Power Quality in Power Systems and Electrical Machines (eBook)

1.8 Power quality improvement techniques

Nonlinear loads produce harmonic currents that can propagate to other locations in the power system and eventually return back to the source. Therefore, harmonic current propagation produces harmonic voltages throughout the power systems. Many mitigation techniques have been proposed and implemented to maintain the harmonic voltages and currents within recommended levels:

• high power quality equipment design,

• harmonic cancellation,

• dedicated line or transformer,

• optimal placement and sizing of capacitor banks,

• derating of power system devices, and

• harmonic filters (passive, active, hybrid) and custom power devices such as active power line conditioners (APLCs) and unified or universal power quality conditioners (UPQCs).

The practice is that if at PCC harmonic currents are not within the permissible limits, the consumer with the nonlinear load must take some measures to comply with standards. However, if harmonic voltages are above recommended levels – and the harmonic currents injected comply with standards – the utility will have to take appropriate actions to improve the power quality.

Detailed analyses of improvement techniques for power quality are presented in Chapters 8 to 10.

1.8.1 High Power Quality Equipment Design

The use of nonlinear and electronic-based devices is steadily increasing and it is estimated that they will constitute more than 70% of power system loading by year 2010 [10]. Therefore, demand is increasing for the designers and product manufacturers to produce devices that generate lower current distortion, and for end users to select and purchase high power quality devices. These actions have already been started in many countries, as reflected by improvements in fluorescent lamp ballasts, inclusion of filters with energy saving lamps, improved PWM adjustable-speed drive controls, high power quality battery chargers, switch-mode power supplies, and uninterruptible power sources.

1.8.2 Harmonic Cancellation

There are some relatively simple techniques that use transformer connections to employ phase-shifting for the purpose of harmonic cancellation, including [10]:

• delta-delta and delta-wye transformers (or multiple phase-shifting transformers) for supplying harmonic producing loads in parallel (resulting in twelve-pulse rectifiers) to eliminate the 5th and 7th harmonic components,

• transformers with delta connections to trap and prevent triplen (zero-sequence) harmonics from entering power systems,

• transformers with zigzag connections for cancellation of certain harmonics and to compensate load imbalances,

• other phase-shifting techniques to cancel higher harmonic orders, if required, and

• canceling effects due to diversity [57–59] have been discovered.

1.8.3 Dedicated Line or Transformer

Dedicated (isolated) lines or transformers are used to attenuate both low- and high-frequency electrical noise and transients as they attempt to pass from one bus to another. Therefore, disturbances are prevented from reaching sensitive loads and any load-generated noise and transients are kept from reaching the remainder of the power system. However, some common-mode and differential noise can still reach the load. Dedicated transformers with (single or multiple) electrostatic shields are effective in eliminating common-mode noise.

Interharmonics (e.g., caused by induction motor drives) and voltage notching (e.g., due to power electronic switching) are two examples of problems that can be reduced at the terminals of a sensitive load by a dedicated transformer. They can also attenuate capacitor switching and lightning transients coming from the utility system and prevent nuisance tripping of adjustable-speed drives and other equipment. Isolated transformers do not totally eliminate voltage sags or swells. However, due to the inherent large impedance, their presence between PCC and the source of disturbance (e.g., system fault) will lead to relatively shallow sags.

An additional advantage of dedicated transformers is that they allow the user to define a new ground reference that will limit neutral-to-ground voltages at sensitive equipment.

1.8.3.1 Application Example 1.6: Interharmonic Reduction by Dedicated Transformer

Figure E1.6.1 shows a typical distribution system with linear and nonlinear loads. The nonlinear load (labeled as “distorting nonlinear load”) consists of two squirrel-cage induction motors used as prime movers for chiller-compressors for a building’s air-conditioning system. Note the thyristor symbol represents the induction motors, although there are no thyristors present: the thyristor symbol represents here a nonlinearity. This load produces interharmonic currents that generate interharmonic voltage drops across the system’s impedances resulting in the interharmonic content of the line-to-line voltage of the induction motors as given by Table E1.6.1. Some of the loads are very sensitive to interharmonics and these must be reduced at the terminals of sensitive loads. These loads are labeled as “sensitive loads.”

Three case studies are considered:

• Case #1: Distorting nonlinear load and sensitive loads are fed from the same pole transformer (Fig. E1.6.2).

• Case #2: A dedicated 1:1 isolation transformer is used between the distorting nonlinear load and sensitive loads (Fig. E1.6.3).

• Case #3: A dedicated 7.62 kV to 120 V pole transformer is used between the distorting nonlinear load and sensitive loads (Fig. E1.6.4).

Solution to Application Example 1.6

Computations are shown in Figs. E1.6.5 and E1.6.6 for the above three cases. As illustrated, for Vload1 = Vload2 the 1128th interharmonic amplitude is V1128 = −120120 (100%) = 2.2%, whereas for Vload3 this interharmonic is only 0.006%. This demonstrates the effectiveness of a dedicated line or transformer, other than an isolation transformer with a turns ratio 1 : 1.

1.8.4 Optimal Placement and Sizing of Capacitor Banks

It is well known that proper placement and sizing of shunt capacitor banks in distorted networks can result in reactive power compensation, improved voltage regulation, power factor correction, and power/energy loss reduction. The capacitor placement problem consists of determining the optimal numbers, types, locations, and sizes of capacitor banks such that minimum yearly cost due to peak power/energy losses and cost of capacitors is achieved, while the operational constraints are maintained within required limits.

Most of the reported techniques for capacitor placement assume sinusoidal operating conditions. These methods include nonlinear programming, and near global methods (genetic algorithms, simulated annealing, tabu search, artificial neural networks, and fuzzy theory). All these approaches ignore the presence of voltage and current harmonics [60,61].

Optimal capacitor bank placement is a well-researched subject. However, limited attention is given to this problem in the presence of voltage and current harmonics. Some publications have taken into account the presence of distorted voltages for solving the capacitor placement problem. These investigations include exhaustive search, local variations, mixed integer...