Today, the utility transformer is so ubiquitous that they almost go unnoticed whether they housed are a green enclosure in the front yard or a gray canister on a pole. But a new type of transformer is lurking on the fringes of the industry.
With rapidly emerging and evolving technologies, keeping up with short-term changes can make it difficult to step back and consider some of the long-term implications. Distributed Energy Resources (DERs) is one of the areas where by extrapolation from some widely known technology developments, we can see some rarely discussed future implications.
Let's talk about transformers from a historic perspective for a minute. In the original AC versus DC debate between Edison and Tesla, one key advantage of AC was the ability to use fairly straightforward transformers to change voltages. Higher voltages were better when power needed to be moved over longer distances (because of reduced I²R losses) while lower voltages were safer and more suitable for use inside the customer premise.
Today, the utility transformer is so ubiquitous that they almost go unnoticed whether they housed are a green enclosure in the front yard or a gray canister on a pole. What is inside the transformer has not changed significantly over the last century. There is a lot of iron for the core, wire for the windings, and oil for additional insulation and cooling.
But a new type of transformer is lurking on the fringes of the industry. Often called solid-state transformers (SSTs), these devices make use of advanced electronics to "build a better mousetrap". With the explosive growth of DER, we have seen significant advances in the area of power electronics and it is now reasonable to believe that the SST may give traditional transformers some competition in the long-term. Let's explore a few of the advantages these devices have.
Voltage Control. With traditional distribution transformers, there is a fixed ratio between the number of turns on the primary and secondary windings. A certain primary voltage will yield a specific secondary voltage. For residential electric distribution systems the secondary voltage is typically around 240 volts center tapped. But you only get the desired secondary voltage if the primary voltage is around its rated value. Because of this, electric utilities put significant effort (and expense) into maintaining primary voltage within a fairly tight range over the entire length of a distribution feeder.
With SSTs, this requirement begins to become less critical. To an extent, these transformers are capable of dealing with much wider ranges of primary voltages while maintaining the desired secondary voltage. This could offer advantages such as allowing longer distribution lines and reducing the need for devices such as load tap changers and voltage regulators.
Volt VAR Optimization. While, in general, lowering the voltage of the system typically results in lower power consumption by various loads, certain loads do not behave this way and may consume even more power as the voltage is lowered. With the SST, optimization of secondary voltages at the transformer level becomes practical because the transformer can be “load aware”. So if you reach a situation where there is a need to reduce total system load, the SST can make an intelligent decision about whether lowering its secondary voltage will achieve the desired result. Today, optimization often occurs at the feeder level, but with SSTs the granularity of optimization narrows to the distribution transformer level instead.
Reactive Power Compensation. Because there is a conversion to DC on both sides of the SST, it is not necessary (or even possible) for reactive power to simply pass through the SST. This fact can allow the SST to help with reactive power issues on both its primary and secondary sides. By adjusting parameters of the SST, they can provide reactive power to help with power factor issues on the primary side. They can also isolate any load-related reactive power issues to the secondary. This can include a range of options, but providing reactive power compensation along with information that can help utilities identify loads that do not meet power-factor requirements would be a reasonable approach.
Power Quality. With a realistic amount of capacitance for short-term storage of DC power, SSTs could offer ride-through capabilities for events lasting less than a few cycles.
Also, if secondary loads drew currents with significant harmonic content, that harmonic content would be isolated and not affect currents on the primary side of the SST.
Finally, since the production of the secondary voltage in an SST has significantly more isolation from fluctuations in the primary voltage than transitional transformers, the ability to isolate and limit surges is another advantage.
Together, these features could provide substantial improvements in power quality for utility customers.
Cold Load Pickup. Today, when power is restored to a feeder that has lost voltage, there can be significant initial current flows because of motor starting and other factors related to cold load pickup. With SSTs, this can be addressed in two ways. First, some type of random or coordinated pickup delay can be implemented so that service on the feeder is restored over a few minutes instead of all at one time. Solid state transforms might also be able to allow some type of “soft start” capability where voltage would ramp up instead of having the SST instantaneously delivering rated voltage on its secondary.
Protection. Traditional transformers are typically protected by some type of fuse. SSTs can add additional protection options such as the use of configurable protection curves that are appropriate for the location on the distribution system and even offer advanced features such as a type of “recloser” function at the transformer level.
Advanced Load Management. With the addition of communications capabilities, SSTs could also help provide tools such as predictive load shedding during system events. If you know transformer-by-transformer what the loads on the secondary are, you can calculate the impact of “dropping” certain transformers at any given time.
Of course there will also be challenges. Today, SSTs are an order of magnitude more expensive than their traditional counterparts. Traditional transformers also tend to be extremely reliable when operated within their ratings and many traditional transformers that have been in service for thirty or more years. Experience has shown that more complex technologies sometimes have challenges equaling the life of simple and proven solutions, especially as they are still being refined.
Also, two-way power flow is an inherent capability of a traditional transformer. With SSTs, unless there is an alternate set of rectifiers and inverters, they can only support power flow in one direction. Some view this as a desirable safety feature, but with the growth of DERs, two-way power flow is likely to be an important element in the future grid.
But experience has shown that with the benefits being so significant, it is likely that the industry will find ways to address these and other issues that may arise. One likely scenario is that the US will see early deployments for targeted customers or feeders where targeted deployment of SSTs offer the most value.
Is your utility ready for solid-state transformers and other emerging technologies? Are you ready to implement the intelligence and communications that will be necessary for these technologies to achieve their full potential?
West Monroe Partners is working with a number of utilities to plan and prepare for these and other emerging technologies. As you look at evolving towards the utility of the future, West Monroe can help you prepare an extensive and comprehensive look at trends and emerging technologies. West Monroe can also help identify which technologies should be watched, evaluated, and deployed along with tools that can ensure initial deployments are targeted to locations where they offer the optimal benefits.