The electrical power cannot be stored in large quantities and consequently, it has to be generated at the same time as the consumers needs it. The generation of electrical energy is performed by means of high power synchronous machines or alternators, whose construction design depends on the type of drive/prime movers i.e. gas, steam, water or by any motor AC/DC. Then the synchronous generator must be connected in parallel with a constant voltage and frequency, on reaching its nominal speed, excitation voltage and current may be increased from zero until the stator voltage is brought up to the same level as that of the network. To obtain this situation the magnitude, the phase rotation and the rotational direction of the two voltages may be achieved. This procedure is termed as synchronization. AC electrical power is generated in power stations, usually far from end users. This power is then transported over long distances using high voltage and low loss transmission lines. High voltage is achieved by using step up transformers and is fed to the transmission lines. Similarly, at user end, step down transformers are used to provide power to the users. This is possible only by using transformers. Transformers are used for stepping up the voltage of the generator to values which are suitable for high voltage systems, for power exchanging between networks, for stepping down the voltages to the medium voltage level and then for feeding the power into the low voltage network. In this laboratory a three-phase transformer is investigated. It consists of three individual poles with different connection possibilities on the primary side and variable secondary voltage. Power transmission lines are used to transmit electrical energy from the power stations to the consumers. Advantage of three-phase systems is that it provides the consumers with two different levels of voltage, so that they can use their equipment in the best possible way. A three-phase model of an overhead power transmission line (with a simulated length of 360 km long, a simulated voltage of 380 kV and a simulated current of 1000 A) is used, with a scale factor of 1:1000. Transmission line characteristics are investigated under various load conditions. Circuit configurations are then connected for the demonstration of various neutral point connections in three-phase mains systems. Different voltage levels are used for transmitting power; the levels are determined by the amount of power and the distance; the higher the transmission voltages, the lower the currents as well as the transmission losses. However the network investment cost increases with the voltage. Asymmetrical short-circuits are also simulated. Other topics covered by this laboratory are reactive power compensation, the basic circuits of power engineering, series and parallel connections of operating equipment (lines, transformers), circuit involving the conversion of delta connections to star connections, circuit involving the conversion of star connections to delta connections, busbars, disconnectors, power circuit breakers, voltage and current transformers. In an Electrical Power System currents and voltages must be monitored constantly to make sure that the values remain within the required limits. This will also switch off the faulty sections from the main supply network. It is important to Electrical Power Technology identify the fault and to isolate the faulty section automatically from the network because this will result in collapse of the entire electrical power system. These protection systems need to be fast and reliable in events of fault in order to avoid any large scale damage. It is important that the protection system identifies and isolates the faulty section without affecting the remaining power system. A number of protective relays are analyzed: under/over voltage time relays, definite time over-current relays, inverse time over-current relays, combined over-current and earth-fault relay, directional relay and earth-fault relays. Large energy consumers like industrial plants, processing plants and manufacturing units are required to provide compensation for reactive power used by them. Modern electrical control circuits also generate harmonics in the network. This laboratory addresses the topics like reactive power compensation, reactive power controllers, measurement of
electrical energy in three phase networks, cost of power consumed by the customers. These topics are analyzed from the theoretical point of view and also by means of practical examples.
Experiments:
Alternator and Parallel Operation Experiments:
- Determination of the effective resistance of stator and exciter windings of the alternator.
- Determination of the mechanical and iron losses of the alternator.
- Recording the open-circuit curve at various speeds.
- Determination of the ohmic and stray losses of the alternator.
- Recording the short-circuit curve at various speeds.
- Calculating the synchronous reactance.
- Recording the response of the alternator
- operating with the excitation and speed kept constant under different types of load.
- Recording the regulation characteristics at different power factors.
- Determination of the conventional efficiency of the alternator using the open-and short-circuit test results.
- Becoming familiar with various lamp circuits used to connect an alternator in parallel to a constant voltage constant-frequency system.
- Parallel operation using a syncnhrono scope.
- Response of the alternator on a constant-voltage constant-frequency system.
Three-Phase Transformer Experiments:
- Determination of the vector group of the three phase transformer.
- Determination of the voltage transformation ratio of the transformer operating at no-load.
- Determination of the current transformation ratio of the transformer operating with shortcircuit.
- Determination of the equivalent circuit quantities based on the consumed active and reactive power.
- Measurement of the effect of the load type and magnitude on the performance of the secondaryvoltage.
- Determination of the efficiency of the transformer.
- Investigation of the zero-impedance of the three phase transformer with various connection modes.
- Examination of the load capacity of the secondary side using a single-phase load with different connection modes on the primary side.
- Determination of the influence of a delta stabilizing winding.
- Demonstration of the possibility of utilizing a, three-phase transformer in economy connection (auto-transformer).
Overhead Line Model Experiments:
- Measurement of the voltage in no-load operation.
- Concept of operating capacitance.
- Line model with increased operating capacitance.
- Measurement of current and voltage relationship of an overhead line in matched-load operation, interpretation of the terms: characteristic wave impedance, Lagging and leading operation, Efficiency and transmission losses.
- Measurement and interpretation of the current and voltage ratios of a transmission
- Measurement of the earth-fault current and the voltage rise of the fault phases.
- Determination of the inductance of an earth-fault neutralizer for the overhead line model.
- Investigation on the performance of a transmission line with a fault and comparison of the current values with those determined during earth-fault with isolated neutral point system.
- Measurement of the fault currents of the results with those for a three-phase fault.
- line during a three-phase short-circuit.
- Measurement and interpretation of the current and voltage ratios of a transmission line with mixed ohmic-inductive and pure inductive loads.
- Measurement and interpretation of the current and voltage ratios of a transmission line with mixed ohmic-capacitive and pure capacitive loads.
- Investigation on the performance of a transmission line with isolated neutral point connection in the case of a fault to earth.
- Investigation on the effect of parallel compensation on the voltage stability at the load
and the transmission losses of the line. - Investigation on the effect of series compensation on the voltage stability at the load.
- Use of measurement techniques to determine the zero-phase sequence impedance of the overhead line model and comparison of this value with the theoretical one.
Alternator and Parallel Operation Experiments:
- Measurement of the voltage distribution in the series connection of two lines without operating capacitance.
- Measurement of the voltage distribution in the series connection of two lines with operating capacitances.
- Measurement of the voltage distribution in the parallel connection of two lines without operating capacitances.
- Measurement of the voltage distribution in the parallel connection of two lines with operating capacitance Busbar System Experiments:
- Operation of a switching station with two busbars and different voltage.
- Busbar transfer with interruption of the power supply to the consumer.
- Busbar coupling and bus transfer without interruption of the power supply to the consumer.
- Switching sequence for disconnectors and power circuit breakers.
Instrument Transformers Experiments:
- Determination of the transformation ratio of a current transformer for various primary currents and investigation on the influence of the load on the transformation ratio
- Explanation of the terms: ratio error (current error), accuracy class and rated accuracy limit factor
- Test on the performance of the current transformer at over-current.
- Assembly of the common current transformer circuit for measurement on three-phase network
- Measurements of the zero-phase sequence current of a three-phase system
- Measurements on a summation current transformer
- Determination of the transformation ratio of a voltage transformer for various primary voltages and investigation on the influence of the load on the transformation ratio
- Explanation of the terms: ratio error (voltage error) and accuracy class
- Assembly of the common voltage transformer circuit for measurements in three-phase network
- Measurement of the residual voltage in a threephase system with a fault to ground
- Assembly of a voltage transformer circuit in open delta connection
- Measurement of the three conductor voltages on symmetrical and asymmetrical loads
- Demonstration of the principle of differential protection
Generator Protective Experiments:
- Overcurrent protection
- Over-voltage and under-voltage protection
- Over-frequency and under-frequency protection
- Unbalanced load protection
- Stator-earth fault protection
- Reverse power protection
Protection of HV Line Experiments:
- Instant time overcurrent protection
- Definite time overcurrent protection
- Inverse time overcurrent protection
- Earth-fault protection
- Under voltage and overvoltage protection
- Unbalanced power protection
- Directional power protection Protection of parallel connected lines Transformer Protection:
- Time overcurrent protection
Power Factor Improvement Experiments:
- Demonstration of the manual operation on the control of reactive power at various inductive load
- Demonstration of the automatic operation on the control of reactive power at various inductive loads and at different sensitivities
Energy Meters and Tariffs Experiments:
- Demonstration of the measurement of active energy consumption
- Demonstration of the measurement of reactive energy consumption
- Determination of the meter’s constant
- Demonstration of the measurement of the maximum demand
- Demonstration of load cut-off operation
Protection of parallel connected lines
Transformer Protection
- Time overcurrent protection