Insulation Co-ordination Studies

Insulation Coordination is the process of determining the proper insulation levels of various components in a power system as well as their arrangements. It is the selection of an insulation structure that will withstand voltage stresses to which the system or equipment will be subjected to, together with the proper surge arrester. The process is determined from the known characteristics of voltage surges and the   characteristics of surge arresters.

        The following standards are used by consultants, while performing the insulation coordination:

• Insulation Co-ordination, Part 1: Definitions, principles and rules IEC 71-1, standard.
• Insulation Co-ordination, Part 2: Application guide IEC 71-2, standard.
• IEEE Guide for the Application of Insulation Coordination. IEEE Std 1313-2-1999

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Motor Starting Studies

The starting current of most AC motors is several times the normal full load current when starting them directly on line at full rated voltage. The starting torque varies directly as the square of the applied voltage. Excessive starting current causes a drop in terminal voltage which may result in the following:

• Failure of motor starting due to low starting torques.
• Unnecessary operation of under voltage relays.
• Stalling of other running motors connected to the network.
• Voltage dips at the power sources and consequent flicker in the lighting system.

Motor starting studies can help in the selection of best methods for motor starting, motor design, and system design thereby minimizing the impact of the motor starting.

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Transient Stability Analysis

The recovery of a power system subjected to a severe large disturbance is of interest to system planners and operators. Typically the system must be designed and operated in such a way that a specified number of credible contingencies do not result in failure of quality and continuity of power supply to the loads. This calls for accurate calculation of the system dynamic behavior, which includes the elector-mechanical, dynamic characteristics of the rotating machines, generator controls, static var compensator, loads, protective systems and other controls. Transient stability analysis can be used for dynamic analysis over time periods from few seconds to few minutes depending on the time constants of the dynamic phenomenon modeled.

The dynamic performance of the system with respect to the disturbance listed below, but not limited to, shall be studied for the following cases:

• 3 phase  fault 
• Unbalanced fault 
• Fault clearance 
• Loss of transformers 
• Loss of lines
• Loss of loads
• Loss of generators
• Load shedding
• Any other relevant contingency

Report and Recommendations from Transient Stability studies

1. Plots of dynamic response of generator rotor angles, frequency, power outputs, voltages, excitation system outputs and governor prime mover outputs
2. Plots of Lines, transformer flows, bus voltages, bus frequencies
3. Plots of the system variables that are of interest from protection point of view like frequencies, fault current seen from over-current relays,etc
4. Recommendation related to protection and control, operating strategy, Load shedding schemes,  control settings of equipments ( for example: power system stabilizer, relay settings etc), based on various study cases considered

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Short Circuit Studies

Short Circuit Studies is critical for the safe, economical and efficient operation of all electrical power systems. In case of a short circuit, excessive electric current will flow through the system and could have disastrous consequences including personnel injury, damaged electrical equipment as well as costly downtime. Short circuit calculations provide currents and voltages on a power system during fault conditions. This information is required to design an adequate protective relaying system and to determine interrupting requirements for circuit breakers at each switching location. Fault conditions can be balanced or un-balanced shunt faults or series (open conductor) faults. Often information about contributions to a fault from rotating machines such synchronous machines, large motors would be required as a function of time to determine making and breaking requirements.

Recommendations and guidelines are given in the IEEE Violet Book (IEEE STD 551-2006) as well as North American ANSI C37.5, ANSI C37.010, ANSI C37.13 and International IEC-60909 guidelines. It also supports conventional short-circuit studies without reference to any particular standards.

The results of these studies are provided in tabular form giving details of making and breaking fault currents requirements at each breaker locations (bus locations). Further, plots of peak fault current, dc component of fault current, symmetrical RMS component of fault currents will be provided with respect to time. These calculations will be performed for all types of shunt faults such as 3 phase faults, single line to ground fault, line to line faults and double line to ground faults.

Apart from determining fault levels at various buses, short circuit studies are useful in determining post fault bus voltages in the entire system, post fault network currents in the entire system, and negative and zero sequence currents in various electrical network elements. These calculations generally provide most of the information needed for protection system design, protection setting calculations and relay coordination. These studies may be carried out for various operating scenarios of the plant with existing earthing system and the performance of the plant can be reviewed and remedies can be suggested.The results of these studies will be provided in tabular form and also on single line diagram of the plant. These calculations will be performed for all types of shunt faults.

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Power Flow Studies

Power flow/Load flow calculations provide active & reactive power flows, bus voltage magnitude and their phase angle at all the buses for a specified power system and operating conditions. These values are typically subject to various factors like regulating capability of generators, synchronous condensers, static VAR compensator, HVDC controls, FACTS controllers, tap changing under-load transformers and specified net interchange between individual operating systems (utilities). Power flow information is essential for the continuous evaluation of the current performance of a power system and for analyzing the effectiveness of alternative plans for system expansion to meet increased load demand. These analyses require the calculation of numerous power flow cases for both normal, and emergency (contingency) operating conditions.

Applications of Power Flow Study and Analysis

• Transmission expansion planning ,operation planning 
• Distribution expansion planning , operation planning 
• Industrial/Commercial distribution system planning, operational planning 
• Network interconnection, Grid interconnection studies 
• Evaluation of energy transactions between various stake holders 
• Energy audit to accurately determine network losses and estimate billing losses if any 
• Sizing of transformers, cables, overhead lines, transformer tap ranges, shunt capacitors, shunt reactors, reactive power management, FACTS devices, HVDC operation 
• System security assessment via static contingency studies 
• Decision making tool in operation planning and operation of the system in load dispatch center 
• Motor starting studies using load flow type analysis, where the starting impedance of the Induction motor is modeled as constant impedance model with starting impedance. 
• Evaluation of static voltage stability using load flow technique

The following general criteria of acceptability of design is used in power flow studies

1.  Voltage Drop at all buses should be within +/- 5% of the nominal rating for all operating conditions considered 
2. No over load conditions of any electrical circuits for all operating conditions considered 
3. Reactive power generation/import/export to be within specified limits for all operating conditions considered 
4. Ensuring quality power supply to all loads, under specified contingency conditions, as per design philosophy adopted.

The following study cases/ power flow outputs are generally considered in power flow studies

1. Extreme operating conditions of maximum and minimum loading conditions will be considered to check the adequacy of the network, even though some of these conditions may not exist during normal operation 
2. Contingency conditions such as outage of lines, transformers and generators will be considered and network adequacy for power evacuation will be assessed 
3. Operating solutions such as transformer taps, generator excitation, shunt reactive power compensations will be provided as needed. 
4. Recommendations for strengthening and equipment upgradations will be provided to meet specific operating requirements. 
5. Summary of load flow studies and concise reports in tabular formats and single line diagram formats will be provided, along with the summary of recommendations

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