How to ride through sags and outages

Electric utility grids rely on circuit breakers that reset automatically to isolate the faults that trees, storms, animals, lightning and auto accidents cause. This self-correction improves electric power reliability by minimizing the number of customers the fault can affect.

When a fault occurs, the affected breakers try to reset themselves within a few seconds of being tripped. They continue to do so a number of times in a coordinated manner to isolate the smallest section of the system around the fault. This technology restores power to many of the affected customers within a few seconds of the initial fault and provides an important benefit from the standpoint of power reliability.

However, these automatic reset breakers raise several power quality issues. One problem is momentary interruptions, which is a loss of voltage (power) for anywhere from 8 milliseconds (half a cycle) to a minute. Another problem is voltage sags, which are reductions in nominal voltage lasting from half a cycle to a minute. The severity of a sag is measured by the amount of voltage drop and its duration.

A costly, complex problem
Several studies researched the cost to industry of these power quality problems. One calculated the annual cost of momentary interruptions to be $52 billion. Another study focused on momentary interruptions and voltage sags to determine the cost per event and the total cost to customers. It revealed that voltage sags cost customers $7,694 per event, which translates to an annual total of $3.2 billion. The cost per momentary interruption is $11,027, and the total annual cost is $1.1 billion.

With the cost of power quality problems this high, it’s no wonder there are solutions that reduce it. Some of the devices that help mitigate voltage sags even claim to provide some momentary interruption ride-through capability. Most of these devices hold the motor starter contactors closed to allow the motors to remain connected to the grid. If the voltage is restored quickly, the motor comes back up to speed and continues to operate.

However, these devices have some issues. Equipment manufacturers insist that if this technology is installed on their equipment, they won’t honor the warranties because, theoretically, the equipment can be damaged if the voltage is restored while the motor is still rotating.

Electromagnetic flux (emf) from the collapsing induction field, system capacitance and residual magnetism in the motor fight the field produced by the reapplied voltage. The instantaneous torque produced may exceed levels anticipated in the design of the motor or the driven machine. When asked for data to support the theoretical situation, equipment manufacturers couldn’t provide any. That’s why we decided to research the matter and gather data about allowing motors to ride through momentary power interruptions.

What really happens?
Our goal was to determine what happens to a three-phase induction motor that is allowed to ride through momentary outages. The independent variables were motor rating, percent load, outage duration and load inertia. We studied 10-, 50- and 75-hp motors as they rode through momentary outages that lasted from one cycle to 100 cycles in 0.25 cycle increments. We selected four motor load operating points -- 25%, 50%, 75% and 100% -- and varied the load inertia values (Table 1) that represent three types of load.

We tested three-phase TEFC squirrel-cage induction motors and used a dynamometer as the load. A torque transducer between the motor and dynamometer measured shaft torque. A 1,024-point-per-revolution encoder attached to the torque transducer acquired shaft speed. A flywheel mounted between the dynamometer and torque transducer varied the load inertia. Our custom-built, solid-state point-on-wave controller (POWC) served two purposes. It controlled the three-phase power supplied to the motor and produced the voltage sags and momentary interruptions. A 12-channel, 1-MHz Nicolet transient analyzer recorded voltage, current, torque and speed data before, during and after the interruption.

We ran tests at each inertia and load combination shown in Table 2. After the motor reached steady-state, the POWC removed the motor supply voltage for a predetermined interruption and immediately reapplied motor supply voltage to return the motor to steady-state conditions. The interruption increment for 100% load on the 10-hp motor was 0.10 of a cycle and 0.25 cycle for the other tests.