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Dip Doctor

Rainbow Technologies Dip Doctor ™

Introduction


The Dip Doctor ™ was developed & patented by Rainbow Technologies, to provide a macro, plant-wide solution to the negative effects of Voltage Dips at large process industries.
 
It provides a "buffer" between supply system generated voltage dips and sensitive load.
 
Suitable for Industrial loads from 5MW to 200MW with proven, audited Payback varying from 4 to 36 months.
 

Voltage Dip Causes and Definitions


Voltage Dips, Voltage Depressions, Brownouts, Sags, Momentary supply interruptions – all terms describing the rapid voltage decrease and eventual increase caused by network faults.
 
When an electrical fault occurs in the supply network, the voltage magnitude decreases (worst case to zero if the fault is nearby) and then increases back to nominal when the fault is "cleared by" the protection relays.
 
 
Dip Doctor

Delivery of Electricity affected by Terrain, Wildlife & Elements
 
Transmission faults are inevitable in Electricity Utilities networks and are caused by weather (lightning), fires, foliage, birds, pollution, equipment degradation & failure, inadequate line design etc. The supply utility should minimize these faults by optimized maintenance, foliage clearance, washing of insulators etc, but in Southern Africa with long transmission lines, the annual number of voltage dips is in the 100's
 
Dip Doctor
Dip Doctor
 
Voltage Dip Definition
 
The definition of a Voltage Dip is shown above, with duration being anything from three 50Hz cycles (0,06 seconds) to 150 cycles (3,00 seconds).
 
The duration is dependent on the sensitivity & efficiency of the protection system and varies from 3 cycles (EHV systems) to 10 cycles (MV) systems for optimized protection, increasing to more than 30 cycles for unoptimised systems.
 
The magnitude is dependent on the proximity of the fault and the degree of interconnection in the supply network.
 
Of course for a 3 phase supply system, there will be 1 phase dips (phase-earth faults), 2 phase dips (phase-phase faults) etc. Various Power Quality lobbies have tried to categorize the dips according to the efficacy of the protection systems and the proximity of the faults. The South African SANS048 standard (previously recommendation NRS048) is shown below with typical dip distribution for a 1 month period.
 
Dip Doctor
 
Voltage Dip Classification
 
 

Voltage Dip Effects

Loads affected by voltage dips are mainly Variable Speed Drives (VSD's) and motors with no-volt coils in their contactors. Typically, VSD's and motors constitute about 75% of all industrial loading, therefore having significant influence on sensitive processes.
 
The VSD's have under-voltage sensing to prevent malfunctions of the power electronics, while motor's have a no-volt coil to open the contactor for under-voltage conditions.
 
The CBEMA sensitivity Curve to supply voltage variations vs time is shown below. Note the Logarithmic scale for duration, where 10-2 and 100 represent 0,01 and 1,00 seconds respectively.
 
 
Dip Doctor
 
CBEMA Curve showing equipment sensitivity for Over & Under voltages
 
 
Equipment suppliers have improved the Voltage Dip immunity of VSD's (Flying Restart's, DC bus-bar buffering etc) and Motors (Slugged no-volt coils, Under-voltage monitored DC supply to contactor coils etc).
 
Individual loads are generally not severely affected by voltage dips, because they can be re-started and the production process can continue within minutes, or they can be fitted with dedicated secure power supplies.
 
However Serial manufacturing processes (eg Paper, Steel & Textile Mills) which have speed dependent, interconnected drives , are seriously affected by voltage dips, with production halted for hours while the process is cleaned up and restarted.
 
This type of Industry requires a "Plant-Wide" solution, which protects not only interconnected individual drives but all the ancillary equipment (eg Bilge Pumps, Cooling fans, PFC, Field Sensors etc).

These relatively insignificant loads tripping due to voltage dips, cause process interruptions initiated by the Distributed Control System trying to prevent potential equipment damage (ie Bilge overflow, over-temperatures etc).
 
Therefore, a process interruption resulting in $10 000 – $100 000 per incident, can be caused by a $10 Bilge or Cooling Pump malfunction.
 
During seasonal bad weather, or temperature inversions, or mating migrations of birds, there can be numerous system faults (ie Voltage Dips) within a 24h period.
 
Production Losses running to Millions of Dollars per year, justifies the installation of the Dip Doctor™ at central load busbars.
 

Dip Doctor ™ Description

Basics
The Dip Doctor essentially utilizes a series reactor (a Current Limiting Reactor or CLR) between the supply and load busbars with a Synchronous Compensator or SCO (unloaded synchronous motor) on the load busbar.
 
The CLR buffers the load from network faults, while the SCO provides instantaneous voltage support for the load.
 
A typical layout is shown schematically below (Blue indicates Dip Doctor™ components).
 
 
Dip Doctor
 
Dip Doctor ™ basic Layout
 

The Dip Doctor ™ is tailored for the specific load and supply system, as well as the types and frequency of voltage dips.
 
It is a fairly complex design process optimising Performance, Reliability, Flexibility, Harmonics, Loading, Cost etc.
 
System Studies
The CLR reduces the load busbar fault level while the SCO increases the fault level with a nett fault level optimized for Dip Performance and availability of SCO unit/s.
 
 
Dip Doctor
System Study for Transient & Steady State Modelling
 
 
 
Hence Protection Grading and Harmonic conditions have to be re-evaluated for the various contingency conditions, as well as all cable & transformer loading conditions.
 
A full system study is undertaken to determine the optimum values of the SCO (dependent on availability of suitable units) and CLR, including system layout and availability of spare switchgear & real estate.
 
 
SCO
Because of the SCO operating regime (thermally unloaded), typically re-furbished synchronous motors are sourced, reducing Capex to less than a third of new equipment (Fit-for-Purpose design).
 
Dip Doctor
 
SCO - 45MVA 11kV synchronous motor (weight = 43000kg)
 
Due to electrical performance requirements, the SCO is usually a large unit, with rating of the same order as the total protected load – typically a medium voltage (MV) unit, with matched step-up transformer to the load voltage.
 
The SCO unit requires a Starting system (Pony Motor), Cooling system, Lubrication system, Excitation Control (AVR), Automation and all the necessary electrical and mechanical Protection systems (temperature, speed, vibration, over/under voltage & currents, oil pressures etc).
 
CLRDip Doctor
The CLR is designed with a continuous thermal rating matching the total plant load. It has to be especially designed to minimize 50Hz losses at full load, as well as having a high reactive (X) impedance to provide the essential buffer.
Under certain conditions (eg loss of SCO automation, trip of SCO), the CLR has to be by-passed with a high speed switch to prevent an inadvertent, self imposed voltage dip.
Thermal protection is included with Alarm & Trip settings, so that Operators can manage the loading prior to tripping.
 
 
 
 
Performance
The Dip Doctor™ reduces the Voltage Dip magnitude, thereby reducing the affect on sensitive loads.
 
Shown below are simultaneous real-time recordings of the same Voltage Dip at two adjacent busbars, one supported (TM2) and the other unsupported (TM1).

The Voltage Dip magnitude is reduced by 50% in this case.
 
 
Dip Doctor
 
Recorded real-time Dip Doctor™ performance
 
The reduction in dip magnitude has a proven exponential reduction in plant trips, due to the shifting of Dips to lower Classification areas as shown below.
 
Dip Doctor

Shifting of Dip Classification
 
References
The table below lists present Dip Doctor™ installations.
All are operating successfully (Kenmare still in progress), meeting the performance & reliability guarantees.
 
Dip Doctor
 
Conclusions
The Rainbow Technologies designed Dip Doctor™ has proven to be a reliable and cost effective solution for problematic affected industries.
 
It provides a Plant wide immunity, eliminating nuisance trips from minor field elements and reduces the following "intangibles" along with the obvious improvement in Production;
 
• Wear & Tear on equipment subjected to rapid voltage changes.
• Maintenance call-outs & overtime costs.
• Spares-holding of fuses, sensors & other dip induced equipment failures.
• Poor quality of production due to varying process changes.
• Loss of market share due to late or non delivery of product.
• Improved environmental emissions related to plant start-ups
 
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