Custom Power: The Utility Solution To Distribution Power Quality

By J. Clouston, A. Sundaram and N.H. Woodley

Power quality problems that result in production losses create a dilemma for both the utility and customer. Since the effects of production loss occurs on the customer's side of the meter, it is on the customer's side where the success of power quality problem mitigation efforts must be measured. The best solution, however, results from the use of system solutions and/or power quality improvement equipment on both sides.

Traditionally, for problems on the utility side, the approach has been to desensitize critical loads while "cleaning up" the circuits. However, once these efforts have been accomplished, there are still many situations where it is not possible to provide improvement. In these cases, customer side solutions usually become very expensive. And building new dedicated circuits or substations is difficult for utilities, and may not even provide the needed degree of improvement (isolation).

A survey of utility industrial and commercial customers was conducted by Westinghouse under a contract with EPRI between May-June, 1992. The purpose of the survey was to determine the criteria used in decisions regarding the purchase, installation, and operation of power conditioning equipment. Customers understand that outages cannot be completely eliminated on the power system. However, they are much less forgiving when their processes are upset by momentary disturbances. And these are usually more frequent than power outages. Most customers said they would prefer a utility-provided solution, with the cost included in their power bill, as an alternative to the purchase, installation, and operation of their own on-site equipment.

Custom Power Products

A solution may be at hand. New technology based on power electronics-based concepts have been developed as part of EPRI's Custom Power Program. Known as Custom Power Products, the technology provides customers, or even groups of customers (industrial or office parks), an opportunity to obtain specified levels of power quality from standard service utility distribution systems.

Custom Power offers the customer: no power interruptions; tight voltage regulation; low harmonic voltages; and acceptance of fluctuating and non-linear loads without effect on terminal voltage

Custom Power Products now being offered include:

- Distribution Static Compensator (DSTATCOM)

- Dynamic Voltage Restorer (DVR)

- Solid-State Breakers (SSB)/ Trans-fer Switch (SSTS)

Figure 1 shows how these devices are implemented on the distribution system to provide power quality improvement at the distribution feeder level for sensitive customer loads.

Distribution Statcom (DSTATCOM)

Utility and customer-side disturbances result in terminal voltage variations, transients, and waveform distortions on the distribution system. DSTATCOM is a fast response, solid-state power controller that provides flexible voltage control for power quality improvements at the point of connection to the utility's 4.16 to 69 kV distribution feeder. DSTATCOM is an alternating, synchronous voltage source that is shunt connected to the distribution feeder circuit via a tie reactance. It can exchange both reactive and real power with the distribution system by varying the amplitude and phase angle of the voltage source with respect to the line terminal voltage. The result is controlled current flow through the tie reactance between DSTATCOM and the distribution lines. This enables DSTATCOM to control the terminal voltage and correct the power factor at substations and customer connection points on the feeder in instantaneous real time.

DSTATCOM is implemented by a solid-state, AC to DC voltage-source inverter utilizing advanced power semiconductor switching devices. It effectively replaces conventional voltage and var control elements, load tap changing transformers, voltage regulators, and switched capacitors. Figure 2 shows a+/-2 MVA inverter assembly that forms the power electronic heart of DSTATCOM. Modular design permits ratings up to +/- 10 MVA.

DSTATCOM can also be equipped with an energy storage device (battery, capacitor, superconducting magnetic energy storage system, flywheel, etc.) to supply real uninterrupted power to downstream customers during temporary outages. To achieve uninterruptable power operation, DSTATCOM and the load are disconnected from the source. Meanwhile, DSTATCOM provides full real power requirement to the load from the rechargeable energy storage subsystem that is connected to its DC terminal. As soon as the utility source voltage is restored, it recharges the energy storage device.

DSTATCOM also provides utility distribution planners with a means for extending the reach of voltage drop-limited systems as well as improving the efficiency of the system. Variations in voltage amplitude along the feeder resulting from load fluctuations can be mitigated by DSTATCOM's ability to function as an ideal shunt compensator.

Since the utility system no longer has to provide the reactive current, circuit capacity is released to allow increased real power transmission. Any harmonic voltages that may be imposed on the system by nonlinear customer loads are also eliminated by DSTATCOM, thereby preventing one polluting customer from disturbing other sensitive loads on the same feeder.

The prototype built by Westinghouse for EPRI has been installed on BC Hydro's 25 kV system, located near a large automated lumber mill, to provide instantaneous voltage regulation, flicker control, and harmonics mitigation. Its effectiveness in reducing flicker, measured as voltage variation at the metering point for the lumber mill, can be seen in Figure 3. The results shown were taken on two different days, measuring non-identical load conditions. Nevertheless, they are indicative of the level of improvement realized through DSTATCOM.

STATCOM Arc-Furnace Voltage Flicker Reduction Application

A larger version of DSTATCOM is the shunt-connected STATCOM which is ideally suited to solve that tricky transmission and distribution system power quality problem Ñ voltage flicker caused by electric arc-furnaces. Because of the nature of the arc furnace load (rapidly varying, largely unbalanced, poor power factor) it must be "isolated" from the utility system to avoid an adverse impact on other customers. In the past, static var compensators have been used, which can help eliminate some of the visible flicker (up to 2 to 1 reduction), but that is the limit of their capability.

STATCOM, through the use of high speed Gate Turn-Off (GTO) thryistor-based inverters, injects 3-phase currents of arbitrary controlled waveshape to compensate for the non-sinusoidal, unbalanced, randomly fluctuating currents of the furnace. Preliminary results indicate that STATCOM can obtain a flicker reduction of up to 7 to 1. This not only provides complete reactive power compensation but also, through real power exchange, smoothes the real power load seen by the utility system. This makes it an "ideal" compensator. Steel producers can increase steel production by operating the furnace at full power without concern for flicker. Increased life on arc furnace equipment results from reducing furnace transformer tap changing operations due to precise STATCOM control. Heat time is reduced as is electrode consumption and refractory wear.

The first STATCOM applied to arc furnace compensation will be rated at +/- 80 MVA coupled with a 60 MVA capacitor bank for an operating range of -20 to +140 MVA. It will be the first installation of its type in the US (and believed to be the largest in the world). Installation is scheduled for late 1997.

Dynamic Voltage Restorer

Installation of the world's first Dynamic Voltage Restorer (DVR) on a major U.S. utility system to protect a critical customer plant load from power system voltage disturbances ushered in a new era of power quality problem-solving. The prototype DVR built by Westinghouse for EPRI was installed in August, 1996, on the Duke Power Company (North Carolina) 12.47 kV system at an automated yarn manufacturing and weaving factory. It provides protection from disturbances coming from the utility distribution system that serves the plant. A DVR is also installed on the Powercor Australia, Ltd. 22 kV (50 Hz) system to protect a large dairy food processing plant.

The DVR employs solid-state power-electronic switching devices and is capable of generating or absorbing independently-controllable real and reactive power at its AC output terminal. Its DC input terminal is connected to an energy source or energy storage device of appropriate capacity. The DVR is a solid-state DC to AC switching power converter that injects a set of three-phase AC voltages in series with the distribution feeder in synchronism with the voltages of the distribution system. The amplitude and phase angle of the injected voltages are variable thereby allowing control of the real and reactive power exchange between the DVR and the distribution system within predetermined positive (power supply) and negative (power absorption) limits.

The reactive power exchanged between the DVR and the distribution system is internally generated by the DVR without any AC passive reactive components, i.e. reactors and capacitors. Real power exchanged at the DVR AC terminals must be provided at the DVR DEC terminal by an external energy source or energy storage system.

In Custom Power applications the DVR is connected in series with the distribution feeder. By injecting voltages of controllable amplitude, phase angle and frequency (harmonic) into the distribution feeder via a series injection transformer, the DVR can "restore" the quality of voltage at its load-side terminals when the quality of the source-side terminal voltage is significantly out of specification for sensitive load equipment. Figure 4 shows how the DVR restored line voltage to critical loads during sags caused by faults on adjacent feeders.

For large variations (sags) in the source voltage, the DVR supplies partial power to the load from a rechargeable energy source attached to the DVR DC terminal. Following a sag the energy storage device is recharged from the AC system by the DVR. Even without stored energy, the DVR can compensate for the variations of terminal voltage by injecting a lagging voltage in quadrature with the load current. This provides continuously variable series capacitive line compensation. The DVR can also limit fault currents by injecting a lagging voltage in quadrature with the fault current, thereby increasing the effective fault impedance of the distribution feeder.

Field experience with the DVR has proven the ability of the equipment to provide protection to critical plant process loads from voltage disturbances on the utility distribution system. An example of such a disturbance is shown in Figure 5. The extremely deep voltage sag occurred on one phase along with a simultaneous voltage swell on the other two phases. The restored voltage also shown in Figure 5 resulted from maximum DVR inverter output (2 MVA) providing 0.53 per unit voltage injection. In this case, the critical plant process loads have been desensitized to provide a robust system capable of surviving up to a 25 per cent (0.75 per unit retained voltage) sag for relatively short duration. The sag restoration shown in Figure 5 did, in fact, save the plant and avoided a costly process interruption.

Solid-State Breaker/Transfer Switch

Advanced current interruption technology, utilizing high power Solid-State Breakers (SSB), offers a viable solution to many of the distribution system problems that result in voltage sages, swells, and power outages. Solid-state, fast-acting (sub-cycle) breakers can instantaneously operate to prevent the spreading of a disturbance, thereby improving power quality performance to other customers. They can also transfer sensitive loads from a normal supply that experiences a disturbance to an alternate supply that is unaffected. The alternate supply may be another utility primary distribution feeder or a standby power supply operated from an integral energy storage system. In this application, the SSB acts as an extremely fast transfer switch that allows the restoration of power of specified quality to the load with 1/4 cycle. When combined with a current limiting reactor or resistor, the SSB can rapidly insert the current limiting device into the distribution line to prevent excessive fault current from developing from sources of high short circuit capacity. Figure 6 shows other potential SSB applications. A prototype SSB built by Westinghouse for EPRI which has been tested on the Public Service Electric & Gas Co. (New Jersey) system. The prototype consists of two parallel-connected circuit branches: a solid-state switch composed of GTOs and a solid-state switch using SCRs in series with a current limiting reactor or resistor.

Custom Power Peak

Custom Power Products are being evaluated by utilities and developers as a means of providing a guaranteed improvement in the quality of electrical service to industrial (or commercial office) park tenants. The "Custom Power Park" would provide significant benefits in improved power quality over comparable "standard" electrical service. In one proposed application, a major utility and developer have estimated that the Custom Power option would only add $0.30/sqft/yr to rental rates of about $20/sqft/yr.

Economics

Justification for Custom Power solutions is based upon cost/benefit analyses that compare installed costs for the solutions against the customer costs incurred by power quality disturbances. Custom Power product installed costs ($kVa load) vary greatly depending upon the application. Generally, costs can be grouped into two categories of power quality improvement equipment: (1) current interruption equipment which includes the SSB and SSTS estimated at $30-80 kVa of load, and (2) voltage control equipment including the DVR and DSTATCOM estimated at $150-250/kVA of load.

Estimates of power quality disturbances costs vary widely. The results of recent European surveys show a lower valued set of costs with a mean value of approximately $6/kVA of load affected by the disturbance. Other U.S. data provides cost of disturbance information (excluding extremely high costs reported from data/information processing installations), and indicates a mean value of approximately $40/kVA.

Figure 7 shows the annual disturbance events to achieve a simple two-year payback which is used by many industries as a "go-no go" project decision criteria. Clearly, the cost/benefit for Custom Power solutions appears to be well within the reach of present technology. Considering that the average events on U.S. utility distribution feeders is greater than 45 per year, the potential for Custom Power solutions seems clear.

Conclusion

As Custom Power Products begin to find application on utility systems, there will be many changes made to the way systems are designed and operated.

The ability to protect both system and sensitive load from each other at the feeder level offers utility planners a powerful lineup of new option that can forestall the difficult acquisition of new right of way and substation sites. In the highly competitive energy supply industry, these options could not have come at a better time.

J. Clouston is with B.C. Hydro; A. Sundaram is with E.P.R.I.; and N.H. Woodley is with Westinghouse Electric. ET