Power Control and Switching
Introduction
Control of electric power, or power conditioning, requires the conversion of electric power from one form to another, that is, DC to AC, one voltage level to another, and so on, by the switching characteristics of various electronic power devices. Some of these devices, such as silicon controlled rectifiers (SCRs), can be operated as switches by applying control voltages to their controlling input (gate terminal of gate- turn-off thyristor (GTO)). Others, such as triggered vacuum and gas gap switches rely on over-voltages or triggering. For large power systems, control is exercised by supervisory control and data acquisition (SCADA) computer systems located at a central location. This master station communicates with remote stations in the system to acquire data and system status and to transmit control information. These centers support the two main objectives of power system control, that is, (1) stable (voltage and frequency) power and (2) economical utilization of the generation capabilities in the system. Typically, system control involves both the generation and load ends of a system. At generating plants this is accomplished by direct control to input to the prime mover, and systemwide it is accomplished by the control centers. Distributing control to the load end provides a localized response to changing requirements at subtransmission and distribution circuits. At this level, faults such as lightning strikes and short circuits can be responded to in a timely manner through relays, circuit breakers, and so forth, thereby protecting system assets.
Development of high-power switches was stimulated by the growth of the electric power distribution networks. Control and protection of these networks involved power handling capabilities in excess of 109 W. Lasers, X-ray equipment, particle accelerators, fusion research, and so forth, have required the switching of power in the form of fast rising pulses and at levels that can exceed 1012 W.
Power control and switching falls into the following general regimes:
✁ Electric power distribution at low frequencies
✁ Pulsed power characterized by fast rising pulses
Switches can be divided into two classes, opening switches and closing switches, with the following characteristics:
✁ Hold-off voltage
✁ Voltage drop during conduction
✁ dv /dt during opening or closing
✁ Trigger voltage requirement, if any
✁ Maximum current
✁ di /dt characteristics
Power electronic circuits can be classified as to function as follows:
✁ Diode rectifiers
✁ DC–DC converters
✁ AC–AC converters
✁ DC–AC converters
✁ AC–DC converters
✁ Switches
In general, these power circuits relate two external systems and, therefore, involve current and voltage relationships at the input and output ports of the power circuit. Thus, control of a power circuit must be defined in context to the external systems to which it is connected.
The switching characteristics of semiconductor devices introduce harmonics into the output of converters. These harmonics can affect the power quality of a circuit and reduction of these harmonics by the use of filters is often necessary. With the increasing utilization of semiconductor devices for power control and switching, the issue of power quality is becoming more important.
Electric Power Distribution
Control of a power system is required to keep the speed of machines constant and the voltage within specified limits under changing load conditions. Control schemes fall into several categories: control of excitation systems, turbine control, and faster fault clearing methods. Design strategies that reduce system reactance are: minimum transformer reactance, compensation of transmission lines using series capacitance or shunt inductance, and additional transmission lines.
One of the prinicipal components of the power distribution system is an AC generator driven by a turbine. Changing load conditions require changes on the torque on the shaft from the prime mover to the generator so that the generator can be kept at a constant speed. The field to the alternator must be adjusted to maintain a constant voltage.
This synchronous machine converts mechanical energy to electrical energy and consists of a stator, or armature, with an armature winding. This winding caries the current Ia supplied by the generator to a system. The rotor is mounted on a shaft that rotates inside the stator and has a winding, called the field winding, that is supplied by a DC current from the exciter. The magnetomotive force (mmf) produced by the field winding current combines with the mmf produced by the current in the armature winding to produce a flux that generates a voltage in the armature winding and provides the electromagnetic torque between the stator and the rotor. This torque opposes the torque of the prime mover, that is, steam or hydraulic turbine.
The basic power circuit is illustrated in Fig. 18.71.
Power from the generator to a system is given by |Vs ||Ia | cos θ , where θ is the power factor angle.
Increasing the exciter current increases E g as shown in Fig. 18.72(a). Note the generator supplies lagging current to the system. In Fig. 18.72(b), the generator is underexcited and supplies leading current to the system, or the generator is drawing lagging current from the system.
If power input to a generator is increased with |E g | constant, the rotor speed will increase and the angle δ will increase. This increase in δ results in a large Ia and lower θ . The generator delivers more power to
Note that for a small δ, a small change in δ results in a larger P than Q.
Reliable electric power service implies that the loads are fed at a constant voltage and frequency at all times. A stable power system is one in which the synchronous machines, if perturbed, return to their original state if there is no net change in power, or stabilize at a new state without loss of synchronization.
The machine rotor angle is used to quantify stability, that is, if the difference in the angle between machines increases or oscillates for an extended period of time, the system is unstable. The swing equation, given by
governs the motion of the machine rotor. J is moment of inertia, δm is mechanical torque angle with respect to a rotating reference, ωm is shaft angular velocity, and Ta is the accelerating torque. Two factors that act as criteria for the stability of a generating unit are the angular swing of the machine during and following fault conditions, and the time it takes to clear the transient swing.
Mechanical torques of the prime movers, steam or hydraulic, for large generators depend on rotor speed. In unregulated machines the torque speed characteristic is linear over the rated range of speeds. The prime mover speed of a machine will drop in response to an increased load, and the valve position must be opened to increase the speed of the machine. In a regulated machine (governor controlled) the speed control mechanism controls the throttle valves to the steam turbine or the gate position for a water turbine. Automatic voltage regulation can be used to adjust the field winding current, thus changing E g as the load on the machine is varied. If the power output of a generator is to be increased while an automatic voltage regulator holds the bus voltage at a constant value, the field winding current must be increased.
The maximum voltage output of a machine is limited by the maximum voltage of the excitor supplying the field winding. Figure 18.73 illustrates control of a power-generating unit.
The performance of a transmission system can be improved by reactive compensation of a series or parallel type. Series compensation consists of banks of capacitors placed in series with each phase conductor of the line and is used to reduce the series impedance of the line, which is the principal cause of voltage drop. Shunt compensation consists of inductors placed from each line to neutral and is used to reduce the shunt susceptance of the line.
Circuit interrupting devices in power systems fall into three categories: fuses, switches, and circuit breakers. Design ratings of these devices are
✁ Power-frequency voltage of the circuit
✁ Maximum continuous current device can carry
✁ Maximum short-circuit current device can interrupt
The operation and control of a power system requires continual analysis of the system parameters to ensure optimal service at the cheapest production cost. Computer studies are essential to this goal. The IEEE Brown book (IEEE, 1980) is an excellent reference source for the power engineer. Stability studies, load flow studies, transient studies, and so forth, are detailed in this source, and the reader is encouraged to refer to this document for detailed information about different aspects of power control.
Pulsed Power
The generation of high-power pulses requires transferring energy stored in electric fields (capacitors) and magnetic fields (inductors). Capacitor charging requires high-voltage sources at relatively modest current ratings, whereas inductive charging requires high-current sources at relatively low volt- ages. Transferring this stored energy to a load re- quires switches capable of holding off high voltages, conducting high currents, and capable of producing pulses with nanosecond or less rise times. An ex- ample of the complexity of controlling high-power pulses is the current effort to generate power with energy derived from the fusion of elements. Ignition of the fusion process requires the simultaneous application of high-energy beams on a single target. Another example of the requirement for fast rising pulses is in the area of a detonator for nuclear weapons. Other applications include lasers, high- energy beams, energy weapons, medical equipment and high-voltage testing of insulators. Solid-state switches, such as SCRs, insulated-gate-bipolar transistors (IGBTs), GTOs, etc., although easy to con- trol, are limited by their voltage/current ratings to the few kilovolt/kiloampere range. Therefore, switches that utilize vacuum, gas, or liquid as the dielectric are commonly used in pulsed power applications. These switches fall into two general categories, opening and closing switches (Fig. 18.74).
There are a number of systems that are utilized to produce fast rising pulses. The basic power energy transfer stage is shown in Fig. 18.75.
The maximum voltage on C2 is given by
where
and V0 is initial voltage on C1. The output of this stage is switched to a subsequent stage with a closing switch. Utilizing this technique an energy transfer system can be designed for a particular load.
Another common high-voltage generating sys- tem found in almost all high-voltage testing facilities is the marx generator (Fig. 18.76). This system charges capacitors in parallel and then discharges them by switching them to a load in series. The switches are usually spherical electrodes with air as the dielectric. The first stage is the control stage and is triggered into conduction at the desired time.
For capacitive loads
The breakdown voltage for air gaps is given by E = 24.5 P + 6.7( P /Reff)1/2 kV/cm where Reff is 0.115R for spherical electrodes and 0.23R for cylindrical electrodes and P is pressure in at mo spheres. To increase the high-voltage characteristics of a switch other dielectrics can be used (Table 18.9).
Another common method of generating high voltage pulses is to charge a transmission line and then switch the stored energy to a load with a closing switch, see Fig. 18.77. For an ideal pulse line the output voltage is given by
where Zc is the characteristic impedance of the line, R is the load, and τ = L /(Zc + R). The rise time of the pulse, 0.1Vmax to 0.9Vmax, is 2.2τ .
There are a number of pulse forming networks (Fig. 18.78) that will produce pulses with near constant amplitudes and fast rise times. These networks approximate the ideal transmission line.
Another type of switch is the opening switch. Fuses and circuit breakers are examples of opening switches, although they are too slow for many pulsed power applications. Opening switches are capable of switching large currents but are difficult to design because during switch opening a significant voltage will develop across the switch. Interrupters used in the power industry rely on a current zero occuring while the switch is opening. If the switch configuration is not such that it can withstand the reoccurring voltage it will close. For pulsed power purposes it is necessary to interrupt currents in the kiloampere, range with nanosecond opening. One such switch is the plasma opening switch. In this switch the plasma channel that sustains conduction between electrodes is degraded to the point that it cannot sustain current flow and the switch opens. This switch is utilized in high-energy beam applications. Other methods such as magnetic interruption and explosive devices are used.
Although the power industry is at a mature stage, the pulsed power arena is a new and dynamic challenge for the power engineer. As more exotic requirements develop for the delivery of high energy in very short periods of time to loads, there will be an increasing demand for improvements in switching technology.
References
Anderson, P.M. and Fouad, A.A. 1977. Power System Control and Stability. Iowa State University Press, IA. Eaton, J.R. and Cohen, E. 1983. Electric Power Transmission Systems. Prentice-Hall, Englewood Cliffs, NJ. IEEE. 1980. IEEE Recommended Practices for Industrial and Commercial Power Systems Analysis. Wiley, New York.
Kassakian, J.K., Schecht, M.F., and Verghere, G.C. 1991. Principles of Power Electronics. Addison-Wesley, New York.
Rashid, M.H. 1988. Power Electronics. Prentice-Hall, Englewood Cliffs, NJ.
Russel, B.D. and Council, M.E. 1978. Power System Control and Protection. Academic Press, New York.
Stevenson, J.D. 1982. Elements of Power System Analysis. McGraw-Hill, New York.
Vitkouitsky, H.M. 1987. High Power Switching. Van Nostrand Reinhold, New York.
Weedy, B.M. 1975. Electric Power Systems. Wiley, New York.
Further Information
IEEE Power Energy Review
IEEE Recommended Practices for Industrial and Commercial Power Systems Analysis
IEEE Transactions on Power Electronics
IEEE Transactions on Power Systems