DISTRIBUTION TRANSFORMERS

Transcription

1 The University of New South Wales School of Electrical Engineering and Telecommunications Industrial and Commercial Power Systems Topic 4 DISTRIBUTION TRANSFORMERS

2 A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors the transformer's coils. 315kVA 22kV pole-mounted transformer (Wilson Transformer) Pad-mounted substation 140MVA 132kV generator transformer

3 Cut-away view of three-phase oil-cooled transformer. The oil reservoir is visible at the top. Radiative fins aid the dissipation of heat.

4 Range of EA standard substation transformers

5 Standard specifications AS : Power transformers General AS : Power transformers - Loading guide for oil-immersed power transformers AS : Power transformers - Application guide AS : Power transformers - Dry-type transformers AS : Loading guide for dry-type power transformers AS : Instrument transformers - Voltage transformers

6 1 LIQUID INSULATION

7 Oil insulated distribution transformer ONAN cooled type (Oil Natural Air Natural)

8 Liquid-insulated transformers Kraft paper wound around winding conductors paper impregnated with liquid dielectric to exclude air bubbles and to provide good thermal circulation for heat dissipation. Use mineral oil in most applications Such oil is highly flammable. Transformers generally limited to Class A materials temperature rise limits of about o C. Outdoor use only. Moisture is a major problem.

9 Mineral oil Refined from petroleum Hydrocarbon compounds: paraffinic, napthenic, aromatic. Additives sometimes added: Inhibitors to retard oxidation of oil Passivators to retard formation of copper sludge Examples: Shell Diala Oil B: straight mineral napthenic solvent refined oil Shell Diala Oil BX: Diala B with added oxidation inhibitor

10 Askarel essentially a Poly-Chlorinated Biphenyl (PCB) artificial insulating oil which is almost nonflammable. toxic effects, particularly if heated or burnt now banned from use in most countries some PCB insulated transformers still in service

11 Polychlorinated Biphenyls (PCBs) Mixtures of synthetic organic chemicals with same basic chemical structure and similar physical properties ranging from oily liquids to waxy solids. Non-flammable, chemically stable, high boiling point and electrical insulating properties.

12 Polychlorinated Biphenyls (PCBs) Used in many industrial and commercial applications including: electrical, heat transfer, and hydraulic equipment; as plasticizers in paints, plastics and rubber products; in pigments, dyes and carbonless copy paper and many other applications Health effects: carcinogenic (cancer) and noncarcinogenic (effects on immune system, nervous system)

13 Silicone oil tetrachloro-benzyl toluene with ~40% trichlorobenzene essentially non-flammable and no toxicity problems most favoured synthetic transformer insulating oil convective heat dissipation coefficient not so good electrical properties very similar to mineral oil more expensive

14 Ester Organic compounds that result from interaction of acids and alcohol Natural or synthetic Bio-degradable, no adverse toxic environmental or aquatic sideeffects Higher flash and fire points than mineral oil More hygroscopic than mineral oil, less affected by moisture Hygroscopic: Readily absorbing moisture, as from the atmosphere Becoming more popular as alternative fluids for transformers

15 H.T.H. : High Temperature Hydrocarbons Ref: ABB Biotemp fluid

16 BIOTEMP Characteristics BIOTEMP is currently in use in small power and distribution transformers across the United States. BIOTEMP is non-toxic to bird, animal and human life and is 97% biodegradable within 21 days when exposed to microbes. The fluid does not contain any petroleum, halogens, silicones or other materials that might adversely effect the environment.

17 2 DRY TYPE INSULATION

18 Open winding type windings in open air paper-insulated or nomex-insulated windings or insulating varnish coating applied to windings Problem with moisture ingress increase dielectric losses in insulation (dielectric dissipation factor) reduce insulation strength

19 Dry-type transformer, open winding

20 Cast resin type cast solid epoxy resin structure much less susceptible to moisture ingress and absorption differential expansion or contraction can cause cracking more costly than open structure dry-type transformer and often more expensive than silicone oil transformers.

21 Dry-type transformer, cast resin

22 SF 6 gas insulated transformer non-flammable used increasingly in buildings and in high-density areas very expensive but very reliable SF 6 is a non-toxic gas with very good electrical insulation properties and thermal transfer properties typically operate at ~2Atm where dielectric strength is similar to oil greenhouse problems SF 6 -N 2 mixture as alternative

23 SF gas insulated transformer

24 Cost

25 Source: Midel web site - Transformer Expectation

26 Losses

27 Losses Comparison of losses of different transformers types [for 1000kVA, 11kV/415V]

28 Source: Electrical Line Magazine Jan/Feb 2005

29 Reduction of insulation life loading effect on operating temperature increased temperature causes increased chemical reactions in insulation and lead to deterioration by changing insulation composition rule of thumb: increase of continuous operating temperature by 10 o C causes reduction of insulation life by ~50%. details in loading guides, e.g. AS (oil-filled transformers) and AS (drytype transformers).

30 Transformer impedance and S/C effect of transformer impedance on prospective fault current is significant typically around 5% (4-6% for Dist. Trans.) usually, leakage inductance component is major contribution to impedance

31 Transformer impedance and S/C

32 Transformer impedance and S/C

33 Tappings on windings for adjusting voltage level tapping points normally on HV windings (Why?) allow ~10% voltage variation, in ~1% steps manually changed (must de-energise and isolate transformer) Tap changer can be installed on neutral point, mid or end points of the winding. on-load tap changer (OLTC) not available

34 tapping points normally on HV windings (Why?) HV winding has more turns which makes voltage control easier HV winding is the outer winding and it is easier to install tap changer on that HV winding has less current compared to LV winding. Thus, easier to manufacture a low current tap changer

35 Transformer with on-load tap changing [rating about 5000kVA]

36 Connections many possible variations of winding connections affect magnitude of voltages and phase shift between primary and secondary. common winding connections (vector group; IEC): Dyn11 (most common, HV delta, LV star, neutral brought out, LV leads HV by 30 o ) Dyn1 Dyn5 Dyn7 star-connected on LV side to eliminate circulating triplen harmonics. HV side almost always delta connected

37 Vector group In the IEC vector group code, each letter stands for one set of windings. The HV winding is designated with a capital letter, followed by medium or low voltage windings designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference. The number is in units of 30 degrees. For example, a transformer with a vector group of Dyn1 has a deltaconnected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.

38 HV winding phase A phasor points at 12 o clock LV winding phase a phasor is oriented according to induced voltage relation which results from the connection. Sense of rotation is counterclockwise, giving the sequence ABC. Ref: Annex D, AS

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40 Cable terminations by means of a cable box LV cable box usually air insulated HV cable box is compound-filled with petroleum grease or similar viscous insulant sealing of box to keep moisture out moulded heat shrink terminations for XLPE cables: easy to construct paper insulated termination required considerable expertise to make

41 Example of MV (11kV) cable terminations

42 Parrallel Operation If transformers used in parallel, ensure they have: same voltage ratios If not, will cause circulating current, overheating same tapping points in use (i.e. same voltage) same vector diagram (same phase shift) If not, line and phase voltages will be intermixed, Overstress insulation. same impedance angle (preferable) If not, will cause unequal loading

43 DISTRIBUTION TRANSFORMERS PART 2 OPERATIONAL CHARACTERISTICS AND EFFICIENCY

44 1 CONSTRUCTION

45 Core and winding structure of a single phase transformer: (a) Core type, (b) Shell type Core Form is the most prevalent type in use. Core construction of 3-phase transformer (a) Core type (b) Shell type or 5 limb core

46 Types of transformer winding (a) Concentric, (b) Sandwich made up of disc sections.

47 2 EQUIVALENT CIRCUITS

48 Ideal transformer with perfect flux coupling between primary and secondary windings. Only winding resistance needed in equivalent circuit

49 Leakage flux

50 Equivalent circuit with leakage inductance

51

52 3 EXCITATION REQUIREMENT

53 (a) Magnetising current I m :

54 (b) Core loss and I c :

55 (c) Total exciting current I o : Full equivalent circuit

56 Equivalent circuit referred to the primary Equivalent circuit referred to the secondary

57 Lumped equivalent circuit

58 Phasor diagram

59

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61 Example: Determine:

62 Solution:

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65 4 TRANSFORMER LOSSES

66 Two components of transformer losses: load (copper) loss in resistance of windings load-dependent scales as square of load current temperature-dependent core (iron) loss in the core material comprise of hysteresis and eddy current losses constant whenever transformer is energized and independent of load

67 Copper loss:

68 Core loss:

69

70 5 TRANSFORMER EFFICIENCY

71 Efficiency Power efficiency

72 Efficiency very efficient, typically 95 99% varies with load maximum efficiency when core loss = load loss Can you explain?

73 Efficiency load will vary (usually) in a cyclic manner hence consider energy efficiency (instead of power efficiency)

74 MEPS AS Power Transformer Part 1.2: Minimum Energy Performance Standard (MEPS) requirement for distribution transformers Power efficiency determined at 50% of rated load at unity power factor. Apply to dry-type and oil-immersed type, 1φ and 3φ power transformers, 10kVA to 2.5MVA, used in 11kV and 22kV networks. Standard also defines minimum efficiency levels for "High Power Efficiency Transformers".

75 Power efficiency for oil-immersed transformers AS Tables 1&3

76 Power efficiency for dry-type transformers AS Tables 2&4

77 6 TRANSFORMER TESTS

78 Open-circuit test for core loss determination require normal operating flux in core, hence need rated voltage applied. no load connected so no load loss contribution in measured power, only (constant) core loss P o. provides P 0, I 0, R c and X m for equivalent circuit

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80 Short-circuit test for load loss determination I 2 (and I 1 ) is rated current, but applied voltage V 1 is impedance voltage level, only ~5%. thus core flux density is ~5% and core loss is negligible. but full rated currents flow in windings so measured power P sc is copper loss in winding resistances only. test requires measurement of P sc, V 1, I 1 and I 2. test results give copper loss, total winding resistance R eq and leakage reactance X eq

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82 EFFECT OF HARMONICS

83 Harmonic problem from non-sinusoidal supply voltage non-sinusoidal current (non-linear loads) losses in transformer are frequency dependent core loss scaled with square of frequency copper loss increase with frequency due to skin effect on the eddy current.

84 effect on core loss of load current harmonics is not generally significant and thus is neglected only load loss increase is considered when derating calculations are performed. thus assume pure sinusoidal supply voltage 2 methods to calculate de-rating factor: CBMEA Crest Factor method IEEE K-factor method

85 CBEMA Crest Factor: Find CF for triangular wave? CBEMA crest factor method is not widely used. The more accurate K-factor method is preferred.

86 K-Factor method Total harmonic distortion of a current waveform: K-factor:

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88 Example: 100kW of PCs supplied from transformer rated at 150kVA and with a typical value of P EC(R) =10%. harmonic current levels caused by the PCs

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91 K-factor transformers specifically designed for use with loads that produce harmonic distortion (without need to de-rate) typically K-factor value of ~15 compare to standard type, K-factor transformers are: more expensive (twice) heavier (20% more) larger lower impedance

92 Fire of a pole-mounted distribution transformer (in Moscow) Source: Intelligent Energy Europe Programme

93 Amorphous metal. Very low loss. Eg. HB1 from Metglass 1.6MVA amorphous iron core transformer (Ireland, 1998) No-load (core) loss = 384W Load (copper) loss = 18.2kW Source: Intelligent Energy Europe Programme

94 Thank you

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98 Ester