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Guide for electrical design engineers

Power Quality Julian Wiatr WP Poland Krakow [email protected]

Emergency and uninterruptible power supplies for buildings and other structures—sizing the source power

Power Quality

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1. INTRODUCTION Prior to designing a power supply system for a building or other structure, a designer shall carry out a detailed analysis concerning the requirements for reliability of power supply of loads to be installed in the designed building or facility. Diverse requirements concerning reliability of supply have enforced classification of loads into power supply categories defined according to the criteria accepted in power management: - Power supply category III loads – upon which an arbitrary long interruption of supply has no adverse effects, - Power supply category II loads – upon which a short supply interruption (up to several minutes) interruption of supply has no adverse effects, - Power supply category I loads – upon which even a short interruption of supply may bring about hazard for human life or substantial material losses due to e.g. interruption of production process. The example of a building where all categories of supply can be found is shown in figure 1.

Fig. 2 Diagram of a motor-generator set with a short starting time 1 – electric motor for the generator and flywheel drive, 2 – generator, 3 – flywheel; 4 – electromagnetic clutch, 5 – internal combustion engine; 6 – control panel and auxiliaries' switchboard

Fig. 1 Power supply of telecommunications facility 1-Automatic standby switching system 2-Generating Set

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2. MOTOR-GENERATOR SETS A motor-generator set is a commonly used source of emergency power supply for buildings and other structures. Its basic components are: • internal combustion engine ― converts fuel chemical energy into

mechanical energy • generator ― converts mechanical energy into electric energy • rotational speed governor • generator voltage regulator • generator field controller • control system • starting system • switchgear.

Commercially available generator sets have rated powers from several kVA to 6 MVA, and are designed for various modes of operation, indoor mounted or installed in a freestanding container. The way a generator set is operated influences several aspects, such as: its lifetime, economic efficiency, reliability, etc. The mode of operation shall therefore be agreed with manufacturer prior to the decision on purchase. The modes of motor-generator sets operation are: •

continuous operation ― the generating set is operated in a continuous manner (non-stop) and any interruptions result from the necessity for repairs or maintenance periodic sporadic,

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intermittent operation ― the generating set is operated in specified, limited time intervals.

•

A generating set can be operated alone or in parallel connection, where two or more sets co-operate, or be synchronized with the power network. In the case of synchronized operation the prior consent from the grid operator shall be obtained and the conditions of co-operation agreed upon. Table 1 gives the key requirements concerning voltage and frequency limit values for particular performance classes as required by provisions of standard ISO 8528-5: Reciprocating internal combustion engine driven alternating current generating sets ‑ Part 5: Generating sets [2]. Standard ISO 8528–1: Reciprocating internal combustion engine driven alternating current generating sets ‑ Part 1: Application, ratings and performance [1], defines four performance classes:

G3

Concerns loads where only basic parameters of voltage and frequency need to be specified, e.g. general-purpose applications like electrical heating. Concerns loads where requirements for electric power supply are similar to those specified for public power networks. In the event of changes in load, temporary deviations of voltage and frequency from their nominal values are acceptable. Examples being lighting systems, pumps, fans, hoists, etc. Concerns loads with enhanced power quality requirements. Examples

G4

Concerns loads with exceptionally high requirements for power quality.

G1

G2

Generating sets are also categorized according to their starting time, i.e. the time that elapses from the instant of voltage loss in a power network to the instant the voltage is applied from the generating set: (i) long duration voltage loss; (ii) short duration voltage loss, and (iii) without voltage loss. Generating sets with long duration voltage loss emergency power supply systems. Such sets are self-starting and shutdown systems. Upon loss network a remote control system activates the 4

are commonly used in provided with automatic of voltage in a power generating set starting

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procedure. In order to facilitate the start-up process, generating sets are provided with heaters to keep the required temperature of the engine block. The heaters supply system comprises a thermostat to ensure constant temperature of the engine block. The power of heaters depends on the generating set power and is defined by the manufacturer. The heaters are supplied from the generating set auxiliaries' switchboard. Loads supplied from the generating set are switched-on by the automatic stand-by switching system and can be switched simultaneously or in sequence. Parameter

U n i G1

Operational limit value for the requirements class: G2 G3 G4

Frequency drop

%

≤8

≤5

≤3

Relative frequency steady state

%

≤ 2.5

≤ 1.5

≤ 0.5

≤+18

≤ +12

≤ +10

≤ -15

≤ -10

≤ -7

Transient frequency deviation

on loss of load (100%) on application of step load

%

Frequency recovery time to the nominal value

s

≤ 10

≤5

≤3

Voltage steady state deviation band

%

≤ ±51)

≤ ±2.5

≤ ±1

≤ +35

≤ +25

≤ +20

≤ -25

≤ -20

≤ -15

≤ 10

≤6

≤4

Transient voltage deviation

on loss of load (100%) on application of step load

Voltage recovery time to the nominal value

Parameters values by agreement between manufacturer and customer

%

s

Note: Complete requirements are given in standard ISO8528-5 [1] 1)

For generating sets with rated powers up to 10 kVA: ≤ 10 %

Table 1 Key requirements concerning voltage and frequency limit values for performance classes of generating sets [2] Upon voltage loss in the power network the control system activates starting procedures with several second delay. Such solution allows avoiding unnecessary starts caused by voltage dips or short supply interruptions. The delay is normally 5-10 seconds. Time elapsing between 5

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the loss of voltage in the power network and the availability of the emergency power usually does not exceed 1 minute. After the voltage recovery in the power network the automatic stand-by switching system switches to the main supply without stopping the generating set. After the switchover the generating set is run idle for about next 3 minutes for cooling the generator. Figure 2 shows diagram of a generating set with short starting time In normal conditions the electric synchronous motor (1) draws energy from power network and drives the generator (2) and flywheel (3). The clutch (4) is disengaged. The generator is running idly. Upon loss of voltage in the power network the switch Q1 closes and the clutch (4) is automatically engaged. Kinetic energy stored in the flywheel is used for starting the internal combustion engine (5), which within a short time begins to drive the generator. Applying the voltage from the generator switches automatically the Q2 switch to emergency power supply and energizes the loads. Normally the time of power supply disruption does not exceed 2 seconds.

Fig. 3 Diagram of a motor-generator set with zero changeover time 1 – electric motor for the generator and flywheel drive, 2 – generator, 3 – flywheel; 4 – electromagnetic clutch, 5 – internal combustion engine; 6 – control panel and auxiliaries' switchboard Fig. 4 Typical installation of a generating set in a room ― the flexible components 1 ‑ antivibration mounts, 2 ‑ flexible connections in the exhaust system, 3 ‑ flexible connections in the heated air discharge

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Figure 3 shows schematic diagram of a generating set with zero time of changeover to emergency power supply. The set comprises a synchronous motor with rated power equal to the generating set generator power. The loads are normally not supplied from the power grid but from the generating set operated in a continuous manner. In normal operating conditions the synchronous motor (1), supplied from the power grid, drives the generator (2) and the flywheel (3). In the event of voltage loss in the power grid the clutch couples the flywheel to the internal combustion engine (5). Kinetic energy stored in the flywheel is used for quick start of the internal combustion engine (5), which now drives the generator. The switch Q2 has only a maintenance function. A generating set can be installed in an outdoor container-type housing placed on a concrete foundation outside the building or in a specially adapted room. In both cases an air intake, exhaust gases discharge and adequate room ventilation are required. This problem shall be solved by a designer according to the requirements of the generating set manufacturer and therefore is outside the scope of this publication. A set installed in a container by the manufacturer is a complete plant in both the electrical engineering and safety terms. Where a room is adapted for the purposes of a generating set installation all the manufacturer's requirements shall be complied with. Figure 4 shows the example of a generating set installation in a room. The room shall be provided with auxiliaries' switchboard, lighting, socket outlets, and control installation for the purposes of ventilation and other equipment, as required. Figure 4 shows a generating set room with wall air intake and air exhaust vent. A generating set produces heat from the following sources: the engine, generator (alternator), radiator, exhaust pipe and silencer. Insufficient ventilation in the room with a running generating set may cause undesired rise of the room temperature that may lead to the engine power reduction and, consequently, the set shutdown. In such case a forced-air cooling, e.g. by means of fans should be applied. The airflow direction within the room should preferable be: the generator => engine =>

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radiator. That solution not only allows the heat produced by the generating set to be removed from the room but also provides combustion air. Figure 5 shows the correct, typical arrangement of a generating set room. The drawing provides example dimensions and distances for installation of the generating set installation with rated power 60-80 kVA.

Fig. 5. The section and floor plan of installation in the room 1 – generating set, 2 – switchboard and control panel, 3 – foundation, 4 ‑ ventilation air outlet, 5 – cable duct, 6 – air intake, 7 – silencer, 8 –exhaust pipe, 9 – flexible element

3. SIZING THE GENERATING SET POWER Sizing the generating set power shall be based on the demanded active power and reactive power of loads to be supplied from the emergency power supply system. The active power demand can be computed from: n

PZ = ∑ k Z ⋅ Pi

(1)

i =1

where:

PZ - active power [kW] kZ

- demand factor [-]

Pi

- active power of the i-th load to be supplied from the emergency power system [kW]. 8

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In the next step the reactive power is determined: n

n

i =1

i =1

QZ = ∑ k Z ⋅ tgϕ i ⋅ Pi =∑ k Z ⋅

1 − 1 ⋅ Pi cos 2 ϕ i

(2)

where:

QZ

- reactive power [kvVar]

cos ϕ i - power factor of i-th load to be supplied from the uninterruptible power supply system [-]. The power factor cos ϕ Z is determined from the active and reactive power values: cos ϕ Z =

PZ

(3)

PZ2 + QZ2

The following step is finding the minimum active power to be provided by generating set generator. Determining thr apparent power from the active power reactive power demand using the formula: SG ≥ P Z2 +QZ2

(4)

may lead to erroneous results. The generator shall be able to deliver the demanded active power PZ and demanded reactive power Q Z if it thus operates at a lower power factor cosϕ Z < cosϕ nG , the utilization of generator active power decreases due to the stator heating. The internal combustion engine is matched to the generator's rated active power i.e. to operation with the rated power factor, therefore if the generator operates at a lower power f ac t or cosϕ Z < cosϕ nG , the engine utilization is lower. The generator relative loading with active power can be described by the utilization factor, which can be calculated from the formula:

p=

cosϕ Z cosϕnG

(5) 9

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The required minimum active power of the generating set shall satisfy the inequality: PG min ≥

PZ p

(6)

The utilization factor p, calculated from the formula (5) shall be inserted to the formula (6). If p ≥ 1, the value 1 has be taken in the formula (6). The value of power factor cos ϕnG shall be taken according to the generating set specification. In case the information on rated power factor is not available, the value cos ϕ nG = 0.8 should be taken. The generating set apparent power shall satisfy the inequality:

S nG ≥

PG min cos ϕ z

(7)

where PG min is the minimum active power to be delivered by the generating set generator, in [kW]. A low value of power factor cos ϕ Z will result in reduction of the generator electromotive force, due to the demagnetising effect of the load current reactive component. If the generator generates a larger reactive power than its nominal value, the load active power should be reduced in order to maintain the nominal voltage magnitude and to not overload the rotor. Within the permissible range of rotor current, the generating set control system controls the generator excitation current to maintain the output voltage at constant level. Therefore the generator operation at power factor cosϕZ < cosϕ nG results in the necessity for increasing the generator's output power in order to deliver the full demanded active power PZ and reactive power Q Z . The use of reactive (particularly inductive) power compensators is not recommended since it may lead to premature damaging of capacitors due to the character of the power supply source operation.

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Where the generating set is intended to supply electric motors its power should be sized considering the motors' starting currents which shall not exceed the generator rated current, taking into account permissible temporary overload specified in the manufacturer's data. Non-linear loads draw distorted currents and, consequently, distort the source current. This distortion results in the presence of harmonics and interharmonics (also subharmonics) in the supply network and final circuits, which, generally are shifted in phase with respect to the voltage. In the effect of harmonics, apart of the active and reactive power, also the deformation power V appears in the system; hence the apparent power cannot be determined as the product of fundamental harmonic voltage and current. The value of deformation power V depends on the current and voltage distortion level, i.e. on the harmonic content and, in polyphase circuits, also on the asymmetry level. In asymmetric loads the power factor cosφ is not the same in all phases. It can be different in each phase depending on the active and reactive power load in a given phase. A detrimental effect of asymmetric loading is the voltage increase above the nominal value in the least-loaded phase. Since the estimation of the additional power resulting from asymmetric loading is difficult, the design should ensure the asymmetry of loads supplied from the emergency generating set will not exceed 20%, according to manufacturers' recommendation. The apparent power of a non-linear load can be found from the relation: (8)

S 2 = P2 + Q2 + V 2

The active power of distorted waveforms is estimated as the sum of active powers of voltage and current harmonics of the same frequency and reactive power is determined from relations (10): ∞

P = ∑ U k I k cos ϕ k k =1

∞

(9)

Q = ∑ U k ⋅ I k ⋅ sin ϕ k k =1

(10)

The apparent power of non-linear loads is: 2

S = P2 + Q2

(11) 11

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In this case the deformation power is V = 0. The graphic illustration of powers P, Q, V, S and S is depicted in figure 6.

Fig. 6. Power tetrahedron for a system with distorted voltage and current waveforms

P – active power [kW]; Q – reactive power, [kvar]; S ‑ apparent power of the linear part of the circuit [kVA]; S – apparent power of the non-linear circuit [kVA]; V – deformation power [kVA]; D – distortion power, defined as D 2 = Q 2 + V 2 Figure 6 explains also that in non-linear circuits the power factor cannot be defined by the formula (3), which holds only for linear circuits:

cos ϕ =

P S

(12)

In non-linear circuits the power factor is defined (cf. figure 6) as: cos Ψ =

P I1 P = ⋅ = 2 S I P + Q2

P 3U n

∞

∑I

2 k

k =0

12

(13)

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where the angle ϕ k is the phase shift of the k-th order harmonic voltage and current, sin ϕ k = 1 − cos 2 ϕ k . The nominal current of a three-phase load that draws a distorted current can be found from the formula: In =

Pn 3U n cos Ψ

(14)

It follows from relations (13) and (14) that with the load's steady state current equal to its nominal value IN and increased distortion of the actual current, the active power the given equipment can be loaded with, should be reduced below its rated value. Therefore non-linear loads drawing distorted current from the generator reduce the generating set active power utilization. The generator's power shall be increased in order to meet the power demand of such loads. The generator's minimum active power required to meet the power demand of these loads should be determined from the formula:

PG min ≥

PZ pW

where: p Pz

(15) – the utilization factor calculated from the formula (5) [-], – active power demanded by loads to be supplied from the emergency power supply system [kW]

PGmin – the generator's required minimum active power [kW] W ≈(

100 )2 100 + THDi %

- deformation index [-],

THDi% – total harmonic current distortion factor [-].

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The generating set power should be determined from the formula (7). Total harmonic distortion THDi% can be found from the formula:

∞

THDi =

∑ (I k =2

I1

k

)2 100%

(16)

where: Ik – rms value of the k-th current harmonic [A] I1 – rms value of the current fundamental harmonic [A] k – harmonic order [-]. Example values of the deformation index W, depending on the THDi% values are given in table 2. Table 2 The deformation index W vs. the THDi% factor THDi% 3% 5% 8% 10 % 15 % 20 % 30 % 40 %

W 0.95 0.91 0.86 0.83 0.76 0.70 0.60 0.51

When the THDi% factor increases, the deformation index W decreases, thus the generator power required to meet the power demand should be increased.

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4. SIZING THE UPS POWER Sizing the UPS rated power shall be based on the demanded active power and reactive power of loads to be supplied from it. The active power demand should be computed from the formula (1) and reactive power demand should be computed from the formula (2). In the next step minimum apparent power is calculated from the demanded active and reactive powers using the formula (17)1: (17)

S nUPS ≥ P Z2 +QZ2

If electric motors are to be fed from the uninterruptible power supply system it shall be capable of meeting the increased active power demand due to the motors starting. Power demand of non-linear loads can be calculated from relation: n

PZ = ∑ k z i =1

Pi Wi

(18)

Motors' starting currents and distorted currents shall be taken into account when sizing the UPS output power to ensure its correct operation. A UPS of insufficient output power, supplying electric motors, will in response to load increase automatically switch to external bypass and therefore cut off the uninterruptible power supply function 2. Also the rated crest factor, which is the ratio between the load's instantaneous peak current and the rms current value, shall be considered when selecting a UPS system. For the currently manufactured UPS units the crest factor value is normally 3. If the crest factor of a current drown from the UPS exceeds its rated value, the UPS operation can be impaired, leading even to its shutdown. _________________________ 1

The apparent power determined from the formula (17) refers to the UPS rated power is

specified in manufactuers’ data sheets as the UPS rated power. 2 The UPS power as specified in manufacturers’ data sheets. The UPS input power isnot equal to its output power. The power drawn by the UPS power supply unit from the power network is larger than the power supplied to loads; this should be taken into account when designing the UPS power supply. When sizing the UPS power supply unit on the basis of the demanded active power it advisable to allow 25 % margin for compensation of temporary increase in power, or possible estimation error.

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Since the UPS shall be able to deliver the demanded active power PZ and demanded reactive power Q Z , thus if it operates at a lower power factor cosϕ Z < cosϕ nG , the utilization of the UPS's active power decreases due to limited capability of the inverter semiconductor switches. The UPS relative loading with active power can be described by the utilization factor:

p=

cosϕZ cosϕnUPS

(19)

The UPS required minimum active power must satisfy the inequality:

PUPS min ≥

PZ p

(20)

The utilization factor p, calculated from the formula (19) shall be inserted to the formula (20). If p ≥ 1, in the formula (20) the value 1 has been taken. The power factor value cos ϕnUPS shall be taken according to the UPS specification. If information on the rated power factor is not available, cos ϕ nUPS = 0.8 should be taken for transformer UPS, or cos ϕ nUPS = 0.9 for transformer-less UPS power supplies incorporating IGBT inverters. The UPS apparent power shall satisfy the inequality:

SnUPS ≥

PUPS min cos ϕnUPS

(21)

where: PUPS min - minimum active power [kW] cos ϕnUPS

- the UPS rated power factor [-].

cos ϕ Z 16

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A low value of power factor results in the inverter overloading and, in consequence, may result in turning it off or switching the UPS to internal bypass circuit. If the UPS delivers reactive power larger than its rated value, the load active power should be reduced in order to maintain the nominal voltage magnitude and to not overload the inverter. Therefore the UPS operation at power factor cos ϕ Z < cos ϕ nG a r i s e s the necessity for increasing its output power in order to deliver the demanded active power PZ and demanded reactive power Q Z . T h e use of reactive (particularly inductive) power compensators is not recommended since it may lead to premature damaging of capacitors due to the character of the power supply source operation. Where the UPS is intended to supply equipment with large inrush currents, its power should be sized considering the inrush currents which shall not exceed the UPS rated current, taking into account permissible temporary overload given in the manufacturer's specification. When the UPS supplies non-linear loads they draw and, consequently, distort the source current. This distortion results in the presence of harmonic and interharmonic currents in the supply network and final circuits, generally being shifted in phase with respect to the voltage. In the effect of harmonics also the deformation power appears in the system, apart of the active and reactive power; hence the apparent power cannot be determined as the product of fundamental harmonic voltage and current. The value of deformation power V depends on distortion current and voltage level, i.e. on the harmonic content and, in polyphase circuits, also on the asymmetry level. In asymmetric loads the power factor cosj is not the same in all phases. It can be different in each phase depending on the active and reactive power load in a given phase. It is difficult to estimate the deformation power caused by asymmetric load, but the present-day transformer-less UPSs with IGBT inverters are immune to the output load asymmetry.

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5. DC POWER SUPPLIES ― TELECOM POWER PLANTS The telecom power plant is a DC power supply whose principle of operation is similar to that of the UPS. It is used to provide 48V DC voltage power supply for telephone exchanges. In the event of voltage loss in the power network, the energy is drawn from batteries being a component of the DC power supply. Sizing a telecom power plant consists in selection of a specified equipment type and determining the number of needed DC power supply units (PSU). For the reliability reasons the number N of PSU modules must be increased by 1, i.e.: N' = N +1

(22)

The basis for all calculations is the active power PC demanded by a telephone exchange to be supplied from the telecom power plant. The formulas below are given to facilitate these calculations: Pc = I ncU nc

PŁB = 52I Ł

(23) (24)

3

(25)

PwejSTK = PC + PŁB IŁ =

N=

I ncT p TŁ

(26)

PweSTK P1

(27)

______________________________ 3

The Number 52 in formula (24) is the value of battery charging voltage.

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Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

(28)

' PwejSTK ≈ N ' P1 = ( N + 1) P1

Where : ' PwejSTK - input active power of telecom power plant [W]

PwejSTK - input active power of telecom power plant necessary to cover the loads' power demand [W] PC

- demanded power of the telephone exchange [W]

N

- the number of required DC power supply units [-]

PŁB

- battery charging power [W]

IŁ

- battery charging current [A]

P1

- power of a single DC power supply unit [W]

Inc

- total current drawn by the telephone exchange [A]

Tp

- battery operation back-up time [h]

TŁ

- battery charging time (to be taken 10 hrs.)

Unc

- the telecom power plant rated voltage [W]

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Figure 7 shows the telecom power plant sizing flowchart.

PUPSwe The telecom power plant input power, which has to be known in order to balance the supplied building power demand, shall be computed from the following formulas:

PwejSTK = (

' PwyjSTK

η

QwejSTK = P

' wyjSTK

+ PŁB )

1 W

tgϕ wejSTK (29)

' S wejSTK = ( PwyjSTK ) 2 + (QwejSTK ) 2

1

Where :

tgϕ wejSTK =

cos ϕ wejSTK

- power factor of the telecom power plant input power [-]

QwejSTK

cos ϕ wejST K 2

−1

- the telecom power plant reactive power [var]

When the telecom power plant is supplied from a generating set, it should be borne in mind that generator's long-duration operation at full loading 20

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and THDi getting close to 8%, may result in premature wear of the generator unless its power is not oversized. A long-duration operation under the generator full load should, therefore, be avoided.

6. UPS – GENERATING SET COMBINATION The reliability of a UPS system can be improved by employing an additional emergency power supply source, i.e. a generating set. Such system ensures safety and confidence that in the event of the normal power supply system failure critical loads will be supplied without interruptions. That solution will protect consumers against losses, often substantial, caused by supply interruptions. A UPS should be sized for the estimated power of loads. It should be remembered that the total power of loads should not exceed the UPS active output power or the apparent output power. It is recommendable to slightly oversize (10-20 %) the UPS system in order to provide a reserve for temporary increase in the loads power or possible estimation errors. A UPS intended for use in co-operation with generating set should be a barrier between loads and the generating set in order to mitigate the impact of distorted currents drawn by non-linear loads (e.g. IT equipment) upon the generating set. The UPS system should ensure that its input current waveform is not influenced by the shape of the load current. The generating set should be capable of meeting full power demand of the UPS and the category II loads. Its power is the sum of the UPS input power consumed under full loading and the power of category II loads.

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PGEN = PUPSwe + PII

(30)

w h e r e : PUPSwe - the UPS input power [kW]

PII

- total power of the category II loads [kW].

The UPS input power is computed from the formula:

PUPSwe =

PUPSwy

ηW

+

PB PUPSwy 0.25PUPSwy = + W ηW W

(31)

w h e r e : PUPSwy - the UPS output active power [kW]

η - the UPS efficiency [-] W

- deformation index, [-]

PB - the UPS additional input power associated with battery charging (at least 25% of the UPS rated power) [kW]. If the UPS has a possibility for extension (its configuration supports increasing the output power), the maximum power shall be taken into account. It is recommended to use UPSs provided with a special interface with generating set that allows limiting of the input current by disabling the battery charging function until the voltage recovery in power grid. In such case a 25% surplus power, needed for the battery charging, is not required4. A UPS intended for co-operation with a generating set should be provided with harmonic filter reducing harmonic content in the input current to ca. 10%; a further reduction is ineffective for it does not significantly improve the UPS–generating set co-operation and therefore is not economically justified. The on-line UPS topology is recommended as the one that exclusively ensures a correct co-operation between the UPS and generating set. It is recommended to use generating sets equipped with electronic rotational speed governors and generators designed for nonlinear load. _____________________________ 4

If the genset is specifically designed to supply non-linear loads the degree of its oversizing can be lower; it should, however, be agreed with a manufacturer. 22

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org EXAMPLE 1 Size the power of a generating set intended for emergency power supply of the below given loads: ο three squirrel cage induction motors with ratings: PNS=7.5 kW; kr = 6; cosj = 0.8; h=0.8; UN =3x400V; sN = 5%; kMr = 2.3 ο UPS with ratings: PN = 15 kW; cosj = 0.95; THDi = 8%; UN =3x400/230 V; h=0.9 ο lighting loads of total power P=5 kW; cosj =0.7 and UN =230V (the loads are grouped symmetrically ensuring equal load in each phase). The rated power of a single motor:

PNS

IN =

3U n cosϕη

75000 = 15.04 A 3 ⋅ 400 ⋅ 0.8 ⋅ 0.9

=

Starting current in delta connection:

I r Δ = k r I N = 6 ⋅15.04 = 90.24 A It is a large current and should be limited. One of the methods is the use of a star/delta starter that reduces the starting current about 3 times. Thus: IrY = I r = 90.24 = 30.08 A

3

3

Due to the large starting current of a single motor, simultaneous start of all motors shall be prevented by means of a control system. Thus, assuming sequential motors starting, the peak power is:

cos ϕ r = cos ϕ

η 1 − sN

M rw 0,90 2,30 + 0.025k r ) = 0.80 ( − 0,025 ⋅ 6) = 0.40 kr 1 − 0,05 6

(

Pszs =

3U N IrY

Qszs=

3U N IrY cos ϕ r

cosϕ r + 2 PNS = 3 ⋅ 30.08 ⋅ 400 ⋅ 0.40 + 2 ⋅ 7500 = 2333.60 W 1 cos ϕ r 2

= 3 ⋅ 400 ⋅ 30.08 ⋅ 0.40 ⋅

− 1 + 2 PNS ∗

1 cos 2 ϕ

−1 =

1 1 − 1 + 2 ⋅ 7500 ⋅ − 1 ≈ 30.33 k var 2 0.40 0.82

The UPS demanded power:

100 100 2 )2 = ( ) ≈ 0.86 W =( 100 + THDi % 100 + 8 PwejUPS =

PwyjUPS Wη

QwejUPS = PwejUPS

+

0,25PwyjUPS W 1 cos ϕ wejUPS 2

=

15000 0.25 ⋅ 15000 + ≈ 23740 W 0.86 ⋅ 0.90 0.86

− 1 = 23740

1 − 1 = 7.80 k var 0.952

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Total demanded active power: PZ = PSZS + PwejUPS + Posw Q Z = Q SZS + Q wejUPS + tgϕPosw

Pz = 23336 + 23740 + 5000 = 31073.60 ≈ 52.08 kW 1 − 1 ⋅ 5.0 ≈ 43.24 k var 0 .7 2

Qz = 30.33 + 7.80 + tgϕ z =

Qz 43.23 = = 0.86 Pz 52.08

cos ϕ z = p=

1 tg ϕ z + 1 2

=

1 = 0.76 0.862 + 1

cos ϕ z 0.76 = = 0.95 cos ϕ nG 0.80

Pz 52.80 = = 55.58 kW p 0.95 P 55.58 SG ≥ G min = = 73.14 kVA cos ϕ z 0.76 PG min ≥

On the basis of the manufacturer's catalogue the above conditions are met by the generating set with rated power 80 kVA.

7. POWER NETWORK ― GENERATING SET CONFIGURATIONS A generating set is an emergency power source that cannot supply electric power to the electric power system. Power network‑generating set configurations shall be therefore designed to preclude parallel operation of sources, or voltage application back to the power system when it is deenergized. Where a generating set is manually started, manual switches shall be used (Fig. 8). Generating sets provided with automatic self-starting and shutdown systems should be equipped with automatic stand-by switching system with mechanical and electrical interlocks. An example of such system is given in figure 9. It should be remembered that since a part of automatic control systems is installed within the set, in the standby mode they shall 24

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be supplied from the power network (heaters, phase detector, etc). These circuits should be provided with the overload and electric shock protection and be arranged the in TN-S system.

Fig. 8. Manual switch power network/generating set Fig. 9. Automatic standby switching system network/generating set

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8. EXAMPLES OF EMERGENCY POWER SUPPLY SYSTEMS Figure 10 shows the diagram of a power supply system with dedicated genset switchboard. That arrangement is used when selected loads are to be supplied from the generating set.

Fig. 10. Diagram of the emergency power supply system supplying dedicated circuits 1 - Automatic standby switching system; 2 - Capacitor banks

In normal conditions switches Q3 and Q5 are opened, whereas switches Q1, Q2 and Q4 are closed. The automatic standby switching system network/network operates in the hidden reserve mode. In effect of a voltage loss in any of the lines feeding the transformers T1 and T2, the switches Q1, Q2 and Q3, Q4 become opened, and the generating set initiates the auto start procedure. Upon completion of the generating set start-up the switch Q5 is closed and critical loads are fed from the generating set. A voltage recovery in one of the power lines causes the switch Q5 to open, and to close switches: Q3, Q4, and Q1 or Q2 ‑ depending on in which line the voltage has occurred. The generating set is run idly for the next 3 minutes to cool the generator windings and then shut down. 26

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If voltage recovery occurs in both lines feeding the transformers T1 and T2, the power supply system returns to its normal operating configuration. Where the generating set is intended supply all loads after a voltage loss in the power network, the power supply system configuration shall be designed as shown in figure 11.

Fig. 11. Diagram of an emergency power system supplying all loads 1 - Capacitor banks;

In the configuration shown in figure 11 the switches Q1, Q2 and Q4 are closed in normal conditions, while the switches Q3 and Q5 are open. The system operation is almost analogous to that of the configuration shown in figure 10. When loads are supplied from the generating set, reactive power compensation capacitor banks are disconnected. Figures 12 and 13 show emergency power systems utilizing more than one generating set.

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Fig. 12. An emergency power system with two generating sets supplying separate sections of the main switchboard 1 - Capacitor banks;

Fig. 13. An emergency power system with several generating sets intended for parallel operation 28

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The solution shown in figure 13 is employed for supplying large loads. In the event of voltage loss in power lines feeding the transformers T1 and T2 the generating sets are individually turned on. The number of running generating sets depends on the loads' power demand controlled by the control system.

REFERENCES 1. ISO 8528–1: Reciprocating internal combustion engine driven alternating current generating sets - Part 1: Application, ratings and performance. 2. ISO 8528-5: Reciprocating internal combustion engine driven alternating current generating sets - Part 5: Generating sets.

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Power Quality Julian Wiatr WP Poland Krakow [email protected]

Emergency and uninterruptible power supplies for buildings and other structures—sizing the source power

Power Quality

Power Quality www.leonardo-energy.org

1. INTRODUCTION Prior to designing a power supply system for a building or other structure, a designer shall carry out a detailed analysis concerning the requirements for reliability of power supply of loads to be installed in the designed building or facility. Diverse requirements concerning reliability of supply have enforced classification of loads into power supply categories defined according to the criteria accepted in power management: - Power supply category III loads – upon which an arbitrary long interruption of supply has no adverse effects, - Power supply category II loads – upon which a short supply interruption (up to several minutes) interruption of supply has no adverse effects, - Power supply category I loads – upon which even a short interruption of supply may bring about hazard for human life or substantial material losses due to e.g. interruption of production process. The example of a building where all categories of supply can be found is shown in figure 1.

Fig. 2 Diagram of a motor-generator set with a short starting time 1 – electric motor for the generator and flywheel drive, 2 – generator, 3 – flywheel; 4 – electromagnetic clutch, 5 – internal combustion engine; 6 – control panel and auxiliaries' switchboard

Fig. 1 Power supply of telecommunications facility 1-Automatic standby switching system 2-Generating Set

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2. MOTOR-GENERATOR SETS A motor-generator set is a commonly used source of emergency power supply for buildings and other structures. Its basic components are: • internal combustion engine ― converts fuel chemical energy into

mechanical energy • generator ― converts mechanical energy into electric energy • rotational speed governor • generator voltage regulator • generator field controller • control system • starting system • switchgear.

Commercially available generator sets have rated powers from several kVA to 6 MVA, and are designed for various modes of operation, indoor mounted or installed in a freestanding container. The way a generator set is operated influences several aspects, such as: its lifetime, economic efficiency, reliability, etc. The mode of operation shall therefore be agreed with manufacturer prior to the decision on purchase. The modes of motor-generator sets operation are: •

continuous operation ― the generating set is operated in a continuous manner (non-stop) and any interruptions result from the necessity for repairs or maintenance periodic sporadic,

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intermittent operation ― the generating set is operated in specified, limited time intervals.

•

A generating set can be operated alone or in parallel connection, where two or more sets co-operate, or be synchronized with the power network. In the case of synchronized operation the prior consent from the grid operator shall be obtained and the conditions of co-operation agreed upon. Table 1 gives the key requirements concerning voltage and frequency limit values for particular performance classes as required by provisions of standard ISO 8528-5: Reciprocating internal combustion engine driven alternating current generating sets ‑ Part 5: Generating sets [2]. Standard ISO 8528–1: Reciprocating internal combustion engine driven alternating current generating sets ‑ Part 1: Application, ratings and performance [1], defines four performance classes:

G3

Concerns loads where only basic parameters of voltage and frequency need to be specified, e.g. general-purpose applications like electrical heating. Concerns loads where requirements for electric power supply are similar to those specified for public power networks. In the event of changes in load, temporary deviations of voltage and frequency from their nominal values are acceptable. Examples being lighting systems, pumps, fans, hoists, etc. Concerns loads with enhanced power quality requirements. Examples

G4

Concerns loads with exceptionally high requirements for power quality.

G1

G2

Generating sets are also categorized according to their starting time, i.e. the time that elapses from the instant of voltage loss in a power network to the instant the voltage is applied from the generating set: (i) long duration voltage loss; (ii) short duration voltage loss, and (iii) without voltage loss. Generating sets with long duration voltage loss emergency power supply systems. Such sets are self-starting and shutdown systems. Upon loss network a remote control system activates the 4

are commonly used in provided with automatic of voltage in a power generating set starting

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procedure. In order to facilitate the start-up process, generating sets are provided with heaters to keep the required temperature of the engine block. The heaters supply system comprises a thermostat to ensure constant temperature of the engine block. The power of heaters depends on the generating set power and is defined by the manufacturer. The heaters are supplied from the generating set auxiliaries' switchboard. Loads supplied from the generating set are switched-on by the automatic stand-by switching system and can be switched simultaneously or in sequence. Parameter

U n i G1

Operational limit value for the requirements class: G2 G3 G4

Frequency drop

%

≤8

≤5

≤3

Relative frequency steady state

%

≤ 2.5

≤ 1.5

≤ 0.5

≤+18

≤ +12

≤ +10

≤ -15

≤ -10

≤ -7

Transient frequency deviation

on loss of load (100%) on application of step load

%

Frequency recovery time to the nominal value

s

≤ 10

≤5

≤3

Voltage steady state deviation band

%

≤ ±51)

≤ ±2.5

≤ ±1

≤ +35

≤ +25

≤ +20

≤ -25

≤ -20

≤ -15

≤ 10

≤6

≤4

Transient voltage deviation

on loss of load (100%) on application of step load

Voltage recovery time to the nominal value

Parameters values by agreement between manufacturer and customer

%

s

Note: Complete requirements are given in standard ISO8528-5 [1] 1)

For generating sets with rated powers up to 10 kVA: ≤ 10 %

Table 1 Key requirements concerning voltage and frequency limit values for performance classes of generating sets [2] Upon voltage loss in the power network the control system activates starting procedures with several second delay. Such solution allows avoiding unnecessary starts caused by voltage dips or short supply interruptions. The delay is normally 5-10 seconds. Time elapsing between 5

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the loss of voltage in the power network and the availability of the emergency power usually does not exceed 1 minute. After the voltage recovery in the power network the automatic stand-by switching system switches to the main supply without stopping the generating set. After the switchover the generating set is run idle for about next 3 minutes for cooling the generator. Figure 2 shows diagram of a generating set with short starting time In normal conditions the electric synchronous motor (1) draws energy from power network and drives the generator (2) and flywheel (3). The clutch (4) is disengaged. The generator is running idly. Upon loss of voltage in the power network the switch Q1 closes and the clutch (4) is automatically engaged. Kinetic energy stored in the flywheel is used for starting the internal combustion engine (5), which within a short time begins to drive the generator. Applying the voltage from the generator switches automatically the Q2 switch to emergency power supply and energizes the loads. Normally the time of power supply disruption does not exceed 2 seconds.

Fig. 3 Diagram of a motor-generator set with zero changeover time 1 – electric motor for the generator and flywheel drive, 2 – generator, 3 – flywheel; 4 – electromagnetic clutch, 5 – internal combustion engine; 6 – control panel and auxiliaries' switchboard Fig. 4 Typical installation of a generating set in a room ― the flexible components 1 ‑ antivibration mounts, 2 ‑ flexible connections in the exhaust system, 3 ‑ flexible connections in the heated air discharge

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Figure 3 shows schematic diagram of a generating set with zero time of changeover to emergency power supply. The set comprises a synchronous motor with rated power equal to the generating set generator power. The loads are normally not supplied from the power grid but from the generating set operated in a continuous manner. In normal operating conditions the synchronous motor (1), supplied from the power grid, drives the generator (2) and the flywheel (3). In the event of voltage loss in the power grid the clutch couples the flywheel to the internal combustion engine (5). Kinetic energy stored in the flywheel is used for quick start of the internal combustion engine (5), which now drives the generator. The switch Q2 has only a maintenance function. A generating set can be installed in an outdoor container-type housing placed on a concrete foundation outside the building or in a specially adapted room. In both cases an air intake, exhaust gases discharge and adequate room ventilation are required. This problem shall be solved by a designer according to the requirements of the generating set manufacturer and therefore is outside the scope of this publication. A set installed in a container by the manufacturer is a complete plant in both the electrical engineering and safety terms. Where a room is adapted for the purposes of a generating set installation all the manufacturer's requirements shall be complied with. Figure 4 shows the example of a generating set installation in a room. The room shall be provided with auxiliaries' switchboard, lighting, socket outlets, and control installation for the purposes of ventilation and other equipment, as required. Figure 4 shows a generating set room with wall air intake and air exhaust vent. A generating set produces heat from the following sources: the engine, generator (alternator), radiator, exhaust pipe and silencer. Insufficient ventilation in the room with a running generating set may cause undesired rise of the room temperature that may lead to the engine power reduction and, consequently, the set shutdown. In such case a forced-air cooling, e.g. by means of fans should be applied. The airflow direction within the room should preferable be: the generator => engine =>

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radiator. That solution not only allows the heat produced by the generating set to be removed from the room but also provides combustion air. Figure 5 shows the correct, typical arrangement of a generating set room. The drawing provides example dimensions and distances for installation of the generating set installation with rated power 60-80 kVA.

Fig. 5. The section and floor plan of installation in the room 1 – generating set, 2 – switchboard and control panel, 3 – foundation, 4 ‑ ventilation air outlet, 5 – cable duct, 6 – air intake, 7 – silencer, 8 –exhaust pipe, 9 – flexible element

3. SIZING THE GENERATING SET POWER Sizing the generating set power shall be based on the demanded active power and reactive power of loads to be supplied from the emergency power supply system. The active power demand can be computed from: n

PZ = ∑ k Z ⋅ Pi

(1)

i =1

where:

PZ - active power [kW] kZ

- demand factor [-]

Pi

- active power of the i-th load to be supplied from the emergency power system [kW]. 8

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

In the next step the reactive power is determined: n

n

i =1

i =1

QZ = ∑ k Z ⋅ tgϕ i ⋅ Pi =∑ k Z ⋅

1 − 1 ⋅ Pi cos 2 ϕ i

(2)

where:

QZ

- reactive power [kvVar]

cos ϕ i - power factor of i-th load to be supplied from the uninterruptible power supply system [-]. The power factor cos ϕ Z is determined from the active and reactive power values: cos ϕ Z =

PZ

(3)

PZ2 + QZ2

The following step is finding the minimum active power to be provided by generating set generator. Determining thr apparent power from the active power reactive power demand using the formula: SG ≥ P Z2 +QZ2

(4)

may lead to erroneous results. The generator shall be able to deliver the demanded active power PZ and demanded reactive power Q Z if it thus operates at a lower power factor cosϕ Z < cosϕ nG , the utilization of generator active power decreases due to the stator heating. The internal combustion engine is matched to the generator's rated active power i.e. to operation with the rated power factor, therefore if the generator operates at a lower power f ac t or cosϕ Z < cosϕ nG , the engine utilization is lower. The generator relative loading with active power can be described by the utilization factor, which can be calculated from the formula:

p=

cosϕ Z cosϕnG

(5) 9

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The required minimum active power of the generating set shall satisfy the inequality: PG min ≥

PZ p

(6)

The utilization factor p, calculated from the formula (5) shall be inserted to the formula (6). If p ≥ 1, the value 1 has be taken in the formula (6). The value of power factor cos ϕnG shall be taken according to the generating set specification. In case the information on rated power factor is not available, the value cos ϕ nG = 0.8 should be taken. The generating set apparent power shall satisfy the inequality:

S nG ≥

PG min cos ϕ z

(7)

where PG min is the minimum active power to be delivered by the generating set generator, in [kW]. A low value of power factor cos ϕ Z will result in reduction of the generator electromotive force, due to the demagnetising effect of the load current reactive component. If the generator generates a larger reactive power than its nominal value, the load active power should be reduced in order to maintain the nominal voltage magnitude and to not overload the rotor. Within the permissible range of rotor current, the generating set control system controls the generator excitation current to maintain the output voltage at constant level. Therefore the generator operation at power factor cosϕZ < cosϕ nG results in the necessity for increasing the generator's output power in order to deliver the full demanded active power PZ and reactive power Q Z . The use of reactive (particularly inductive) power compensators is not recommended since it may lead to premature damaging of capacitors due to the character of the power supply source operation.

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Where the generating set is intended to supply electric motors its power should be sized considering the motors' starting currents which shall not exceed the generator rated current, taking into account permissible temporary overload specified in the manufacturer's data. Non-linear loads draw distorted currents and, consequently, distort the source current. This distortion results in the presence of harmonics and interharmonics (also subharmonics) in the supply network and final circuits, which, generally are shifted in phase with respect to the voltage. In the effect of harmonics, apart of the active and reactive power, also the deformation power V appears in the system; hence the apparent power cannot be determined as the product of fundamental harmonic voltage and current. The value of deformation power V depends on the current and voltage distortion level, i.e. on the harmonic content and, in polyphase circuits, also on the asymmetry level. In asymmetric loads the power factor cosφ is not the same in all phases. It can be different in each phase depending on the active and reactive power load in a given phase. A detrimental effect of asymmetric loading is the voltage increase above the nominal value in the least-loaded phase. Since the estimation of the additional power resulting from asymmetric loading is difficult, the design should ensure the asymmetry of loads supplied from the emergency generating set will not exceed 20%, according to manufacturers' recommendation. The apparent power of a non-linear load can be found from the relation: (8)

S 2 = P2 + Q2 + V 2

The active power of distorted waveforms is estimated as the sum of active powers of voltage and current harmonics of the same frequency and reactive power is determined from relations (10): ∞

P = ∑ U k I k cos ϕ k k =1

∞

(9)

Q = ∑ U k ⋅ I k ⋅ sin ϕ k k =1

(10)

The apparent power of non-linear loads is: 2

S = P2 + Q2

(11) 11

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In this case the deformation power is V = 0. The graphic illustration of powers P, Q, V, S and S is depicted in figure 6.

Fig. 6. Power tetrahedron for a system with distorted voltage and current waveforms

P – active power [kW]; Q – reactive power, [kvar]; S ‑ apparent power of the linear part of the circuit [kVA]; S – apparent power of the non-linear circuit [kVA]; V – deformation power [kVA]; D – distortion power, defined as D 2 = Q 2 + V 2 Figure 6 explains also that in non-linear circuits the power factor cannot be defined by the formula (3), which holds only for linear circuits:

cos ϕ =

P S

(12)

In non-linear circuits the power factor is defined (cf. figure 6) as: cos Ψ =

P I1 P = ⋅ = 2 S I P + Q2

P 3U n

∞

∑I

2 k

k =0

12

(13)

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where the angle ϕ k is the phase shift of the k-th order harmonic voltage and current, sin ϕ k = 1 − cos 2 ϕ k . The nominal current of a three-phase load that draws a distorted current can be found from the formula: In =

Pn 3U n cos Ψ

(14)

It follows from relations (13) and (14) that with the load's steady state current equal to its nominal value IN and increased distortion of the actual current, the active power the given equipment can be loaded with, should be reduced below its rated value. Therefore non-linear loads drawing distorted current from the generator reduce the generating set active power utilization. The generator's power shall be increased in order to meet the power demand of such loads. The generator's minimum active power required to meet the power demand of these loads should be determined from the formula:

PG min ≥

PZ pW

where: p Pz

(15) – the utilization factor calculated from the formula (5) [-], – active power demanded by loads to be supplied from the emergency power supply system [kW]

PGmin – the generator's required minimum active power [kW] W ≈(

100 )2 100 + THDi %

- deformation index [-],

THDi% – total harmonic current distortion factor [-].

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The generating set power should be determined from the formula (7). Total harmonic distortion THDi% can be found from the formula:

∞

THDi =

∑ (I k =2

I1

k

)2 100%

(16)

where: Ik – rms value of the k-th current harmonic [A] I1 – rms value of the current fundamental harmonic [A] k – harmonic order [-]. Example values of the deformation index W, depending on the THDi% values are given in table 2. Table 2 The deformation index W vs. the THDi% factor THDi% 3% 5% 8% 10 % 15 % 20 % 30 % 40 %

W 0.95 0.91 0.86 0.83 0.76 0.70 0.60 0.51

When the THDi% factor increases, the deformation index W decreases, thus the generator power required to meet the power demand should be increased.

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4. SIZING THE UPS POWER Sizing the UPS rated power shall be based on the demanded active power and reactive power of loads to be supplied from it. The active power demand should be computed from the formula (1) and reactive power demand should be computed from the formula (2). In the next step minimum apparent power is calculated from the demanded active and reactive powers using the formula (17)1: (17)

S nUPS ≥ P Z2 +QZ2

If electric motors are to be fed from the uninterruptible power supply system it shall be capable of meeting the increased active power demand due to the motors starting. Power demand of non-linear loads can be calculated from relation: n

PZ = ∑ k z i =1

Pi Wi

(18)

Motors' starting currents and distorted currents shall be taken into account when sizing the UPS output power to ensure its correct operation. A UPS of insufficient output power, supplying electric motors, will in response to load increase automatically switch to external bypass and therefore cut off the uninterruptible power supply function 2. Also the rated crest factor, which is the ratio between the load's instantaneous peak current and the rms current value, shall be considered when selecting a UPS system. For the currently manufactured UPS units the crest factor value is normally 3. If the crest factor of a current drown from the UPS exceeds its rated value, the UPS operation can be impaired, leading even to its shutdown. _________________________ 1

The apparent power determined from the formula (17) refers to the UPS rated power is

specified in manufactuers’ data sheets as the UPS rated power. 2 The UPS power as specified in manufacturers’ data sheets. The UPS input power isnot equal to its output power. The power drawn by the UPS power supply unit from the power network is larger than the power supplied to loads; this should be taken into account when designing the UPS power supply. When sizing the UPS power supply unit on the basis of the demanded active power it advisable to allow 25 % margin for compensation of temporary increase in power, or possible estimation error.

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Since the UPS shall be able to deliver the demanded active power PZ and demanded reactive power Q Z , thus if it operates at a lower power factor cosϕ Z < cosϕ nG , the utilization of the UPS's active power decreases due to limited capability of the inverter semiconductor switches. The UPS relative loading with active power can be described by the utilization factor:

p=

cosϕZ cosϕnUPS

(19)

The UPS required minimum active power must satisfy the inequality:

PUPS min ≥

PZ p

(20)

The utilization factor p, calculated from the formula (19) shall be inserted to the formula (20). If p ≥ 1, in the formula (20) the value 1 has been taken. The power factor value cos ϕnUPS shall be taken according to the UPS specification. If information on the rated power factor is not available, cos ϕ nUPS = 0.8 should be taken for transformer UPS, or cos ϕ nUPS = 0.9 for transformer-less UPS power supplies incorporating IGBT inverters. The UPS apparent power shall satisfy the inequality:

SnUPS ≥

PUPS min cos ϕnUPS

(21)

where: PUPS min - minimum active power [kW] cos ϕnUPS

- the UPS rated power factor [-].

cos ϕ Z 16

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A low value of power factor results in the inverter overloading and, in consequence, may result in turning it off or switching the UPS to internal bypass circuit. If the UPS delivers reactive power larger than its rated value, the load active power should be reduced in order to maintain the nominal voltage magnitude and to not overload the inverter. Therefore the UPS operation at power factor cos ϕ Z < cos ϕ nG a r i s e s the necessity for increasing its output power in order to deliver the demanded active power PZ and demanded reactive power Q Z . T h e use of reactive (particularly inductive) power compensators is not recommended since it may lead to premature damaging of capacitors due to the character of the power supply source operation. Where the UPS is intended to supply equipment with large inrush currents, its power should be sized considering the inrush currents which shall not exceed the UPS rated current, taking into account permissible temporary overload given in the manufacturer's specification. When the UPS supplies non-linear loads they draw and, consequently, distort the source current. This distortion results in the presence of harmonic and interharmonic currents in the supply network and final circuits, generally being shifted in phase with respect to the voltage. In the effect of harmonics also the deformation power appears in the system, apart of the active and reactive power; hence the apparent power cannot be determined as the product of fundamental harmonic voltage and current. The value of deformation power V depends on distortion current and voltage level, i.e. on the harmonic content and, in polyphase circuits, also on the asymmetry level. In asymmetric loads the power factor cosj is not the same in all phases. It can be different in each phase depending on the active and reactive power load in a given phase. It is difficult to estimate the deformation power caused by asymmetric load, but the present-day transformer-less UPSs with IGBT inverters are immune to the output load asymmetry.

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5. DC POWER SUPPLIES ― TELECOM POWER PLANTS The telecom power plant is a DC power supply whose principle of operation is similar to that of the UPS. It is used to provide 48V DC voltage power supply for telephone exchanges. In the event of voltage loss in the power network, the energy is drawn from batteries being a component of the DC power supply. Sizing a telecom power plant consists in selection of a specified equipment type and determining the number of needed DC power supply units (PSU). For the reliability reasons the number N of PSU modules must be increased by 1, i.e.: N' = N +1

(22)

The basis for all calculations is the active power PC demanded by a telephone exchange to be supplied from the telecom power plant. The formulas below are given to facilitate these calculations: Pc = I ncU nc

PŁB = 52I Ł

(23) (24)

3

(25)

PwejSTK = PC + PŁB IŁ =

N=

I ncT p TŁ

(26)

PweSTK P1

(27)

______________________________ 3

The Number 52 in formula (24) is the value of battery charging voltage.

18

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

(28)

' PwejSTK ≈ N ' P1 = ( N + 1) P1

Where : ' PwejSTK - input active power of telecom power plant [W]

PwejSTK - input active power of telecom power plant necessary to cover the loads' power demand [W] PC

- demanded power of the telephone exchange [W]

N

- the number of required DC power supply units [-]

PŁB

- battery charging power [W]

IŁ

- battery charging current [A]

P1

- power of a single DC power supply unit [W]

Inc

- total current drawn by the telephone exchange [A]

Tp

- battery operation back-up time [h]

TŁ

- battery charging time (to be taken 10 hrs.)

Unc

- the telecom power plant rated voltage [W]

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Figure 7 shows the telecom power plant sizing flowchart.

PUPSwe The telecom power plant input power, which has to be known in order to balance the supplied building power demand, shall be computed from the following formulas:

PwejSTK = (

' PwyjSTK

η

QwejSTK = P

' wyjSTK

+ PŁB )

1 W

tgϕ wejSTK (29)

' S wejSTK = ( PwyjSTK ) 2 + (QwejSTK ) 2

1

Where :

tgϕ wejSTK =

cos ϕ wejSTK

- power factor of the telecom power plant input power [-]

QwejSTK

cos ϕ wejST K 2

−1

- the telecom power plant reactive power [var]

When the telecom power plant is supplied from a generating set, it should be borne in mind that generator's long-duration operation at full loading 20

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

and THDi getting close to 8%, may result in premature wear of the generator unless its power is not oversized. A long-duration operation under the generator full load should, therefore, be avoided.

6. UPS – GENERATING SET COMBINATION The reliability of a UPS system can be improved by employing an additional emergency power supply source, i.e. a generating set. Such system ensures safety and confidence that in the event of the normal power supply system failure critical loads will be supplied without interruptions. That solution will protect consumers against losses, often substantial, caused by supply interruptions. A UPS should be sized for the estimated power of loads. It should be remembered that the total power of loads should not exceed the UPS active output power or the apparent output power. It is recommendable to slightly oversize (10-20 %) the UPS system in order to provide a reserve for temporary increase in the loads power or possible estimation errors. A UPS intended for use in co-operation with generating set should be a barrier between loads and the generating set in order to mitigate the impact of distorted currents drawn by non-linear loads (e.g. IT equipment) upon the generating set. The UPS system should ensure that its input current waveform is not influenced by the shape of the load current. The generating set should be capable of meeting full power demand of the UPS and the category II loads. Its power is the sum of the UPS input power consumed under full loading and the power of category II loads.

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PGEN = PUPSwe + PII

(30)

w h e r e : PUPSwe - the UPS input power [kW]

PII

- total power of the category II loads [kW].

The UPS input power is computed from the formula:

PUPSwe =

PUPSwy

ηW

+

PB PUPSwy 0.25PUPSwy = + W ηW W

(31)

w h e r e : PUPSwy - the UPS output active power [kW]

η - the UPS efficiency [-] W

- deformation index, [-]

PB - the UPS additional input power associated with battery charging (at least 25% of the UPS rated power) [kW]. If the UPS has a possibility for extension (its configuration supports increasing the output power), the maximum power shall be taken into account. It is recommended to use UPSs provided with a special interface with generating set that allows limiting of the input current by disabling the battery charging function until the voltage recovery in power grid. In such case a 25% surplus power, needed for the battery charging, is not required4. A UPS intended for co-operation with a generating set should be provided with harmonic filter reducing harmonic content in the input current to ca. 10%; a further reduction is ineffective for it does not significantly improve the UPS–generating set co-operation and therefore is not economically justified. The on-line UPS topology is recommended as the one that exclusively ensures a correct co-operation between the UPS and generating set. It is recommended to use generating sets equipped with electronic rotational speed governors and generators designed for nonlinear load. _____________________________ 4

If the genset is specifically designed to supply non-linear loads the degree of its oversizing can be lower; it should, however, be agreed with a manufacturer. 22

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org EXAMPLE 1 Size the power of a generating set intended for emergency power supply of the below given loads: ο three squirrel cage induction motors with ratings: PNS=7.5 kW; kr = 6; cosj = 0.8; h=0.8; UN =3x400V; sN = 5%; kMr = 2.3 ο UPS with ratings: PN = 15 kW; cosj = 0.95; THDi = 8%; UN =3x400/230 V; h=0.9 ο lighting loads of total power P=5 kW; cosj =0.7 and UN =230V (the loads are grouped symmetrically ensuring equal load in each phase). The rated power of a single motor:

PNS

IN =

3U n cosϕη

75000 = 15.04 A 3 ⋅ 400 ⋅ 0.8 ⋅ 0.9

=

Starting current in delta connection:

I r Δ = k r I N = 6 ⋅15.04 = 90.24 A It is a large current and should be limited. One of the methods is the use of a star/delta starter that reduces the starting current about 3 times. Thus: IrY = I r = 90.24 = 30.08 A

3

3

Due to the large starting current of a single motor, simultaneous start of all motors shall be prevented by means of a control system. Thus, assuming sequential motors starting, the peak power is:

cos ϕ r = cos ϕ

η 1 − sN

M rw 0,90 2,30 + 0.025k r ) = 0.80 ( − 0,025 ⋅ 6) = 0.40 kr 1 − 0,05 6

(

Pszs =

3U N IrY

Qszs=

3U N IrY cos ϕ r

cosϕ r + 2 PNS = 3 ⋅ 30.08 ⋅ 400 ⋅ 0.40 + 2 ⋅ 7500 = 2333.60 W 1 cos ϕ r 2

= 3 ⋅ 400 ⋅ 30.08 ⋅ 0.40 ⋅

− 1 + 2 PNS ∗

1 cos 2 ϕ

−1 =

1 1 − 1 + 2 ⋅ 7500 ⋅ − 1 ≈ 30.33 k var 2 0.40 0.82

The UPS demanded power:

100 100 2 )2 = ( ) ≈ 0.86 W =( 100 + THDi % 100 + 8 PwejUPS =

PwyjUPS Wη

QwejUPS = PwejUPS

+

0,25PwyjUPS W 1 cos ϕ wejUPS 2

=

15000 0.25 ⋅ 15000 + ≈ 23740 W 0.86 ⋅ 0.90 0.86

− 1 = 23740

1 − 1 = 7.80 k var 0.952

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Total demanded active power: PZ = PSZS + PwejUPS + Posw Q Z = Q SZS + Q wejUPS + tgϕPosw

Pz = 23336 + 23740 + 5000 = 31073.60 ≈ 52.08 kW 1 − 1 ⋅ 5.0 ≈ 43.24 k var 0 .7 2

Qz = 30.33 + 7.80 + tgϕ z =

Qz 43.23 = = 0.86 Pz 52.08

cos ϕ z = p=

1 tg ϕ z + 1 2

=

1 = 0.76 0.862 + 1

cos ϕ z 0.76 = = 0.95 cos ϕ nG 0.80

Pz 52.80 = = 55.58 kW p 0.95 P 55.58 SG ≥ G min = = 73.14 kVA cos ϕ z 0.76 PG min ≥

On the basis of the manufacturer's catalogue the above conditions are met by the generating set with rated power 80 kVA.

7. POWER NETWORK ― GENERATING SET CONFIGURATIONS A generating set is an emergency power source that cannot supply electric power to the electric power system. Power network‑generating set configurations shall be therefore designed to preclude parallel operation of sources, or voltage application back to the power system when it is deenergized. Where a generating set is manually started, manual switches shall be used (Fig. 8). Generating sets provided with automatic self-starting and shutdown systems should be equipped with automatic stand-by switching system with mechanical and electrical interlocks. An example of such system is given in figure 9. It should be remembered that since a part of automatic control systems is installed within the set, in the standby mode they shall 24

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

be supplied from the power network (heaters, phase detector, etc). These circuits should be provided with the overload and electric shock protection and be arranged the in TN-S system.

Fig. 8. Manual switch power network/generating set Fig. 9. Automatic standby switching system network/generating set

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8. EXAMPLES OF EMERGENCY POWER SUPPLY SYSTEMS Figure 10 shows the diagram of a power supply system with dedicated genset switchboard. That arrangement is used when selected loads are to be supplied from the generating set.

Fig. 10. Diagram of the emergency power supply system supplying dedicated circuits 1 - Automatic standby switching system; 2 - Capacitor banks

In normal conditions switches Q3 and Q5 are opened, whereas switches Q1, Q2 and Q4 are closed. The automatic standby switching system network/network operates in the hidden reserve mode. In effect of a voltage loss in any of the lines feeding the transformers T1 and T2, the switches Q1, Q2 and Q3, Q4 become opened, and the generating set initiates the auto start procedure. Upon completion of the generating set start-up the switch Q5 is closed and critical loads are fed from the generating set. A voltage recovery in one of the power lines causes the switch Q5 to open, and to close switches: Q3, Q4, and Q1 or Q2 ‑ depending on in which line the voltage has occurred. The generating set is run idly for the next 3 minutes to cool the generator windings and then shut down. 26

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

If voltage recovery occurs in both lines feeding the transformers T1 and T2, the power supply system returns to its normal operating configuration. Where the generating set is intended supply all loads after a voltage loss in the power network, the power supply system configuration shall be designed as shown in figure 11.

Fig. 11. Diagram of an emergency power system supplying all loads 1 - Capacitor banks;

In the configuration shown in figure 11 the switches Q1, Q2 and Q4 are closed in normal conditions, while the switches Q3 and Q5 are open. The system operation is almost analogous to that of the configuration shown in figure 10. When loads are supplied from the generating set, reactive power compensation capacitor banks are disconnected. Figures 12 and 13 show emergency power systems utilizing more than one generating set.

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Fig. 12. An emergency power system with two generating sets supplying separate sections of the main switchboard 1 - Capacitor banks;

Fig. 13. An emergency power system with several generating sets intended for parallel operation 28

Emergency & Uninterruptible Power Supplies for Buildings & Other Structures - sizing the source power www.leonardo-energy.org

The solution shown in figure 13 is employed for supplying large loads. In the event of voltage loss in power lines feeding the transformers T1 and T2 the generating sets are individually turned on. The number of running generating sets depends on the loads' power demand controlled by the control system.

REFERENCES 1. ISO 8528–1: Reciprocating internal combustion engine driven alternating current generating sets - Part 1: Application, ratings and performance. 2. ISO 8528-5: Reciprocating internal combustion engine driven alternating current generating sets - Part 5: Generating sets.

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