A feasibility study determines whether the proposed system upgrades, future upgrades and present distribution equipment will meet the present and future system requirements.

This study includes, research and evaluates the different voltage levels available from utility. Own generation may be considered in some cases. Voltage levels, UPS loads, SWGR double-end-feedings, emergency systems, MCC schedules, transformer location, will be considered. Facility owner will be advised of the reliability of the different distribution schemes, and the possible impact in the production process.

A short circuit study is performed to determine the maximum fault currents that would be present in the power system during a system disturbance. As plant expansion occurs, loads may be moved and larger ones added, leading to increased levels of available short circuit currents. In addition, the power company supplying the plant may have increased the available fault capacity due to the enlargement of its own system. The possibility of increasing the amount of short circuit current available into a fault by these changes is the major reason for a periodic system study.

If the short circuit capacity of the system exceeds the capacity of the protective device, a dangerous situation exists for both plant personnel and system equipment. Under fault conditions, the protective devices would attempt to interrupt the fault current, which could cause a violent failure. Therefore, the need and importance of determining the short circuit capabilities of a system cannot be stressed enough.

The protective device evaluation study compares the calculated short circuit currents with the manufacturer’s published withstand and interrupting ratings for a given protective device (circuit breaker or fuse). This comparison is necessary to verify that the system components will not be damaged when subjected to larger currents than they are expected to handle. The evaluation will pinpoint any problem areas, and corrective action then can be planned. Usually several alternatives exist to correct system deficiencies.

Proper coordination of protective devices should be a built-in feature of a system design. Protective device coordination, also known as selectivity, is the ability of the closest upstream protective device to detect and clear the system fault without the operation of another protective device further upstream in the power system. As backup protection, if the closest device fails to operate for some reason, the next set of protective devices should be coordinated so they will operate before extensive damage results. Many cases of unexplained outages or heavily damaged equipment are accepted without question or knowledge that the real reason may be improper coordination of protective devices.

A poorly coordinated system will result in nuisance outages and greater damage to apparatus when a system fault does occur. Unnecessary costs are added to this already bad situation due to unnecessary downtime and equipment repair or replacement.

Proper coordination usually means the proper specification of settings for adjustable protective relays. Device coordination also means choosing the correct type of device to begin with, or the substitution of the actual device if this existing device cannot be set for the proper result.

There are many considerations to starting a motor other than effectively connecting it to the line voltage. Nuisance tripping and excessive running currents, as well as dimming of lights, are signs that a power system is not performing properly.

During starting, an AC induction motor or AC synchronous motor will draw greater-than-normal running current, typically about 600% of rated full-load current and will last as long, although diminishing in magnitude, as the motor takes to come to full speed. If the motor is started with a mechanical load connected to the shaft, inrush current will be drawn for a longer period of time. However, it will not be greater in magnitude than if the motor was started with no load.

The power system should be able to supply inrush to any motor on the system while supplying normal service for the rest of the system. If the system does not have sufficient capacity, there will be excessively low voltage drops and insufficient capacity for motor starting.

A load flow study is especially valuable for a system which involves multiple load centers. The load or power flow study is an analysis of the system’s capability to adequately supply the connected load. The study will provide useful information about real and reactive power flow, bus voltages, and power factor in each branch of the system.

1. Other types of information gained in a load flow study are:
2. The optimum tap settings for power transformers.
3. The need and justification for shunt capacitors or synchronous motors to supply the reactive power requirements.
4. Optimum size and location of power factor correction equipment.
5. The possibility and extent of overloads on transformers, generators, and tie circuits during normal and emergency conditions.
6. The present load on each feeder for consideration and location of future loads.

The load flow study, like all system studies, usually is performed on a digital computer, which produces a printout that lists voltage, megawatt (MW), and megavar (MVAR) values at each bus location. The total system losses, as well as individual line losses, also are tabulated. Transformer tap positions are selected to insure the correct voltage at critical locations such as motor control centers.

A system stability study is appropriate for systems with large amounts of generation. This type of study determines the ability of ability of local generation to stay in synchronization with the remainder of the power system during and shortly after a system disturbance. Emergency generators generally are not considered in stability studies.

This study is essential when adding or upgrading generators within the facility and is an integral part of the generator specifications. This study may evaluate overcurrent relay settings and or modifications to the protection scheme associated with the generators and utility. This time-based analysis will determine relay settings that will allow the generator out-of-step protection to clear the faulty condition before stability is lost or the over-current relays on the utility operate.

A harmonic study can determine whether harmonic currents will cause problems. Variable frequency drives (VFD), uninterruptible power supplies, and any power semiconductor equipment produce high frequency currents (harmonics) and have an inherently poor power factor. Ironically, shunt capacitors, which are the usual remedy for poor power factors, sometimes are adversely affected by the harmonic currents that flow in a power system when either VFD or rectifiers are used.

The harmonics can overexcite the capacitors and the system’s inductances under certain conditions. The net result could be failure of electrical equipment due to overheating or overvoltage, as well as noise in telephone and computer communication circuits. From a harmonic study, recommendations can be made concerning the need to install power line filters or to rearrange equipment in order to separate harmonic sources from appliances.

The single-line diagram is updated to accurately illustrate the power system; in fact, it is an important maintenance document in any plant. When any change or addition is made to a power system, the one-line diagram should be updated immediately to show that change. All people concerned with the maintenance and operation of the electrical system should be given revised copies on a regular basis. Single-line diagrams should be reviewed and updated periodically.

The results of detailed engineering studies are of no value unless the results are implemented within the system. In cases where miscoordination for low voltage power breakers is of primary concern and where the devices are in good mechanical condition, retrofitting the older-type series or dashpot trip units with modern static trip units is very beneficial. The modern static trip units are much more reliable, have a narrower bandwidth in their time-current characteristics and settings, and are not affected by vibration, dirt, or aging of oil or elastic parts. They make the breaker less susceptible to contact chatter and are tested and calibrated easily.