Before we continue our deep dive into the design and operation of the electric energy system, we want to address some basic concepts. Electric energy systems respond very quickly; response time is measured in milliseconds. Mechanical systems respond much slower; acceleration time is measured in seconds. Power system operators need to match the response of mechanical systems to the limitations of electrical systems.
In this article, we intentionally represent the transmission and distribution system as a cloud, rather than the traditional line diagram, because we are discussing impacts over a wide area that includes numerous electric facilities. Read on to learn more.
Steady State Electric Energy System Models
A typical electric energy delivery system model that illustrates uncompensated customer loads and energy sources is included as figure 1.
Figure 1 shows an electric energy system uncompensated model.
Because electric utility revenue is based on kilowatt hours sales, and losses are based on current flow, electric utilities install power factor correction capacitors to reduce current flow and increase efficiency. A typical electric energy delivery system model that illustrates power factor compensation is included as figure 2. This figure illustrates actual power system operation during steady state conditions.
Figure 2 shows an electric energy system power factor compensated model.
Electric Energy System Models After a Fault
When faults occur, energy transfer within the power grid is distorted. A new temporary operating condition is established as soon as the fault is cleared. If the fault is in load area 2 in figure 3, there will be a slight increase in power to reaccelerate motors that slowed down or stalled during the energy transfer distortion caused by the fault.
If motors stalled rather than simply slowed down, a substantial increase in excitation energy will be needed during recovery. If nearby power and excitation energy is available, recovery will occur.
Figure 3 shows an electric energy system during recovery conditions.
When a fault is cleared, grid voltage recovers instantaneously and new, temporary power and excitation energy transition patterns are established. These temporary patterns evolve as motors reaccelerate. Current, power, and excitation energy of motors are a function of motor speed, while current and power to lights, heaters and other resistive loads are a function of voltage.
If nearby recovery power and excitation energy is insufficient, nearby energy sources may trip offline as shown in figure 4. If the demand in load area 2 has not decreased and energy sources in areas 2 and 3 trip offline, a wide area blackout will occur.
Figure 4 shows an electric energy system during delayed recovery conditions.
Compared to short circuit calculations and load flow analysis, fault recovery simulations are tedious. Integrated system response is a function of load type and load level, in addition to the configuration of the power grid and location of energy sources.
To determine motor reacceleration parameters, mechanical loads connected to each motor must be understood. Reaccelerating a pump motor with discharge valves open may require more energy and time than accelerating a pump motor with discharge valves closed.
Reaccelerating an intake fan motor with dampers open may require more energy and time than accelerating a fan motor with dampers closed. Reaccelerating an air conditioner compressor with a charged condenser may not be possible.
We recommend that electric utilities:
Categorize fault recovery loads as fans, pumps, compressors, heaters, etc.
Develop models for each load type that represent at least 10% of connected load.
Include lumped load models for each load type at each equivalent load point.
Include fault recovery calculations in their standard system models.
To learn more about the risk of wide area blackouts from a mismatch in energy demand and availability, check out our article Before Renewable Energy Expands, FIDVR Must be Resolved.
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