Ynyn0+d Transformer

[Guilleuma Deeken]

SituationI have recently encountered a fairly unusual installation where a YNyn0 transformer has been used to supply a low voltage pump installation incorporating a large VFD.

The transformer was initially chosen to offset harmonic distortion with an adjacent Dyn11 supplying an identical VFD.

The transformer has been installed with the primary neutral connection floating in an effort to prevent zero sequence currents from causing nuisance tripping at the supply.

My concern is that, at least in theory, the zero sequence impedance on the secondary of the transformer will be high, and will not permit enough current to drive the protection in the event of an earth fault.

The add to the complexity of the situation, the upstream supply transformer is earth fault limited to 10 amps. There is no reticulated neutral on the 22kV.

The three potential solutions I am considering are:

1) Earth the primary star point of the pump transformer (however I'm concerned this will allow some fault current to bypass the upstream NER)

2) Carry out zero sequence impedance testing to accurately model any flux linkage through the tank, to accurately determine how much current would flow

3) Replace the transformer (obviously the costly option)

As I don't often encounter this vector group in my line of work, I'm interested in the thoughts of anyone with experience in this area.

Transformer details:

22kV/433V 50Hz

750kVA

5.7%

ONAN

Answer

You should give consideration to grounding the high-voltage neutral. If the YNyn0 transformer is supplying a single, 3-phase, VFD load and there are no other loads which are connected phase-to-neutral on the low voltage side of the transformer, then blocking zero-sequence harmonics will not be required because the VFD rectifier will not produce zero-sequence harmonic currents during balanced conditions.

The upstream supply transformer (supplying the 22 kV) uses a NER to limit the ground fault level to 10 A. When the YNyn0 transformer is grounded on both sides, the NER will also limit the ground fault level on the 433 V side to approximately 508 A at 433 V. You will need to review this to determine if your ground fault protection will have selectivity.

The answer to that question can be found in the zero sequence model of an YNyn0 transformer.

When both neutrals are connected to ground, the transformer will provide a path for zero-sequence current supplied from the HV side to the LV side. When the HV neutral is disconnected and isolated from ground, the zero-sequence circuit path will be controlled by the magnetic circuit formed by the reluctance of the iron tank of the transformer and free-space which will be very high, essentially an open circuit, and the zero-sequence current which flows to a ground fault on the LV side will be very small.

In your case, when the both neutrals of the YNyn0 transformer are solidly grounded, the ground fault level on the 433 V system will be controlled mostly by the NER on the 22 kV side which will be something smaller than the 508 A value.

I have a transformer test report in which the zero sequence impedance has measured according to IEEE standard. YNyn0 Transformer has rating of 132/11kV, 45MVA, 28% impedance with no delta winding.In the test report there is no single value for zero sequence impedance but have three derived impedance values. They have measured Z1no, Z1ns and Z2no.That is impedance measured from primary side while secondary open and short, and impedance measured from secondary side while primary opened. Eventually calculated Z1, Z2, and Z3. These are the steps of deriving.

If the Z3 component is very large, like would be found in a 5-leg core or shell-type unit, use Z12 and ignore Z3.If Z3 is low enough to affect your calculations, like in a 3-leg core design, implement it as a Ynyn0d 3-winding transformer in DIGSILENT PowerFactory. Use the standard pi-to-tee equations to calculate the effective zero sequence impedances Z0ps, Z0pt, Z0st for entry in PF.

At present, among the 6-35kV voltage levels, the distribution transformer with dyn1 and ynyn0 winding connection is the most common. As we all know, the distribution transformer winding with Dyn11 connection has obvious advantages over Ynyn0 connection, which suppresses the third harmonic, reduces the zero sequence impedance, and is more conducive to improving the sensitivity of single phase short circuit current action of the circuit breaker at the low-voltage side, making it widely used in engineering design in recent years.

According to the experience, the over-current protector is also used as the single-phase grounding protection at the low-voltage side. For the transformer with winding Dyn11 (hereinafter referred to as D, yn11 transformer), its sensitivity coefficient can meet the requirements; while for the transformer with winding ynyn0, its sensitivity coefficient can meet the requirements or not.

Statistics from power supply companies referring to supply interruptions at the end customer show that e.g.in Germany about 80% of the interruptions are caused by failures in the medium-voltage system.The System Average Interruption Duration Index(SAIDI) describes the total time of all interruptions for customers, divided by the number of customers. Typical values for a German municipal utility are 10 minutes of annual outage per customer.In other regions of the world the outage times reaches from hours to days.

Yyn0: means high voltage Y connection, that is, star connection, low voltage is also y connection, the low voltage neutral point is derived, and the phase angle on the high and low voltage side is 0 o'clock above the clock.

Dy11: It means high-voltage D connection, that is, triangular connection, and low-voltage connection is also y-connection. The phase angle on the high and low voltage side is the angle at 11 o'clock above the clock, that is, 30 degrees.

There are three connection methods for transformer high and low voltage: star, triangle and zigzag connection. For high-voltage windings, use the symbols Y, D, and Z (uppercase); for medium-voltage and low-voltage windings, use y, d, and z (lowercase). When there is a neutral point, it is expressed by YN, ZN (high-pressure neutral point) and yn, zn (low-voltage neutral point). One of the two windings of the autotransformer with a common part having a lower rated voltage is indicated by the symbol a. Transformers are combined in the order of high voltage, medium voltage, and low voltage winding connections to form the winding connection group. For example: high voltage is Y, low voltage is yn connection, then the winding connection group is Yyn. In addition, the clock method indicates that the phasor relationship between the high and low voltage sides is the connection group.

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The connection group for a three-phase electrical transformer refers to the specific arrangement of the primary and secondary windings of the transformer. It defines how windings are connected to form a particular configuration and is essential to ensure the correct voltage and phase ratios between input and output.

The choice depends on the specific application, voltage requirements, and electrical system design. Different countries and regions may have their own standards and preferences for transformer connection groups, so local practices should also be considered.

e- The "Z" connection group: is a specialized transformer connection that is primarily used for applications where a balanced two-phase supply from a three-phase source is required. It's not as commonly used as other connection groups like Yy or Dy, but it has specific applications:

The "angle" of a connection group refers to the relative phase change between the primary and secondary windings of the transformer. It is an important feature that defines how windings are connected to create specific voltage and phase relationships between the input and output sides of the transformer.

At POLYLUX, in order to be able to offer a solution adapted to the needs of our customers and the requirements of their electrical projects, we can manufacture transformers on request with the connection groups that are necessary depending on the application.

A three-phase electrical system is necessary to generate and transmit electric power for use by businesses and industries from long distances. By connecting the windings in different ways, three-phase voltages (and currents) can be increased or decreased using powerful three-phase transformers.

In a delta connection, the three windings of the transformer are connected in a triangular shape, forming a closed loop. Each winding is connected between two phases of the three-phase power system. This type of connection is used when the load is balanced and does not require a neutral connection.

In a wye connection, one end of each winding is connected together to form a common point called the neutral, while the other ends are connected to the three phases of the power system. This type of connection is used when the load is unbalanced or requires a neutral connection.

There is also a third type of connection called a Zigzag connection, which is a variant of the delta connection that provides a neutral connection. It is used in some special applications where a neutral is required but not available in the power system.

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