Power system disturbances

DC link isolates the inverter from power system disturbances... 

FIGURE 6.5 Power system disturbances resulting from Table 6.10a (A) generation seventeen cases (B) power supply seventeen cases and (C) system operation twenty-three cases. 

Types of disturbances for each equipments are categorized in Table 6.7a. It is clear from the table that halts and locks (freeze) of equipments are about half of the total disturbances, and those including malfunctions (not operated too) reach 70% of the total disturbances. The aforementioned disturbances of control equipments in Table 6.1a result in troubles of power supply and system operation as in Figure 6.5, which shows that more than 10% of the control circuit disturbances cause power system disturbances. As far as a control equipment disturbance is closed within a control circuit, the disturbance costs not... 

These applications have considerably more stringent performance requirements than any other application. Circulating water pumps, boiler feed pumps, forced-draught (FD) and induced-draught (ID) fans, pulverizers (ball mills) and condensate pumps are components in a thermal power station that may require extra safety in a standard motor to make it able to fulfil these requirements and withstand abnormal service conditions and system disturbances. Abnormal operating conditions may be one or more of the following ... 

In a large power station, connected to a transmission network through a power grid, system disturbances are a common feature. Foi details see Section 17.5. 

G) For a generator connected to a power system, abnormal frequency operation may be the result of a severe system disturbance. An isolated unit, however, can operate at a low or high frequency, due to an incorrect speed control adjustment or a malfunctioning of the speed control device. 

In actual operation, disturbances on a power system, causing sudden changes in the system parameters, are quite frequent and may generate temporary overvoltages and voltage surges, as summarized above. The system disturbances may be of two types, external or internal, as explained below. 

But as the harmonics do exist in the system, they do affect an inductive load. They may also disturb a communications network as a result of capacitive coupling, whose effects are magnified in the presence of capacitor units in the power system. It is therefore considered relevant to discuss this subject in more detail to make the harmonic study more informative. 

The response of the fuel cell to system disturbances or load swings also must be considered whether it is connected to a dedicated load or to the utility s grid. Demonstrated fuel cell power conditioning responses are (33) ... 

Transient — Subcycle disturbance in the AC waveform evidenced by a sharp, brief discontinuity of the waveform. This may be of either polarity and may be additive or subtractive from the nominal waveform. Transients occur when there is a sudden change in the voltage or the current in a power system. Transients are short-duration events, the characteristics of which are predominantly determined by the resistance, inductance, and capacitance of the power system network at the point of interest. The primary characteristics that define a transient are the peak amplitude, the rise time, the fall time, and the frequency of oscillation. Figure 1.12 shows a transient voltage waveform at the output of a power transformer as the result of switching-in of a motor containing power factor correction capacitors. 

Power frequency disturbances are low-frequency phenomena that result in voltage sags or swells. These may be source or load generated due to faults or switching operations in a power system. The end results are the same as far as the susceptibility of electrical equipment is concerned. Power system transients are fast, short-duration... 

Transverse mode noise is noise present across the power wires to a load. The noise is referenced from one power conductor to another including the neutral wire of a circuit. Figure 7.4 depicts common and transverse mode noises. Transverse mode noise is produced due to power system faults or disturbances produced by other loads. Transverse mode noise can also be due to conversion of common mode noise in power equipment or power lines. Some electrical loads are also known to generate their own transverse mode noise due to their operating peculiarities. 

The equations above are shown for the steady state. However they apply equally well for the transient and sub-transient states provided the substitutions for E, E, Ej, X, X, Xj and X" are made systematically. Such substitutions are necessary in the digital computation of transient disturbances in power systems, those that are often called transient stability studies . 

Protective devices, usually relays or fuses, are installed at supply points in a power system to accurately detect and quantify a disruptive disturbance in the system. The variable most freqnently used for detection is the supply line current, and in most sitnations this is detected throngh the nse of current transformers. Occasionally direct acting devices are used e.g. fuses for voltages up to about... 

Figures D.3 and D.6 show the responses of frequency in the first one second. Both machines respond in much the same way in the first half second. This is dne to the fact that this part of the response is open loop and is mainly determined by the mechanical inertia and the size of the disturbance, as discussed in Chapter 21 of Reference 1 see also snb-section 2.5 herein. Also shown in these two figures are typical setting levels for underfrequency (81) multi-stage relays. In addition to the setting levels the relays shonld also have time delay settings, so that coordination with other power system equipment can be achieved, e.g. automatic voltage regulators of generators, automatic re-acceleration of induction motors, see also sub-section 7.6 herein. For the settings shown the relays would respond in the range of about 70 to 150 milliseconds, which is typically about half the response...

The series resonance phenomenon can occur when a disturbance occurs as a phase to ground fault in power system operation. Resonance occurs when the inductive reactance equals the capacitive reactance [Riidenberg, 1950 Enriquez Harper, 1978 Boylestad, 1998 Fuentes Rosado, 1988 Halladay and Shih, 1985 Hayt Kemmerly, 1990 Strum Ward, 1973 Dodger Norriss, 1988 Kraft Heydt, 1984]. 

Key, T. The Effects of Power Disturbances on Computer Operation, IEEE Industrial and Commercial Power Systems Conference, Cincinnati, lune 7,1978. 

FIGURE 10.179 A dual feeder standby power system using a motor-generator set to provide power fail ride-through and transient-disturbance protection. Switching circuits allow the m-g set to be bypassed, if necessary. 

Noise produced by backup power systems can be a serious problem if not addressed properly. Standby generators, motor-generator sets, and UPS systems produce noise that can disturb budding occupants and irritate neighbors or landlords. 

Uninterruptible power system (UPS) A power processing device used to protect a sensitive load from disturbances on the utility AC Hne. A UPS system typically consists of a battery bank that drives an inverter to produce power for the load, line switching hardware, and supervisory control circuitry. 

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