By Kathy Siciliano, Positron Inc.
There is a lightning storm. Suddenly, the telephone line coming into a substation fails and communications equipment is damaged. What went wrong?
This substation has fallen victim to ground potential rise (GPR). Each year, electrical damages caused by GPR cost power companies millions of dollars in lost revenue and equipment damage. Furthermore, there is a disruption in service to customers to contend with.
What Is GPR?
In a nutshell, when a fault or lightning occurs and a current reaches a substation grid, the result, according to Ohm’s law, is a potential rise. V equals R*I, where I is the surge current, R is the impedance of the ground grid and V is the resulting potential rise.
If the equipment is all tied to the same ground grid and is not referenced to any external ground, then it will not be damaged due to GPR. However, wire-line telecommunications, which are connected through equipment bonded to the substation’s ground grid, are also terminated at a central office (CO) by copper pairs. This CO is the remote earth, and the copper wire-line is a conductor tied between two ground planes. Therefore, a difference in potential between the two ground planes will cause a current to flow, up from the ground at the substation, through the equipment and out onto the wire-line. This is dangerous to personnel and can damage equipment.
Using an analogy, we can compare this situation to two glasses filled with water, one representing the ground plane at the substation, the other, the ground plane at the CO. Imagine one glass cup on a shelf, and the other lower on the table. If there is no connection between the two, then no matter what happens to the water levels in the glasses (comparing variations in potential), no water will flow between the glasses (meaning no current will flow). However, if the two glasses are connected by means of a straw (i.e., connecting the two ground planes by means of a copper phone line), then sudden increases in the water level of the first glass will mean that the water will flow down the straw (i.e., current on the wire-line) to the second glass. Anything tied to that straw would get wet. In the same way, anything tied to the wire-line will see the current. The only way to prevent this is to put a barrier in the straw. This is what isolation devices do.
Protecting Communications Infrastructure
While proper grounding is essential and standard communication protection methods, used properly, are critical at these sites, they are unfortunately ineffective in protecting equipment from GPR. For example, shunting devices normally are placed at each end of a cable communication facility and are designed to direct foreign voltage impulses into a grounding system. During a GPR, these devices merely offer an additional path to remote ground reference, and actually provide a path for current to flow in the reverse direction from which they were intended to operate. Thus, no matter how good standard protection devices are, equipment or cable facilities will become part of an electrical path between the GPR and remote ground. The only effective protection scheme against GPR is an isolation device.
The next step is defining what tools are available to help solve GPR problems. A series of field-proven national standards provide methods for protecting people and equipment from GPR. The most important and useful standards include:
Although most of these standards address protection from GPR due to 60-Hz fault currents, lightning strike energy applications are basically the same when considering higher frequency impedance. Both currents generate a GPR and can potentially harm personnel and damage or destroy communication facilities.
The above standards define when a high voltage interface (HVI) device is necessary for wire-line protection. In general, an HVI should be installed when the calculated GPR is above 1,000 V peak asymmetrical, or the identified service performance objective (SPO) is for Class B, and must always be deployed for a SPO Class A.
It should be stressed that failure to comply with national standards can have serious legal repercussions, should a GPR incident cause injury to personnel or damage to property. Safety issues must be considered when designing and installing communication systems. When designing a communication system, communications or protection engineers must be aware of their responsibility with respect to the specification of a safe installation, respecting all applicable standards.
In summary, there are three issues, which must be considered before a protection scheme can be designed and implemented:
- 1. Is the site a likely candidate for GPR? The answer is yes if a wire-line communication link enters a high voltage area or one that is prone to lightning.
2. What is the calculated level of GPR at this site? If it is evaluated at greater than 1,000 V peak asymmetrical, then high voltage isolation is required by IEEE standard 487, rather than using shunting devices.
3. What level of service performance objective is required at the site? If it is Class B, HVI protection should be installed rather than using shunting devices. If the service is Class A, then an HVI is a MUST.
The Wire-line Isolator Concept
The basic objectives for the protection of wire-line facilities are to ensure personnel safety, protect the telecommunications plant and terminal equipment, maintain reliability of service, and accomplish these in the most economic way.
A good wire-line isolator will use state-of-the-art technology to isolate and protect telephone facilities and personnel from the hazardous voltages associated with GPRs. Inserted into the wire-line link at the terminal end, the wire-line isolator unit breaks the copper continuity of the telephone line as it enters the high-voltage site, thus eliminating the conductive bridge, which links the high-voltage site and the CO ground planes. Basically, it acts as a dam between the local site services (also called station side) and the central office (called CO side), allowing all of the communication signals to pass through transparently, but preventing any fault currents from passing on the phone lines. It accomplishes this either through an isolating transformer or a fiber optic link.
What Does All This Mean?
Basically, it comes down to protecting your people and your investment. That’s why all the protection, grounding and isolating equipment is installed at a site. Wire-line isolation is one more factor to consider when designing and installing a communication system. The question to ask is: Will I have a wire-line communication line coming into my substation? If the answer is yes, then you need to look at wire-line isolation. à¯£à¯£
Kathy Siciliano is the marketing manager for Positron Inc., a company that has been protecting critical infrastructures for the past 35 years.