Pomona, CA May 24 & 25, 2016 LTC Applications - Location, Series & Preventative Auto s siemens.com/answers
Introduction Tap changer at active part Example of 3-phase tapchanger Page 2
Winding Configurations Primary > High voltage, many turns Secondary > Heavy current, more copper
Winding Configurations Tap Changers and Tap Windings: DETC or OLTC? OLTC vs. external Regulator? Separate or integral tap windings? Page 4
Reactance-Type on-load Tap changer (LV) Page 5
Resistance-type OLTC (HV or LV) Page 6
Function of the Preventive Auto-transformer Enables the tap changer to move from one position to an adjacent position Without breaking the tap connection Without causing a winding short circuit. Provides an intermediate voltage step between the tap winding steps.
Components of the Preventive Auto The PA has 2 equal windings connected in series as an Auto The windings are inter-wound to reduce the impedance to load current. main transformer tap winding P4 P1 Core Selector switches PA I L The voltage drop due to load current is insignificant. Winding 2 Vacuum interrupter P3 By-pass switches Winding 1 P2 I L P
Non-bridging position P4 P1 I L /2 I L /2 Currents in the 2 windings are in opposing directions. P3 P2 The windings are inter-wound to reduce the impedance to a low level to avoid voltage drop across the PA. 1 No voltage is applied to the windings and there is no core flux.
Bridging position P4 I L /2+I C P1 I L /2-I C When bridging, voltage applied to 2 windings in series. Selected voltage @middle of selected tap voltages. This circulates a current (I C ) through both windings, tap winding section, and by-pass switches. P3 P2 Circulating current limited by PA s magnetizing impedance. If used standard transformer core, impedance would be very high and circulating current would be small (<1%). 6 It s necessary to reduce magnetizing impedance to allow reasonable circulating current,. This done by introducing airgaps in PA s core leg. The transformer now has the magnetizing characteristics of a gapped core reactor.
Loss Evaluation at Nominal Rating Three possibilities for loss evaluation 1. Bridging 2. Non-bridging 3. Average Recommend average of bridging and non-bridging position losses.
Components: Helical windings with CTC
Components: Disc windings with simple conductors
Components: Rectangular core and windings Core E Foil windings E with windings fitted Top yoke, clamps and tie rods fitted
Noise level Noise occurs whenever legs excited (on bridging position). Noise also produced during commutation; it sounds like a grunt lasting about 1 sec. Noise created by compression of gap material, - Can be reduced by making gap material very hard, -OR- -Some designs use very high compression forces & softer gap material. As gaps are magnetically compressed, leg blocks & yokes are subject to vibration. Anti-vibration mountings may be used to reduce vibration transmitted to the tank.
Types of regulation Reversing Regulation Similar to linear arrangement, Selector contact moves up or down. There is also a reversing switch which reverses the sense of regulating winding. Maximum tap position > Regulating winding is 100% in positive sense. Mid tap position > none of regulating winding is in circuit Minimum tap position > when 100% regulation is in negative sense Page 16
Autotransformer Line End Regulation HV Do autotransformers require flux remaining constant across all tap positions? MV Can position regulating winding at MV terminal, rather than at neutral. Disadvantage - regulating winding (& tap changer) directly exposed to transients (lightning strikes) from MV terminal Page 17 120kV and 230KV voltages require 3 x single-phase tap changers (high currents). Maybe also Metal Oxide Varistors (MOV s) across regulating winding (lightning surges)
Autotransformer with Neutral End regulation HV Better alternative - place cheaper 3-phase tap changer at neutral end. MV With graded insulation, neutral terminal & OLTC have lower insulation levels. Disadvantage - number of turns in both MV and HV circuits changes simultaneously. Without one winding acting as a constant voltage, the transformer becomes a variable flux transformer. If tertiary winding is used, its output voltage will vary, unless a booster is fitted to compensate. Page 18
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In-tank Tap changer Cut-away Showing Completed Tap changer Regulating Leads Connections Motor Drive Page 20
Tap changer Basics Mounting flange Elbow pipes for connection to conservator Diverter Housing Connections for transformer leads Selector The main components are the selector, diverter, drive mechanism and surge relay. The selector is in the main bulk oil. The diverter, which makes and breaks the current path, is in a separate oil compartment with it s own conservator & surge relay. The drive mechanism houses the drive motor which moves tap switches, and controls and indications. Page 21
Tap changer Motor Drive Unit Page 22 1. HAND CRANK 2. INSPECTION LAMP 3. SHAFT FOR HAND CRANK 4. OPERATION COUNTER 5. POSITION COUNTER 6. DRAG HANDS 7. TAP CHANGE INDICATOR 8. DOOR HAND FOR LAMP 9. MOTOR PROTECTION DEVICE 10. RAISE/LOWER CONTROL SWITCH 11.HEATING PANEL 12. SWING-FRAME LATCH 13. DRAWINGS 14.COVER PLATE 15. PADLOCKING FACILITY 16.QUICK RELEASE SCREWS
Tap changer Separate Oil Systems of OLTC and expansion tank Separation wall Siliga gel breather OLTC expansion tank Protective relay Diverter switch oil Silica gel breather Main Buchholz relay Main transformer oil Page 23
Tap changer Protective Relay OLTC Surge Relay A type RS 2001 surge relay is fitted in the pipe work between the divertor compartment and the OLTC conservator. The surge relay generates a trip signal on a fault which is latched. A red flag can be seen in the relay window when it has tripped. Testing Remove the terminal box lid to reveal the test and reset buttons. Pressing the test button latches the switch in the trip position. It must be reset. TERMINAL BOX INSPECTION WINDOW Page 24
Examples for Tap Changers OLTC V III 200 D (MR) OLTC M III 350/M III 500 (MR) OLTC RM III 600 (MR) Page 25
Siemens US s Contact James McIver Principal Application Engineer E T TR US 6860 Bermuda Road, STE 100 Las Vegas NV 89119 USA Mobile: (702) 241-0157 E-mail: james.mciver@siemens.com siemens.com/transformers Page 26