Consumer load models are not accurately depicted by electric utilities, and therefore are not appropriately considered when engineers design the electric power grid. During power system design, electric utilities need to perform a complete analysis of consumer load models, recognizing the different operating conditions of the electric power grid: normal operations, cold load pickup, and fault recovery.
In this article, we present an innovative curve to guide the development of system models. Prescient’s updated curve considers information provided by equipment manufacturers, which has not been used previously to drive grid designs within electric utilities.
For example, the Information Technology Industry Council (ITIC) provides voltage boundary curves that illustrate the voltage tolerance envelope applicable to single phase 120 volt equipment. ITIC curves are plotted from zero volts to 600 volts, and from 1 microsecond to twenty four hours. ITIC curves are used by engineers who design personal computers, televisions, and other devices that are connected to 120 volt circuits in business and residences.
Manufacturers of pumps, fans, and compressors provide speed torque curves that are used to size electric motors. Mechanical engineers use these curves when designing water treatment facilities, large air conditioning systems, food processing facilities, etc.
By integrating the information from the curves of other industries, electric utilities can more appropriately design transmission and distribution systems. Read on to learn more about Prescient’s updated curves.
Robust Electric Grid Design
Basic premises when using voltage boundary curves and speed torque curves are that power system voltage is stable, and that loads are switched on and off at random. However, to design resilient electric power systems, electric utility engineers need to consider three operating conditions: normal operations, cold load pickup, and fault recovery. We’ll explore these three conditions later in this blog.
This means that electric utility engineers need to utilize time-voltage performance curves for electric system loads, as seen in Figure 1, to understand metrics that describe the response of residential, business, and industrial loads. Table 1 lists some important characteristics for a variety of components. Figure 1 and Table 1 are valid representations for most residential, business, and industrial loads.
The key variables listed in Figure 1 are (tabulated from high to low at 100 milliseconds):
Upper voltage limit – component service life will be reduced when terminal voltage exceeds this value.
Increased motor start time – motor start time will increase when terminal voltage is less than this value.
Increased output power – component output power will be increased when terminal voltage exceeds this value (20% for resistive devices, 5% for induction motors).
Compressor stall – compressor motors will stall when terminal voltage is less than this value for 100 milliseconds.
Lower voltage limit – component performance will be compromised when terminal voltage is less than this value.
Motor starter drop out – fan, pump and compressor motor starters will drop out when terminal voltage is less than this value for 25 milliseconds (manual intervention may be needed to restart the motor).
Reduced output power – component output power will be reduced when terminal voltage is less than this value (20% for resistive devices, 5% for induction motors).
Fan/pump speed reduction – fan and pump motors will slow down when terminal voltage is less than this value for 100 milliseconds.
Power Grid Operating Conditions
Let’s take a closer look at the three operating conditions of the electric power grid: normal operations, cold load pickup, and fault recovery.
1. Normal Operations
During normal operations, components listed in Table 1 start and stop on a random basis, as illustrated in Figure 2. On a peak load day, a typical 12.47 KV distribution line supplies electric power to 1,000 air conditioners with less than 50 air conditioners starting at the same time.
Some air conditioners operate continuously. Some air conditioners start/stop several times each hour. When a small portion of connected loads start/stop at the same time, the impact on the nationwide grid is tolerable.
2. Cold Load Pickup
Cold load pickup is the term used to describe the instant that power is restored to consumers who are out of power for several hours while equipment repairs are completed. On a peak load day all air conditioners, refrigerators, freezers, fans, etc. supplied via a 12.47 KV distribution line will start simultaneously as soon as power is restored. Cold load pickup can be as much as 200% of peak load and persist for an hour or more until normal load cycling returns.
Figure 3 illustrates cold load pickup. As cold load pickup is initiated manually, the impact on the power grid is addressed before a circuit breaker is closed to restore power.
3. Fault Recovery
Fault recovery is the term used to describe the instant that faults are cleared from the electric power grid. The assumption is that most of the loads that were energized prior to a fault will continue to draw power after the fault is cleared. The complication is that air conditioners and other motor driven loads will slow down or stall during the short time so that the flow of electric power is interrupted, as seen in Figure 4.
The concern is that fault recovery can be delayed when thousands of air conditioners attempt to reaccelerate simultaneously.
Worst Case Conditions
From the perspective of wide area blackout prevention, the most challenging condition is a three phase fault on a 345 KV, 500 KV, or 765 KV facility that is adjacent to a major metropolitan area. If a three phase fault occurs on a 345 KV, 500 KV, or 765 KV facility in a major metropolitan area, load controllers in residences and industrial buildings will drop out and load shedding will occur.
However, if a three phase fault occurs on a 345 KV, 500 KV, or 765 KV facility 25 miles away from a major metropolitan area, load controllers in residences and industrial buildings will not drop out, load shedding will not occur, and simultaneous reacceleration of thousands of fans, pumps, and air conditioners will occur. This is what leads to a wide area blackout.
Next Steps: Staged Fault Testing
Analyzing a power grid with thousands of fans, pumps, air conditioners, process heaters, etc. served by traditional power generators in tandem with renewable energy sources is challenging, even with today’s data logging and computing capability.
A better approach is to perform staged fault tests each year in the spring. The purpose of these tests would be to minimize uncertainties in the response of customer loads and to prepare for real world conditions. In our next article, we’ll describe how to implement staged fault testing in more detail.