Power Systems Energy Converters and Drives

Question: Prepare a critical appraisal report comparing the following alternative network reinforcement methods: A) The existing case, no reinforcement, Figures A3A-n (n=1 to 3). ERACS network code: A3A B) A 25 MVAr capacitor, B4Comp, on B4 for reactive compensation, Figs A3B-n. ERACS network code: A3B C) A second 33 kV line, L2, identical to L1 between buses B2 and B3, Figs A3C-n. ERACS network code: A3C D) Use of both methods (C) and (D) above to control B4 voltage, Figs A3D-n. ERACS network code: A3D Answer: Infinite Bus: An infinite bus means a large system whose voltage and frequency remains constant independent of the power exchange between the synchronous machine and the bus, and independent of the excitation of the synchronous machine. A synchronous machine (generating or motoring) while operating at constant power supplies positive reactive power into the bus bar (or draws negative reactive power from the bus bar) when overexcited. An under excited machine, on the other hand, feeds negative reactive power into the bus bar(or draws positive reactive power from the bus bar). The existing radial network is represented with four bus bars on a 100 MVA, 50 Hz base. The supply is represented as an infinite bus, nominal voltage 132 kV, a fault level of 2000 MVA and X: R ratio of 20 (bus B1/INF and generator Inf Bus), which under certain worst-case supply conditions may fall to a voltage of 0.98 pu. A single 132/66 kV transformer (T1) and 66 kV feeder line (L1) supplies the load on B4 at 33 kV via a 66/33 kV step down two winding transformer (T2) between B3 and B4. The design load on B4 is rated at 30 MVA at 0.8 pf lag. Generally, for a load with a power factor of less than 0.95 more reactive powers is required. For a load with a power factor value higher than 0.95 is considered good as the power is being consumed more effectively and a load with a power factor of 1.0 or unity is considered perfect and does not use any reactive power. Thus the existing network and plant are in need of review so that additional load can be supplied at B4 without allowing the voltage at the 33 kV busbar B4, to fall below 1.0 pu. Three reinforcement methods have been considered: A 25 MVAr capacitor, B4Comp, on B4 for reactive compensation or shunt compensation. A second 33 kV line, L2, identical to L1 between buses B2 and B3 Both the above methods to control B4voltage Shunt Compensation: Reactive power can be leading or lagging. While it is the active power that contributes to the energy consumed, or transmitted, reactive power does not contribute to the energy. Reactive power is an inherent part of the total power. Reactive power is either generated or consumed in almost every component of the system, generation, transmission, and distribution and eventually by the loads. The impedance of a branch of a circuit in an AC system consists of two components, resistance and reactance. Reactance can be either inductive or capacitive, which contribute to reactive power in the circuit. Most of the loads are inductive, and must be supplied with lagging reactive power. It is economical to supply this reactive power closer to the load in the distribution system. Reactive power compensation in power systems can be either shunt or series. Shunt capacitors are employed at substation level for the following reasons: 1. Voltage regulation - The main reason that shunt capacitors are installed at substations is to control the voltage within required levels. As the load varies, voltage at the substation bus and at the load bus varies. Since the load power factor is always lagging, a shunt connected capacitor bank at the substation can raise voltage when the load is high. The shunt capacitor banks can be permanently connected to the bus (fixed capacitor bank) or can be switched as needed. Switching can be based on time, if load variation is predictable, or can be based on voltage, power factor, or line current. 2. Reducing power losses- Compensating the load lagging power factor with the bus connected shunt capacitor bank improves the power factor and reduces current flow through the transmission lines, transformers, generators, etc. This will reduce power losses (I2R losses) in this equipment. 3. Increased utilization of equipment- Shunt compensation with capacitor banks reduces kVA loading of lines, transformers, and generators, which means with compensation they can be used for delivering more power without overloading the equipment. Thus as per the values calculated after having 25MVAr capacitor and B4 compensator the values of load/B4 voltage magnitude and of B3 and B2 is .998 to 1.00pu, which is the desired and basic requirement of the sample radial network. Thus the voltage at B4/load is not falling below 1.0pu. Series Compensation: Series compensation is defined as insertion of reactive power elements into transmission lines and provides the following benefits: Reduces line voltage drops Limits load-dependent voltage drops Influences load flow in parallel transmission lines Increases transfer capability Reduces transmission angle Increases system stability Series compensation aims to directly control the overall series line impedance of the transmission line. The AC power transmission is primarily limited by the series reactive impedance of the transmission line. A series-connected can add a voltage in opposition to the transmission line voltage drop, therefore reducing the series line impedance. It is easy to see that capacitor reduces the effective line reactance. Reactive voltage drops of a series reactance added in a line is I2X. It is positive if X is inductive and negative if X is capacitive. So a series capacitive reactance reduces the reactance voltage drop of the line. Thus voltage drop in a line gets reduces (gets compensated) i.e. minimization of end voltage variation and prevents voltage collapse. Thus from the above outputs we can see that the voltage at B4 is maintained at 1.0 pu. So the adding of a same KV line in parallel to the existing line, also minimize the voltage drop in the line. When both the reinforcements are used simultaneously i.e. B4 compensator and a 33KV addition line the load voltage is .996 pu which is nearly to 1.0 pu. But the better results are shown when these reinforcements opted individually. The existing network additional load can be supplied at B4 without allowing the voltage at the 33 kV bus bar B4, to fall below 1.0 pu by implying any of the above methods.