Integration of High Level of Renewables into the T&D System, A Vermont Experience

Presented By:
Christopher E. Root

COO Vermont Electric Power Company
TechCon 2020

Changes in the way we produce, store, manage and use electricity are transforming the way the electric power industry operates the grid. Among the key trends: retiring base-load power plants, increased distributed generation, rising investment in energy efficiency and demand response, and technological innovation in heat pumps, electric vehicles and storage. This paper outlines the experience of the utilities in the state of Vermont, USA as its solar and wind generation is now at a level of installed capacity of 50% of its peak load.

Background on Vermont Energy Picture

The state of Vermont has ambitious renewable goals in three areas; electricity, heating and transportation. These goals target to reduce the carbon emissions from these three areas by 90% by 2050. The strategy is to encourage high levels of photovoltaic and wind electric generation along with storage programs to “clean” the electric sector and then leverage this for heating and transportation. At this time, the electric peak load is approximately 1000 MW in both the summer and in the winter. It has been at this level for several years. Active conservation programs and strong peak load management programs by customers and utilities alike have constrained peak load growth.

The state has 16 distribution utilities which include one large investor-owned utility (Green Mountain Power) and two electric co-ops, one large municipal utility and many smaller municipal electric departments. All of these groups are joint owners of the transmission company in Vermont (Vermont Electric Power Company or VELCO). VELCO owns over 700 miles of transmission (115kV, 230kV and 345kV), 52 high voltage substations, 1700 miles of fiber optic communications lines, one HVDC terminal and maintains a HVDC line with Canada. VELCO is the transmission operator in the state and operates under the direction and guidelines of the independent system operator in New England (ISO-NE).

The New England region’s high reliance on natural gas poses a particularly significant challenge. On days of low solar and wind production, natural gas-fired generation supplies more than 60% of the region’s power. Due to limited pipeline capacity and few delivery points for liquefied natural gas (LNG) into the region, fuel availability to electric generators is constrained on the coldest days when firm supply goes first to space heating leading to short term concerns with capacity adequacy.

Vermont’s Internal Generation Resources

An aggressive program to increase renewable energy was started in the early 2010’s and after the largest generation plant in the state closed in 2014 (Vermont Yankee Nuclear Plant of 640MW), the efforts for more renewables intensified. The table below shows the nameplate capacity of the generation in the state in 2019. Also described is whether it is weather dependent and inverter based.

vermont's resources

The table describes the generation supply which is largely renewable and carbon free. Over 50% of the peak load in the state could be met with just the solar and wind capacity if they were to operate at max at the same time at peak. This cannot occur as the load demand peaks are now consistently after dark year-round. Vermont is a net importer of energy at all hours of the year from its neighboring states and with Canada over a 222MW HVDC terminal.

The Vermont “Duck Curve”

High renewable penetration has fundamentally changed the timing of daily peaks. Traditionally, load peaked around 2 p.m. when commercial energy demand reached its highest point. Today load peaks after sundown.

More critical to grid operators, the daily load shape changes dramatically on sunny days, as shown in the below figure. This is the infamous “duck curve.” Where loads used to ramp up in the morning, peak in mid-afternoon, and drop gradually, on a sunny day, mid-day loads drop to the bottom of the duck’s belly, and then rise to their daily peak when the sun goes down.

april load comparison

The chart shows the increasing levels of photovoltaic generation in the state and its impact on the net load over a 24-hour period over several progressive years. 2017 was the first year which the midday net load was lower than in the middle of the night. In 2019, the state set a new low net load of a sunny Sunday afternoon in the Spring. It can be seen that the peak time of electrical consumption is after dark when the photovoltaics shut down. A final point to make is to notice the flat curve during the day. This is a day when there were thick clouds and it was raining. This significantly reduces the level of solar generation and the difference between a sunny day and a rainy day is significant. The below chart from two days in the same week in 2018 shows how different it can be.

velco load curves (overcast vs. sunny days)

Renewable integration challenges

As stated, the high levels of renewables have created a number of new technical challenges. Many of these are the same being worked on by utilities around the world and some are unique to Vermont.

As the reliance of generation dependent on weather conditions has increased, several new challenges have emerged. First, unlike Southern California or Hawaii, Vermont is a northern state and is home to many world class ski areas. This is because it snows many times in the winter. This results in snow accumulating on solar panels making them not available for use until the snow melts off, which can take several days after a storm. Additionally, the lower hours of sunlight during the winter limits the generation levels. Historically, the daily peaks in the winter occur between 6PM and 7PM when it is dark and cold. There is no available solar generation to assist during these times. Unfortunately, on the very coldest nights there tends to be little wind to offset the unavailability of PV generation. Generally, wind generation is highest and consistent in the fall, winter and spring.

Since the transmission system has not changed significantly in the last 5 years, but the growth of this distributed renewable generation has, constraints have emerged on the T&D system where some renewables have had to be curtailed to ensure system reliability. This happens most acutely in Vermont in the spring when the melting snow swells the rivers and the run of the river hydro plants are at full output. It is frustrating to many groups in the state that during these times that the renewable energy is abundant, it is not available due to these constraints. It is hoped that over time the policymakers and utility operators and planners will come to an agreement to fund solutions to this situation to allow for maximum use of renewables.

An additional challenge of renewables is intermittency. Wind generation in Vermont is located on mountain ridges and the output is not constant as the wind tends to have some micro gusts embedded in it naturally. This leads to small varying outputs from the turbines.

A similar situation occurs with solar plants when clouds move through. The below chart is of a 20MW photovoltaic plant on a partly cloudy day. It is interesting at the rate at which the generation level drops as the cloud shields the blue sky for a period. When this generation drops, some other resource in the ISO-NE system must make up the difference in order to keep the system in equilibrium.

barker solar generation

A final challenge that has been emerging is the lack of visibility of behind-the-meter net generation (roof top solar installations) and utility-scale generation <5MW connected to the distribution system but lack telemetry to the transmission grid operators. Except for large solar farms, most solar generation is connected at the local distribution level and, therefore, is invisible to transmission system operators in real-time. System operators can only monitor the net load. Without real-time data on this so-called “behind the meter” generation, operators cannot know what to expect in the case of a disturbance. If generation suddenly trips offline, contingency analysis simulations may under-predict the resulting load levels coming onto the system.

The power industry and its partners are developing better predictive tools based on production data from intermittent generators even as installed renewable capacity is growing. In addition, recent changes to interconnection standards (IEEE 1547) will require updated “ride-through” and voltage control capabilities even for smaller inverter-based installations, which will keep these resources online through minor perturbations of the grid. This is no doubt just the beginning of many adaptions that will be needed to adapt to an increasingly distributed grid.

Voltage control

Voltage regulation is a critical aspect of utility reliability. State regulations require all utilities to maintain steady state voltage with set limits, which has traditionally been achieved using capacitors, load tap changers on substation transformers, capacitors on lines and in substations, and circuit tap voltage regulators on lines. These all are relatively slow-acting methods of voltage control. Today, solid-state inverters, which are features of most renewable generation, technically allow quick voltage adjustments if the capability is utilized.

IEEE Standard 1547 was modified in 2018 to allow utilities to set interconnection requirements for distributed resources’ behavior in a disturbance, as well as for voltage regulation, VAR generation, and other factors. Often, high levels of distributed generation can cause high voltage at the distribution substations as utility devices struggle to control voltage under low loads. The changes in IEEE 1547 give distribution utilities the authority they needed to address voltage and ride-through issues raised by large amounts of inverter-based generation, but utilities must now take the next step; they must adapt their interconnection rules to implement this authority. Only then will distributed generation installers have the criteria and settings to work smoothly with the existing utility voltage control strategies.

Similar issues can arise on the transmission system. Due to the topology of the Vermont transmission system, voltage control is a significant issue, which has been addressed by deploying several technologies including switched capacitors, phase shifting transformers to limit flows, synchronous condensers for fault current sources, voltage control and VAR management, switchable shunt reactors, a Static Synchronous Compensator (STATCOM), and a Static Var Compensator (SVC). Below is a picture of a SVC connected to the 115Kv system in Vermont.

Figure 1 Static VAR compensator (SVC) at Ascutney, Vermont

With the challenges of controlling voltages in this new environment, utilities can make good use of non-utility-owned, inverter-based generation to assist in the control of voltages to all customers. Additionally, utilities will have to turn to faster voltage controlling technologies to keep voltages steady. Failure to make these changes threatens power quality for customers.

Evolution and growth of storage

Many different battery technologies are being developed and deployed today. Although today’s costs that are still relatively high, like most emerging technologies, costs are likely to drop over time, and storage will likely become a significant part of our energy future. Today, the amount of storage (battery) in the state is approximately 17.2MW with several new storage projects under develop.

The characteristics of the various battery storage options, and therefore their value streams, vary greatly. Some batteries achieve high output for a short duration; others are more effective for large amounts of storage delivered over a long time. Some technologies are readily available today, while others are still in the R&D phase. Regulatory treatment of these assets as either generation or T&D assets (or both) will be a significant debate over the next year.

Vermont’s largest distribution utility, Green Mountain Power, has invested heavily in storage as part of its plan to reduce system peaks, and increase distributed energy and system resilience. GMP is working at both the utility scale and customer ends of the spectrum. The below picture shows the utility’s 4 MWh battery system installed in Rutland, Vermont, at their Stafford Hill Solar facility.

Figure 2 GMP's Stafford Hill Solar Farm and 4 MWh battery installation

For individual customers, GMP has installed over 2000 Tesla Powerwalls to customers within its service territory. For $15 a month for ten years, or a one-time payment of $1,500, GMP and Tesla install a Powerwall battery in the customer’s home, providing backup power during grid outages. In exchange, participating customers agree to allow GMP to share access to the Powerwall in order to reduce the company peak energy consumption and lower costs to all customers.

With the reducing costs of storage technologies and increasing need for short term energy sources to rectify some of the issues this paper has described, storage will play an important role in the future. How it is used and controlled will be an important development of the next few years.

Figure 3 GMP is offering 2000 Tesla Powerwalls at $15 per month to customers


Vermont is one of the smallest states in the US, but in terms of focus on reducing carbon emissions, it is one of the leaders in the electricity area. This northern state has levels of solar and wind similar to the large western and southern states in terms of percentages. It has the additional burden of having less consistent weather. This has made the state’s utilities deal with some unique issues.

Many interesting technical challenges face today’s electric power industry. The industry is seeking—and finding—creative solutions to the uncertainties ahead.

Unknowns such as electrification of transportation and heating will have a major impact on demand, as will the continued growth of distributed energy. Storage will be a key part of the solution, even though it is a time-shifter, not an addition of new energy and its growth will be limited until costs come down.

It is an exciting time to be in the industry. Electricity is and will continue to be essential to today’s society and will have a valuable contribution to the climate change solution.

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