Industrial Power-System Protection--Grounding Industrial Electrical Power Systems

Introduction

We’ve already briefly covered that the phase-to-ground faults in a system can limit the ground fault current depending on adding external impedance between neutral and the ground. This section briefly covers the various methods of grounding that are adopted in the electrical systems. In the following clauses, the star-connected transformer is shown which are widely used in power distribution. The grounding methods are also applicable in case of generators, whose windings are also invariably star connected.

The following table highlights the possible problems that can occur in a system due to the common faults and the solutions that can be achieved by adopting system grounding.

Problems:

Phase faults: High fault currents

Only limited by inherent impedance of power supply.

Ground faults: Solid grounding means high ground fault currents; Only limited by inherent zero sequence impedance of power system.

Consequence:

1. Heavy currents damage equipment extensively - danger of fire hazard.

2. This leads to long outage times - lost production, lost revenue.

3. Heavy currents in ground bonding gives rise to high touch potentials - dangerous to human life.

4. Large fault currents are more hazardous in igniting gases - explosion hazard.

Solutions

Phase segregation:

Eliminates phase-to-phase faults.

Resistance grounding: Means low ground fault currents - can be engineered to limit to any chosen value.

Benefits:

1. Fault damage now minimal - reduces fire hazard.

2. Lower outage times - less lost production, less lost revenue.

3. Touch potentials kept within safe limits - protects human life.

4. Low fault currents reduce possibility of igniting gases - minimizes explosion hazard.

5. No magnetic or thermal stresses imposed on plant during fault.

6. Transient overvoltages limited - prevents stressing of insulation, breaker restrikes.

Grounding devices

Solid grounding

In this case, the neutral of a power transformer is grounded solidly with a copper conductor.

++++ Solid grounding of power transformer

Advantages:

• Neutral held effectively at ground potential.

• Phase-to-ground faults of same magnitude as phase-to-phase faults; so no need for special sensitive relays.

• Cost of current limiting device is eliminated.

• Grading insulation towards neutral point N reduces size and cost of transformers.

Disadvantages:

• As most system faults are phase-to-ground, severe shocks are more considerable than with resistance grounding.

• Third harmonics tend to circulate between neutrals.

Resistance grounding

A resistor is connected between the transformer neutral and ground:

• Mainly used below 33 kV.

• Value is such as to limit an ground fault current to between 1 and 2 times full load rating of the transformer. Alternatively, to twice the normal rating of the largest feeder, whichever is greater.

++++ Resistance grounding

Advantages:

• Limits electrical and mechanical stress on system when an ground fault occurs, but at the same time, current is sufficient to operate normal protection equipment.

Disadvantages:

• Full line-to-line insulation required between phase and ground.

Reactance grounding

A reactor is connected between the transformer neutral and ground:

• Values of reactance are approximately the same as used for resistance grounding.

• To achieve the same value as the resistor, the design of the reactor is smaller and thus cheaper.

++++ Reactance grounding

Arc suppression coil (Petersen coil)

A tunable reactor is connected in the transformer neutral to ground:

• Value of reactance is chosen such that reactance current neutralizes capacitance current. The current at the fault point is therefore theoretically nil and unable to maintain the arc, hence its name.

• Virtually fully insulated system, hence current available to operate protective equipment is so small as to be negligible. To offset this, the faulty section can be left in service indefinitely without damage to the system as most faults are ground faults of a transient nature, the initial arc at the fault point is extinguished and does not restrike. N IF IC

++++ Arc suppression coil (Petersen coil)

Sensitive watt-metrical relays are used to detect permanent ground faults.

Grounding via neutral grounding compensator

This provides an ground point for a delta system and combines the virtues of resistance and reactance grounding in limiting ground fault current to safe reliable values.

++++Grounding via neutral grounding compensator

Evaluation of grounding methods

The grounding method is called effectively grounded when it’s directly connected to ground (solidly grounded) without any passive component in between. Non-effective grounding refers to, the method of grounding through a resistance, reactance, transformer, etc.

Following table compares the grounding methods.

Evaluation of Relative Merits of Effective and Resistive Grounding

1. Subject: [Rated voltage of system components, particularly power cables and metal oxide surge arresters Ground fault current magnitude; Degree of damage, because of an ground fault; Step and touch potentials during ground fault; Inductive interference on and possible damage, to control and other lower-voltage circuits; Relaying of fault conditions; Cost]

2. Effective Grounding: [Need not exceed 0.8Um -- Approximately equal to three-phase fault current (typically 2-10 kA); High degree of damage at fault point and possible damage to feeder equipment; High step and touch potentials; High probability; Satisfactory; Lower initial cost but higher long-term equipment repair cost ]

3. Resistive Grounding: [Must be at least 1.0Um for 100s Reduced ground fault current magnitude (typically 300-900 A) Lesser degree of damage at fault point and usually no damage to feeder equipment

Reduced step and touch potentials:

Lower probability; Satisfactory; Higher initial cost but lower long-term equipment repair cost, usually making resistive grounding more cost-effective ]

++++ gives the touch potential with solid grounding. ++++ shows the touch potentials with resistance R introduced in the neutral. Here the ground fault current is limited by the resistance R, so only reduced current flows to the ground. However, it’s a normal practice to adopt solid grounding method at low voltages (up to say 600 V) and resistance grounding is adopted for higher voltages (up to 33 kV). The other methods of grounding (reactor, transformer, etc.) are generally adopted in the cases of voltages beyond 33 kV. Cost invariably determines the grounding method.

Ground bond 1000 V 1000 V 1000 A REB =1? 1000 V

++++ Touch potentials - solid grounding Ground bond 1000V 25 V 25 V 2.5A; 2.5A REB =10? R

++++ Touch potentials - resistive grounding

The main reason for adopting solid grounding is because, the resistance grounding cannot be used for single-phase loads, whereas most of the LV distribution mainly households, etc. comprise of single-phase loads. Nevertheless, resistance grounding is considered at low voltages in industrial environments, where three-phase loads are connected and the process conditions don’t accept frequent shutdowns due to ground faults. Though it’s true that the power interruptions can be kept low with the use of resistive grounding method, human protection demands that the power to be isolated in case of ground faults.

That is one more reason for using solid grounding in utility distribution transformers.

Effect of electric shock on human beings

Electric shock and sensitive ground leakage protection

There are four major factors, which determine the seriousness of an electric shock:

1. Path taken by the electric current through the body

2. Amount of current

3. Time the current is flowing

4. The body's electrical resistance.

The most dangerous and most common path is through the heart. Persons are not normally electrocuted between phases or phase-to-neutral, almost all accidents are phase-to- ground.

Arm to arm; Arm to leg

++++ Dangerous current flows

++++ shows the four stages of the effect of a current flow through the body:

1. Perception - tingling - about 1 mA

2. Let-go threshold level - about 10 mA

3. Non-let-go threshold level - 16 mA

4. Constriction of the thoracic muscles - death by asphyxiation and ventricular fibrillation - about 70-100 mA. 1mA 9mA 16mA 70-100mA

++++ Effects of current flow through the body

++++ shows the normal electro-cardiogram - one pulse beat - at 80 bpm = 750 ms.

1. QRS phase: normal pumping action

2. T phase: refractory or rest phase - about 150 ms

3. Death could occur if within this very short period of 150 ms a current flow was at the fibrillation level.

++++ Electro-cardiogram

++++ shows the resistance of the human body - hand-to-hand or hand to foot.

Consider an example of a man working and perspiring, he touches a conductor at 300 V (525 V phase to ground). 300 V divided by 1000 ? = 300 mA. It’s important to remember that, it’s the current that kills and not voltage.

Hand to hand or hand to foot

Very dry skin Average Very moist skin Volts Resistance (ohms)

++++ Resistance of human body

Sensitive ground leakage protection

Grounding does not ensure that humans will be protected when coming in contact with a live conductor. Though there may be relays, which are set to sense the ground leakages, invariably their settings are high. Hence ground leakage circuit breakers (ELCB) or residual current circuit breakers (RCCB) are adopted where possibility of human interaction to a live conductor is high. These breakers work on the core balance current principle.

++++ illustrates the operation of the core balance leakage device. When the system conditions are normal, the phase current and neutral current will be equal and in phase. Hence the CT won’t detect any current under normal conditions since IL + IN = 0 (vector sum).

IL = IN + IF ; IF = IL - IN

++++ Principles of core balance protection

The ELCB comprising of core balance CT is mounted at the source end. When a human comes in contact on any part of the line, a part of the current will start flowing through the body. It will result in unbalance of the currents entering and returning to the CBCT of the ELCB. If the fault current IF, flows through the human body, IN is reduced by this amount. Relay is operated by this unbalance quantity, and immediately trips the ELCB. It’s normal that the ELCBs are molded breakers similar to the miniature circuit breakers including, CBCT mounted inside. It’s also common that the CBCT can be mounted outside and the unbalanced current can be taken to trip a separate relay namely ground leakage relay.

The above example considered a single-phase system. However, the principle is the same for three-phase systems with neutral, where also the vector sum of the three-phase currents (IR + IY + IB) and the returning neutral current IN will be zero. All the phase and neutral conductors are taken in through the CBCT so that the CBCT does not sense any current under normal conditions. In the event of any leakage in any phase, the CBCT immediately detects unbalance and causes the breaker to trip.

The ELCBs are available with sensitivity of 30 mA, 100 mA and 300 mA. For human protection, 30 mA ELCBs is recommended, since currents flowing above 30 mA in a human can cause serious damage including death.