EE 466 Power Systems Planning & Optimization PDF

EE 466: POWER SYSTEMS PLANNING & OPTIMIZATION SECTION 3: TECHNICAL ASPECTS OF POWER SYSTEM PLANNING Dr. E. K. ANTO  3.0: Technical Considerations The various technical aspects must be considered for both normal (steady-state) and abnormal (fau...

EE 466: POWER SYSTEMS PLANNING & OPTIMIZATION SECTION 3: TECHNICAL ASPECTS OF POWER SYSTEM PLANNING Dr. E. K. ANTO  3.0: Technical Considerations The various technical aspects must be considered for both normal (steady-state) and abnormal (faulty) operating conditions The technical considerations include: 1. Modeling of network components 2. Load flow analysis – (has been given a detailed treatment already) 3. Voltage drop 4. System earthing – (has been given a detailed treatment already) 5. Fault analysis – (has been given a detailed treatment already) 6. Overvoltages and overcurrents 7. Estimation of load growth/Load forecasting - (to be given special consideration as a Section topic) 2  MODELLING OF NETWORK COMPONENTS  3.1: Modelling of Network Components (1/1) Modelling is the mathematical representation of the components of the system In the modelling, one makes use of ‘equivalent circuits’ for various components, These equivalent circuits are then combined, in order to represent the interconnection of the components in the actual electrical network. After combining them, network/circuit theories are used in the analyses of the system performance 4  LOAD FLOW ANALYSIS  3.2: Load Flow Analysis (1/1) Normally, load flow studies provide information concerning the system when it is operating under normal or steady-state conditions. With simulation or analytical tools like load flow analysis and contingency analysis, the planner can evaluate the capability of each alternative expansion plan for providing the level of performance desired 6  VOLTAGE DROP  3.3: Voltage Drop (1/2) As far as voltage drop in cables and overhead lines is concerned, it is important to obtain the following information: i. The accurate route lengths for the cables and lines; ii. the maximum continuous current the conductors will have to carry. It should be recalled that the VOLTAGE DROP IS INFLUENCED BY the: i. load current ii. load power factor iii. dimensions ( R and X) of the linking conductor Voltage drop in the circuit must not be overlooked, as this can have a bearing on the conductor size chosen. 8  3.3: Voltage Drop (2/2) It is also important to remember that some electrical equipment, such as direct-on-line (DOL) start squirrel-cage induction motor, can take a starting current that is some 6 to 7 times the equipment’s normal running current. This means that for a short period of time, the voltage drop may be higher than the 5 % permitted by regulation. An excessive reduction in voltage, even for a short period of time, may have an adverse effect on the starting characteristics of the motor loads and in certain circumstances could affect its correct operation. Again, a sudden drop in voltage can have negative effect on electronic equipment. 9  OVERVOLTAGES  3.4: Overvoltages (1/1) Overvoltage is the condition when the peak value of the voltage applied to a device exceeds the limits defined in a standard or specification. Consideration may be given for 3 TYPES OF OVERVOLTAGE: 1. Power-frequency overvoltage 2. Switching overvoltage 3. Lightning overvoltage A look will also be taken at consequences of overvoltages We shall also treat protective measures to address overvoltages 11  3.4.1: Power-Frequency Overvoltages (1/1) By definition, power-frequency overvoltages include all overvoltages with frequency under 500 Hz. These overvoltages may be CAUSED BY THE FOLLOWING: 1) Insulation breakdown faults – (phase-ground faults) 2) Resonance overvoltage by fault – Under certain resonant conditions, internal overvoltages can be produced in a network. 3) Ferroresonance – a rare non-linear oscillatory phenomenon arising from a circuit containing a capacitor and a saturable inductance (e.g. a power transformer). 4) Overcompensation of reactive power 5) Break of the neutral conductor 6) Faults on alternator regulators or tap-changing transformer 12  3.4.2: Switching Overvoltages (1/1) Switching overvoltages are produced by rapid modifications in the network structure as a result of a switching activity, be it for protection in the event of a fault or for maintenance purposes. (For example, opening protective devices, etc.). The following THREE (3) DISTINCTIONS are made: i. Switching overvoltage at normal load. ii. Overvoltage produced by the normal switching operations (intentional switching) in the system. iii. Overvoltages due to the tripping (unintentional, fault- induced switching) in the system. 13  3.4.3: Lightning Overvoltages (1/1) Lightning is a natural phenomenon occurring during storms. Lightning overvoltages are thus due to atmospheric or lightning strikes. When traveling along a line, the voltage waves are attenuated and distorted, and have steep fronts of the order of hundreds of kilovolts per microsecond 14  3.4.4: Consequences of Overvoltages (1/1) The consequences of overvoltages are varied according to the period of application, repetitivity, magnitude, mode and frequency. Some of the CONSEQUENCES OF OVERVOLTAGE include: 1) Dielectric breakdown – causing significant permanent damage to equipment (electronic components, etc.) 2) Degradation of equipment through ageing – repetitive rather than destructive overvoltages 3) Long interruptions – caused by the destruction of equipment (leads to loss of sales for distribution company, loss of production for industrial companies) 4) Disturbances – in control systems and low current communication circuits 5) Electrodynamic and thermal stress 15  3.4.5: Protection for Overvoltages (1/1) 1) A substation can be shielded from direct lightning strokes by a system of overhead earthwires as LIGHTNING ARRESTERS. 2) Protection against transient overvoltages is given by SURGE DIVERTERS: i. that shunt the lightning current to earth but ii. resist the power-frequency current, so that protective devices are not operated and circuits not interrupted unnecessarily. 3) SHIELDING alone does not provide complete protection, and is usually COMBINED WITH SURGE DIVERTERS. On the other hand, conditions sometimes justify the use of surge diverters alone. 4) In areas of low isokeraunic levels (areas with reduced susceptibility to lightning strokes), ROD GAPS are used instead of surge diverters to save cost. 16  OVERCURRENTS  3.5: Overcurrents (1/1) An overcurrent is either an overload or a short-circuit/fault current. The OVERLOAD CURRENT is: i. an excessive current that flows under normal conditions, but ii. one that is confined to the normal conductive paths provided by the conductor and other components and loads of the distribution system. A SHORT-CIRCUIT CURRENT is one which flows outside the normal conductive paths, and usually due to fault or abnormal condition. 18  3.5.1: Overloads (1/1) Overloads are most often between one and six times the normal current level. Overloads may be transient or continuous. Transient overloads are usually caused by harmless temporary surge currents that occur when motors are started-up or transformers are energized. Continuous overloads can result from defective motors (such as worn motor bearings), overloaded equipment, or too many loads on one circuit. 19  3.5.2: Short-Circuits (1/1) The ampere interrupting capacity (AIC) rating of a circuit breaker or fuse is the maximum short-circuit current which the breaker will interrupt safely. This AIC is at rated voltage and frequency. Whereas overload current can occur at rather modest levels, the short-circuit or fault current can be many hundred of times larger than the normal operating current. Typically, a high level fault may be 50,000 A (or larger). 20  EXERCISES  3.6: EXERCISES (1) State any 5 planning objectives that must be considered to ensure a good planning of an electrical power system. (2) Mention any 5 technical considerations that need to be taken in the planning of a power transmission system, and explain the significance of those considerations in the overall planning of the power system. (3) What 3 main categories of overvoltages do you know of? Discuss very briefly how they occur under each category. (4) Enumerate 5 consequences of overvoltages that a planning engineer will seek to avoid. (5) Mention any 4 protective measures for addressing overvoltages 22  END OF SECTION 3 Jan 2014