Basler Electric SSE Shunt Static Exciter-Regulator

Introduction

The Basler Electric Shunt-Static Exciter-Regulator (SSE) controls and regulates the generator output

voltage by controlling the generator field voltage. Standard features include underfrequency or volt/hertz

operation, manual control, and provisions for paralleling. The capabilities and parameters of each model

are detailed in Table 1-1. A power input transformer is supplied as a part of the SSE.

Options

The list of options described below contains some of the options that are available on all units and may be

on your unit.

• Failed rectifier detector

• Auto / manual nulling meter

• Manual tracking motor operated potentiometer

• Over / under excitation limiting

• AC shut-down contactor

• Field flashing contactor

• Paralleling current transformers

• VAR / power factor controller

• Primary fusing for power transformer when sold in cubicle

• Metering

• Potential transformer

• Cubicles

• MOC potentiometers

• Overexcitation protective relay

• Underexcitation protective relay

Operating Power

Three-phase ac is applied to the power isolation transformer to provide operating power to the regulated

power supply and the semiconverter power rectifier bridge.

When the generator is placed into operation, residual voltage will pass through the normally closed contacts

of K1. It is then rectified and applied to the generator field. When the generator output voltage builds up to

a sufficient value, K1 (connected across the output of the regulator power supply) energizes, and the

contacts open, removing the voltage build-up circuits. Thereafter, the power bridge output is established by

the gating pulses from the firing circuits. The minimum residual voltage required for build-up without flashing

is approximately 5% of nominal.

Flash and Overflash Protection (SSE 250 Series Only)

This circuit provides 125 Vdc to the generator field during start-up. When the generator output voltage rises

to 35% of nominal, the flashing voltage is removed and the static exciter regulator takes over. If the output

voltage does not build up in 25 seconds, the flashing voltage removed by the overflash protection circuitry.

Semiconverter Power Rectifier (Power Bridge)

The conduction time of the SCRs in the power bridge determines the amount of output current supplied to

the generator exciter field. Conduction time is controlled by the timing of gating pulses from the firing circuits.

The greater the conduction time of the SCRs, the greater the output current. The less the conduction time,

the less the output current.

Firing Circuit

The circuit compares a dc signal representative of the SCR output with a reference input established by the

manual adjust control (R5). Gating pulses to the power bridge are developed as a result of the comparison

of the two inputs. As R5 is adjusted to demand a higher or lower nominal generator output, the firing circuits

determine whether the gating pulses need to be applied earlier or later in the SCR firing cycle to establish

the new operating level. The power bridge responds by providing the correct output to the generator field.

When the auto / manual mode switch is in manual, R5 (essentially a manual voltage control) and the firing

circuits function independently of the rest of the exciter-regulator and the excitation system does not

regulate the system ac voltage. (Refer to Section 3. Controls)

Generator Sensing

Single-phase generator sensing voltage is connected at TS3-E1 and TS3-E3. Three-phase generator

sensing is connected to TS3-E1. TS3-E2. and TS3-E3. The sensing inputs are applied to the primary of

sensing transformers T3 and T4. The output voltage from the transformer(s) secondary windings is rectified

and a representative sample, taken from a voltage divider network, is applied to the automatic voltage

regulator. Selection of single-phase or three-phase sensing is done with a jumper on TS2 (Refer to Section

3. Jumper Connections).

Automatic Voltage Regulator (AVR)

The AVR senses any change in generator output voltage and translates such changes into a dc error

voltage. When the auto / manual mode switch is auto, the dc voltage is sent to the firing circuits where it

modifies the gating pulse output originally established by the manual adjust control. When the AVR

determines that the generator output voltage is low, the gating pulses are applied to the power bridge SCRs

earlier in the cycle to cause a greater dc output. When the generator output voltage is high, the gating

pulses are applied later in the cycle causing less current in the generator field.

Underfrequency Limiter

The output from one winding of the generator sensing transformer is applied to the underfrequency limiter

circuit. It is the function of this circuit to reduce the generator output voltage when the generator is operated

at less than rated speed. The percentage of generator output voltage that will be obtained for a specific

reduction in frequency is illustrated in Figures 1-1 and 1-2. The horizontal axis of the graph represents

generator frequency and the vertical axis represents the percent of nominal generated output voltage. Volts

per-hertz operation is obtained by jumping the terminals located on TS3. Limiting for 50 or 60 Hz is obtained

by proper positioning of the jumper on TS4. (Refer to Section 3. Jumper Connections)

Parallel Compensation

When two generators are operating in parallel, if the field excitation on one generator becomes excessive

and causes a circulating current flow between generators, this current will appear as a lagging power factor

(inductive) load to the generator with excessive field current and as a leading power factor (capacitive) load

to the other. The parallel compensation circuit causes the exciter-regulator to decrease the field excitation

on the generator with the lagging power factor load and increases the excitation on the other generator to

minimize the circulating currents.