Electrical Grounding Using the High-resistance (HRG) Method

The high-resistance grounding (HRG) method for electrical power systems has some of the same advantages as ungrounded systems. These advantages include the reduction of equipment damage (due to the low value of the ground-fault current), and no need for immediate removal of the first ground-fault, with the additional benefit of achieving acceptable values of transient overvoltages.

 

Image courtesy of Pixabay

 

 

What is the HRG Method?

The high-resistance grounding (HRG) method consists of inserting a resistor into a three-phase generator, power transformer, or grounding transformer neutral to limit the single line-to-ground fault current to a low value.

The original intent of the HRG method was to continue the operation of a system with a ground-fault present on one phase and suppress transient overvoltages. It emerged during the search for ways to reduce hazards to personnel, minimize damage to electrical infrastructure, and improve service continuity.

The first ground-fault activates an alarm (sound and visual), to alert maintenance personnel. Depending on the protection philosophy, the fault persists until a safe planned shutdown or an overcurrent device trips the faulted circuit after a predetermined time.

 

Where is the HRG Method Used?

HRG has uses in the electrical industry and in commercial and industrial applications.  The main usages are in industrial processes where continuity of service is vital, as is the protection of rotating equipment, like motors and generators. The resistor reduces iron burning damage, damp out oscillations, and confines transient overvoltages to less than two-and-a-half times the standard line-to-neutral voltage.

In some industrial operations, a sudden power failure may send the process out of control, producing toxic chemical releases, fires, or explosions. Loss of electrical supply may render some machinery inoperative if the product they handle solidifies inside.

 

Installing HRG According to the NEC

Electrical installations governed by the National Electrical Code (The NEC), must meet Sections 250-36 and 250-187. Section 250.36 allows the use of high-impedance grounded neutrals (usually a resistor) in three-phase AC systems of 480V to 1000V meeting the following requirements:

(1) Only qualified persons service the installation

(2) Equipped with ground detectors

(3) No line-to-neutral loads.

 

This section also gives installation rules concerning the location of the grounding impedance, conductor insulation, and ampacity, grounding connection, conductor routing, bonding jumper routing and size, and grounding electrode conductor connection.

Section 250.187 regulates the systems and circuits of over 1000V. The conditions to follow are equivalent to lower voltages, and this section also lists installation rules.

 

Design Considerations for HRG

The common practice to achieve HRG's goals is to select a resistance value to allow a single line-to-ground fault current through the resistor equal to or slightly higher than the capacitive charging current of the system.

To reach this condition, use a value of resistance equivalent or somewhat less than the capacitive reactance to ground. This procedure balances two conflicting requirements: to let enough fault-current flow to avoid unwanted transient overvoltages and, simultaneously, to keep it low to minimize the damage at the ground-fault, especially when the fault remains on the system for some time.

Typical capacitive charging currents for industrial plants or auxiliary systems of power generation stations range from less than 1A to 20A, depending on the system voltage and size. Utility systems involve greater conductor length with higher values of charging currents. In large systems with high charging currents, HRG may not be achievable.

Charging currents may be determined by testing existing systems or using tables when in the design phases. In the latter case, always measure the real charging current after the system installation.

Although the core reason for using HRG is to avoid unexpected shutdowns, by no means should the single line-to-ground fault be overlooked. Although small, the current may cause considerable damage if the failure remains uncleared, escalating to a destructive short-circuit. The short-circuit may also happen with a second ground-fault at a different phase. To avoid this, clear the fault in a matter of hours.

 

Components of an HRG package

Some manufacturers produce packaged HRG systems. Depending on the user’s needs and budget, these packages include a grounding resistor, a ground-fault detector and relay, visual indicators, optical arc-flash detection sensor, fault-tracking system, and a grounding transformer to provide a neutral on ungrounded systems. They are for use in low- and medium-voltage power distribution systems supplying three-phase loads or line-to-line single-phase loads. Voltages range from 480V to 5kV.

For higher voltages, the systems are custom made and provided with sensitive ground relaying.

 

Calculations with the Method of Symmetrical Components

 

Figure 1 shows a simplified circuit diagram with the connection between the sequence networks and the current distribution for a single line-to-ground fault.

Figure 1. Connection and current distribution in the sequence networks

 

In a previous article focusing on the Peterson coil method, we confirmed that only the zero-sequence network is significant when the ground-fault current goes back to the source through the natural distributed capacitance of the system. HRG is no exception. Also, the transformer reactance to zero-sequence currents (Xt) is very low and ignored when compared to 3R.

The resultant zero-sequence impedance is:

Zₒ = 3R ∙ (-j Xₒc)/3R - j Xₒc.

 

Where Should the Resistor Be Placed?

There are three typical arrangements for resistor placement.

The first arrangement is the easiest way to get neutral. It involves using the neutral of a wye-connected power transformer or generator. The resistor will be placed directly in the neutral. This method is appropriate for new systems rated 5kV or less.

The second arrangement employs one distribution transformer in the source neutral with the resistor located in the low-voltage side. The value of resistance in the LV side is small but seen by the fault current as a high resistance when reflected in the HV side. During a line-to-ground fault, the transformer will see line-to-neutral voltage. The transformer voltage rating may be line-to-neutral or line-to-line voltage. This arrangement allows a suitable voltage (120V or 240V) to supply power to the protective relays and other hardware. Use it in systems with a single supply, either one unit generator or transformer.

The third arrangement pertains to delta systems, multiple generators connected to a single busbar, or several power sources. It uses three distribution transformers connected wye-delta to the substation busbar, and the resistance placed in the secondary broken-delta. As in the second method, the value of resistance in the LV side is small but seen by the fault current as a high resistance when reflected in the HV side. During a line-to-ground fault, the three transformers will see line-to-line voltage and must be rated accordingly. This method allows a convenient voltage to supply power to the protective relays and other hardware.

Another type of grounding transformer bank, commonly used, is a zigzag configuration.

 

The rules in electrical installations governed by the NEC are:

• Between 480V and 1000V, follow Section 250-36 (A): "The grounding impedance shall be installed between the grounding electrode conductor and the system neutral point. If a neutral point is not available, the grounding impedance shall be installed between the grounding electrode conductor and the neutral point derived from a grounding transformer."
• Over 1000V, follow Section 250-187 (A): "The grounding impedance shall be inserted in the grounding electrode conductor between the grounding electrode of the supply system and the neutral point of the supply transformer or generator."

 

An Example Using the Three HRG Arrangements

An example, solved via the three arrangements mentioned above, will help to understand the mechanics of HRG.

A 13.8kV industrial power distribution grid has a total charging capacitance to ground (calculated with tables) of 0.658 microfarad/phase. At a rated-frequency of 60Hz and neglecting the transformer zero-sequence impedance, calculate:

1. Zero-sequence capacitive reactance per phase (X0c)
2. Capacitive charging current per phase (I0c)
3. Value of resistance in the zero-sequence network (3R)
4. The real value of resistance to be connected to the neutral (R)
5. Zero-sequence impedance (Zₒ)
6. Ground-fault current for a fault in phase a (If = 3I0)
7. Current through the resistor (3I0R) and the total capacitance (3I0C) during fault
8. Power dissipation in the resistor (P) during fault

 

First Arrangement

The first arrangement involves inserting the resistor in the neutral of the sys

In this 13.8kV example, the first arrangement is not economically wise because the voltage in the neutral, during a ground-fault, will require an expensive resistor and protective relaying hardware. Note that though methods two and three are preferred, these computations are illustrative.

1. -jX0c = -j/120∙π∙C = -j10⁶/120∙π∙0.658 = -j4 031.40 Ω/phase
2. jI0c = jVLL/√3∙X0c = j13 800/√3∙4 031.40 = j1.976 A/phase
3. Use 3R = 4 031.40 Ω
4. 4 031.40/3 = 1 343.80 Ω
5. Zₒ = 4 031.40 ∙ (-j4 031.40)/4 031.40 – j4 031.4 = 16 252.186∠-90°/5 701.26∠-45° = 2 850.63∠-45° Ω
6. I0 = VLL/√3∙ Zₒ = 13 800∠0° V/√3∙ 2 850.63∠-45° Ω = 2.795∠45° A; If = 3I0 = 3∙2.795∠45° = 8.390∠45° A
7. 3I0R = Ɩ3I0Ɩ cos 45° = 8.390 ∙ 0.707 = 5.930∠0° A; 3I0C = Ɩ3I0Ɩ sen 45° = 8.390 ∙ 0.707 = 5.930∠90° A
8. P = (3I0R)² ∙ R = (5.929)² ∙ 1 343.80 = 47 239 W = 47.24 kW

 

Figures 2 and 3 show the system under normal conditions. With equal distributed capacitances to the ground (balanced system), a symmetrical set of charging currents flows in the lines. These currents are identical, displaced 120°, and add to zero. No current flows through R and the neutral stays at ground potential.

Figure 2. Circuit diagram showing capacitance (charging) currents under normal conditions

Figure 3. Phasor diagram under normal conditions

 

The single line-to-ground fault in phase a disrupts this symmetry. Phase a is now at ground potential and causes a potential shift of the neutral and in phases b and c — the fault shorts out the capacitance of phase a and no charging current flows.

Line-to-line voltages excite the capacitance of phases b and c, and the currents flowing through them are increased by √3 and their phase relationship changes to 60°. The resulting total system charging current is √3 times each of them, and 3 times the charging currents under normal conditions.

The resistor R sees a voltage VaN, and the current through it has the same phase angle. Figures 4 and 5 show this state of affairs.

Observe that the magnitude of the current flowing in the resistor is equal to the total system charging current. This magnitude, or higher, is expected to control transient overvoltages.

Figure 4. Circuit diagram showing currents under a single line-to-ground fault in phase a

Figure 5. Phasor diagram under a single line-to-ground fault in phase a

 

You may compute the answers to questions 6 and 7 using fundamental circuit analysis. See Figure 6.

Figure 6. Circuit diagram showing currents under single line-to-ground fault in phase a

 

Ib = Vab∠30°/Xc ∠-90° = 13 800 ∠30°V/4 031.40∠-90° Ω = 3.423 ∠120° A

Ic = Vac∠−30°/Xc ∠-90° = 13 800 ∠−30°V/4 031.40∠-90° Ω = 3.423 ∠60° A

Current through the resistor: IR = VaN∠0°/R∠0° = 13 800∠0°V/√3∙1 343.80∠0° Ω = 5.930∠0° A

Current through the total capacitance: Ib + Ic = 3.423∠120° A + 3.423∠60° A = 5.930∠90° A

Current at the fault: If = Ib + Ic + IR = 3.423∠120° A + 3.423∠60° A + 5.930∠0° A = 8.390∠45° A

 

Second Arrangement

The second arrangement involves grounding through a single-phase distribution transformer connected in the source neutral with a resistor inserted in the low-voltage side. Some calculations are the same as method one. We'll focus on the additional calculations. See Figure 7.

Fig. 7 Circuit diagram showing currents flowing in the distribution transformer and through the resistor

 

The standard voltage choices for the high-voltage side are 7.97kV and 13.8kV. For low voltage, 120V, and 240V. Pick a 13.8kV:120V transformer for this example.

 

Resistance at HV side: RHV = 1 343.80 Ω

Resistance at LV side: RLV = RHV ∙ (VLV/VHV)² =1 343.80 Ω ∙ (120/13 800)² = 0.102 Ω

Current at HV side: IHV = 5.930 A

Current at LV side: ILV = IHV  ∙ (VHV/VLV) = 5.930 A ∙ (13 800/120) = 682 A

Voltage in LV side: VLV = ILV ∙ RLV = 682 A ∙ 0.102 Ω = 69.56 V

Power dissipated by the resistor: P = (ILV)² ∙ R = (682 A)² ∙ 0.102 Ω = 47 442 W = 47.44 kW

Transformer kVA rating: VA = VLV ∙ ILV = 69.56 V ∙ 682 A = 47 439 VA = 47.44 kVA

 

Select the next standard ratings. The power rating will be short-time, for tripping service, or continuous-duty if the ground fault will persist until the next planned shutdown.

 

Third Arrangement

The third arrangement involves grounding through three distribution transformers, connected star-delta to the substation busbar, and a resistor inserted in the secondary broken delta. Some calculations are the same as method one. We'll focus on the additional calculations. See Figure 8.

Fig. 8 Circuit diagram showing currents flowing in the distribution transformers and through the resistor

 

Voltage choices are 13.8kV for the high-voltage side and, for low voltage, 120V, and 240V. Pick three 13.8kV:120V transformers for this example. The current rating should be enough to carry I0RHV. 

 

The current 3I0R splits in the three HV windings, with each winding carrying I0R.

I0RHV = 5.930 A/3 = 1.976 A

I0RLV = I0RHV ∙ (VHV/VLV) = 1.976 A ∙ (13 800/120) = 227.24 A

 

Under fault conditions, the two transformers connected to phases b and c see 13.8kV, and the transformer connected to phase a is shorted out and sees 0V.

 

Voltage on the LV side (across the resistor): VR = 120∠-60° + 120∠-120° = 208∠-90 V

Resistance on LV side: R = ƖVRƖ/ƖI0RLVƖ = 208V/227.24A = 0.915 Ω

Power dissipated by the resistor: (I0RLV)² ∙ R = (227.24A)² ∙ 0.915Ω = 47 249 W = 47.25 kW

Transformer kVA rating: VA = VLL∙ I0RHV = 13 800V ∙ 1.976 A = 27 269 VA = 27.27 kVA x 3 units.

 

Select the next standard ratings. The power rating will be short-time, for tripping service, or continuous-duty if the ground fault will persist until the next planned shutdown.

Note that the computed ohmic value of resistance is different in the three methods, but the power dissipated is the same.

Manufacturers may supply tapped resistors to allow adjustments after testing the real system when in operation.

 

Favorable Performance Traits and Drawbacks

HRG limits the first ground-fault current to a value that will not trip the faulted circuit instantly but will sound an alarm (sound and visual) to alert maintenance personnel. Hopefully, this results in a safe, planned shutdown. In addition to improving availability, an advantage over the ungrounded method is that the system will not be subject to destructive transient overvoltages that may cause additional ground faults.

Although there are various schools of thought, the best practice is to detect and clear the fault as soon as possible. Leaving the fault too long can cause catastrophic damage and put personnel at risk. In the case of rotating machines, a single phase-to-ground fault in the stator can progress to a phase-to-phase fault that destroys the windings and burns the magnetic iron. Double failures are more likely when the resistance is high and the insulation is weak.

During a ground-fault, the neutral shifts and sound phases see the line-to-line voltage. This circumstance implies that most equipment must have the same insulation as ungrounded systems. In addition to the extra cost, this can be a problem when converting an old system, without the required insulation levels, to HRG.

In the past, the task of finding the fault was clumsy and time-consuming. But today, new HRG packages integrate fault-tracing technology for secure ground-fault location.

 

Areas of Application

Use HRG in processes requiring continuity of power and when converting ungrounded systems to improve their performance (primarily fault damage and transient overvoltages), placing special attention to ground relaying. Also, apply HRG in generator-transformer unit systems, to prevent burning damage caused by the ground-fault current, and in utility station auxiliaries.

In systems rated 5kV and below, use HRG to keep them in operation after the first ground fault, controlling transient overvoltages. The maintenance procedures must be able to remove the ground faults quickly. HRG is not the answer to poor maintenance practices. Above 5kV, use sensitive ground-fault relays to trip the circuit breakers of the faulted section.

Do not use HRG when the system supplies line-to-neutral loads because of the neutral displacement during the fault. Single-phase loads are acceptable when supplied from line-to-line voltage.

In general, do not apply HRG in utility transmission and distribution power systems.

 

Hybrid High-resistance Grounding (HHRG)

In an upcoming article, we'll see that the primary industrial practice in the USA is grounding medium-voltage generators through low-resistance (LRG). The usual arrangement is multiple generators connected to medium-voltage distribution busbars.

LRG provides adequate ground-fault current to stabilize the neutral shift and allows the correct operation of the ground-fault protection scheme. However, when the fault is inside the generator, LRG cannot prevent the burning damage caused by the ground-fault current.

Per the statistics, the dominant type of fault in the generator stator windings is a short-circuit to ground. This fault produces severe damages that require stator lamination repairs and probable process downtime.

An IEEE/IAS Working Group has proposed the hybrid high-resistance grounding (HHRG) method.

The HHRG objective is to minimize the damage to generators when subjected to internal ground-faults. With the HHRG method, the conventional system is LRG, reacting rightly for external ground faults. In the event of an internal generator ground-fault, the grounding quickly switches to HRG.

When a generator is close to the end of its life span, it may exhibit poor insulation properties. A way of extending its life for some years is by switching to HRG only and changing its protective relay scheme accordingly.

For more information, see Switching Transient Analysis and Specifications for Practical Hybrid High-resistance Grounded Generator Applications by the IEEE/IAS Working Group, Presented at the 2009 IEEE IAS Pulp & Paper Industry Conference in Birmingham, AL.

 

A Review of HRG Characteristics and Applications

High-resistance grounding can be useful in power systems supplying critical processes that cannot suddenly stop. HRG also reduces the electrodynamic stresses on materials, the induced voltages on telecommunication lines, and the thermal deterioration of electrical circuits and equipment. It also cuts hazards to personnel. The main advantage of HRG over the ungrounded method is its ability to control transient overvoltages coming from arcing-ground faults.

The value of the resistor is selected to allow current to flow equal to or higher than the system charging current. The charging current may be obtained by testing the existing network or using tables when in the design phases.

Sections 250-36 and 250-187 of the NEC allow the use of HRG in three-phase systems that do not supply line-to-neutral loads. The NEC also requires adequate maintenance, supervision, and ground detectors. Commercial packages, from 480V to 5kV, provide all the hardware to comply with the NEC. For higher voltages, some manufacturers offer tailor-made arrangements.

The resistor may be placed directly in the generator or transformer neutral or on the low voltage side of several distribution transformer configurations. Wye-delta and zigzag are typical transformer configurations to get a neutral on ungrounded systems.

Overall, HRG is only recommended in industrial critical processes and power station auxiliary systems.

HHRG consists of adding high resistance as a complement to low-resistance grounded systems to protect the generators' windings and iron cores.