I’ve said this before, but it’s worth repeating: Utilities deliver three things: voltage, frequency and reliability. The first two items impact the third. And, frequency – at least in an interconnected system with plenty of inertia like what we have in continental North America – is pretty easy to manage because it’s the same throughout the power system. Here in the Western interconnection where I live, that means the frequency is the same in Denver, Las Vegas, San Diego and Vancouver, BC.
Voltage is not like that. You have to have what’s called a reactive power balance, and reactive power is done with things volt amperes reactive, or VARS, which are power with a current that leads or lags the voltage on the system. If VAR current lags voltage , it pulls voltage down. If it leads voltage, it brings voltage up. Unlike real power – which controls frequency – there is no storage for VARs. If you get a shortage of VARS anywhere in the power system, the voltage will drop instantly.
More power, more complexity
Now, here’s a little bit of history about VARs. Prior to about 1950, the grid was dominated by low voltage lines (138 kV and lower). Those lines did not produce or absorb a lot of reactive power.
By the time I worked as a power plant operator in the 1960s, the system that I was operating could control all voltage from the generating station. When dispatch called me and told me to raise voltage, I could raise the generator voltage, and that raised everything – including the plugs in peoples’ houses – about 200 miles away in several directions. It was simple. Few people paid attention to voltage control. It was all about frequency and power.
By 1970, we started seeing a lot of lines at 230 kV to 735 kV, and these lines are interesting. They have some characteristics that were somewhat ignored initially but they have proven to be really significant.
The first of those characteristics is that high voltage transmission lines themselves produce VARs, and they increase or decrease the amount with voltage squared. If you raise the voltage by 10 percent, high voltage lines produce 21 percent more VARs, and that drives the voltage up further because VARs drive voltage. If you drop the voltage by 10 percent the lines deliver 19 percent fewer VARs, which allows the voltage to fall further. The amount of VARs produced by a long 500 kV line is far more than any generator could generate.
On the other hand, high voltage lines also can consume VARs, particularly if they are highly loaded, and VAR consumption goes up with the square of the power (current). If a line is loaded lightly, as it usually is at night, it produces huge amounts of VARs and the voltages are pushed up. The generator operator can do almost nothing to pull it down because the amounts are too big. At the same time, at peak load, some lines consume more VARs than they produce, and the result can be low voltage. Again, the generator operator can do little to correct the situation.
Power engineers generally agree: You cannot transmit a VAR over long lines. The VAR loss is too large, and marginal loss can exceed 80 percent.
These factors and characteristics of VARs are becoming increasingly important because of the proliferation of intermittent solar energy coming onto the grid. A sudden drop in generation – which happens with the variable generation of renewables – will drop local voltage instantly. Even neighborhood-by-neighborhood cloud cover can create instability if there are enough solar generators on local feeders.
But, if you could take local measurements and know what kind of voltage your customers are experiencing, then you could add devices to help correct voltage problems locally. What you need is devices that are in continuous, two-way communication with the grid operator but still have enough intelligence to do calculations and, based on conditions and pre-programming, take fast action.
This is the idea behind distributed energy resource management systems – or DERMS - which have voltage and frequency sensors on each distributed energy resource and also tracks the entire network’s capacity, ramp rate, output and consumption every few seconds. With platforms like these, grid operators can have another voltage-correcting tool just about as significant as those generators I operated in the 1960s. And, unlike those old-time, centralized generators, this approach makes local corrections to local voltage excursions. Isn’t progress grand?