THREE-PHASE GROUND-FAULT BASICS In this section we will discuss the basics of grounding three phase systems. We’ll look at Ungrounded, Solidly Grounded, and Resistance Grounded systems. Then we will discuss a method of adding an Artificial Ground to Ungrounded transformer secondary. C:\Data\...\GF_BasicsOHd.ppt

UNGROUNDED (DELTA) SYSTEM Transformer Secondary Feeder (with capacitance to ground) This is the basic circuit of an Ungrounded system. Typically this is a Delta-Wound Transformer Secondary. The presence of distributed capacitance, represented here as Lumped Capacitance, of the feeder cable and the load, means that there is always a current known as charging current flowing, even with the load disconnected. Should a Ground Fault occur in an ungrounded system there is no return path and therefore no fault current. C:\Data\...\GF_BasicsOHd.ppt

UNGROUNDED SYSTEM: INDICATION Typical 3-Light Indication for an Ungrounded System When you see a three-light setup like this you’ll know that you are dealing with an ungrounded system. Under normal, non-faulted, operation all three lights glow (eg: 347V). When a ground fault occurs, the light in the faulted phase extinguishes, as there is no phase-to-neutral voltage. The other two glow more brightly, as the neutral point has essentially been forced to the faulted phase’s voltage, with the result that the voltage across these lights is line-to-line voltage (eg: 600V). C:\Data\...\GF_BasicsOHd.ppt

UNGROUNDED SYSTEM: NORMAL OPERATION & FAULTED OPERATION All phases are at line-to-neutral voltage above ground (eg: 347V) •A & B phases are at line-line voltage above ground (eg: 600V) Under normal, non-faulted, operation of an Ungrounded System, a neutral point is established with respect to line voltage, via distributed capacitance. The electrical stress placed on the insulation of the load, say a motor, is the system’s line-to-neutral voltage, say, 347 V. With a phase faulted to ground, the potential of that phase is zero volts. However, the electrical stress placed on the insulation of two phases of the load is now the system’s line-to-line voltage, say, 600V. This elevated electrical stress may cause the insulation of another phase to break down, resulting in phase-to-phase fault,or phase-to-phase-to ground, more damage to the load, and greater danger to personnel. • Phase C @ ground potential • No fault current (no return path to source) Neutral point established by distributed capacitance Normal operation Ground fault on phase C C:\Data\...\GF_BasicsOHd.ppt

UNGROUNDED (DELTA) SYSTEM: FAULTED OPERATION WITH TRANSIENT OVERVOLTAGE •A & B phases are: > Line-line voltage Above ground • Phase C > ground voltage • Intermittent fault current • Personnel danger An arcing fault can cause a transient overvoltage condition. This condition is a result of the charging of system capacitance. Voltages up to 10 times system voltage can occur. This is a situation very dangerous to equipment and to personnel. System insulation is placed under extreme stress. This overvoltage is impressed upon other loads in the system and can cause unpredictable damage. (eg: Loss of Communications or Data Systems) Intermittent ground fault on phase C Normal operation C:\Data\...\GF_BasicsOHd.ppt

UNGROUNDED SYSTEMS: Advantages & Disadvantages Difficult to detect ground- faults — no fault current Running with a ground-fault increases stress on insulation, leading to phase-to-phase faults Intermittent fault may cause transient overvoltage Advantages: Ability to run with one phase faulted to ground

SOLIDLY GROUNDED SYSTEM A D A solidly Grounded system has the neutral of a Wye-wound transformer secondary bolted to ground. Wye-Wound Transformer Secondary Feeder (with capacitance to ground) C:\Data\...\GF_BasicsOHd.ppt

SOLIDLY GROUNDED SYSTEM WITH A GROUND FAULT The transformer-neutral-to-ground connection allows fault current to flow through a low impedance path. This current eliminates the problem of transient overvoltage and reduces personnel hazard. This current may also be detected by ground-fault monitoring devices. However fault current is limited only by fault impedance, which may be very low. For example the current in a 600 V system with a 2 ohm fault would be: IF = 600/(3 x 2) = 173 A. This may result in the violent failure of equipment. IF X Transformer Secondary Feeder (with capacitance) Current Limited Only by System impedance C:\Data\...\GF_BasicsOHd.ppt

SOLIDLY GROUNDED SYSTEMS Advantages: Detect ground-faults -Annunciate -Trip Eliminate transient overvoltages Disadvantages: Potentially large ground-fault currents Cannot run with a ground fault Possible equipment damage

RESISTANCE GROUNDED SYSTEM L O A D The use of a Neutral Grounding Resistor adds further protection to the electrical system. NGR’s are typically rated in terms of let-through current under continuous or ten second operation (10s resistors are only used when ground-faults trip the system). For example, a system may use a 25 A, continuous NGR. Such a system has prospective ground-fault current limited to only 25 A, and retains the ability to detect that current with protective relays. In addition the presence of the NGR itself can be detected by a properly designed monitoring device. NGR Wye-Wound Transformer Secondary Feeder (with capacitance to ground) C:\Data\...\GF_BasicsOHd.ppt

RESISTANCE-GROUNDED SYSTEM WITH A GROUND FAULT IF The ground-fault current in a Resistance Grounded system is limited to the NGR’s let-through current. If a 25 A NGR is used, 25 A is the maximum current that can flow through the ground fault. This may prevent damage to equipment, perhaps reducing a motor replacement to a rewind. This is the RECOMMENDED SYSTEM as it affords maximum safety and allows monitoring of the neutral-to-ground connection of the transformer, through the NGR. X NGR Transformer Secondary Feeder (with capacitance) Current Limited to NGR Let-Through C:\Data\...\GF_BasicsOHd.ppt

RESISTANCE-GROUNDED SYSTEMS Advantages: Detect ground-faults -Annunciate -Trip Eliminate transient overvoltages Limited fault current May continue to run with a ground-fault Minimize damage Detect faulted resistors Disadvantages: Cost of Neutral-Grounding Resistor

ESTABLISHING AN ARTIFICIAL NEUTRAL USING A ZIGZAG TRANSFORMER D Transformer Secondary (Delta-wound or wye-wound with inaccessible neutral) NGR An Ungrounded Delta System can be retrofitted with an “artificial neutral”, via a Zigzag Transformer. This neutral can then be resistance grounded, giving the adapted system the benefits of a Resistance Grounded System. The zigzag transformer need be sized only for system voltage and prospective fault current (ie: NGR let-through). Eg: an artificial neutral for a 600 V system with a 15 A NGR needs to be rated at 600/(1.73) V x 15 A = 5 kVA. Startco Engineering can provide ready-to-install artificial neutral, monitored systems. Zigzag Transformer C:\Data\...\GF_BasicsOHd.ppt

ZIGZAG TRANSFORMER ZIGZAG CONNECTION ZIGZAG TRANSFORMER  A  B  C b Circuit representations of zigzag transformer. Zigzag transformers have very low impedance to zero sequence current at power frequencies. Zigzag transformers draw very little current (magnetizing current only) when no ground-fault is present.  N ZIGZAG TRANSFORMER C:\Data\...\GF_BasicsOHd.ppt

ZIGZAG TRANSFORMER SYSTEM WITH A GROUND FAULT X Transformer Secondary Ground Fault Current is, at maximum, NGR let-through. This protection is achieved at minimal cost. NGR’s should be continually monitored to ensure the integrity of the grounding system, and of the ground-fault protection systems in place downstream from the transformer. Zigzag Transformer NGR IF Current Limited to NGR Let-Through C:\Data\...\GF_BasicsOHd.ppt

CHARGING CURRENT: IDEAL SYSTEM IC IA LOAD IA+IB+IC=0 IB A Ideal System: - all current flows from, and returns to the source, through the CT - ammeter ‘A’ reads 0 amps.

CHARGING CURRENT: REAL SYSTEM IC IA LOAD IA+IB+IC=0 IB A XCB XCA XCC XC=capacitive impedance to ground Real System: - includes distributed capacitance to ground (shown as lumped capacitance) - all current flows from, and returns to the source, through the CT - ammeter ‘A’ reads 0 amps.

CHARGING CURRENT: REAL SYSTEM IC IA LOAD IA+IB+IC=0 IB XCB XCA XCC A2 Charging Current: - Current that will flow into the grounding connection when one phase of an ungrounded system is faulted to ground - Here charging current will be measured by ammeter A2 - Typical values are: 0.5 A/1000 kVA for low-voltage systems and 1.0 A/1000 kVA for medium-voltage systems - Ammeter A will continue to read zero C:\Data\...\GF_BasicsOHd.ppt

A FAULT ON THE LOAD SIDE OF CT A READS ZERO LOAD

A FAULT ON THE SUPPLY SIDE OF CT LOAD The ammeter will read the sum of the capacitive currents in the unfaulted phases This value is the charging current of all equipment on load side of CT

SINGLE-LINE DIAGRAM OF A THREE-FEEDER UNGROUNDED SYSTEM Charging current I1 LOAD 1 Feeder 1 Reads I1 A1 Charging current I2 LOAD 2 Feeder 2 Reads I2 A2 Charging current I3 LOAD 3 Feeder 3 Reads I1+I2 A3 There is little difference in currents between faulted and unfaulted feeders C:\Data\...\GF_BasicsOHd.ppt

CHARGING CURRENT: REAL GROUNDED SYSTEM IC IA LOAD IA+IB+IC= IZC IB ICB ICA ICC ICA+ICB+ICC = IZC IZC A XCB XCA XCC XC=capacitive impedance to ground Real Grounded System: - includes distributed capacitance to ground(shown as lumped capacitance) - zero-sequence current may bypass the CT - ammeter ‘A’ reads IZC: 0 amps if XCA= XCB =XCC, not 0 if a capacitance imbalance exists. - can be the cause of nuisance tripping. Set relays to trip higher.

A RESISTANCE-GROUNDED SYSTEM Single-Feeder System with a Ground Fault LOAD NGR IR A Ammeter Reads IR

SINGLE-LINE DIAGRAM OF A THREE-FEEDER RESISTANCE-GROUNDED SYSTEM LOAD 1 2 3 NGR FEEDER 1 I1 Reads I1 FEEDER 2 IR I2 Reads I2 FEEDER 3 Reads I1+I2+IR It is easy to determine which feeder is faulted