Power System Analysis

Thursday, 1 December 2011

Harmonic Measurements, Analysis and Filter Design

Harmonics in power systems can result in undesirable influence such as Capacitor heating/failure, Telephone interference, Rotating equipment heating, Relay mis-operation, Transformer heating, Switchgear failure, Fuse blowing. The main sources of harmonics in power system are static power converters, arc furnaces, discharge lighting and any other load that requires non-sinusoidal current. In order to limit the harmonic current propagation in to the network, harmonic filters are placed close to the source of the harmonic currents. Harmonic filters provide low impedance paths to harmonic currents and thus prevent them from flowing into the power network. Harmonic analysis program computes indices such as total voltage harmonic distortion factor at system buses to evaluate the effect of the harmonic sources and to evaluate the effectiveness of the harmonic filters. Also, driving point impedance plots of the buses of interest are generated to identify whether series or parallel resonance phenomenon occurs at any harmonic frequency of interest.

Approach to Harmonic Analysis

In the first step, existing and functional networks harmonic current measurements are performed at selected points to identify the harmonic currents injected into the network by the harmonic sources. These measurements reflect harmonic currents for one operating configuration and the loads prevailing at the time of measurements only. These may not represent conservative estimates of harmonic currents available.
In the second step, the measurement information of the first step will be used along with design data of harmonics (where available) from non-linear loads generating harmonic currents. A computer network model will be prepared as per IEEE standards and the effect of various harmonic sources at various harmonic orders will be examined. Various harmonic distortion factors will be computed as outlined in relevant IEEE standards. The advantage of computer model and simulation is that it can take care of large number of operating configurations and conservative estimates of harmonic currents, which cannot be covered by field measurements. Field measurements of the first step can however be used to validate the computer model developed.
In the third step, harmonic driving point impedances of all buses of interest will be generated at various harmonic orders and plots of the driving point impedances will be generated with respect to a range of harmonic orders [orders 1 through 50]. These plots indicate series and parallel resonance conditions in network.
In the fourth step, analysis of results of the first 3 steps will be carried out and solutions needed to solve any harmonic related problems will be obtained. These solutions are verified by using the computer model developed. The problems that might arise could be excessive harmonic distortion factors beyond relevant IEEE specified standards, existence of resonance conditions close to an exciting harmonic frequency. Where these problems are encountered, solutions will be provided by introduction of harmonic filters and its design will be verified again by using the computer model developed. Recommendations include specifications on sizing of individual components of the harmonic filters.

Switching Over Voltage Study

Switching Over Voltage Study is based on computer transient simulation using the EMTP/ATP program. Switching under different loading conditions and load rejection at important 400 kV and 220 kV buses should be considered. The influence of surge arresters on the switching surges will also be investigated through conventional modeling. Simulation plots will be assessed and the worst switching conditions will be reported.
These studies are generally performed to assess the transients associated with:

Energization of overhead transmission lines and study of associated transients, surge arrester ratings, transient mitigation methods.
Energization of capacitor banks / reactors in industrial or public utility facilities.
Transients associated with various switching actions such as fault application and clearance.

Lightning Over Voltage Study
Lightning over voltage will be determined mainly by transient power system analysis [EMTP studies] , including statistical switching calculation taking account of the time difference between first and last pole to close or open.

Grounding Mat Design
The grounding study is strictly based on the procedure recommended in IEEE std.80 or IEC 479 for safety analysis.

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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

Protection and Relay Co-ordination Studies

Overcurrent phase and earth fault relay coordination is necessary to achieve proper fault identification and fault clearance sequence. These relays must be able to distinguish between the normal operating currents including short time overcurrents that may appear due to certain equipment normal operation (example: Motor starting currents, Transformer inrush currents) and sustained overcurrent due to fault conditions. During fault conditions, these relays must operate quickly isolating the faulted section of the network only and allowing for continued operation of the healthy circuits. In the event of failure of primary relays meant for isolating the fault within its primary zone of protection, backup relays must operate after providing for sufficient time discrimination for the operation of primary relays. Hence, the operation of backup relays must be coordinated with those of the operation of the primary relays. The flexible settings of the relays (namely plug or tap setting, the time dial setting and possibly selection of suitable time-current operating characteristics), must be set to achieve the objectives stated in this section.

Once the relays are coordinated, the discrimination in the operation of primary and backup relays and their coordination with the maximum possible load currents will be plotted on the time current characteristics (TCC’s). The results of the relay coordination will be provided in the form of recommended settings and TCC’s. Relay coordination needs to be evaluated for maximum and minimum fault conditions and for various possible network configurations. Where a network has several levels of primary and backup relay levels, the source end relay operation can become quite delayed due to successive time discrimination at down stream load end coordination levels. In such cases it may be necessary to ensure isolation of fault at the earliest by possibly coordinating the source end relays with much faster dedicated equipment relays in the down stream (example differential protection of transformers).

Relay coordination may need to provide suitable instantaneous settings where it is possible that instantaneous settings can positively discriminate between the faults in the primary and subsequent zones. This is essential to ensure that the instantaneous settings will not act for faults outside its primary protective zone bypassing the needed discrimination between the primary relay to which it is a backup.

The following protection aspects will be covered in the study

Overcurrent Phase (51, 50) and Earth (51N, 50N) Fault Protection – Relay coordination with High set settings, and definite time releases. Coordination will also consider coordination with maximum load currents, motor starting current and motor starting time; transformer inrush currents, safe stalling time and current of motors, thermal withstand characteristics of the equipments.
Motor protection
Transformer protection
Unit Protection
Distance Protection

The complete protection system study of the plant will be performed considering the following criterias:

Accepted Standard Engineering practices in protection
Applicable IEEE standards
Information available in the relay application guides, and relay catalogues of the relay manufacturer. This criterion will be generally more used in determining relay settings in comparison to the other methods specified for equipment protections.

Evaluation of Load Shedding Schemes

The existing and proposed load shedding schemes will be evaluated using the transient stability simulation, with dynamic models of generators, controllers and loads. The following factors will be considered in evaluating the load shedding schemes and providing the recommendations

Governor/prime mover controllers will be modeled in simulation
Spinning reserves will be modeled appropriately in the governor/prime mover model
Frequency deviation, Rate of change of frequency deviation, Bus voltage variations as function of time will be considered
The power balance factor will also be considered in designing and evaluating the load shedding schemes.

Load shedding is essential only during under conditions with frequency still decreasing and never to be used when frequency is recovering or increasing. This factor will be considered appropriately while designing load shedding schemes.

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

Plots of dynamic response of generator rotor angles, frequency, power outputs, voltages, excitation system outputs and governor prime mover outputs
Plots of Lines, transformer flows, bus voltages, bus frequencies
Plots of the system variables that are of interest from protection point of view like frequencies, fault current seen from over-current relays,etc
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