This is part 2 in a series on staged fault testing.
As more distributed renewable energy sources are connected to the grid, and heat waves and other severe weather events linked to climate change increase in frequency, the risk of wide area blackouts caused by preventable relay misoperations and unrecognized modelling errors increases. Though the last major wide area blackout occurred in 2011, the risk increases each year as hotter summers become the norm. The possibility of another major wide area blackout can be minimized by implementing well-planned staged fault testing.
Staged fault testing validates the integrity of complex protective relaying schemes. Rather than hoping that protective relay schemes work correctly when a naturally occurring fault happens, electric utilities should perform staged fault testing to preemptively verify protective relay system operation.
Let’s compare traditional protective relay analysis with staged fault test analysis.
Traditional Protective Relay Analysis
The electric energy grid is a network of transmission lines that are equipped with relayed circuit breakers, as illustrated in figure 1. Protective relays associated with transmission lines and buses are designed to actuate when a fault occurs, and to open as many circuit breakers as needed to isolate a short-circuited facility.
Figure 1 shows a simplified electric energy grid. In the figure, □ represents a relayed circuit breaker.
Before lines and buses are energized, protective relays and circuit breakers are thoroughly tested and certified as ready for energization.
When a new transmission line is placed in service, such as Line 6 in figure 1, electric utilities input transmission line data into Load Flow, Short Circuit, and Recovery models to determine electric grid performance. Then, as time passes, operational and model data are compared.
Let’s take a closer look at each of the three model types:
1. Load Flow Comparisons
Comparing operating data with Load Flow models is easy and can be completed as soon as a transmission line is energized. All that needs to be done is to compare MW, MVAR, and KV values in the model with real world values. Discrepancies between actual and model data are indicators that the model needs to be refined.
2. Short Circuit Model Comparisons
Comparing short circuit data with short circuit models is challenging because faults only occur once every few years, and uncovering discrepancies between actual and model data is tedious and time consuming.
To compare data after a fault, the protective relay engineer must first download data from every protective relay that may have detected the fault. Then the short circuit model must be modified to represent the actual fault condition. After that, the protective relay engineer must compare relay data with model data on a line-by-line and phase-by-phase basis.
3. Recovery Model Comparisons
Comparing operating data with Recovery models is also challenging. Three phase faults that provide relevant data occur infrequently. Operating data is not granular enough to compare MW, MVAR, and KV values in the Recovery model with real world values.
Digital fault recorders, which contain a good deal of relevant data, are located throughout the electric grid; however, they log milliseconds of fault data, not tens of seconds of recovery data. Ultimately, uncovering discrepancies between actual and model data typically takes place at a far later time than is helpful.
The Solution: Staged Fault Testing
When an intentional short circuit, also known as a staged fault, is placed on an energized line, the operability and security of many protective relaying schemes can be assessed.
Figure 2 shows a staged fault test being performed on Line 6, near Bus 3. This test verifies the performance of protective relaying schemes associated with Line 6, and the security of schemes associated with other lines and buses. In addition, recovery data can be recorded and used to verify Recovery models.
Figure 2 shows a staged fault test, represented by the flash symbol, on the simplified electric energy grid from figure 1.
Charting Staged Fault Test Results
After the staged fault illustrated in figure 2, short circuit models and “Go / No Go” results should be assessed before Line 6 is placed in service. Results from the short circuit model for three phase faults and single phase to ground faults can be tabulated in Table 1.
For Line 6 to receive “Go” status, actual transmission line short circuit current should be within +/-10% of the listed value, and actual bus voltage should be within +/-5% of the listed value. Actual relay times should not be more than 8 milliseconds longer than the listed value.
Short circuit current, voltage, and operation of relays associated with existing, unfaulted transmission lines are monitored to assure that control scheme logic and relay firmware have been properly enabled, and that facilities are properly modelled. Existing facilities are not assessed as “Go / No Go."
For the staged fault illustrated in figure 2, recovery data can be tabulated in Table 2: Bus Data, and Table 3: Line Data. The expectation is that the electric grid should recover in less than 1.0 second when staged fault tests are conducted during mild weather conditions. During hot and cold weather conditions, recovery may take longer. Model data should be compared with 1 second data and 5 second data.
Staged Fault Testing: Vital for Grid Security
Staged fault testing allows electric utilities to assess protective relay schemes' functionality before a naturally occurring fault happens. Waiting for a fault to occur to know whether protective relay schemes will function correctly is risky, and could lead to a wide area blackout during peak load conditions.