Connecting plant power to DeltaV

The power and grounding practices in this article have been established over decades to provide excellent protection against electrical surges and transients in industrial installations. The electrical system installation must meet the local area codes and regulations to insure personnel safety and protection of property. The most common methodology is based on a solidly grounded AC power distribution system as recognized by the National Electrical Code (NEC) NFPA 70, the Canadian Standards Association (CSA), the International Electrotechnical Commission (IEC), the Institute of Electrical and Electronics Engineers Inc. (IEEE), and most world area electrical codes. Other grounding methodologies can also provide good alternatives for customer requirements. For more information, refer to the Types of ground systems section in Chapter 5 of this manual.
Industrial power is supplied through a three-phase (3 ∅) transformer and stepped down to a nominal plantwide voltage level. The National Electric Code (NEC) and IEC define low voltage (LV) as at or below 1000 VAC. CSA C22.1-15 defines low voltage as any voltage exceeding 30 V but not exceeding 750 V. The following table, adapted from a table in IEC 60038 Standard Voltages, lists the harmonized global voltage levels for low-voltage AC power. Many industries have separate feeds from two substations or one substation along with generators to provide redundant power.
Table 1 Standardized global power

a. In Europe and many other regions, most facilities with 220/380 V and 240/415 V systems have madethe transition to 230/400 V.
b. In Europe and many other regions, most facilities with 380/660 V systems have made the transition to 400/ 690 V.c. 200 V and 220 V are also used in some countries.
d. 100/200 V are also used with 50 Hz and 60 Hz systems in some countries.
To maintain the highest power factor, maximize efficiency, and reduce harmonics and unwanted noise, it is always best to maintain a balanced system. The secondary side of most 3 ∅ LV isolation transformers in North America is a 120/208 VAC or 277/480 VAC 60 Hz wye with a solidly grounded system. Europe typically distributes low-voltage (LV) power through 230/400 VAC 50 Hz system.To maintain optimal control for a distributed control system (DCS), the DCS must be powered from a separately derived source. This separately derived power can be provided from three-phase AC feeds, singlephase AC feeds, or distributed DC feeds. Separately derived power is very important for reducing unwanted interference.AC power to the DeltaV bulk power supplies can also be provided from a separately derived AC floating or resistance grounded system, however, DeltaV’s DC system must have its DC return solidly grounded to meet and maintain EMC and certifications for safety.

AC power from a grid

Worldwide power is a constantly evolving entity. Today, power is produced from generation, such as hydroelectric; nuclear, coal, or natural gas through steam turbine; wind; or photovoltaic. The three-phase voltage is then stepped up through a transformer and delivered on transmission lines at high-voltage (HV) or ultra-highvoltage (UHV) alternating current with a fundamental frequency of either 60 Hz or 50 Hz. This HV or UHV is ultimately reduced to a medium voltage at the distribution substations. Many industrial customers are supplied medium voltage which is further reduced to meet their plants requirements, typically 400 to 600 VAC.
Regional and national power grids exist throughout the world. This network of power is connected, rerouted, and distributed at the substations to produce power, as shown for example in the following figure. To make this system work in unison, the various power sources must synchronize their generation frequency, voltage, and phase.
Figure 1 Typical power generation and distribution

An integrated control system must maintain a relatively steady state condition by rapidly reacting to transient events and the dynamic conditions introduced both from the sources and loads connected to the grid. A 1976 IEEE report on power disruption listed the following most common causes of disruptive events.
Table 2 Causes of power disruptions

Substations are configured and interconnected radially with lateral services; through loops; or in networked grids of interconnected feeders supplied from several substations. Power may be provided to Industrial customers from any of these types of distribution systems or their facility may be the sole recipient of one of these distribution networks.
Generating plants, transmission systems, distribution substations and integrated control systems use switches, relays, and fuses to redistribute power and, in most cases, reapply power to correct for disruptions. Many disruptions are intermittent and cleared once power is restored to the circuit. For example, if a tree limb falls across a transmission line causing a temporary short: 1) the short is detected; 2) the circuit with the short is removed from the power network; 3) a reclosing breaker is activated to reestablish the original power source to the network after the fault has cleared. This type of situation can cause multiple power quality issues for control systems, such as interruptions, sags, undervoltage, swells on reclosure, or transients from the event or reclosure attempts.
Care should be taken when deriving control system power from multiple sources. If the sources are from two independent distribution substations, a localized power control interconnect system between the distribution substations should be in place to assure equality of voltage, frequency, and phase. If the phase from source one is not completely matched to that of source two, there may be additional neutral current from these separately derived systems with a commonly grounded neutral.

DeltaV AC distribution and grounding system

A well-engineered AC distribution system meets or exceeds all electrical codes and standards. For a typical DeltaV node being powered from the plant’s AC distribution system, the power should be supplied through an isolation transformer or UPS with a good AC ground network established at or near the transformer or UPS.
AC conductors are routed from the AC source to the main disconnect panel (containing the main disconnect breaker or fuse) and then into branch disconnect panels. For large DeltaV systems, multiple branch disconnect panels should always be used. Multiple branch panels enable you to dedicate power to particular areas of the system, its enclosures, and to DeltaV workstations. Figure 2 below is an isolated three-phase system with redundant power. A double-conversion UPS provides isolated power to the primary. An isolation transformer and transfer switch provide a maintenance bypass for UPS service. The secondary power is provided with an isolation transformer. The loads of the three-phase power system should be balanced if sourcing single-phase power as shown in figure below.
Figure 2 Isolated 3-phase system with redundant power

When connecting multiple grounding systems together, such as the plant ground grid and the lightning grounding in the figure above and the DeltaV Instrument Ground (DIG), the lightning ground should have its bonding connection as far as practical from the DIG bonding location on the plant ground grid.
The following figure shows an isolated single-phase system with redundant power. When connecting singlephase power from three-phase plant power distribution, it is important to choose the correct phasing between legs of the service to maintain proper load balancing. A balanced load on the three-phase power reduces harmonics. Harmonics in the system can result in transformer overheating and other power-quality issues.
Figure 3 Isolated single-phase system with redundant power

DC distribution system

Many facilities supply power to control systems through a distributed DC power bus. DC busses typically range from 24 VDC to 28 VDC. These lower DC voltages are safer for personnel in case of shock, and they lessen the risk of fire and explosion. Disadvantages of DC distribution include higher costs, higher susceptibility to noise, and higher power losses. The voltage at the DC source is always greater than the voltage at the various taps throughout the plant due to resistance losses with increased distance.
Adding a separate DC/DC power supply between the DC distribution taps and DeltaV has the following advantages:
• It establishes a separately derived zero-equipotential ground reference. Many DC distribution systems do not have grounded DC returns. Additionally, DC systems that are grounded at the distribution center return may not be at the same ground potential as the DeltaV Instrument Ground (DIG).
• It isolates DeltaV equipment from other equipment also connected to the DC bus in the followingways:
o Stabilizes voltage fluctuations caused by other equipment.
o Provides isolation from noise on the power lines.
• DeltaV certifications for safety, EMC, ATEX/IECEx and Low Voltage all require a solidly grounded DC system. (ATEX is the name commonly given to the two European Directives for controlling explosive atmospheres. IECEx is a system that provides an internationally accepted means of proving compliance with IEC standards.)
DC/DC power supplies must be adequately sized for the DeltaV system, must have the proper certifications for the applicable area, and should be 100 ft (about 30 m) or less from the DeltaV equipment to minimize the susceptibility to noise.
For noisy environments, consider additional surge protection. Low-voltage DC lines are more susceptible to noise than are higher-voltage power lines.
Figure 4 Bussed DC power

Floating AC and high-resistance grounded systems typically with marine applications

Marine power as it applies in this manual refers to shipboard or offshore platform power. Most marine applications for control systems use low voltage (LV) < 1000VAC for the power source. However, marine propulsion is often derived from medium voltage (MV) generation. Redundancy is provided from MV generator-sets which are stepped-down through transformers to provide the LV power systems for control.
Reasons for using a floating ground are to maintain high availability and to prevent oxidation or plating through electrolysis. Marine applications typically use floating AC to obtain high power availability. An example of a floating AC system as described in IEC 60364-1 is an IT power system. DeltaV bulk power supplies can be used in many power schemes including floating IT power designs.
Warning: Floating or high resistance ground systems allow for a single ground fault to occur without loss of power to critical systems. A second fault will result in loss of power and/or hazard to personnel and property. Fault sensing and alarming will allow maintenance to locate and correct the ground fault.
Some equipment failure conditions can produce an imbalance in the line currents when using floating power. For example, bearing wear will cause a rotational imbalance that will affect the line currents. When a nonhazardous current imbalance is detected, an alarm alerts personnel to the need to troubleshoot and clear the issue. This sensing is important to maintain power availability to the process.
Detection of current imbalance is accomplished by measuring leakage with current monitors, such as those listed below, which compare the phase to phase and phase to ground current. The resultant current or net current will be zero. Minor imbalances are a result of wire insulation breakdown or load issues. When an imbalance is observed, action can be taken depending on the severity of the issue. If the imbalance is minor, such as early signs of insulation breakdown, the cable can be replaced at the next scheduled preventive maintenance if the issue does not progress.
The following figure shows a marine application that incorporates surge protection, earth leakage detection, and a solution for phase imbalance or loss of a phase.
Figure 5 Three-phase floating power for marine applications

The surge protection device (SPD) is solidly grounded and if properly maintained will not be a source of leakage. In other words, leakage detection from the SPD to the structure is not generally required because the ground connection is only used in the event of a high voltage surge. The SPD at the first disconnect from the Medium Voltage (MV) distributed 3-phase bus can be either type 1 or type 2 depending on the conditions at the site. The SPD will attenuate surges from phase to phase or phase to hull/structure.
A single isolated ground path should be maintained from the isolation transformers, UPS, transfer switch through to the distribution disconnect to ensure that any leakage current is detected through dedicated ground wires. Earth leakage current will be detected by the current transformer (CT) located between the disconnect and the DIG at the structure. The ground bus in the DeltaV cabinet will be isolated and monitored by a leakage detector. Once leakage is detected the source should be located and eliminated.
Current imbalance relays as shown in the DeltaV cabinets can be set to monitor phase imbalance. This imbalance can be an indication of an issue such as loss of one of the three phases or a component failure.
Offshore applications use the hull of the ship or platform as the ground reference. Leakage current to and throughout the structure will result in a build-up of non-conductive oxidation, i.e. electrolysis. Therefore, it is important to detect and eliminate the cause of the leakage.
The following detection methods are used to maintain the integrity of the hull, to assure the availability of the process, and to protect personnel and property:
Differential ground fault protection is a process of detecting an imbalance in the pole current with respect to a ground, either functional earth or protective.
Differential current monitor is the method and/or device to measure the pole current imbalance by means of current transformers (CT). This is the most general term for current monitoring in that it can measure a single pole with respect to ground; pole to pole; multiple pole current sharing; or multiple pole and the ground leakage.
Differential relays are used in conjunction with Differential current monitors to achieve a level of protection in cases of current imbalance by interrupting circuits with faults. In cases with multiple power sources they may also be used to reconfigure circuit power from the fault to the non-fault circuits.
Residual current devices (RCD) – Measures imbalance in power lines. These RCDs will detect if the current supplied has an equal and opposite current through the return pole conductor(s). If leakage is detected, the RCD interrupts all poles. The interrupt can be fast acting or delayed. Two types of RCDs are RCCB and RCBO listed below.
Residual-current circuit breaker (RCCB) opens power circuits when a current imbalance exists between the poles.
Residual-current circuit breaker with overcurrent protection (RCBO) – protect against current imbalance between the poles, short-circuit, overloads, and earth faults by interrupting power circuits during an event.
Residual current relays are used in conjunction with Residual current devices to achieve a level of protection in cases of current imbalance by interrupting circuits with faults. In cases with multiple power sources they may also be used to reconfigure circuit power from the fault to the non-fault circuits.
Earth Leakage detectors (ELD) are used in a grounded power system, such as low voltage AC power, DC power, or signal wires.
Ground Fault Circuit Interrupter (GFCI) as defined by Article 100 of the NEC “A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device.” The Class A devices will trip when the current to ground is <= 6mA. GFCIs are primarily used in North America with applications in residential and office environments. The GFCIs are in the form of receptacle or breaker and can be considered a non-inclusive subset of differential ground fault protection.
Another method of increasing power availability is using a high-resistance grounding system. If you use floating or high-resistance ground, insulation and interrupt devices must be sufficiently sized to accommodate the possibility of line-to-line fault conditions. Residual current devices (RCDs) should also be incorporated on the supply lines to quickly detect leakage current and provide power interruption if required.
Although AC power and isolated input or output field power can be floating, the DeltaV DC power must remain solidly grounded.
Warning: Use only DeltaV isolated AC I/O products with floating or high-resistance ground. DeltaV AC discrete I/O products are tested and certified for use with solidly grounded AC systems and should not be used on a floating or high-resistance ground.
Note: Isolated AC channels are permitted.
Emerson bulk power supplies can provide up to 1500 VDC isolation from AC power and must be installed in accordance with the manufacturers’ instructions. AC power and grounding is governed by the applicable codes and regulations and is independent of the DeltaV DC power requirements.
If you are using a high-resistance ground, install a surge protection device (SPD) with filters immediately before the DeltaV bulk power supply. SPDs protect the integrity of the DeltaV system from re-strike transients. Restrike transients are generated in HRGs to find system faults.