What is Grid islanding scheme ? Why it is required ?

In recent years, industrial system are getting advanced and increases their capacity day by day. In order to meet the process and production requirement more power is needed. As result to run continuous process industries are usually parallel their in house generator with local grid to fulfill the requirements.

The main objective to connect grid in parallel with in-house generator are as under,

• To Black start of island (Plant),
• To Exporting / Evacuating extra generated power from generator to grid,
• To Meet the extra load requirement of plant,
• To cater fluctuating load of plant,
• Fulfill the momentary requirement of heavy load like HT Motors, Rectiformers.

However, The connecting grid in parallel with in house generation have it own benefits, at the same time one shall keep in mind if proper protection is not provided, generator may have subjected risk and chance of damage during disturbance, fault, and any switching operation take place in grid.

These disturbances may lead to frequency and voltage collapse of the system, in worst case cascade tripping leading to complete blackout of plant.

What is grid islanding scheme ?

In order to safeguard our in house generation during grid side disturbance dedicated protection scheme is very much essential. The process of disconnecting the Plant from the Utility (during parallel operation) in case of Grid disturbances is called Grid Islanding Scheme.

In simple word, Grid Islanding scheme means as the name indicates it isolates grid from our island (Power plant) in case of any grid side disturbance or fault.

Fig : Typical Grid Islanding scheme

The causes of disturbances in grid network may be,

• Load and Generation imbalance
• Load Fluctuation in grid
• Loss of grid power / mains power
• Large switching operation
• Symmetrical or Asymmetrical fault in infinite grid
• Voltage dip for a long duration of 1s or more or Rise in voltage above the normal value along with zero real power import from grid to the plant.

It is essential to isolate the island from the grid as quickly as possible to save the generator and production of plant.

Why grid islanding is required ?

Grid Islanding scheme is a set of protective relays, connected at the grid incomer- these relays will sense a disturbance in the grid and give a trip command to the incomer breaker whenever the grid disturbance exceeds a set limit. By opening the incomer breaker, the plant is isolated from the disturbed grid and we can keep our generator safe.

Selection of grid islanding relay is very vital process as it decides the proper operation of grid islanding. The selected protective device for grid islanding scheme shall having protection function like directional over current & earth fault Protection, Reverse power protection, voltage vector shift protection, rate of change of frequency protection, under / over voltage protection, under / over frequency protection, negative phase sequence protection, residual over voltage protection.

Important points to be kept in mind while implementing and engineering of grid islanding scheme,

• Condition of parallel operation to be established in protective relay via circuit breaker contact (52a)
• Grid islanding setting shall not operative when grid is operating in solo mode
• Grid islanding scheme shall be take place within safe limit of Critical Clearing Time (CCT) of the generator which derived through transient stability study
• Frequency, voltage and overcurrent protection functions of grid islanding relay shall be coordinated with generator protection relays. At the same time it is important to ensure that grid islanding scheme shall be inoperative for plant Internal faults, and transient conditions. Also, all grid islanding protection setting are require to be fine tuned at site.
• Changeover of governor and AVR mode is envisage followed by grid islanding operation.
• After grid islanding our main focus shall be shifted on to meet generation and load balance (plant which are importing power from grid). It can be taken care by cutting down non critical load of the system through dedicated load sheeding scheme.

Hence, It is required to have dedicated protection scheme for industrial plant which have parallel connectivity of grid with in house generator, to keep our generator safe from grid side disturbances. Also, after grid islanding our concern shall be shifted to match load generation balance using dedicated load shedding scheme by isolating non-critical load from generator which prevent generator from overloading.?

Reference:

1. IEEE paper on Islanding and Load shedding schemes for captive power plants by K. Rajamani and U. K. Hambarde
2. Protection Relay Application Guide - Micom P341 Schneider Electric make