load transmission line is energized, or reenergized after a line fault. The resistors are operated by the same operating mechanism as the main contacts. Preinsertion resistors were previously sometimes used on circuit breakers for capacitor banks, reactor banks and transformers. For these applications, however, controlled switching is now widely used as a powerful means to reduce the switching transients. Modern SF6 circuit breakers also have better switching properties than old circuit breaker types. This has generally made preinsertion resistors superfluous for these applications. New technologies may also eliminate the need for Preinsertion resistors for line circuit breakers. In many cases controlled switching can replace the resistors and reduce the voltage transients to the same extent as, or even better than, resistors.

RECLOSING In the case of a fault on the transmission line, the circuit breakers at both ends of the line are opened and one of the two circuit breakers interrupts the fault. Provided that the circuit breakers are single-pole operated, the protection system may be arranged to open only the faulted phase(s), and leave the healthy phase(s) unaffected. As a result, the line will still have some transmission capability left. This will help to maintain stability in large, heavily loaded transmission systems. If all three poles of the circuit breaker are opened, the poles not interrupting the fault are switching a no-load transmission line. When the circuit breaker recloses, the poles switching the no-load transmission line are now energizing the transmission line. As a worst case, this energizing may occur when the trapped charge on the line is at opposite polarity. The poles energizing the faulted line are energizing a transmission line without trapped charge. One way of limiting the voltage transients associated with the energizing of no-load transmission lines is by using Preinsertion resistors. The value of the resistance is in the order of the surge impedance of the line (a typical resistance value is 400 Ω) and the Preinsertion time 8 – 12 ms to guarantee that the voltage wave has traveled out and back a couple of times before the main contacts close.

Alternative for PIR SWITCHING SURGE CONTROL OF LONG TRANSMISSION LINES Long EHV lines are prone to high overvoltage's along the line during switching operations. Traditionally, the solutions have been higher line insulation or the use of Preinsertion resistors for the switching circuit breakers. The modern solution,

maintenance--free, is the use of which is more economical and maintenance arresters at the line ends and, where required, at other points along the line. Here too, PEXLIM (Polymer housed) arresters have obvious advantages in view of their low weight and flexible erection as they can be attached directly to the breakers or the lines.

Factors Governing the Selection of Circuit Breakers The selection of the type of circuit breakers is governed mainly by the following important factors: 1)Use of pre-insertion resistors to control the switching surge over-voltage ; 2)Requirement of inherent restrike-free operation under all conditions ; 3)Consistent characteristics ; 4) 5) 6) 7)

Simple and reliable mechanism ; Operating speed ; Ease in maintenance ; Reliability and life of plant in view of future developments.

1. USE OF PRE – INSERTION RESISTORS: The importance of pre-insertion resistor in reducing over-voltages is of extreme significance and is well-known. It must, therefore, have the greatest reliability so far as switching-in action is concerned. Failure of this item will mean outage of breaker besides the possible damage which might result due to over voltages. The sequence of operation, control of insertion time and synchronizing the complete action are matters of great precision.

Method of Transient Control 1. Zero-Crossing Breaker 2. Pre-Insertion Resistor

Method of Transient Control Pre-Insertion Resistor

"RESISTORS" (1) are typically inserted into the capacitivecapacitive-energizing circuit through the closing of "RESISTOR CONTACT" (2) for 5 ms to 15 ms, prior to the closing of the "MAIN CONTACT" (3).

PRE INSERTION RESISTOR

Unless any abnormality is noticed during condition monitoring tests, no internal inspection of the PIR contacts of resistors is required. The insulators are to be kept clean and proper tight connection with the circuit breaker has to be ensured.

PRE INSERTION RESISTOR

PIR OPERATING LINK

 Energisation of Long Transmission Lines When an initially uncharged, unloaded long line is charged, then over-voltages to the order of twice the normal voltage are impressed, imposing an onerous duty on the power system. Higher voltages can occur if the line has trapped charges. When a line is switched on to an energised network, a voltage wave is impressed on it. The impressed wave will be reflected at the far end of the line, and if this is open, the voltage will increase two-fold. In the case of the threephase circuit, the situation becomes more severe if the circuit breaker poles do not close simultaneously. The wave on one phase will produce an induced wave on the other phase. The highest over-voltages occur at the open end of the unloaded line when this is being energized or re-energized. Tests have confirmed that these voltages can exceed even 3 p.u., when no special means to reduce them are used. With the help of pre-insertion resistors, the switching voltages can be reduced to less than 2 p.u. The best method of reducing the over-voltages generated on the energisation of the lines is to equip the circuit breaker with pre-insertion resistors (see Fig. 1), to ensure that the closing takes place in two stages. In the first stage, the resistor is included in series with the line to damp the waves. In the second stage, the resistor is short-circuited. The optimum value of the preinsertion resistance is usually of the same order of magnitude as that of the surge impedance of the line and the insertion time should be at least 10 milliseconds.

Fig. 1: Energisation of unloaded lines—basic phenomena The switching over-voltages occurring with energisation and re-energisation of the lines are caused by the system phenomenon and are thus independent of the circuit breaker used. Other means like the controlled synchronous closing of the circuit breaker poles can also be used.

Pre insertion resistance timings of CB is monitored to indicate any problem in close/open timings of the resistance with respect to the main contacts.




PIR time before main contact closing, PIR over lap time with main contact, PIR time before opening of the main contact.

MEASUREMENT OF CB OPERATING TIMINGS INCLUDING PIR TIMINGS PRECAUTIONS a) There should not be any joint in testing cables. b) Test leads should not touch any live part. c) Never connect the test set to Energized equipment. d) The ground cable must be connected first and removed at last. e) High voltage plugs should be free from moisture during installation and operation. f) Circuit Breaker Analyzer body should be earthed ( if separate earth is provided). g) It should be ensured that whole testing equipment along with testing procedures are available at testing site. Testing must be carried out in presence of testing personnel only. h) Surface/terminals should be cleaned where the connections for testing are to be made. i) Clean earth point with sand paper/wire brush where earth terminal is to be provided. j) Ensure that all the poles trip simultaneously through single close/trip command.

MEASUREMENT OF CB OPERATING TIMINGS INCLUDING PIR TIMINGS TESTING PROCEDURE a)Make connections as shown in the figure-14 below. It is to be ensured that R, Y, B phase marking cables are connected with the proper place in the CB analyser and colour codes are to be maintained for all the three poles of CB. b) Make connections for recording operating timings of Auxiliary contacts. c) Extend power supply to Circuit Breaker Analyzer. d) Give closing command to closing coil of CB and note down the PIR and main contact closing time. Take the print out from the analyser. e) Give tripping command to trip coil-I of CB & note down the main contact tripping time. f) Give tripping command to trip coil-II of CB & note down the main contact closing time. g) Note down the timings for `CO', and `OCO' by giving respective commands. CO command to be given without time delay but 300ms time delay to be given between O and CO operation in testing for OCO. h) To find out opening time of PIR contacts, PIR assembly has to be electrically isolated from Main contacts and then PIR contacts are to be connected to separate digital channels of the Analyzer.

MEASUREMENT OF CB OPERATING TIMINGS INCLUDING PIR TIMINGS TESTING PROCEDURE a)Make connections as shown in the figure-14 below. It is to be ensured that R, Y, B phase marking cables are connected with the proper place in the CB analyser and colour codes are to be maintained for all the three poles of CB. b) Make connections for recording operating timings of Auxiliary contacts. c) Extend power supply to Circuit Breaker Analyzer. d) Give closing command to closing coil of CB and note down the PIR and main contact closing time. Take the print out from the analyser. e) Give tripping command to trip coil-I of CB & note down the main contact tripping time. f) Give tripping command to trip coil-II of CB & note down the main contact closing time. g) Note down the timings for `CO', and `OCO' by giving respective commands. CO command to be given without time delay but 300ms time delay to be given between O and CO operation in testing for OCO. h) To find out opening time of PIR contacts, PIR assembly has to be electrically isolated from Main contacts and then PIR contacts are to be connected to separate digital channels of the Analyzer.

EVALUATION OF TEST RESULTS A) CLOSING TIMINGS Closing timings and Discrepancy in operating times of PIR and main contacts should not exceed the permissible limits as specified in the DOC NO: D-5-02-XX-01-03 (Acceptable/Permissible limits for Maintenance test results of Substation Equipment). In any case,main contacts should not close prior to closing of PIR contacts and PIR contacts should not open prior to closing of main contacts. In case, contact bouncing is observed in operating timings for PIR and main contacts, same should be rectified by tightening the cable connections.

B) TRIPPING TIMINGS Trip time and pole discrepancy in operating timings should not exceed beyond permissible value given in Doc. No. D-5-02-XX-01-03. In case of ABB, NGEF and CGL make CBs, while tripping, PIR contacts should not open after opening of main contacts.

C) 'CO' TIMINGS CO timings should be within permissible limits as specified by different manufacturers. If operating timings of CB poles are not within limits, same may be corrected by: 1. Equalizing the SF6 gas pressure in all the poles 2. Adjusting plunger movement of trip/ close coils 3. Adjustment in operating mechanism 4. Changing of trip/ close coils (if required) It is also important to measure timings of auxiliary contacts from the point of view of variations w.r.t. the main contacts.

PIR SPECIFICATION ::420 kV circuit breakers wherever specified shall be provided with single step preinsertion closing resistors to limit the switching surges to a value of less than 2.3 p.u. The value of the pre-insertion resistor and the duration of preinsertion time shall be as given in clause . the resistor shall have thermal rating for the following duties: i) TERMINAL FAULT Close .... 1 Min ........ Open ...... Close open 2 min ........ close ....... 1 Min........ open close open. C-1 min – O-C0 – 2 min – C-1 min –OCO. ii) RECLOSING AGAINST TRAPPED CHARGES Duty same as under (i) above. The first, third and fourth closures are to be on deenergised line while second closing is to be made with lines against trapped charge of 1.2 p.u. of opposite polarity. iii) OUT OF PHASE CLOSING One closing operation under phase opposition that is with twice the voltage across the terminals. iv) No allowance shall be made for heat dissipation of resistor during time interval between successive closing operations. The resistors and resistor supports shall perform all these duties without deterioration. Calculations and test reports of resistors proving thermal rating for duties specified above shall be furnished alongwith the bid. The calculations shall take care of adverse tolerances on resistance values and time settings.

PrePre-insertion resistor requirement for 420kV CB i) Rating (ohms) : 400 ii) Minimum pre-insertion time (ms) : 8ms iii) Opening of PIR contacts a) PIR Contacts should open immediately after closing of main circuits. b) Atleast 5 ms prior to opening of main contacts at rated air/gas pressure, where the PIR Contacts remain closed.