Resistance grounding
A neutral of the star-connected network is connected to the ground or earth through a resistor is called a resistance grounding system.
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| Resistance grounding |
The resistance neutral grounding system is used for a voltage of 2.2 kV to 33kV and where the capacity of the power source is more than 5 MVA.
The resistor used in resistance grounding is metal or liquid resistance. When the resistance is made from metal it consists of some inductance and due to this inductance, the insulation of equipment is damaged when there are lightning surges on the overhead transmission lines. This problem is solved by using liquid resistance instead metal resistance.
By connecting resistance between the neutral and earth, we can limit the fault current. The resistor connected between the neutral and earth is called a Neutral grounding resistor(NGR).
The value of resistance used in resistance grounding should not be very high and very low. Very low resistance in resistance grounding makes the system to solidly grounded. In this condition, very high fault current flows to the earth and fault current should not be limited and it damages the system equipment. if the resistance of the system is very high it makes the system to ungrounded. this condition also damages the system equipment. The value of resistance is increase with the increase in the system voltage in order to limit the high short current or unbalanced current during the fault in the power system.
The value of resistance is taken such that ground-fault current is limited. In a resistance grounding system, the fault current is limited to 5 to 10 % of fault which occurs with a three-phase line to line fault. The resistance grounding is used for limiting the earth fault current.
Advantages of resistance grounding
1. The resistance grounding system protects the system equipment from transient overvoltages.
2. In resistance grounding arcing ground risk is minimized and protect the system from a ground fault.
3. The resistance grounding system improves the stability of the system.
4. In a resistance grounding system, inductive interference with neighbouring communications lines should be reduced because due to the resistor, fault current value is reduced.
5. In resistance grounding, transient overvoltage faults are converted to the controlled current fault.
6. In a resistance grounding system, a low value of resistance allows the use of discriminative protective relays.
7. The resistance grounding improves safety.
Disadvantages of resistance grounding system
1. The resistance grounding system is more costly than the solidly grounded system.
2. In a resistance grounding system, a large amount of energy loss takes place in the resistor during the fault. Sometimes it is become difficult to dissipate this energy into the atmosphere.
3. During the earth fault, the neutral of the system is displaced, so there is a need for a lightning arrester and high voltage insulation of equipment which is increase the cost of the system.
Reactance grounding
An impedance or reactance is connected between the neutral of the system and ground is called resistance grounding. This method is also used to limit the high fault current.
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| Reactance grounding |
The reactance grounding method is employed for transmission whose fault current is very high and voltage is level between 3.3 kV to 33kV. The reactance used in reactance grounding is highly inductive.
The reactance grounding is used where the charging current of the circuit is high. In the reactance grounding system reactance is added in series with neutral of the system which increases the lagging current and the lagging current naturalized the capacitive current.
The selection of reactance in the reactance grounding system is a very important part. If the selected reactance value is smaller then the system work as a solidly or effectively grounded system.If the selected value is more than the system it works as an isolated system.
In a reactance grounding system, to minimize the transient overvoltages and switching surges, the ground-fault current should not be less than 25 % of the three-phase to phase fault current. where the system has a low capacitance value, reactance grounding is preferred.
In reactance grounding, the fault current is limited and also reactance grounding provides the phase opposition between the capacitive ground current and faults current. In resistance grounding, the only fault current is limited.
From the ratio of zero sequence reactance to positive sequence reactance we can say the system is reactance grounded or solidly grounded. If the ratio of Zero sequence reactance to positive sequence reactance is more than 3, then the system is reactance grounded. If this ratio is less than 3, then the system is solidly grounded.
The reactance grounding system is used in the synchronous condenser, lightly loaded transmission line, underground cables which are having high charging current and also used in circuits having high charging current.
Advantages of reactance grounding system
1. The reactance grounding system provides the proper relay operation.
2. In the reactance grounding system, the arcing ground is limited because by using reactance grounding capacitive current is reduced.
3. In the reactance grounding system, transient and switching faults are converted to controllable ground faults.
4. In the reactance grounding system, earth fault protection is provided due to low fault current.
5. In the reactance grounding system, fault currents is reduced so the disturbance with the communication lines is reduced.
6. In the reactance grounding system, the fault current interrupted by the circuit breaker is low.
7. In the reactance grounding system, partial grading of insulation of equipment is required.
Disadvantages of reactance grounding
1. The transient overvoltages are not suppressed if the value of the reactance is more which will damage the insulation of equipment.
2. In reactance grounding, due to reactance value of surge voltages increase during the switching operation.
3. In reactance grounding, under fault conditions very high transient voltages are produced.
4. In reactance grounding, fault current and capacitive charging current are not neutralized properly so some arcing ground remains in the system.
5. In the reactance grounding system, more fault current is required to operate relaying system than the fault current required in the resistance grounding system.