Dead Bus ParelingGLPT 6174 en

 

Our energy working for you.

TM

Power topic #GLPT-6174-EN | Technical information from Cummins Power Generation

Random Access vs Dead Bus Paralleling   White Paper  By Rich Scroggins, Technical Advisor – Sales Application Engineer

 The ove overwh rwhelmi elming ng maj majorit orityy of of paralleled generator sets in operation today use a paralleling method known as Random Access Paralleling to synchronize and connect generator sets to a paralleling bus. An alternate paralleling method known as dead bus or dead field paralleling is being promoted by some generator set and control manufacturers as having some performance advantages with respect to random access paralleling, specifically speed to parallel and the ability to energize large banks of transformers. In this paper we will examine the capabilities of these two paralleling methods.

Random Access Paralleling In a random access paralleling system all generator sets receive a start command at the same time and independently build up their voltage and speed to rated values at which point they are ready to close to the paralleling bus. The generator sets will not be in sync with each other so the generator set controls must have some kind of arbitration scheme which allows only one generator set to close to the dead bus. When one generator set “wins” the arbitration it sends a signal to the other generator sets preventing them from closing their breakers and then closes its own paralleling breaker to the bus. At this point the other generator sets recognize that the bus is now live and they synchronize and close to the bus. In a random access system it is not predetermined which generator set will be the one to close to the dead bus. It is a robust paralleling method because if a single generator set fails or is slow to come up to speed the rest of the generator sets are not affected. There is no single point of failure.

 

Dead Bus Paralleling In a dead bus paralleling system generator sets start with their paralleling breakers closed to the bus. All generator sets start with their excitation circuits disabled. This allows generator sets to be connected in parallel without being in sync because no voltage is being generated. As engines reach a preset speed the generator set controls turn on and ramp up excitation levels. This causes the voltage on the bus to build up and forces the generator sets to come into sync with each other. There is a variation of this method known as dead field paralleling in which the generator sets start with the paralleling breakers open and then close them as the engine starter disengages. “Exciter paralleling”, “run up synchronization” synchronizatio n” and “close before excitation” are other terms that describe this same basic paralleling algorithm. Because there is no need for arbitration or synchronizing multiple generator sets, dead bus paralleling can bring a paralleling bus to rated speed and voltage quickly. This, however, is a less robust method of paralleling as each generator set represents a single point of failure. A control or excitation system fault on any generator set compromises the entire system. Most control systems that use dead bus or dead field paralleling employ a timer based system for removing generator sets that fail to start. If the engine has failed to reach a predetermined speed before the delay expires the control opens the breaker disconnecting the failed generator set from the paralleling bus, allowing the rest of the generator sets to energize the bus. If the breaker fails to open however however,, the unpowered alternator will be connected directly to the bus effectively putting a low impedance three phase fault on to the system as the bus backfeeds the alternator.. Similarly a failure of the excitation system alternator that prevents the alternator from building up voltage will have the same effect. With this type of system the control, excitation system and paralleling circuit breaker for each generator set all represent single points of failure for the entire system. This is in contrast to a random access system where the generator sets are not connected to the bus until they are ready to load so any failed generator set will have no effect on any working generator set.  The other risk associated associated with dead bus paralleling paralleling is the potential for circulating currents between generator sets causing diode or MOV failures, nuisance tripping of breakers or other failures of the

 AVR or excitation excitation system. Circulating Circulating currents currents are caused by generators building up internal voltage at differentt rates as excitation is increased. Although differen the terminal voltages of the paralleled generator sets will be the same because they will be electrically connected, the internal voltages of the generators may be different due to differen differentt internal reactances of the generators, different exciter resistances, differen differentt levels of saturation, differen differentt residual voltage levels of the generators at the start of the excitation ramp, and different transient characteristics of the engine as it is ramping up to nominal speed. Current will flow from generators with higher internal voltage to generators with lower internal voltage resulting in some generators being back fed which causes stress to the windings and excitation system. In a random access paralleling system the closed loop load sharing algorithm will effectively eliminate any circulating current. During the excitation ramp in a dead bus paralleling system the voltage reference of the AVR is increased linearly with no feedback. The control is not correcting for any of these differences so there could be substantial current circulating between the generator sets.

Speed of Paralleling

 The sequence of operations operations in a random access access paralleling system consists of three main steps: 1. Generator sets start and ramp up to rated speed and voltage. 2. As generators reach reach rated speed and voltage they arbitrate with each other so that only one generator set is allowed to close to the dead bus. 3. The remaining generator sets synchronize synchronize and close to the energized paralleling bus. Cummins diesel generator sets are designed to meet NFPA NFP A 110 (Standard for Emergency and Standby Power Systems) requirements that life safety loads are restored restored within 10 seconds of a power outage.  The time it takes to ramp up speed speed and voltage will typically range from 6 to 9 seconds depending on generator set size and model. This is the longest segment of the total time to parallel and dead bus paralleling does not result in faster ramping to rated speed and voltage.  The first start arbitration time depends somewhat on how many generators will parallel in the system. With four generators using the Cummins PowerCommand®  3.3 controller, controller, we see an average time of 1.1 seconds between the first generator being ready to load and the first generator closing its breaker to a dead paralleling bus.

power.cummins.com ©2016 Cummins Power Generation

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

   2    0

 

 The time it takes to synchronize synchronize the remaining remaining generators to the paralleling bus depends in part on how much their phase angles differ from the phase angle of the connected generator generator.. This phase angle difference is an uncontrolled variable and therefore the total paralleling time will have some variation. Cummins experience has shown that this synchronizing synchronizin g time ranges from less than 1 second up to 6 seconds. Because each generator set synchronizes synchronize s to the bus independently the number of generator sets in the system does not impact the synchronizing synchronizin g time.

Figure 1 shows a transformer equivalent circuit. circuit.  As voltage is applied to the primary primary terminals of the transformer the level of inrush current that the transformer will draw is a function of the magnetizing impedance of the transformer (Xm). The magnetizing impedance is non-linear. non-linear. It varies due to saturation effects. As the flux in the transformer t ransformer core is driven to saturation levels the impedance becomes very low resulting in the high level of inrush current.  The initial phase angle angle of the voltage waveform waveform and the residual flux in the transformer core at the instant that voltage is applied to the t he transformer primary winding also affect the level of inrush current. Initial phase angle and residual flux result in a dc offset of the flux in the core of the transformer which decays as the transformer becomes energized. The effects of the initial phase angle and the residual flux may be additive or may cancel each other out. The result of this is that the same transformer will draw different different levels of inrush current each time it is energized and each of the three phases will draw different levels of inrush current.

Energizing Transformers When energizing transformers the key concern is the transformer inrush current. The high level of current during magnetization can result in nuisance tripping of overcurrent or undervoltage devices. The level of inrush current is very difficult to model and predict because it depends on a number of factors that are highly variable and not well documented.

 

R1

X1

 

I1

Magnetizing Current

I2

Rm

Xm

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

Figure 1. Transformer equivalent circuit

power.cummins.com ©2016 Cummins Power Generation

   3    0

 

Transformer Inrush Current

3000

2000

1000

  p   m    A      t 0   n   e   r   r   u    C -1000

-2000

-3000

-4000 0

1

 

2

 

3

4

5

6

 

7

8

 

9 15

Time - Seconds

Figure 2 – Inrush Current of a 50 MVA trans transformer former being energized by a 2.5 MW generator set.

Estimating Inrush Current  Another challenge in estimating a maximum maximum inrush current is that data on transformer saturation is generally not published so estimations and assumptions must be made. As an example, SKM system analysis software simply prompts the users to enter a single point to represent represent inrush current for analyzing system protection and coordination schemes. In their documentation they recommend 8 - 12 times transformer full load current for 0.1 seconds. This is a reasonable approximation approximation for

Conventional wisdom on how large of a transformer a single generator set can energize varies between 5 and 20 times the kVA rating of the generator set. The limiting factor typically is that the peak inrush current will be high enough to trip a circuit breaker. breaker. Cummins Power Generation has modeled transformer inrush current when energized by a generator set. Figure 2 is a graph developed by a model of a 2.5 MW generator set energizing a 50 MVA transformer.

transformers which are fed by a stiff source such as a utility substation transformer transformer.. When energizing a transformer with a synchronous generator the inrush current will be limited by the physics of the generator generator.. Characteristics of the alternator decrement curve and the excitation system need to be considered in estimating inrush current.

From the graph we see that the instantaneous peak current is about 3300 amps. The alternator reactances and the magnetizing impedance of the transformer limit the inrush current. The three differen differentt colors in the graph represent the three phases and illustrate the concept of the inrush current being different on each of the three phases as the initial phase angle at the instant of connecting the source will be different for each of the phases.

power.cummins.com ©2016 Cummins Power Generation

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

   4    0

 

Genset Voltage 4200 4100 4000

   t    l   o    V     e   g   a    t    l   o    V

3900 3800 3700 3600 3500 3400 0

1

2

3

4

5

6

7

8

9

5

6

7

8

9

Time - Seconds

Genset Frequency 60.5

60

  z    H 59.5     y   c 59   n   e   u   q 58.5   e   r    F 58

57.5 0

1

2

3

4

Time - Seconds

Figure 3 – Voltage and frequency recovery after a transformer inrush event.

Figure 3 displays the voltage and frequency recovery during this inrush event. It shows that the t he voltage recovers to nominal in about a second and a half and frequency recovers recovers in about 3 seconds. The dip in the voltage recovery corresponds to the under frequency roll off function in the voltage regulator. In this model we see a voltage dip of about 20% and a frequency dip of about 2 Hz. The voltage is recovered recover ed and stabilized within about 1.5 seconds.  The peak inrush current current is about 7.6 times the generator set full load current rating. In this application overcurrent overcurr ent protective devices would need to have their instantaneous trip settings higher than 7.6 times the nominal current to prevent nuisance tripping. When the generator set control has integral overcurrent overcurr ent protection the instantaneous overcurr overcurrent ent setting of protective devices may be set to a high level without risk of damaging the alternator windings or properly sized cables. The Cummins PowerCommand  AmpSentry function is UL listed as an overcurrent overcurrent protective device and provides this protection.

Random Access Paralleling in Power Po wer Plant Applicat Applications ions In a random access paralleling application the first generator set to connect to the paralleling bus is required to energize all of the transformers in the system. The previous example consisted of a 2.5MW (3.125 MVA) genset energizing a 50 MVA transformer; a transformer roughly 16 times the size of the generator set. The model would yield the same result for any combination of transformers connected in parallel at the terminals of the genset with a summed power rating of 50 MVA (such as 10 5 MVA transformers).

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

power.cummins.com ©2016 Cummins Power Generation

   5    0

 

 A more common topology in in power plant applications applications is one in which each generator set is connected directly to a single transformer t ransformer and the transformer secondary windings are all connected in parallel to a common bus. Figure 4 is an example of this topology. Under steady state operation each generator is providing magnetizing current for only one transformer, however, during a black start using random access paralleling a single generator will be energizing all of the transformers.

G

G

G

G

G

0.4Kv

0.4Kv

0.4Kv

0.4Kv

0.4Kv

2.744MVA

2.744MVA

2.744MVA

2.744MVA

2.744MVA

8.7%

8.7%

8.7%

8.7%

8.7%

T-NEA

T-NEA

T-NEA

T-NEA

T-NEA

Yd5

Yd5

Yd5

Yd5

Yd5

0.4/30kV

0.4/30kV

0.4/30kV

0.4/30kV

0.4/30kV

3.15MVA

3.15MVA

3.15MVA

3.15MVA

3.15MVA

6%

6%

6%

6%

6%

On start up the generator sets go through the arbitration process to determine which genset is the first to close to the bus. When that first generator closes it energizes its transformer. As voltage builds up on the secondary side of that t hat transformer the other transformers become energized through their secondary windings. Although the total number of transformers that a single generator is energizing may be the same, the inrush current seen by the generator will be significantly less in this topology because the total current is limited by the transformer that is connected directly to the generator set.  This topology was used as a black start system for a wind farm. Figure 5 is a partial one line drawing from that application. In this application a single

Figure 4 – Common Co mmon power plant topology

500 kW generator set is connected to a 1500 kVA step up transformer transformer.. The high voltage winding of that transformer is connected to 11 x 2700 kVA transformers. The single 500 kW generator set successfully energizes 31.2 MVA of transformers, or nearly 50 times the generator set kVA rating.

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

Figure 5 – Wind farm single line diagram

power.cummins.com ©2016 Cummins Power Generation

   6    0

 

Performance of Random  Access Paralleling Systems Dead bus paralleling is often specified as a method for energizing large transformer banks and paralleling multiple generators quickly. quickly. Cummins generator sets with Power Command controls using random access paralleling are able to match dead bus paralleling performance in both of these areas. One such example consists of 14 2.5 MW generator sets each connected to a step up transformer and paralleled on the high voltage side of the transformers. With this system 35 MW of power was brought on line in less than 15 seconds. In this application the first generator set was ready to load in 7.5 seconds. Arbitration between generator sets took 1.5 seconds so the first generator set closed its breaker to begin energizing all of the transformers just over 9 seconds after the system started. As the remaining generator sets sensed that voltage was available on the low voltage side of their connected transformer they synchronized to that voltage and closed their breakers. b reakers. All generator sets were on line 14.25 seconds after starting.

Conclusions Dead bus paralleling has been used in the past as a low cost paralleling method because generators could be paralleled without the use of an external synchronizer.. As generator set controls evolved to synchronizer the point where the synchronizer function became integral to the control, the cost advantage of a dead bus paralleling system was negated. Recently there has been increased interest in energizing large transformer banks and bringing large systems on line quickly which has made dead bus paralleling seem attractive, however dead bus paralleling introduces failure modes that are not present in random access paralleling systems.  Appropriate overcurrent overcurrent pr protection otection settings allow single generator sets to energize transformers many times their size, particularly when the generator set controls have a current regulation function such as Cummins PowerCommand AmpSentry. Systems controlled by Cummins PowerCommand controls have consistently demonstrated the capability to parallel multiple generator sets within 10 – 15 seconds, even when energizing transformers is part of the sequence. A well designed system using random access paralleling offers the best combination of system speed and reliability.

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

power.cummins.com ©2016 Cummins Power Generation

   7    0

 

 About the author

Rich Scroggins is a Technical Advisor in the Application Engineering group at Cummins Power Generation. Rich has been with Cummins for 18 years in a variety of engineering and product management roles. Rich has led product development and application work with transfer switches,

switchgear controls and networking and remote monitoring products and has developed and conducted seminars and sales and service training internationally on several products. Rich received his bachelors degree in electrical engineering from the University of Minnesota and an MBA from the University of St. Thomas.

power.cummins.com ©2016 Cummins Power Generation Inc.  All rights reserved. Cummins Cummins Power Generation Generation and Cummins are registered trademarks of Cummins Inc. “Our energy working for you.” is a trademark of Cummins Power Generation. GLPT-6174-EN (02/16)

   N    E      4    7    1    6      T    P    L    G    #   c    i   p   o    T   r   e   w   o    P  

   8    0