Last week, we discussed the risk of wide area blackouts from renewable energy transfer to the grid. We concluded that staged fault testing is the best way to prevent wide area blackouts from occurring during peak load conditions. Before we take a deep dive into exactly how staged fault testing will decrease the risk of wide area blackouts, it is important to understand normal grid operating conditions and typical fault recovery conditions.
In this article, we first review electric energy grid operating conditions: steady state, fault, and fault recovery. These three operating conditions must be considered when dispatching energy production facilities. They are especially important to consider as more renewable energy production sources are connected to the grid because renewables do not always respond to fault conditions in the same way that traditional, fossil fuel generating plants do.
Next, we dive into fault conditions and recovery. When motors stall during a fault, energy transmitted to consumers decreases; after the fault, the simultaneous reacceleration of motors leads to increased energy demand, sometimes more than the grid is prepared to provide. This can lead to a wide area blackout when the mismatch between production and consumption persists for more than 10 seconds.
Let’s take a closer look at these important considerations, and improve your background knowledge for our upcoming series on staged fault testing.
Steady State Operating Conditions
The electric energy grid operates in a quasi-steady state condition 99.99% of each year. This means that MW and MVAR production and consumption are balanced as indicated by stable frequency (60HZ) and stable voltage (near nominal). The word “quasi” is used to indicate that changes are slow enough that they can be analyzed as constant conditions.
During normal operating conditions, load ramps up and down, as illustrated in Figure 1, while consumers go about their daily activities. Induction generators and inverters used at renewable energy production facilities work well when the power system is in normal operating conditions.
Figure 1 shows normal, steady state operating conditions for the electric energy grid on any given day.
Fault Conditions
Fault conditions, also known as short circuits, create unbalanced MW and MVAR conditions and unstable voltage. The electric energy grid operates during fault conditions less than 0.0001% of each year. However, a single three phase fault, during peak load periods, with delayed clearing, can lead to a massive issue that results in a wide area blackout. This can be likened to playing Yahtzee: the possibility of rolling five dice with the same number is 0.0001% per roll.
Most faults are cleared within 100 milliseconds. In worse case conditions, faults have persisted for as long as one minute. If faults are not quickly isolated by opening circuit breakers, the electric energy grid collapses.
During fault conditions, energy production sources transmit reduced amounts of energy to consumers. Lights dim, induction motors powering fans and pumps slow down, and some induction motors powering air conditioners slow down while others stall.
Fault Recovery Conditions
Fault recovery conditions persist for less than 0.001% of each year. During fault recovery, the energy production / consumption balance may be distorted. When post-fault energy production and consumption are almost equal to pre-fault energy production and consumption, the electric power grid will return to steady state conditions almost immediately.
However, when post-fault energy production is less than pre-fault energy production or when post-fault energy consumption is greater than pre-fault energy consumption, returning to steady state conditions becomes tenuous. Figure 2 illustrates a temporary change in energy consumption during fault recovery. This temporary change can occur on any day, at any time when a fault occurs.
Figure 2 shows a temporary change in energy consumption during fault recovery, illustrated by the small spike between midnight and noon.
After a Fault: Grid Recovery Considerations
When three phase faults persist on 230 KV, 345 KV, 500 KV or 765 KV systems for more than 250 milliseconds, collapse of the electric grid is likely. Note, 250 milliseconds equal 15 cycles at 60 Hertz and 5 revolutions of a 1200 RPM induction motor.
The two keys to understanding recovery are knowing how energy production facilities will respond to a fault, and how many fans, pumps, and air conditioners will attempt to reaccelerate simultaneously. We discussed motor reacceleration in our last post, Renewable Energy and Risk of Wide Area Blackouts. Let’s take a closer look at fault recovery at traditional and renewable energy production facilities.
Fault Recovery at Energy Production Facilities
When a traditional energy production facility recovers from a fault, synchronous generators will continue to produce MW while increasing voltage to produce additional MVAR, unless an MVAR limiter actuates. If an MVAR limiter is actuated, generator terminal voltage will be reduced to the value needed to remain with MVAR limits.
When a renewable energy production facility with induction generators recovers from a fault, the induction generators will continue to produce MW while extracting more MVAR until voltage recovers.
When a renewable energy production facility with inverters recovers, inverters will continue to produce MW and MVAR as long as inverter current is less than 110% of rated MW amps. If voltage drops to 95% of rated, MVAR capability drops to 70% of rated as inverters are current limited.
Shunt capacitors, installed for power factor correction, will only produce 90% of rated MVAR when voltage drops to 95%.
When recovering from a fault at all types of energy production facilities, MW production facilities will continue operating. However, MVAR sources will be able to produce fewer MVARs during fault recovery than during normal, steady state operating conditions.
The issue is this: MW can be thought of as power that is passed to consumers, while MVAR sets the stage for power delivery. When a fault occurs, the stage is distorted and must be reset before normal steady state conditions can be re-established.
Up Next: Exploring Staged Fault Testing
It is important to understand normal grid conditions, fault conditions, fault recovery conditions, and grid recovery conditions because the electric energy grid experiences step changes that should not be evaluated as quasi-steady steady conditions. This is especially important to note before we discuss staged fault testing.
Next week, we will begin a series on staged fault testing: how to perform it, associated benefits, and more. Though not performed today, data gathered through staged fault testing could offer significant benefits to the electric power industry.
Questions about normal grid conditions, faults on the grid, or staged fault testing? Contact us!
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