Protecting the Power Grid: High Altitude EMP Filters

High altitude electromagnetic pulse, or HEMP, are electromagnetic fields having extremely fast rise times, around 1 nanosecond, and relatively very long decay times, over a minute or longer. The initial pulse can have field strengths of 50 kV/m, and decay to a constant 10-100 V/km after 1 second. By comparison, a lightening induced pulse will have risetimes on the order of 1 microsecond, 1000 times slower than the HEMP event. This should be noted since the surge arrestors used on the power grid tend to work well for lightning but respond much too slow to handle the very fast risetime and duration of the E1 portion of HEMP.

The issues created by HEMP are wide ranging. A handheld meter or cell phone may be affected by the initial pulse and intense field, but be unaffected by the long duration, low level field that follows. Electrical equipment which used power from the power grid can be affected by the intermediate time frame of the field. This affect appears as a voltage surge on power lines, similar to those created by lightning strikes. However, the intensity of HEMP E1 pulses can be three times or more greater than the design protection Basic Insulation Level (BIL) for lightning.

The main transmission power grid, having high voltage wires covering vast areas and regions, may experience little effect by the initial pulse though the field is very intense. However, the transformers and substations used to convert the high voltage transmission grid down to the medium voltage distribution grid are more susceptible to the initial fast risetime and high intensity of the E1 pulse

This is due to the nature of the power equipment, which consists of transformers, relays, and other industrial equipment which is relatively unaffected by radio frequency energy and protected from surge or lightning events, but have not been designed to handle high frequency, high amplitude E1 pulse. The control systems for the grid may be affected by the direct fields of the initial pulse, since they have computer-based controllers and communication systems which would be disrupted by such an intense field. Without shielding, filtering and especially transient protection specifically designed for the E1 pulse, these controls are vulnerable to HEMP.

The transmission power grid could be susceptible to the long duration, low level field of the long pulse. The long duration fields created by HEMP are similar to those created by geomagnetic disturbances (GMD) and coronal mass ejection (CME), discharges (CMD), and the like. These fields can couple very low frequency current (less than 1 Hz) in the high voltage power transmission lines. The transformers at either end of the power lines experience a near DC current through them, that can cause the transformer to saturate or distort the voltage provided by the transformers.

Protection of equipment, especially crucial infrastructure communications and command installations, becomes critical. Power lines must be filtered to protect equipment from the impulses and injected currents which damage sensitive electronics. External intense fields must be blocked from exposing the electronics to damage. This may require either a quality shielded enclosure to install the electronics in, or an entire shielded facility where personnel can work during and through the HEMP event.

To provide guidance for this protection, MIL-STD 188-100 series of documents was created. Early versions of MIL-STD 188-125 had two public domain sections: part 1 of the document addresses HEMP hardening for fixed facilities; part 2 addresses transportable systems. The same or similar information is provided in UK DEF STAN 59­188. The latest revision of the military standard, MIL-STD 188-125-1A, is classified.

The purpose of these standards is to establish the “minimum requirements and design objectives for high-altitude electromagnetic pulse (HEMP) hardening of fixed ground-based facilities that perform critical, time-urgent command, control, communications, computer, and intelligence (C4I) missions.”1 The document provides the minimum requirements needed to avoid HEMP threats, giving instructions on how to verify the installation and construction of the subsystems. The goal is to assure the design hardness for the complete facility.

Facility hardening to HEMP requires high quality shielding. The minimum shielding effectiveness is 80 dB from 10 to 1000 MHz. A solid steel box can easily meet this requirement, however there are many aspects to consider which will degrade the quality of the shield if not properly installed or designed. The most important considerations are the various points of entry.

For a hardened working facility, there is a need for human access into the facility. Entry doors must be properly designed to establish shielding once closed. But when using a single access door, while the door is open, almost no shielding is provided inside the facility. To maintain shielding effectiveness at all times, a double door system is used, and both doors shall not be open at the same time. Both doors and the vestibule between the doors must be part of the same shielded enclosure. The contact surfaces of these doors are often brass with a ridge that slides between beryllium copper finger stock. These surfaces can and will become dirty and degrade over time, requiring regular cleaning and maintenance.

Inside the facility, conditioned air needs to be brought in. The entry ports for the air need to be large enough, and in enough locations, to provide fresh air and to remove built up heat being generated by personnel and electronics used. Maintaining shielding requires passing air through honeycomb vents. These air vents have excellent shielding effectiveness, typically 120 dB up to 10 GHz or higher. The honeycomb structure creates many waveguides beyond cutoff channels for the air to flow through.

A waveguide beyond cutoff is a metal opening which has dimensions too small for the wavelength of the radio frequency signal to propagate well. This effect occurs when the half wavelength of the radiated field is longer than the largest dimension of the opening. It is best when the length of the opening is five times the longest dimension. When this is true, attenuation over 100 dB is possible. For shielding effectiveness to meet the requirements of the standards, a maximum dimension of the opening cannot exceed 10 cm.

Waveguides are used for openings in ventilation, or other non-conductive materials such as a routing path for fiber optic cables. However, if a conductive material is placed through this opening, say a wire, the shielding effectiveness is violated, and benefits of a waveguide are eliminated.

Power and signal lines will need to be brought into the facility – the critical Point of Entry, or POE. Obviously, power must be supplied on a conductor, typically copper. Conductors are antennas and pick up radio frequency currents from the fields they are exposed to. This energy can then rebroadcast on the inside of the facility and couple or conducted directly to sensitive electronics. Signal lines typically use copper wire, but whenever possible, they shall be converted to fiber optic at the POE of the shielded facility.

Filtering conductors at the point of entry into the facility is critical in order to maintain the quality of the shield. The filter properties must be for the entire frequency range the shield is needed. Therefore, the filter must have greater than 80 dB of effectiveness to greater than 1 GHz. Shield room filters available from commercial manufacturers can provide power line and signal filters which may meet the basic attenuation needs, however HEMP has specific Pulsed Current Injection (PCI) requirements most commercial filters are not able to supply.

Therefore, all lines, shielded and unshielded, penetrating the facility, shall be filtered unless they use fiber optical cables. The filters shall be tested using a pulsed current injection method. Testing is performed on each filter line to ground as well as common mode, which includes all wires in a bundle, with respect to ground.

The test pulses are divided into three characteristics entitled Short Pulses, Intermediate Pulses, and Long Pulses. Short Pulse risetimes are 20 nS, with a half wave pulse widths of 500 nS. Intermediate Pulses have a 1.5 μS risetime, with a 3 mS pulse width. Long Pulses use a 0.2 second risetime and a half amplitude pulse width of 20-25 seconds. Injected currents are 250-5000 amps peak short circuit current depending on the pulse. Source impedance for the short pulse is ≥60 Ω, and for the long wave is ≥5 Ω.

The test load impedance listed in the public domain versions of the standards is defined as 2 Ω for the Short Pulse and 50 Ω for the other pulses. This is an important aspect of the filter, since if the load impedance is very low, the line to chassis impedance at those high speed time frames must be even lower in order to shunt the currents to chassis instead of allowing them to pass through the filter. Series impedances can benefit the filter design, such as using inductance in series after a shunting element. This increases the total impedance by appearing in series with the load, so that even if a short appears across the output, some series impedance exists to help shunt the current in the filter. The difficulty occurs in the design of that inductive element to be able to handle the current pulse without saturation.

The shunting elements must also be able to handle the peak current, but also the long duration current of the intermediate pulse. Having half amplitude pulse wide greater than 1 mS requires significant power dissipation. Most common transient suppression devices are not equipped to handle this much energy.

Shield room construction is a well-established technology which has been used for decades around the world. The challenge exists in the maintenance and survivability of the enclosure through the filtering and protection of all power and signal lines which must be brought into the chamber. Thus, filter design must be able to survive the extreme nature of a HEMP event.

Footnotes

  1. From MIL-STD 188-125-1 paragraph 1.1 – Purpose

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