Three phase transformer NKD, Lecture notes of Electric Machines

Download Three phase transformer NKD and more Lecture notes Electric Machines in PDF only on Docsity! 10/3/2013 1 1 Department of EEE, CUETNipu Kumar Das Electrical Machine-I Three Phase Transformer EEE-233 2 Department of EEE, CUETNipu Kumar Das THREE PHASE TRANSFORMER Why Three Phase Transformer?
Almost all electrical power generation and most of power transmission throughout the world today is three phase ac circuits. Advantages of 3φ AC Power Systems Over Single Phase AC Power Systems: 1) It is possible to get more power per pound of metal from a 3φ machine. 2) Power delivered to 3φ load is constant at all times, instead of pulsing as it does in single phase system. 3) 3φ Systems also make the use of 3φ induction easier by allowing them to start without special auxiliary starting windings . 10/3/2013 2 3 Department of EEE, CUETNipu Kumar Das THREE PHASE TRANSFORMER Construction: 3φ transformer circuits can be construct in one of two ways: 1. Three single-phase transformers connected in a three-phase bank 2. Three sets of windings wrapped on a common core 4 Department of EEE, CUETNipu Kumar Das THREE PHASE TRANSFORMER Windings of a single phase transformer:
have only two coils namely primary and secondary.
Primary is energized with single phase supply and load is connected across the secondary. Windings of a in a 3-phase transformer :
there will be 3 numbers of primary coils and 3 numbers of secondary coils.
These 3 primary coils and the three secondary coils are t o be properly connected so that the voltage level of a bal anced 3-phase supply may be changed to another 3-phas e balanced system of different voltage level. 10/3/2013 5 9 Department of EEE, CUETNipu Kumar Das G ROUP 1: 00 PHASE SHIFT . 10 Department of EEE, CUETNipu Kumar Das GROUP 2: 1800 PHASE SHIFT . 10/3/2013 6 11 Department of EEE, CUETNipu Kumar Das GROUP 3: 300 PHASE SHIFT (lag) , 12 Department of EEE, CUETNipu Kumar Das GROUP 4: + 300 PHASE SHIFT (lead) . 10/3/2013 7 13 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER Y-Y CONNECTION .  In Y-Y connection, the primary voltage on each phase of the transformer is given by The secondary phase voltage is The primary phase voltage is related to secondary phase voltage by the turns ration of the transformer is defined as: )1(3 P V LP V φ= )2(3 S V LS V φ= )3( 3 3 YYina S V P V LS V LP V −== φ φ 14 Department of EEE, CUETNipu Kumar Das Star connected primary with secondary coils left alone . 10/3/2013 10 19 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER Y- ∆ CONNECTION Y-∆ connection :
The primary voltage on each phase of the transformer is
The secondary phase voltage is
The voltage ratio of each phase is:
The overall voltage ratio is )1(3 P V LP V φ= )2( S V LS V φ= )3( 3 3 ∆−== Yina S V P V LS V LP V φ φ )3(a S V P V = φ φ 20 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER Y- ∆ CONNECTION
The Y-∆ connection has no problem with third harmonic components due to circulating currents in ∆. It is also more stable to unbalanced loads since the ∆ partially redistributes any imbalance that occurs.
One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-phase transformers since transfor mers secondary voltages must be in-phase to be paralleled. Therefore , we must pay attention to these shifts.
In the U.S., it is common to make the secondary voltage to lag the pri mary voltage. The connection shown in the previous slide will do it. 10/3/2013 11 21 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER Y-∆ CONNECTION . 22 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER ∆-Y CONNECTION )1( P V LP V φ= )2(3 S V LS V φ= )3( 33 Yin a S V P V LS V LP V −∆== φ φ ∆-Y connection :
The primary voltage on each phase of the transformer is
The secondary phase voltage is
The voltage ratio of each phase is:
The overall voltage ratio is
The same advantages and the same phase shift as the Y-∆ connection. )3(a S V P V = φ φ 10/3/2013 12 23 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER ∆- ∆ CONNECTION ∆-∆ connection :
The primary voltage on each phase of the transformer is
The secondary phase voltage is
The overall voltage ratio is No phase shift, no problems with unbalanced loads or harmonics. )1( P V LP V φ= )2( S V LS V φ= )3(∆−∆== in S V P V LS V LP V a φ φ 24 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER Usual connections for three-phase transformers Star/star (Yy0, Yy6) This is the most economical one for small high voltage transformers. Insulation cost is highly reduced. Neutral wire can permit mixed loading. Triplen harmonics are absent in the lines. These Triplen harmonic currents cannot flow, unless there is a neutral wire. wye-wye-delta (Y-Y-∆) Frequently used to interconnect high voltage networks (240 kv/ 345 kv). The delta winding filters the 3rd harmonics, equalizes the unbalanced current, and provides a path For ground curnt wye-delta (Y-∆) Frequently used as step down (345 kv/69 kv) delta-delta (∆-∆) This is an economical configuration for medium voltage (15 kv), transformers. Large amount of unbalanced load can be met with ease. Mesh permits a circulating path for triplen harmonics thus attenuates the same. It is possible to operate with one transformer removed in open delta or Vee connection meeting 58 percent of the balanced load. delta-wye (∆-Y) This arrangement is very common for power supply transformers. The delta winding permits triplen harmonic currents to circulate in the closed path and attenuates them 10/3/2013 15 29 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER: PER-UNIT SYSTEM ii. Voltage regulation of a 3-phase transformer : The rated phase current on the primary side can be found as: The rated phase voltage on the (Y connected) secondary of the transformer is: When referred to the primary (high-voltage) side, this voltage becomes %100× − = s aV s aV P V VR φ φφ A V VA base V base S I 208,1 800,133 000,50 , 3 = × == φ φ V VL V V 120 3 208 3 ===φ V S Va s V 138003 ' == φφ 30 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER: PER-UNIT SYSTEM Assuming that the transformer secondary winding is working at the rated voltage and current, the resulting primary phase voltage is The voltage regulation, therefore, is: iii. In the per-unit system, the output voltage is 1∠00, and the current is 1∠- 360). Therefore, the input voltage is V jIeqjXIeqRS aV p V 073.2506,14 036208.1800036208.12.1140013800 ∠= −∠×+−∠×+∠=++= φφφφ %1.5%100 13800 1380014506 %100 =×−=× − = s aV s aV P V VR φ φφ 073.2051.1 036107.0036101.0001 ∠= −∠×+−∠×+∠= j p Vφ 10/3/2013 16 31 Department of EEE, CUETNipu Kumar Das 3-PHASE TRANSFORMER: PER-UNIT SYSTEM Thus, the voltage regulation in per-unit system will be: The voltage regulation in per-unit system is the same as computed in volts %1.5%100 1 1051.1 =×−=VR 32 Department of EEE, CUETNipu Kumar Das Transformer ratings Transformers have the following major ratings: i. Apparent power; ii. Voltage; iii. Current; iv. Frequency. 10/3/2013 17 33 Department of EEE, CUETNipu Kumar Das Transformer ratings: Voltage and Frequency . The voltage rating of a transformer serve two functions a) To protect the winding insulation from breakdown due excessive voltage applied to it. b) related to the magnetization curve and magnetization current of the transformer flux Magnetization current If a steady-state voltage is: tVtV M ωsin)( = applied to the transformer’s primary winding, the transformer’s flux will be An increase in voltage will lead to a proportional increase in flux. However, after some point (in a saturation region), such increase in flux would require an unacceptable increase in magnetization current! t pN M V pN t dttV ω ω φ cos1)( )(∫ −== 34 Department of EEE, CUETNipu Kumar Das Transformer ratings: Voltage and Frequency Therefore, the maximum applied voltage (and thus the rated voltage) is set by the maximum acceptable magnetization current in the core. We notice that the maximum flux is also related to the frequency: Therefore, to maintain the same maximum flux, a change in frequency (say, 50 Hz instead of 60 Hz) must be accompanied by the corresponding correction in the maximum allowed voltage. This reduction in applied voltage with frequency is called derating. As a result, a 50 Hz transformer may be operated a t a 20% higher voltage on 60 Hz if this would not cause insulation damage. pN V ω φ max max = 10/3/2013 20 39 Department of EEE, CUETNipu Kumar Das Instrument transformers
Two special-purpose transformers are uses to take measurements: potential and cur rent transformers.
A potential transformer has a high-voltage primary, low-voltage secondary, and ve ry low power rating. It is used to provide an accurate voltage samples to instrument s monitoring the power system.
A current transformer samples the current in a line and reduces it to a safe and mea surable level. Such transformer consists of a secondary winding wrapped around a ferromagnetic ring with a single primary line (that may carry a large current )runni ng through its center. The ring holds a small sample of the flux from the primary li ne. That flux induces a secondary voltage.
Windings in current transformers are loosely coupled: the mutual flux is much smaller than the leakage flux. The voltage and current ratios do not apply although the secondary current is directly proportional to the primary.
Current transformers must be short-circuited at all times since very high voltages can appear across their terminals. 40 Department of EEE, CUETNipu Kumar Das THREE PHASE TRANSFORMER 10/3/2013 21 41 Department of EEE, CUETNipu Kumar Das THREE PHASE TRANSFORMER 42 Department of EEE, CUETNipu Kumar Das THREE PHASE TRANSFORMER 10/3/2013 22 43 Department of EEE, CUETNipu Kumar Das THANK YOU