Summer Revisited: The laws of physics don't change with the seasons

shutterstock / bohbeh
photo credit: shutterstock / bohbeh

With the fourth heat wave of the past summer in the Northeast becoming a distant memory, some of the power-related issues that surfaced then are still worth revisiting. The laws of physics don’t change with the seasons, even if our focus does. I’ve included a few questions you may have asked yourself during the summer and their explanations below.

Why do electric utilities reduce the voltage during high-temperature times?

What used to be called “brownouts,” and are labeled in the IEEE 1159 standard as “sustained undervoltages,” occur when the electrical demand exceeds the ability to provide it without causing damage to the equipment supplying the power. It is a reduction in voltage supplied to the point of common coupling being lower than the ANSI C84.1 limits. Brownouts get their name from the days when most lighting came from incandescent bulbs whose light output gets visibly dimmer or “browner” when the voltage is reduced 10% or more.

Multiple parts of the electrical-supply system can be at risk during a demand overload. The current demand can exceed that of the generators and the transformers at the transmission and distribution levels. The increased environmental heat during such times further derates the capacity of the transformers.

The wires, especially the transmission wires with the long spans between towers, will stretch out as heated up from the increased current flow, due to their impedance. As they dissipate more power within them, the metal expands, causing the wire to droop lower, and increase the voltage drop across them. The wires can droop so low that it becomes a safety issue. In Ohio, transmission wires drooped so low they contacted trees, which led to events that became the August 2003 Northeast blackout.

How do loads react to reduced nominal voltages?

There are basically three types of loads: linear, constant power and nonlinear. The simplest to understand is linear. The power consumed is proportional to the voltage times the current, along with a power factor that reduces the effective use of the power as it lowers from 1.0. An incandescent light bulb is a linear load with a power factor of nearly 1. By reducing the voltage supplied to it, the current and subsequently the power consumed will go down. The electric supplier effectively reduces the demand on the generator and the wires when voltage is lowered to a linear load.

A constant power load, such as an induction motor when the voltage is reduced a small amount, will increase the current demand with a reduced voltage to keep the power nearly constant. Such an increase in current means that the electric utility didn’t really do much to reduce the power demand on the generator and actually increases the losses in the wires and transformers. Forty years ago, the dominant load type was from electrical motors.

A nonlinear load—such as the power supplies for IT equipment and other electronic loads, including adjustable speed drives—changes its current demand based on the voltage level provided and the demand of the load that it powers. A laptop in sleep mode draws a small amount of current only in the middle or peak of the voltage sine wave. This is a greatly reduced power amount compared to a linear load drawing current throughout the cycle, but it is rich in harmonic currents. As the computer is operating with the CPU and hard drive in high utilization, the width of the current pulse gets wider. This changes the characteristic of the harmonic currents while increasing the total current demand. This type of load has become the predominate load in most commercial sites. It is one similar to the constant power, in that a voltage reduction causes an increase in current, as well as being constantly changing in overall current demand and harmonic currents, which are also a significant factor in the derating of transformers.

Why do the electric utilities want to put in taller utility poles/towers?

In the same manner that electric utility uses a sustained voltage to protect its equipment, an increased voltage can reduce the current demand on the supply system. Doubling the voltage will result in a four times increase in the power available with the same size wiring. That is why large loads in residential dwellings, such as HVAC and electric ovens, operate phase-to-phase at 240V versus 120V for most equipment. On the distribution system, 26 kV has four times the power capacity of 13 kV. However, the higher voltage requires higher distances between the wires and the rest of the environment. Transmission towers for 345 kV lines are significantly higher than for 230 kV lines. Of course, both cases cause those in the area to get upset about the perceived dangers of higher voltage poles. Curiously though, they generally don’t consider reducing their electrical demand, nor the electrical fields of the cellphone constantly placed right against their brains. Next month we will look into ways to reduce the electrical demand of your facility all year round, not just when heat waves come through each summer. It doesn’t just reduce your expenses, it also reduces the possibility of brownouts and the need for more of those taller, “monstrous” poles.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.