Transformer

Transformers - Pad Mounted

A pad-mounted transformer is a type of electrical transformer that is installed on a concrete pad above ground level, rather than being installed overhead on a pole. These transformers are commonly used for distributing electricity in residential and commercial areas, and are also increasingly being used in distributed energy resource (DER) interconnections. Sizing, protecting and installing transformers used in large solar interconnections is a critical task that requires careful consideration of various factors such as system voltage, current, and power flow. In this article, we will discuss the process of sizing, protecting and installing transformers for large solar interconnection systems and the factors that need to be taken into account.

 

1. Power and voltage rateing

The first criterion to consider when selecting a transformer for a large-scale solar interconnection is the transformer's power rating. The power rating is the maximum amount of power that the transformer can handle and is measured in VA (volt-amperes) or kVA (kilovolt-amperes). It is important to choose a transformer with a power rating that is appropriate for the size of the solar installation, taking into account factors such as the number of solar panels, the expected total AC output of the DER system, and any future expansion plans. Depending on design and the project specific, the total AC outout of the DER system can be no more than 20% of the transformer nameplane power rate.

 

Another important criterion to consider is the transformer's voltage rating. The voltage rating is the maximum voltage that the transformer can operate and is measured in kV (kilovolts). The voltage rating of a transformer must match the voltage of the installed inverter and grid system.


The transformer protection device such as a fuse must be able to handle the short-circuit current of the system. Short-circuit current is the maximum current that can flow in the system during a short-circuit event. The the transformer protection device must be able to interrupt the short-circuit current to protect the transformer and grid from possible damage. The transformer must also be able to operate under the harmonics. Harmonics are high-frequency currents that can cause damage to the system and reduce the efficiency of the transformer.


2. Protection

The transformer must be able to provide the necessary protection and control functions for the system. This includes the ability to isolate the transformer from the rest of the system in the event of a fault, as well as the ability to monitor the performance of the transformer and the system as a whole. The transformer must also be protected against overcurrents and short-circuits. This is accomplished by installing overcurrent protection devices, such as fuses or circuit breakers, in the transformer. These devices will automatically open the circuit if an overcurrent or short-circuit occurs, preventing damage to the system.


Another important aspect of protection is to install protective relays in the transformer to detect abnormal conditions and take appropriate actions to prevent damage to the system. These relays are designed to detect abnormal conditions such as overcurrents, short-circuits, and ground faults. They can also detect other abnormal conditions such as under-voltage, over-voltage, and loss of voltage. Arc flash protection and ground fault protection are also important for the safety of personnel working on or around the transformer. Arc flash protection systems are designed to detect and mitigate the risk of arc flash hazards, which can cause severe burns or even death. Ground fault protection systems are designed to detect and interrupt ground faults, which can also be a serious safety hazard.

 

3. Efficiency

The efficiency of a transformer is the ratio of the power output to the power input, and it is measured as a percentage. The higher the efficiency of a transformer, the less energy is lost as heat, and the more energy is available to be used by the system. It is important to choose a transformer with a high efficiency rating to ensure that the system is as efficient as possible.

 

4. Termal rating

Another important criterion to consider is the transformer's thermal rating. The thermal rating is the maximum temperature that the transformer can handle before it becomes damaged.

 

5. Noise level

The noise level is the amount of noise that the transformer generates and is measured in decibels (dB). The lower the noise level, the less disruptive the transformer will be to the surrounding environment. It is important to choose a transformer with a low noise level to minimize the impact on the surrounding environment.

 

6. Size and weight

The transformer's size and weight are also important criteria to consider. The size and weight of a transformer are important because they determine how easy it is to transport and install the transformer. It is important to choose a appropriate transformer pad according to the trasformer size and weight for the DER installation.

 

7. Installation

The transformer must be installed in a location that is easily accessible for maintenance and repair. The transformer must be:


•    Installed on a level and stable surface to ensure proper operation.

•    Installation should provide required NEC/Utility clrearence for proper operation and maintenance.
•    Properly grounded to ensure the safety of personnel working on or around the transformer.
•    Properly ventilated to ensure that it does not overheat.
•    Properly connected to the rest of the system, including the iverters, LV switchboard or switchgear, and the utility grid.

•    Properly grounded to protect against electrical hazards

 

 

 

 Transformer winding types and grounding requirements used for large solar interconnections

Transformer winding types are categorized based on the number of windings on the transformer's primary and secondary sides. The two most commonly used transformer winding types are single-phase and three-phase transformers. Single-phase transformers have one primary winding and one secondary winding. They are typically used in small-scale solar interconnection systems, and are known for their simplicity and ease of use. Single-phase transformers are often used to step down the voltage level of the electrical power generated by solar panels, making it safe to use in homes and small businesses.


Three-phase transformers have three primary windings and three secondary windings. They are typically used in larger solar interconnection systems and are known for their high efficiency and ability to handle large amounts of electrical power. Three-phase transformers are often used to step up the voltage level of the electrical power generated by solar inverters, making it suitable for transmission to the grid. Grounding is an essential aspect of transformer installation in any size of solar interconnection systems. A proper grounding system is necessary to protect against electrical hazards and ensure the safe and reliable operation of the system.

 

The two most common connection methods for transformers used in large solar interconnection systems are the wye-wye and the wye-delta.

 

1. Wye-wye connection

The wye-wye connection is the most common method used in large solar interconnection systems. In this method, the primary and secondary windings of the transformer are connected in a wye configuration and are grounded at the neutral point. A wye-wye connection in transformers provides the advantage of having a neutral point, which allows for a more versatile connection options and better system stability. This connection allows for the use of three-phase systems and the ability to connect to unbalanced loads, as well as the ability to ground one side of the system for safety. Additionally, it also allows for a better voltage regulation and power factor correction. This method provides a high level of protection against electrical hazards.

 

1. Wye-delta connection

The wye-delta connection method is sometimes used in large solar interconnection systems. In this method, the primary winding of the transformer is connected in a wye configuration, while the secondary winding is connected in a delta configuration. The neutral point of the primary winding is grounded, while the neutral point of the secondary winding is not grounded.


When installing a transformer in a large solar interconnection system, it is important to comply with the local electrical codes and regulations. The transformer must be installed in a location that is easily accessible for maintenance and repair and is safe for personnel working on or around the transformer. The transformer must be grounded properly to protect against electrical hazards. In addition, the transformer should be equipped with protection devices such as fuses, circuit breakers and overcurrent protection devices.

 

3. Grounding

Grounding is the process of connecting a conductor, such as a wire, to the earth or a reference point to establish a zero potential. The purpose of grounding in a transformer is to provide a safe path for electrical current to flow in the event of a short circuit or other fault. One of the main reasons for grounding a transformer is to protect personnel and equipment from electrical hazards. In the event of a short circuit or other electrical fault, a grounded transformer can provide a safe path for the electrical current to flow, reducing the risk of electrical shock and damage to equipment.

 

There are several different methods for grounding a transformer, including single-point grounding, multi-point grounding, and neutral grounding. Single-point grounding is the most common method and involves connecting the transformer to a single grounding point, such as a grounding rod. Multi-point grounding involves connecting the transformer to multiple grounding points, and neutral grounding involves connecting the transformer to a neutral point on the electrical system. When grounding a transformer, it's important to use the right materials and techniques to ensure that the grounding is effective. For example, copper wire is often used as the grounding conductor due to its high conductivity and resistance to corrosion. The grounding conductor should also be of the appropriate size and length to ensure that it can handle the current that may flow through it during a fault.

 

 

Components of a pad mounted transformer used in DER/BESS interconnection

 

Understanding the different sections of a transformer and how they work is crucial for choosing the right transformer for a large scale DER interconnection.

 

1. Core: The core is made up of a stack of thin sheets of electrical steel, which provide a path for the magnetic flux. The core is the heart of the transformer, as it is responsible for linking the primary and secondary windings of the transformer. The core also helps to reduce the core loss, which is the loss of energy due to the resistance of the core material.


2. Primary winding: The primary winding is the coil of wire that is connected to the electrical source, such as a solar generation. The primary winding is responsible for receiving the electricity from the DER and transferring it to the core. The number of turns in the primary winding will depend on the voltage level of the electricity received and the voltage level required for the load.

 

3. Secondary winding: The secondary winding is the coil of wire that is connected to the load, such as a power grid. The secondary winding is responsible for transferring the electricity from the core to the load. The number of turns in the secondary winding will depend on the voltage level required for the load.


4. Insulation: The insulation is a material that is placed between the primary and secondary windings to prevent electrical current from flowing between them. The insulation also helps to prevent the transformer from overheating and to maintain the electrical safety of the transformer.

 

5. Cooling system: Transformers are typically cooled using one of several methods:

 

 - Air cooling: This is the most common method of cooling transformers. Air coolers, also known as radiators, are installed on the transformer's core and windings to dissipate heat into the air. The transformer's fan is used to circulate air over the core and windings, which cools them down.

 - Oil cooling: Some transformers use oil as a coolant. Oil is circulated through the transformer's core and windings to remove heat. The oil is then cooled in an oil cooler, and then circulated back through the transformer.

 - Forced air cooling: In this method, a fan is used to force air over the transformer's core and windings, which helps to dissipate heat.

 - Natural air cooling: Natural air cooling relies on the natural convection of air to dissipate heat from the transformer.

 

The choice of cooling method depends on the transformer's size, power rating, and the operating environment. In addition, the oil-immersed transformer are cooled by radiation and convection from the oil to the ambient air.


6. Tap changer: The tap changer is a device that allows the transformer's output voltage to be adjusted. This can be done by changing the number of turns in the secondary winding. The tap changer is useful for fine-tuning the voltage level of the electricity that is sent to the load.

Operation of pad-mounted transformer for DER interconnections

 

Pad-mounted transformers are used in DER interconnections to convert the low-voltage electricity (480-600 Vac) produced by the DER system to the higher voltage needed for distribution to homes and businesses. The transformer typically includes a primary winding, which is connected to the high-voltage input, and a secondary winding, which is connected to the low-voltage output. The transformer also includes a metal enclosure to protect the transformer components and to provide a weatherproof barrier.

 

DER interconnections refer to the connection of distributed energy resources such as solar panels, wind turbines, and energy storage systems to the electric grid. These types of systems are becoming more popular as a way to increase the amount of renewable energy on the grid and to reduce dependence on fossil fuels.

 

In a DER interconnection, the pad-mounted transformer is typically connected to the electric grid through a set of switches called a "switchgear." The switchgear allows the DER system to be disconnected from the grid when necessary, such as during maintenance or when the system is not producing electricity. The switchgear also provides protection for the transformer and the grid by automatically disconnecting the DER system if there is a fault or other problem. Pad-mounted transformers are designed to be durable and long-lasting, with a typical lifespan of around 25 years. They are relatively easy to maintain, with regular inspections and cleaning being the main tasks required.

 

 

Testing and Maintenance

 

Regular maintenance is essential to ensure safe and reliable operation of the transformer and the system as a whole.

 

  1. Regular visual inspections should be conducted to check for any signs of damage or wear, such as leaks, corrosion, or overheating.

  2. For oil-cooled transformers, oil samples should be taken periodically and analyzed for contaminants, such as water or debris, which can indicate a problem with the transformer.

  3. The transformer's bushings and terminals should be inspected for any signs of wear or damage, and should be cleaned as needed.

  4. The transformer's grounding system should be inspected and tested to ensure that it is properly functioning and providing adequate protection against electrical hazards.

  5. Regular cleaning of the transformer's exterior surfaces can help to prevent the build-up of dirt and debris, which can impede cooling and contribute to corrosion.

  6. The transformer's fans and other mechanical components should be inspected and maintained to ensure that they are functioning properly.

  7. Utility companies have a maintenance program in place for their pad-mounted transformers, including regular inspections, testing, and maintenance activities. It is important to have qualified personnel to perform the maintenance and testing on pad-mounted transformers, as it requires specialized knowledge and equipment.

  8. It is important to keep accurate records of all maintenance and testing activities for pad-mounted transformers, including the date of the activity, the results of the activity, and any repairs or replacements made. It is important to follow the manufacturer's recommendations for maintenance and to use the recommended materials and parts when performing maintenance on the transformer. Regular monitoring of the transformer's load, temperature and other parameters can help to detect potential issues and schedule maintenance accordingly. It's important to schedule maintenance during times when the transformer's load is low to minimize the impact on the electric service.

 

 

 

 

 

(1) DER
DER, or Distributed Energy Resources, refer to decentralized energy generation and storage systems that are connected to the electric grid. Examples of DER include solar panels, wind turbines, and batteries. These resources can be owned and operated by utilities, businesses, or individuals, and they can be used to generate electricity, store it, or both.

DER is becoming increasingly popular as a way to reduce dependence on fossil fuels and increase the amount of renewable energy on the grid. By generating and storing energy locally, DER can also help to reduce transmission and distribution losses, and improve power quality and reliability. In addition, DER can also help to reduce the need for expensive transmission and distribution upgrades, and it can provide a way to increase the amount of renewable energy on the grid.

DER systems can also be used in conjunction with demand response programs, where customers are incentivized to reduce their electricity use during periods of high demand. This can help to reduce the need for expensive peaker power plants, and it can also help to improve the overall efficiency of the electric grid.

 

(2) CT and PT

In medium voltage switchgear, the CT (Current Transformer) and PT (Potential Transformer) are used for measuring and protecting electrical equipment. The CT is used to measure current by reducing the current to a more manageable level, while the PT is used to measure voltage by reducing the voltage to a standard level. Both the CT and PT are used to provide accurate current and voltage measurements to protection relays and metering equipment, allowing for safe and efficient operation of the electrical system.