Grounding transformers

Why grounding transformers are necessary for large wind farms with multiple turbines

   When we think of wind farms, images of majestic towers with huge rotating blades crossing the horizon probably come to mind. Engineers are no exception, as their focus is on location, procurement, erection and connection of towers, turbines and blades. Many people don't know that grounding transformer is often overlooked in the design and installation of a wind farm, as evidenced by the fact that 90% earthing transformers for wind farms are purchased after the start of the installation of the main structure. However, those who neglect proper grounding planning do so at their own risk. In reality, millions of dollars in damages can be due to an earth fault, so grounding issues should be high on the list of concerns for anyone developing a wind farm.

Why are grounding transformers needed?
   In simple terms, a grounding transformer is used to provide grounding for either an ungrounded star or a delta-connected system. Grounding transformers are commonly used for:

-Providing the shortest path to earth with relatively low resistance, thereby maintaining the system neutral at or near ground potential.
 - Limitation of the magnitude of transient overvoltages during repeated earth fault.
 - Current source for earth fault.
 -If necessary, connect loads between phase and neutral.
   If a single earth fault occurs in an ungrounded or isolated system, there is no return path for the short-circuit current, so no current flows. The system will continue to operate, but the other two healthy lines will increase the square root of three, overvolting the insulation of the transformer and other related components in the 173% system. Metal Oxide Varistors (MOVs), solid-state devices used to suppress surges / surges (lightning arresters), are particularly susceptible to heat damage from leakage through blocks, even if the voltage rise is not enough to breakdown. The grounding transformer provides grounding to prevent this.

Grounding transformers are required for large wind farms with multiple turbines, where the substation transformer is often the only ground source for the distribution system. A grounding transformer located on the turbine column provides an earth path in case the tower becomes isolated from the system ground.

When a ground fault on a collector cable causes the substation circuit breaker for that cable to open, the wind turbine string becomes isolated from the ground source. Turbines do not always detect this fault or the fact that the column is insulated and not grounded. As a result, the generators continue to supply power to the collector cable, and the voltage between the good cables and ground rises well above the normal voltage value. As a result, the costs can be staggering.

According to one source at Iberdrola, the world leader in wind energy development, the loss of revenue for a chain of 10 turbines alone could exceed $ 10,000 a day. Taking into account dismantling and replacement, the cost of the equipment could approach an additional $ 40,000 for a transformer. A typical wind farm configuration is actually somewhat similar to a carriage wheel with ring, hub and spokes. The outer ring of the wheel is like a fence around a wind farm, and the hub in the center is where the collector is located, which connects to the grid. The spokes are the radial lines on which each wind turbine is located. Typically, each radial turbine column is connected to an earthing transformer as shown in fig. 1.

Order transformer calculation

Correct design
   Grounding transformers usually have one of two configurations: a zig-zag winding (Zn) (with or without auxiliary winding) or a star-connected winding (Ynd) (with a delta-connected secondary winding that may or may not be used to supply auxiliary power) ). Both options are shown in Fig. 2.схема заземляющего трансформатора

   The current trend in the design of wind farms is to connect the primary winding in a star with a secondary winding in a delta. In our experience, there are several reasons why 2-winding star-connected grounding transformers seem to be more popular than zigzag designs.

This includes:

   - Double winding transformers are considered more readily available for replacement or retrofitting.
   -Lack of understanding of zig-zag configuration means that engineers tend to work with more understandable diagrams.
   -The double-winding star-connected design allows secondary loading and dispensing, while the zig-zag design does not.
   Not all manufacturers provide zig-zag grounding options to potential customers, even those for whom such a configuration may be most appropriate.
Zig-zag connection geometry is useful for limiting third harmonic circulation and can be used without delta winding or without the 4- or 5-bar core structure commonly used for this purpose in distribution and power transformers. Eliminating the need for a secondary winding can make this option less expensive and compact compared to a similar double winding earthing transformer. In addition, the use of a zig-zag transformer provides grounding with a smaller device than a double-winding Y-Delta transformer, which provides the same zero sequence impedance.

On the other hand, wye-connected earthing transformers require either a delta-connected secondary or a 4- or 5-pin core design to provide a return flow path for the unbalanced load associated with this primary connection. Since it is often desirable to supply auxiliary power from the secondary winding of the grounding transformer, this advantage may make it preferable to use a double winding grounding transformer instead of a zigzag connection. Both zigzag and double-winding earthing transformers can be designed with auxiliary power supply - this can be a star or delta load.

A solid earthed system using an earthing transformer offers many safety improvements over an ungrounded system. However, a single grounding transformer lacks the current-limiting capacity of a resistive grounding system. For this reason, neutral ground resistors are often used in conjunction with a ground transformer to limit the magnitude of the neutral ground fault current. Ohm values must be specified to ensure that the earth fault current flows sufficiently high to ensure reliable operation of the protective relaying equipment, but low enough to limit thermal damage.

Definition of a grounding transformer
  When choosing a grounding transformer for your wind farm, be sure to consider the following key parameters:

Primary voltage Is the system voltage to which the grounded winding is to be connected. Be sure to include the base impulse level of the transformer (BIL), which measures its ability to withstand lightning strikes. In some cases, the BIL will be driven by equipment considerations such as 150kV BIL ratings in 34.5kV wind farms due to the limitation on dead front connectors.

Rated kilovolt-amperes (kVA)... Since an earthing transformer is usually a short-term device, it is smaller and less expensive than a continuous-duty transformer of the same kVA rating. For this reason, earthing transformers are often not calculated in terms of kVA, but in terms of DC and short-time current ratings. Regardless of how you judge it, the grounding transformer must have a rated direct current of the primary phase without exceeding its temperature limit. This load includes core magnetizing current, capacitive charging current for cables, and any auxiliary load, if applicable. The higher this value, the larger and more expensive the transformer. Typical DC currents range from 5 A to several hundred. Be sure to include additional download requirements.

Continuous neutral current - The continuous neutral current is defined as three times the phase current or, in other words, the zero sequence current. This is usually considered zero if the system is balanced. However, for design purposes of an earthing transformer, it is the value that is expected to flow in the neutral circuit without interrupting the protective circuits (resulting in zero current) or earth leakage current, which is not a symmetrical function. ... Again, this value is needed to calculate the thermal power of the grounding transformer.

Damage current and duration - This value is needed to calculate the transient heating resulting from a fault in the system and should be determined based on the engineering study of the system. Typical values range from a few hundred amperes to several thousand amperes, with durations expressed in seconds rather than cycles. For example, typically 400A for 10 seconds. The duration of the damage is a critical parameter for the transformer designer. Where protection schemes use an earthing transformer for trip functions, a relatively short time (5 to 10 seconds) is indicated. On the other hand, if an earthing transformer is used in an earth fault signaling circuit, a constant or extended duration of the earth fault current will be required.

Impedance - impedance can be expressed as a percentage or ohms per phase. In any case, it should be selected so that the phase voltages in good condition during an earth fault are within the permissible temporary overvoltage of the transformer and associated equipment such as arresters and terminal connectors. Values that can range from 2.5% to nearly 10% must be provided by the system designer.

Primary winding connection - be sure to indicate the type of primary connection: zigzag or grounded. Before making a decision, consider the factors discussed earlier regarding the situations for which a particular configuration might be most appropriate.

Secondary connection - specify secondary voltage and connection, if applicable. In addition, be sure to consider the size of the auxiliary load connected for Zn or star connected primary windings.

If there is an option to use a two-winding transformer without a secondary load, determine if the delta winding can be "buried" (ie not led out), or only one insulator should be led out for tank grounding or testing. 

Important features and options
In addition to the design features discussed, there are a number of other considerations or considerations that you should consider when designing a wind farm grounding transformer.

Inform your supplier if you need a pedestal-mount transformer with built-in tamper-evident or substation design.
Consider whether the grounding transformer will be placed outdoors or indoors. Even outdoor units require special attention when placed next to other structures.
Select the appropriate fluid type for your application. Options include mineral oil, silicone, and natural ester fluids.
Consider connection options and choose the best one for the site. Options range from dull front, live front and blade terminals. The location of the terminals can be under the cover or on the side wall, open or closed.
The temperature rise is assumed to be 65 ° C - adjust the design if necessary.
Consider site height or any special environmental concerns.
Special paint as required.
Neutral Grounding Resistors - The rated voltage NGR should be equal to the line voltage of the grounding transformer to earth. The rated current and duration must match the rating of the earthing transformer. Remember to set the rated current high enough to exceed the cable charging current and the magnetizing current of the grounding transformer.



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