PARALLEL OPERATIONS OF TRANSFORMER PRIMER INFORMATION

The theoretically ideal conditions for paralleling transformers are:

1. Identical turn ratios and voltage ratings.

2. Equal percent impedances.

3. Equal ratios of resistance to reactance.

4. Same polarity.

5. Same phase angle shift.

6. Same phase rotation.

Single-Phase Transformers

For single-phase transformers, only the first four conditions apply, as there is no phase rotation or phase angle shift due to voltage transformation.

If the turns ratio are not same a circulating current will flow even at no load. If the percent impedance or the ratios of resistance to reactance are different there will be no circulating current at no load, but the division of load between the transformers when applied will no longer be proportional to their KVA ratings.

Three-Phase Transformers

The same conditions hold true for three phase transformers except that in this case the question of phase rotation and phase angle shift must be considered.

Phase Angle Shift

Certain transformer connections as the wye-delta or wye-zigzag produce a 30º shift between the line voltages on the primary side and those on the secondary side. Transformers with these connections cannot be paralleled with other transformers not having this shift such as wye-wye, delta-delta, zigzag-delta, or zigzag-zigzag.

Phase Rotation

Phase rotation refers to the order in which the terminal voltages reach their maximum values. In paralleling, those terminals whose voltage maximums occur simultaneously are paired.

Power Transformer Practice

The preceding discussion covered the theoretically ideal requirements for paralleling. In actual practice, good paralleling can be accomplished although the actual transformer conditions deviate by small percentages from the theoretical ones.

Good paralleling is considered attainable when the percentage impedances of two winding transformers are within 7.5% of each other. For multi-winding and auto-transformers, the generally accepted limit is 10%.

Furthermore, in power transformers of normal design the ratio of resistance to reactance is generally sufficiently small to make the requirement of equal ratios of negligible importance in paralleling.

When it is desired to parallel transformers having widely different impedances, reactors or auto-transformers having the proper ratio should be used. If a reactor is used it is placed in series with the transformer whose impedance is lower. It should have a value sufficient to bring the total effective percent impedance of the transformer plus the reactor up to the value of the percent impedance of the second transformer.

When an auto-transformer is used, the relative currents supplied by each transformer are determined by the ratio of the two sections of the auto-transformer. The auto-transformer adds a voltage to the voltage drop in the transformer with the lower impedances and subtracts a voltage from the voltage drop in the transformer with the higher impedance. Auto-transformers for use in paralleling power transformers are specially designed for each installation. The wiring diagram showing the method of connecting the auto-transformer is usually furnished.

In general, transformers built to the same manufacturing specifications as indicated by the nameplate may be operated in parallel.

Connecting transformers in parallel when the low voltage tension is comparatively low requires care that the corresponding connecting bars or conductors have approximately the same impedance. If they do not, the currents will not divide properly.

Information Courtesy of ABB Power Transformers

IMPEDANCE MATCHING TRANSFORMER SELECTION BASIC AND TUTORIALS

Select a transformer with the correct turns ratio to match the 8- resistive load in Fig. 4.3 to the Thevenin equivalent circuit of the source.

Calculation Procedure
1. Determine the Turns Ratio
The impedance of the input circuit, Zi, is 5000 . This value represents the Thevenin impedance of the source. The load impedance, ZL , is 8 .

To achieve an impedance match, the required turns ratio is

Therefore, the impedance-matching transformer must have a turns ratio of 25:1.

Related Calculations. The maximum power transfer theorem (Sec. 1) states that maximum power is delivered by a source to a load when the impedance of the load is equal to the internal impedance of the source.

Because the load impedance does not always match the source impedance, transformers are used between source and load to ensure matching.

When the load and source impedances are not resistive, maximum power is delivered to the load when the load impedance is the complex conjugate of the source impedance.

PARALLEL OPERATIONS OF POWER TRANSFORMERS CONSIDERATION

The theoretically ideal conditions for paralleling transformers are:

1. Identical turn ratios and voltage ratings.

2. Equal percent impedances.

3. Equal ratios of resistance to reactance.

4. Same polarity.

5. Same phase angle shift.

6. Same phase rotation.

Single-Phase Transformers
For single-phase transformers, only the first four conditions apply, as there is no phase rotation or phase angle shift due to voltage transformation.

If the turns ratio are not same a circulating current will flow even at no load.  If the percent impedance or the ratios of resistance to reactance are different there will be no circulating current at no load, but the division of load between the transformers when applied will no longer be proportional to their KVA ratings.

Three-Phase Transformers

The same conditions hold true for three phase transformers except that in this case the question of phase rotation and phase angle shift must be considered.

Phase Angle Shift
Certain transformer connections as the wye-delta or wye-zigzag produce a 30º shift between the line voltages on the primary side and those on the secondary side.  Transformers with these connections cannot be paralleled with other transformers not having this shift such as wye-wye, delta-delta, zigzag-delta, or zigzag-zigzag.

Phase Rotation
Phase rotation refers to the order in which the terminal voltages reach their maximum values.  In paralleling, those terminals whose voltage maximums occur simultaneously are paired.

Power Transformer Practice
The preceding discussion covered the theoretically ideal requirements for paralleling.  In actual practice, good paralleling can be accomplished although the actual transformer conditions deviate by small percentages from the theoretical ones.

Good paralleling is considered attainable when the percentage impedances of two winding transformers are within 7.5% of each other.  For multi-winding and auto-transformers, the generally accepted limit is 10%.

Furthermore, in power transformers of normal design the ratio of resistance to reactance is generally sufficiently small to make the requirement of equal ratios of negligible importance in paralleling.

When it is desired to parallel transformers having widely different impedances, reactors or auto-transformers having the proper ratio should be used.  If a reactor is used it is placed in series with the transformer whose impedance is lower.  It should have a value sufficient to bring the total effective percent impedance of the transformer plus the reactor up to the value of the percent impedance of the second transformer.

When an auto-transformer is used, the relative currents supplied by each transformer are determined by the ratio of the two sections of the auto-transformer.  The auto-transformer adds a voltage to the voltage drop in the transformer with the lower impedances and subtracts a voltage from the voltage drop in the transformer with the higher impedance.

Auto-transformers for use in paralleling power transformers are specially designed for each installation.  The wiring diagram showing the method of connecting the auto-transformer is usually furnished.

In general, transformers built to the same manufacturing specifications as indicated by the nameplate may be operated in parallel.

Connecting transformers in parallel when the low voltage tension is comparatively low requires care that the corresponding connecting bars or conductors have approximately the same impedance.  If they do not, the currents will not divide properly.

MATCHING TRANSFORMERS FOR PARALLEL AND BANK OPERATIONS BASIC AND TUTORIALS

PARALLEL AND BANK  OPERATIONS TRANSFORMERS MATCHING 
How To Match Transformers For Banking and Parallel Operation?


The following rules must be obeyed in order to successfully connect two or more transformers in parallel with each other:


1. The turns ratios of all of the transformers must be nearly equal.
2. The phase angle displacements of all of the transformers must be identical.
3. The series impedances of all transformers must be nearly equal, when expressed as ‘‘%Z’’ using the transformer impedance base.

The first two rules are required so that the open-circuit secondary voltages of the transformers are closely matched in order to avoid excessive circulating currents when the parallel connections are made.

The last rule is based on the fact that for a given voltage rating and %Z, the ohmic impedance of a transformer is inversely proportional to its KVA rating. When transformers having the same %Z are connected in parallel, the load currents will split in proportion to the KVA ratings of the units.

Therefore, transformers with different KVA ratings can be successfully operated in parallel as long as their %Z values are all approximately the same.


Example
(This example is based on an actual event.)
Two three-phase 10,000 KVA 66,000Δ-12,470Y volt transformers were in parallel operation in a substation. The primaries of the two transformers are connected to a 66 kV transmission line through a single air break switch.

This switch is designed to interrupt magnetizing current only, which is less than 1 A. The transformers were being removed from service and the secondary loads had been removed. A switchman then started to open the air break switch, expecting to see a small arc as the magnetizing current was interrupted.

Instead, there was a loud ‘‘bang’’ and there was a ball of flame where the air break switch contacts had vaporized. Something was obviously wrong.

Upon closer inspection, it was revealed that the two transformers had been set on widely different taps: The first transformer was on the 62,700 V primary tap and the second transformer was on the 69,300 V primary tap.

Both transformers had a 7% impedance. Because the turns ratios were unequal, a circulating current was set up even without any secondary load. The opencircuit secondary voltage difference, assuming 66 kV at the transformer primaries, is calculated below.

ΔEs = 66,000 x ( 12,470/ 62,700 - 12,470/69,300 ) V = 1250 V = 0.10 per unit

The per-unit circulating current in the secondary loop is equal to ΔEs divided by the sum of the per-unit impedances of the two transformers:

Ic = 0.10/ 0.14 = 0.714 per unit


Converting Ic into amperes:
Ic = 0.714 x 10,000 KVA/(12.47 kV 1.732) = 331 A per phase

Since Ic flows in a loop in the secondary circuit, the current out of the secondary of the first transformer equals the current into the secondary of the second transformer. But since the turns ratios are not equal, Ic does not get transformed into equal and opposite currents at the primaries.

Primary current of first transformer
65.8 A per phase
Primary current of second transformer
59.6 A per phase
The net current through the air break switch, IAB, is the difference in the primary currents:
IAB 65.8 A per phase 59.6 A per phase 6.2 A per phase

The current through the air break switch supplies the I2c Xs reactive losses of both transformers and therefore lags the primary voltage by 90°. The resulting current exceeded the interrupting rating of the switch, causing it to fail.

PARALLEL OPERATION OF TRANSFORMER LOAD DISTRIBUTION TUTORIALS

LOAD DISTRIBUTION OF TRANSFORMER DURING PARALLEL OPERATION 
How To Compute The Load Distribution of Transformer During Parallel Operation?

If parallel operated transformers have the same voltage ratio but different short circuit impedance, then the load is distributed among them in such a way that each transformer accepts a specific level of load for which the short-circuit impedance becomes the same for all the parallel operated transformers.

When none of the parallel operated transformers is permitted to be overloaded, the transformer with the minimum short-circuit impedance must operate at maximum under its rated power. Consequently, the load distribution is given by the following equation:

Example 1.8

Let us assume that three transformers operate in parallel. The first transformer has 800 kVA rated power and 4.4% short-circuit impedance. 

The rated power and the short-circuit impedance of the other two transformers is 500 kVA and 4.8%, and 315 kVA and 4.0%, respectively. 

Calculate the maximum total load of the three transformers.

Solution

From the above, it is concluded that the maximum total load (1460 kVA) represents 90.4% of the total installed power (1615 kVA).

It should be noted that, in order for the maximum total load to be equal to the total installed power, the transformers must have the same short-circuit impedance.

POWER TRANSFORMER QUESTION AND ANSWER TUTORIALS PART 1

QUESTION AND ANSWER ABOUT POWER TRANSFORMER
Power Transformer Question and Answer

1 . What is a transformer and how does it work?

A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle.

A transformer has no moving parts and is a completely static solid state device, which insures, under
n o rmal operating conditions, a long and t ro u b l e - f ree life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core.

When voltage is introduced to one coil, called the primary, it magnetizes the iron core . A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

2 .What are taps and when are they used?
Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Standard tap arrangements are at two-and-one -half and five percent of the rated primary voltage for both high and low voltage conditions.

For example, if the  transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating.

The standard ASA and NEMA designation for taps are “ANFC” (above normal full capacity) and “B N FC” (below normal full capacity).


3 . What is the difference between “ I n s u l a t i n g,” “I s o l a t i n g,” and “Shielded Winding” transformers?
Insulating and isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two.

A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise. This is especially important in critical applications such as computers, process controllers and many other microprocessor controlled devices.

All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers ,
w hose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.

4. Can transformers be operated at voltages other than nameplate voltages?
In some cases, transformers can be operated at voltages below the nameplate rated voltage. In N O case should a transformer be operated at a voltage in excess of its nameplate rating unless taps are provided for this purpose.

When operating below the rated voltage, the K VA capacity is reduced correspondingly. For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts.

If the transformer was originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.

5. Can 60 Hz transformers be operated at 50 Hz?
ACME transformers rated below 1 KVA c a n be used on 50 Hz service. Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to the higher losses and resultant heat rise. Special designs are required for this service.

However, any 50 Hz transformer will operate on a 60 Hz service .