Transformer Load Stress from Renewables

As renewable energy sources like solar and wind power become more popular, electricity grid infrastructure is facing new challenges. One such challenge is the increased potential for distribution transformer overloading. Distribution transformers play a major role in stepping medium voltage electricity down to the lower voltages used by homes and businesses. However, many existing transformers were not designed with large amounts of renewable generation in mind. The addition of distributed renewable generation sources like rooftop solar can lead to reverse power flows and excessive load on distribution transformers. If not addressed, this issue could become a bottleneck that prevents further renewable energy adoption. This article provides an overview of the distribution transformer overload challenge and explores potential solutions.

The Role of Distribution Transformers

Distribution transformers are common pieces of equipment found throughout electricity distribution systems. They are the final step-down transformers that convert medium-level voltages of 5-35 kV down to the 120/240 V used by end consumers. A typical distribution transformer rated at 50 kVA to 500 kVA will serve anywhere from 10 up to 100 homes or small businesses.

Distribution transformers are designed to only flow electricity in one direction, from the higher distribution voltages down to the lower utilization voltages. Traditionally, electricity has flowed uni-directionally from large centralized power plants through transmission networks, into distribution grids, and finally into homes and businesses. Distribution transformers have been designed around these well-established power flow patterns.

Challenges from Distributed Renewable Generation

The growth of distributed renewable energy sources like rooftop solar photovoltaics (PV) is beginning to challenge traditional power flow techniques. Whereas power plants generate electricity at high voltages, distributed renewables generate power at the same low voltages that distribution transformers deliver. This can lead to bidirectional and uneven power flows that distribution transformers were not originally designed for.

Specifically, high levels of distributed solar PV generation during midday can cause reverse power flow from low voltages back into the distribution grid. This can significantly increase loading on distribution transformers beyond their nameplate rating. Overloading transformers for long periods degrades insulation, shortens lifespan, and increases failure rates. Thermal models indicate that consistently loading transformers above 120 % of their rating markedly increases the risk of failure.

Current efforts to expand renewable energy adoption could exacerbate distribution transformer overloads. Analysis by the National Renewable Energy Laboratory (NREL) found that a five-fold increase in rooftop solar deployment would double the number of transformers that experience overload conditions for over 10 % of the year. Overloading risks being most acute in sunny areas with high solar PV adoption, like California and Hawaii. Preventing widespread distribution transformer degradation and failure is thus critical for enabling more renewable distributed generation.

Main Impacts

Transformer Overheating
Overloading the transformer can cause its windings to overheat, leading to accelerated insulation degradation and reduced transformer life.
Increased Transformer Losses
Overloading the transformer increases its load losses, which can reduce the overall efficiency of the distribution system.
Voltage Rise Issues
The reverse power flow from distributed PV systems can cause voltage rise issues, potentially exceeding the acceptable voltage limits and affecting power quality.
Protection Coordination Challenges
The bidirectional power flow introduced by renewable energy sources can disrupt the coordination of protection devices, leading to potential issues with fault detection and isolation.

Potential Solutions

Fortunately, there are solutions utilities can implement to prevent distribution transformer overloads as renewable generation expands. These strategies focus on either reducing loading on existing transformers or upgrading transformers to handle higher bidirectional power flows.

One effective approach is the installation of monitoring and control systems to manage the timing of electricity exports from distributed generation sources. Smart inverters can play a crucial role here, as they have the capability to curtail solar PV output when transformers are nearing their maximum load. California and Hawaii, for instance, have already established requirements for smart inverters to enable such active management, which also includes reactive power support and voltage regulation.
By maintaining optimal voltage levels throughout the distribution system, these smart inverters help reduce the strain on transformers. Hawaii has mandated advanced inverter functionalities to provide voltage ride-through capabilities and reactive power support in light of increasing solar PV installations.

In addition to smarter control systems, upgrading transformer hardware is another viable solution. Utilities can opt to install larger capacity transformers specifically designed for back-feed scenarios common with distributed generation. New liquid-filled transformers, featuring advanced cooling systems, can handle greater loads, while higher-cost hermetically sealed transformers offer a cost-effective upgrade in areas with high overloading risk.

Moreover, utilities are increasingly deploying distributed energy resource management systems (DERMS) to optimize operations. By providing visibility into the location, size, and output of distributed generation sources, DERMS enable more intelligent control and dispatch of assets such as voltage regulators and capacitors, effectively preventing transformer overload. As renewable penetration continues to rise, wide-scale adoption of DERMS will be key.

Complementing these strategies are several others, such as smart charging of electric vehicles (EVs), which coordinates EV charging during off-peak hours to reduce strain on transformers.

Energy storage systems, like battery energy storage systems (BESS), can absorb excess power generated by renewables, thereby reducing reverse power flow and preventing overload.

Volt-VAr optimization, employing advanced control algorithms and power electronics-based devices, actively manages voltage and reactive power in the distribution network. Dynamic transformer rating, utilizing dynamic thermal models and real-time monitoring, allows for adjusting the transformer's rated capacity based on actual operating conditions, increasing utilization without compromising lifespan.

As a last resort, distributed generation curtailment can be employed during periods of high generation and low demand to reduce reverse power flow and prevent transformer overload. Curtailment can be done by reducing the output of local power generators, such as solar panels.
This is typically achieved by adjusting the output of solar inverters, which are responsible for converting solar DC power to AC, thereby limiting the amount of energy fed into the grid. In some cases, curtailment might involve temporarily switching off certain generators or enforcing limits on how much power distributed generation sources can contribute.
These measures ensure that the grid remains balanced and that key equipment, like distribution transformers, is not subjected to excessive stress due to reverse power flow.

Takeaway

Distribution transformer overloading is an emerging challenge as renewable distributed generation accelerates. While concerning, there are actionable near and long-term solutions available to avoid widespread failures. Utilities must begin implementing transformer load monitoring and control technologies, while also upgrading hardware and incorporating DERMS capabilities.

At CLOU, we offer a comprehensive range of products and solutions to support these efforts, from battery energy storage systems and energy metering equipment to data acquisition and distribution solutions. With prudent planning and investment, the existing distribution system can evolve to enable much greater renewable energy adoption while maintaining reliability. The outcome will be ongoing progress toward decarbonization goals, all while preserving an affordable and resilient energy delivery infrastructure.

If you have any inquiries or need further information about our products and solutions, please do not hesitate to reach out to us. We are here to assist you and welcome your valuable thoughts and comments.

Until then, keep shining bright like a solar panel on a sunny day!