Interconnection of large electric utilities with small-dispersed power supply sources is practiced in a number of countries; except power poor Bangladesh! In Bangladesh, unfortunately we have not interconnected dispersed small individual power plants to our national grid. This is an overall loss to the nation of at least over 200MW of readily available power for meeting power shortage, particularly during maximum demand period, without any major capital investment to connect these existing skid-mounted units to our power grid! It is indeed a shame for us particularly with our perennial power shortage.

The situation reflects the lack of technical competence and sagacity of our power ministry and the various organizations under it for failing to utilize this resource already available at our doorstep!

This article reviews some of the protective guidelines for power grid integration, which is needed for a stable power system. We must at the earliest incorporate and maximize power availability by harnessing all sources of electrical power be it a conventional (engine driven) photovoltaic, wind or solar based. The issues discussed here are based on studies carried out in the US, which has a number of large independent, but interconnected electric utilities. Small isolated power sources are also connected to these utility grids as potential power resources available on tap.

Interconnection Guidelines
The guidelines for safe and effective interconnection with isolated generation sources typically have to satisfy the following objectives.
• Protecting utility system from short circuit current produced by isolated generation units during utility system faults.
• Avoiding “islanding” when an isolated generating source and a part of the utility operates in isolation from the overall utility system.
Islanding may occur when a fault is detected by the utility system and not the isolated generator, or due to accidental opening of utility supply due to equipment failure, human error or malicious intent. Utilities avoid islanding for the following reasons:
• Utility system looses control of voltage and frequency in the island environment. This could lead to potential cause of damage to customer’s equipment with the utility having no control over it.
• Islanding may be an interference in restoring normal service by the utility system.
• Islanding could create unwanted accidents to system line-workers through energizing a line that should other wise be de-energized.
• Reclosing utility breaker to an island could re-trip the breaker and /or damage the isolated generating unit due to out-of-phase breaker closure.

Voltage and Frequency Protection
Most utility require under and over voltage and frequency protection in their setup. Some utilities may accept proven internal protection of the isolated power source. For small isolated units some utilities may omit under voltage requirements, if the isolated units output is of very small capacity, compared to the large system power capacity source or sources.
The protection is basically an islanding protection step, which can be looked at in two ways:
a) When voltage and frequency “window” for the isolated generation unit is very narrow, then the probability of an islanding situation with the isolated generation source practically disappears. In this case the frequency tolerance is maintained at ±0.5 Hz.
b) If values of the voltage and frequency window will not cause any known damage to customer’s equipment even under islanding. Where these views are accepted, the frequency tolerance may be between ± 2 to 3Hz.

Reverse Power
Islanding detection can be done through reverse power sensing at the isolated generation site. This could be of various types for tripping for excessive power coming form the isolated generation site. Excess power flow from the isolated site beyond its maximum rating is an indication of an islanding situation.

Reverse power flow is a simple, one-function protection that can detect an islanding situation. Usually it is a part of the main breaker set up where the switchgear has a current transformer measuring the main breaker current and reacting to it.

Primary Ground Fault Protection
A separate ground fault sensing device may be needed for a larger capacity isolated generating unit in relation to the local grid. Basically this requirement is another issue related to islanding. However if
islanding itself can be prevented through other measures then the requirement for a separate primary ground fault detection system may be superfluous. Similarly re-closing blocking is not needed if formation of islanding situation is prevented.

Relay Testing
Field-testing of relay is an established practice in good power system operation. This is a must where the system is based on electro-mechanical relays. These devices need periodic cleaning, contact burnishing as well as occasional recalibration to protect setting drift.

Microprocessor-based protective relays which came in the 1980’s don’t require maintenance and its setting does not drift. However till a better level of confidence develops; it may be worth to test the microprocessor based relays; may be at longer intervals.

The Inverter Approach
Based on the above discussions, it is quite possible to define an inverter based protection system that could meet most of the utility needs for the isolated generation unit. The control features to be made available could be:
• Over and under voltage and frequency drift with adjustable set points and time delays, protected by passwords or other means to ensure unnecessary tampering.
• Dedicated protection for detecting islanding not based on frequency setting alone and which does not trip at low frequency’s between 46 to 50 Hz.
• Isolating protection circuits from the power circuitry to ensure protection for testing protective function easily.

To incorporate inverter based protection one needs to be aware of following relevant characteristics of inverters. These are fault current delivery, tendency to run on, harmonic content and results of failure in the inverter.

Conclusions
Utilities generally need over and under voltage and frequency protection for interconnection with isolated generation units. If this is part of any inverter set-up, it must be on site settable and testable.

The other imposed requirements are meant to deal with islanding situation. For inverters if it is not included in the inverter set up, it needs to be separately incorporated for protections against unwanted islanding.
Standards needs to be uniformly set up for all power producers which are interconnected to dispersed isolated power sources. These may somewhat vary between utilities, but by and large the basic protection protocol has to be similar to ensure that the interconnected system is properly protected.


S. A. Mansoor: Director Engineering, Partex Group