Last week, we talked about renewable energy transfer from the production source to the grid. We noted that the two best options for renewable energy transfer are induction generators and inverters.
This week, we’ll explore a notable concern related to increased renewable energy sources powering the grid: the fact that induction generators and inverters can exacerbate low voltage conditions during recovery from fault to pre-fault conditions. This may not be an issue at all times of the year, but on a hot summer’s day, exacerbated pre-fault conditions could lead to a wide area blackout when thousands of air conditioners restart at the same time.
Let’s take a closer look at exactly how this can happen. First, we’ll discuss the way that energy is consumed from the electric energy grid.
Energy Consumption: A Closer Look
Energy consumption is categorized as real power, MW, and excitation energy, MVAR. Real power, MW, is consumed by induction motors, synchronous motors, heaters, furnaces, power supplies (such as AC/DC converters for laptops), lights, refrigerators, and more. Excitation energy is consumed by induction motors and generators, power supplies, and transformers. Excitation energy can either be consumed or produced by synchronous motors.
The energy production/consumption flow path illustrated in Figure 1 is dynamic. When faults occur the transfer capability of the electric energy grid is temporarily degraded. When faults are cleared, energy consumption is temporarily increased.
Figure 1 shows the energy production/consumption flow path.
Inrush energy, which exceeds operating energy, is required every time that a load is switched on. Inrush energy for a 100-watt incandescent lightbulb can be 1600 watts for 8 milliseconds. Inrush energy for a one horsepower induction motor can be 1000 watts and 3000 vars for 10 seconds, while full load energy is only 746 watts and 360 vars.
The amount and duration of inrush energy to a motor is a function of mechanical load and operating condition. For example, induction motors can start in less than one second if the shaft is not connected to a load. When the shaft is connected to a window fan, the start time can be a few seconds. When the shaft is connected to a water pump with an open discharge valve, the start time can be twenty to thirty seconds.
Normal Air Conditioner Operation
In residential areas, a typical neighborhood substation supplies energy to 5,000 customers. During August, approximately 4,000 air conditioners will be operating at the same time.
The electric energy grid is designed to accommodate random motor starts, as each motor is started and stopped based on local conditions. Because all air conditioners are started and stopped based on indoor temperatures, neighborhood electric energy systems are designed so that 10% of air conditioners can start at almost the same time.
After a Fault: Air Conditioner Acceleration Concerns
The electric energy system is not designed for all air conditioners to start at exactly the same time because this is rarely the case. However, after a fault occurs, this is exactly what happens.
When a three-phase fault occurs on transmission facilities in August, all 4,000 air conditioners described above can stall if a fault persists for more than 150 milliseconds. The restart of these 4,000 air conditioners at the same time would overload a system that is designed to only support 400 units starting at one time.
This will create low voltage conditions throughout the electric energy grid for 20 seconds or more. Shunt capacitors that are installed for power factor correction will be less effective as MVARs produced by capacitors decrease when voltage decreases. The result would be cascading low voltage, leading to a wide area blackout.
Renewable Energy Transfer and Risk of Blackouts
Unless designers include fault recovery in voltage control algorithms, induction generators and inverters can exacerbate low voltage conditions during recovery to pre-fault conditions. Remember, induction generators and inverters are the best option for transferring electric energy to the grid, especially from renewables. However, without implementing the proper algorithms that account for the conditions described above, the risk of a wide area blackout is high.
Shunt capacitors used to correct power factor of induction generators may only produce 60% of nameplate vars. Inverter capability may be reduced to 30% of nameplate vars if algorithms are focused on watt production.
Calculations and simulations are developed using assumptions and clarifications that are based on the experience of the person who prepares the calculation. For example, the percentage of energy consumed by air conditioners can be estimated by comparing weekday load in April to weekday load in August. When this metric is used in Philadelphia, 50% of energy is consumed by air conditioners in August. When this metric is used in Phoenix, 65% of energy is consumed by air conditioners in August.
How to Address this Issue
Despite induction generators and inverters being the best option to transfer energy from renewable energy sources to the electric energy grid, these devices do not completely alleviate the risk of wide area blackouts during peak load conditions.
One solution to address this issue is to conduct staged fault testing during non-peak load times. This will enable the collection of data that minimizes inaccuracies introduced by previous experience.
For the next several weeks, we will discuss staged fault testing in detail. Follow along with our blog to learn more! And please contact us with your questions. We are always happy to discuss these issues and solutions in more detail.