Your choice of either solid, low-, or high-resistance grounding is based on the power system application and degree of power interruption tolerated.
Solid, low-, or high-resistance grounding? That is the question asked by those involved in the design or retrofit of power systems. The answer depends upon certain important factors. To make the correct choice, the designer must have a complete understanding of system configurations, favorable performance traits, and drawbacks. Also required is the relative importance of the powered process or load.
The table shown on the facing page includes a comparison of the characteristics of these various grounding methods. Let's look at them more closely.
Historical information
Most older industrial plants were powered by ungrounded, 3-phase, 3-wire, delta power systems. Many of these systems are still in use today. This system choice was based on two factors. First, it made the most efficient use of conductor copper. Second, no fault current flowed when the first ground-fault occurred, which was, and still is, considered an advantage in some applications, although a shock hazard is introduced.
However, multiple motor failures in numerous industrial plants were seen and were due to severe overvoltages caused by arcing or resonant ground faults on the ungrounded systems. To prevent these overvoltages, many power system neutrals were grounded, usually solidly. There were many factors that contributed to the change to solidly grounded systems and these factors are still important today.
First, solid grounding very effectively limits the maximum phase-to-ground voltage. Second, it allows phase-to-neutral loads to be served without encountering dangerous neutral-to-ground voltages under ground-fault conditions. Third, simple and effective ground relaying systems can be used to isolate the defective portion of the system under ground-fault conditions.
Solid grounding limitations
There are, however, some limitations to solid grounding. In medium voltage (MV) systems (2400V through 35kV), even with good ground-fault relaying, the damage at the point of fault can be excessive. In fact, this problem led to the common use of low-resistance grounding, which allows the passing of anywhere from several hundred to several thousand amperes of ground-fault current. This practice reduces fault damage to acceptable levels while maintaining enough ground-fault current flow to effectively relay off the defective portion of the system.
In addition, solidly grounded, low voltage (LV) systems in the 480 to 600V range have two other problems. The first problem stems from application concerns. Some users prefer to maintain service, if possible, with a ground fault present on the system, or at least to arrange for an orderly, controlled shutdown. This is especially true for such continuous process industries as electric power generation, oil refining, chemical and steel manufacturing, and the paper industry. Since many of these power systems are worked hot, electricians are exposed to a considerable flash hazard from a possible line-to-ground fault caused by a misplaced tool.
Second, since most such systems rely on the phase overcurrent devices to protect against ground faults, it's possible to have a destructive arc of several thousand amperes in magnitude for several minutes duration without initiating an automatic trip.
To overcome the problems of unwanted shutdown, flash hazard, and burndown while still maintaining the transient overvoltage protection of a grounded system, high-resistance grounding was developed.
Pro high-resistance arguments
High-resistance grounding involves the grounding of the system neutral through a resistance that limits ground-fault current flow to a value equal to or slightly greater than the capacitive charging current of the system. This value is chosen because it is the lowest level of ground-fault current flow at which system overvoltages can be effectively limited. Increasing the current flow improves overvoltage control at the expense of increasing damage at the point of fault; decreasing the current flow reduces point-of-fault damage at the expense of greater risk of overvoltage.
High-resistance grounding is applicable to LV and MV power distribution systems serving 3-phase, 3-wire loads or line-to-line, single-phase loads. It effectively controls transient overvoltages during ground faults, minimizes arcing damage and flash hazard at point of fault, and allows continued operation of the system with a ground fault present at voltages of 5kV and below.
Components of high-resistance grounding system
A high-resistance grounding system consists of five basic parts: a system neutral, a grounding resistance, a fault-detector and [TABULAR DATA OMITTED] alarm scheme, a fault-locating scheme, and packaging for these components. Strictly speaking, only the first two items are required; however, the grounding system's usefulness is severely limited without the other three items.
System neutral. By far the easiest way to obtain a system neutral is to use the neutral of a wye-connected power transformer or generator supplying the system. On any new system, it's recommended that this method be used.
On existing delta-connected systems (or on new systems that must be delta-connected to allow paralleling with existing systems), a neutral may be derived by using a bank of three small transformers connected in wye on the primary and in delta on the secondary. The primary voltage rating must be equal to the system line-to-line voltage since the transformers connected to the ungrounded phases will see that voltage under conditions of solid ground fault on one phase. The secondary should be rated 120V for convenience of fault detection. The kVA rating should be chosen such that the rated primary current of the transformer equals or exceeds 1/3 of the selected system ground current, since the ground current divides equally among the three transformers.
For example, if you decided to ground a 2400V system so that 10A of ground current can flow, the transformer size required is equal to the quantity 2400 times 10, divided by 3, or 8000VA. Thus, three standard 10kVA transformers would be used.
Grounding resistance. The grounding resistance determines the value of ground-fault current that will flow. Since the desired value is dependent on the system capacitive charging current, the charging current must be determined before the resistor can be selected. The only accurate method of determining this current for any given system is measurement.
Since measurement is not possible during the design stages of an installation, normal practice is to estimate the capacitive charging current, provide a tapped resister that allows several settings in the range of the estimated current, make the necessary measurements, and set the resister at installation time.
Sufficient data have been accumulated from system measurements to allow fairly accurate estimates of system capacitive charging currents for various systems.
Typical values of capacitive charging currents have been found to be as follows.
* 480V systems: usually less than 1A with maximum of about 5A.
* 2400V and 4160V systems: 2 to 7A.
* 13.8kV systems: 10 to 20A.
These values are for in-plant power systems, such as auxiliary systems for generating systems or distribution systems for industrial plants. Utility distribution systems would exhibit higher values because of the greater length of conductor involved.
(July 1994 issue, "What To Know About High-Resistance Grounding.")
Determining required resistance
After the system charging current has been estimated and a value of ground-fault current selected, the value of the required resistance is determined. For 480V systems, a very practical grounding resistor can be made from four 77 ohm resistors rated 750W, 240V each. These can be connected in various series-parallel arrangements to produce the appropriate current flow.
LV delta-connected systems. It's more common to connect a grounding transformer bank to the 480V secondary and insert the resistance between the neutral of this bank and ground, as shown in Fig. 1, with no load connected to the secondary. As shown, ground-fault current can be limited to 1.2A by a resistance of 277 divided by 1.2, or 230.8 ohms. Thus, three 77 ohm resistors connected in series will provide this value.
MV delta-connected systems. For MV systems having a power transformer with a delta-connected secondary, a grounding transformer bank is connected to the power transformer secondary, and the grounding resistance is connected in the secondary of this bank, as shown in Fig. 2. This permits the fault-detecting and locating circuitry components to be operated at the secondary voltage level. With this connection, the secondary current may be calculated by multiplying the transformer primary current by the transformer ratio. This is the current through the grounding resistor, and its value establishes the continuous current rating of the grounding resistor. The voltage across the resistor under ground-fault conditions is 1.732 times the secondary voltage of the grounding transformer bank, or 208V for a 120V rating. The grounding resistance required can be determined from these values of current and voltage. For the example shown in Fig. 2, the transformation ratio is 4160 divided by 120, or 34.67 to 1. For a fault current of 5A, primary current will be 5 divided by 3, or 1.67A; secondary current will be 1.67 times 34.67, or 57.9A. The required grounding resistance will be 208 divided by 57.9, or 3.6 ohms. This will be seen by the fault current as a high resistance when reflected in the primary.
MV wye-connected systems. For a power transformer with a wye-connected secondary, the primary of a single-phase grounding transformer is connected between the neutral point and ground, and the resistor is connected in the secondary circuit, as shown in Fig. 3. The primary voltage rating of the transformer must be at least equal to the line-to-neutral system voltage and may be equal to the line-to-line system voltage, if that is more convenient. The kVA rating must be chosen so that the rated primary current of the transformer is not exceeded by the system ground-fault current. The secondary voltage rating may be either 120 or 240V. The secondary current under ground-fault conditions will be the system ground-fault current multiplied by the transformer ratio. The secondary voltage under ground-fault conditions will be the system line-to-neutral voltage divided by the transformer ratio. Using these values, the resistance and wattage of the ground resistor can be calculated. The values shown in Fig. 3 are the results for a ground-fault current of 5A. Note that the ohmic value is different from that of Fig. 2, but the wattage required is the same.
As in LV systems, it's common practice for manufacturers to supply a tapped resistor that covers the range of expected values. Field measurements will determine the final setting.
High-resistance grounding do's and don'ts
The following guidelines should be followed when using a high-resistance grounding system.
* Use high-resistance grounding to limit transient overvoltages without shutting down grounded equipment on occurrence of first ground fault (5kV and below).
* Use sensitive ground-fault relays to trip breakers feeding faulted system elements at voltages above 5kV.
* Enforce maintenance procedures for locating and removing ground faults promptly upon detection.
* Test all systems for actual system capacitive charging current upon installation and set grounding resistor accordingly.
* Do not use high-resistance grounding where 3-phase, 4-wire loads must be served.
* Do not use high-resistance grounding as a substitute for proper system maintenance.
* Do not provide additional ground connections on other electrical equipment when using high-resistance grounding equipment. Ground only at grounding resistor.
SUGGESTED READING
EC&M Articles:
"What To Know About High-Resistance Grounding" July '94 issue.
Baldwin Bridger, Jr., P.E. is Technical Director, Powell Electrical Manufacturing Co., Houston, Tex. and past president of the IEEE Industry Applications Society.