Control oil vent

Overspeed trip

Local turbine trip lever

Excitation fault

Bearing vibration

Back-up overspeed

EHC major fault trip

Condensate pot level high

Condensate hotwell level high

Low vacuum

Thrust bearing wear

Loss of lub oil

Exhaust temp high

Emergency P.B.

Busbar zone

Biased differential


33kV overcurrent


33kV restricted earth fault


33kV earth fault


Buchholz gas


Winding temp


Oil temp


Overall circulating current


Negative phase sequence


Reverse power


Voltage restrained overcurrent (VOC DT)

Loss of excitation


Earth fault


Over voltage


Under frequency


Field switch

Typical arrangement protection and tripping logic 60MW. steam turbine generator Figure 21.4 Protection and tripping logic for a 60MW. steam turbine generator

Figure 16.4 Protection and tripping logic for a 60 MW steam turbine generator

Figure 16.5 Protection and tripping logic for a 2 MVA transformer

Where: 0 = cos-1 (PF). The correction required is:

Figure 16.9 illustrates the amount of power factor correction required per 100 kW of load to correct from one power factor to another.

The degree of correction necessary for any particular installation will depend upon the circumstances. In economic terms the costs and prospective benefits can be simply set out as:

Capital Running Savings

Capital cost Maintenance Reduction in of equipment demand charges

Installation costs Depreciation Reduction in losses

Under excited

Motor operating at lagging power factor

Motor mode, drawing active power

Motor operating at leading power factor

Generator operating at leading power factor

Generator mode, contributing active power

Generator operating at lagging power factor

Over excited

Figure 16.7 Synchronous machine operating modes

Figure 16.8 Synchronous machine operation. (a) Synchronous machine acting as a motor (over-excited); (b) synchronous machine acting as a generator (over-excited)

16.7.3 Types of control

Capacitors as bulk units can be connected to the supply busbar via a fuse switch, molded-case circuit breaker of air circuit breaker. In this type of installation control is purely manual, and in cases of a reasonably constant load and where the amount of power factor correction is limited such a manually controlled system is perfectly adequate. The supply authority may, however, require to be informed that a capacitor bank is permanently connected to the supply. Capacitors are more generally connected either in banks controlled from a VAR sensitive relay or across individual loads (e.g. motors).

When connected as switchable banks the rating of each step of the capacitor bank must be selected with care. It is important that the control relay settings are matched to the ratings of each capacitance step in order to prevent hunting (i.e. continuously switching in and out at a particular load point). When capacitors are connected to one particular load (usually a motor) the capacitor bank can be located at the motor, adjacent to but separate from the control switchgear or within the control switchgear itself.

When located at the motor the capacitor bank will be normally cabled from the motor terminal box, so that the size of the motor cable can then be selected on the basis of the reduced-power factor corrected current drawn by



0.80 0.81 0.82 0.83 0.84 0.85 0.86 0.87 0.88 0.89 0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1.0

Initial power factor

Figure 16.9 Amount of power factor correction required the motor/capacitor combination. However, whenever a capacitor is connected across an individual motor circuit, by whatever means, the setting of the motor overload device must be chosen to take account of the corrected current rather than the uncorrected motor full-load current.

16.7.4 Potential problems

Control devices

Capacitors, circuit breakers and HRC fuses must be selected with care for use with capacitor circuits, with contractors chosen on the basis that the capacitor current can rise by 25 per cent above nominal line current. Equally, HRC fuses for capacitor applications should be de-rated by a factor of 1.5.

When choosing high-voltage circuit breakers for capacitor control it is necessary to select the full-load current of the breaker, taking into account variations in supply voltage, tolerance on rating manufacture and harmonic currents. Again, a de-rating factor of 1.5 is usually considered adequate. The capacitor manufacturers will insist that the control device is re-strike free and that the control device has been tested to IEC 56, Part 4. When the control device is a high-voltage circuit breaker, capacitor manufacturers frequently recommend a maximum between initial current makes and final contact closure (typically, 10m/s).

Motor circuits

In instances where a capacitor is connected directly across the terminal of the motor, the capacitor can act as a source of excitation current after the control device is opened. In order to prevent this the capacitor rating should not exceed 90 per cent of the motor no-load magnetizing current.


A capacitor bank will represent reducing impedance to currents of increasing frequency. Such reducing impedance, if matched with a similarly increasing inductance impedance of a transformer or a supply system, can cause a resonant condition. In plants where equipment produces harmonic current, a full survey of the installation is recommended prior to installation of the capacitors.

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