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NERC PRC-010-2 – Undervoltage Load Shedding: Concerns and Improvements

In the latest installment of this series on NERC’s Reliability Standards, I focus on PRC-010-2 – Undervoltage Load Shedding. It is vital for NERC to recognize that wide area blackouts will occur until root cause issues are addressed, ideally through innovations that are inspired by reliability standards.

Follow along to read about my concerns with PRC-010-2, and my recommended improvements.


PRC-010-2 Stated Purpose


The stated purpose of NERC Reliability Standard PRC-010-2 is:


“To establish an integrated and coordinated approach to the design, evaluation, and reliable operation of Undervoltage Load Shedding Programs (UVLS Programs).”


Requirement R.1 includes the following:


"The evaluation shall include, but is not limited to, studies and analyses that show:


1.1. The implementation of the UVLS Program resolves the identified undervoltage issues that led to its development and design.


1.2. The UVLS Program is integrated through coordination with generator voltage ride-through capabilities and other protection and control systems, including, but not limited to, transmission line protection, autoreclosing, Remedial Action Schemes, and other undervoltage-based load shedding programs."


Concerns with Reliability Standard PRC-010-2


Like other reliability standards, PRC-010-2 would benefit from updating the jargon used throughout from legalese to that of the user, in this case, protective relay engineers and transmission system operators.


Every electric utility installs redundant protective systems that trip facilities when short circuits occur as well as when underfrequency occurs. Some electric utilities install undervoltage load shedding schemes that are designed to actuate when system models reveal that load shedding will prevent system collapse.


Let’s take a closer look at three related concerns with PRC-010-2.


Concern 1: Defense in Depth


The electric energy grid has been designed to withstand a limited variety of component failures. Generally, this includes single phase faults, three phase faults, single phase faults with a stuck circuit breaker, and three phase faults with a stuck circuit breaker.


At most voltage levels, design base conditions do not include other unlikely failures, even though these unlikely failures have occurred. This includes discharged substation batteries, incorrect relay logic, misaligned circuit breaker auxiliary contacts, etc.


Concern 2: Lack of System Recovery Models


Models have not been developed to show recovery of the electric energy grid in the event that a short circuit has not been cleared in 250 milliseconds. The requirement for recovery models is lacking in multiple of NERC’s Reliability Standards, including PRC-010-2 and CIP-014-3 – physical security.


Concern 3: No Mandate for Overloaded Facility Shutoffs


The National Electric Safety Code (NESC), unlike the National Electric Code (NEC), does not mandate that overloaded facilities are automatically tripped when overloads occur. In fact, FERC has required ambient-adjusted ratings be developed to maximize energy transfer across transmission lines.


How to Improve NERC PRC-010-2


The following actions will substantially improve the effectiveness of PRC-010-2.


1. Add the requirement that electric utilities install undervoltage schemes that shed load when voltage is less than 90% of nominal for more than one second.

Basis:

Unlikely events, such as the inadvertent opening of a 500 KV air break switch as happened at North Gila Substation, will create an energy transient. Immediately, nearby motors will slow down or stall; then, when the fault is cleared, many motors will attempt to simultaneously reaccelerate. This will overly stress the power grid and lead to a wide area blackout, as occurred in the 2011 Southwest blackout.

2. Add the requirement that every transmission operator study and understand:

  1. Unlikely events that have occurred.

  2. How such events can result in collapse of the electric energy grid unless undervoltage load shedding is utilized to mitigate the collapse.

Basis:

Unlikely events that have resulted in wide area blackouts include inadvertent opening of a three phase air break switch or inadvertent closing of a three phase ground switch during prearranged switching. Unlikely events that almost resulted in wide area blackouts include stuck circuit breakers that were not detected by circuit breaker failure schemes.


3. Add the requirement that transmission system owners prepare bifurcated recovery models based on calculated bus voltages during fault conditions.

Basis:

Voltage is depressed across a wide area whenever a three phase fault occurs. The amount of load that trips offline or attempts to simultaneously reaccelerate after the fault is cleared is a function of fault location, grid configuration, and load type.


Modern computer-based models need to be developed to automatically change load models based on fault voltage using the following parameters:


1. Fault voltage greater than 80%:

  • Compressor motors, pump motors, and fan motors slow down and reaccelerate when voltage recovers.

2. Fault voltage between 40% and 80%:

  • Compressor motors stall and restart when voltage recovers.

  • Pump motors and fan motors slow down and reaccelerate when voltage recovers.

3. Fault voltage less than 40%:

  • Compressor motors, pump motors, and fan motors trip offline and remain off until restarted.

All three of the above conditions will occur at different locations every time a three-phase short circuit occurs.


4. Add the requirement that time delays of undervoltage relays be based on power factor as a trigger for immediate actuation.

Basis:

During normal operations, the power factor of the load supplied by any high voltage power transformer will be near unity. If load power factor drops to less than 70% after a fault has been cleared, this is an indication that many motors supplied via a specific substation are reaccelerating simultaneously.


If power factor has not recovered to more than 80% in ten seconds, collapse of the electric energy grid is imminent.


A Better Purpose Statement for NERC PRC-010-2


A better purpose statement for NERC Reliability Standard PRC-010-2 would be:


“To establish an integrated and coordinated approach to the design, evaluation, and reliable operation of Undervoltage Load Shedding Programs (UVLS Programs).”


Undervoltage load shedding schemes, just like underfrequency load shedding schemes, are needed at many locations throughout the electric energy grid.


In our next article in this series, I’ll outline concerns and improvements to NERC Reliability Standard, MOD-031-3 — Demand and Energy Data.


Questions about my expertise or recommendations? Please email me directly or submit our contact us form.

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