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Medium and Low Voltage Grounding Methods. The ASSET Company August 20, 2010 Establish a voltage relationship between the system neutral and ground. Overvoltage.

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Presentation on theme: "Medium and Low Voltage Grounding Methods. The ASSET Company August 20, 2010 Establish a voltage relationship between the system neutral and ground. Overvoltage."— Presentation transcript:

1 Medium and Low Voltage Grounding Methods

2 The ASSET Company August 20, 2010 Establish a voltage relationship between the system neutral and ground. Overvoltage protection for system component insulation Controlled single-phase-to- ground fault current magnitude Establish a voltage relationship between energized phase conductors and ground. System Grounding

3 The ASSET Company August 20, 2010 Establish a consistent reference plane for all system components. Personnel safety Optimum single-line-to- ground fault current distribution Safe conduction of lightning discharge currents a.k.a. bonding Equipment Grounding

4 The ASSET Company August 20, 2010 Cable Sheath Neutral Wire Water Pipes Building Steel lg Human Physiological Response: 1 ma – threshold of sensation 6-9 ma let go currents 9-25 ma – muscular contraction ma – ventricular fibrillation Body resistance: 5000 ohms or higher Perceptable gradient voltage: 50 volts & above Harmful gradients: 375 volts and above Controlling factors o Magnitude of Potential Gradients- magnitude of ground fault current phase-neutral voltage complexity of return path o Duration of Potential Gradients- protective device settings fuse, relay, & breaker operation Z

5 The ASSET Company August 20, 2010 Cable Sheath Neutral Wire Water Pipes Building Steel IgIg Distribution of I g : Kaufmanns work showed that 90-95% of the fault current will return through the cable sheath and/or neutral wire I g = I C + I N + I P + I S Example: I g = A I S = 0.05 x = 1000 A For V s > 100 V, Z > 0.1 Ohms Power cable- ground shield at both ends to help equalize electric potential along cable length, but be mindful of magnitude and duration Communication cable- do not ground communication cable at both ends to avoid circulating current that would act as noise ICIC ININ IPIP ISIS Z

6 The ASSET Company August 20, 2010 Cable Sheath Neutral Wire Water Pipes Building Steel IgIg Conclusions: Ineffective grounding at any voltage sets the stage for personnel injury or death. The likelihood of these distracting gradients is insignificant at low voltages Low voltage systems can be solidly grounded without undue concern for distracting potential gradients. Ineffective grounding at higher voltages can set the stage for potential gradient shocks which are severe enough to distract personnel in the workspace ICIC ININ IPIP ISIS Z

7 The ASSET Company August 20, 2010 Cable Sheath Neutral Wire Water Pipes Building Steel IgIg Ineffective grounding at any voltage sets the stage for personnel injury or death. The likelihood of these distracting gradients is insignificant at low voltages Low voltage systems can be solidly grounded without undue concern for distracting potential gradients. Effective grounding at higher voltages can set the stage for potential gradient shocks which are severe enough to distract personnel in the workspace ICIC ININ IPIP ISIS Z System Neutral Ground- an intentional electrical connection between the neutral of the power system and ground Grounded System- a system in which one conductor, usually the neutral, is intentionally connected to ground. Ungrounded System- a system in which none of the electrical conductors is intentionally connected to ground Solidly Grounded Neutral- a direct electrical connection between the neutral and ground with no added impedance Resistance Grounded Neutral- an electrical connection in which a resistor is inserted between neutral and ground Reactance Grounded Neutral- an electrical connection in which an inductive reactance is inserted between neutral and ground Capacitance Grounded Neutral- an electrical connection in which a capacitor is inserted between neutral and ground Note: There is an inherent distributed capacitance between each conductor and ground. Hence, an ungrounded system is really capacitively grounded s ss

8 The ASSET Company August 20, 2010 Effective Grounding- grounding such that the steady-state operating voltage on the healthy phases of the power system during a single-line-to-ground fault will not exceed 140% of the open-circuit line-to-neutral RMS voltage. and Both must be met

9 - Industrial Power Systems Handbook General Electric Company © Donald Beeman, editor Since power sources are fewer in number than loads and are less likely to be disconnected, they are preferred as grounding points.

10 Forms of Neutral Grounding -G Fault Magnitude Transient Over- voltages Arrester Applications (% of V L-L ) System Protection Selectivity Comments Ungrounded 0 Very high100%NoneNot Recommended Solidly I 3 < 140%80%Generally Good Common at high voltages and low voltages Low Resistance 100 – 1200 ANot Excessive100%Generally Good Common at medium voltages High Resistance 2 – 10 ANot Excessive100%Requires Special Equipment Alarm application for continuity must trip >5kV Reactive I 3 Not Excessive100%Generally Good Special case – rarely need Resonant 0 Not Excessive100%Special Treatment Special case – very rarely need Capacitive I 3 High100%Generally Good Special case – very rarely need The ASSET Company August 20, 2010

11 or Ungrounded Solidly Grounded The ASSET Company August 20, 2010

12 The ASSET Company August 20, 2010

13 Low Resistance High Reactance High Resistance Ground Low XLXL R The ASSET Company August 20, 2010

14 A B C A B C Resonant Neutral Grounding Capacitive Neutral Grounding The ASSET Company August 20, 2010

15 The ASSET Company August 20, 2010 R A B C 400 A 10 sec V R =0 Effectively Grounded R A B C 400 A 10 sec VRVR V R = 7970 Volts If V = IR, and you wish to limit I to 400 Amps, 7970 = 400 R R = 19.9 Resistor must be insulated at One terminal for 8000 V and to pass 400 Amps for 10 seconds without damage 7970 A B C A B C A B C VRVR A B C

16 The ASSET Company August 20, 2010 For Low Resistance Resistors ANG

17 IBM Substation Neutral Grounding Resistor

18 The ASSET Company August 20, 2010 Impedance Transformer Effectively grounded under normal conditions 7970:240 V Grounding Transformer Lets limit ground fault current to 10 amps

19 The ASSET Company August 20, 2010 If I G is 10 amps, then the power through the transformer is: Power in the secondary is the same: Transformer spec : 75 kVA or 100 kVA 7970 : 120 V Resistor spec: Rated for 240 VAC operation for 10 seconds IGIG IGIG IRIR

20 The ASSET Company August 20, Low voltage or 5 kV process plant distribution - Delta winding transformers - Generators

21 The ASSET Company August 20, 2010 Scott-T Connection -Small, lightweight -Economical -Off-the-shelf in common ratings -Not practical for unusual applications (i.e. voltage, frequency, current levels) -Limited to Low-R applications Y - Transformer -Standard transformer -Applications are readily field-designed -Can be used for any grounding mode -Grounding resistor can be inside delta if single phase units are used in Hi-R scheme -Offers option for redundant backup protection Zig-Zag Potential Transformers -Custom designed -Can fit any application -Can be designed to provide full reactive limitation with no external impedance -Can be used for effectively grounded system -Usually required for metering -Economical -Thermal ratings suitable for -highly restricted schemes only -Application may not provide desired limitation of transient overvoltages because the grounded wye winding is high impedance H 1, 1 H 2, 2 H 3, 3 X2X2 X2X2 X1X1 X1X1 X3X3 X3X3 X2X2 X2X2 X1X1 X1X1 X3X3 X3X3 H 1, 1 H 2, 2 H 3, X2X2 X2X2 X3X3 X3X3 H1H1 H1H1 X1X1 X1X1 H2H2 H2H H3H3 H3H3 X2X2 X2X2 3 3

22 This wye-delta transformer connection doesnt limit fault current, except the winding impedance of the grounding transformer. The ASSET Company August 20, V

23 Low resistance ground fault limiting – same type calculations as before. The ASSET Company August 20, V

24 Legend AM- Ammeter CPB- Control Power Breaker CR- Main Contactor CT- Current Transformer HR- Horn Relay HRX- Auxiliary Relay MR- Meter Relay PR- Pulsing Relay PT- Potential Transformer R3- Fault Time Delay R4- Pulsing Adjustment TR- Timing Relay UV- Undervoltage The ASSET Company August 20, 2010

25 Placing a Resistor in the transformer secondary will limit the primary ground fault current. The ASSET Company August 20, 2010

26 The ASSET Company August 20, 2010 H0H0 H3H3 H2H2 A C B H1H1 R= X1X1 X6X6 X4X4 X5X5 X3X3 X2X2 Transformer ratio is GRY – 120 V delta. This is V to ground on the primary, and is the voltage on each winding. The secondary voltage of 120 V is the voltage across each winding.

27 The ASSET Company August 20, 2010 Under non-faulted balanced system conditions, the voltage at the corner delta = V = 0 120

28 The ASSET Company August 20, 2010 B When one phase suffers a bolted fault, a phase for instance, the voltage vectors change: C v v 208 V The resulting voltage at the delta corner rises to 208 V. For a Resistor with 208 V across it, the current through it is 1963 Amps. (The resistor is rated 208 V, 1960 Amps, 10 sec.)

29 The ASSET Company August 20, V B 7970 C v v The resulting voltage across the resistor is 120 V, and the resistor has 1,132 Amps through it. v v V 69.3 V Ours is actually a V L-L system:

30 The ASSET Company August 20, V B 7970 C v v v v V 69.3 V The resulting voltage across the resistor is 120 V, and the resistor has 1,332 Amps through it. This system is a high impedance grounding system that limits current to 10 Amps.

31 The relay settings for the job were: 300 Amp pickup on the secondary side, which equates to 2.6 Amp primary. 50 V setting on the 59 G relay is equal to A voltage element, looking at voltage across the resistor, and a current element, looking at current through the resistor, are used in conjunction for redundant ground fault detection. Voltage element is much more sensitive than current element The ASSET Company August 20, 2010

32 The ASSET Company August 20, 2010 High Voltage Bus Normal central station practice – no generator breaker GSU neutral not required on generator side Saves cost of startup transformer Availability of suitable breakers GSU neutral grounding required on generator side

33 The ASSET Company August 20, 2010 Low Voltage (< 1000 volts) Medium Voltage (through 15kV) High Voltage (> 15kV) 1.Solid Grounding 2.High Resistance 1. Low resistance grounding 2. High resistance grounding 3. Effective grounding 1.Effective grounding (at the source)


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