Bench Talk

PV panels are becoming a significant contributor of electricity but introduce new operational challenges for utilities

The century-old U.S. electricity grid is the largest interconnected machine on Earth. But this infrastructure––comprising more than 9,200 electric generating units with more than 1,000 gigawatts of generating capacity connected to nearly 300,000 miles (483,000 km) of transmission and distribution lines––is facing the largest disruption in its history as the way power is generated undergoes a revolution.

The grid, like those of almost all developed nations, was built to cater for centralized power generation in the form of relatively few several-hundred-megawatt- and even fewer gigawatt-rated power stations. High-voltage, long-distance transmission lines move power to local substations where the the voltage is stepped down for distribution to industry and consumers. It’s a system that’s set up for one-way delivery of power - from generator to consumers.

All that’s changing, primarily as a response to climate concerns. At the latest, in a long line of U.N. Climate Change Conferences dating back to Berlin in 1995, Heads of Government at the 2015 Paris version agreed to reduce carbon emissions in an attempt to limit the planet’s average temperature rise to 1.5 °C. Much of the effort to do this in developed nations is focused on reducing fossil fuel powered electricity generation (a significant contributor to the world’s carbon output) with “renewable energy” such as solar or wind power.

The result of this initiative (which builds on earlier declarations) is that select areas of the world––some sunny (Australia, Hawaii, Italy) and some not so sunny (Germany, New Jersey, Ontario)––are subject to a high penetration of residential solar (photovoltaic or PV) panel installations - turning consumers into “prosumers” (alternating between producing and consuming power depending on the variability of incident sunlight). Australia and Hawaii lead the world in residential PV penetration with 7.5 percent and 2.8 percent of all households employing the technology, respectively.

Such significant PV penetration has a major impact on generation capacity. An electricity provider in Brisbane, Australia, for example, reported that in mid-2015 there was 1000 MW of domestic PV capacity on a 5000 MW network. This capacity is able to deliver up to seven percent (around 1464 GWh) of the daily energy demand. One-way power flow is definitely a thing of the past.

In addition, PV generation changes power production from a centralized operation to a distributed one. In the U.S., a few thousand large power stations are now complemented by millions of “microgenerators” producing power from panels on their roofs. To make matters even more complex, PV generation is also characterized by high variability (when the sun goes behind clouds generation drops off) requiring utilities to be on constant alert to switch to other sources of supply.

Utilities are only just getting to grips with the implications of a bidirectional, distributed and variable power supply. That’s a challenge because regulators demand electricity companies ensure the power supply meets tight voltage and current tolerances; for example, voltages are not allowed to “sag” below or “swell” above tight voltage thresholds. Moreover, “power quality”––a measure of the purity of the power supply––must also be closely controlled. For example, many countries have standards that state total harmonic distortion (THD – the presence of unwanted harmonics of the supply frequency in the AC supply) must be no greater than two percent. Other measures of power quality, such as total demand distortion (TDD), must be similarly kept below mandated limits. Underwriting power quality when the supply is flowing in from centralized generation to consumers is one thing, when it’s randomly switching between that centralized generation and non-utility distributed generation, it’s quite another.

PV panels generate power by the photoelectric effect; photons from the sun are absorbed by atoms in the panel, releasing electrons which produce a current when flowing across the junction between the p- and n-type semiconductors making up the assembly. The panel is divided into numerous cells to increase the voltage and current generated. Many factors influence the panel’s performance, including the power density of the incident sunlight and the ambient temperature, but perhaps the most critical component is the inverter which converts the panel’s DC voltage to the AC voltage carried over the grid.

Unfortunately, while there are some excellent systems on the market, there are also many lower-quality inverters that impinge on the efficiency of the panel and, worse still, introduce the kind of harmonics that utilities don’t want in their systems. Multiplied across tens of thousands of PV systems, these harmonics add up a lot of unregulated energy injected into the grid, generating heat that eventually damages equipment and insulation. This in turn raises maintenance costs and increases the risk of outages. Consumers’ equipment, particularly those using medium and large electric motors, can also be affected.

In addition to the introduction of unwanted harmonics from less-than-perfect inverters, high incidence of PV penetration commonly creates two other problems: voltage swells and “islanding”.

Like harmonics, swells can also cause damage to network assets and customer devices. Islanding is an even bigger danger. It happens when supplies bringing power into an area are interrupted but the local PV generators continue to feed the local grid. The result is an energized ‘island’ isolated from the network. Such a situation is hazardous because it causes damage to grid-protection equipment such as circuit breakers if they attempt to switch live high-voltage conductors, delayed restoration of service to affected customers outside the island, increased danger to maintenance crews repairing faults, degradation of power quality within the island, and increased potential of damaging voltage swells.

The long-term solution to these challenges is a smart grid, defined by the U.S. National Institute of Standards and Technology (NIST) as “a modernized grid that enables bidirectional flows of energy and uses two-way communication and control capabilities that will lead to an array of new functionalities and applications”. Among those functionalities will be real time monitoring of power quality enabling the smart grid to automatically predict when excessive harmonics, voltage swells and islanding are about to occur and triggering the operation of switchgear to temporarily isolate PV generation before asset or consumer equipment damage occurs.

But the cost and complexity of a smart grid––not to mention the sheer size of the rebuilding task and conservative regulation––means it will take decades to fully implement. In the meantime, utilities will need to get tough on low-quality PV systems and work hard to improve their own operational procedures because prosumers are here to stay.