Voltage Stability in a Changing World

I posted an article on wind and solar and there have been several questions on firming intermittent sources. This is a coming issue that may not easy or cheap to solve. Batteries can certainly do the job and may well turn out to be one of the easier solutions, but cost will be a significant factor.  Whatever the case, the firming source will have to be fairly close to the intermittent source and not a large generator 100 miles away, even though from a balance perspective, that would easily meet the need.

The problems are interesting were predictable, but largely ignored by many people. It is about the dreaded and poorly understood VAR. VARs are “reactive power” – just another meaningless pair of words – a VAR is actually a unit of energy that goes toward the load for ¼ cycle, then turns around and comes back for the next ¼ cycle, then goes again for the next ¼ cycle, and back again for the last ¼ cycle. VARs are often called watt-less energy – because they constantly deliver and return watts – but over time there is NO net delivery of energy. They flow to and from a device that will store and return energy rapidly. There are two common types of storage devices that absorb (or supply) VARs, and these are capacitors and inductors. Interestingly, they store and return at exactly opposite parts of each cycle, so one can effectively cancel the other if connected and sized appropriately. If fully matched, power engineers call this unity power factor, radio engineers call it resonance, but the result is the same.

Why do I talk about VARs? There are two key balances that must be maintained in an electric grid… watts generated – and used (or stored) are continuously in balance and VARs created or absorbed are also balanced. Any power imbalance causes a frequency deviation, and when I was young, that made all clocks run a little fast or a little slow, depending on the direction of the frequency error. The power balance is maintained for an entire interconnected area. The VAR balance must be maintained locally, and any local surplus or shortage will push the voltage up or down. Utilities have methods and systems that maintain these two separate balances on a continuous basis.

One important distinction between watts and VARs is the fact that watts ultimately come from a generator, while VARs can be created or absorbed locally using capacitors or inductors. But both watts and VARs require Amps to deliver them, with the result that most utilities try to minimize VAR transfers to minimize system losses.

So how does this have anything to do with firming intermittent generation?  As it turns out, quite a lot.

As the continent has become heavily connected with HV transmission lines, new problems have appeared. High voltage lines look electrically like a capacitor connected between the phases, and a series inductance in the line itself. Energizing a line that is carrying no power will look like a big capacitor, and it will “spew” VARs at both ends, driving voltages up. As the line starts to carry power, the series inductance of the line absorbs VARs. The equations are simple. The capacitor creates VARs proportional to the square of the voltage (CV2) , so a line that has double the voltage of a nearby line of the same construction and length will create 4 times the VARs. On the other hand, the inductance in the line absorbs VARs in proportion to the square of the current (LI2), so if there is no current in the line, no VARs will be absorbed. As the line is loaded up, the VARs absorbed by the line inductance increase, and may soon equal the VARs created by the capacitance of the line. This power level is known as the Surge Impedance Level (SIL), and many utilities have limits on the line loading, based on the SIL. This is common in the west, while in the east, where line lengths are generally shorter, and transmission systems denser, the limit is a thermal limit – when the cable gets hot.

In recent years, we have seen rapid growth in high voltage lines, meaning that the VARs produced or used in transmission lines is growing rapidly.

So now back to renewable firming. Intermittent generation can change quite rapidly, and if it does, there is a need to manage the power from somewhere else. The source may be nearby batteries, or it may involve demand management (turning water heaters or EV chargers up or down) or it may require power from some distant generating station. If the latter is the case, and the change in power delivered is rapid, the transmission lines delivering the power may suddenly start to absorb or release a lot of VARs and cause significant voltage issues at locations along the way. This has been a significant problem in areas where large hydro utilities are used to provide regulation service or intermittent firming from a distance. Hydro can generally respond very quickly and can be an ideal firming or balancing source for intermittent generation. But if the distance between the hydro and the intermittent source is large, voltages in between may be difficult to manage.

At the same time, it brings up another problem. When I was young and worked as a dispatcher, if one plant and associated transmission went “down,” we could simply power the lines to the station from the remote end, power up the plant, start it and everything worked well. But the lines to the plants were often 60 or 138 kV and were easy to power up.

Today, large remote hydro generating stations often require 2 or more generators to be running and synchronized to the bus before the transmission line can be energized, because of the large “charging current” (required by the line capacitance). In the past, very few generating stations were equipped for “black start” capability, but today, where the generating stations are remote, and have long transmission lines, the plants must have large diesel or similar capacity to be able to meet “black start” requirements.

In looking at the solar and wind systems that I have seen, most are not capable of black starting, and even those that are, have limited capacity for reactive power. It may become difficult to charge a transmission line where all of the power source at the end is renewable, such as wind or solar.

Utility planners are well aware of these issues, and yet I seem to hear many renewable advocates assuming that these issues are trivial. They ae not. They can likely all be solved, but a carefully designed plan will be needed.

We all want to make this work, but it seems that there remains a challenge in getting people on the “same train” with the same goals in mind. That will NOT be easy. Many utility people are slowed by what I refer to as “utility inertia” and they resist all change, while advocates of renewable are quite the opposite, wondering why there is a problem at all. There are real problems, but there are also solutions.

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