Last week, we discussed the importance of electric utilities preparing for the inevitable, whether that is climate change related severe weather, widespread system disruption due to sabotage, or rare, but likely, power system events that could lead to a wide area blackout. To better prepare for this range of disruptive and costly events, electric utilities must update their power system models.
Specifically, power system models must have improved accuracy. By creating several models for a range of conditions, electric utilities can achieve two goals:
Estimating whether voltage will be within range when specific conditions occur.
Estimating whether load current will exceed facility ratings when specific conditions occur.
Including voltage and current parameters in updated models will also help to achieve these goals. Making these three changes will improve the accuracy of power system models. Follow along to learn more.
Include A Range of Conditions in Power System Models
The accuracy of electric utilities’ load flow models depends on relevance of data used to develop each model. For example, transformer models are only somewhat accurate, as impedances are entered for a single tap position but are not changed when the tap position changes. Similarly, transmission line models are only somewhat accurate, as resistance is entered for a single conductor temperature and not changed as conductor temperature changes.
Creating accurate load models is a challenge because actual loading is unknown. The solution is to create several load models that represent a range of loads. To create the most accurate and widely applicable models, electric utilities would be wise to model a range of conditions, including:
80% of peak load
90% of peak load
110% of peak load
Goal 1: Estimate Voltage Range
One goal of these updated models is to estimate whether voltage will be within range when specific conditions occur. There are no mandatory values for transmission system voltage and distribution system voltage. There are only a few locations where transmission system voltage can be controlled.
Consider, for example, a 230 KV system. Voltage can be controlled by energy production facilities and by 765KV / 230KV, 500KV / 230KV and 345KV / 230 KV transformers that are equipped with load tap changers.
Distribution system voltage can be controlled at every neighborhood substation with transformers that are equipped with load tap changers.
Goal 2: Estimated Load Current
The second goal of the updated models is to estimate whether load current will exceed facility ratings. Load current limits are determined for transmission facilities in compliance with NERC Reliability Standard FAC 008 - Facility Ratings. Load current limits for distribution lines are determined in accordance with the National Electrical Safety Code (NESC).
Multiple models that display a range of power system conditions will help utilities determine if load current will exceed facility ratings during various periods of energy demand.
Include Voltage and Current Parameters
Electric utilities accurately calculate quasi-steady state voltage and current. Quasi refers to the fact that voltage and current values change slowly as measured in minutes or hours. Transient values that change in seconds or milliseconds are difficult to represent accurately. During normal system operation, this is not a concern as all loads are independently stopped, started, and controlled.
A Monte Carlo simulation, a mathematical technique that predicts possible outcomes of uncertain events, is acceptable during normal operating conditions. However, when three phase faults occur on transmission facilities, Monte Carlo simulations are no longer valid. This is because controllers, motor starters, etc. will be prepositioned to simultaneously reaccelerate all motors that were running before the short circuit occurred.
Should this occur after a three phase fault, there is a high probability that a wide area blackout could occur. Updates to models must incorporate voltage and current parameters so that the response of the power system to a three phase fault can be understood before such an event occurs.
Updated Models are Vital for Electric Utility Response
As climate change worsens, demand for electric energy will only increase; at the same time, the conditions in which the grid is operating will change rapidly. It is vital that electric utilities have accurate models of power system response to various grid conditions and events so that they are prepared to respond to these changing conditions.