Consulting-Specifying Engineer - January 2009 - (Page 42) Load addition Recovery time Engine rpm (frequency) Load removal % overshoot % dip Recovery time Transient response (time) Figure 3 The time required for the generator set to return to its normal steady-state speed is called recovery time. Source: Stanley Consultants • What actions can the operator take when the emergency power system does not function as expected? Control of generator and switchgear The emergency power system does not instantaneously become available upon the loss of the normal power supply source. It takes some time to get the emergency generator system started. Starting requirements will vary depending on the application. Emergency generator sets can typically start to accept loads when they reach approximately 90% to 95% of rated frequency, at which point the breaker closure is initiated. Emergency backup generators will operate with the engine governor set for isochronous control for load frequency control, which maintains the engine-generator at a constant speed with no governor droop, while not synchronized to the electrical power system. Should the backup generator units be required to operate in parallel with the main electrical grid, either for changeover or periodic testing purposes, then the governor will have to be switched to speed droop regulation mode. Droop is when the governor compensates for speed with an increasing load but relies on other generation for frequency control. Speed is lowest at full load and highest at no load. Droop is expressed as a percentage of rated speed. A 2% to 3% droop is typical. Most modern solid-state governors can readily operate in either mode. Whenever a load is applied to or removed from a generator set, the engine speed, voltage, and frequency will experience a transient condition or a temporary change from its steady-state condition. When a significant load is applied, the engine speed temporarily reduces, causing a momentary frequency. There is also a voltage dip until the emergency power system exciter compensates for the increased reactive power (var) demand. The degree of this dip depends on the power capacity and dynamic characteristics of the emergency engine generator set. When the emergency loads are removed, the engine speed increases momentarily, causing system overshoot before returning to its steady-state condition. The time required for the generator set to return to its normal steady-state speed is called recovery time (see Figure 3). Typically, depending on equipment manufacturers, the maximum allowable voltage dip for the generator control system may be as low as 30%, and the maximum frequency dip is about 25%. Modern equipment restricts the electrical system voltage and frequency to a much tighter margin. Thus, the acceptable percent and duration of voltage and frequency dip, and type of load to be connected, are also important criteria that need to be considered for the design and control of the emergency power system. The control of load pickup and removal becomes an important factor in maintaining emergency power supply system stability and power quality. Starting of certain load feeders may be inhibited when the online generators have insufficient capability. When the online generators have sufficient capability, load feeders may be automatically reclosed in a controlled manner after load shedding. Avoid addition and removal of large blocks of emergency loads. If response time and load priority permit, the largest emergency load should be added first, followed by smaller emergency load blocks. Allow sufficient time delay in the sequencing of emergency load pickup and load shedding. Sensitive electronic equipment must be on UPS or other power-conditioning devices. Design engineers and facility owners may choose to employ a sophisticated emergency power system control, designed to stabilize critical power systems by monitoring frequency and power sources from utility plus generator capacity versus total circuit load. Load priority setting Load priority is a decision that the facility operator must make based on its impact to the emergency load supporting functions, considering load characteristics and emergency power system limitations. The facility operator has to decide which loads are critical and have priority for emergency power system loading. The order of emergency generator units’ startup and shutdown is also important. If the same generator unit is always started first, it will accumulate hours and maintenance expenses at a higher rate than the remaining units. To make the operating hours among the units more uniform, the first unit to start can 42 Consulting-Specifying Engineer • JANUARY 2009
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