PROTECTING ELECTRICAL EQUIPMENT IN GENERATING STATION

Transmission system plays an important role in supply of power to the consumers through the vital link between the generating stations and the distribution system (Refer Figure 1).

Summarizing the generation, transmission and distribution voltages, we have:

• Generating voltage:  11, 13.2, 15.75 or 21 kV.
• High voltage transmission: 400 kV, 220 kV, 132 kV, 66kV, 33 kV
• High voltage or primary distribution: 11 kV.
• Low-voltage distribution: A.C. 415/230, 3-phase, 4-wire
• The standard frequency for A.C. working is 50 Hz

Earth Fault: It is an inadvertent fault between the live conductor and the earth. When earth fault occurs, the electrical system gets short-circuited and the short-circuited current flows through the system. The fault current returns through the earth or any electrical equipment, which damages the equipment. It also interrupts the continuity of the supply and may shock the user. To protect the equipment and for the safety of people, fault protection devices are used in the installation.

Causes of earth fault

The main cause of earth fault in an overhead transmission and distribution line or any electrical equipment is the failure or puncture of the insulator. Insulators are used in the equipment to provide insulation between live conductors and metallic body that are already connected with the earth connection surface. So, if the insulation of insulator fails or in other words if the insulator is punctured then the fault current will flow through the live conductor and metallic part to the earth creating an earth fault.

Another reason for earth fault is that sometimes overhead transmission lines break due to any unusual loads on line and hence fall to the ground. So, in this case also live conductor gets in direct contact with the ground and creates a major earth fault in the electrical system.

Effect of earth fault

Whenever an earth fault occurs in the electrical system than during earth fault, the System gets short, and hence huge amount of short-circuit current flows through the system. That huge current damages electrical equipment, which comes in contact with a loop of earth fault circuits and also it interrupts the continuity of the power supply.

Generator earth fault protection

A typical layout of generating station is shown in Figure 2. The high capacity generators are of double winding or single winding per phase, star connected with star point grounded. The zero sequence impedance of a synchronous generator is generally of half value than the positive sequence impedance. So, single phase ground fault current is higher per phase compared to three phase fault. Also a ground fault on one phase results into rise of voltage on other two phases by about 173%, putting other two phases into danger of insulation failure.

This high fault current can not only damage winding, may also cause damage to core, making repair of faulty generator a very costly affair.

To overcome the problem of flowing of high current during an earth fault, the neutral is grounded through any equipment, i.e., through grounding transformer or resistor to control flow of earth fault current. However, many type of grounding techniques are used viz:

• High resistance grounding
• Low resistance grounding
• Low reactance grounding
• Resonant grounding
• Effectively ground
• Unground
• Hybrid grounding – In this method, resistance switching from low to high is used, as per the condition.

A 247 MVA, 15.75 kV, Hydrogen/ water cooled Generator with Slip Ring Excitation is shown in Figure 3.

Methods to detect earth fault in generators

The first method is a combination of neutral displacement voltage and third harmonic comparison or third harmonic under voltage detection method. In this method, 100% stator earth fault is achieved by combining two relay elements, as below:

• The first unit is connected across the secondary winding of distribution transformer connected between generator winding and ground and in general it is known as Neutral Grounding Transformer (NGT). The primary voltage rating of this NGT is line-line or line-earth voltage of generator terminal voltage and secondary is of the rating of 120-480 volt. An inductor is also connected across secondary winding of NGT. The value of this inductor is so selected that, it nullifies capacitive fault currents during a fault and only active component is made available. It also avoids Ferro resonance in connected generator potential transformers.

The secondary voltage obtained during a fault is fed to a relay, which operates when the voltage limit crosses the setting limit. Normally setting of relay is of IDMT characteristic. (Refer Figure 4)

• The problem with the first unit is that, the earth faults near to neutral point of generator remain undetected, as no much voltage is available there to provide sufficient driving force to operate a voltage relay. This problem is resolved by putting another unit, which compares third harmonics of generator phase side to third harmonics of generator neutral side. The 3 phase generator PT voltage is fed on relay terminals and voltage from NGT is fed on another terminal of relay. From these voltages, third harmonic component is filtered out in relay and ratio of both the third harmonic components is compared. In case of earth fault towards neutral of generator, third harmonics obtained from NGT get suppressed and ratio of third harmonics gets disturbed, causing operation of relay. (Refer Figure 5).

However, about 5% of generator winding, near to 85% portion of generator winding from phase side, remains unprotected by this unit also, as whenever earth fault occurs in this area, the phase side and neutral side third harmonics get suppressed, in such a manner that, ratio remains unchanged. This area of winding is covered by the first unit i.e., Neutral Displacement Unit. The unprotected zone is figured out in Figure 6.

The latest and nowadays mostly adopted method is sub harmonic voltage injection principle. In this method, a signal of low frequency generally of 20 Hz is injected. This principal also gives better coverage than earlier mentioned third harmonic comparison based principle. By adopting this principle, it is also possible to detect earth fault, while machine is under stand still condition or on barring speed i.e., on turning gear. This method is deployed with high resistance grounding for large generators. The working principle is shown in Figure 7. In injection principle, low frequency and low voltage injection is used. By using low frequency/low voltage injection, following aspects are fulfilled:

• Human safety by injecting low voltage, generally 50 V. In case generator is in stand still condition and someone accidentally touches, person will remain safe.
• Low frequency injection rejects high frequency signals to ground and injection source do not interfere with normal operation.

There are many methods of injection of sub harmonic voltage, the most common method‘s connection diagram is shown in Figure 8.

In this way, injection principle based 100% generator stator earth fault protection gives protection against earth faults, while generator is in stand still, unexcited condition too. The new age protection systems also continuously monitor generator winding capacitance and grounding system health parameters also. It makes injection principle based 100% Generator Stator earth Fault Protection most reliable and now a days, it is widely used.

Transformer earth fault protection

The transformers are protected by various protections against various faults. The deployment of protection depends upon the capacity of transformer (Refer Figure 9).

Following protections are used for transformer protection:

• Fuses: Fuses are generally used in small distribution transformers up to capacity of 1000 KVA. While using fuses as primary protection care should be taken to select fuse rating – such as to meet out full load capacity of transformer, as well as short time over load conditions and magnetizing inrush current. High rupturing capacity fuses are used with suitable characteristic.
• Over Current Relay: This protection is used in medium capacity transformers and which are fed through some type of circuit breaker. The over current relay used may be of IDMT characteristic or with High set Instantaneous operating characteristics or combination of both. This provides high speed clearance of fault. The CT circuit is so connected that residual earth fault protection is also achieved.
• Restricted Earth Fault Protection: Sometimes, especially in case of star connected transformer winding, the conventional earth fault fails to provide adequate protection against earth fault. By deploying Restricted Earth Fault Protection (REF protection), the degree of protection is very much improved. In this protection, phase side CTs of three phase are connected in residual connection and the current from neutral CT works as bias current. Hence, this protection remains immuned to external earth faults, occurring out of installed CT zone. Here it is also to be noted that, if transformer is stat-star connected, than for both sides separate REF protection must be deployed, as zero sequence components cannot transfer through transformer winding. The connection diagram for REF is as follow:

Let us consider a star winding transformer, which is protected by a Restricted Earth Fault Protection with EFR protecting device as shown in the Figure 10.

When an external fault F1 occurs in the network, I₁ and I₂ flow through the secondary side of the CTs. The resultant of I₁ and I₂ will be zero. However, if an internal fault F₂ occurs inside the protective zone, only I₂ flows and I₁ is neglected. The resultant current I₂ passes through the earth fault relay, which senses the fault current and protects the restricted portion of winding. The relay setting is usually 15% more than the rated winding current. To avoid the magnetizing inrush current, the stabilizing resistance must be in series with the relay.

Differential Protection: The REF protection and Differential protection and REF protection both work on the principle of Kirchhoff’s law i.e., sum of currents flowing in a conducting circuit is zero (Refer Figure 11). The differential protection is used in transformers capacity of 10000 kVA or above. To deploy differential protection, as both side CT’s i.e., HV and LV are used, following points are to be kept in consideration and must be resolved:

• Phase shift correction, if different connections are used on both side.
• Current ratio correction, as CT ratio of HV & LV side differs.
• Effect of magnetizing inrush current during initial charging.

The problem of operation of differential relay during initial charging and due to effect of magnetizing inrush is resolved by using:

• Incorporating Time Delay: It was used in very early days but this time delay also affect actual operation of relay during fault condition. So, this scheme is not used anywhere.
• Harmonic Restraint: Second harmonics are always present in inrush current irrespective of degree of core saturation. Normal fault currents do not contain second or other even harmonics. So this second harmonic restraint feature is used to block the relay operation during initial charging of transformer. Study says there is as much as 72% of inrush current is second harmonics. In case if there is a fault in transformer, prior to charging, this percentage of second harmonics will fall down and will cause relay to operate.

In differential protection it is important to mention here that the earthing of neutral point of each CT core plays a vital role. In the absence of proper earthing the differential protection may operate without actual fault.

• Buchholz Relay: This protection is the oldest protection and is still used without any much change. It is basically a mechanical type of protection and its operation for alarm or trip depends on the quantity of gas generated inside transformer tank, which depends upon the severity of internal fault. (Refer Figure 12)

• Winding / Oil Temperature: In case of failure of cooling system of transformer, which may be due to power supply failure of fan & pumps or choking of oil pipe line, temperature of oil and winding will start increasing. At a lower stage this system will give alarm and if the problem is not resolved, it will initiate trip signal, thus safeguard transformer from further damage due to temperature fatigue.(Fig 13).

• Over fluxing Protection: If due to some abnormal condition, transformer is fed by normal voltage and reduced frequency, over fluxing will occur resulting into core heating and damage to winding due to heat. Probability of this phenomenon is very high with generating transformers directly connected to generators. In this Over Fluxing Protection relay is deployed that measures voltage to frequency ratio. The settings are so selected that it permits about 10 -15% overvoltage condition operation.
• Pressure Reducing Device (PRD) Or Rate of Rise Of Pressure: It is also a mechanical type of protection and a membrane is deployed. In case, due to internal fault, if pressure inside transformer tank rises drastically, this membrane gets punctured and oil is splashed out, thus reducing pressure inside and preventing tank from explosion. A limit switch like device is connected in the path of oil, thus while oil escape, also operates this relay, which ultimately cause trip. (Refer figure 14)

AC HT Motor Protection

Various high rating AC motors usually used in the generating station are shown in Figure 15. Following faults may occur in AC HT Motors:

Here only earth fault protection shall be discussed. For HT motor earth fault protection, two methods are adopted:

• Residual CT Connection: This type of earth fault protection is deployed when system permits sensitivity higher than 20% of motor rated current. Here care should be taken that relay should not trip due to spill current, which may occur due to unequal saturation of CTs and different lead resistance. A stabilizing resistance can be deployed in series with protection relay. The residual connection circuit is displayed here in Figure 16.

• Core Balance CT Method: In this method a core balance CT is used. This method is used where high resistance grounding is done to limit fault current.  In this method all three phases and sheath earthing is surrounded by a CT. Care should be taken that the sheath earthing must pass through CT. It must be ensured that sheath earthing taken from inside the CT, otherwise earth fault relay will operate as soon as supply is given to motor. The circuit arrangement is as shown below in Figure 17.

Following care should be taken, if motor is fed through three phase HT cable:

• The HT cable sheath and armour must be earthed at single point only. Double point earthing will cause circulating current and thus premature failure of cable due to heating.
• The three cores of cable, at both ends, the semi-conductor layer must be removed up to a length of at least 50 cm otherwise earth fault may occur between cable lug and gland.
• Most of the earth faults in cables are observed at glands. In case of cable earth fault, glands should be inspected carefully.

Conclusion

Earth fault protection is not only most essential for machine/equipment safety, but also essential for human safety and system reliability. A single system disturbance on account of uncleared fault or a single causality, affects the whole system and society. As well as it also imposes financial implications.

Therefore, being an electrical professional, it is the duty of each and every one to take appropriate action to keep protection system healthy, at every moment. It is also the duty of every electrical professional to keep aware the society and people around to make sure that electrical safety should not be compromised, at any point of use.

Harsh Samvedi is working in M.P. Power Generating Co.Ltd., Jabalpur and has an experience of over 36 years in the field of  Testing & Commissioning of Electrical Equipment. He has experience of commisioning of 210, 500 & 600 MW thermal power plants and various capacity hydel power plants. He is having vast experience of dealing with  generator protection and high capacity HT motor protection.

Dr. Rajesh Kumar Arora completed his PhD in grounding system design from UPES, Dehradun. He is  also a certified Energy Manager and Auditor and has worked in 400kV and 220kV substation for more than 14 years in Delhi Transco Limited (DTL). Presently he is working in D&E (Design and Engineering) Department of DTL.