Classification and Types of UPS – Part Three

In the previous article “Classification and Types of UPS – Part One”, We explained the following points:

• Applicable Standards for UPS Systems
• What is a UPS?
• Why do we need a UPS?
• UPS Rating
• Classification of UPS

 

In addition, we stated that UPS is classified according to:

1. Voltage range,
2. No. of phases,
3. Mobility,
4. Technological design,
5. Physical Size/capacity,
6. Form factor/ configurations,
7. Topology,
8. Distribution Architecture,
9. Use of transformers.

 

We already explained the first four classifications in the above-mentioned article.

Also, in the article “Classification and Types of UPS – Part Two”, We explained the following classifications:

• Physical Size/capacity,

• Form factor/ configurations.

In addition, we stated that according to the Form factor/ configurations, the UPS Systems have five famous configurations, which are:

1- “N” System Configuration

 

2- “N+1” System Configuration, which includes:

·         Isolated Redundant Configuration (N +1)

·         Parallel Redundant Configuration (1+1)

·         Parallel Redundant Configuration (N +1)

·         Parallel Redundant Configuration (N +2) and so on

 

3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

 

4- Parallel Redundant with STS Configuration

·         Parallel Redundant Configuration (1+1) + STS

·         Parallel Redundant Configuration (N+1) + STS

 

5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N

 

Moreover, in article Part Two, we explained the first configuration; “N” System Configuration and today, we will continue explaining the other configurations of UPS Systems.

 

2-  “N+1” System Configuration

 

 

The “N+1” System Configuration has many sub-Configurations included under it like:   1- Isolated Redundant Configuration (N +1) Using two UPS systems may be with different Ratings and different manufacturers; one on duty and the other is isolated or standby, feeding load through single bus.   2- Parallel Redundant Configuration (1+1) Using two UPS systems with same Rating from the same manufacturer, both on duty and operating in parallel, feeding load through single bus.   3- Parallel Redundant Configuration (N +1) Using more than two UPS systems with same Rating from the same manufacturer, all on duty and operating in parallel, feeding load through single bus.   4- Parallel Redundant Configuration (N +2) and so on   Using Two or more than two UPS systems with same Rating from the same manufacturer, all on duty and operating in parallel, feeding load through dual bus.

 

 

2.1  Isolated Redundant Configuration (N +1)   It is sometimes called the Hot Standby Configuration; this is a way to achieve a level of redundancy for a previously non-redundant “N” System configuration without completely replacing the existing UPS.   Fig-1 Hot Standby Configuration The hot standby configuration is the simplest form in providing a redundant configuration. In Hot standby configuration, we utilize two UPS systems (Fig.1), the normal system power flow to the critical load is through UPS #1 (Main or Primary UPS) and UPS#2 (Isolation or Secondary UPS) acts as standby UPS. If UPS #1 should fail, or if the load current exceeds the UPS's full load rating, the static switch will transfer the critical load to the static bypass (which is the output of UPS #2) within 4-5 milliseconds. When the UPS #1 is rectified, the loads will be transferred back to UPS#1 automatically without any interruption to the loads. Under normal conditions, UPS #2 is idle but always on and ready to assume the critical load through the transfer of static switch (UPS #1). If UPS #2 should fail, its static switch will sense the loss of function and transfer the load to the system bypass source (see Fig.2). The system bypass can be supplied through either a voltage conditioning (regulating) transformer or a non-regulating shielded isolation transformer through Static Var Compensators SCVS. Fig-2 Isolated Redundant UPS Configuration The isolated redundant design concept does not require a paralleling bus, nor does it require that the modules have to be the same capacity, or even from the same manufacturer   Criteria Score Comments Reliability 4 Reduces the probability of the loss of power to the critical load. However, this configuration does not overcome the risk of common output bus which could lead to a single point of failure. The secondary UPS is always in standby condition, the performance of the UPS under fault conditions is a risk as the system’s components performance on load is not knows Complexity 4 Simplest of the redundant UPS configurations Maintainability 5 System configuration allows for either UPS systems to be maintained while the critical load is being fed from a protected power source. Functionality 4 Protects the load from all types of Power Quality issues and also gives the flexibility for converting to standalone configuration in future. Redundancy Level 100%      Advantages: Flexible product choice, products can be mixed with any make or model, Provides UPS fault tolerance, No synchronizing needed, Relatively cost effective for a two-module system.   Disadvantages: Reliance on the proper operation of the primary module's static bypass to receive power from the reserve module, Requires that both UPS modules’ static bypass must operate properly to supply currents in excess of the inverter's capability, The secondary UPS module has to be able to handle a sudden load step when the primary module transfers to bypass. (This UPS has generally been running with 0% load for a long period of time. Not all UPS modules can perform this task making the selection of the bypass module a critical one), Complex and costly switchgear and associated controls, Higher operating cost due to a 0% load on the secondary UPS, which draws power to keep it running, A two module system (one primary, one secondary) requires at least one additional circuit breaker to permit choosing between the utility and the other UPS as the bypass source. This is more complex than a system with a common load bus and further increases the risk of human error, Two or more primary modules need a special circuit to enable selection of the reserve module or the utility as the bypass source (Static Transfer Switch), Single load bus per system, a single point of failure.

 

 

2.2  Parallel Redundant Configuration (1+1)   Parallel redundancy refers to the simultaneous operation of two UPS systems operating in parallel. Figure 3–Parallel Redundant Configuration, illustrates two UPS systems operating in parallel.   Fig-3 Parallel Redundant Configuration (1+1) In this scheme both UPS systems are supplying approximately 50% of the combined AC load. The failure of either UPS systems would result in the entire load being assumed by the healthy UPS. The failure of both UPS systems would result in the load being transferred to the alternate power source via the static transfer switch.     Criteria Score Comments Reliability 3 Advantages of this configuration include features such as load sharing between UPS systems and the fault clearing capability of the overall system is higher because both units can provide short circuit current to the load side protective devices. A negative of this system configuration is a reliance on a single output bus. A failure on the output bus will result in a loss of power to the critical load(s). Complexity 3 Communication has to be established between the UPS systems to achieve proper load sharing and to quickly detect and remove the faulty unit from the parallel output bus making this configuration a much more complex design. Maintainability 5 System configuration allows for either UPS systems to be maintained while the critical load is being fed from a protected power source. Functionality 4 Protects the load from all types of Power Quality issues Redundancy Level 100%

 

 

2.3  Parallel Redundant Configuration (N +1)   Parallel redundancy refers to the simultaneous operation of more than two UPS systems with same rating from the same manufacturer operating in parallel. Figure 4 – Parallel Redundant Configuration, illustrates three UPS systems operating in parallel.   Fig-4 Parallel Redundant Configuration (N+1) In this scheme each of the UPS systems will share equally the combined AC load. The failure of any one of the UPS system would result in the entire load being assumed by the remaining healthy UPS. The failure of two UPS systems would result in the load being transferred to the alternate power source via the static transfer switch. This capability allows any one module to be removed from the bus and be repaired without requiring the critical load to be connected to straight utility. The UPS module manufacturer also provides the paralleling board for the system. The paralleling board may contain logic that communicates with the individual UPS modules, and the UPS modules will communicate with each other to create an output voltage that is completely synchronized. The parallel bus can have monitoring capability to display the load on the system and the system voltage and current characteristics at a system level. The parallel bus also needs to be able to display how many modules are on the parallel bus, and how many modules are needed in order to maintain redundancy in the system. There are logical maximums for the number of UPS modules that can be paralleled onto a common bus, and this limit is different for different UPS manufacturers. The system is N+1 redundant if the “spare” amount of power is at least equal to the capacity of one system module; the system would be N+2 redundant if the spare power is equal to two system modules; and so on   Criteria Score Comments Reliability 3 Advantages of this configuration include features such as load sharing between UPS systems and the fault clearing capability of the overall system is higher because both units can provide short circuit current to the load side protective devices. A negative of this system configuration is a reliance on a single output bus. A failure on the output bus will result in a loss of power to the critical load(s). Complexity 3 Communication has to be established between the UPS modules to achieve proper load sharing and to quickly detect and remove the faulty unit from the parallel output bus make this configuration a much more complex design. Maintainability 5 System configuration allows for each UPS systems to be maintained while the critical load is being fed from a protected power source. Functionality 4 Protects the load from all types of Power Quality issues Redundancy Level 67%      In an N+1 system configuration there is an opportunity for the UPS capacity to grow as the load grows. Capacity triggers need to be set up so that when the percentage of the capacity in place reaches a certain level, (acknowledging that delivery times for some UPS modules can be many weeks or even months), a new redundant module should be ordered. The larger the UPS capacity, the more difficult a task this can become. Large UPS modules weigh thousands of pounds and require special rigging equipment in order to set them into place. There would typically be a spot reserved in the UPS room for this module. This type of deployment needs to be well planned as placing a large UPS module into any room comes with some risk. System efficiency can be an important factor to consider in the design of redundant UPS systems.  Lightly - loaded UPS modules are typically less efficient than a module that is loaded closer to its capacity. The below table shows the typical running load for a system using various UPS module sizes, all feeding a 240 kW load. As can be seen in the table, the module size chosen for a particular application can seriously affect the system efficiency. The efficiency of any particular UPS at low loads varies from manufacturer to manufacturer, and should be investigated during a design process.     UPS Modules in Parallel Mission Critical Load Total UPS System Capacity % Each UPS Module is Loaded 2 X 240 KW 240 KW 480 KW 50% 3 X 120 KW 240 KW 360 KW 66% 4 X 80 KW 240 KW 320 KW 75% 5 X 60 KW 240 KW 300 KW 80% Table: Parallel Redundant Configuration (N +1)   Advantages: Higher level of availability than capacity configurations because of the extra capacity that can be utilized if one of the UPS modules breaks down, Lower probability of failure compared to isolated redundant because there are less breakers and because modules are online all the time (no step loads), Expandable if the power requirement grows. It is possible to configure multiple units in the same installation, The hardware arrangement is conceptually simple, and cost effective.   Disadvantages: All modules must be of the same design, same manufacturer, same rating, same technology and configuration Still single points of failure upstream and downstream of the UPS system, The load is exposed to unprotected power during maintenance of the UPS, batteries or downstream equipment, which usually takes place at least once a year with a typical duration of 2- 4 hours, Lower operating efficiencies because no single unit is being utilized 100%, Single load bus per system, a single point of failure, Most manufacturers need external static switches in order to load-share equally between the UPS modules; otherwise they will share within a wide window of 15%. This adds to the cost of the equipment and makes it more complex, Most manufacturers need a common external service bypass panel. This adds to the cost of the equipment and makes it more complex.

 

 

 

2.4  Parallel Redundant Configuration (N +2) ) and so on   Using Two or more than two UPS systems with same Rating from the same manufacturer, all on duty and operating in parallel, feeding load through dual bus.   Same like (N+1) but the spare power is equal to two system modules; and so on

  

 

3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

 

 

The dual bus configuration, Figure-5 utilizes two or more UPS systems operating in parallel with two independent distribution systems. This is the most reliable and most expensive design in the industry. Fig-5 Dual Bus Configuration   Criteria Score Comments Reliability 4.5 This system configuration eliminates the potential for single point failures associated with single output bus failures that the other configurations are susceptible too with the Perception & distributed configuration. Complexity 4 From a complexity standpoint, this system ranks slightly below the hot standby and slightly above parallel redundant configuration. Maintainability 5 System configuration allows either UPS systems to be maintained while the critical load is being fed from a protected power source. Functionality 4 Protects the load from all types of Power Quality issues Redundancy Level 100%

  

In the next Article, I will continue explaining other Configurations of UPS System. So, please keep following.