How to select the proper neutral-grounding resistor for a high-resistance grounded electrical system
Rami Hakam, P.Eng., Applications Engineer, Custom Products, Littelfuse Startco –
High-resistance grounding (HRG) makes a power distribution system safer and more reliable than the alternatives. High-rresistance grounding can limit point-of-fault damage, eliminate transient overvoltages, reduce the arc flash hazard, limit voltage exposure to personnel, and provide adequate tripping levels for selective ground-fault detection and coordination. While the decision to use HRG may be a “no-brainer,”implementing it is not. In particular, the selection of a proper neutral-grounding resistor (NGR) requires a certain amount of design knowledge. This article will explain how an HRG system works and how to calculate the value of the NGR for particular applications.
What is resistance grounding?
In a resistance-grounded system (Fig. 1) the neutral point (either the center of a wye-connected transformer or, for a delta-connected transformer, an artificial neutral created with the aid of a zig-zag transformer) is connected to ground via a resistor (Fig. 2). When a ground fault occurs the unfaulted phases will assume the phase-to-phase voltage with respect to ground, the neutral point of the transformer will assume the phase-to-ground voltage andground-fault current will flow through the NGR. The magnitude of this current is determined by the voltage across the NGR divided by its resistive value.
Where is HRG required?
High-resistance grounding is widely used in mining around the world, including Canada, the U.S., Chile, Peru, Brazil, China (open-pit), Mongolia, Australia, and India. It is a recommended practice for use in mining as described by the IEEE 3003 Standard: Power Systems Grounding (formerly known as the IEEE Green Book). The electrical codes for mining in Canada (CSA M421) and in the USA (MSHA) both require the use of high-resistance grounding. High-resistance grounding is also becoming widely applied outside of mining; for example, the IEC/ISO/IEEE 80005-1 standard recommends the use of high-resistance grounding for high-voltage connections feeding ships from shore. The recommended use of high-resistance grounding is a frequent topic of peer-reviewed papers, round-table discussions, and presentations at IEEE events, including the Petroleum and Chemical Industry Technical Conference and the Pulp and Paper Industry Conference. Various regulations and standards around the world also commonly require monitoring the continuity of the NGR.
Designing a high-resistance-grounding system
The main challenge in designing an HRG system is determining the proper ohmic value for the neutral-grounding resistor. The primary rule is that the NGR should be sized so that ground-fault current is equal to or slightly greater than the system charging current. But what is system charging current (a), and how is it measured (b)?
(a) As shown in Fig. 3 each phase of a three-phase system exhibits a certain amount of distributed capacitance to ground, shown here as three capacitors. A capacitive current flows through these, but does not show up on a current transformer installed around the three phases (zero-sequence current transformer) because the three phase currents sum to zero.
If one of the phases shorts to ground causing a ground fault, the charging current for the other two phases will flow through the ground fault.
(b) This makes for a direct way to measure the charging current: introduce a deliberate ground fault on an ungrounded system and the resulting current through the ground fault will be the charging current of the system, as shown in Fig. 4. The shorting wire should be AWG 8 to minimize its voltage drop. The grounding connection should include a 5 to 10 amp current-limiting fuse rated for the full system voltage, a variable resistance, and a switch. At its maximum value, the variable resistance should be such as to limit the charging current to half of its estimated magnitude. Set the resistor to its maximum value, and then close the switch. Gradually reduce the resistor setting to zero; this will help prevent transient overvoltages during the test. Zero sequence ammeter A1 will continue to show zero, and ammeter A2 will measure the charging current. After taking the reading, gradually bring the resistor back to its maximum value before opening the switch. Repeat the process on each of the other phases, and choose the one with the highest current for the rest of the calculations. Please note that the system is isolated from ground during the test.
In a system that has not yet been built, ground fault current must be estimated. For a quick estimate, use the following typical values: 0.5 A/1000 kVA for low-voltage systems and 1.0 A/1000 kVA for medium-voltage systems. For a more precise estimate, use manufacturer’s data to sum up the different circuit elements that contribute to charging currents, such as cable capacitance per 100 feet, surge arrestors, motors, etc.
Calculating the value of the NGR
To detect high-impedance faults and provide machine-winding protection, the desired ground-fault-current pickup level for the ground-fault relay should be less than 20% of the prospective ground-fault current. To put it another way, a good rule of thumb is to multiply the desired ground-fault-current pickup level by an acceptable tripping ratio (say, 5x), and then use the next-largest available standard let-through current rating for the neutral-grounding resistor. For low- to medium-voltage systems, standard NGR current ratings are typically 1, 2, 5, 10, 15, and 25 amps.
For example, consider a system with a charging current of 0.5 amps (which is quite common on a 480 V system) and a desired ground-fault pickup level of 1 amp. Using a trip ratio of 5, the value of the NGR should be selected to allow five times the desired pickup level current, or 5 amps.
Another example: in potash mining the trailing cables can become quite long, and as such will have correspondingly large charging currents in the range from 1 to 2 amps (or more). The tripping current on each feeder must be above charging current to avoid nuisance or sympathetic tripping; a tripping value of 3 A would be reasonable. . In this case the NGR should be selected for a let-through current five times the desired pickup level, or 15 amps.
It is worth noting that the 2009 edition of Canadian Electrical Code (CEC) rule 10-1102 says that for systems up to 5 kV the system may continue to operate with NGR currents up to 10 amps, but that a visual or audible alarm must be activated;
The case of mobile equipment in mining
In mining applications, a mobile or movable piece of equipment is supplied power through a resistively grounded system. What’s more, the ground-fault voltage must be limited to a maximum of 100V (M421 4.5.6.a & Annex A Figure A.1). Ground-fault voltage is defined as the maximum ground-fault current multiplied by the resistance in the ground path from the equipment to the supply. Depending on the size of the portable cabling used and the length of that cable, the resistance of the ground wire will limit the magnitude of the allowable ground fault current. For systems with large charging current, this could cause what is known as sympathetic tripping, and as such the method of selecting the NGR value will have to be adjusted.
If the value of the charging current on a specific feeder is higher than the tripping point on the ground-fault relay of that feeder, sympathetic tripping occurs. Consider the example shown in Fig. 5.
The charging current of the system is (I1+I2+I3). The value of the NGR is calculated as Ir =5 x (I1+I2+I3) and the set point of the ground-fault relays on the feeders is 20% of Ir.
If a ground fault occurs on Feeder 3, then Feeders 1 and 2 will each see their charging currents I1 and I2 flow through the current transformer. If I1, for example, is higher than 20% of Ir, then the protective relay for that feeder will detect the current and trip.
The obvious solution is to increase the value of the current flowing through the NGR. However, due to the limit on the ground-fault voltage, that is not always feasible. Thus the solution is to calculate the NGR value based on the largest charging current value among all the feeders. So let’s assume the feeder 1 has the largest charging current value, I1, then the NGR is selected such as Ir = 5 x I1.
An HRG system can improve safety, aid in compliance with regulations, and reduce downtime by providing a more stable distribution system. The critical part to consider is that these systems must be properly designed and receives proper maintenance. The use of a dedicated NGR continuity monitor is one recommended method to ensure a continuously safe and reliable operation of the system.