It is an unfortunate characteristic of rig123.net' target='_blank'>SCR systems that the Power Factor (the relationship between real and reactive power) is poor - often down as low as 0.4 - which maeans that the KW capacity of the engines is under-utilised. This often leads to frustration because the rig runs out of power, yet the engines may be only delivering half of their full-load capacity. This article explains how the Power Limit system works and what can be done to ensure it is working at its optimal level.

In general terms, the KW the system consumes is provided by the engine. The other limitation on power delivery is the current capacity of the generator, which is often expressed as a VA or KVA value. SCR systems are designed with something like a 0.7 Power Factor rating; in other words, the KW of the engine will be 0.7 x the KVA rating of the generator. This goes some way to compensating for the poor Power Factor, but often is insufficient during operations where where the Mud Pumps are heavily loaded. However, measures are still available to ensure that the system is delivering what it can within these physical limitations, one one thing to ensure is that the Power Limit system is working optimally.

Theory of Operation

Fig 1: Caterpillar 3412 Engine and Generator

The purpose of the Power Limit is to prevent rig blackouts in the event of sudden increases in load. The original philosophy also was to allow the minimum number of engines to be run and maximise efficiency but most drilling operators prefer not to run up against these limits and demand as much power as is available. The Power Limit system measures the KW load of the highest loaded engine, and the current load of the highest loaded generator and compares this to a pre-set reference to determine if Power Limiting of the SCRs should occur. Furthermore, some systems have an initial 60% KW limit, which then opens up to 100% once this threshold has been reached under ramped control. This is to allow the engine turbo-chargers to spin up. Once above this threshold only the 100% limit is in operation, unless the load dips below 60% in which the ramp is re-set.

One other addition to the basic Power Limit is a 90% switch setting, which reduces the overall KW limit to 90%. The thinking here was to compensate for engines which had become old and a bit tired or due a maintenance overhaul.

The Power Limit system ONLY affects the SCR output; there is no control over other AC loads.

Power Limit Calibration

It follows, therefore, to ensure the Power Limit card is correctly calibrated. The procedure for calibrating the Power Limit system is beyond the scope of this article, and should not be undertaken by unqualified personnel. This is a job for your service engineer. For a typical system with CAT D399 engines, the engine will have a 930KW rating, and the generator will be rated at 1500KVA which equates 1443A maximum load. When fully in Power Limit, at least one of the generator section meters should show either maximum KW (930KW in our typical system) or maximum current (1443A in our example). If not, there may be a problem with the calibration of the Power Limit system, but there may not - the calibration may have been set deliberately low to compensate for equipment which is a bit old and tired and can't quite deliver nameplate ratings.

Load Sharing

Fig 2: Power Factor Triangle

Hopefully, when the rig is fully loaded, the load will be shared equally amongst the engines and generators. All the engines should have KW values within 50KW of each other. The generator KVAR meters should also be showing values within 50 KVARs of each other. If not - remember that the Power Limit system only looks at the highest loads - it will be the highest loaded engine or generator (they may not be the same unit) which dictates when Power Limit occurs. The KW load sharing is usually not adjustable, but it is absolutely vital that, when a generator is put on line, the voltage adjust potentiometers are adjusted to balance up the KVAR. Once adjusted they should remain balanced unless there is a problem with the KVAR/voltage regulation on any one generator which may be caused by a faulty exciter or exciter controller. Either way, if load sharing is poor, Power Limiting will prevent the rig developing full power.

Power Factor Correction

Power factor correction has been deployed on a number of systems in the form of capacitor banks to compensate for the poor power factor. Although this might make the numbers look better, there is some doubt as to whether or not it increases the useable power from the limited amount of generating capacity available. About 1500KVARs of reactive load is required to make a significant difference.