Insulation Coordination Analysis

Insulation coordination study is particularly important when overhead lines are involved.

The objective of the study is to examine possible overvoltage conditions following contingencies and to ensure that sufficient insulation and protection are provided.

Insulation coordination study is not yet carried out by TNB. In this guidebook, insulation coordination study has been proposed to be carried out and therefore TNB is encourage to explore further.

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The protective requirement must be based on the need to detect system faults and malfunctions both within the DG installation as well as the TNB distribution feeder. On detection of fault or malfunction, the relays must trip appropriate circuit breakers to isolate the faulty section to minimize equipment damage and safety hazards during the faults whilst maintaining power supply continuity on healthy parts of the system.

Although the design and types of protection for the DG installation including its generating units is the responsibility of the DG Developer, TNB must ensure that these protections are properly coordinated for reliable and safe operation of the distribution feeder to protect TNB equipment and safety of other TNB customers. The basic philosophies for the types and design of the protection schemes are that:

a) For any internal fault within the DG installation, the DG must not cause problems to the utility system and the customers.

b) For any distribution fault, the generator must be protected from any damaging effects.

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The following paragraphs describe the various functions of the required protections under various types of faults and conditions.

Under voltage (UV) and Under Frequency (UF) relays are designed to trip the generator when the distribution feeder is taken off. When the feeder is supplying load greater than the capacity of the generator, under frequency and under voltage are expected to occur and UV and UF relays will operate to trip the generator (see figure 12.1). The setting of the under frequency trip (Hz) must be based on the recommendation of the manufacturer.

DG S

Main Intake

DG Plant

Feeder 1

Feeder 2

Feeder 4

Feeder 5

Figure 11.1: Illustration of UV and UF relays operation

When the feeder load is sufficient to be supplied from the generator under islanded operation, UV and UF relays may not operate. Therefore, under the current operational practice, this condition where flow to the feeder at the source will result in sustained islanded operation of the DG Plant with feeder must be avoided by rescheduling the DG export to the distribution network.

If the resulting feeder load could be totally supplied by the generator under islanded operation, this may present a hazard to personnel. Generator damage would be likely when the feeder breaker is reclosed. In distribution systems, feeder breakers are not equipped with dead line check to prevent reclosing on live feeder. An alternative to dead line check relays is an automatic transfer trip that upon opening of the utility feeder breaker, a signal is provided to trip the generator. Any islanded operation required later must be performed based on operation and safety procedures agreed by both the generator and the utility.

If the feeder load to be supplied by the generator is less than the generation, over-frequency will occur and therefore OF relay is required as illustrated in figure 12.2. The setting of the OF relay must also be based on the recommendation of the generator manufacturer.

BR3 BR1

BR2

BR4

BR5

2MW 3MW

Total load 5MW

UF UV 1. BR3 open

2. DG to supply 5MW with 3MW turbine power limit then:

• Underfrequency trip; or/and

• Undervoltage trip.

DG

Figure 12.2: Illustration of OF relay operation

OV relays are installed on the DG side to protect against over-voltage resulting from a sudden loss of load. However, the generator voltage regulator will take care of the over-voltage by reducing excitation. Therefore, the over-over-voltage relay would be useful when the voltage regulator is defective or limited that it would result in sustained over-voltage.

Transient over-voltages due to switching or lightning should be catered for by the design of the distribution and DG systems insulation.

To prevent damage on the prime mover due to motoring of the generator during reversal of power, RP or directional relays are installed. Time delay must be incorporated to prevent nuisance tripping during synchronization of the generator.

Combined over current and earthfault (OCEF) relays are employed for protection of over-current and earthfault in both directions. IDMT relays equipped with instantaneous trip are used in this case. For large generators provided with its own unit protection, the OCEF relays are used as backup for the generator internal fault. On distribution system fault, both generator OCEF and feeder OCEF would see these fault currents. Coordination of the generator OCEF relays with that of feeders would become more difficult due flow of fault currents from both sources into fault. It is normal practice that on a distribution fault on the feeder, the feeder OCEF is allowed to trip first followed by the generator. If the fault is cleared and the generator operates in isolation then frequency and voltage relays would likely to operate depending on the generation-demand balance. However, if inter-trip is provided, the generator would also be tripped out.

Loss of synchronism manifested into generator over-speed or under-speed that would be detected by the generator mechanical speed relays.

BR3

2. DG to supply only 1MW with 3MW turbine power:

• Overfrequency trip.

Field failure (FF) relays is employed to detect malfunction of the generator excitation field.

Upon loss of excitation, the generator rotor accelerates to above synchronous speed where it continues to generate power as an induction generator. Loss of field is normally detected by an undercurrent relay connected to a shunt in the field circuit.

Negative phase sequence relays are employed to detect excessive unbalanced loading of the generator.

To summarise, the requirements of protection are as follows:

i) Undervoltage (UV) ii) Over-voltage (OV) iii) Under Frequency (UF) iv) Reverse Power (RP) v) Over-current (OC) vi) Earthfault (EF)

vii) Step-up transformer differential protection - above 5MVA requires unit protection (TD)

viii) Loss of system synchronization/Field failure relay (FF)

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Figure 12.3 shows the functional details of the required DG protection scheme. All the relaying code used follows the TNB protective device code.

Figure 12.3: Distributed Generator protection schemes

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“The Protection and Control: Code of Practice (2^nd Edition)” was launched in May 2003 as a main guidebook to standardize the policies, schemes and practices on the protection, control and their supporting equipment for all TNB core business, Generation, Transmission and Distribution Divisions.

All TNB primary equipment shall be protected against damages from any type of faults. For all external parties such as IPPs, co-generators and bulk customers, their primary equipment shall also be protected against damage from any type of faults and shall follow the setup and schemes specified as in the guidebook.