POWER DISTRIBUTION--Transformers in Distribution Systems

Transformers in distribution systems include distribution substation trans formers and distribution transformers.

DISTRIBUTION SUBSTATION TRANSFORMERS

Distribution substation transformers come in a wide variety of ratings. Some of the typical characteristics of distribution substation transformers are given in Tbl. 3.

TBL. 3 Typical Characteristics of Distribution Substation Transformers (Power distribution engineering: fundamentals and applications

Class, kV Voltage, kV Rating of High Voltage Winding 34.5 to 230 kV Rating of Low Voltage Winding 2.4 to 46 kV MVA Rating (OA) 2.5 to 75 MVA Transformer Impedance 5 to 12 % Number of Transformers in Substation 1 to 4 Loading OA, OA/FA, OA/FA/FOA, OA/FA/FA High Side Protection Circuit Switches, Circuit Breakers, Fuses Relay Protection Overcurrent, Differential, Under-Frequency Feeder Protection Circuit Breakers, Reclosers

ILL. 15 Three-phase 22.9 kV_/ 4.16 kVY distribution substation transformer rated 12 MVA OA/ 16 MVA FA1/20 MVA FA2. The transformer has fixed taps on the high-voltage side and an LTC on the low-voltage side.

ILL. 16 Voltage regulators at the 69/13.8 kV Lunenburg distribution substation, Lunenburg MA. The regulators are General Electric Type VR1 with ratings of 7.96 kV (line to neutral), 437 A

Distribution substation transformers usually contain mineral oil for insulating and cooling purposes (older transformers manufactured prior to 1978 originally contained askarels with high PCB content, but many of these have either been retired or re-classified as non-PCB transformers using perchloroethylene). In some units, an inert gas such as nitrogen fills the space above the oil, in order to keep moisture and air out of the oil, and the transformer tank is sealed. Some sealed transformers have a pressure relief diaphragm that's designed to rupture when the internal pressure exceeds a specified value, indicating possible deterioration of the insulation. Sealed transformers may also have a sudden pressure relay to either provide an alarm or de-energize the transformer when the internal pressure suddenly increases above a specified threshold.

Many distribution substation transformers have load tap changers (LTCs) that automatically regulate voltage levels based on loading conditions. Ill. 15 shows a distribution substation transformer that has an internal LTC on the low-voltage side. Some distribution substations have distribution substation transformers with fixed taps and separate voltage regulators. A voltage regulator is basically an autotransformer with taps that automatically raise or lower voltage, operating in a similar way as LTCs on distribution substation transformers. Ill. 16 shows a voltage regulator at a distribution substation. In addition to voltage regulators for distribution substations, there are also pole-mount voltage regulators that can be placed on feeders.

Some outdoor distribution substation transformers are equipped with a tank on the top of the transformer called a ''conservator,'' in which expansion and contraction of the oil takes place. Condensation of moisture and formation of sludge occur within the conservator, which is also provided with a sump pump to draw o¤ the moisture and sludge.

Distribution substation transformers have MVA ratings that indicate the continuous load that the transformers carry without exceeding a specified temperature rise of either 55_ C (for older transformers) or 65_ C (for newer transformers) above a specified ambient (typically 40_ C). Also, distribution substation transformers are typically equipped with external radiators with fans and /or oil circulating pumps, in order to dissipate heat generated by copper and core losses. These transformers have multiple MVA ratings that include the following:

1. OA rating (passive convection with oil circulating pumps and fans off).

2. FA rating (with fans on but oil circulating pumps off ).

3. FOA rating (with both fans and oil circulating pumps on). Some units, such as the one shown in Ill. 15, may have two FA ratings, a lower FA rating with one of two sets of fans on, and a higher FA rating with both sets of fans on. Also, some units have water-cooled heat exchangers. The nameplate transformer impedance is usually given in percent using the OA rating as the base MVA [5].

EX. 1 Distribution Substation Transformer Rated Current and Short Circuit Current A three-phase 230 kV_/34.5 kV Y distribution substation transformer rated 75 MVA OA/100 MVA FA/133 MVA FOA has a 7% impedance. (a) Deter mine the rated current on the low-voltage side of the transformer at its OA, FA, and FOA ratings. (b) Determine the per unit transformer impedance using a system base of 100 MVA and 34.5 kV on the low-voltage side of the transformer. (c) Calculate the short-circuit current on the low-voltage side of the transformer for a three-phase bolted fault on the low-voltage side.

Assume that the pre-fault voltage is 34.5 kV.

SOLUTION

_ 34:5 = 13.07 kA per phase

b. The transformer impedance is 7% or 0.07 per unit based on the OA rating of 75 MVA. Using (3.3.11), the transformer per unit impedance on a 100 MVA system base is:

Z pu System Base = 0.07(100/75)=0.09333 per unit

c. For a three-phase bolted fault, using the transformer ratings as the base quantities,

Isc3j = 1.0 / 0.07 = 286 per unit

= 286 = 7:372

= 105.31 kA/phase

Note that in (c) above, the OA rating is used to calculate the short-circuit current, because the transformer manufacturer gives the per unit transformer impedance using the OA rating as the base quantity. 9 Most utilities have a planning and operating policy of loading distribution substation transformers within their nameplate OA/FA/FOA ratings during normal conditions, but possibly above their nameplate ratings during short-term emergency conditions. If one transformer has a scheduled or forced outage, the remaining transformer or transformers can continuously carry the entire substation load.

Typically there are two emergency loading criteria for distribution sub station transformers:

1. A two-hour emergency rating, which gives time to perform switching operations and reduce loadings.

2. A longer-duration emergency rating (10 to 30 days), which gives time to replace a failed transformer with a spare that's in stock.

As one example, the distribution substation shown in the photograph at the beginning of this Section has two transformers rated 9 MVA OA/12 MVA FA1/15 MVA FA2. The practice of the utility that owns the substation is to normally operate the substation at or below 15 MVA. As such, if there is a forced or scheduled outage of one transformer, the other transformer can supply all six 4.16-kV feeders without being loaded above its FA2 nameplate of 15 MVA. For this conservative operating practice, emergency transformer ratings above nameplate are not used.

Some utilities operate their distribution substation transformers above nameplate ratings during normal operating conditions, as well as during emergency conditions. ANSI/IEEE C-57.91-1995 entitled, IEEE Guide for Loading Mineral-Oil-Immersed Transformers identifies the risks of transformer loads in excess of nameplate rating and establishes limitations and guidelines, the application of which are intended minimize the risks to an acceptable limit [21, 22].

EX. 2 Distribution Substation Normal, Emergency, and Allowable Ratings

As shown in Ill. 17, a distribution substation is served by two 138-kV sub-transmission lines, each connected to a 40 MVA (FOA nameplate rating) 138 kV_/12.5 kV Y distribution substation transformer, denoted TR1 and TR2. Both TR1 and TR2 are relatively new transformers with insulation systems designed for 65_ C temperature rises under continuous loading conditions. Shunt capacitor banks are also installed at 12.5-kV bus 1 and bus 2.

The utility that owns this substation has the following transformer loading criteria based on a percentage of nameplate rating:

1. 128% for normal summer loading.

2. 170% during a two-hour summer emergency.

3. 155% during a 30-day summer emergency.

(a) Assuming a 5% reduction for unequal transformer loadings, determine the summer ''normal'' rating of the substation. (b) Determine the ''allowable'' summer rating of the substation under the single-contingency loss of one transformer. (c) Determine the 30-day summer emergency rating of the sub station under the single-contingency loss of one transformer.

ILL. 17 Distribution substation for Ex. 2

SOLUTION

a. During normal operations, both transformers are in service. Using a 5% reduction to account for unequal transformer loadings, the summer normal substation rating is 1:28 _ð 40 þ 40 Þ_ 0:95 = 97 MVA. With both transformers in service, the substation can operate as high as 97 MVA without exceeding the summer normal rating of 128% or 51.2 MVA for each transformer.

b. The summer allowable substation rating, based on the single-contingency loss of one transformer, is 1:7 _ 40 = 68 MVA. The transformer that re mains in service is allowed to operate at 170% of its nameplate rating (68 MVA) for two hours, which gives time to perform switching operations to reduce the transformer loading to its 30-day summer emergency rating. Note that, even though the normal summer substation rating is 97 MVA, it's only allowed to operate up to 68 MVA, so that a transformer will not exceed its two-hour emergency rating in case the other transformer has an outage.

c. The 30-day summer emergency rating of the substation is 1:55 _ 40 = 62 MVA. When one transformer has a permanent failure, the other can operate at 62 MVA for 30 days, which gives time to replace the failed transformer with a spare that's in stock. 9

ILL. 18 Three conventional single-phase pole mount 25-kVA transformers. The transformers are wired to form a three-phase bank rated 75 kVA, 4160V_-208/120 V grounded Y, which supplies secondary service for commercial customers. The transformers are supplied from a 4160-V primary through fused cutouts, with surge arresters mounted vertically on the sides of the transformer tanks

DISTRIBUTION TRANSFORMERS

Distribution transformers connect the primary system (2.4 to 46 kV) to the secondary system (480 V and lower). Distribution transformers may be in stalled outdoors on overhead poles (pole-mount), outdoors at ground level on pads (padmount transformers), indoors within buildings, or underground in manholes and vaults.

Pole-mount transformers for overhead distribution are liquid-filled transformers that can be either single-phase or three-phase, depending on the load requirements and the primary supply configuration. Pole-mount distribution transformers may be manufactured as conventional transformers with no integral surge protection, overload protection, or short circuit-protection, or alternatively as completely self-protected (CSP) transformers.

For conventional pole-mount transformers, the protective devices are mounted external to the transformer. Typically a fuse cutout, which is a combination of a fuse and a switch, is installed adjacent to the conventional distribution transformer to disconnect it from the primary under overload conditions or an internal transformer failure. Similarly, a surge arrester is in stalled adjacent to the conventional transformer primary to protect it against transient overvoltages due to switching and lightning surges. Ill. 18 shows three conventional single-phase pole-mount distribution transformers that are wired to form a three-phase bank rated 75 kVA supplying 120/208 V overhead secondary service for commercial customers.

For CSP transformers, a primary fuse is located within the transformer tank. The surge arrester is mounted outside the tank, but connected to the primary bushing. Circuit breakers on the secondary side of CSP trans formers provide protection from overloads and are coordinated with primary fuses.

Padmount transformers for underground distribution are liquid-filled or dry-type transformers that can be either single-phase or three-phase, outdoors or indoors. Single-phase padmount distribution transformers are typically designed for underground residential and commercial distribution systems where safety, reliability and aesthetics are especially important.

Three-phase padmount distribution transformers are compact power centers usually for large commercial or industrial applications. Ill. 19 shows a three-phase liquid-filled padmount transformer that supplies 480/277 V underground secondary service to an industrial plant. Dry type padmount distribution transformers, whose insulation is solid (for example glass, silica, epoxy, or polyester resins) are primarily used where safety is a major concern, in close proximity to people such as at schools, hospitals, commercial buildings, and industrial plants, both indoors and outdoors.

ILL. 19 Three-phase oil-filled padmount transformer shown with doors closed (a) and open (b). This padmount, rated 1,500 kVA OA, kV_- 480/277 V grounded Y with internal fuses on the high-voltage side, supplies secondary service to an industrial plant

ILL. 20 General Electric 500 kVA, 13.8 kV delta 120/208V grounded Y network transformer from utility stock.

TBL. 4 Standard Distribution Transformer kVA Ratings.

Network transformers are large (300-to-2,500 kVA) liquid-filled, three phase distribution transformers that are designed for use in underground vaults or in specially designed rooms within buildings to supply power to either secondary networks or spot networks. Their voltage ratings vary from 4.16-to-34.5 kV_ or grounded Y for the high-voltage windings, and either 216 grounded Y/125 V or 480 grounded Y/277 V for the low-voltage windings. Network transformers are designed to be connected through network protectors that are integrally mounted on the transformer. Ill. 20 shows a network transformer from utility stock. Network transformers are built as either ''vault type'' (suitable for occasional submerged operation) or ''subway type'' (suitable for continuous submerged operation).

Tbl. 4 shows typical kVA ratings of distribution transformers. The kVA ratings of distribution transformers are based on the continuous load the transformers can carry without exceeding a specified temperature rise of either 55_ C (for older transformers) or 65_ C (for newer trans formers above a specified ambient temperature (usually 40_ C). When in service, distribution transformers are rarely loaded continuously at their rated kVA as they go through a daily load cycle. Oil-filled distribution transformers have a relatively long thermal time constant; that's , the load temperature rises slowly during load increases. As such, it's possible to load these transformers above their kVA ratings without compromising the life expectancy of the transformer. ANSI/EEE Std. C57.92-1981 is entitled IEEE Guide for Loading Mineral-Oil-Immersed Overhead and Pad-Mounted Distribution Transformers Rated 500 kVA and Less with 65_C or 55_ C Average Winding Rise.

Tbl. 5 shows a typical loading guide, based on this standard.

Note that in accordance with Tbl. 5, short-time loadings can be as high as 89% above the nameplate kVA rating for short durations. Also note that dry-type distribution transformers, which are not considered as rugged as liquid-filled units of the same rating, are not normally loaded above their kVA ratings.