FIDVR and IEC 61850

by Nirmal-Kumar Nair, New Zealand, and Alex Apostolov, USA

Its impact on the electric grid has been further analyzed by a task force of specialists and published in a NERC (North American Electric Reliability Corporation) Technical Reference Paper on Fault-Induced Delayed Voltage Recovery. It includes the following definition provided by the Transmission Issues Subcommittee. Fault-Induced Delayed Voltage Recovery is a voltage condition initiated by a fault and characterized by:

  • Stalling of induction motors
  • Initial voltage recovery after the clearing of a fault to less than 90 percent of pre-contingency voltage
  • Slow voltage recovery of more than two seconds to expected post-contingency steady state voltage levels

Considering that air conditioning loads are typically connected to the low voltage level of the electric power grid, it is clear that short circuit faults at the transmission and distribution systems will result in voltage sags at the low voltage level and stall the induction motors of the air-conditioners. When an induction motor with constant torque stalls, it draws reactive power from the grid. When such an event occurs in an area with a large number of air-conditioners, like in the desert areas of Southern California, Florida or any other region with high temperatures and/or humidity, it may have a severe impact even at the bulk level of the electric power grid. (Figure 1).

What makes it both challenging and interesting to protection specialists is the fact that what needs to be done after a short circuit fault somewhere in the grid depends on the point of view of the different actors involved in clearing the disturbance.
A stalled motor draws excessive reactive power from the grid and in a locked-rotor condition draws five - six times their typical steady-state current. A large number of motors in this state can cause the system voltage to be significantly depressed for several seconds after the clearing of the fault, which can lead to a wide area system disturbance. From the point of view of the control of the electric power grid all stalled induction motors need to be disconnected after the voltage drop caused by a short circuit fault in order to stop the significant reactive power draw and help the recovery of the system voltage. Reducing the duration of the voltage sag by reducing the fault clearing time is something that may help with controlling the level of FIDVR. (see Figure 4). From the point of view of the users at the distribution and low voltage level of the grid, the sympathetic tripping of the protection devices on healthy feeders with significant motor load is something that should be avoided by the implementation of more intelligent protection schemes that can adapt to the increased current drawn by the stalled induction motors while still maintaining their sensitivity during normal operating conditions.

In order to determine the required actions during and following the clearing of short circuit faults it is necessary for the planning departments of electric power utilities to perform dynamic stability system studies to predict the severity of the FIDVR and specify the required actions by the protection system. However, one of the major problems that our industry is facing is the lack of good understanding of the type and characteristics of the loads at the distribution and low voltage level. That is why currently there are some ongoing efforts to develop better dynamic load models that can be used in FIDVR studies.  With increasing penetration of inverter interfaced devices like roof-top solar and storage batteries, that have requirements of anti-islanding settings, equivalent load modeling development is becoming more necessary and involved. (see Figure 3).

Grid Level Solutions to FIDVR
One of the recommendations of the NERC paper is to reduce the fault clearing time as much as possible, because the fault duration and location are critical factors that influence the FIDVR risk. This is where IEC 61850 can help in supporting strategies to reduce the expected fault duration at different locations in the transmission grid in a very efficient way, thus reducing the levels of FIDVR exposures. When a short circuit fault occurs on a transmission line connected to a substation with many induction motors loads connected to the distribution grid, the voltage drop caused by the fault needs also to be considered in the analysis of their performance during the fault and after it is cleared.
Distance protection is the typical protection used on transmission lines. While the Zone 1 operates without time delay, when the fault is in Zone 2 of the protected transmission line (especially on shorter lines), the time delayed trip will depend on the time delay setting which may be in the range of 300 – 400 msec. Such a delayed trip will result in the increased duration of the voltage sag experienced by induction motors stalling and leading to a wide area FIDVR. An accelerated protection scheme can significantly reduce the fault clearing time and its effect on the motor loads.

 

Power. Flexible. Easergy.
BeijingSifang June 2016